CA1265269A - Tdm communication system for efficient spectrum utilization - Google Patents
Tdm communication system for efficient spectrum utilizationInfo
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- CA1265269A CA1265269A CA000531565A CA531565A CA1265269A CA 1265269 A CA1265269 A CA 1265269A CA 000531565 A CA000531565 A CA 000531565A CA 531565 A CA531565 A CA 531565A CA 1265269 A CA1265269 A CA 1265269A
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Abstract
TDM COMMUNICATION SYSTEM FOR EFFICIENT
SPECTRUM UTILIZATION
Abstract A time division multiplexed (TDM) communication system (100) is disclosed, which apportions radio frequency communication channels into at least two time slots (Fig. 2). Voice signals for transmission on this system are analyzed and vo-coded (406) into a digital signal that is transmitted during one or more of the time slots. Received messages are recovered from at least one of these time slots and the voice message synthesized (433) from the vo-coded signal. In this manner multiple voice messages may be transceived in a time division multiplexed manner on a single narrowband communication channel.
SPECTRUM UTILIZATION
Abstract A time division multiplexed (TDM) communication system (100) is disclosed, which apportions radio frequency communication channels into at least two time slots (Fig. 2). Voice signals for transmission on this system are analyzed and vo-coded (406) into a digital signal that is transmitted during one or more of the time slots. Received messages are recovered from at least one of these time slots and the voice message synthesized (433) from the vo-coded signal. In this manner multiple voice messages may be transceived in a time division multiplexed manner on a single narrowband communication channel.
Description
SPECTRU~q UTILIZATION
Back~round of the Invention This invention relates generally to two-way radio communication and more particularly to time division multiplexed digital communication and i~ more particularly directed to a communication system for the efficient utilization of the ~re~lency speatrum.
Those skilled in the art will appreciate the congested and crowded nature o~ the available frequency spectrum. ~he Federal Communication Commission (FCC) have continually sought ways to reallocate the available spectrum or as~ign previously reserved spectrum to relieve thi~ cong~stion. This condition is particularly noticeable in metropolitan areas where a large number of radio u~ers are concen~rated in a small geographic area.
ona proposal the FCC is considexing is sharing a portion of the UHF television spectrum with the land mobile market (FCC docket 85-172). Another consideration is the reallocation of the land mobile re erve frequencies in the 896-902 MHz region to private land mobile uses (FCC
docket 84-1233).
Another alternative for the FCC is to redefine the standard for land mobile communication channels.
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Back~round of the Invention This invention relates generally to two-way radio communication and more particularly to time division multiplexed digital communication and i~ more particularly directed to a communication system for the efficient utilization of the ~re~lency speatrum.
Those skilled in the art will appreciate the congested and crowded nature o~ the available frequency spectrum. ~he Federal Communication Commission (FCC) have continually sought ways to reallocate the available spectrum or as~ign previously reserved spectrum to relieve thi~ cong~stion. This condition is particularly noticeable in metropolitan areas where a large number of radio u~ers are concen~rated in a small geographic area.
ona proposal the FCC is considexing is sharing a portion of the UHF television spectrum with the land mobile market (FCC docket 85-172). Another consideration is the reallocation of the land mobile re erve frequencies in the 896-902 MHz region to private land mobile uses (FCC
docket 84-1233).
Another alternative for the FCC is to redefine the standard for land mobile communication channels.
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Currently, the standard for land mobile communication is a ~hannel having a bandwidth of 25 kHz. However, the FCC
may redefine this standard to use 12.5 kHz (or possibly 15 kHz) channels. The theory behind this "band-split" is to effectively double the number of channels in any newly allocated frequency spectrum. Potentially, as "older"
spectrum is reallocated, all communications equipment will be required to op~rate in the 12.5 kHz channel bandwidth.
Although ~acially atkractive, a band-split to double the available number of channels is not without cost. Present day communication devices operate with a su~ficient frequency guard-band tha~ protects against adjacent-channel interference ~given the frequency stability of the transmitters). Of cour~e, the band-split would also reduce the frequency guard-band tending to lead to higher adjacent-channel interference.
Even assuming a greater than a two-to-one improvement in transmitter ~requency stability, and high selectivity arystal filters for the receivers, adjacent-channel performance may be degraded by a band-split. Thus, there exists sub~tantial teahnological barriers that must be overcoma to provide a radio with comparable performance sp~ci~ications at a competitive cost in the marketplace.
Therefore, a sub~tantial need 2xists in the market to develop a communication system that will provide an increase in the number of available communication channels that is compatible with present day 25 kHz channel bandwidths.
Summary of the Invention Accordingly, it is an object of the present invention to provide a spectrally efficient communication system.
It is a further object of the present invention to provide a communication sys~em readily adaptable to improved coding techniques.
Currently, the standard for land mobile communication is a ~hannel having a bandwidth of 25 kHz. However, the FCC
may redefine this standard to use 12.5 kHz (or possibly 15 kHz) channels. The theory behind this "band-split" is to effectively double the number of channels in any newly allocated frequency spectrum. Potentially, as "older"
spectrum is reallocated, all communications equipment will be required to op~rate in the 12.5 kHz channel bandwidth.
Although ~acially atkractive, a band-split to double the available number of channels is not without cost. Present day communication devices operate with a su~ficient frequency guard-band tha~ protects against adjacent-channel interference ~given the frequency stability of the transmitters). Of cour~e, the band-split would also reduce the frequency guard-band tending to lead to higher adjacent-channel interference.
Even assuming a greater than a two-to-one improvement in transmitter ~requency stability, and high selectivity arystal filters for the receivers, adjacent-channel performance may be degraded by a band-split. Thus, there exists sub~tantial teahnological barriers that must be overcoma to provide a radio with comparable performance sp~ci~ications at a competitive cost in the marketplace.
Therefore, a sub~tantial need 2xists in the market to develop a communication system that will provide an increase in the number of available communication channels that is compatible with present day 25 kHz channel bandwidths.
Summary of the Invention Accordingly, it is an object of the present invention to provide a spectrally efficient communication system.
It is a further object of the present invention to provide a communication sys~em readily adaptable to improved coding techniques.
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2`~
It is a further object of the present invention to provide a communication system that operates in a 25 kHz channel bandwidth that maximizes spectral efficiency.
Accordingly, thQse and other objects are achieved 5 in the present time division multiplex communication system.
Briefly, according to the invention, a time division multiplexed (TDM) communicakion system is disclosed, which apportions radio frequency communication channels into at least kwo time slots. Voi~e signals for transmission on this system are analyzPd and vo-coded into a digital signal that is transmitted during one or more of the time slots. Received messages are recovered from at least one of these time slot~ and the voice message syntheæized from the vo-coded signal. In this manner multiple voice messages may be transcei.ved in a time diYision multiplexed manner on a single 25 kHz bandwidth channèl.
Brief De~cripkion o~ the ~rawinqs The features of the present invention which are believed to be novel are set ~orth with particularity in the appended claim~. The invention, together with further ob~ QCt~ and advantages thereof, may be understood with reference to the following description, taken in con~unction with the accompanying drawings, and the several figures of which like referenced numerals identi~y like elements, and in which:
Figure 1 is a block diagram of a TDM
communication system according to the invention;
Figure 2 is an illustration of the preferred organization of a communication ¢hannel;
Figure 3a i6 an illustration of the pref~rred . 35 organization of the slot overhead for a primary to remote - transmission;
2`~
It is a further object of the present invention to provide a communication system that operates in a 25 kHz channel bandwidth that maximizes spectral efficiency.
Accordingly, thQse and other objects are achieved 5 in the present time division multiplex communication system.
Briefly, according to the invention, a time division multiplexed (TDM) communicakion system is disclosed, which apportions radio frequency communication channels into at least kwo time slots. Voi~e signals for transmission on this system are analyzPd and vo-coded into a digital signal that is transmitted during one or more of the time slots. Received messages are recovered from at least one of these time slot~ and the voice message syntheæized from the vo-coded signal. In this manner multiple voice messages may be transcei.ved in a time diYision multiplexed manner on a single 25 kHz bandwidth channèl.
Brief De~cripkion o~ the ~rawinqs The features of the present invention which are believed to be novel are set ~orth with particularity in the appended claim~. The invention, together with further ob~ QCt~ and advantages thereof, may be understood with reference to the following description, taken in con~unction with the accompanying drawings, and the several figures of which like referenced numerals identi~y like elements, and in which:
Figure 1 is a block diagram of a TDM
communication system according to the invention;
Figure 2 is an illustration of the preferred organization of a communication ¢hannel;
Figure 3a i6 an illustration of the pref~rred . 35 organization of the slot overhead for a primary to remote - transmission;
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Figure 3b is an illustration of the pre~erred organization o~ the slot overhead ~or a remote to primary transmission;
Figure 4 is a block diagram of a remote unit according to the invention;
Figure 5 is a block diagram of a primary unit according to the invention;
Figure 6 is a bloc~ diagram of a single ~requency primary unit according to the invention;
Figure 7 ~s a block diagram o~ the pre~erred embodiment o~ the controller of Figure~ 4-~;
Figures 8a-8c are ~low diagrams of the steps executed by the controller o~ Figure 4;
Figure 9 is a flow diagram o~ khe steps executed by the controller of Figures 6 or 7D
Detailed Description of the Pre~erred Embodiment In Figure 1 there is shown a block diagram o~ the time di~ision multiplexed (TDM) system 100 o~ the present invention. The system is comprised essentially o~ a repeater 102, a mobile unit 104, a base station 106 and a portable 108. As used herein, a portable unit (108) is de~ined to be a communication unit typically designed to be carried about the person~ A mobile unit (104) is a transceiving unit designed to be carried in vehicles, and a base station (106) is contemplated to be a permanent or semi-permanent installation at a fixed location. The mobile 104, the base station 106 and the portable unit 108 arQ hereinafter collectively re~erred to as remote units, and the repeater 102 is hereinafer raferred to as the primary station. The remote unit~ communicate via the primary station using radio freguency tRF) channels that are divided into at least two time slots. The RF
channels used by the pre~ent inven~ion are contemplated to be standard narrowband land mobile channels. These channels are typically understood to be communication
Figure 3b is an illustration of the pre~erred organization o~ the slot overhead ~or a remote to primary transmission;
Figure 4 is a block diagram of a remote unit according to the invention;
Figure 5 is a block diagram of a primary unit according to the invention;
Figure 6 is a bloc~ diagram of a single ~requency primary unit according to the invention;
Figure 7 ~s a block diagram o~ the pre~erred embodiment o~ the controller of Figure~ 4-~;
Figures 8a-8c are ~low diagrams of the steps executed by the controller o~ Figure 4;
Figure 9 is a flow diagram o~ khe steps executed by the controller of Figures 6 or 7D
Detailed Description of the Pre~erred Embodiment In Figure 1 there is shown a block diagram o~ the time di~ision multiplexed (TDM) system 100 o~ the present invention. The system is comprised essentially o~ a repeater 102, a mobile unit 104, a base station 106 and a portable 108. As used herein, a portable unit (108) is de~ined to be a communication unit typically designed to be carried about the person~ A mobile unit (104) is a transceiving unit designed to be carried in vehicles, and a base station (106) is contemplated to be a permanent or semi-permanent installation at a fixed location. The mobile 104, the base station 106 and the portable unit 108 arQ hereinafter collectively re~erred to as remote units, and the repeater 102 is hereinafer raferred to as the primary station. The remote unit~ communicate via the primary station using radio freguency tRF) channels that are divided into at least two time slots. The RF
channels used by the pre~ent inven~ion are contemplated to be standard narrowband land mobile channels. These channels are typically understood to be communication
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channels having a bandwidth of 25 kHz (for duplex, the channel ~r~guency pairs are spaced 45 MHz apart in the 800 MHz band). of course, other channel bandwidths and spacings are possible, however, the present invention contemplates the use of standard land mobile channel requirements thereby obviating the need for any new FCC
allocations or requirements~
Those skilled in the art can appreciate that human speech contains a large amount o~ redundant in~ormation. To most ef~iciently utilize the ~requency spectrum it is desirable to remove as much of the redundant information as pocsible prior to transmission.
The message i8 then reconstructed at the reaeiving end Prom the transmitted essential speech infor~ation.
Speech production can be modeled as an excitation signal (i.e~, air from the lungs) driving a filter (the vocal tract), which possesses a certain resonant structure.
The spoken sound ahanges with time since the ~ilter varies with time. The excitation is noise-like ~or unvoiced sounds (i.e., consonants) and appears as a periodic excitation ~or voiced sounds (~or example vowels). Therefore, to reduce the amount o~ bandwidth required to send a volced 5ignal, the spectral characteristics of the signal must be analyzed and the nature o~ the exaitation signal mu~t be determined.
Prior communica~ion systems have employed speech digitation techniques such as pulse code modulation (PCM) or continuously variable slope delta (CVSD) modulation to attempt to replicate the time waveforms of the speech signals. However, these techniques suffer the detriment of requiring data rates from 12 kbps to 64 kbps. The current state of the art in land mobile communications is a data rate of 12 kbps to 16 kbps on a 25 kHz channel.
This allows the transmission of one voice signal using CVSD. Those skilled in the art will appreciate that the combination of more efficient voice coding (for example coding in the range of 2.4 kbps to 9.6 kbps) and more
channels having a bandwidth of 25 kHz (for duplex, the channel ~r~guency pairs are spaced 45 MHz apart in the 800 MHz band). of course, other channel bandwidths and spacings are possible, however, the present invention contemplates the use of standard land mobile channel requirements thereby obviating the need for any new FCC
allocations or requirements~
Those skilled in the art can appreciate that human speech contains a large amount o~ redundant in~ormation. To most ef~iciently utilize the ~requency spectrum it is desirable to remove as much of the redundant information as pocsible prior to transmission.
The message i8 then reconstructed at the reaeiving end Prom the transmitted essential speech infor~ation.
Speech production can be modeled as an excitation signal (i.e~, air from the lungs) driving a filter (the vocal tract), which possesses a certain resonant structure.
The spoken sound ahanges with time since the ~ilter varies with time. The excitation is noise-like ~or unvoiced sounds (i.e., consonants) and appears as a periodic excitation ~or voiced sounds (~or example vowels). Therefore, to reduce the amount o~ bandwidth required to send a volced 5ignal, the spectral characteristics of the signal must be analyzed and the nature o~ the exaitation signal mu~t be determined.
Prior communica~ion systems have employed speech digitation techniques such as pulse code modulation (PCM) or continuously variable slope delta (CVSD) modulation to attempt to replicate the time waveforms of the speech signals. However, these techniques suffer the detriment of requiring data rates from 12 kbps to 64 kbps. The current state of the art in land mobile communications is a data rate of 12 kbps to 16 kbps on a 25 kHz channel.
This allows the transmission of one voice signal using CVSD. Those skilled in the art will appreciate that the combination of more efficient voice coding (for example coding in the range of 2.4 kbps to 9.6 kbps) and more
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e~ficient data transmission (18 kbp~ to 24 kbps on a 25 kHz channel) would allow the transmission o~ two or more voice signals in 25 kHz o~ ~re~uency spectrum.
Prior techinques indicate splitting the communication channels into narrow fre~uency segments, each being the minimum to allow one digitized voice path.
These techniques have two distinct disadvantages. First, narrow channel~ and wide channels do not mlx well within a system so that a gradual tran~ition ~rom wider to lo narrower channels is accompanied by increased co-channel and adjacent channel interference. Secondly, any particular choice of a narrower standard channel bandwldkh "freezes" the state of the art. That is, simply redefining and fixing the standard ~andwidth ~or land mobile communications prohibits advantageous exploitation of technological improvements without another reassignment or redefinition of communication standards.
The present invention keeps the current standard ~or land mobile communication channels while splitting the time among users according to tlle ~raction of the chann~l bit rate required ~or on voice signal. This method has the advantage~ of pre~erving the present level o~ inter~erQnce protection and allowing splitting (in time) as often a~ needed to take ~ull advantage of advances in the state of the art of coding and data transmission.
The present invention contemplates vo-coding the voice signal to minimize the speech data rate. As used herein, vo-coding mean~ the analysis and synthesis of voice, which either utilizes a vocal track model, or quantize~ sub-bands of a speech wave~orm to remove redundant speech information thereby enabling the transmission of the required voice information in a reduced bandwidth.
~ typical example o~ a vo-coder employing a vocal track model is a linear predictive coder ~LPC). An LPC
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analyzer typically operates on blocks of digitized voice, detarmining the model parameters that are applicable during a paxticular block, and transmitting thQse parameters to a synthesizer at the receiving unit. The synthesizer reconstructs the speech signal by using the parameters received. Since the model parameters vary slowly with time compared to the speech waveform, the redundancy of the speech is removed.
A typical example of a vo-coder employing speech sub-band quantitization is a sub-band coder (SBC). In an SBC analyzer, sub-bands of a speech waveform are quantized and a determinakion is made concerning the amoun~ Or speech ener~y in each sub-band. Only those sub-bands having an energy content above a predetermined threæhold are transmitted thereby enabling transmission in a reduced bandwidth. Accordingly, vo-coding provides a further reduction in the speech data rate by using a coding technique basQd upon specific speech characteristics, transmitting only ~he perceptually important in~ormation contained in a speech signal.
Vo-coding allows a su~ficiently low speQch coding rate to enable the division of a 25 kHz channel bandwidth, thereby providing a spectrally e~iciant communication system.
Re~erring now to Figure 2, there is shown an RF
communication channel 200 subdivided into 8 time sub-slot~. Each time sub-slot 1-8 has associated with it an overhead data portion 202 which contains a signalling protocol to be hereina~ter defined. Once the RF channel is divided into a predetermined number of time sub-slots (8 in the preferred embodimen~) they are grouped into subsets that ~orm communication time slots employed by the actual system users.
Those skilled in the art will appreciate that vo-coding a voice at various coding rates may affect the perceived quality of the received speech. Accordingly, spaech vo-coded in a 9.6 kbps sub-band coder may be o~
~ CM-00183N
higher perceived quality than 2.4 kbps LPC coded speech.
There~ore, the present invention contemplates grouping ~he 8 tlma sub-slots into subsets as required by the particular vo-coder utilized. An exemplary arrangement of slot assignments i8 illustrated in Figure 2 (reference 202). Sub-slot~ 1-4 have been combined to form slot la, which may provide toll quality speech for the users of a system. Slot lb and slot lc are formed by combining two sub slots (5~6 and 7~8 respectively) that may provide speech o~ a lesser quality that i~ still acceptable to a particular user. Accordingly, the air-time billing rate may vary depending upon the quality of speech required in a particular user environment. Moreover, as technology improves and the quality of speech for a lower bit rate vo-coder is enhanced, ~urther ~ubdivisions may be readily employed since the system wa~ designed originally to operate with a greater number of time slots (i.e., ultimately the ~.time sub-slots would be communication time slots).
Re~erring now to Figures 3a and 3b, there is shown the preferred embodiment of ths overhead data in~ormation (202 o~ Figure 2) for both the primary-to-remote, and remote-to-primary transmissions.
Figura 3a illustrate~ the primary-to-remote data overhead 300. The data overhead begins with a propagation delay 302. Typically, the maximum propagation time delay will be de~ined by the particular system coverage designed into a particular implementation. Typically, system range is predominately responsible for determining the propagation delay. For example, the two-way propagation delay for distant remote units (60 miles) may be twelve bits with 18 kbps signalling. If the vo-coded signal recaived at the primary station (repeater) were simply repeated, the message delay would become a function of 3~ the distance o~ the transmitting remote unit. The receiving remote uni~s would be required to correctly detexmine where the message information resided within .
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the slot to correctly recover the voice messaye.
Accordingly, the present invention contemplates a system wherQin the primary ~tation repeat~ the information at a fixed point in the slot. All remote units synchronize to the primary station's transmitted signal.
Following the propagation delay 302 is the transmit key time 304. The transmit key time 304 representq khe time required to switch a unit between thP
transmit and receive frequency. This is typically considered to be a hardware limitation, and in the preferred embodiment is lo 22 milli-seconds (ms) in duration. Those skilled in the art will appreciate ~hat the actual number of bitR transmitted will depend on the data rate used. Of course, as improved power amplifiers and ~re~uency synthe6izers are designed, the transmit key time may decrease to a lesser duration. The bit synchronization pa~tern 306 follows the transmit key 304.
The bit sync portion o~ the data overhead 300 rapresents a digital pattern re~uired to obtain bit synchronization between a transmitting unit and a receiving unit. In the preferred embodiment, the bit sync portion 306 consists o~ 1.22 ms of an alternating logic-one logic-zero pattern. After acquiring bit synchronization, the receiving unit must al80 have ~rame synchronization to properly decods one or more time slot~. In the pre~erred embodiment o~ the present invention the ~rame synchronization portion 308 consists of a predetermined digital word~ The receiving unit must correctly receive the frama sync portion 308 in a majority decision fashion (3 out of 5 in the preferred embodiment) in order to properly acquire ~rame synchronization. Synchronizing in this manner allows an acceptabla system falsing rate utilizing a minimized number o~ data bit~ to ~orm the synchronization word. ~fter frame synchroniza~ion, the receiving remote unit receives the sub~rame ID code 310.
The sub~rame ID code contains information which is used by a remote unit to control and direct the receiving ~_ ~s ~_D ~
circuitry to operate on at lea~ one TDM 510~. Of aour~, a~ uBtrated in Figure 2, ths receiving remote unit may be informed, via the subframe ID 310, that it will group a plurality of time sub-slots into a single user slot. ~ter correctly synchronizing and decoding an a~signment to at least ona TDM ~lot, the remote receives the vo~coded speech 312, which follows the data ov~rhead 300.
In Figure 3b, the data overhead 314 for the remote-to-primary station transmission is illustrated.
lo The daka overhead 314 i8 similar to the data overhead 300 o~ Figure 3a except that the pxopagation delay 302 is not requirad since the primary station repeats all messages at the same point in the time slot, and the subframe ID
310 is not required since slot assignment i~ performed by the primary station (repeater). Following the frame synchroni2ation portion 308 (of the remote-to-primary station datA overhead 314) the remote unit transmits the vo-coded VoicQ me~sage.
In Figure 4 there i5 shown a block diagram of a remote unit 400. The heart of the remote unit 400 is the controller 402 (a more detailed illustration and disaus~ion of which follows hereinafter). To transmit, a ~peech signal i~ first input via a microphone 404. The speech i~ analyzed by a vo-coder analyzer 406, which is enabled by ~he controller 402 via connection 407. The vo-coder analyzer may be any suitable coder and in the preferred embodiment is an LPC or SBC vo-coder. The controller 402 takes the vo~coded information, which is in digital form, and routes i~ to the transmit buf~er 408 via data line 410. The digitized speech information is stoxed in the transmit buffer 408 at whatever coding rate is selected for th~ vo-coder analyzer 406. Typical examples of vo-coding data rates include, but are not limited to, 9~6, 4.8, and 2.4 kbps. When the transmit buffer 408 has reached a predetermined capacity limit, the informa~ion is extracted by the con~roller 402 via ~ CM-00183N
connection 412 and routed to the transmitter 414. Of course, the controller 402 preambles the speech in~ormation by the data overhead portion 202 as illustrated in Figure 2. The controller 402 couples the transmi~ter 414 to an antenna 416 via the switch 418.
Alternatively, the switch 418 could be replaced with a duplexer (or the like) to continually couple the transmitter and receiver to the antenna. In this manner, the data overhead and speech information are transmitted at a selected transmission data rate, which must be at least twice that of the vo-coding data rate.
Alternately, data information (already in digital form) may be transmitted in the same manner via data source 420. Moreover, a combination of vo~coded speech and data may alternatively be sent as determined by a particular usar.
To receive information from a time slot, the controller 402 couples the antenna 416 to a receiver 422 via the switch 418. The receiver ~2 is coupled both to the controller 402 and a clock recovery means 424, which may be any suitabla clock recovery means that will synchronize the controller 402 to the received in~ormation using the bit syna or frame sync portions.
once synchronized, the controller 402 takes the received vo-coded speech ~or digital data) and routes it to the receive bu~fer 426 via connection 428. This information is cloc]ced into the receive bu~fer 426 at a suitable data rate, which typically may be the transmission data rate.
The in~ormation is extracted from receive buffer 426 via connection 430 and routed through the controller 402 to the vo-coding synthesizer 432. Of course, the information must be ex~racted at a data rate identical to that which the speech information was vo-coded. The synthesizer 432, enabled by the controllar 402 by connection 433, operates on the essential speech components to synthesize the voice signal. This signal is applied to a speaker 434 that allows the message to be received by the operator. If, however, data was tran~mitted during a TDM slot, the data sink 436, which may be a printer or monitor device, accepts the data and di~plays it for the operator.
Referring now to Figure 5, there is shown a repeater 500 suitable for usQ in the TDM communication system of the present invention. The controller 502 controls the operation of the repeater 500. The system re~erence 504 provides the aontroller 502 with the clock signal, which is used to determine the transmission data rate. Operationally, a vo-coded signal iR received from at least one time slot on a first frequency and travels from the antenna 506 through the duplexer 508 to a receiver 510. The receiver 510 is coupled to a clock recovery device 512 and the controller 502. The controller accepts the received data signal from the receiver 510 at the data rate determined by the clock recovery davice 512 and supplies it to the transmitter 514. Tha transmitter 514 repeats the signal including the overhead 202 in at least one time ~lot on second frequency (at a transmission da~a rata determined by the controller 502) through the duplexer 508 to the antenna 506.
Re~erring now to Figure 6, a single frequency repeater ~SFR) suitable for use in the TDM system of the present invention is shown. The repaater 600 is controlled by the controller 602, which takes a master clock signal from the system reference 604. A signal is received via antenna 606 and routed via the switch 608 to thQ receiver 610. The receiver 610 supplies signals to the clock recovery means 612 and the controller 602. The received vo-coded ignal is stored in a bu~fer 618 via connection 620 at the received data rate as de~ermined by the clock recover means 612. The vo-coded message is stored in the buf~er 618 until a subsequent time slot, at which time the buffer 618 is emptied by the controller 602 via connection 622 at a predetermined data rate, which i ~ypically the tran~mission data rate. The conkroller 602 then routes the buf~ered signal to the tranRmitter 614. The transmitter 610 sends the signal to the antenna 606 via the switch 608, which has been S coupled to the transmitter via the controller 602 through connection 624. Accordingly, in an SFR, the transmitter 614 and receiver 610 aro multiplexed to the antenna 606 a duplexer is not required. Those skillad in the art will appreciate that either the multiple frequency repeater or 19 the single fxequency repeater may be used alternately or in combination in any particular TD~ system.
Re~erring now to Flgure 7 there i5 shown a block diagram of a controller 700 suitable ~or use in either a primary or remote unit. The controller 700 is comprised o~ a microprocessor 702, such as an MC6801 manu~actured by Motorola, Inc. The microprocessor 702 is supplied a clock signal by clock source 704. The system reference (see Figs. 5 an~ 6) is routed to the frame marker 706 and tha Synchronous Serial Data Adaptor ~SSDA) 708.
Microprocessor 702 is coupled to the frame marker 706 and the SSDA 708 via an address bus 710 and a data bus 712.
The frame marker 706 i9 used to generate ~he ~rame synchronization in~ormation contained in the data overhead a~ was de~cribed in con~unc~ion with Figure 2.
The frame marker 706 can be any convenient device and may be, for example, a programmable timer module (PTM), such ag an MC6840 manu~actured by Motorola, Inc. The SSDA 708 is used in the controller 700 to accept da~a ~rom the mioroprocessor 702 and communicate the data serially to the transmitter 714. In the pre~erred embodiment, the SSDA is an MC685~ manu~actured by Motorola, Inc. The SSDA 708 is also coupled to the clock recovery and data detector 716. The clock recovery data detector 716 is coupled to the receiver 718 and is used to supply the received synchroniza~ion information and received - vo=coded voice signals to the SSDA 708. Thus, the SSDA
is used in both the transmit and receive modes to route data accordingly. The clock recovery and data detector 716 is also coupled to the frame sync detec~or 720. The frame sync detector 720 receives data from the data datector and clock rscovery device 716 and is used to look for the frame sync marker in the received vo-coded signal. When frame synchronization is achieved, the frame sync detector 720 alerts the microprocessor 702 via connection 722. Once the clock recovery device and the frame ~ync detector have both synchronized, the vo-coded signal can be either repeated (a~ in the primary stations of Figs S or 6), or received and routed to the vo-coder synthe~izer to recover the voice signal (as in the remote u~it of Fig~ 4).
Referring now to Figures 8a-8c, there is shown a flow diagram of the steps executed by a controller utilized in a remote unit. In Figure 8a, the routine begins with the initialization step 800, which is executed during first time operation or after a reset.
The initialization step 800 programs any ~re~lency synthesizers and loads various ID codes that may be employed during the operation of the controller. The routine next proceeds to deci~ion 802, which checks to see if the repeater is active. The remote unit determines i~ tha repeater is active via the bik Syllc circuity that operatss on the bit sync portion of the data overhead (see Figure 3). ~ positive bit sync indication occurs if tha repeater i9 operatiny (i.e., transmitting). Of course, if the repeater were inactive, the remote unit would not be abla to obtain bit sync.
Referring again to Figure 8a, if the repeater is not active the routine proceeds to decision 804 to detect whether the push-to-talk (PTT) switch has been actuated to initiate a communication. If the determination of decision 804 is that the PTT switch is not actuated, the routine returns to reference letter A and decision 80~.
The routine will continue in ~his loop un~il the PTT
switch is actuated at which time the routine procaeds to step 805. In step 805, the predetermined repeater key-up code i5 transmitter to activate the repeater. The key-up code may be any suitable code and, of course, if a particular implementation the repeater is always activated, step 805 could be omitted. In the preferred embodiment of ~he present invention, the repeaters are inactive (i.e., off the air) if no remote unit is transmitting. This conserves energy and increases the mean time between failure (MTBF) of the primary station.
Of course, the repeaters could be designed to operate continuously thereby eliminating the need of an activation code. After transmitting the repeater key-up code, ~he routine proceeds to decision 806. Decision 806 determines whether or not synchronization has been achieved. Both bit synchronization and frame synchronization are required for an affirmative determination in decision 806 (however, bit sync may have already been established in decision 802). Frame sync is determined by a ma~ority determination based on a threa-o~-~ive correct receptions of the frame sync word (see Figure 3). If synchronization i9 established, the routine proceeds to step 808, which enables the analyzer of the particular vo-coder employed. Following the enabling of the vo-coding analyzer, the routine proceeds to decision 810, which determines whether the PTT switch has been activated. If the switch has been activated, the routine routes to reference letter B of Figure 8b (to transmit). If the PTT ~witch is not activated, the routine proceeds to reference letter C of Figure 8c (to receiye).
Referring now to Figure 8b, the steps involved during the transmit mode of the controller are shown.
The routine begins in step 812, which takes the digitized speech informa~ion from the vo-coding analyzer. The vo-coded speech is s~ored in the buffer t408 of Figure 4) in step 814 at the vo-coding data ra~e. Decision 816 determines whether the ~uffer is sufficiently full to begin transmitting. In the preferred embodiment, the buffar is deem~d to bs full (ready) when at least one-hal~ of one slot of vo-coded data has been buffered.
If declsion 816 determines that the buffer is not sufficiently full, the routine returns to the reference letter B to receive more vo-coded speech from the analyzer in step 812. I~ the determination of decision 816 is that the buffer i5 sufficiently full, the routine proceeds to decision 818 to determine whether the present time lot is the assigned slot of a particular unit.
The time slots must be assigned so that the mobile controller knows how many of the sub-slots (1-8) to combine for this particular communication slot. If the pr~ent time slot is not the unit's assigned time slot, the routine proceeds to decision 817 to check for sync.
If decision 817 determines that sync has been lost, the routine proceeds to reference letter A. Otherwise, the routine proceeds to reference letter B. I~ decision 818 determines that the present time ~lot is the unit's assigned time slot, the routine proceeds to step 819 to determine whether the unit is still in frame sync. The unit will have a valid ~rame sync if it has correctly recelved five of the past nine frame sync words. If decision 819 datermines that the unit has dropped ~rame sync, control returns to reference letter B. If the unit has held sync, tho routine proceeds to step 820, which formats the data overhead preamble as previously described in conjunction with Figure 3. Following the data overhead formatting of step 820, step 822 transmits a single burst on the TDM channel by transmitting ~he overhead and vo-coded speech taken from the buffer at the transmission data rate. After this single slot is burst on to the TD~ channel, decision 824 determines whether the buffer is empty. I~ the bu*fer is not empty, the routine returns to ra~erence le~ter B which takes more speech and continues to transmit. If the buffer is empty, the routine returns to reference letter A of i ~;r~
FigurQ 8a which det~rmines whether the repeater is active.
In Figure 8c, the steps executed by the mobile controller for the receive operation are shown. Routine begins in step 826 which receives the vo-coded signal from one or more time slots in the TDM channel. Step 828 update~ the slot assignments for the device employing the controller. In the preferred embodiment, thi represen~s updating a memory location which sontain~ the number of sub-slots (1-8) that may be combined in various arrangements to form communication slots for the TDM
device. ~he routine next proceeds to decision 830 to determine whether or not synchronization has been maintained. An affirmative dete~mination results if the unit has correctly received five of the past nine frame sync words. If there is synchronization, the routine proceeds to decision 832 to determine whether the communication davice is muted or whether the squelch is open to allow reception of the message. ThosQ skilled in the art will appreciate various metho~s of squelch are known. One technique would consist of detecting whether khe received signal i~ valid data or noise. An alternative would be to use a form of continuous squelch, commonly re~erred to as "digital private line" (DPL).
Another alternakive would be to employ begin-of-message (BOM) and end-of-message (EOM) data words pre-ambled and post-ambled to the mes~age, respectively. Basically, any suitable squelch system is acceptable to the presenk invention to operate as decision 832. If the squelch is muted, the routine returns to reference letter D of Figure 8a. However, if the squelch is unmuked the routine proceeds to set 834 where the vo-coded signal is placed in the buf~er (426 of Figure 4) at the received data rate. Step 836 remove~ khe buffered ~ignal from khe buffer at the vo-coding data rate and presents it to the vo-coding synthesizer (432 in Figure 4). The vo-coding synthesizer reconstructs the original voice message and ~ CM-00183N
presents i~ to ~he operator either via a speaker of othsr means. Following the completion of the synthesized me~sage, the routine returns to reerence letter D of Figure ~a.
Re~erring now to Figure 9, the steps executed by the primary controller trepeater) are illustra~ed. The routine beglns in decision 900, which determines whether the key up code has been received from a particular remote unit. I~ the key up code is not received, the lo repeater waits (i.e., off the air) until a key up code is received. Assuming, however, that the key up code was received, the routine proceeds to step 902, which starts the frame marker and keys up the transmitter. Step 904 transmits ona burst o~ the data overhead defined in Figure 3 containing the TDM slot assignmen~ ~or ~he remote unit. A~ter the remote unit receives sync and a slot assignment, the remote unit transmits the data overhead and TDM.vo-coded data message to the repeater.
Accordingly, declsion 906 determines whether the synchronization (both bit and ~rame) from the mobile has been received in the pres~nt time slot. I~ sync has been received, the routino proceeds to step 908, which resets a transmitter time--out-timer, which may be present to prevent tha transmitter from transmitting either permanenkly or for prolonged periods. The routine then proceeds to ~tep 910, whiah receives the TDM vo-coded data ~rom ths particular slot (or group of slots) assigned by tha repeater. Step 912 retransmits or repeats the TDM data in another time slot on either the same ~requency or in the same or dif~erent time slot on a second fre~uency depending upon which type of repeater is employed. Following the retransmission of step 912, the routine returns to reference letter A, which again sends one burst o~ data overhead wi~h the time slo~ assignment and continues in this loop until there is no more vo-coded data to transmit.
Referring again to decision 906, if the decision of step 906 i5 that the synchronization was not received in the present time 910t, the routine proceed~ to deci~ion 914, which determine~ whether or not the repeater transmitter i5 still key~d. The repeater transmitter may not be keyed if th~ time-out-timer has expired or a dekey code has been received (i~ any such code is employed). I~ the determination of decision 914 i~ that the repeater is still keyed, an alternating loyical one and logical zero pattern are transmitted in the flrst sub-slot in step 916. Following step 916 The data over-head and slot assignment are transmitted in each of the sub-slots that form ~he particular time slot used. Since th~ data over-head will not fill a sub-slot, an alternating logical one and logical zero pa~tern is 5 US2 to ~ill each sub-slot. Following step 918 the routine returns to re~erQnce letter A, which will again send one burst of data overhead with the time slot assignment to thQ mobile unit, and then to decision 906 to recheck if the repeatar has properly received synchronization ~rom the remote unit. I~ the determination o~ dQcision 914 i~ that the repeater is no longer keyed, the routine returns to reference letter B, which again will await the kQy Up code before the repeater is operational again.
While a particular embodiment o~ the invention has been described and shown, it should be understood that the invention i9 not limited thereto since many modi~ications may be made. It is therefore contemplated to cover by the present application any and all such modificationR that may fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.
What is claimed is:
e~ficient data transmission (18 kbp~ to 24 kbps on a 25 kHz channel) would allow the transmission o~ two or more voice signals in 25 kHz o~ ~re~uency spectrum.
Prior techinques indicate splitting the communication channels into narrow fre~uency segments, each being the minimum to allow one digitized voice path.
These techniques have two distinct disadvantages. First, narrow channel~ and wide channels do not mlx well within a system so that a gradual tran~ition ~rom wider to lo narrower channels is accompanied by increased co-channel and adjacent channel interference. Secondly, any particular choice of a narrower standard channel bandwldkh "freezes" the state of the art. That is, simply redefining and fixing the standard ~andwidth ~or land mobile communications prohibits advantageous exploitation of technological improvements without another reassignment or redefinition of communication standards.
The present invention keeps the current standard ~or land mobile communication channels while splitting the time among users according to tlle ~raction of the chann~l bit rate required ~or on voice signal. This method has the advantage~ of pre~erving the present level o~ inter~erQnce protection and allowing splitting (in time) as often a~ needed to take ~ull advantage of advances in the state of the art of coding and data transmission.
The present invention contemplates vo-coding the voice signal to minimize the speech data rate. As used herein, vo-coding mean~ the analysis and synthesis of voice, which either utilizes a vocal track model, or quantize~ sub-bands of a speech wave~orm to remove redundant speech information thereby enabling the transmission of the required voice information in a reduced bandwidth.
~ typical example o~ a vo-coder employing a vocal track model is a linear predictive coder ~LPC). An LPC
~ 7 - CM-00183N
analyzer typically operates on blocks of digitized voice, detarmining the model parameters that are applicable during a paxticular block, and transmitting thQse parameters to a synthesizer at the receiving unit. The synthesizer reconstructs the speech signal by using the parameters received. Since the model parameters vary slowly with time compared to the speech waveform, the redundancy of the speech is removed.
A typical example of a vo-coder employing speech sub-band quantitization is a sub-band coder (SBC). In an SBC analyzer, sub-bands of a speech waveform are quantized and a determinakion is made concerning the amoun~ Or speech ener~y in each sub-band. Only those sub-bands having an energy content above a predetermined threæhold are transmitted thereby enabling transmission in a reduced bandwidth. Accordingly, vo-coding provides a further reduction in the speech data rate by using a coding technique basQd upon specific speech characteristics, transmitting only ~he perceptually important in~ormation contained in a speech signal.
Vo-coding allows a su~ficiently low speQch coding rate to enable the division of a 25 kHz channel bandwidth, thereby providing a spectrally e~iciant communication system.
Re~erring now to Figure 2, there is shown an RF
communication channel 200 subdivided into 8 time sub-slot~. Each time sub-slot 1-8 has associated with it an overhead data portion 202 which contains a signalling protocol to be hereina~ter defined. Once the RF channel is divided into a predetermined number of time sub-slots (8 in the preferred embodimen~) they are grouped into subsets that ~orm communication time slots employed by the actual system users.
Those skilled in the art will appreciate that vo-coding a voice at various coding rates may affect the perceived quality of the received speech. Accordingly, spaech vo-coded in a 9.6 kbps sub-band coder may be o~
~ CM-00183N
higher perceived quality than 2.4 kbps LPC coded speech.
There~ore, the present invention contemplates grouping ~he 8 tlma sub-slots into subsets as required by the particular vo-coder utilized. An exemplary arrangement of slot assignments i8 illustrated in Figure 2 (reference 202). Sub-slot~ 1-4 have been combined to form slot la, which may provide toll quality speech for the users of a system. Slot lb and slot lc are formed by combining two sub slots (5~6 and 7~8 respectively) that may provide speech o~ a lesser quality that i~ still acceptable to a particular user. Accordingly, the air-time billing rate may vary depending upon the quality of speech required in a particular user environment. Moreover, as technology improves and the quality of speech for a lower bit rate vo-coder is enhanced, ~urther ~ubdivisions may be readily employed since the system wa~ designed originally to operate with a greater number of time slots (i.e., ultimately the ~.time sub-slots would be communication time slots).
Re~erring now to Figures 3a and 3b, there is shown the preferred embodiment of ths overhead data in~ormation (202 o~ Figure 2) for both the primary-to-remote, and remote-to-primary transmissions.
Figura 3a illustrate~ the primary-to-remote data overhead 300. The data overhead begins with a propagation delay 302. Typically, the maximum propagation time delay will be de~ined by the particular system coverage designed into a particular implementation. Typically, system range is predominately responsible for determining the propagation delay. For example, the two-way propagation delay for distant remote units (60 miles) may be twelve bits with 18 kbps signalling. If the vo-coded signal recaived at the primary station (repeater) were simply repeated, the message delay would become a function of 3~ the distance o~ the transmitting remote unit. The receiving remote uni~s would be required to correctly detexmine where the message information resided within .
- g - CM-00183N
the slot to correctly recover the voice messaye.
Accordingly, the present invention contemplates a system wherQin the primary ~tation repeat~ the information at a fixed point in the slot. All remote units synchronize to the primary station's transmitted signal.
Following the propagation delay 302 is the transmit key time 304. The transmit key time 304 representq khe time required to switch a unit between thP
transmit and receive frequency. This is typically considered to be a hardware limitation, and in the preferred embodiment is lo 22 milli-seconds (ms) in duration. Those skilled in the art will appreciate ~hat the actual number of bitR transmitted will depend on the data rate used. Of course, as improved power amplifiers and ~re~uency synthe6izers are designed, the transmit key time may decrease to a lesser duration. The bit synchronization pa~tern 306 follows the transmit key 304.
The bit sync portion o~ the data overhead 300 rapresents a digital pattern re~uired to obtain bit synchronization between a transmitting unit and a receiving unit. In the preferred embodiment, the bit sync portion 306 consists o~ 1.22 ms of an alternating logic-one logic-zero pattern. After acquiring bit synchronization, the receiving unit must al80 have ~rame synchronization to properly decods one or more time slot~. In the pre~erred embodiment o~ the present invention the ~rame synchronization portion 308 consists of a predetermined digital word~ The receiving unit must correctly receive the frama sync portion 308 in a majority decision fashion (3 out of 5 in the preferred embodiment) in order to properly acquire ~rame synchronization. Synchronizing in this manner allows an acceptabla system falsing rate utilizing a minimized number o~ data bit~ to ~orm the synchronization word. ~fter frame synchroniza~ion, the receiving remote unit receives the sub~rame ID code 310.
The sub~rame ID code contains information which is used by a remote unit to control and direct the receiving ~_ ~s ~_D ~
circuitry to operate on at lea~ one TDM 510~. Of aour~, a~ uBtrated in Figure 2, ths receiving remote unit may be informed, via the subframe ID 310, that it will group a plurality of time sub-slots into a single user slot. ~ter correctly synchronizing and decoding an a~signment to at least ona TDM ~lot, the remote receives the vo~coded speech 312, which follows the data ov~rhead 300.
In Figure 3b, the data overhead 314 for the remote-to-primary station transmission is illustrated.
lo The daka overhead 314 i8 similar to the data overhead 300 o~ Figure 3a except that the pxopagation delay 302 is not requirad since the primary station repeats all messages at the same point in the time slot, and the subframe ID
310 is not required since slot assignment i~ performed by the primary station (repeater). Following the frame synchroni2ation portion 308 (of the remote-to-primary station datA overhead 314) the remote unit transmits the vo-coded VoicQ me~sage.
In Figure 4 there i5 shown a block diagram of a remote unit 400. The heart of the remote unit 400 is the controller 402 (a more detailed illustration and disaus~ion of which follows hereinafter). To transmit, a ~peech signal i~ first input via a microphone 404. The speech i~ analyzed by a vo-coder analyzer 406, which is enabled by ~he controller 402 via connection 407. The vo-coder analyzer may be any suitable coder and in the preferred embodiment is an LPC or SBC vo-coder. The controller 402 takes the vo~coded information, which is in digital form, and routes i~ to the transmit buf~er 408 via data line 410. The digitized speech information is stoxed in the transmit buffer 408 at whatever coding rate is selected for th~ vo-coder analyzer 406. Typical examples of vo-coding data rates include, but are not limited to, 9~6, 4.8, and 2.4 kbps. When the transmit buffer 408 has reached a predetermined capacity limit, the informa~ion is extracted by the con~roller 402 via ~ CM-00183N
connection 412 and routed to the transmitter 414. Of course, the controller 402 preambles the speech in~ormation by the data overhead portion 202 as illustrated in Figure 2. The controller 402 couples the transmi~ter 414 to an antenna 416 via the switch 418.
Alternatively, the switch 418 could be replaced with a duplexer (or the like) to continually couple the transmitter and receiver to the antenna. In this manner, the data overhead and speech information are transmitted at a selected transmission data rate, which must be at least twice that of the vo-coding data rate.
Alternately, data information (already in digital form) may be transmitted in the same manner via data source 420. Moreover, a combination of vo~coded speech and data may alternatively be sent as determined by a particular usar.
To receive information from a time slot, the controller 402 couples the antenna 416 to a receiver 422 via the switch 418. The receiver ~2 is coupled both to the controller 402 and a clock recovery means 424, which may be any suitabla clock recovery means that will synchronize the controller 402 to the received in~ormation using the bit syna or frame sync portions.
once synchronized, the controller 402 takes the received vo-coded speech ~or digital data) and routes it to the receive bu~fer 426 via connection 428. This information is cloc]ced into the receive bu~fer 426 at a suitable data rate, which typically may be the transmission data rate.
The in~ormation is extracted from receive buffer 426 via connection 430 and routed through the controller 402 to the vo-coding synthesizer 432. Of course, the information must be ex~racted at a data rate identical to that which the speech information was vo-coded. The synthesizer 432, enabled by the controllar 402 by connection 433, operates on the essential speech components to synthesize the voice signal. This signal is applied to a speaker 434 that allows the message to be received by the operator. If, however, data was tran~mitted during a TDM slot, the data sink 436, which may be a printer or monitor device, accepts the data and di~plays it for the operator.
Referring now to Figure 5, there is shown a repeater 500 suitable for usQ in the TDM communication system of the present invention. The controller 502 controls the operation of the repeater 500. The system re~erence 504 provides the aontroller 502 with the clock signal, which is used to determine the transmission data rate. Operationally, a vo-coded signal iR received from at least one time slot on a first frequency and travels from the antenna 506 through the duplexer 508 to a receiver 510. The receiver 510 is coupled to a clock recovery device 512 and the controller 502. The controller accepts the received data signal from the receiver 510 at the data rate determined by the clock recovery davice 512 and supplies it to the transmitter 514. Tha transmitter 514 repeats the signal including the overhead 202 in at least one time ~lot on second frequency (at a transmission da~a rata determined by the controller 502) through the duplexer 508 to the antenna 506.
Re~erring now to Figure 6, a single frequency repeater ~SFR) suitable for use in the TDM system of the present invention is shown. The repaater 600 is controlled by the controller 602, which takes a master clock signal from the system reference 604. A signal is received via antenna 606 and routed via the switch 608 to thQ receiver 610. The receiver 610 supplies signals to the clock recovery means 612 and the controller 602. The received vo-coded ignal is stored in a bu~fer 618 via connection 620 at the received data rate as de~ermined by the clock recover means 612. The vo-coded message is stored in the buf~er 618 until a subsequent time slot, at which time the buffer 618 is emptied by the controller 602 via connection 622 at a predetermined data rate, which i ~ypically the tran~mission data rate. The conkroller 602 then routes the buf~ered signal to the tranRmitter 614. The transmitter 610 sends the signal to the antenna 606 via the switch 608, which has been S coupled to the transmitter via the controller 602 through connection 624. Accordingly, in an SFR, the transmitter 614 and receiver 610 aro multiplexed to the antenna 606 a duplexer is not required. Those skillad in the art will appreciate that either the multiple frequency repeater or 19 the single fxequency repeater may be used alternately or in combination in any particular TD~ system.
Re~erring now to Flgure 7 there i5 shown a block diagram of a controller 700 suitable ~or use in either a primary or remote unit. The controller 700 is comprised o~ a microprocessor 702, such as an MC6801 manu~actured by Motorola, Inc. The microprocessor 702 is supplied a clock signal by clock source 704. The system reference (see Figs. 5 an~ 6) is routed to the frame marker 706 and tha Synchronous Serial Data Adaptor ~SSDA) 708.
Microprocessor 702 is coupled to the frame marker 706 and the SSDA 708 via an address bus 710 and a data bus 712.
The frame marker 706 i9 used to generate ~he ~rame synchronization in~ormation contained in the data overhead a~ was de~cribed in con~unc~ion with Figure 2.
The frame marker 706 can be any convenient device and may be, for example, a programmable timer module (PTM), such ag an MC6840 manu~actured by Motorola, Inc. The SSDA 708 is used in the controller 700 to accept da~a ~rom the mioroprocessor 702 and communicate the data serially to the transmitter 714. In the pre~erred embodiment, the SSDA is an MC685~ manu~actured by Motorola, Inc. The SSDA 708 is also coupled to the clock recovery and data detector 716. The clock recovery data detector 716 is coupled to the receiver 718 and is used to supply the received synchroniza~ion information and received - vo=coded voice signals to the SSDA 708. Thus, the SSDA
is used in both the transmit and receive modes to route data accordingly. The clock recovery and data detector 716 is also coupled to the frame sync detec~or 720. The frame sync detector 720 receives data from the data datector and clock rscovery device 716 and is used to look for the frame sync marker in the received vo-coded signal. When frame synchronization is achieved, the frame sync detector 720 alerts the microprocessor 702 via connection 722. Once the clock recovery device and the frame ~ync detector have both synchronized, the vo-coded signal can be either repeated (a~ in the primary stations of Figs S or 6), or received and routed to the vo-coder synthe~izer to recover the voice signal (as in the remote u~it of Fig~ 4).
Referring now to Figures 8a-8c, there is shown a flow diagram of the steps executed by a controller utilized in a remote unit. In Figure 8a, the routine begins with the initialization step 800, which is executed during first time operation or after a reset.
The initialization step 800 programs any ~re~lency synthesizers and loads various ID codes that may be employed during the operation of the controller. The routine next proceeds to deci~ion 802, which checks to see if the repeater is active. The remote unit determines i~ tha repeater is active via the bik Syllc circuity that operatss on the bit sync portion of the data overhead (see Figure 3). ~ positive bit sync indication occurs if tha repeater i9 operatiny (i.e., transmitting). Of course, if the repeater were inactive, the remote unit would not be abla to obtain bit sync.
Referring again to Figure 8a, if the repeater is not active the routine proceeds to decision 804 to detect whether the push-to-talk (PTT) switch has been actuated to initiate a communication. If the determination of decision 804 is that the PTT switch is not actuated, the routine returns to reference letter A and decision 80~.
The routine will continue in ~his loop un~il the PTT
switch is actuated at which time the routine procaeds to step 805. In step 805, the predetermined repeater key-up code i5 transmitter to activate the repeater. The key-up code may be any suitable code and, of course, if a particular implementation the repeater is always activated, step 805 could be omitted. In the preferred embodiment of ~he present invention, the repeaters are inactive (i.e., off the air) if no remote unit is transmitting. This conserves energy and increases the mean time between failure (MTBF) of the primary station.
Of course, the repeaters could be designed to operate continuously thereby eliminating the need of an activation code. After transmitting the repeater key-up code, ~he routine proceeds to decision 806. Decision 806 determines whether or not synchronization has been achieved. Both bit synchronization and frame synchronization are required for an affirmative determination in decision 806 (however, bit sync may have already been established in decision 802). Frame sync is determined by a ma~ority determination based on a threa-o~-~ive correct receptions of the frame sync word (see Figure 3). If synchronization i9 established, the routine proceeds to step 808, which enables the analyzer of the particular vo-coder employed. Following the enabling of the vo-coding analyzer, the routine proceeds to decision 810, which determines whether the PTT switch has been activated. If the switch has been activated, the routine routes to reference letter B of Figure 8b (to transmit). If the PTT ~witch is not activated, the routine proceeds to reference letter C of Figure 8c (to receiye).
Referring now to Figure 8b, the steps involved during the transmit mode of the controller are shown.
The routine begins in step 812, which takes the digitized speech informa~ion from the vo-coding analyzer. The vo-coded speech is s~ored in the buffer t408 of Figure 4) in step 814 at the vo-coding data ra~e. Decision 816 determines whether the ~uffer is sufficiently full to begin transmitting. In the preferred embodiment, the buffar is deem~d to bs full (ready) when at least one-hal~ of one slot of vo-coded data has been buffered.
If declsion 816 determines that the buffer is not sufficiently full, the routine returns to the reference letter B to receive more vo-coded speech from the analyzer in step 812. I~ the determination of decision 816 is that the buffer i5 sufficiently full, the routine proceeds to decision 818 to determine whether the present time lot is the assigned slot of a particular unit.
The time slots must be assigned so that the mobile controller knows how many of the sub-slots (1-8) to combine for this particular communication slot. If the pr~ent time slot is not the unit's assigned time slot, the routine proceeds to decision 817 to check for sync.
If decision 817 determines that sync has been lost, the routine proceeds to reference letter A. Otherwise, the routine proceeds to reference letter B. I~ decision 818 determines that the present time ~lot is the unit's assigned time slot, the routine proceeds to step 819 to determine whether the unit is still in frame sync. The unit will have a valid ~rame sync if it has correctly recelved five of the past nine frame sync words. If decision 819 datermines that the unit has dropped ~rame sync, control returns to reference letter B. If the unit has held sync, tho routine proceeds to step 820, which formats the data overhead preamble as previously described in conjunction with Figure 3. Following the data overhead formatting of step 820, step 822 transmits a single burst on the TDM channel by transmitting ~he overhead and vo-coded speech taken from the buffer at the transmission data rate. After this single slot is burst on to the TD~ channel, decision 824 determines whether the buffer is empty. I~ the bu*fer is not empty, the routine returns to ra~erence le~ter B which takes more speech and continues to transmit. If the buffer is empty, the routine returns to reference letter A of i ~;r~
FigurQ 8a which det~rmines whether the repeater is active.
In Figure 8c, the steps executed by the mobile controller for the receive operation are shown. Routine begins in step 826 which receives the vo-coded signal from one or more time slots in the TDM channel. Step 828 update~ the slot assignments for the device employing the controller. In the preferred embodiment, thi represen~s updating a memory location which sontain~ the number of sub-slots (1-8) that may be combined in various arrangements to form communication slots for the TDM
device. ~he routine next proceeds to decision 830 to determine whether or not synchronization has been maintained. An affirmative dete~mination results if the unit has correctly received five of the past nine frame sync words. If there is synchronization, the routine proceeds to decision 832 to determine whether the communication davice is muted or whether the squelch is open to allow reception of the message. ThosQ skilled in the art will appreciate various metho~s of squelch are known. One technique would consist of detecting whether khe received signal i~ valid data or noise. An alternative would be to use a form of continuous squelch, commonly re~erred to as "digital private line" (DPL).
Another alternakive would be to employ begin-of-message (BOM) and end-of-message (EOM) data words pre-ambled and post-ambled to the mes~age, respectively. Basically, any suitable squelch system is acceptable to the presenk invention to operate as decision 832. If the squelch is muted, the routine returns to reference letter D of Figure 8a. However, if the squelch is unmuked the routine proceeds to set 834 where the vo-coded signal is placed in the buf~er (426 of Figure 4) at the received data rate. Step 836 remove~ khe buffered ~ignal from khe buffer at the vo-coding data rate and presents it to the vo-coding synthesizer (432 in Figure 4). The vo-coding synthesizer reconstructs the original voice message and ~ CM-00183N
presents i~ to ~he operator either via a speaker of othsr means. Following the completion of the synthesized me~sage, the routine returns to reerence letter D of Figure ~a.
Re~erring now to Figure 9, the steps executed by the primary controller trepeater) are illustra~ed. The routine beglns in decision 900, which determines whether the key up code has been received from a particular remote unit. I~ the key up code is not received, the lo repeater waits (i.e., off the air) until a key up code is received. Assuming, however, that the key up code was received, the routine proceeds to step 902, which starts the frame marker and keys up the transmitter. Step 904 transmits ona burst o~ the data overhead defined in Figure 3 containing the TDM slot assignmen~ ~or ~he remote unit. A~ter the remote unit receives sync and a slot assignment, the remote unit transmits the data overhead and TDM.vo-coded data message to the repeater.
Accordingly, declsion 906 determines whether the synchronization (both bit and ~rame) from the mobile has been received in the pres~nt time slot. I~ sync has been received, the routino proceeds to step 908, which resets a transmitter time--out-timer, which may be present to prevent tha transmitter from transmitting either permanenkly or for prolonged periods. The routine then proceeds to ~tep 910, whiah receives the TDM vo-coded data ~rom ths particular slot (or group of slots) assigned by tha repeater. Step 912 retransmits or repeats the TDM data in another time slot on either the same ~requency or in the same or dif~erent time slot on a second fre~uency depending upon which type of repeater is employed. Following the retransmission of step 912, the routine returns to reference letter A, which again sends one burst o~ data overhead wi~h the time slo~ assignment and continues in this loop until there is no more vo-coded data to transmit.
Referring again to decision 906, if the decision of step 906 i5 that the synchronization was not received in the present time 910t, the routine proceed~ to deci~ion 914, which determine~ whether or not the repeater transmitter i5 still key~d. The repeater transmitter may not be keyed if th~ time-out-timer has expired or a dekey code has been received (i~ any such code is employed). I~ the determination of decision 914 i~ that the repeater is still keyed, an alternating loyical one and logical zero pattern are transmitted in the flrst sub-slot in step 916. Following step 916 The data over-head and slot assignment are transmitted in each of the sub-slots that form ~he particular time slot used. Since th~ data over-head will not fill a sub-slot, an alternating logical one and logical zero pa~tern is 5 US2 to ~ill each sub-slot. Following step 918 the routine returns to re~erQnce letter A, which will again send one burst of data overhead with the time slot assignment to thQ mobile unit, and then to decision 906 to recheck if the repeatar has properly received synchronization ~rom the remote unit. I~ the determination o~ dQcision 914 i~ that the repeater is no longer keyed, the routine returns to reference letter B, which again will await the kQy Up code before the repeater is operational again.
While a particular embodiment o~ the invention has been described and shown, it should be understood that the invention i9 not limited thereto since many modi~ications may be made. It is therefore contemplated to cover by the present application any and all such modificationR that may fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.
What is claimed is:
Claims (27)
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:
1. In a time division multiplex communication system which apportions radio frequency communication channels into at least two time slots for communicating voice signals vo-coded at a predetermined rate, V, a transceiving device comprising:
means for providing a clock signal having a frequency at least twice that of the vo-coding rate;
means for receiving a vo-coded signal from a communication channel having a predetermined maximum data rate,C, during at least one time slot in accordance with a time division multiplex protocol defining N time slots, where N is a positive integer less than or equal to C/V, at a rate approximately equal to said clock signal to provide a received signal, said receiving means including means for synchronizing the device to said received signal, means for buffering said received signal to provide a buffered signal;
means for transmitting said buffered signal onto said communication channel during at least one of said N
time slots at a rate approximately equal to said clock signal, said transmitting means including means for generating and preambling a data signal which includes at least a synchronization signal to said buffered signal;
means for controlling the device whereby the device operates as a single frequency repeater.
means for providing a clock signal having a frequency at least twice that of the vo-coding rate;
means for receiving a vo-coded signal from a communication channel having a predetermined maximum data rate,C, during at least one time slot in accordance with a time division multiplex protocol defining N time slots, where N is a positive integer less than or equal to C/V, at a rate approximately equal to said clock signal to provide a received signal, said receiving means including means for synchronizing the device to said received signal, means for buffering said received signal to provide a buffered signal;
means for transmitting said buffered signal onto said communication channel during at least one of said N
time slots at a rate approximately equal to said clock signal, said transmitting means including means for generating and preambling a data signal which includes at least a synchronization signal to said buffered signal;
means for controlling the device whereby the device operates as a single frequency repeater.
2. In a time division multiplex communication system which apportions radio frequency communication channels into at least two time slots for communicating voice signals vo-coded at a predetermined rate, V, a transceiving device comprising:
means for providing a clock signal having a frequency at least twice that of the vo-coding rate;
means for receiving a vo-coded signal from a first communication channel having a predetermined maximum data rate, C, during at least one time slot in accordance with a time division multiplex protocol defining N time slots, where N is a positive integer less than or equal to C/V, at a rate approximately equal to said clock signal to provide a received signal, said receiving means including means for synchronizing the device to said received signal;
means for transmitting said received signal onto a second communication channel also having said predetermined maximum data rate, C, during at least one time slot in accordance with said time division multiplexed protocol defining said N time slots, at a rate approximately equal to said clock signal, said transmitting means including means for generating and preambling a data signal which includes at least a synchronization signal to said received signal;
means for controlling the device whereby the device operates as a time division multiplex repeater.
means for providing a clock signal having a frequency at least twice that of the vo-coding rate;
means for receiving a vo-coded signal from a first communication channel having a predetermined maximum data rate, C, during at least one time slot in accordance with a time division multiplex protocol defining N time slots, where N is a positive integer less than or equal to C/V, at a rate approximately equal to said clock signal to provide a received signal, said receiving means including means for synchronizing the device to said received signal;
means for transmitting said received signal onto a second communication channel also having said predetermined maximum data rate, C, during at least one time slot in accordance with said time division multiplexed protocol defining said N time slots, at a rate approximately equal to said clock signal, said transmitting means including means for generating and preambling a data signal which includes at least a synchronization signal to said received signal;
means for controlling the device whereby the device operates as a time division multiplex repeater.
3. In a time division multiplex communication system which apportions radio frequency communication channels into at least two time slots for communicating voice signals, an improved transceiving device comprising:
means for receiving a vo-coded signal from a communication channel having predetermined maximum data rate, C, during at least one time slot in accordance with a time division multiplex protocol defining N time slots, where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, at a rate approximately equal to twice said selected coding rate to provide a received signal, said receiving means including means for synchronizing the device to said received signal;
means for analyzing a voice signal at said selected coding rate to provide a vo-coded signal;
means for generating and preambling a data signal which includes at least a synchroniztion signal to said vo-coded signal;
first and second buffering means for buffering respectively said received signal at approximately twice said selected coding rate, and said vo-coded signal at said selected coding rate;
means for transmitting said buffered vo-coded signal onto said communication channel during at least one of said N time slots at a rate approximately equal to twice said selected coding rate;
means for synthesizing a recovered voice signal from said buffered received signal at a rate approximately equal to said selected coding rate; and means for controlling the time division multiplex transceiving device, whereby the device operates in a first mode to repeat said vo-coded signal received during said first time slot during said second time slot, and operates in a second mode to simulate full duplex operation on a single communication channel.
means for receiving a vo-coded signal from a communication channel having predetermined maximum data rate, C, during at least one time slot in accordance with a time division multiplex protocol defining N time slots, where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, at a rate approximately equal to twice said selected coding rate to provide a received signal, said receiving means including means for synchronizing the device to said received signal;
means for analyzing a voice signal at said selected coding rate to provide a vo-coded signal;
means for generating and preambling a data signal which includes at least a synchroniztion signal to said vo-coded signal;
first and second buffering means for buffering respectively said received signal at approximately twice said selected coding rate, and said vo-coded signal at said selected coding rate;
means for transmitting said buffered vo-coded signal onto said communication channel during at least one of said N time slots at a rate approximately equal to twice said selected coding rate;
means for synthesizing a recovered voice signal from said buffered received signal at a rate approximately equal to said selected coding rate; and means for controlling the time division multiplex transceiving device, whereby the device operates in a first mode to repeat said vo-coded signal received during said first time slot during said second time slot, and operates in a second mode to simulate full duplex operation on a single communication channel.
4. In a time division multiplex communication system which apportions radio frequency communication channels into at least two time slots for communicating voice signals, a transceiving device comprising:
means for transmitting a vo-coded signal onto a communication channel having a predetermined maximum data rate, C, in accordance with a time division multiplex protocol defining N time slots where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, said vo-coded signal being temporarily buffered at a first rate and transmitted at a rate exceeding 2 V during at least one of said N time slots, said transmitting means including means for analyzing a voice signal at said selected coding rate, V, to provide said vo-coded signal, and means for generating and preambling a data signal, which includes at least, a synchronization signal to said vo-coded signal;
means for receiving and buffering a vo-coded signal from the communication channel in accordance with said time division multiplex protocol during at least one of said N time slots to provide a received signal, said synchronization signal, and means for processing said received signal at said selected coding rate, V, to synthesize a recovered voice signal form said received signal; and means for intercoupling and controlling said transmitting means and said receiving means such that the device operates to repeat said received signal received in at least one of said N time slots during at least one other time slot thereby providing a single frequency repeater.
means for transmitting a vo-coded signal onto a communication channel having a predetermined maximum data rate, C, in accordance with a time division multiplex protocol defining N time slots where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, said vo-coded signal being temporarily buffered at a first rate and transmitted at a rate exceeding 2 V during at least one of said N time slots, said transmitting means including means for analyzing a voice signal at said selected coding rate, V, to provide said vo-coded signal, and means for generating and preambling a data signal, which includes at least, a synchronization signal to said vo-coded signal;
means for receiving and buffering a vo-coded signal from the communication channel in accordance with said time division multiplex protocol during at least one of said N time slots to provide a received signal, said synchronization signal, and means for processing said received signal at said selected coding rate, V, to synthesize a recovered voice signal form said received signal; and means for intercoupling and controlling said transmitting means and said receiving means such that the device operates to repeat said received signal received in at least one of said N time slots during at least one other time slot thereby providing a single frequency repeater.
5. The device of claim 4, wherein the frequency of said selected coding rate comprises 4.8 kHz.
6. The device of claim 4, wherein the frequency of said selected coding rate comprises 2.4 kHz.
7. The device of claim 4, wherein said analyzing means comprises an LPC analyzer.
8. The device of claim 4, wherein said analyzing means comprises an SBC analyzer.
9. The device of claim 4, wherein said processing means comprises an LPC synthesizer.
10. The device of claim 4, wherein said processing means comprises a SBC synthesizer.
11. The device of claim 4, wherein said transmitting and receiving means are constructed and arranged to operate on a communication channel having a 25 kHz bandwidth.
12. In a time division multiplex communication system which apportions radio frequency communication channels into at least two time slots for communicating voice signals, a transceiving device comprising:
means for transmitting a vo-coded signal onto a communication channel having a predetermined maximum data rate, C, in accordance with a time division multiplex protocol defining N time slots where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, said vo-coded signal being temporarily buffered at a first rate and transmitted at a rate exceeding 2 V during at least one of said N time slots, said transmitting means including means for analyzing a voice signal at said selected coding rate, V, to provide said vo-coded signal, and means for generating said preambling a data signal, which includes at least a synchronization signal to said vo-coded signal;
means for receiving and buffering a vo-coded signal from the communication channel in accordance with said time division multiplex protocol during at least, one of said N time slots to provide a received signal, said synchronizing signal, and means for processing said received signal at said selected coding rate, V, to synthesize a recovered voice signal from said received signal; and means for duplexing said transmitting means and said receiving means such that the device operates to repeat said received signal received from at least one of said N
time slots on a first frequency in at least one of said N
time slots on a second frequency.
means for transmitting a vo-coded signal onto a communication channel having a predetermined maximum data rate, C, in accordance with a time division multiplex protocol defining N time slots where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, said vo-coded signal being temporarily buffered at a first rate and transmitted at a rate exceeding 2 V during at least one of said N time slots, said transmitting means including means for analyzing a voice signal at said selected coding rate, V, to provide said vo-coded signal, and means for generating said preambling a data signal, which includes at least a synchronization signal to said vo-coded signal;
means for receiving and buffering a vo-coded signal from the communication channel in accordance with said time division multiplex protocol during at least, one of said N time slots to provide a received signal, said synchronizing signal, and means for processing said received signal at said selected coding rate, V, to synthesize a recovered voice signal from said received signal; and means for duplexing said transmitting means and said receiving means such that the device operates to repeat said received signal received from at least one of said N
time slots on a first frequency in at least one of said N
time slots on a second frequency.
13. The device of claim 12, wherein the frequency of said selected coding rate comprises 4.8 kHz.
14. The device of claim 12, wherein the frequency of said selected coding rate comprises 2.4 kHz.
15. The device of claim 12, wherein said analyzing means comprises an LPC analyzer.
16. The device of claim 12, wherein said analyzing means comprises an SBC analyzer.
17. The device of claim 12, wherein said processing means comprises an LPC synthesizer.
18. The device of claim 12, wherein said processing means comprises an SBC synthesizer.
19. The device of claim 12, wherein said transmitting and said receiving means are constructed and arranged to operate on a communication channel having a 25 kHz bandwidth.
20. In a time division multiplex communication system which apportions radio frequency communication channels into at least two time slots for communicating voice signals, a transceiving device comprising:
means for transmitting a vo-coded signal onto a communication channel having a predetermined maximum data rate, C, in accordance with a time division multiplex protocol defining N time slots where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, said vo-coded signal being temporarily buffered at a first rate and transmitted at a rate exceeding 2 V during at least one of said N time slots; said transmitting means including means for analyzing a voice signal at said selected coding rate, V, to provide said vo-coded signal, and means for generating and preambling a data signal, which includes at least a synchronization signal to said vo-coded signal;
means for receiving and buffering a vo-coded signal from the communication channel in accordance with said time division multiplex protocol during at least one of said N time slots to provide a received signal, said receiving means including means for synchronizing to at least a portion of said synchronization signal, and means for processing said received signal at said selected coding rate, V, to synthesize a recovered voice signal from said received signal; and means for controlling and selectively intercoupling said transmitting means and said receiving means, such that the device operates in a first mode to repeat said received signal received in at least one of said N time slots during at least one other time slot, and operates in a second mode to simulate full duplex communication on a single channel.
means for transmitting a vo-coded signal onto a communication channel having a predetermined maximum data rate, C, in accordance with a time division multiplex protocol defining N time slots where N is a positive integer less than or equal to C/V, where V comprises a selected coding rate, said vo-coded signal being temporarily buffered at a first rate and transmitted at a rate exceeding 2 V during at least one of said N time slots; said transmitting means including means for analyzing a voice signal at said selected coding rate, V, to provide said vo-coded signal, and means for generating and preambling a data signal, which includes at least a synchronization signal to said vo-coded signal;
means for receiving and buffering a vo-coded signal from the communication channel in accordance with said time division multiplex protocol during at least one of said N time slots to provide a received signal, said receiving means including means for synchronizing to at least a portion of said synchronization signal, and means for processing said received signal at said selected coding rate, V, to synthesize a recovered voice signal from said received signal; and means for controlling and selectively intercoupling said transmitting means and said receiving means, such that the device operates in a first mode to repeat said received signal received in at least one of said N time slots during at least one other time slot, and operates in a second mode to simulate full duplex communication on a single channel.
21. The device of claim 20, wherein the frequency of said selected coding rate comprises 4.8 kHz.
22. The device of claim 20, wherein the frequency of said selected coding rate comprises 2.4 kHz.
23. The device of claim 20, wherein said analyzing means comprises an LPC analyzer.
24. The device of claim 20, wherein said analyzing means comprises an SBC analyzer.
25. The device of claim 20, wherein said processing means comprises an LPC synthesizer.
26. The device of claim 20, wherein said processing means comprises an SBC synthesizer.
27. The device of claim 20, wherein said transmitting and said receiving means are constructed and arranged to operate on a communication channel having a 25 kHz bandwidth.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US843,961 | 1986-03-25 | ||
| US843,882 | 1986-03-25 | ||
| US06/843,961 US4742514A (en) | 1986-03-25 | 1986-03-25 | Method and apparatus for controlling a TDM communication device |
| US06/843,882 US4754450A (en) | 1986-03-25 | 1986-03-25 | TDM communication system for efficient spectrum utilization |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1265269A true CA1265269A (en) | 1990-01-30 |
Family
ID=27126450
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000531565A Expired CA1265269A (en) | 1986-03-25 | 1987-03-10 | Tdm communication system for efficient spectrum utilization |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA1265269A (en) |
| IL (1) | IL81835A (en) |
-
1987
- 1987-03-09 IL IL81835A patent/IL81835A/en not_active IP Right Cessation
- 1987-03-10 CA CA000531565A patent/CA1265269A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| IL81835A (en) | 1990-11-05 |
| IL81835A0 (en) | 1987-10-20 |
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