MXPA02001199A - Method and apparatus supporting tdd/tty modulation over vocoded channels. - Google Patents

Abstract

A method used to encode/decode a low activity communication signal - such as a Baudot tone - for transmission over a telecommunications system (100). The telecommunications system (100) may include any number of wireless links. Once the system (100) is noticed that a low activity signal needs to be transmitted, each vocoder used in the system (100) to encode/decode the signal performs a unique encoding/decoding process. In one embodiment, frames containing errors adversely affecting a signal are delivered to the vocoder and the "soft bits" contained therein are used to determine the original signal transmitted. In another embodiment, encoding of the signal may include encoding the signal using redundancy with the encoded signal being spread across multiple vocoder frames.

Description

METHOD AND APPARATUS THAT SUPPORTS TDD / TTY MODULATION IN VOCODIFIED CHANNELS FIELD OF THE INVENTION Generally, the present invention relates to the field of telecommunication devices for the deaf (TDDs) or teletypewriters (TTYs). More particularly, the invention relates to the modification of the operation of standard vocodi ff ers to allow the reliable transport of TDD / TTY signals within a telecommunications system. The system can include wireless links.

BACKGROUND OF THE INVENTION Many deaf or hearing impaired people use communication terminals constructed and designed specifically to allow them to communicate over standard telephone lines. Such devices, referred to as telecommunication devices for the deaf (TTDs) or teletypewriters (TTYs), are collectively referred to as TDDs in this application. Typically, TDDs include a keyboard and a screen connected to a telephone through a modem (modulator / demodulator). The modem is incorporated within the TDD and is connected directly to a telephone line or is coupled by an acoustic coupler to a hand-held handset. TDDs are capable of transmitting information over telephone lines by means of coded tones to other TDDs connected at opposite ends of the telephone line through another modem. These tones are referred to as low activity communications because the frequency and amplitude envelopes remain relatively constant. The code and protocol that is in conventional use disseminated for TDD communications is an idiosyncratic one. The code set, known as Baudot, and the communications protocol (TDD protocol) were historically developed at a time when many telecommunications devices for the deaf were based on mechanical or electromechanical devices rather than on electronic devices. In accordance with the above, the TDD protocol was constructed for a set of constraints that are no longer relevant to the devices present. These restrictions work to create a code protocol and a telecommunications network of users and devices that operate under that protocol, which is somewhat outdated. Traditionally, TDD communications are carried out at 50 Baud (45.5 baud in some countries), representing a transfer of 6 characters per second. Other protocols now available for TDD communications incorporate higher baud rates, such as ASCII (American Standard Code for the Exchange of Information) and improved Baudot protocols. Regardless of this, a set of normal TDD communication characters consists of characters that are 5 bits long. These characters are analogous to a letter in an alphabet where each letter represents a word or idea. A character is grouped with bits of header information before transfer, in which each group of bits to be transferred has a duration or unit interval equal to 22 milliseconds. For example, under the conventional TDD protocol, a group of bits to be transferred comprises 8 bits: an initial bit (a source bit or zero bit), five bits that represent the character, and at least one and i bits that mark the stop point of the transfer group. Compared to modern telecommunications systems, TDD transmissions are given at the snail pitch. A major problem is that the TDD signals are substantially constant. These monotonous, slow-paced signals can create havoc in digital telecommunications systems that transmit signals from high activities to very high speeds, and especially in telecommunications systems that include wireless links. An example of such a telecommunications system is a code division multiple access (TDMA) system having a large number of subscriber units wireless Each subscriber unit has a transceiver and communicates within the system through satellite repeaters or terrestrial stations referred to as cells. Each cell includes a physical plant called a base station. A cell covers a limited geographical area and directs calls transported by subscriber units to and from the telecommunications network through a mobile switching center. When a subscriber moves within the geographical area of a new cell, the address of that subscriber's call can eventually be made through the new cell through a process called "transfer". A subscriber unit, generically referred to as a cell phone, transmits a signal that is received by a base station. The signal is transmitted to a mobile switching center that routes the signal to a public switched telephone network (PSTN) that includes telephone lines or other subscriber units. Similarly, a signal can be transmitted from the PSTN to a subscriber unit through a base station and a mobile switching center. The interface between the subscriber unit and the base station is referred to as the air interface. The Telecommunications Industry Association (TIA) has provided a standard for the processing of CDMA calls by the air interface entitled "IS-95 Mobile Station - Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System." Supplementary articles to IS-95 are provided as Telecommunications Service Bulletins (TSB). Standard IS-95 + TSB74 includes the provisions for the negotiation of the service by the air interface and is incorporated herein for reference. The negotiation of the service is critical to successfully transmit any communication, especially a low-activity TDD communication, by a digital telecommunications system. One problem with modern systems, including the one mentioned above, is that a vocoder - a device used in the system to encode a voice or an analogue TDD signal in a digital signal, and to decode a digital signal in a voice or in a an analog TDD signal - has difficulty in handling the substantially monotone and low speed signal dictated by the TDD protocol. In current systems, a low-activity communications signal such as a TDD communication would probably be treated by the vocodi fier as background noise or signal interference and discarded. What is needed is an invention that can be easily integrated into existing communication systems and that can be capable of reducing frame error rates by invoking a protocol to be used by vocoders during the transmission of the communication signal of low activity The invention should be compatible with wireless telecommunication modulation systems, such as CDMA systems, serving a large number of system users. A more robust description of the CDMA systems and techniques used in the multiple access communication systems can be found in the U.S. Patent. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated herein by reference thereto. In addition, the invention must also be compatible with other modulation systems and techniques used in other types of communication systems, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and modulation schemes. of amplitude (AMPS).

BRIEF DESCRIPTION OF THE INVENTION Broadly speaking, the present invention involves the modulation of a low activity communication by a telecommunications system that uses coding signals and increased transmission power levels. More particularly, the invention relates to a method using specialized coding, decoding, or both, in a low activity communications signal to minimize the error rate of a transmitted signal. The invention also provides for decoding a low activity signal by looking at the "soft bits" contained in the wrong frames, or in the frames adjacent to a bad frame, in an attempt to determine the content of the original frame. Some disclosed embodiments of the invention provide unique decoding methods for a TDD signal that was encoded using standard coding protocol. In one embodiment, the decoder can compare a frame containing transmission errors (bad frame) with a vocoded frame coming from a known TDD signal and determine the vocoded frame that was most likely transmitted. In another embodiment, the decoder can review the adjacent frames to determine the vocoded frame that was most likely transmitted but received in error. In yet another embodiment, the decoder can be modified to include a signal enhancer or repeater that "cleans" the corrupted bits in the transmitted frame before the decoding methods are applied in the transmitted frames. And while a TDD communication is described throughout this application, it should be understood that any low or slow activity communication may be transmitted using this invention.

Another embodiment of the invention is provided for decoding as described above but invokes vocoder parameters that are different from the standard vocoder parameters. When a TDD signal is received, the encoder switches to the "Baudot encoding mode", notifies the decoder of the protocol change, and uses the channel coding redundancy to further improve the decoder's chances of determining the correct TDD signal sent if it is contained in a wrong plot. This version of the invention replaces the standard vocoder parameters with "signatures" of the vocoder that are better spaced, thus making it easier to distinguish between tones. Another version of the invention is provided to encode a TDD signal in the vocoder frames using redundancy, but by coding through several vocoder frames. The information is distributed through "N" frames so that if a frame is lost, the decoder can extract the necessary information from the adjacent frames to determine the content of the lost frame. Still another version of the invention provides a cost effective system for combining the aforementioned coding and decoding methods of the invention with methods for controlling transmission power levels. Modifying the design of a standard vocoder chip set within a mobile station is expensive, which could obstruct the implementation of the aforementioned encoding and decoding methods. However, a communications system may be implemented in which the aforementioned methods are used in the base stations of the system, but not in the mobile stations. The frame error rates for the low activity communication signals transmitted from the base station to the unmodified vocoder in a mobile station can be minimized using methods to control the transmission power levels.

The invention provides its users with numerous advantages. One advantage is that a TDD message can be transmitted using a digital transmission medium that has wireless links. Still another advantage is that a TDD device can be connected to a mobile device or a subscriber unit, such as a digital cellular phone, connected to the telecommunications system by a wireless link. The invention also provides a number of other advantages and benefits that should become more apparent after reviewing the following detailed descriptions of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The nature, objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description in connection with the accompanying drawings, in which like reference numerals designate similar parts throughout. of the same, wherein: Figure IA is a block diagram of hardware components and interconnections of a telecommunication system incorporating wireless links according to one embodiment of the invention; FIG. IB is a block diagram of a decoder capable of implementing the coding and encoding methods of the present invention coupled to a prior art announcement apparatus according to one embodiment of the invention; Figure 2 illustrates a typical TDD communications device of the prior art used in accordance with one embodiment of the invention; Figure 3 shows a traffic channel frame format for a rate set to 1 used by a variable rate vocoder; Figure 4 is a flow chart of a method aspect according to one embodiment of the invention; Figure 5 illustrates a block diagram of a wireless telecommunications system configured in accordance with an embodiment of the invention; Figure 6 illustrates a block diagram of a wireless telecommunications system configured in accordance with an embodiment of the present invention; and Figure 7 is a flow chart of a method for controlling transmission power levels between a base station and a mobile station.

DETAILED DESCRIPTION OF THE INVENTION Figures 1 to 7 illustrate examples of various methods and apparatus aspects of the present invention. For ease of explanation, but without intending any limitation, these examples are described in the context of a TDD communications device attached to a digital telecommunications system incorporating wireless links, an example of which is described below.

HARDWARE COMPONENTS AND INTERCONNECTIONS Figure 1 illustrates one type of telecommunications system 100 that includes wireless links and a TDD (TDD) communication device 200 as used in the present invention. As shown in detail in Figure 2, TDDs generally include a keyboard and a screen that are connected to a telephone through a modem (modulator / demodulator). The modem is incorporated into the TDD and is connected directly to a telephone line or is coupled by an acoustic coupler to a normal handset. The TDDs are capable of transmitting information over the telephone lines by means of coded tones to other TDDs, such as the TDD 102 shown in Figure 1, connected to the opposite ends of a telephone line through another modem. In digital telecommunications systems using wireless links, the TDD 200 may be coupled to a subscriber unit 104 that is used in the telecommunications system 100 to transmit the received signals. Exemplary embodiments of a subscriber unit 104 are digital signal telephones, such as the Q-800 manufactured by Qualcomm Incorporated, and commonly referred to as cellular telephones. The subscriber unit 104 as shown in Figure 1 includes a warning apparatus 106 communicatively coupled to the circuitry of the subscriber unit 104. A wired connection 108 can be used to connect the TDD 200 to the subscriber unit 104 through the apparatus 106, or a device port can be used. Examples of such device warning and device ports are described in the U.S. Patent application. entitled "METHOD AND APPARATUS FOR ESTABLISHING TTD / TTY SERVICE OVER VOCODED CHANNELS" Serial No. 09 / 114,344, filed July 13, 1998, assigned to the assignee of the present invention and incorporated herein by reference. The device port can be configured to receive a junction of the low activity communications device such as a pin, connector or receiver. These elements are commonly used today to connect telephone and computer equipment, and are readily available by electronic providers. The device port interfaces with the annex to communicatively connect a low activity communications device (not shown) such as the TDD 200 with the subscriber unit 104 of the telecommunications system 100. The device port allows exchange of information between a low activity communications device and the subscriber unit 104. Irrespective of whether a device port or wired connection is used, the message apparatus 106 allows the system 100 to be notified that a TDD signal needs to be transmitted. Returning to Figure 1, after the warning apparatus 106 receives the low activity communications signal, the signal is processed by the subscriber unit 104. Very basically, a signal for transmission is created that includes the information contained in the signal of low activity. Because the telecommunication system 100 has been advised that a signal of low activity is being transmitted, the system is adapted to ensure that a decipherable transmission occurs. For example, an analog signal received from the analog circuitry 228 shown in Figure 2 will typically be experienced by a signal or "voice" processing including digitizing the signal, establishing a transmission power level for protection against signal fading during the transmission, including signal and filtering. These functions can be carried out by the circuitry (not shown) of the subscriber unit 104 that includes a vocoder. Depending on the received signal, a variable rate vocoder - generically referred to in this application as a vocoder - can dynamically determine and negotiate the service within the telecommunications system 100 to provide successful transmission and decoding of the signal. This negotiation involves establishing the values of multiple parameters, such as the rate that the vocoder must use, the transmission power and the compression technique. A detailed description regarding signal processing for transmission in telecommunication systems can be found in the standard of the Electronic Industry Association TIA / EIA / IS-95-A entitled Mobile Stat ion-Based Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems ", referred to as" IS-95"and incorporated in the present invention for reference, and other transmission standards, which include the standard vocoder protocol, are well known in the art. However, when a signal of low activity is received, a vocoder can identify the signal either as a noise, a pause, or a signal not intended to be transmitted simply, a vocoder does not know which service to use because it can not identify the signal from Low activity received By notifying the system 100 that a signal of low activity is being sent, the vocoder will establish the necessary service to ensure the best possible transmission and decoding of the signal After the low activity communications signal has been processed and that the service has been determined, a signal can be transmitted using an antenna 112 over a wireless link 114. The digitized signal is received by another antenna 116 at a remote location, such as a base station 118, and is processed by a base station circuitry (not shown) that includes a vocoder 120. Various configurations of the base station circuitry for Telecommunication systems are well known in the art, and a further understanding can be found in TIA / EIA / IS-95 referred to above. Upon processing the signal after reception, a low activity signal may be sent which reflects the information contained in the low activity signal transmitted to the low activity device 102 via a communications link 120. A second device is shown. notices 106 coupled to the base station 106. This is provided for a low activity signal to be sent from the low activity communications device 102 back to the TDD communication device 200. The communications link 120 appears bifurcated to emphasize that the base station 118 may not connect directly to low activity device 102. Base station 118 is generally connected to a standard PSTN switching station commonly used by telephone companies for telephone call coordination and low activity communications device 102 connects to the PSTN. In another embodiment, a second mobile station (not shown) connected to the low activity communications device 102 may be linked to the base station 118. In addition, the telecommunications system may include mobile switching stations as mentioned above. Shown in Figure 2 is a schematic block diagram of the circuitry of a typical TDD device 200, either standard or enhanced TDD, operating in accordance with the present invention. In the TDD device 200 of Figure 2, a keyboard 202 is provided within which the user may include input data characters. The keyboard output 202 is connected to a processor 204 which serves to control the circuit elements contained in Figure 2. The characters that are received or transmitted by the processor 204 are also displayed on a screen 206. Optionally, the same characters received or transmitted can be played back on a device such as printer 208. Some TDD devices may not have a printer, although their standard for TDDs must have a visual display of some kind so that a user can see the characters typed and received. Accordingly, the keyboard 202 functions as an input source of data characters to the processor 204 while either or both of the screen 206 and the printer 208 serve as local destinations for the data stream characters. The processor 204 may be connected by a suitable data and address bus which is typically used for this type of application by a person skilled in the art. In Figure 2, the bus 210 connects a read-only memory (ROM) with a non-volatile random access memory (NVRAM) 214. The appropriate control lines 216 and 218 are connected from the processor 204 to the ROM 212 and the NVRAM 214 that provides Interactive control of these units. The ROM 212 is intended to permanently store the program dictating the operation of the processor 204 as well as some data by the program. For example, special character strings can be stored for machine-to-machine communication and to synchronize two TDDs in an improved operational mode. The NVRAM 214 is used as a buffer, a floating storage place for the input and output of data from the TDD 200 device, and for the storage of standard messages as they are entered by the user through the keyboard 202 and intended for the fast . Other circuitry configurations may be used, such as combining the microprocessor 202 with the ROM 212 and the NVRAM 214 into a single integrated circuit. Also connected to the processor 202 in Figure 2 in Figure 2 is a telephone keypad 220 that allows entry of telephone numbers to be dialed by the processor 202 through telecommunications system 100. A standard handset 224 is supported on a rack 226 which incorporates a switch (not shown) that indicates whether the handset 224 is in use and is therefore removed from the frame 226. The processor 204 communicatively connects through the analog circuitry 228 to the telecommunications system 100. This connection is shows as a wired connection 230, but can be any type of connection that can communicatively link the analog circuitry 228 with the telecommunication system 100. The analog circuitry 228 provides a connection between the handset and the processor 202 that allows both the tones of Baudot as the dial tone to be received by the telecommunication system 100. The analog circuitry 228 provides a voice information interface to and from the handset 224. The analog circuitry 228 of the TTD device 200 is connected to the telecommunications system 100 using a connector such as the device described above. Notwithstanding the foregoing specific descriptions, those skilled in the art having the benefit of this disclosure will recognize that the apparatus described above may be implemented in a telecommunication system of different construction without being insulated from the scope of the present invention. As a specific example, the multiple subscriber unit 104 may be linked to the base station 118, or the low activity communications device 200 may be integrated with the subscriber unit 104.

OPERATION After a TDD signal is received, the vocoders used by the system 100 are notified during signal processing or they detect that a low activity signal has been received for the transmission and can use a frame format of eighth rate traffic channel to transmit the signal. However, the following methods can be adapted for the fourth-to-total rate traffic channel transmissions, as described below. Figure 3 shows a typical variable rate vocoder frame format for a traffic channel using a rate set to 1. The variable rate vocoder produces one frame every 20 milliseconds using Code Excited Linear Prediction (CELP) techniques. They are well known in the art. The frames can be formatted in eighth, quarter, half or full rate formats depending on the voice activity. If a Baudot tone is received, the variable rate vocodi ficator will generally detect low activity and use the eighth rate format, assuming that the standard vocoder currently in use can detect that a signal is being sent. Commonly, a Baudot signal will be treated as noise and will generally be ignored. Total rate refers to the fact that each bit contained in each frame is not repeated. The average rate refers to sending the same number of bits per frame, but each bit is repeated once in the frame; that is, each unique bit will appear twice in the frame. The quarter rate refers to each single bit that appears four times per frame, and so on. The more repetitively a bit of information was sent, the lower the total information sent by the frame will be. At full rate the signal is sent at a higher power because a certain bit is sent only once. This total rate power level is referred to as the reference power for purposes of this application. Because the bits are repeated at lower rates, a reduced power level is used because the power for each repeated bit accumulates in the frame. Assuming that a fixed minimum power is used for the transmission, a total rate transmission will contain more frame errors than would be contained in an average rate transmission of the same information. Typically, the power level is established depending on a frame error rate (FER) selected for the transmitted signal as received at a remote location, also referred to as the target of the transmitted signal, such as the subscribing unit. A desired FER is selected because when a low activity signal is sent, the current FER increases the use of the current methods. This selected FER range is between an error rate of 0.1% and 1-0%, but may be higher or lower if necessary for the preservation of the quality of the transmitted signal. Preferably, an FER of 0.2% is desirable for low activity signals. In the present invention, by implementing the specialized coding and decoding techniques, the frame error rate is controlled. In case the described techniques fall short of the desired FER - in this case defining FER as the total number of erroneous frames even after the reconstruction of the vocoder frame information - methods for adjusting the levels of the vortex can also be used. transmission power together with specialized coding and decoding techniques. Typically, the vocoders will be secured at full rate and the transmission power will be increased to transmit low activity signals. It should be understood that any required increase will be even less than the increase required if the present coding / decoding techniques were not implemented A. Decoder that uses soft bits. In a modality, when a TDD call is received, the system 100 is notified or detects the type of call. The system 100 processes the call coming from the TDD 200 unit for transmission using standard processing techniques known in the art. When the frame is received at a remote point, for example the base station 118, the call is decoded using the present invention. If a frame error has occurred in the physical layer, that is, if the frame does not pass the checksum as described in IS-95, the frame will be sent to the vocoder 120 for decoding. Sending the wrong frame to the vocoder is not currently done in the implementations of the IS-95 standard. The bits contained in a bad frame are referred to as "soft bits" because they may not all be in error and information from them can be collected individually to reconstruct information contained in the wrong frames.

However, detecting or being notified that a TDD call has been received, the vocoder decoder in the present invention processes the erroneous frames by looking at the received vocodi? Er parameters and comparing these parameters against the "signatures" or tones of the TDD modulation signals as seen in the vocoder parameter space. This compares the vocoder parameters of stored vocoded TDD tones with those received. This comparison results in a determination that is made in order to know which TDD signal was most likely received. For example, suppose a vocoding representation of a Baudot tone of "0" is represented as sixteen "0" s in sequence, and that the representation of a Baudot tone of "1" is represented as sixteen "1" s . The present method considers that these are the signatures of space-parameter-voice. For the following examples, three layers are identified as: vocoder frame borders I -voc frame? N '- I Baudot tone borders: | - baudot? X '- | , and vocodi ficator parameter received: 000000000000000 or 11111111111111111. Assume that the vocodi ficator decoder receives the following parameters:. { wrong plot} | -voc raster l- | -voc raster 2- | -voc raster 3- | -voc plot 4- | -voc t '1' - | -baudot '0' - | -baudot '0' - | -baudot '1' - | -baudot '0' - | 111111110000000000000000000000000000011000011111 1111110000000000000000? ? ? ? ? [frame errors] The decoder recognizes the borders of baudot tone and recognizes that the parameters received for the second baudot? 0 'are closer to? 0' than to l '. The decoder is decided by the baudot tone 0 'and modifies the suspected error bits before decoding. For the next baudot? L 'tone, the decoder recognizes that the vocoder parameters are closer to l' than a? 0 'and modifies the bits according to the above. The decoder now uses the following sequence to produce a corrected TDD signal:. { corrected plot} | -voc plot 1- | -voc plot 2- | -voc plot 3- | -voc plot 4- | -voc t '1' - | -baudot '0' - | -baudot '0' - | -baudot '1' - | -baudot '0' - | 111111110000000000000000000000000000001111111111 1111110000000000000000 This example shows the error transitions to occur in a frame boundary, which is not always the case. If these transitions commonly fall within one frame, another version of the invention may be used as explained below. If the decoder of the vocoder receives an erratic frame in which the erroneous bits are contained within the frame, the vocoder can look for adjacent frames that are not wrong or "good" to reconstruct the wrong frame. The adjacent frame will not contain a portion of the baudot tone that was lost in the wrong frame. For example, suppose that the following signal is received:. { wrong plot} | -voc frame 1- I -voc frame 2- I -voc frame 3- I -voc frame 4- | -voc t '1' - | -baudot '0' - I -baudot '0' - I -baudot '1' - | -baudot '0' - | 111111110000000000111111111111000000000111111111 1111110000000000000000? AAAAAAAAAAA [frame errors] The vocoder parameters for the second tone of baudot? 0 'are too ambiguous to make an accurate decision about the tone because the number of? 0's is almost the same as the number of? L's in the Vocoder frame parameters. To make a better determination, the vocoder searches the next adjacent frame (speech frame 3) and determines that the tone seems to continue as a "0" within this frame. The decoder therefore decides that this means a baudot tone 0 'in the last half of the vocoder frame 2. As seen in the flow chart of Figure 4, after it is determined whether a signal is being received of low activity in tasks 402 and 404, the decoder continuously monitors and updates the pitch boundaries of baudot received in task 408. Otherwise, any non-low activity signal is processed using traditional methods. If a bad frame is received as detected in the physical layer, the frame is assigned an N flag and the vocoder examines the bad frame in task 410. If a "reliable" decision can be made as to whether the frame is or not a baudot? 0 'or? l' tone, such as when the frame parameters are quite different, then the wrong frame is modified to reflect the parameters of the decision. A reliable decision is one that falls within a prewritten probability to obtain the original plot parameters. For purposes of this invention, the desired probability will be found in the range of 51% certainty. If a modification is made, the method returns to task 402 and determines the next signal. If a reliable decision can not be made as shown in task 412, the vocoder checks the next adjacent frame N + l or, alternatively, N-l. If this frame is good in task 416, the decision to modify the wrong frame is made in task 418 based on the parameters contained within the frame N + 1, or alternatively, the frame N-1. If none of the next two adjacent frames is good, then a next more reliable decision is made based on the parameters contained within the next adjacent frame N + 1 and accordingly the frame parameters N 's are modified.

B. Encoder and decoder that use soft bits. The implementation of decoder in this embodiment of the invention is similar to that described above. However, to further reduce the error rate and improve the accuracy and reliability of the encoded signal, the encoder also takes advantage of the "soft bits". When the vocoder encoder detects that baudot tones are going to be sent, the encoder changes to a "mode to encode baudot tone". In this mode the encoder decides whether the tone received to encode is a? 0 'or an l'. The encoder then sends this decision to the decoder using a vocoder frame, but using channel coding redundancy to improve the decoder's chances of determining the appropriate baudot tone. Even if the decoder receives a tone in a wrong frame, it will have a greater chance of determining the correct tone sent due to the decision sent in advance. In a simplified example, if the encoder detects that a baudot? L 'is to be transmitted, it sends a series of ls to the decoder. The series can be of any length, but it should be sufficient in order that the decoder can operate as described above in section A if necessary This version of the invention replaces the standard vocoder parameters with vocoder "signatures" that they are spaced better (that is, easier to differentiate), making it easier to decide between two tones even when the frames are in error.

C. Encoder and decoder that do not use soft bits. This embodiment of the invention is another version of the methods described in sections A and B, but the soft bits from any vocoder frame that is wrong to process are not given to the decoder. In this case, when the vocoder encoder detects a baudot? L 'or 0' tone, the vocoder also encodes the tone in a vocoder frame using redundancy, but coding can be done in many vocoder frames. The I's and 0's are distributed in a certain number of frames M so that if a frame is lost, the decoder can extract the necessary information from the adjacent frames. The following example shows the distribution that takes place in four frames, but any number of frames could be used. Assume that the encoder detects the following baudot tones for transmission: 1 1 0 0 1 The encoder encodes the frames as explained below for transmission to the decoder: | -voc raster l- | -voc raster 2- | -voc raster 3- | -voc raster 4- I voc raster 5 I -baudot '1' - | -baudot '1' - I -baudot '0' - | -baudot '0' - | -baudot '1' - 11111111000000001111000000001111. In this example, the vocoder frame parameters are segmented for each frame where four bits represent the detected baudot tone in a particular vocoder frame. The sixteen bits represent the detected baudot tones derived from the last four vocoder frames: | baudot for frame N-3 | baudot for frame N-2 | -baudot for frame N-l | baudot for frame N |. To represent baudot tones not corresponding to the vocoder frame boundaries, the invention uses the following four-bit sequence where XXYY indicates that the code in the current vocoder frame reflects a baudot code of? X 'followed by a baudot code of? Y ': I -voc frame 1- | -voc plot 2- | -voc plot 3- | -voc plot 4- | voc t '0' - | -baudot '1' - | -baudot '1' - | -baudot '0' - | -baudot '0' - I D. Modified vocoder with increased transmission power. In another embodiment, the prohibitive cost of modifying the chip set in a mobile station to carry out the coding and decoding processes described above in sections A, B and C is advantageously minimized. The coding and decoding methods of sections A, B and C can be advantageously carried out by the use of a standard vocoder that communicates connectively with a signal enhancer, such as an estimator or a repeater or some other device capable of performing a signal improvement function. In addition, the methods of sections A, B, or C can be carried out by using a modified vocoder. It will be apparent to one skilled in the art that a vocoder can be modified to additionally incorporate the functions of a signal enhancer, i.e., to estimate whether the corrupted bits received by the communications unit were originally transmitted as? 0's or? l's. The system of Figure 5 illustrates one embodiment of the invention. The transmission from the base station 550 to the mobile station 510 is referred to as the forward link and the transmission from the mobile station 510 to the base station 550 is referred to as the reverse link. In the reverse link, the unmodified vocoder 520 in the mobile station 510 encodes the Baudot signal in the standard vocoder parameters and transmits the vocoder parameters to the base station 550. The modified vocoder 560 receives the encoded Baudot signal and improves the Baudot signal to recover the corrupted bits. Then a clean version of the Baudot signal is generated. In the forward link, the unmodified vocoder 580 in the base station 550 encodes the Baudot signal using vocoder parameters and transmits the vocoder parameters to the mobile station 510. The modified vocoder 530 receives the coded Baudot signal and improves the Baudot signal to recover the corrupted bits. A clean version of the signal is then generated. However, using modified signal enhancers or vocoders in both the forward link and the reverse link to carry out the decoding methods described in sections A, B, and C can be prohibitively expensive to produce. In still another embodiment of the invention, the frame error rate of the system can be reduced by using a signal enhancer at a base station on the reverse link, and by using power control techniques to adjust the transmission power levels of the signal from the base station to the mobile station in the forward link. In the reverse link of the communication system of Figure 6, the base station 655 reconstructs the original Baudot signal contained within a coded vocoder frame according to the methods described in sections A, B and C. However, in the forward link, the base station 655 transmits the encoded Baudot signal using power control techniques as described in the US Patent Application No. 09 / 114,344, entitled "METHOD AND APPARATUS FOR ESTABLISHING TDD / TTY SERVICE OVER VOCODED CHANNELS", assigned to the assignee of the present invention and incorporated herein by reference. Figure 7 is a flow diagram of a method for controlling the transmission power between the base station and the mobile station. The method begins at task 702, where a Baudot signal is received by the mobile station 605. After the signal is received, the vocoders used by the mobile station 605 during the signal processing are set within a rate total in task 704. In this mode, the transmission power does not decrease from the transmission power used by the telecommunications system for the total rate transmissions in task 710. The power level is typically established based on an FER selected for the transmitted signal as received at the mobile station 605. A desired FER is selected because when a Baudot signal is sent, the current character error rate of the Baudot signal is approximately 9 or 10 times that of the FER . This selected FER range is between an error rate of 0.1% and 1.0%, but may be lower if necessary for the preservation of the quality of the transmitted signal. Preferably, a 0.2% FER is desirable for transmitting Baudot signals. If the FER exceeds the range selected in the task 712, the mobile station 605 notifies the base station 655 in a conventional manner during task 706 that a system adjustment is needed to reduce the FER. Accordingly, an adjustment is made to task 708. An adjustment typically includes increasing the transmit power for the total rate transmission, but may also include adjusting other known parameters to reduce the FER. If the FER is acceptable in task 712, the transmission of the signal can continue in task 714 and the dynamic adjustments to the telecommunications system continue through the transmission of the entire transmitted signal 710. Otherwise, when the transmission ends of the Baudot signal, the vocodi ficators are opened, and the telecommunication system returns to normal operation. Although it has been shown what are currently considered the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications without being insulated from the scope of the invention as defined by the appended claims.

Claims (17)

1. NOVELTY OF THE INVENTION Having described the invention as antecedent, the content of the following claims is claimed as property: CLAIMS 1.
2. A method for communicating a signal of low activity communications noticed in a telecommunications system, including the telecommunications system mobile stations and base stations, characterized in the method because it comprises: transmitting an original frame represented by an original bit sequence; receiving a first transmitted frame, the first transmitted frame comprising a first transmitted bit sequence, the first transmitted sequential bit sequence substantially equal to the original bit sequence; improving the first transmitted bit sequence to produce an improved bit sequence; and processing the improved bit sequence to obtain additional information used to identify the original bit sequence in the original frame.
3. The method according to claim 1, characterized in that the step for processing the improved bit sequence further comprises: comparing the improved bit sequence with a low activity communication signal bit sequence; determine if the improved bit stream is statistically reliable; and if the improved bit sequence is statistically reliable, then modify the improved bit sequence to identify the original bit sequence in the original frame.
4. The method according to claim 1, characterized in that the improved bit sequence is labeled frame N and the step for processing the improved bit sequence further comprises: comparing the improved bit sequence with a low signal sequence of communications signals known activity to produce a comparison result; if the result of the comparison indicates statistical reliability, then modify the improved bit sequence based on the result of the comparison to reflect the bit sequence of the known low-activity communication signal; but if the result of the comparison is not statistically reliable, then a frame N + l neighboring the frame N is processed to determine if the frame N + l contains some error; but if the frame N + l does not contain errors; then determining a low activity communications signal defined by a sequence of bits contained in the frame N + 1; and modifying the improved bit sequence of the N frame based on the bit sequence contained in the N + 1 frame.
5. The method according to claim 3, characterized in that the results of the comparison indicate statistical reliability if the probability that the bit sequence of the known low-activity communications signal reflects the original bit sequence is 51% or greater.
6. The method according to claim 1, characterized in that the step for transmitting the original frame is carried out by a mobile station and the step for receiving the first transmitted frame is carried out by a base station.
7. The method according to claim 1, characterized in that the method further comprises the step to replace the coding / decoding parameters of the standard vocoder used by a plurality of vocoders with vocoder signatures, wherein the sequenced bits contained within the signatures of The vocoder is further spaced from the sequenced bits contained within the parameters of the standard vocoder, preceding the replacement step to the transmission step.
8. The method according to claim 6, characterized in that the step to replace the parameters of the standard vocoder further comprises: communicating with a first vocoder that is transmitting a low activity communications signal; and encoding the low activity communications signal with a second vocoder, determining the second vocoder if a bit is a 0 or a 1, using the coding carried out redundancy of channel coding within an information bit field of the original frame to improve the probability that the first vocoder decodes the original frame, also notifying the coding to the first vocoder that a signal is being transmitted. low activity The method according to claim 6, characterized in that the step for processing the improved bit sequence further comprises the steps for: comparing the improved bit sequence with a sequence of communication signal bits of low known activity; and modifying the improved bit sequence based on the comparison to identify the original bit sequence.
9. 9. A method for communicating a low activity communication signal noticed in a telecommunications system, including the telecommunications system mobile stations and base stations, characterized in that it comprises: transmitting a first original frame from a mobile station, in the that the first original frame is represented by a first original bit sequence; receiving a first frame transmitted on a base station, the first transmitted frame comprising a first transmitted bit sequence, the first transmitted sequential bit sequence substantially equal to the first original bit sequence; improving the first bit sequence transmitted within the base station to produce an improved bit sequence; sending the improved bit sequence to a first vocoder within the base station; processing the improved bit sequence with the first vocoder to obtain additional information used to identify the first original bit sequence in the first original frame; and transmitting a second original frame from the base station at an increased transmission power level.
10. The method according to claim 9, characterized in that the sfor transmitting the second original frame from the base station at an increased transmission power level further comprises the s for: securing all the vocoders used in the processing of the second frame original within a total rate; and transmitting the second original bit sequence at an increased transmit power level to maintain a minimum target frame error rate for the second original bit sequence.
11. The method according to claim 9, characterized in that the improved bit sequence is labeled frame N and the sfor processing the improved bit sequence further comprises: comparing the improved bit sequence with a low signal sequence of communications signals known activity to produce a comparison result; and if the result of the comparison indicates statistical reliability, then modify the improved bit sequence based on the result of the comparison to reflect the bit sequence of the known low-activity communications signal; but if the result of the comparison is not statistically reliable, then a frame N + l neighboring the frame N is processed to determine if the frame N + l contains some error; but if the frame N + l does not contain errors; determining a low activity communications signal defined by a sequence of bits contained in the frame N + 1; and modifying the improved bit sequence of the N frame based on the bit sequence contained in the N + 1 frame.
12. The method according to claim 11, characterized in that the results of the comparison indicate statistical reliability if the probability that the bit sequence of the known low-activity communications signal reflects the original bit sequence is 51% or greater.
13. The method according to claim 11, characterized in that the sfor transmitting the second original frame from the base station to an increased transmit power level also comprises the s for: securing all the vocoders used in the processing of the second one. original plot within a total rate; and transmitting the second original bit sequence at an increased transmit power level to maintain a minimum target frame error rate for the second original bit sequence.
14. The method according to claim 13, characterized in that the minimum target frame error rate is less than 10%.
15. 15. Apparatus for communicating a signal of communications of low activity in a telecommunications system-, characterized in the apparatus because it comprises: means for coding an original frame of the signal of communications of low activity within an original sequence of bits; means for transmitting the original bit sequence; means for receiving a first transmitted frame, the first transmitted frame comprising a first transmitted bit sequence; and means for improving the first bit sequence transmitted with a signal enhancer to produce a clean signal.
16. 16. The apparatus according to the claim 15, characterized in that the apparatus further comprises: means for comparing the improved bit sequence with a sequence of communications signal bits of known low activity; means for determining whether the improved bit sequence is statistically reliable; means for verifying the statistical reliability of the improved bit sequence; and means for modifying the improved bit sequence to identify the original bit sequence in the original frame.
17. 17. Apparatus for communicating a low-activity communications signal noticed in a telecommunications system, characterized in that the apparatus comprises: a vocoder for encoding an original frame of the low-activity communications signal within an original sequence of bits; a transmitter for transmitting the original bit sequence; a receiver for receiving a first transmitted frame, the first transmitted frame comprising a first transmitted bit sequence; and a signal enhancer for improving the first transmitted bit sequence in order to produce a clean signal.