CA2248015C - A random access scheme for mobile satellite communications - Google Patents
A random access scheme for mobile satellite communications Download PDFInfo
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- CA2248015C CA2248015C CA002248015A CA2248015A CA2248015C CA 2248015 C CA2248015 C CA 2248015C CA 002248015 A CA002248015 A CA 002248015A CA 2248015 A CA2248015 A CA 2248015A CA 2248015 C CA2248015 C CA 2248015C
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- transmitter
- receiver
- control station
- access
- access channel
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Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/204—Multiple access
- H04B7/212—Time-division multiple access [TDMA]
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/204—Multiple access
- H04B7/212—Time-division multiple access [TDMA]
- H04B7/2121—Channels assignment to the different stations
- H04B7/2123—Variable assignment, e.g. demand assignment
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/204—Multiple access
- H04B7/212—Time-division multiple access [TDMA]
- H04B7/2125—Synchronisation
Abstract
A method for transmitting random access signals from a mobile unit to a satellite or other control station which requires the transmission of less information than in conventional terrestrial cellular or proposed mobile satellite systems. According to the method, the mobile unit acquires synchronization from the satellite and transmits reduced random access information to the satellite, and the satellite calculates time adjustment information for the mobile units based on the reduced random access information. The random access information is reduced by representing a slot number to each mobile unit, and by transmitting only the odd or even status of the frame number desired by the mobile unit.
Description
A RANDOM ACCESS SCHEME
FOR MOBILE SATELLITE COMMUNICATIONS
Field of the Invention The present invention relates to data transmission for mobile satellite communications. In particular, the present invention provides an improved method for transmitting random access signals used for call initiation or call paging acknowledgement in mobile satellite communication systems.
Background of the Invention Referring to FIG. 1, a typical mobile radiocommunication system is shown.
The typical system includes a number of base stations similar to base station 110 and a number of mobile units or stations similar to mobile 120. Voice and/or data communication can be performed using these devices or their equivalents. The base station includes a control and processing unit 130 which is connected to the MSC
(mobile switching center) 140 which in turn is connected to the public switched telephone network (not shown).
The base station 110 serves a limited geographic coverage area, and includes a plurality of voice channels handled by voice channel transceiver 150 which is controlled by the control and processing unit 130. Also, each base station includes a control channel transceiver 160 which may be capable of handling more than one control channel. The control channel transceiver I60 is controlled by the control and processing unit I30. The control channel transceiver 160 broadcasts control information over the control channel of the base station to mobiles locked to that control channel. The voice channel transceiver broadcasts the traffic or voice channels which can include digital control channel location information.
When the mobile 120 first enters an idle mode, it periodically scans the control channels of base stations like base station 110 for the presence of a paging burst addressed to the mobile 120. The paging burst informs mobile 120 which base station to lock on or camp to. The mobile 120 receives the absolute and relative information broadcast on a control channel at its voice and control channel transceiver 170. Then, . , .. ~ 1 the processing unit 1$0 evaluates the received control channel iDformation which includes the characteristics of the candidate cells and determines which cell the mobile should lock to. The received control channel information not only includes absolute information concerning the base station with which it is associated, but ,also contains relative information concerning adjacent coverage areas with which the control channel is associated. These adjacent coverage areas are periodically scanned while rzzonitoring the primary cozttrol channel to determine if there is a more suitable candidate. Additional information relating to specifics of mobile and base statiotz implementations can be found in U.S. Patent No. 5,745,523 to Dent et al., entitled "Mufti-Mode Signal Processing." It will be appreciated that the base station may be replaced by one or more satellites in a satellite-based mobile radiocommunication system.
To increase communication system capacity, digital communication and multiple access techniques such as Freguency Divisiun Multiple Access (FDMA), IS Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) may be used. The objective of each of these multiple access techniques is to combine signals from different sources onto a common transmission medium in such a way that, at their destinations, the different channels can be separated without mutual interference. In a FDMA system, users share the radio spectrum in the frequency domain. Each user is allocated a part of the frequency band which is used throughout a conversation. In a TDMA system, users share the radio spectrum in the time domain. Each radio channel or carrier frequency is divided into a series of time slots, and individual users are allocated a time slot during which the user flay access to the entire frequency band allocated for the system (wiaehand TDMA) or only a part of the band (n,arrowband TDMA)_ Each tine slot contains a "burst" of information from a data source, e,g., a digitally encoded portion of a voice conversation. The time slots arc grouped into successive TDMA fraraes having a predetermined duration. The number of time slots in each TDMA frame is related to the number of different users that can sirriultaneously share the radio channel. If each slot in a TDMA
frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user. In a CDMA
system, each user is assigned a unique pseudorandom user code to uniquely access the frequency time domain. Examples of CDMA techniques include spread spectrum and frequency hopping.
Transmission of signals in a TDMA system occurs in a buffer-and-burst, or discontinuous-transmission, mode: each mobile unit transmits or receives only during its assigned time slots in the TDMA frames on the mobile unit's assigned frequency.
At full rate, for example, a mobile station might transmit during slot 1, receive during slot 2, idle during slot 3, transmit during slot 4, receive during slot 5, and idle during slot 6, and then repeat the cycle during succeeding TDMA frames. The mobile unit, which may be battery-powered, can be switched off (or "sleep") to save power during the time slots when it is neither transmitting nor receiving.
In a TDMA system, the successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user's digital traffic channel, which is considered to be a logical channel assigned to the user. The organization of TDMA channels, using the GSM standard as an example, is shown in FIG. 2. The TDMA channels include traffic channels TCH and signalling channels SC. The TCH channels include full-rate and half rate channels for transmitting voice and/or data signals. The signalling channels SC transfer signalling information between the mobile unit and the satellite (or base station). The signalling channels SC
include three types of control channels: broadcast control channel (BCCHs), common control channels (CCCHs) shared between multiple subscribers, and dedicated control channels (DCCHs) assigned to a single subscriber. A BCCH typically includes a frequency correction channel (FCH) and a synchronization channel (SCH), both of which are downlink channels. The common control channels (CCCHs) include downlink paging (PCH) and access grant (AGCH) channels, as well as the uplink random access channel (RACH). The dedicated control channels DCCH include a fast associated control channel (FACCH), a slow associated control channel (SACCH), and a standalone dedicated control channel (SDCCH). The slow associated control channel is assigned to a traffic (voice or data) channel or to a ki'1.1i>\:F:I'1-~IIL\C:IIEV ~W :Il- :1-:CA 02248015 1998-09-01 ' J.-lJ r3J
_:s:5.-i-1-1F~~:.a1:3 , - "r , Y;,,.,,.
standalone dedicated control channel (ST?CGH). The SACCH channel provides power and frame adjustment and control information to the mobile unit.
The random access channel R4CH is used by the xttiobiles to request access cu the system The RACH logical channel is a unidirectional uplink channel i,from the S mobile to the base station or satellite), and is sharrx3 by separate mobile units (one RACH p~,r cell is sufficient in typical systems: even during p~rio~is of heavy use).
Mobile emits continuously monitor the status of the R.4CH channel to deterrrzine if the channel is busy or idle. if the'RACH channel is idle, a mobile unit desiring au;ess sends access data, such as the uzubile's identit'~Caiion number. call establyshed cause, and the desired telephone number, on the RACE-1 to the base station or satellite. The MSC receives this information from the base station or satellite and assigns an idle votce channel to the mobile station, and transnuts the channel identification to the mobile through the base station or satellite over the access grant channel AGCH so chat the mobile station ca.ti mne itself to the new channel. .411 time slots on the RACH
1~ uplink channel are used for mobile access requests, either on a contention basis ur on a reserved basis. Contention-basis access schemes include ALOHA, CSMA, CSMA-CD, and other access protocols. Reserved-basis access is described in U.S.
Patent Mo. >.~20,Rfui, entitled "Method of Effecting Random Access is a Mobile Radio System." One important feature of RACH operation is that reception of some downlink information is required, whereby mobile stations receive real-time feedback for every burst they send on the uplin.k. This is known as Layer 2 ARQ, or automatic repeat request, on the RACH. The downlink information preferably comprises t~Nenty-two bits that may be thought of as another downlink sub-channel dedicated to carrying, in the downlink, Layer 2 infurtnation specific to the uplink. This flow of information, which can be called shared channel feedback, enhances the throughput capacity of the RACH so that a mobile station can quickly determine whether any burst of any access attempt has been successfully received.
In conventional terrestrial cellular communi~tioas systems such ag GSM, a random access channel (RACH) burst includes 68.25 bits as a guard time to prevent ." ,~ i_ ',.! (s r _ _ _ ,..~;~_T
FOR MOBILE SATELLITE COMMUNICATIONS
Field of the Invention The present invention relates to data transmission for mobile satellite communications. In particular, the present invention provides an improved method for transmitting random access signals used for call initiation or call paging acknowledgement in mobile satellite communication systems.
Background of the Invention Referring to FIG. 1, a typical mobile radiocommunication system is shown.
The typical system includes a number of base stations similar to base station 110 and a number of mobile units or stations similar to mobile 120. Voice and/or data communication can be performed using these devices or their equivalents. The base station includes a control and processing unit 130 which is connected to the MSC
(mobile switching center) 140 which in turn is connected to the public switched telephone network (not shown).
The base station 110 serves a limited geographic coverage area, and includes a plurality of voice channels handled by voice channel transceiver 150 which is controlled by the control and processing unit 130. Also, each base station includes a control channel transceiver 160 which may be capable of handling more than one control channel. The control channel transceiver I60 is controlled by the control and processing unit I30. The control channel transceiver 160 broadcasts control information over the control channel of the base station to mobiles locked to that control channel. The voice channel transceiver broadcasts the traffic or voice channels which can include digital control channel location information.
When the mobile 120 first enters an idle mode, it periodically scans the control channels of base stations like base station 110 for the presence of a paging burst addressed to the mobile 120. The paging burst informs mobile 120 which base station to lock on or camp to. The mobile 120 receives the absolute and relative information broadcast on a control channel at its voice and control channel transceiver 170. Then, . , .. ~ 1 the processing unit 1$0 evaluates the received control channel iDformation which includes the characteristics of the candidate cells and determines which cell the mobile should lock to. The received control channel information not only includes absolute information concerning the base station with which it is associated, but ,also contains relative information concerning adjacent coverage areas with which the control channel is associated. These adjacent coverage areas are periodically scanned while rzzonitoring the primary cozttrol channel to determine if there is a more suitable candidate. Additional information relating to specifics of mobile and base statiotz implementations can be found in U.S. Patent No. 5,745,523 to Dent et al., entitled "Mufti-Mode Signal Processing." It will be appreciated that the base station may be replaced by one or more satellites in a satellite-based mobile radiocommunication system.
To increase communication system capacity, digital communication and multiple access techniques such as Freguency Divisiun Multiple Access (FDMA), IS Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) may be used. The objective of each of these multiple access techniques is to combine signals from different sources onto a common transmission medium in such a way that, at their destinations, the different channels can be separated without mutual interference. In a FDMA system, users share the radio spectrum in the frequency domain. Each user is allocated a part of the frequency band which is used throughout a conversation. In a TDMA system, users share the radio spectrum in the time domain. Each radio channel or carrier frequency is divided into a series of time slots, and individual users are allocated a time slot during which the user flay access to the entire frequency band allocated for the system (wiaehand TDMA) or only a part of the band (n,arrowband TDMA)_ Each tine slot contains a "burst" of information from a data source, e,g., a digitally encoded portion of a voice conversation. The time slots arc grouped into successive TDMA fraraes having a predetermined duration. The number of time slots in each TDMA frame is related to the number of different users that can sirriultaneously share the radio channel. If each slot in a TDMA
frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user. In a CDMA
system, each user is assigned a unique pseudorandom user code to uniquely access the frequency time domain. Examples of CDMA techniques include spread spectrum and frequency hopping.
Transmission of signals in a TDMA system occurs in a buffer-and-burst, or discontinuous-transmission, mode: each mobile unit transmits or receives only during its assigned time slots in the TDMA frames on the mobile unit's assigned frequency.
At full rate, for example, a mobile station might transmit during slot 1, receive during slot 2, idle during slot 3, transmit during slot 4, receive during slot 5, and idle during slot 6, and then repeat the cycle during succeeding TDMA frames. The mobile unit, which may be battery-powered, can be switched off (or "sleep") to save power during the time slots when it is neither transmitting nor receiving.
In a TDMA system, the successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user's digital traffic channel, which is considered to be a logical channel assigned to the user. The organization of TDMA channels, using the GSM standard as an example, is shown in FIG. 2. The TDMA channels include traffic channels TCH and signalling channels SC. The TCH channels include full-rate and half rate channels for transmitting voice and/or data signals. The signalling channels SC transfer signalling information between the mobile unit and the satellite (or base station). The signalling channels SC
include three types of control channels: broadcast control channel (BCCHs), common control channels (CCCHs) shared between multiple subscribers, and dedicated control channels (DCCHs) assigned to a single subscriber. A BCCH typically includes a frequency correction channel (FCH) and a synchronization channel (SCH), both of which are downlink channels. The common control channels (CCCHs) include downlink paging (PCH) and access grant (AGCH) channels, as well as the uplink random access channel (RACH). The dedicated control channels DCCH include a fast associated control channel (FACCH), a slow associated control channel (SACCH), and a standalone dedicated control channel (SDCCH). The slow associated control channel is assigned to a traffic (voice or data) channel or to a ki'1.1i>\:F:I'1-~IIL\C:IIEV ~W :Il- :1-:CA 02248015 1998-09-01 ' J.-lJ r3J
_:s:5.-i-1-1F~~:.a1:3 , - "r , Y;,,.,,.
standalone dedicated control channel (ST?CGH). The SACCH channel provides power and frame adjustment and control information to the mobile unit.
The random access channel R4CH is used by the xttiobiles to request access cu the system The RACH logical channel is a unidirectional uplink channel i,from the S mobile to the base station or satellite), and is sharrx3 by separate mobile units (one RACH p~,r cell is sufficient in typical systems: even during p~rio~is of heavy use).
Mobile emits continuously monitor the status of the R.4CH channel to deterrrzine if the channel is busy or idle. if the'RACH channel is idle, a mobile unit desiring au;ess sends access data, such as the uzubile's identit'~Caiion number. call establyshed cause, and the desired telephone number, on the RACE-1 to the base station or satellite. The MSC receives this information from the base station or satellite and assigns an idle votce channel to the mobile station, and transnuts the channel identification to the mobile through the base station or satellite over the access grant channel AGCH so chat the mobile station ca.ti mne itself to the new channel. .411 time slots on the RACH
1~ uplink channel are used for mobile access requests, either on a contention basis ur on a reserved basis. Contention-basis access schemes include ALOHA, CSMA, CSMA-CD, and other access protocols. Reserved-basis access is described in U.S.
Patent Mo. >.~20,Rfui, entitled "Method of Effecting Random Access is a Mobile Radio System." One important feature of RACH operation is that reception of some downlink information is required, whereby mobile stations receive real-time feedback for every burst they send on the uplin.k. This is known as Layer 2 ARQ, or automatic repeat request, on the RACH. The downlink information preferably comprises t~Nenty-two bits that may be thought of as another downlink sub-channel dedicated to carrying, in the downlink, Layer 2 infurtnation specific to the uplink. This flow of information, which can be called shared channel feedback, enhances the throughput capacity of the RACH so that a mobile station can quickly determine whether any burst of any access attempt has been successfully received.
In conventional terrestrial cellular communi~tioas systems such ag GSM, a random access channel (RACH) burst includes 68.25 bits as a guard time to prevent ." ,~ i_ ',.! (s r _ _ _ ,..~;~_T
collisions with traffic bursts on adjacent time slots. The guard bits allow the random access burst to be received at the base station within a predicted time slot, for a mobile station within 35 Km of the associated base station. Specifically, the mobile phone acquires synchronization from a downlink channel, such as the broadcast control channel (BCCH). The mobile then transmits a signal on the RACH channel on a time slot some number of time slots after the initial synch information.
The offset for transmitting the IRACH signal can be a random offset or a specified fixed offset. The transmitted RACH burst will be received at the base station within the predicted time slot if the distance between the base station and the mobile is as specified above. The guard bits are necessary because the ItACH burst is sent before the mobile has received a time advance command from the base station.
In mobile satellite communications the beams from a single satellite are coverage areas analogous to cells in a terrestrial system. Typically each satellite will have 140 beams, each beam providing a coverage area of about 500 Km or more in diameter. Because of the size of a beam, there is an end-to-end delay within a beam of about 6 ms, resulting in a time ambiguity within a beam of ~3ms. Therefore, the use of guard bits as in a conventional system does not resolve collision and ambiguity problems, since a slotted ItACH in a satellite system would require a very long guard time.
Summary of the Invention The present invention provides a method for transmitting random access information in a satellite mobile communication system. According to the method, the mobile unit acquires synchronization from a satellite or other control station, based on the high power short message service (HP-SMS) burst or other suitable signal. After some fixed or random time offset, the mobile unit transmits the necessary random access information, including a desired frame and slot number, on a dedicated RACH carrier. Based on the information received from the mobile, the satellite or control station calculates the differential delay between users and send appropriate time advance or retard information to the particular users.
According to an aspect of the present invention there is provided a method for acquiring access by a transmitter/receiver to a TDMA communication system, comprising the steps of receiving a synchronization signal from a control station, transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted TDMA frame for communication between the transmitter/receiver and the control station, and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data.
According to another aspect of the present invention there is provided a method for acquiring access by a transmitter/receiver to a TDMA communication system, comprising the steps of receiving a synchronization signal from a control station, transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted frame and slot number using a single information bit, and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data, wherein a maximum transmission delay between the transmitter/receiver and the control station is less than the length of a TDMA
frame.
According to a further aspect of the present invention there is provided a method for acquiring access by a transmitterlreceiver to a TDMA communication system, comprising the steps of receiving a synchronization signal from a control station, transmitting access data from the transmitter/receiver to the control station on a predetermined time slot, determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access data, and granting access to the transmitter/receiver.
Sa Brief Description of the Drawings The foregoing objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which:
FIG. 1 is a block diagram of an exemplary mobile radiocommunication system;
FIG. 2 is a diagram showing the organization of channels in a typical GSM
digital radiocommunication system;
FIG. 3 is a diagram of a satellite-based mobile radiocommunication system in which the signal transmission method of the present invention may be implemented;
FIG. 4 is a flow chart describing a random access method according to an embodiment of the present invention; and FIG. 5 is a flow chart describing a random access method according to an alternate embodiment of the present invention.
Detailed Description of Preferred Embodiments While the following description is directed toward a short message service implemented in a satellite-based radiocommunication system, it will be appreciated that the present invention may also be applied to other types of communication systems.
Referring now to FIG. 3, a satellite-based mobile communication system in which the method of the present invention may be implemented is shown. The system includes one or more satellites 10, a mobile switching center MSC which exchanges communication signals with one or more satellites and is connected to the PSTN
(public switched telephone network; not shown), and any number of mobile units MUn. In such a system, communication links for exchanging voice or data can be established between a mobile unit and either a standard telephone or a second mobile unit through one satellite, multiple satellites, or a combination of one or more satellites and the PSTN. Such a system may be desirable to achieve a broad geographical coverage in which few or no base stations are present, and additional base stations are not practical, such as in rural areas.
Referring now to FIG. 4, a flow chart describing the steps of an exemplary method of the present invention is shown. The process begins at step 100, in which the mobile unit acquires synchronization information from the satellite. The synchronization information can be acquired from a variety of source signals S transmitted by the satellite such as, for example, a high power short message service (HP-SMS) burst transmitted as described in United States Patent No 6,046,990 to Chennakeshu et al., entitled "High Penetration Transmission Method For A
Radiocommunication System". In step 102, the mobile unit then transmits on the uplink RACH channel one or more RACH bursts which preferably include IO information such as coded message data (which includes information and redundancy bits), coded frame information identifying a predicted frame on which the mobile unit desires to transmit (after a random number of frames}, and coded slot information identifying a predicted slot on which the mobile unit desires to transmit (after a random number of slots). In step 104, the satellite calculates the time adjustment 15 necessary from the coded frame information and coded slot information using any suitable method for calculating time adjustments.
Alternatively, the mobile unit may transmit only a single frame bit to indicate whether the frame number is to be an odd-numbered frame or an even-numbered frame. This single bit information is sufficient to enable the satellite to determine the 20 necessary time adjustment for differential delays less than twice the length of a TDMA frame, a characteristic of most regional mobile satellite systems.
However, for a satellite beam having a relatively small size and small differential delay, this scheme will result in high collision probability. To avoid this, a mobile unit may choose a particular time slot to transmit the RACH burst based on 25 the mobile's own identification number, e_g., the temporary mobile station identifier (TMSI) or the international mobile station identifier (IMSI). In this way, the RACH
traffic can be evenly distributed over the duration of a frame to reduce the collision prohability. Since the mobile unit's identification number is known to the base station, the satellite or base station knows which time slot the mobile unit is 30 transmitting the RACH burst on, and can determine the necessary time adjustment for the mobile unit.
RCS. 10~ ::I'\-~1~ L\CiiE\ (;? : ~ i- -;-:1CA 02248015 1998-09-O1 - +~:) F3.3 '?:_35)x)-Vii()-:;~ I l a,~'f ~~~'~' r~,: us) L~:~G,lry~.; ~. ';.''~J ~.:i;~ ~.
.i:'.,..V.,=.L~'l.i~.l;!. i .'2 In the i;ase where tha maximum differential delay within a beam; of the entire system is always less than a frame, the mobile unit need not indicate the predicted frame ~or slot number on which it desires to transmit.
Tlie random access transmission scheme ~3n be further simplified by predeterniining that all transmissions from a particular mobile unit will occur in a given let of a given frame. By predeternninizig the transmission slot anal frame number, the random access channel information need not include coded time slot information, further reduciz:g the information required in the RACH bursts. By transzrutting only the even or odd status of the desired frame and preassigning a ztut number to e-ach mobile unit, the raztdom acrxss channel information (coded fra_~ne and slot numbers) can be reduced to a Single information bit. Accordingly, tt~e need for guard bits is unnecessary and is substantially reduced by the method of the present invention, thereby increasing transmission efficiency.
Referring nuw to FIG. 5, a flow chart describing the steps of an alternate embodiment of the method of the present iimention. Tue ernbodimetit of FIG. 5 differs from that of FIG. 4 in that the FIG. 5 embodiment includes a step 101 for receiving a page signal prior to transmitting tb~e random access information to the satellite in step 102.
While the foregoing description includes numerous details and specific;ities, it ZO is to he uederstood that these are for purposes of ezplanation only, and are not to be construed as limitations of the invzntion. Numerous modifications to the described examples will be readily apparent to one of ordinary skill in the art which do not depart from the scope of the invention, 3s defined by the following claims.
_,,;..':.'~ :.:,-_I~:s-i
The offset for transmitting the IRACH signal can be a random offset or a specified fixed offset. The transmitted RACH burst will be received at the base station within the predicted time slot if the distance between the base station and the mobile is as specified above. The guard bits are necessary because the ItACH burst is sent before the mobile has received a time advance command from the base station.
In mobile satellite communications the beams from a single satellite are coverage areas analogous to cells in a terrestrial system. Typically each satellite will have 140 beams, each beam providing a coverage area of about 500 Km or more in diameter. Because of the size of a beam, there is an end-to-end delay within a beam of about 6 ms, resulting in a time ambiguity within a beam of ~3ms. Therefore, the use of guard bits as in a conventional system does not resolve collision and ambiguity problems, since a slotted ItACH in a satellite system would require a very long guard time.
Summary of the Invention The present invention provides a method for transmitting random access information in a satellite mobile communication system. According to the method, the mobile unit acquires synchronization from a satellite or other control station, based on the high power short message service (HP-SMS) burst or other suitable signal. After some fixed or random time offset, the mobile unit transmits the necessary random access information, including a desired frame and slot number, on a dedicated RACH carrier. Based on the information received from the mobile, the satellite or control station calculates the differential delay between users and send appropriate time advance or retard information to the particular users.
According to an aspect of the present invention there is provided a method for acquiring access by a transmitter/receiver to a TDMA communication system, comprising the steps of receiving a synchronization signal from a control station, transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted TDMA frame for communication between the transmitter/receiver and the control station, and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data.
According to another aspect of the present invention there is provided a method for acquiring access by a transmitter/receiver to a TDMA communication system, comprising the steps of receiving a synchronization signal from a control station, transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted frame and slot number using a single information bit, and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data, wherein a maximum transmission delay between the transmitter/receiver and the control station is less than the length of a TDMA
frame.
According to a further aspect of the present invention there is provided a method for acquiring access by a transmitterlreceiver to a TDMA communication system, comprising the steps of receiving a synchronization signal from a control station, transmitting access data from the transmitter/receiver to the control station on a predetermined time slot, determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access data, and granting access to the transmitter/receiver.
Sa Brief Description of the Drawings The foregoing objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which:
FIG. 1 is a block diagram of an exemplary mobile radiocommunication system;
FIG. 2 is a diagram showing the organization of channels in a typical GSM
digital radiocommunication system;
FIG. 3 is a diagram of a satellite-based mobile radiocommunication system in which the signal transmission method of the present invention may be implemented;
FIG. 4 is a flow chart describing a random access method according to an embodiment of the present invention; and FIG. 5 is a flow chart describing a random access method according to an alternate embodiment of the present invention.
Detailed Description of Preferred Embodiments While the following description is directed toward a short message service implemented in a satellite-based radiocommunication system, it will be appreciated that the present invention may also be applied to other types of communication systems.
Referring now to FIG. 3, a satellite-based mobile communication system in which the method of the present invention may be implemented is shown. The system includes one or more satellites 10, a mobile switching center MSC which exchanges communication signals with one or more satellites and is connected to the PSTN
(public switched telephone network; not shown), and any number of mobile units MUn. In such a system, communication links for exchanging voice or data can be established between a mobile unit and either a standard telephone or a second mobile unit through one satellite, multiple satellites, or a combination of one or more satellites and the PSTN. Such a system may be desirable to achieve a broad geographical coverage in which few or no base stations are present, and additional base stations are not practical, such as in rural areas.
Referring now to FIG. 4, a flow chart describing the steps of an exemplary method of the present invention is shown. The process begins at step 100, in which the mobile unit acquires synchronization information from the satellite. The synchronization information can be acquired from a variety of source signals S transmitted by the satellite such as, for example, a high power short message service (HP-SMS) burst transmitted as described in United States Patent No 6,046,990 to Chennakeshu et al., entitled "High Penetration Transmission Method For A
Radiocommunication System". In step 102, the mobile unit then transmits on the uplink RACH channel one or more RACH bursts which preferably include IO information such as coded message data (which includes information and redundancy bits), coded frame information identifying a predicted frame on which the mobile unit desires to transmit (after a random number of frames}, and coded slot information identifying a predicted slot on which the mobile unit desires to transmit (after a random number of slots). In step 104, the satellite calculates the time adjustment 15 necessary from the coded frame information and coded slot information using any suitable method for calculating time adjustments.
Alternatively, the mobile unit may transmit only a single frame bit to indicate whether the frame number is to be an odd-numbered frame or an even-numbered frame. This single bit information is sufficient to enable the satellite to determine the 20 necessary time adjustment for differential delays less than twice the length of a TDMA frame, a characteristic of most regional mobile satellite systems.
However, for a satellite beam having a relatively small size and small differential delay, this scheme will result in high collision probability. To avoid this, a mobile unit may choose a particular time slot to transmit the RACH burst based on 25 the mobile's own identification number, e_g., the temporary mobile station identifier (TMSI) or the international mobile station identifier (IMSI). In this way, the RACH
traffic can be evenly distributed over the duration of a frame to reduce the collision prohability. Since the mobile unit's identification number is known to the base station, the satellite or base station knows which time slot the mobile unit is 30 transmitting the RACH burst on, and can determine the necessary time adjustment for the mobile unit.
RCS. 10~ ::I'\-~1~ L\CiiE\ (;? : ~ i- -;-:1CA 02248015 1998-09-O1 - +~:) F3.3 '?:_35)x)-Vii()-:;~ I l a,~'f ~~~'~' r~,: us) L~:~G,lry~.; ~. ';.''~J ~.:i;~ ~.
.i:'.,..V.,=.L~'l.i~.l;!. i .'2 In the i;ase where tha maximum differential delay within a beam; of the entire system is always less than a frame, the mobile unit need not indicate the predicted frame ~or slot number on which it desires to transmit.
Tlie random access transmission scheme ~3n be further simplified by predeterniining that all transmissions from a particular mobile unit will occur in a given let of a given frame. By predeternninizig the transmission slot anal frame number, the random access channel information need not include coded time slot information, further reduciz:g the information required in the RACH bursts. By transzrutting only the even or odd status of the desired frame and preassigning a ztut number to e-ach mobile unit, the raztdom acrxss channel information (coded fra_~ne and slot numbers) can be reduced to a Single information bit. Accordingly, tt~e need for guard bits is unnecessary and is substantially reduced by the method of the present invention, thereby increasing transmission efficiency.
Referring nuw to FIG. 5, a flow chart describing the steps of an alternate embodiment of the method of the present iimention. Tue ernbodimetit of FIG. 5 differs from that of FIG. 4 in that the FIG. 5 embodiment includes a step 101 for receiving a page signal prior to transmitting tb~e random access information to the satellite in step 102.
While the foregoing description includes numerous details and specific;ities, it ZO is to he uederstood that these are for purposes of ezplanation only, and are not to be construed as limitations of the invzntion. Numerous modifications to the described examples will be readily apparent to one of ordinary skill in the art which do not depart from the scope of the invention, 3s defined by the following claims.
_,,;..':.'~ :.:,-_I~:s-i
Claims (20)
1. A method for acquiring access by a transmitter/receiver to a TDMA
communication system, comprising the steps of:
receiving a synchronization signal from a control station;
transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted TDMA frame for communication between the transmitter/receiver and the control station; and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data.
communication system, comprising the steps of:
receiving a synchronization signal from a control station;
transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted TDMA frame for communication between the transmitter/receiver and the control station; and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data.
2. The method of claim 1, wherein the access channel data identifies the predicted TDMA frame using a single data bit indicative of an even-numbered or odd-numbered status of the predicted TDMA frame.
3. The method of claim 1, wherein a slit number on which the transmitter/receiver will transmit is preassigned.
4. The method of claim 1, wherein the transmitter/receiver is a mobile station and the control station is a satellite.
5. The method of claim 1, wherein the synchronization signal is a message signal containing coded message data.
6. The method of claim 1, wherein a transmission delay between the transmitter/receiver and the control station is less than twice the length of a TDMA
frame.
frame.
7. The method of claim 4, wherein the transmitter/receiver transmits the access channel data over a random access channel on a dedicated carrier frequency.
8. The method of claim 1, further comprising the step of receiving a page signal from the control station prior to the step of transmitting.
9. The method of claim 1, wherein a slot number on which the transmitter/receiver will transmit is determined by the transmitter/receiver based on a identification value known to the control station.
10. A method for acquiring access by a transmitter/receiver to a TDMA
communication system, comprising the steps of:
receiving a synchronization signal from a control station;
transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted frame and slot number using a single information bit; and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data, wherein a maximum transmission delay between the transmitter/receiver and the control station is less than the length of a TDMA frame.
communication system, comprising the steps of:
receiving a synchronization signal from a control station;
transmitting access channel data from the transmitter/receiver to the control station, the access channel data identifying a predicted frame and slot number using a single information bit; and determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access channel data, wherein a maximum transmission delay between the transmitter/receiver and the control station is less than the length of a TDMA frame.
11, The method of claim 10, wherein a slot number on which the transmitter/receiver will transmit is preassigned.
12. The method of claim 10, wherein the transmitter/receiver is a mobile station and the control station is a satellite.
13. The method of claim 10, wherein the synchronization signal is a message signal containing coded message data.
14. The method of claim 10, wherein the transmitter/receiver transmits the access channel data over a random access channel on a dedicated carrier frequency.
15. The method of claim 10, further comprising the step of receiving a paging signal from the control station prior to the step of transmitting.
16. The method of claim 10, wherein the slot number on which a transmitter/receiver will transmit is determined by the transmitter/receiver based on an identification value known to the control station.
17. The method of claim 1, wherein the transmitter/receiver is a mobile station and the control station is a base station of a cellular communication system.
18. The method of claim 10, wherein the transmitter/receiver is a mobile station and the control station is a base station of a cellular communication system.
19. A method for acquiring access by a transmitter/receiver to a TDMA
communication system, comprising the steps of:
receiving a synchronization signal from a control station;
transmitting access data from the transmitter/receiver to the control station on a predetermined time slot;
determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access data; and granting access to the transmitter/receiver.
communication system, comprising the steps of:
receiving a synchronization signal from a control station;
transmitting access data from the transmitter/receiver to the control station on a predetermined time slot;
determining, at the control station, a time adjustment necessary for communication between the transmitter/receiver and the control station based on the access data; and granting access to the transmitter/receiver.
20. The method of claim 19, wherein the access data identifies a predicted frame and time slot using a single information bit.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/611,490 US5822311A (en) | 1996-03-05 | 1996-03-05 | Random access scheme for mobile satellite communications |
| US08/611,490 | 1996-03-05 | ||
| PCT/US1997/002189 WO1997033383A1 (en) | 1996-03-05 | 1997-02-12 | A random access scheme for mobile satellite communications |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2248015A1 CA2248015A1 (en) | 1997-09-12 |
| CA2248015C true CA2248015C (en) | 2005-06-21 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002248015A Expired - Lifetime CA2248015C (en) | 1996-03-05 | 1997-02-12 | A random access scheme for mobile satellite communications |
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| US (1) | US5822311A (en) |
| EP (1) | EP0885493B1 (en) |
| JP (1) | JP4405592B2 (en) |
| KR (1) | KR100669295B1 (en) |
| CN (1) | CN1184750C (en) |
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| CA (1) | CA2248015C (en) |
| DE (1) | DE69736171T2 (en) |
| ID (1) | ID16765A (en) |
| WO (1) | WO1997033383A1 (en) |
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1996
- 1996-03-05 US US08/611,490 patent/US5822311A/en not_active Expired - Lifetime
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1997
- 1997-02-12 AU AU21897/97A patent/AU712723B2/en not_active Expired
- 1997-02-12 EP EP97914764A patent/EP0885493B1/en not_active Expired - Lifetime
- 1997-02-12 DE DE69736171T patent/DE69736171T2/en not_active Expired - Lifetime
- 1997-02-12 KR KR1019980706958A patent/KR100669295B1/en not_active IP Right Cessation
- 1997-02-12 JP JP53176897A patent/JP4405592B2/en not_active Expired - Lifetime
- 1997-02-12 CA CA002248015A patent/CA2248015C/en not_active Expired - Lifetime
- 1997-02-12 WO PCT/US1997/002189 patent/WO1997033383A1/en active IP Right Grant
- 1997-02-12 CN CNB971941998A patent/CN1184750C/en not_active Expired - Lifetime
- 1997-02-27 ID IDP970599A patent/ID16765A/en unknown
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| CN1184750C (en) | 2005-01-12 |
| AU712723B2 (en) | 1999-11-11 |
| DE69736171D1 (en) | 2006-08-03 |
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| JP2000506346A (en) | 2000-05-23 |
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| JP4405592B2 (en) | 2010-01-27 |
| EP0885493B1 (en) | 2006-06-21 |
| WO1997033383A1 (en) | 1997-09-12 |
| US5822311A (en) | 1998-10-13 |
| KR19990087524A (en) | 1999-12-27 |
| EP0885493A1 (en) | 1998-12-23 |
| DE69736171T2 (en) | 2006-11-09 |
| CA2248015A1 (en) | 1997-09-12 |
| CN1217104A (en) | 1999-05-19 |
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