WO1998056124A1 - Systeme d'affectation de canaux - Google Patents

Systeme d'affectation de canaux Download PDF

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Publication number
WO1998056124A1
WO1998056124A1 PCT/GB1998/001607 GB9801607W WO9856124A1 WO 1998056124 A1 WO1998056124 A1 WO 1998056124A1 GB 9801607 W GB9801607 W GB 9801607W WO 9856124 A1 WO9856124 A1 WO 9856124A1
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WO
WIPO (PCT)
Prior art keywords
base station
mobile
frequency
timeslot
timeslots
Prior art date
Application number
PCT/GB1998/001607
Other languages
English (en)
Inventor
Luke Ibbetson
Luis Lopes
Robert Joyce
Original Assignee
British Telecommunications Public Limited Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by British Telecommunications Public Limited Company filed Critical British Telecommunications Public Limited Company
Priority to AU77788/98A priority Critical patent/AU7778898A/en
Publication of WO1998056124A1 publication Critical patent/WO1998056124A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
    • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
    • H04B7/2643—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using time-division multiple access [TDMA]
    • H04B7/2656—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using time-division multiple access [TDMA] for structure of frame, burst

Definitions

  • This invention relates to channel allocation systems, suitable for use in cellular mobile radio telephone systems.
  • individual mobile telephones communicate with the fixed part of the network, specifically the radio base stations, over radio links.
  • Each radio link has a given frequency, which is allocated to the relevant radio base station.
  • the same radio frequency is also used by other radio base stations; the frequency re-use pattern, that is the allocation of frequencies to base stations, being such that co-channel interference is minimised.
  • a radio base station may have more than one frequency allocated to it. However, the available number of frequency channels is limited. In areas of low traffic demand a base station may have only one or two frequencies allocated to it whereas a base station in an area of higher demand would require a larger number of frequencies to be allocated. Higher demand may also be accommodated by providing base stations at more closely spaced locations, having lower power outputs to minimise the effective range of each base station so that co-channel interference is still avoided.
  • a voice signal requires less bandwidth than that provided by a frequency channel, and therefore most cellular radio systems use time division multiplex systems to allow several mobile units to communicate with a base station on the same radio frequency.
  • GSM Global System for Mobile Radio
  • eight voice channels can be carried on each frequency.
  • each mobile unit can be allocated to a logical channel using one such slot in each frame.
  • Digitisation of the speech signal allows it to be compressed so that it can be accommodated in the allotted timeslot.
  • each frame is 4.6 milliseconds in duration, and each of the eight timeslots in a frame comprises 148 digital bits.
  • Radio transmissions are prone to a number of degradations caused by the propagation characteristics of radio waves.
  • co-channel interference that is interference between two signals using the same radio frequency
  • a mobile unit at an elevated position may be within range of more than one base station operating at the same frequency, because the base stations have to have sufficient power to allow reception by mobile units within their notional range of coverage which do not have a direct (line-of-sight) path to them.
  • a mobile unit MS1 operating to a base station BS1 but also in range of a base station BS2 operating on the same frequency will detect signals transmitted from base station BS2 intended for other mobile units e.g MS2, as well as those transmitted from the base station BS1 .
  • the base station BS2 will detect interference from the mobile station MS1 , as well as the signal intended for it, which is being transmitted from the mobile station MS2.
  • fading is a phenomenon caused by multipath interference (that is interference between radio signals which travel over two or more different paths between the transmitter and receiver.
  • multipath interference that is interference between radio signals which travel over two or more different paths between the transmitter and receiver.
  • one of these paths may be the direct " ne-of-sight" path from the transmitter
  • Other paths are generated by reflection of the radio waves off objects such as buildings, mountains, vehicles or vegetation. The differences in path length cause constructive and destructive interference patterns.
  • the existing GSM system uses a system known as "frequency hopping” in which a mobile unit does not communicate with its base station on a single frequency throughout a call, but switches from one frequency to another according to an established pattern, which may either be cyclic or pseudo-random. (A "pseudo-random" pattern is one which is cyclic over a long period, typically several hours.) This "hopping" ensures that any frequency- dependant degradation of the signal is shared out equally among all the users, instead of being concentrated on those users which happen to have been allocated to the frequency on which the problem arises.
  • Frequency hopping cannot be used in all situations.
  • a base station may have only one frequency allocated to it.
  • a more general problem is that one of the frequencies allocated to each base station carries a control channel (known as the BCCH) which is used for setting up calls to and from mobile units, and in particular for maintaining communication with mobile units which have not currently got a call in progress.
  • the BCCH allocated to each base station is permanently allocated to a timeslot (timeslot 1 ) at a given frequency. This is necessary so that a mobile unit can establish initial contact with the base station in question.
  • the mobile unit must have information as to the frequency on which the BCCH of the local base station operates so that it can receive synchronisation instructions, and be directed as to which frequency and timeslot the mobile unit is to communicate on.
  • the BCCH of that base station also transmits details of the BCCH allocated to each neighbouring base station so that the mobile unit can listen out for these neighbouring base stations' BCCHs and arrange a handover if appropriate.
  • any frequency hopping pattern must avoid moving the BCCH to another frequency or moving a mobile unit to the BCCH. If the frequency hopping pattern excludes the frequency on which the BCCH is carried altogether, then all other logical channels (i.e. the other timeslots) on the frequency will not hop either. Alternatively, the frequency hopping pattern used for timeslot 1 must be different from that used for the other timeslots, since there is one fewer frequency available, again in order to avoid the BCCH being involved in the frequency hopping pattern. In particular, in a base station having only two frequencies, any mobile unit allocated to timeslot 1 will not be able to 'hop' between frequencies at all, because it could only exchange with the frequency which is permanently allocated to the BCCH.
  • a mobile telephone system comprising a plurality of base stations and a plurality of mobile units capable of radio communication with the base stations, each base station being allocated at least one radio frequency for radio communication with one or more of the mobile units, using a time division multiplex system, the time division multiplex system having a repeated frame structure comprising a plurality of timeslots, wherein the system is arranged such that at least one of the base stations exchanges signals with mobile units in different timeslots in successive frames, and at least one timeslot and frequency being reserved for a control channel, such that no mobile unit operating on a reserved frequency is allocated to a reserved timeslot.
  • a method of operating a mobile telephone system comprising a plurality of base stations and the plurality of mobile units capable of communication with the base stations, in which a base station is allocated one or more radio frequencies for communication with mobile units using a time division multiplex system, the time division multiplex system having a repeated frame structure comprising a plurality of timeslots, wherein signals are exchanged between a mobile unit and a base station in different timeslots in successive frames, and at least one timeslot and frequency are reserved for a control channel such that no mobile unit operating on a reserved frequency is allocated to a reserved timeslot.
  • a mobile telephone for use in a mobile telephone system, having means for exchanging radio signals with a base station using a division multiplex system, the time division multiplex system have a repeated frame structure comprising a plurality of timeslots, and having means for exchanging signals with a respective base station in different timeslots in successive frames, having means for communicating with a base station on a control channel having a predetermined reserved frequency and timeslot and being arranged such that the allocated channel for exchange of radio signals does not operate on a reserved frequency and reserved timeslot simultaneously.
  • a base station for a mobile telephone system comprising means for exchanging radio signals with one or more mobile telephones using a time division multiplex system, the time division multiplex system having a repeated frame structure comprising a plurality of timeslots, and arranged such that the base station exchanges signals with individual mobile units in different timeslots in successive frames, wherein at least one timeslot and frequency are reserved for a control channel, and the sequence of available timeslots allocated for communication with an individual mobile unit is arranged such that a reserved frequency and timeslot combination are not both allocated to the same mobile unit simultaneously.
  • only one timeslot is reserved for use as a control channel, in only one of the available frequencies.
  • control channels each use a different timeslot/frequency pair, but it is possible for two control channels on different frequencies to use simultaneous timeslots, and for two control channels to use the same frequency, provided they use different timeslots.
  • the allocation of each timeslot to a mobile unit preferably follows a cyclic pattern, which may proceed stepwise through the available timeslots in each frame or may be a pseudo-random pattern.
  • the pattern may be combined with frequency hopping to allow hopping throughout the time/frequency domain.
  • the effect of co-channel interference can be reduced, because interference between two logical channels will only occur when both channels occupy the same timeslot (as seen by the receiver, allowing for any time-lag).
  • Co-channel interference may still occur in each timeslot if they are all in use in the interfering channel, but the interference will be from different logical channels in successive timeslots.
  • the interfering signal will therefore be less coherent than if it came from a single logical channel, and therefore more like random noise, making it easier to extract the real signal.
  • Figure 1 is a schematic diagram showing parts of a typical cellular radio system, illustrating co-channel interference
  • Figure 2 is a schematic diagram illustrating multipath interference
  • Figure 3 is a schematic diagram illustrating a conventional frequency hopping system
  • Figure 4 shows a simple variable timeslot pattern in conjunction with variable frequency Figures 5a to 5d show a further hopping pattern according to the invention.
  • Figures 6a and 6b show another hopping pattern, providing two control channels
  • Figure 7 shows another frequency/timeslot hopping pattern, also according to the invention.
  • FIG. 1 there are shown four base stations, BS1 , BS2, BS3 and BS4 and two mobile stations MS1 and MS2.
  • Base stations are typically controlled remotely by a control system arranged to control a plurality of such base sites.
  • the mobile station MS1 is in radio communication with the base station BS1 and the mobile station MS2 is in communication with the base station BS2.
  • Base stations BS1 and BS2 share a frequency f 1 and both mobile stations MS1 and MS2 have been allocated to that frequency.
  • the geographical locations and power outputs of the base stations BS1 and BS2 will be selected such that co- channel interference is minimised.
  • the system will generally be designed so that base stations using the same frequency do not have coverage zones which are adjacent.
  • a mobile station MS1 is, in normal use, unlikely to be able to receive a strong signal strength from both BS1 and BS2, and similarly only one of the base stations BS1 and BS2 will be able to receive a strong signal from the mobile station MS1 .
  • Other base stations using different frequencies e.g. base stations BS3, BS4 on frequencies f 2 , f 3 will in general cover regions where signal strengths from base stations BS1 , BS2 would be similar.
  • situations can arise when a mobile station MS1 is in radio range of two base stations BS1 , BS2 operating at the same frequency f-
  • transmissions from the mobile station MS1 will be detected at the base station BS2 and will interfere with any transmissions from the mobile station MS2, which is transmitting on the same frequency and timeslot. (Note that because of the different path lengths mobile station MS1 will not be synchronised with the base station BS2, but it will nevertheless interfere with one timeslot of the signal received at the base station BS2. Similarly, transmissions from the base station BS2 to the mobile station MS2 will be detected by the mobile station MS1 .
  • Such situations can arise in particular where the mobile station is in direct line of sight with a base station at some distance away from it, for example on a high building or a hill, when the base station is configured for communicating with mobile stations at much shorter range but not in direct line of sight (for example in a built- up area).
  • FIG. 2 illustrates schematically the phenomenon of multipath interference, also known as fading.
  • the base station BS1 and the mobile unit MS1 are in radio communication with each other. It will be seen that transmissions from base station 1 can travel in a direct line of sight over a path P1 .
  • the radio signal over this path may be attenuated by ground level objects such as foliage or topography, such that the signal received at mobile station 1 is attenuated.
  • a second radio path P2 may exist to the mobile station 1 by way of reflections off buildings etc. This path may also be attenuated. It is also possible for both paths P1 and P2 to be indirect. Because of the different path lengths of the path P1 and P2 the radio signals transmitted from the base station BS1 over the two paths do not arrive in phase.
  • Constructive interference where the path lengths differ by a whole number of wavelengths results in the signal being reinforced
  • destructive interference where the path lengths differ by an odd number of half wavelengths results in partial or complete loss of the radio signal.
  • the mobile unit MS1 moves, it will pass through areas of constructive and destructive interference. If it is stationary if may by chance be in an area of destructive interference (known as a deep fade) and the signal strength will be very much reduced. An attempt to rectify this by increasing the gain renders the mobile unit prone to co-channel interference from other base stations in the area working on the same frequency.
  • Frequency hopping has been found to overcome the problem of fading better than a reactive system in which frequencies are changed only in reaction to a fade.
  • the mobile station may be moving, by the time a switch to another frequency has taken place in a reactive system then the need for the change may well have passed, and indeed the change may be to a frequency for which the mobile station is entering a zone of fading.
  • Figure 3 shows a typical frequency hopping pattern for a base station having two frequencies available, which is the most common configuration in typical "GSM" networks. Some base stations have more than two frequencies available, but the problem to be explained is at its greatest in base stations where only two frequencies are available.
  • a mobile station A allocated to timeslot t 5 the mobile station is allocated in the first frame to a frequency f In the following frame it is switched to the other frequency f 2 , and it then returns in the third time frame to the frequency f-i .
  • Another mobile unit may also operate in timeslot t 5 , using the frequency unoccupied by the mobile station A at any given time. Similarly other pairs of mobile units may use the other timeslots t 2 to t 8 .
  • timeslot t-i is allocated, for frequency f-i to a base site control channel
  • BCCH which provides control information to mobile units which are not currently engaged in a full radio link with the base station.
  • mobile units In order for mobile units to first establish contact with the base station, they need to identify the base site control channel, and for this reason the base site control channel must always remain on the same frequency. However, this means that if a mobile unit B uses this time slot t-i in the second frequency channel f 2 , it is unable to frequency-hop because this would require it to use the base site control channel which is already preallocated in timeslot t-
  • the mobile unit B being unable to frequency hop, is therefore more vulnerable than the other mobile stations to signal degradation from fading, co- channel interference, and other phenomena.
  • the timeslot t-i (for frequencies other than that allocated to the BCCH) is allocated to mobile stations according to interference measurements from other active mobile stations, without knowledge of BCCH logical channel location or regard to frequency hopping freedom, and the frequency hopping pattern used in other timeslots is suspended or modified for this timeslot to avoid the mobile unit operating on the BCCH. This reduces the average channel quality, as logical channels allocated to timeslot t-i have fewer frequencies to hop between.
  • the frequency hopping pattern as shown in Figure 3 to be used by mobile station A is indicated to the mobile station when it establishes contact with the base station.
  • the base site control channel BCCH includes, as part of the control data transmitted to the mobile stations under the control of that base station, information regarding the channel to be allocated, including the timeslot and the frequency or frequency hopping pattern.
  • FIG. 4 shows a channel allocation scheme according to the invention. Eight successive frames of the transmissions are shown, again for two frequencies f-i , f 2 . Each frame has eight timeslots ⁇ to t 8 . Again the broadcast control channel (BCCH) is permanently allocated to timeslot tt at frequency f- ] in each successive frame. Both mobile stations A and B alternate between the two frequencies f-i and f 2 . However, unlike in the prior art arrangement they are not permanently allocated to the same timeslot. In the arrangement shown in Figure 4, in each frame the mobile station is allocated to the timeslot following the timeslot allocated to it in the previous frame.
  • BCCH broadcast control channel
  • This pattern is cyclic, so that when mobile station A reaches timeslot t 8 in frame 4, in the next frame (frame 5) it is allocated to the first timeslot t-, .
  • the system is arranged such that the timeslot/frequency allocated to the BCCH is omitted in the cycle, so that base station A goes from frequency f 2 in timeslot t 8 , to frequency f 2 in timeslot t 2 , thereby skipping timeslot t ⁇ frequency f-, .
  • base station B reaches timeslot t 8 in frame 8
  • it is next allocated to timeslot t-i and frequency f 2 in frame 9.
  • some mobile units repeat the timeslot pattern after seven frames (e.g. mobile unit A) and others after eight frames (e.g. mobile unit B).
  • Figure 5 shows a complete cycle of the channel hopping pattern of Figure 4.
  • the left hand side shows the full channel allocation
  • the right-hand side is identical, but only the positions of three channels, (1 , 6, and 1 1 ) are shown, to allow their movement to be more clearly understood.
  • the odd-numbered channels e.g. 1 , 1 1
  • the even-numbered channels e.g. 6
  • the full pattern therefore repeats every 56 frames, this being the lowest common multiple of 7 and 8 (0.2576 seconds): the first frame in Figure 5a is repeated in the last frame of Figure 5d.
  • Figure 6 (made up of two parts, Figure 6a and Figure 6b) shows a similar allocation pattern for a system with two control channels on radio frequency f 1 ; allocated to timeslots t-, and t 5 . (It is usually convenient to provide all control channels on one frequency and to have them equally spaced in time, to allow them to be readily recognised, but other arrangements are also within the scope of the claims).
  • the left hand side shows the full pattern
  • the right hand side shows the movements of channels 1 ,6, and 1 1 only.
  • the even-numbered channels (for example channel 6) skip two timeslots, t ⁇ and t 5 , so they repeat after only six frames, so the full pattern repeats after 24 frames (the lowest common multiple of 6 and 8), or 0.1 104 seconds.
  • the last frame shown in Figure 6b is the same as the first in Figure 6a.
  • Figure 7 shows another timeslot/frequency allocation pattern. Again the base station has eight timeslots t-, to t 8 , but this time the base station has five available frequencies f-i to f 5 . A mobile unit requiring a logical channel is allocated to a given frequency/timeslot allocation for the first frame of the transmission and then follows the slots in the numerical order shown, returning after slot 39 to slot
  • the pseudo-random sequence and also the position in the pseudo-random sequence must be identified to the mobile unit by the base station, so that the mobile unit can follow the pattern
  • the pseudo random sequence of the timeslots may have a different repeat period to the pseudo random sequence of the frequencies, so that the whole pattern repeats very rarely indeed.
  • the number of frames in a full repeat is the lowest common multiple of the number of frames in each of the frequency and timeslot repeat patterns. For example, if the frequencies and timeslots each repeat over a period of about an hour, such that the number of frames in each pattern is approximately 800,000 frames, but selected to have no common factors, (e.g. 799,995 and 800,008 frames respectively), the complete pattern will repeat in approximately 640,000,000,000 frames, or 800,000 hours, which is more than ninety years.
  • each logical channel has all frequencies allocated to the base station available to it.
  • no mobile stations are permanently allocated to timeslot t-i , which would require that they have one of the available frequencies permanently barred to them for the duration of the call.
  • the timeslots with which it will interfere will be different in each frame, and will therefore, in general, interfere with different mobile stations in each frame, thereby reducing the effects of the interference on any given logical channel.
  • Different timeslot hopping patterns may be used at each base station, in order to avoid accidental synchronisation of logical channels at different base stations.
  • the interfering signal which is synchronised with an individual timeslot of the logical channel to which the mobile station is allocated will relate to a different logical channel at the interfering station for other timeslots.
  • each successive timeslot of the interfering signal will relate to a different logical channel so the interfering signal will be mere noise and not a coherent speech (or data) signal and will thus be less intrusive.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

Dans un système de téléphonie mobile, une station de base communique avec plusieurs unités mobiles par l'intermédiaire de plusieurs canaux de communications partageant la même fréquence radioélectrique, au moyen du multiplexage par répartition dans le temps (TDM). Ce système peut être utilisé conjointement au saut de fréquences, dont l'utilisation a jusqu'ici été limitée par la nécessité pour la station de base d'opérer sur plusieurs fréquences, et par l'obligation de réserver un canal pour les fonctions de contrôle (canal BCCH), ce qui empêche les canaux du créneau temporel correspondant de n'importe quelle autre fréquence utilisée par ladite station de base de basculer sur la fréquence du canal BCCH. Chaque unité mobile utilise un créneau temporel différent pour chaque nouvelle trame de transmission. Ce système permet de réduire les effets de brouillage dans le même canal. En changeant de créneau temporel et en utilisant une séquence qui évite le canal BCCH, on peut alors interchanger tous les canaux, même si ladite station de base utilise seulement une ou deux fréquences.
PCT/GB1998/001607 1997-06-03 1998-06-03 Systeme d'affectation de canaux WO1998056124A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU77788/98A AU7778898A (en) 1997-06-03 1998-06-03 Channel allocation system

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GBGB9711521.6A GB9711521D0 (en) 1997-06-03 1997-06-03 Channel allocation system
GB9711521.6 1997-06-03

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000036766A1 (fr) * 1998-12-17 2000-06-22 Alcatel Procede de synchronisation dans un systeme de telecommunication
EP1463358A1 (fr) * 2003-03-28 2004-09-29 Siemens Aktiengesellschaft Procédé et station de base pour le transfert de données MBMS

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EP0247790A2 (fr) * 1986-05-27 1987-12-02 Fairchild Weston Systems Inc. Système de communication sécurisé pour plusieurs unités éloignées
EP0480505A1 (fr) * 1990-10-09 1992-04-15 Philips Communication D'entreprise Système de transmission radioélectrique comportant une pluralité de dispositifs d'abonné
EP0533545A1 (fr) * 1991-09-17 1993-03-24 Matra Communication Station de base pour système de communication à accès multiple par répartition dans le temps
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US5291475A (en) * 1992-03-27 1994-03-01 Motorola, Inc. Slot hopped FD/TD/CDMA
WO1995007013A1 (fr) * 1993-09-01 1995-03-09 Telefonaktiebolaget Lm Ericsson Selection de voies dans un systeme de communications cellulaire
WO1995031878A1 (fr) * 1994-05-11 1995-11-23 Nokia Telecommunications Oy Procede et agencement pour la transmission de donnees de grande vitesse dans un systeme de telecommunications mobile amrt
EP0687078A2 (fr) * 1994-06-08 1995-12-13 Nokia Mobile Phones Ltd. Système de transmission de paquets de données dans un système téléphonique radio TDMA
US5546384A (en) * 1993-06-09 1996-08-13 Alcatel Mobile Communications Step by step method of controlling frequency redefinition in a cellular mobile radio system

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Publication number Priority date Publication date Assignee Title
EP0247790A2 (fr) * 1986-05-27 1987-12-02 Fairchild Weston Systems Inc. Système de communication sécurisé pour plusieurs unités éloignées
EP0480505A1 (fr) * 1990-10-09 1992-04-15 Philips Communication D'entreprise Système de transmission radioélectrique comportant une pluralité de dispositifs d'abonné
EP0533545A1 (fr) * 1991-09-17 1993-03-24 Matra Communication Station de base pour système de communication à accès multiple par répartition dans le temps
WO1993017507A1 (fr) * 1992-02-28 1993-09-02 Telefonaktiebolaget Lm Ericsson Procede de communication d'un systeme radio mobile cellulaire amrt par saut de frequence
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EP0564339A1 (fr) * 1992-03-31 1993-10-06 Matra Communication Procédé de radiocommunication AMRT
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EP0687078A2 (fr) * 1994-06-08 1995-12-13 Nokia Mobile Phones Ltd. Système de transmission de paquets de données dans un système téléphonique radio TDMA

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000036766A1 (fr) * 1998-12-17 2000-06-22 Alcatel Procede de synchronisation dans un systeme de telecommunication
FR2787660A1 (fr) * 1998-12-17 2000-06-23 Cit Alcatel Procede de synchronisation dans un systeme de telecommunication
EP1014601A1 (fr) * 1998-12-17 2000-06-28 Alcatel Procédé de synchronisation dans un système de télécommunication
EP1463358A1 (fr) * 2003-03-28 2004-09-29 Siemens Aktiengesellschaft Procédé et station de base pour le transfert de données MBMS

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AU7778898A (en) 1998-12-21
GB9711521D0 (en) 1997-07-30

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