CA2463797A1 - Communication methods and apparatus - Google Patents
Communication methods and apparatus Download PDFInfo
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- CA2463797A1 CA2463797A1 CA002463797A CA2463797A CA2463797A1 CA 2463797 A1 CA2463797 A1 CA 2463797A1 CA 002463797 A CA002463797 A CA 002463797A CA 2463797 A CA2463797 A CA 2463797A CA 2463797 A1 CA2463797 A1 CA 2463797A1
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Abstract
in a wireless communications system, transceivers transmit short bursts to a base station, which determines timing corrections from the time of receipt of the burst and transmits the timing corrections to the respective transceivers. In one aspect, the base station indicates to the trasceivers a plurality of time slots, each transceiver selects one of the time slots at random, formats a burst including an indicator of the selected time slot and transmits the burst in that slot. In another aspect, the base station transmits to each transceiver a timing uncertainty value, which determines how the timing correction will be modified by the tranceiver as the interval since last receiving a timing correction increases. Data bursts are transmitted in a format comprising a first unique word, a content field and a second unique word, in that order. The bursts are transmitted in a TDMA channel format which can accommodate both short and long bursts in a block format of constant periodicity.
Description
1 . o ~O~Pe4U1~1I~ATIO~aT IeWTI-IOi.)~ A~II~ APPARA?LJ~
The present In~TentIOi~I relates to communications apparatus and methods.
particularly but not e~:clusively for wireless communications. particularly but not exclusively via satellite.
A problem associated r~~ith communications systems in vrhich different transmitters share a time-divided channel resource is that timing misalignments may cause interference°
between the transmitters. The mzsalYgrIrrlents may Eie caused by drift i.n the transmitter clocks, or variations in the propagatioY~ delay from different transmitters to a common receiver. In time-divided multiple access ('TI~MA) ~c,harlrYels, a glaard band is usuall~~
provided between adjacent time slots, so that timin.;7 differences of less.
than the guard band time between transmissions in adjacent slots do not cause interference. I-Iowever, the guard bands occupy band~-idth which could otherwise be' used to carry traffic, so reliance on guard bands alone to avoid interference is not suitable for systerrYS where a high bandwidth channel is shared by many transmitters.
The document ~J~ 5790939 describes a TL~I'vIA based satellite communication system including a timing correction protocol. The systerr~ broadcasts timing corrections tcs mobile terminals. residual errors in the timing of individual terminals acre corrected following transmission by the mobile terminals in s conteratYOn acceSS
channel. A gateway measures the residual timing er or and z~eports the e~r~or back to tile relevant mobile terminal.
Another problem associated with bursts in fC'Oh.!IA chanrlc°.ls is that, if adjacent bursts do overlap, the interference between the bur:>ts generally prevents either from being demodulated and decoded successfully. Burst forma-~s for each time slot may include a unique u,~ord which aids acquisition of the burst. as described for example i:rl LJS 5661'764, 25 but the advantage of the unique word is lost if the burst interferes with an adjacent burst.
The document G13 ?~'70~315 describes a cellular mobile radio system with a packet resez-~ration multiple access protocol, in whicl-a user traffic can be carried by both single and double slots, allocated dynarrYicaIIy by the base station according to Ioad requirements.
however, if the slot allocation is entireay flexible arid can include slots o;f differing Lengths, the timing alignment of bursts in those slots becams~s complex.
According to aspects ox the present invention, there is provided a timin=~
correction method in a communications system. in which tra~bsceivers transmit short bursts to a base station, which determines timirgg corrections from the tirr.e of receipt of the bursts and transmits the timing ~:orrections to the respective transceivers.
In one aspect, the base station indicates to ~t~e transceivers a plurality oftime slots:
each transceiver selects one of the time slots at ran,doz~, formats a burst including a indicator of the selected time s'ot and transmits thc: burst in that slot. The base station can therefore determine the timing with which each trc~~sca fiver transmitted the burst, but the probability of collision between bursts is reduced since they are spread across the plurality 1 Q of time slots.
In another aspect, the base station transmits to each transceiver a tinning uncertaintay value, which determines hove the timing correction a~ili be modi:~ied by the transceiver as the interval since last receiving a timing correction increases. Prf:ferably, i~;~the modification determined by the timing uncertainty value increases beyond a predetermined 15 threshold, the transceiver itilvibits transmission oth~e;~° than to reqL~est a timing correction.
These measures ad~-antageously reduce the likelihood of interference between slots, due to timing misalignment.
The above aspects of the prese~~?t invention extend individually to those parts of the method ~~hich are carried out by tl:e transceiver, those parts which are carried out by the 20 network vs.~ith which the transmitter co3 ~municates, and apparatu:>
arranged to carry out those individual parts of the method.
According to another aspect of the present it~.vexltiorl, there is provided a signal having a format comprising a first unique word, a content Meld and a second unique word, in that order. Preferably, there are no other fiends ire the burst before the :first or after the 25 second unique word which are necessary for the demodulation arid decoding of the burst;
this has the advantage that, if~ either the beginning c>r the end of the burst overlaps with another burst, it may still be possible to read the data content of t:he burst correctly. The content field may carry user data andlor signalling :information. There may be an additional field before the first unique word and/or after the se;coa~d unique word, but these are iQ preferabl;,- auxiliary melds which are not essential to the decoding ofthe content feld. For example, there may be a constant power preamble art the beginning of the burst to assist with power control ira the transmitter. This aspect of the present invention extends to methods of formatting and/or tnansrni~.~ting such a signal, and apparatus arranged to perform such methods. -According to another aspect of the present invention, there is provided a TI~MA
channel format v~~hiclm: can accornrnodate both short and long bursts in a block format of constant periodicity.
Specifac eynbodiments of the present invention will now be described with reference to the accor.~panying drawings, in ~~hich:
Figure 1 is a diagram of components of a satellite comrrrunication system 1 G incorporating embodiments of the preswnt in~~entior~;
Figure 2 shows the char.~raels used for communication bet~~~een the SAN and the IvIA~1's irl a packet data ser'~ice implemented in thf: system of Fi;~ure 1;
Figure 3 is a diagram of transmitter and receiver° channel units in a satellite access node (SA~1) of the system of Figure 19 Figure 4 is a diagram of transmitter and receiver cl-rannel units in a Mobile Access bode {M.~~1) of the system of Figure l;
Figures ~a to ~d show the structure of one of the L~~'SP channels of Figure 4;
Figure 6a sho~~~s the burst structure ofa 5 rrLS burst in one ofthe MF;SP
channels of F figure 4;
Figure 6b shows the burst structure of a 20 ms burst in one of the 1~IESP
channels of Figure 4;
Figure '~ is a tinning diagram illt.astratia~g the operation of an initial timing correction protocol for correcting the tinning of transmissions in the ~~ESP channels:
Figure 8a is a timing diagram illustrating the: tirr~ing of a t3°ansmission in one of the ?5 MESS' channels irnrrrediately following a timing co~°rection;
Figure 8b is a timing diagram illustrating thf: timing of a transmission in one of the MESF channels at an interval after a timing correction, where there is tirraing uncertainty;
Figures 9a to 3c are timing diagrams showing different collision scenarios between bursts of a conventional format in adjacent T~IvIA 'slots; and Figures l 0a to ~ f~c are tinning diagrams sho~aring the equivalent collision scenarios between bursts of a for.nat according to an embodiarmnt of the present invention.
SVStem ~verViet~' Figure 1 shoes the principal elements of a satellite communications system in an embodiment of the present invention. A plurality of Mobile Access Nodes (MAID') ~
communicate via a satellite 4 ~~ith a satellite earth station, hereinafter referred to as a Satellite Access Node (SAN) 6. The satellite 4 may for example be an lnmarsat-:iTM
satellite, as described for example in the article 'Launch of a New ~enerataon' by J R
Asker, TRANSAT, Issue ifs, January I g9~, pages 15 to 187 published by Inmarsat, the contents of which are included herein by reference. The satellite 4 is geostationary and projects a pluralit~~ of spot bcarns SB (five spot beans in the case: of an .Tnn~arsat-3TM
satellite) and a global beam CB, which encompasses the coverage areas of the spot beams SB, on the earths surface. The MAN's 2 may be portable satellite terminals having manually steerable antennas, of the type currently available for use with the Inmarsat mini-MTM service but with modifications as described hereafter. There may be a plurality of SAN's 6 ~~ithin the coverage area of each satellite ~~~ and capable of supporting communications wvith the M~.N"s ? and there may also be further geostationary satellites 4 with coverage areas vs~hich may or may not overlap that of the exemplary satellite 4. Each SAN 6 may form pa~~t of an lntr4arsat Land Earth Station (LES) and share RF
antennas and modulation:~demodulation equipment with conventional parts of the LES. Each 20 provides an interfatv~° between the communications link through the satellite 4 and one or more terrestrial net~~o~ks 8. so as to connect the M~'~t~l s 2 to terrestrial access nodes (TAN) I0, which are connectable directly or indirectly throe~gl~ further networks to any of a number of communications services, such as Internet, :STN or ISI)N-based services.
Channel Tvpes Figure 2 shows the channels used for communication between a sample one of the MAN's 2 and the SAN 6. All communications under this packet data service from the MAN 2 to the SAN 5 are carried on one or more slcl~:, of°one or more TI~MA channels, referred to as MESP channels (mobile earth station - packet channels), Each MESh channel 30 is divided into 40 ms blocks, divisible into 20 ms blocks. Each 20 ms block carries either one 20 ms burst or four' S ms bursts, in a format whirl' will be described below.
AlI communications under this packet data service from v:~he S AN C to the MAC' '_' are carried on one or more slots of one or more TLaM channels. referred to as LESP
channels (land earrh station - packet channels). Th~.~ slots are eac~~ ~0 ms long, and comprise two subframes of equal length.
S For the purposes of channel sec-up and othf:r network sig:r~alling, the MAN
The present In~TentIOi~I relates to communications apparatus and methods.
particularly but not e~:clusively for wireless communications. particularly but not exclusively via satellite.
A problem associated r~~ith communications systems in vrhich different transmitters share a time-divided channel resource is that timing misalignments may cause interference°
between the transmitters. The mzsalYgrIrrlents may Eie caused by drift i.n the transmitter clocks, or variations in the propagatioY~ delay from different transmitters to a common receiver. In time-divided multiple access ('TI~MA) ~c,harlrYels, a glaard band is usuall~~
provided between adjacent time slots, so that timin.;7 differences of less.
than the guard band time between transmissions in adjacent slots do not cause interference. I-Iowever, the guard bands occupy band~-idth which could otherwise be' used to carry traffic, so reliance on guard bands alone to avoid interference is not suitable for systerrYS where a high bandwidth channel is shared by many transmitters.
The document ~J~ 5790939 describes a TL~I'vIA based satellite communication system including a timing correction protocol. The systerr~ broadcasts timing corrections tcs mobile terminals. residual errors in the timing of individual terminals acre corrected following transmission by the mobile terminals in s conteratYOn acceSS
channel. A gateway measures the residual timing er or and z~eports the e~r~or back to tile relevant mobile terminal.
Another problem associated with bursts in fC'Oh.!IA chanrlc°.ls is that, if adjacent bursts do overlap, the interference between the bur:>ts generally prevents either from being demodulated and decoded successfully. Burst forma-~s for each time slot may include a unique u,~ord which aids acquisition of the burst. as described for example i:rl LJS 5661'764, 25 but the advantage of the unique word is lost if the burst interferes with an adjacent burst.
The document G13 ?~'70~315 describes a cellular mobile radio system with a packet resez-~ration multiple access protocol, in whicl-a user traffic can be carried by both single and double slots, allocated dynarrYicaIIy by the base station according to Ioad requirements.
however, if the slot allocation is entireay flexible arid can include slots o;f differing Lengths, the timing alignment of bursts in those slots becams~s complex.
According to aspects ox the present invention, there is provided a timin=~
correction method in a communications system. in which tra~bsceivers transmit short bursts to a base station, which determines timirgg corrections from the tirr.e of receipt of the bursts and transmits the timing ~:orrections to the respective transceivers.
In one aspect, the base station indicates to ~t~e transceivers a plurality oftime slots:
each transceiver selects one of the time slots at ran,doz~, formats a burst including a indicator of the selected time s'ot and transmits thc: burst in that slot. The base station can therefore determine the timing with which each trc~~sca fiver transmitted the burst, but the probability of collision between bursts is reduced since they are spread across the plurality 1 Q of time slots.
In another aspect, the base station transmits to each transceiver a tinning uncertaintay value, which determines hove the timing correction a~ili be modi:~ied by the transceiver as the interval since last receiving a timing correction increases. Prf:ferably, i~;~the modification determined by the timing uncertainty value increases beyond a predetermined 15 threshold, the transceiver itilvibits transmission oth~e;~° than to reqL~est a timing correction.
These measures ad~-antageously reduce the likelihood of interference between slots, due to timing misalignment.
The above aspects of the prese~~?t invention extend individually to those parts of the method ~~hich are carried out by tl:e transceiver, those parts which are carried out by the 20 network vs.~ith which the transmitter co3 ~municates, and apparatu:>
arranged to carry out those individual parts of the method.
According to another aspect of the present it~.vexltiorl, there is provided a signal having a format comprising a first unique word, a content Meld and a second unique word, in that order. Preferably, there are no other fiends ire the burst before the :first or after the 25 second unique word which are necessary for the demodulation arid decoding of the burst;
this has the advantage that, if~ either the beginning c>r the end of the burst overlaps with another burst, it may still be possible to read the data content of t:he burst correctly. The content field may carry user data andlor signalling :information. There may be an additional field before the first unique word and/or after the se;coa~d unique word, but these are iQ preferabl;,- auxiliary melds which are not essential to the decoding ofthe content feld. For example, there may be a constant power preamble art the beginning of the burst to assist with power control ira the transmitter. This aspect of the present invention extends to methods of formatting and/or tnansrni~.~ting such a signal, and apparatus arranged to perform such methods. -According to another aspect of the present invention, there is provided a TI~MA
channel format v~~hiclm: can accornrnodate both short and long bursts in a block format of constant periodicity.
Specifac eynbodiments of the present invention will now be described with reference to the accor.~panying drawings, in ~~hich:
Figure 1 is a diagram of components of a satellite comrrrunication system 1 G incorporating embodiments of the preswnt in~~entior~;
Figure 2 shows the char.~raels used for communication bet~~~een the SAN and the IvIA~1's irl a packet data ser'~ice implemented in thf: system of Fi;~ure 1;
Figure 3 is a diagram of transmitter and receiver° channel units in a satellite access node (SA~1) of the system of Figure 19 Figure 4 is a diagram of transmitter and receiver cl-rannel units in a Mobile Access bode {M.~~1) of the system of Figure l;
Figures ~a to ~d show the structure of one of the L~~'SP channels of Figure 4;
Figure 6a sho~~~s the burst structure ofa 5 rrLS burst in one ofthe MF;SP
channels of F figure 4;
Figure 6b shows the burst structure of a 20 ms burst in one of the 1~IESP
channels of Figure 4;
Figure '~ is a tinning diagram illt.astratia~g the operation of an initial timing correction protocol for correcting the tinning of transmissions in the ~~ESP channels:
Figure 8a is a timing diagram illustrating the: tirr~ing of a t3°ansmission in one of the ?5 MESS' channels irnrrrediately following a timing co~°rection;
Figure 8b is a timing diagram illustrating thf: timing of a transmission in one of the MESF channels at an interval after a timing correction, where there is tirraing uncertainty;
Figures 9a to 3c are timing diagrams showing different collision scenarios between bursts of a conventional format in adjacent T~IvIA 'slots; and Figures l 0a to ~ f~c are tinning diagrams sho~aring the equivalent collision scenarios between bursts of a for.nat according to an embodiarmnt of the present invention.
SVStem ~verViet~' Figure 1 shoes the principal elements of a satellite communications system in an embodiment of the present invention. A plurality of Mobile Access Nodes (MAID') ~
communicate via a satellite 4 ~~ith a satellite earth station, hereinafter referred to as a Satellite Access Node (SAN) 6. The satellite 4 may for example be an lnmarsat-:iTM
satellite, as described for example in the article 'Launch of a New ~enerataon' by J R
Asker, TRANSAT, Issue ifs, January I g9~, pages 15 to 187 published by Inmarsat, the contents of which are included herein by reference. The satellite 4 is geostationary and projects a pluralit~~ of spot bcarns SB (five spot beans in the case: of an .Tnn~arsat-3TM
satellite) and a global beam CB, which encompasses the coverage areas of the spot beams SB, on the earths surface. The MAN's 2 may be portable satellite terminals having manually steerable antennas, of the type currently available for use with the Inmarsat mini-MTM service but with modifications as described hereafter. There may be a plurality of SAN's 6 ~~ithin the coverage area of each satellite ~~~ and capable of supporting communications wvith the M~.N"s ? and there may also be further geostationary satellites 4 with coverage areas vs~hich may or may not overlap that of the exemplary satellite 4. Each SAN 6 may form pa~~t of an lntr4arsat Land Earth Station (LES) and share RF
antennas and modulation:~demodulation equipment with conventional parts of the LES. Each 20 provides an interfatv~° between the communications link through the satellite 4 and one or more terrestrial net~~o~ks 8. so as to connect the M~'~t~l s 2 to terrestrial access nodes (TAN) I0, which are connectable directly or indirectly throe~gl~ further networks to any of a number of communications services, such as Internet, :STN or ISI)N-based services.
Channel Tvpes Figure 2 shows the channels used for communication between a sample one of the MAN's 2 and the SAN 6. All communications under this packet data service from the MAN 2 to the SAN 5 are carried on one or more slcl~:, of°one or more TI~MA channels, referred to as MESP channels (mobile earth station - packet channels), Each MESh channel 30 is divided into 40 ms blocks, divisible into 20 ms blocks. Each 20 ms block carries either one 20 ms burst or four' S ms bursts, in a format whirl' will be described below.
AlI communications under this packet data service from v:~he S AN C to the MAC' '_' are carried on one or more slots of one or more TLaM channels. referred to as LESP
channels (land earrh station - packet channels). Th~.~ slots are eac~~ ~0 ms long, and comprise two subframes of equal length.
S For the purposes of channel sec-up and othf:r network sig:r~alling, the MAN
2 also communicates u~~ith a network co-ordination station (NCS) S, as is known in the Inrrtarsat Mini-MT's service. The SAl'J 6 communicates through the network i~ to a regional land earth station {IZ.LESJ ~ which communicates ~~ith the NCS 5 so as to perform channel set-up and other network signalling.
Satellite Link Interface The present embodiments concern in particular a set of protocols and algorithms for the interface over the satellite link between the MAN's 2 and the SAN 6 to which the MAN's 2 are connected. This interface can be considered as a series of communications layers: a physical layer, a medium access control {MAC) layer and a service connection layer.
SAN Channel Unit Figure 3 shows the functions ~°ithin the SAhi 6 of a transmitter channel unit ST, 20 which performs the transmission of data packets over a single frequency channel of the satellite link, and a receiver channel unit SR, which performs the reception of data packets aver a single frequency channel of the satellite link. Preferably, the SAN 6 includes multiple transmitter channel units S'h and receiver s:hannel units SR so as to be able to provide communications services to a sufficient nu:rnber of MAN's 2.
2S A hardware adaptation layer (I~AL) 10 pro~~:~des an interface between the channel units and higher level software, and controls the settings of the channel units. In the transmitter channel ~znit ST, the FIAL 1 ~ outputs data bursts Td which are scrambled by a scrambler i2, the output timing of v~~hich is controlled by a frame timing function 14 which also provides frame timing control signals to the outer transmitter channel units ST. The 30 scrambled data bursts are then redundancy encoded by an encoder 16, by means for example of a turbo encoding algorithm as described in PCTIGB97/03551.
The data a_nd parity bits arc output from the encoder 16 to a transmit s~~nchronisin.~T
function I ~ which outputs the data and parity bits ~s acts of four bits far modulation by a I6QAM modulator 2d. ~,Tniquc ~vcrd (U~J) symbols arc also input to the modulator 2~
according to a slot format w-hich is described belov~~. T he output timing of the encoder .I 6.
5 transmit synchroniser I ~ and modulator 2Q is cont~~°olled by the ~~IAL 10, which also selects the frequency of the transmit c:~aannel by controlling a transmit frequency synthesiser ?2 to output an upconversion frequency signal. This frequency signal is combined with the output of the modulator 2D at an upconvertcr 24, tans output of ~rhich is transmitted by an 1ZF antenna (not sl~°~o~~~n) to the satellite 4.
I(? In the receiver channel unit S~, a frequency channel is receivcfi by an RF
antenna (not shown) and downconvcrtcd by mixing with a doe~Jnconversion frequency signal at a downconverter 2d. The dow-nconversion frequenc;;~ signal is generated by a reception freq=~ency signal syrsthesiscr 2~, the output frequeT:~cy ofw~hich is controlled by the I~AI
I 0.
I j In order to demodulate the received bursts correctly, the timing of reception of the bursts is predicted by a receive timing controller 2R, which receives the frame timing control information from the fi°arne timing functio:rQ l ~ and parameters of the satellite 4 from the :l-IAI. 3 0. These parameters fief ne the po:~ition of the satellite ~ and of its beams and allow the timing of arrival of data bursts from ~~c IRAN"s 2 to the SAN 6 to be 20 predicted. The propagation delay from the SAN 6 tc3 the satellite 4 r~aries cyclically over a 24 hour period as a result ofthe inclination of the ;z~atelliteys orbit. This delay variation is similar for alI of the ;'~SAN's 2 and is therefore used to modify tl:.e reference timing of the IVIESP channels, so that the timing of the individual ~IAN's 2 does not need to be modified to compensate for ~~az-iations in satclli~:e position.
25 T'he predicted timing informat:son is output to each of the receive channel units SIg.
The received bursts are of either 5 ms or 20 ms dw~atio;t~ according to ~
scheme controlled by the SAN b. The I~ AL I 0 provides information about the expected slot types to a slot controller 32, which also receives information fronr~ the receive tinning controller 29.
Figure s shows separate reception paths fog- ~ ms and 20 :rns bur:yts' references to 3t3 functions on each of these paths will be denoted by the suffixes cz and i~
respectively. The slot controller 32 selects vahich reception path to u:~e for each received burst according to t the predicted length of the burst. The burst is demodulated by a 16QAM
demodulator 34al34b and the timing of the burst is acquired by a LJVs~' acquisition stage 36a%36b. Once the start and end of the burst/ is determined, the buxst is turbo-decoded by a decoder 38aJ38b and descrambled by a descra;-nbler 40a/~Ob. The recovered 5 or ~'0 ms data burst is then received b~° the :~Ah, 10.
MAN Channel Unit Figure 4 shows the functions within one oh the MAN°s 2 of a receiver channel unit:
MR and a transmitter channel unit M ~~'. The MAN ? may have only one each of the receiver and transmitter channel unit, for reasons of compactness and cost, but if increased bandwidth capacity is required, multiple receiver and transmitter channel units may be incorporated in the MAN 2.
In the receiver channel unit M~ a signal is received by an antemia (not shown) andf dov~n-converted by a down--converter ~2 which receives a down..conversion frequency 15 signal from a receive frequency signal synthesiser ~4, the frequency of which is controlled by an M AN hardware adaptation layer 46. The do~~an-converted :9ignal is demodulated by a 16QAM demodulator 48 which outpuas the parallel bit valaes of each symbol to a UW
detection stage ~0, where the timing of the received simnal is detected by identii'r~ing a unique word (LW,-~ in the received siga~al. The timing information is sent to a frame and 20 symbol timing unit 52 which stores timing information and controls the timing of the later stages of processing of the signal, as sl~os~~n in Fign.re ~4. Once thc: block boundaries of the received data have been detected, the received bloc;l~s are turbo decoded by a decoder 54, descrambled by a descrambler 56 and output as received bursts to the I-h~IIJ
46.
In the transrnitrter channel unit IvIT, data for bursts of ~ or 20 ms duration are output 25 from the HAL 46. Separate paths identified by the suffixes a and b are shown in Figure 4 for the 5 and 20 ms bursts respectively. The data is scrambled by a scrambler 48a/48b and encoded by a turbo encoder 50a/SOb. Unique Word~.s (iJW~ are added as dictated by the burst format at step ~2a/52b and the resultant data stream is mapped onto the transmission signal set at step ~~aI54b and filtered at step 56ai5e31~. The transmlSSlon taming is controlled 30 at a transmission tirnirgg control step 58a/j8b. At tl;~as step, the TI:7MA
slot position is controlled by a slot control step 60 according to a designated slot position indicated by the 1-lAL 46. A timing offset is output by the HAL 46 t~r3d is supplied to a tidming adjustment step 62 which adjusts the timing of the slot control step 60. This viming offset is used to compensate for variations in propagation delay caused Yoy the relative position of the ~I Any 2, the satellite 4 and the SAN 6 and is controlled by a signalling protocol.
as will be described in greater detail below. The sets of data bits are output at a tirrie deter-nined according to the slot timing and the timing adjustment to a 16QAIvI modulator 6=1. The modulated symbols are upconverted by an upconverter 66 to a transmission channel frequency determined by a frequency output 'by a transmission frequency synthesiser 68 controlled by the 1-lAL ~6. 'The upconverted signal :is transmitted to the satellite 4'ny an antenna (not shownj.
LESP Channel Format Figure ?a shows the frame structure of one of the LESP channels. Each frame LPF
has a duration of 8~ ms and has a header consisting of a constant .unique word '~~ which 1 ~ is the same for all frames. The unique word LJ'W is ~:,sed for frame acquisition, to resolve phase ambiguity of the output of the demodulator 48 and to synchronise the descrambler 56 and the decoder ~~.
Figure ~b shows the structure oj~ each frame., which consists of the unique word U'N of -~0 symbols, followed by 88 blocks of 29 syn ~.bols each followed by a single pilot 20 symbol PS, terminating in 8 symbols so as to make up the total frame length to 2688 symbols, of which 2~6() are data symbols. These data symbols are divided, as shown in Figure ?c, into two stabframes SFI, SF~.' each encoded separately by the encoder 16, each of j 12Q bits, making 1;'80 symbols. The. encoder 16 has a coding :rate of ~J.5C937j, so that each subframe is encoded from an input: block 1131, :f~2 of'?608 bits, as sho~~rn in Figure 2j jd. This structure is surn.marised below in Table 1:
Table I a LRSP F'rarrae Format Modulation 16~AM
Data Rate (lcbit/s) 6~.?
Interface fra.rne ~0 9ength I;rns~
Interface Frame Size j I 20 (bits) Subframe length (ms) 4fl Input Bits per Subfrarne26fl8 Coding Rate ~ 0.509'75 ~
~utp~at IBit per Sufratne~ S I2fl l ~utput Symbol Per Subfrarne ~ I2~fl Frame hength (ms)~ ~fl ~
Data Symbol per Frarne~j60 a Pilot Symbol Insertion~ 1/(29+I) Date Pilot Symbols per .
F'ram~
~TV6~ svanbois 4~J
lFrame Sire 26i;~
Symbol Rate (lssyrrels,)3~.6 MESP Channel format The MESP channel structure is based on 40 ms blocks with a channel timing s referenced to the timing of the associated LESP channel as received by the MAN's 2. Each 4fl ms block can be divided into two 2fl ms slots, f;ach of which can be further divided into four ~ ms slots. and the division of each block into slots is determined flexibly by higher level protocols. 1=figure 6a shows the format of a j ms burst, consisting of a pre-burst guard time GI of 6 symbols, a preamble C~J of 4 symbols. an initial unique word UW1 of 20 I O symbols, a data subframe of I 12 symbols, a anal unique word UW2 of 20 symbols and a post-burst guard time G2 of 6 symbols.
The preamble C~R% is not intended for synclxronisation purposes by receivers (for example, the demodulators j0a, ~flb) gut convenie;ntly provides a constant power level signal to assist the automatic level control of a high-power amplifier (HPA, not shown) in I5 the transmitting MAN 2. In one example, each of t:he symbols of the preamble C'W has the:
value (0,I.0,0). In an alternative format, the preamble may consist of less than 4 symbols and the symbol times not used by the preamble C~J~' are added to the pre-burst and post-burst guard times ~I, ~2. For example, the preamlue C~' mae- be omitted altogether and the pre-and post-burst guard times increased to 8 symbols each.
5 The unique words include only the symbols ( I , I . I , I ), which is mapped onto a phase of 4a° at maximum amplitude, and (0,I,0,1), ~.~~hirh is mapped onto a phase of 22~°
at maximum amplitude. Hence, the unique words are effectively BPSI~.
modulated.
although the symbols are modulated by the 16QArn modulator 64. Indicating the ( 1, I , l .l ) symbol as ( 1 ) and the (0, I ,0, I ) symbol as (0), the icutial unique word ~
W I comprises the 10 sequence I0I OI I I00I 1 I 1 I 100I00, while the final unique word UW2 comprises the sequence of symbols i O I I I O l I O1 O I 100001 I I .
The 5 ms burs° is designed for carrying shoe signalling messages or data messages;
the structure is summarised below in Table 2:
I S Table 2 - 5 s Burs9: Structure Modulation 16QAM
Input Bits per Butst s ~ I9?
Coding rate 3,~;' r C)utput Bits per Burst 44~~
~utput Symbols per Subfrarne112 , Preaanble Initial LJ~'~' (symbols)20~
I< final LTW (symbols) 20 .hotel syrnhols I ~'2 Total Guard Tirne (syrnb~ols)12 Symbol Rate (ksym/s) 33.6 Slot Length (rns) Figure 6b shows the structure of a 20 ms burst of the MESP channel. The same reference numerals will be used to denote the parts of the structure corresponding to those of the 5 ms burst. The jtructure ~~onsists of a pre-burst guard time ~ I .of 6 symbols, a preamble C~% ofd ss~mbols. arx initial unique word 1.1~~JI of 40 symbols, a data subframe of X96 symbols. a final unique word of 20 symbols and a post-burst guard time G?
of 6 symbols. The structu;:°e is sumrr~arised below in Tai~le 3:
Table 3 - 20 ans durst Structure odulati~aa~ 16Q
---Input I$its per durst f~
_ 11 Coding rate ~ 92 i 1~'?~
~utput ~3its per burst 234 t)utput Symbols per X96 Subframe Preamble w ~I
Initial U'~' (symbols 40 j Final U'9Y (symbolsj~ 3~
l ~'otal symbols 660 ~
f _ 1 Tatal Guard Time (symbolsi Syanbol Rate (l~ymfs~ 33.6 !
blot Length (ms) ( 20 The preamble Cue' has the same form and hrarpose as that of the > ms burst.
The initial unique word L~1 comprises the sequence:
while the final unique word X72 comprises the sequence 11101.11000001 l O100I0, using 10 the same convention as that of the ~ ms burst.
MESP Timing Correction As shown above, the MESP slot structure incorporates a very shock guard time of about 0.24 ms at each end. ~lowever, the difference ia~ the SAN 6 to MAh' 2 propagation 15 delay between the MAN 2 being at the sub-satellite point and at the edge of coverage is about 40 ms for a geostationary satellite, so the po sition of each MAN '' will affect the timing of reception of transmitted bursts in the IvIESP channel, and may cause interference between bursts from MAN's 2 at different distances from the sub-satellite point. ~loreove~r the satellite, although nominally, geostationary, is subject to perturbations which introduce a small inclination to the orbit and cause the distance between the satellite 4 and the S A'~"
6, and between the satellite =~ and the ?vl.4N ?. to oscillate. although the position of the SAN 6 is fixed and that of the satellite ~ can be predicted, the MAN°s are mobile and therefore their positions change unpredictably, and their clocks are subject to fitter and drift.
A timing correction protocol is used b~° the ~AN 6 to measure the propagation delas° from the M A~\~" '~ and send a timing correction value to the MAN '' to compensate for differences in propagation delay between the different MAN's '?, so as to avoid interference between bursts from different I~.~AN's caused by misalignment with the slots.
The protocol will now be illustrated with reference to the timing diagram of Figure 7.
Figure 7 shows LESP frames LPF includinf; subframes SFI, SF? and initial unique words L W. When the s~.AN ? is switched on. or is able to acquire one of the LESP
channels after an inter~°al of not being able to do so, the MAN 2 receives (step 70) a 40 ms LESP subframe SF including return schedule information which dictates the slot usage of a lj corresponding MESh channel. Return schedule information is transmitted periodically with a periodicity controlled by the SAN 6. The subframe SF includes the designation of a block of at least nine continuous ~ ms slots as a timing acd~aisi.tion group consisting of random access slots not assigned to any specif c MAN 2. The MESP return schedule to which the subframe SF relates begins 1?0 ms after the beginning of reception of the subframe SF.
This 1?0 ms period allows 90 ms for the MAN 2 to demodulate the LESP subframe SF
(step 7?) and 30 ms for the MAN 7 to initialise itself for transmission (step 74).
At the beginning of the MESP return sched~zle there is allocated a timing allocation group of 5 ms slots. Initially, it is assumed that Y~he MAN ? has the maximum timing uncertainty of 40 ms, corresponding to eight S ms slots. Therefore, the Ie~IAN
2 can only transmit after the first eight slots of the timing acquisition group. and cannot transmit at all in acquisition groups containing less than nine slots, so as to avoid interfering with transmissions in slots pAeceding the timing acquisition group.
The MAN 'Z randomly. selects (step 7g) one of'the slots of the timing acquisition group following the ftrst eight slots and transmits (step '~9) a burst in the selected slot, the burst including an indication of the slot selected. In the example shown in Figure 7, the slots of the timing acquisition group are numbered f~om ~0 to M-l, where M is the number 1j , .
of slots in the timing acquisition group. and the number R, selected at random from 8 to Iv1-1. is transmitted in the burst at step 79. The burst rraav also indicate the t~°pe of the mobile., such as land-based, maritime or aeronautical.
The SAN 6 receives and records the time cef arrival of thcr burst transmitted by the MAN 2. From th.e slot number R indicated in the laurel, the SAN 6 calculates the differential propagation delay to that MAN 2. Since the timing of transmission of the burst u~as (120 + R x ~) ms after the time ofreception of the LESP subframe SF, the timing of reception TR of tlae burst is approximately (2 x Icy + ~ +120 + ~ x 1t) ms after the time of transmission of the LESP subframe LI'SF, adhere IMP is the differential propagation delay to that MAN 2 and C is a delay which is the same for all the MAN"s in. a group, and includes various factors such as the propagation delay to and from the satellite 4 and the retransmission delay of the satellite 4. Hence, in this example, tl-~e differential propagation delay is calculated as:
DP=TR-~-120-SxFg (1) 1 ~ The SAN 6 then transmits to the MAN 2 a data packet indicating a timing correction offset X in the range 0 to 40 ms. The offset replaces the initial timing offset of 40 ms in step 76, for subsequent transmissions. The M AN 2 receives the timing correction offset and adjusts its :ransmission timing accordingly.
If the burst transmitted by the MAN ? intro°fere s with a burst transmitted by another MAN 2 also attempti:zg to receive a timing correction, the SAN ~6 may riot be able to read the contents of either burst and in that case will not transmit a tuning offset correction to either M.~N 2. If the it~AN '? does not receive a tirr~ing offset correction from the SAN 6 within a predetermined time, the MAN 2 waits for random interval within a predetermined range before attempting to transmit ,a br.~rst in the next subsequently°
available timing acquisition group. The predetermined range of intervals is determined by a signalling packet transmitted by the SAN 6 which indicates maximum and minimum intervals to be observed by MAN's 2 after a first unsuccessful transmission before attempting retransmission. together with a further .waiting interval to be added to the total waiting interval each time a further retransmission is made following an unsuccessful transmission.
Figure 8a illustrates the transmission timin;?. of one of the M AN's '~ which has previously received a timing correction offset value X. As in Figure 7, tl.~e MAN ? receives (step 80) the LESP subframe SF which includes return schedule information. The MAST ?
demodulates (step 8?) the LESP subframe LPSF and initialises (step 84) its transmitting j channel unit. during a total allotted tirrae of I20 ms after tl~ae beginning of reception of the LESP subframe LPSF. The MAN 2 calculates the start of the MESP return schedule as being ( 120 + X) s from the beginning of reception of the subframe SF which carries the retuni schedule information. The MAN 2 therefore waits for the timing offset period X
(step 86) after the end of the 120 ms period before being able to transmit.
In this example, the return schedule dictated by the LESP subfrarne. LPSF
includes a four ~ ms slots, followed by a ~0 ms slot. If the 1!!CAN ~ has been allocated a 20 ms slot, then ~it will transmit (step 88) in. the designated 20 ms slot; if the MAN ~
has been allocated a 5 ms slot, then it will transmit in the designated ~ ms slot. Alternatively, if the 5 ms slots are designated as being random access slots and the. MAN 2 has a short Lpacket that is due i 5 to be sent to the SAN 6, the MAN 'g selects one of the four slots at random and transmits in that slot (step 89).
If the SAN 6 detects from the transmission by the MAN 2 that a correction in the timing offset is needed, for exai~nple if the time ber~reen the start of the burst and the slot boundanT as measured b~~ the SAN 6 is less than a predetermined number of symbols, the SAN 6 indicates a new timing correction to the MAN % in a subsequent data packet. This may be indicated as an absolute timing offset X ~r as a relative ti.rning offset to be added or subtracted from the current value of X..
Timing Uncertainty 2j In the timing correction offset burst the SAI~3 6 transmits to the MAN 2, together with the timing offset, a timing uncertainty rate RU indicating the rate at which the timing of the MAN 2 is likely to change. For example, the timing uncertainty rate may represent a number of symbols per second by which the MAN ~ is likely to change its timing. The SA_N 6 determines the timing uncertainty rate frown the class of the MAN ?
(e.g. land mobile, aeronautical) and other factors such as the inclination of the orbit of the satellite 6.
l~
The MAN ? times the interval elapsed since the last timing correction was received and multiplies this by the timing uncertainty rate ~~, to give a timing uncertainty tL;. where to = MIN (T - Tc x Flu, 40 ms) (2) where T is the current time and Te is ~~e time at which the last correction ~~~as received.
j The MIN function means that the timing uncertainty cannot exceed the maximum uncertainty of 40 rr~s.
T'ne timing offset X is reduced by the timing uncertainrd~ to such that:
X = MIN(X~ - tu, 0) (3) where X~ is the initial value of X indicated in the last timing correction, the MIN function 1 Q ensuring that X car~~ot fall belov~ zero.
Figure 8b illustrates the transmission timing of one of the MAN's ~' with timing uncertainty. Steps ~0 to ~4 correspond to those shown in Figure ~a and their description will not be repeated. At step ~6, the MAN 2 calculates the MESF return schedule as starting ( 120 -°- X) rns after the beginni~ig of reception of the subfi°ame SF, using the value of X as reduced by the timing uncertainty tL,. As a result of the timing uncertainty tu, the MAN 2 must ignore the first I slots of a random access group, where I = II~TT[(ts - to + tu)~tsj (4) is is the slot duration of ~ ms and tG is the guard time GI, ~~hich is 6 symbol periods in this case.
20 In the example shown in Figure fib, there are four 5 ms slots at the start of the MESI~' return schedule. but to is 7 ms, s~ that the first two slots must be ignored. The MAN
2 can then only transmEt in the third anca fourth slots.
If the timing uncertain;y to is greater than a predetermined value, such as the value of the guard time, the MAN 2 reverts to the random accc,ss 'timing correction request 2~ process shown in Figure 7 and inhibits transmission in time slots allocated exclusively to itself, except where a sufficient number of these are concatenated so that their total length can accammodate bath the timing uncertainty and the burst itself, until a new timing correction offset has been received from the SAN C. I3owever, the protocol differs from that of Figure 7 in that the Mfi~N 2 uses its current timing offset X instead of returning to 30 the default value of 4~? rns in step '76. This protocol reduces the chance of interference between bursts in allocated slots.
In the above embodiment, the timing OffsE°$ ~ is reduced by the timing uncertainm tL for all transmissions by the :i~AN ~.'. In an alter~~ati~v embodiment, the timing offset ?~ is reduced by the timing uncertainty tU r3nly for transmissions by the MAST '~ in random access slots, while the original timing offset A~ received in the last timing correction j message from the SAN 6 is applied when transmitting in allocated slots. In this alternative embodiment, it is important to distinguish between timing correction messages initiated by the SAN 6, after detection of a transmission by the NIAN "~ in arr allocated slot too close to the slot boundary, and timing correction messages sent by the SAN E in response to a timing correction request by the MAN 2, which wall have a different timing offset from the transmissions in allocated slots. Therefore, the SAN 6 indicates in the timing correction message ~a~hether this is being sent in response to a request by the MAN 3, or was initiated by the SAN 6. The IRAN 2 then determines the new timing offset ?LO from the timing offset indicated in the timing correction message according to how the timing correction messaue was initiated.
1j Unique Word Structure As shorten in Figures 6a and 6b, each MESP burst includes an initial unique word UW 1 and a final unique word UW2. This format is particularly advantageous for TDMA
channels with short guard times between slots. By ~w°ay of comparison.
Figures 9a to 9c 2G show a conventional burst structure with initial uni~:lue word onlg-, respectively with no collision, two-burst collision and three~burst collision, ~avhile Figtcres l 0a to l Oc show the equivalent situations with both an initial and final U'~6% structure.
As shown in Figure gb, if burst ? transmitted in slot 2 is delayed because of timing error, the data contents of burst 2 interfere with the LTW of burst 3 in slot 3 and may both 2~ be corrupted, possibly preventing the data contents of burst 3 from being read correctly as a result of a failure to acquire the symbol timing of burst 3. I-Iowever, in the situation shown in Figure l Ob, the final UW of burst 2 interferes with the initial U W of burst 3, but in both bursts the data and one of the unique words is uncorrupted, giving: a good chance of reading both data bursts.
In the situation shown in Figure qe, burst 2 is delayed and burst 4. is advanced, both as a result of timing errors. The data content of bursts 2 and 3, and the unique words of bursts 3 and 4, are corrupted so that it will be diffic:alt to read any of the data contents of bursts ? to 4. In contrast. in the situation shown in figure l Oc, the final U~' of burst ?. both unique words ofbursi 3 and the initial unique wor~a ofburst 4 are comapted.
l~ievertheless, if the timing of bursts ? and 4 can be acquired from the uncorrupted unique words. the .
corrupted unique words of bursts ? and 4 can be synthesised and subtracted from the received signal of burst 3, allowing the corrupted uniqc~e words of burst 3 to be recovered and the data content of burst 3 to be read successfully.
The use of twc> unique words per burst also provides the advantages of time diversity: in the presence of fading or impulsive noise, the chance of two separate unique 10 words being comapted is less than that of one uniqv~ce word of the combined length. The two unique words can be detected independently ao.d the results crombined before a timing decision is made.
In order to der_zodulate a received burst, the SAh 6 needs to estimate the carrier amplitude, phase and frequency. The estimated channel state is also used by the decoder 1 f 38a/38b. Since there is an UW present at both the bcginning and end of each burst, the channel state at bath the beginning and end of the ~mrst can be determined.
and optionally the channel state tl~.roughout the burst can be interpolated from these. This may result in improved demodulation and decoding performance. ~ urthermore, timing slip between the beginning and the end of the data burst can be detected; this is advantageous where there is 20 considerable drift in the transmiver or receiver clock. commonly., the channel state cannot be estimated from the data burst itself, because the energy of the data portion is typically too low.
As a further advantage, tl~e proposed unique word structure gives improved performance with high-power amplifiers (IiPA). A common problem with HPA's is their 25 slow ramp-up/dow~n at the beginning and end of a bnurst. This may result in distortion or attenuation of symbols at the start and end of a burst. If these symbols were carrying encoded data, their distortion could lead to loss of tl-m whole encoded data in the burst.
With the proposed structure, only some of the UW symbols will be distorted, which is less likely to cause loss of the whole burst.
30 As a Iess advantageous alternative, additional fields may be transmitted in each burst either before the initial UW or after the final U~~I of the burst, or both. The additional fields may be additional data fields carryinz' addi~:aonal data or signalling, or may cam flxrther burst format signals designed to assist in the demodulation and.~'or decoding of the data content of the burst. I-lowever, such addition~.l fields are evalnerable to interference and preferably should not carry data or signalling esse~xztiai. for dcmcsdulation and/or decoding of the burst.
The above embodiments have been descrii~ed ~~ith reference to certain lnmarsatT"'' systems purely by ~~ay of example and aspects of ti°~e present invention are in no '~~ay limited thereto. For example, it will be readily understood to the skilled person that the problem of timing correction occurs in geo-stationary, geosyncrzronous and non-geostationazy satellite systems and aspects of the 1 aresent in'~enti.on are applicable to these systems. Moreover, timing errors can occur for reasons such as clock instability as well as relative movement between satellites, base stations and wireless transceivers, so that aspects of the present invention are also applicablf: to wireless communication systems not using satellites as relay stations. such as terrestrial communications systems or systems involving alternative relay stations such as balloons or other aircraft.
Although the above embodiments have beew described with reference to a TDMA
channel format, it u-ill be readily understood by the skilled person that the problem of interference as a result of timing error can occur with other chara,nel forr~aats, such as combined TDMA-CDMA, slotted Aloha and other time-divided formats and that aspects of the present invention are also applicable to such formats.
The description of the above embodiments iz~eludes a detailed description of the transmission formats of LESP and MESP channels. Aspects of these chef forrraats are particularly advantageous for packet data transmission via satellite, particularly via geostationary satellite and have been selected after considerable investigation of alternative formats, but may also be advantageous in different cor°~texts. Dr the other hand, it will be apparent that some aspects of the present invention are entirely independent of the specific channel formats used.
iRr'hile the apparatus of the specific embodiments has been described in terms of functional blocks, these blocks do riot necessarily correspond to discrete hardware or software objects. As is well known, most baseband functions may in practice be performed 19 . .
by° suitably programmed I~S~''s or ger~eraI purpose processors and the software may be opzimised for speed rather tha.~ str~czure.
Satellite Link Interface The present embodiments concern in particular a set of protocols and algorithms for the interface over the satellite link between the MAN's 2 and the SAN 6 to which the MAN's 2 are connected. This interface can be considered as a series of communications layers: a physical layer, a medium access control {MAC) layer and a service connection layer.
SAN Channel Unit Figure 3 shows the functions ~°ithin the SAhi 6 of a transmitter channel unit ST, 20 which performs the transmission of data packets over a single frequency channel of the satellite link, and a receiver channel unit SR, which performs the reception of data packets aver a single frequency channel of the satellite link. Preferably, the SAN 6 includes multiple transmitter channel units S'h and receiver s:hannel units SR so as to be able to provide communications services to a sufficient nu:rnber of MAN's 2.
2S A hardware adaptation layer (I~AL) 10 pro~~:~des an interface between the channel units and higher level software, and controls the settings of the channel units. In the transmitter channel ~znit ST, the FIAL 1 ~ outputs data bursts Td which are scrambled by a scrambler i2, the output timing of v~~hich is controlled by a frame timing function 14 which also provides frame timing control signals to the outer transmitter channel units ST. The 30 scrambled data bursts are then redundancy encoded by an encoder 16, by means for example of a turbo encoding algorithm as described in PCTIGB97/03551.
The data a_nd parity bits arc output from the encoder 16 to a transmit s~~nchronisin.~T
function I ~ which outputs the data and parity bits ~s acts of four bits far modulation by a I6QAM modulator 2d. ~,Tniquc ~vcrd (U~J) symbols arc also input to the modulator 2~
according to a slot format w-hich is described belov~~. T he output timing of the encoder .I 6.
5 transmit synchroniser I ~ and modulator 2Q is cont~~°olled by the ~~IAL 10, which also selects the frequency of the transmit c:~aannel by controlling a transmit frequency synthesiser ?2 to output an upconversion frequency signal. This frequency signal is combined with the output of the modulator 2D at an upconvertcr 24, tans output of ~rhich is transmitted by an 1ZF antenna (not sl~°~o~~~n) to the satellite 4.
I(? In the receiver channel unit S~, a frequency channel is receivcfi by an RF
antenna (not shown) and downconvcrtcd by mixing with a doe~Jnconversion frequency signal at a downconverter 2d. The dow-nconversion frequenc;;~ signal is generated by a reception freq=~ency signal syrsthesiscr 2~, the output frequeT:~cy ofw~hich is controlled by the I~AI
I 0.
I j In order to demodulate the received bursts correctly, the timing of reception of the bursts is predicted by a receive timing controller 2R, which receives the frame timing control information from the fi°arne timing functio:rQ l ~ and parameters of the satellite 4 from the :l-IAI. 3 0. These parameters fief ne the po:~ition of the satellite ~ and of its beams and allow the timing of arrival of data bursts from ~~c IRAN"s 2 to the SAN 6 to be 20 predicted. The propagation delay from the SAN 6 tc3 the satellite 4 r~aries cyclically over a 24 hour period as a result ofthe inclination of the ;z~atelliteys orbit. This delay variation is similar for alI of the ;'~SAN's 2 and is therefore used to modify tl:.e reference timing of the IVIESP channels, so that the timing of the individual ~IAN's 2 does not need to be modified to compensate for ~~az-iations in satclli~:e position.
25 T'he predicted timing informat:son is output to each of the receive channel units SIg.
The received bursts are of either 5 ms or 20 ms dw~atio;t~ according to ~
scheme controlled by the SAN b. The I~ AL I 0 provides information about the expected slot types to a slot controller 32, which also receives information fronr~ the receive tinning controller 29.
Figure s shows separate reception paths fog- ~ ms and 20 :rns bur:yts' references to 3t3 functions on each of these paths will be denoted by the suffixes cz and i~
respectively. The slot controller 32 selects vahich reception path to u:~e for each received burst according to t the predicted length of the burst. The burst is demodulated by a 16QAM
demodulator 34al34b and the timing of the burst is acquired by a LJVs~' acquisition stage 36a%36b. Once the start and end of the burst/ is determined, the buxst is turbo-decoded by a decoder 38aJ38b and descrambled by a descra;-nbler 40a/~Ob. The recovered 5 or ~'0 ms data burst is then received b~° the :~Ah, 10.
MAN Channel Unit Figure 4 shows the functions within one oh the MAN°s 2 of a receiver channel unit:
MR and a transmitter channel unit M ~~'. The MAN ? may have only one each of the receiver and transmitter channel unit, for reasons of compactness and cost, but if increased bandwidth capacity is required, multiple receiver and transmitter channel units may be incorporated in the MAN 2.
In the receiver channel unit M~ a signal is received by an antemia (not shown) andf dov~n-converted by a down--converter ~2 which receives a down..conversion frequency 15 signal from a receive frequency signal synthesiser ~4, the frequency of which is controlled by an M AN hardware adaptation layer 46. The do~~an-converted :9ignal is demodulated by a 16QAM demodulator 48 which outpuas the parallel bit valaes of each symbol to a UW
detection stage ~0, where the timing of the received simnal is detected by identii'r~ing a unique word (LW,-~ in the received siga~al. The timing information is sent to a frame and 20 symbol timing unit 52 which stores timing information and controls the timing of the later stages of processing of the signal, as sl~os~~n in Fign.re ~4. Once thc: block boundaries of the received data have been detected, the received bloc;l~s are turbo decoded by a decoder 54, descrambled by a descrambler 56 and output as received bursts to the I-h~IIJ
46.
In the transrnitrter channel unit IvIT, data for bursts of ~ or 20 ms duration are output 25 from the HAL 46. Separate paths identified by the suffixes a and b are shown in Figure 4 for the 5 and 20 ms bursts respectively. The data is scrambled by a scrambler 48a/48b and encoded by a turbo encoder 50a/SOb. Unique Word~.s (iJW~ are added as dictated by the burst format at step ~2a/52b and the resultant data stream is mapped onto the transmission signal set at step ~~aI54b and filtered at step 56ai5e31~. The transmlSSlon taming is controlled 30 at a transmission tirnirgg control step 58a/j8b. At tl;~as step, the TI:7MA
slot position is controlled by a slot control step 60 according to a designated slot position indicated by the 1-lAL 46. A timing offset is output by the HAL 46 t~r3d is supplied to a tidming adjustment step 62 which adjusts the timing of the slot control step 60. This viming offset is used to compensate for variations in propagation delay caused Yoy the relative position of the ~I Any 2, the satellite 4 and the SAN 6 and is controlled by a signalling protocol.
as will be described in greater detail below. The sets of data bits are output at a tirrie deter-nined according to the slot timing and the timing adjustment to a 16QAIvI modulator 6=1. The modulated symbols are upconverted by an upconverter 66 to a transmission channel frequency determined by a frequency output 'by a transmission frequency synthesiser 68 controlled by the 1-lAL ~6. 'The upconverted signal :is transmitted to the satellite 4'ny an antenna (not shownj.
LESP Channel Format Figure ?a shows the frame structure of one of the LESP channels. Each frame LPF
has a duration of 8~ ms and has a header consisting of a constant .unique word '~~ which 1 ~ is the same for all frames. The unique word LJ'W is ~:,sed for frame acquisition, to resolve phase ambiguity of the output of the demodulator 48 and to synchronise the descrambler 56 and the decoder ~~.
Figure ~b shows the structure oj~ each frame., which consists of the unique word U'N of -~0 symbols, followed by 88 blocks of 29 syn ~.bols each followed by a single pilot 20 symbol PS, terminating in 8 symbols so as to make up the total frame length to 2688 symbols, of which 2~6() are data symbols. These data symbols are divided, as shown in Figure ?c, into two stabframes SFI, SF~.' each encoded separately by the encoder 16, each of j 12Q bits, making 1;'80 symbols. The. encoder 16 has a coding :rate of ~J.5C937j, so that each subframe is encoded from an input: block 1131, :f~2 of'?608 bits, as sho~~rn in Figure 2j jd. This structure is surn.marised below in Table 1:
Table I a LRSP F'rarrae Format Modulation 16~AM
Data Rate (lcbit/s) 6~.?
Interface fra.rne ~0 9ength I;rns~
Interface Frame Size j I 20 (bits) Subframe length (ms) 4fl Input Bits per Subfrarne26fl8 Coding Rate ~ 0.509'75 ~
~utp~at IBit per Sufratne~ S I2fl l ~utput Symbol Per Subfrarne ~ I2~fl Frame hength (ms)~ ~fl ~
Data Symbol per Frarne~j60 a Pilot Symbol Insertion~ 1/(29+I) Date Pilot Symbols per .
F'ram~
~TV6~ svanbois 4~J
lFrame Sire 26i;~
Symbol Rate (lssyrrels,)3~.6 MESP Channel format The MESP channel structure is based on 40 ms blocks with a channel timing s referenced to the timing of the associated LESP channel as received by the MAN's 2. Each 4fl ms block can be divided into two 2fl ms slots, f;ach of which can be further divided into four ~ ms slots. and the division of each block into slots is determined flexibly by higher level protocols. 1=figure 6a shows the format of a j ms burst, consisting of a pre-burst guard time GI of 6 symbols, a preamble C~J of 4 symbols. an initial unique word UW1 of 20 I O symbols, a data subframe of I 12 symbols, a anal unique word UW2 of 20 symbols and a post-burst guard time G2 of 6 symbols.
The preamble C~R% is not intended for synclxronisation purposes by receivers (for example, the demodulators j0a, ~flb) gut convenie;ntly provides a constant power level signal to assist the automatic level control of a high-power amplifier (HPA, not shown) in I5 the transmitting MAN 2. In one example, each of t:he symbols of the preamble C'W has the:
value (0,I.0,0). In an alternative format, the preamble may consist of less than 4 symbols and the symbol times not used by the preamble C~J~' are added to the pre-burst and post-burst guard times ~I, ~2. For example, the preamlue C~' mae- be omitted altogether and the pre-and post-burst guard times increased to 8 symbols each.
5 The unique words include only the symbols ( I , I . I , I ), which is mapped onto a phase of 4a° at maximum amplitude, and (0,I,0,1), ~.~~hirh is mapped onto a phase of 22~°
at maximum amplitude. Hence, the unique words are effectively BPSI~.
modulated.
although the symbols are modulated by the 16QArn modulator 64. Indicating the ( 1, I , l .l ) symbol as ( 1 ) and the (0, I ,0, I ) symbol as (0), the icutial unique word ~
W I comprises the 10 sequence I0I OI I I00I 1 I 1 I 100I00, while the final unique word UW2 comprises the sequence of symbols i O I I I O l I O1 O I 100001 I I .
The 5 ms burs° is designed for carrying shoe signalling messages or data messages;
the structure is summarised below in Table 2:
I S Table 2 - 5 s Burs9: Structure Modulation 16QAM
Input Bits per Butst s ~ I9?
Coding rate 3,~;' r C)utput Bits per Burst 44~~
~utput Symbols per Subfrarne112 , Preaanble Initial LJ~'~' (symbols)20~
I< final LTW (symbols) 20 .hotel syrnhols I ~'2 Total Guard Tirne (syrnb~ols)12 Symbol Rate (ksym/s) 33.6 Slot Length (rns) Figure 6b shows the structure of a 20 ms burst of the MESP channel. The same reference numerals will be used to denote the parts of the structure corresponding to those of the 5 ms burst. The jtructure ~~onsists of a pre-burst guard time ~ I .of 6 symbols, a preamble C~% ofd ss~mbols. arx initial unique word 1.1~~JI of 40 symbols, a data subframe of X96 symbols. a final unique word of 20 symbols and a post-burst guard time G?
of 6 symbols. The structu;:°e is sumrr~arised below in Tai~le 3:
Table 3 - 20 ans durst Structure odulati~aa~ 16Q
---Input I$its per durst f~
_ 11 Coding rate ~ 92 i 1~'?~
~utput ~3its per burst 234 t)utput Symbols per X96 Subframe Preamble w ~I
Initial U'~' (symbols 40 j Final U'9Y (symbolsj~ 3~
l ~'otal symbols 660 ~
f _ 1 Tatal Guard Time (symbolsi Syanbol Rate (l~ymfs~ 33.6 !
blot Length (ms) ( 20 The preamble Cue' has the same form and hrarpose as that of the > ms burst.
The initial unique word L~1 comprises the sequence:
while the final unique word X72 comprises the sequence 11101.11000001 l O100I0, using 10 the same convention as that of the ~ ms burst.
MESP Timing Correction As shown above, the MESP slot structure incorporates a very shock guard time of about 0.24 ms at each end. ~lowever, the difference ia~ the SAN 6 to MAh' 2 propagation 15 delay between the MAN 2 being at the sub-satellite point and at the edge of coverage is about 40 ms for a geostationary satellite, so the po sition of each MAN '' will affect the timing of reception of transmitted bursts in the IvIESP channel, and may cause interference between bursts from MAN's 2 at different distances from the sub-satellite point. ~loreove~r the satellite, although nominally, geostationary, is subject to perturbations which introduce a small inclination to the orbit and cause the distance between the satellite 4 and the S A'~"
6, and between the satellite =~ and the ?vl.4N ?. to oscillate. although the position of the SAN 6 is fixed and that of the satellite ~ can be predicted, the MAN°s are mobile and therefore their positions change unpredictably, and their clocks are subject to fitter and drift.
A timing correction protocol is used b~° the ~AN 6 to measure the propagation delas° from the M A~\~" '~ and send a timing correction value to the MAN '' to compensate for differences in propagation delay between the different MAN's '?, so as to avoid interference between bursts from different I~.~AN's caused by misalignment with the slots.
The protocol will now be illustrated with reference to the timing diagram of Figure 7.
Figure 7 shows LESP frames LPF includinf; subframes SFI, SF? and initial unique words L W. When the s~.AN ? is switched on. or is able to acquire one of the LESP
channels after an inter~°al of not being able to do so, the MAN 2 receives (step 70) a 40 ms LESP subframe SF including return schedule information which dictates the slot usage of a lj corresponding MESh channel. Return schedule information is transmitted periodically with a periodicity controlled by the SAN 6. The subframe SF includes the designation of a block of at least nine continuous ~ ms slots as a timing acd~aisi.tion group consisting of random access slots not assigned to any specif c MAN 2. The MESP return schedule to which the subframe SF relates begins 1?0 ms after the beginning of reception of the subframe SF.
This 1?0 ms period allows 90 ms for the MAN 2 to demodulate the LESP subframe SF
(step 7?) and 30 ms for the MAN 7 to initialise itself for transmission (step 74).
At the beginning of the MESP return sched~zle there is allocated a timing allocation group of 5 ms slots. Initially, it is assumed that Y~he MAN ? has the maximum timing uncertainty of 40 ms, corresponding to eight S ms slots. Therefore, the Ie~IAN
2 can only transmit after the first eight slots of the timing acquisition group. and cannot transmit at all in acquisition groups containing less than nine slots, so as to avoid interfering with transmissions in slots pAeceding the timing acquisition group.
The MAN 'Z randomly. selects (step 7g) one of'the slots of the timing acquisition group following the ftrst eight slots and transmits (step '~9) a burst in the selected slot, the burst including an indication of the slot selected. In the example shown in Figure 7, the slots of the timing acquisition group are numbered f~om ~0 to M-l, where M is the number 1j , .
of slots in the timing acquisition group. and the number R, selected at random from 8 to Iv1-1. is transmitted in the burst at step 79. The burst rraav also indicate the t~°pe of the mobile., such as land-based, maritime or aeronautical.
The SAN 6 receives and records the time cef arrival of thcr burst transmitted by the MAN 2. From th.e slot number R indicated in the laurel, the SAN 6 calculates the differential propagation delay to that MAN 2. Since the timing of transmission of the burst u~as (120 + R x ~) ms after the time ofreception of the LESP subframe SF, the timing of reception TR of tlae burst is approximately (2 x Icy + ~ +120 + ~ x 1t) ms after the time of transmission of the LESP subframe LI'SF, adhere IMP is the differential propagation delay to that MAN 2 and C is a delay which is the same for all the MAN"s in. a group, and includes various factors such as the propagation delay to and from the satellite 4 and the retransmission delay of the satellite 4. Hence, in this example, tl-~e differential propagation delay is calculated as:
DP=TR-~-120-SxFg (1) 1 ~ The SAN 6 then transmits to the MAN 2 a data packet indicating a timing correction offset X in the range 0 to 40 ms. The offset replaces the initial timing offset of 40 ms in step 76, for subsequent transmissions. The M AN 2 receives the timing correction offset and adjusts its :ransmission timing accordingly.
If the burst transmitted by the MAN ? intro°fere s with a burst transmitted by another MAN 2 also attempti:zg to receive a timing correction, the SAN ~6 may riot be able to read the contents of either burst and in that case will not transmit a tuning offset correction to either M.~N 2. If the it~AN '? does not receive a tirr~ing offset correction from the SAN 6 within a predetermined time, the MAN 2 waits for random interval within a predetermined range before attempting to transmit ,a br.~rst in the next subsequently°
available timing acquisition group. The predetermined range of intervals is determined by a signalling packet transmitted by the SAN 6 which indicates maximum and minimum intervals to be observed by MAN's 2 after a first unsuccessful transmission before attempting retransmission. together with a further .waiting interval to be added to the total waiting interval each time a further retransmission is made following an unsuccessful transmission.
Figure 8a illustrates the transmission timin;?. of one of the M AN's '~ which has previously received a timing correction offset value X. As in Figure 7, tl.~e MAN ? receives (step 80) the LESP subframe SF which includes return schedule information. The MAST ?
demodulates (step 8?) the LESP subframe LPSF and initialises (step 84) its transmitting j channel unit. during a total allotted tirrae of I20 ms after tl~ae beginning of reception of the LESP subframe LPSF. The MAN 2 calculates the start of the MESP return schedule as being ( 120 + X) s from the beginning of reception of the subframe SF which carries the retuni schedule information. The MAN 2 therefore waits for the timing offset period X
(step 86) after the end of the 120 ms period before being able to transmit.
In this example, the return schedule dictated by the LESP subfrarne. LPSF
includes a four ~ ms slots, followed by a ~0 ms slot. If the 1!!CAN ~ has been allocated a 20 ms slot, then ~it will transmit (step 88) in. the designated 20 ms slot; if the MAN ~
has been allocated a 5 ms slot, then it will transmit in the designated ~ ms slot. Alternatively, if the 5 ms slots are designated as being random access slots and the. MAN 2 has a short Lpacket that is due i 5 to be sent to the SAN 6, the MAN 'g selects one of the four slots at random and transmits in that slot (step 89).
If the SAN 6 detects from the transmission by the MAN 2 that a correction in the timing offset is needed, for exai~nple if the time ber~reen the start of the burst and the slot boundanT as measured b~~ the SAN 6 is less than a predetermined number of symbols, the SAN 6 indicates a new timing correction to the MAN % in a subsequent data packet. This may be indicated as an absolute timing offset X ~r as a relative ti.rning offset to be added or subtracted from the current value of X..
Timing Uncertainty 2j In the timing correction offset burst the SAI~3 6 transmits to the MAN 2, together with the timing offset, a timing uncertainty rate RU indicating the rate at which the timing of the MAN 2 is likely to change. For example, the timing uncertainty rate may represent a number of symbols per second by which the MAN ~ is likely to change its timing. The SA_N 6 determines the timing uncertainty rate frown the class of the MAN ?
(e.g. land mobile, aeronautical) and other factors such as the inclination of the orbit of the satellite 6.
l~
The MAN ? times the interval elapsed since the last timing correction was received and multiplies this by the timing uncertainty rate ~~, to give a timing uncertainty tL;. where to = MIN (T - Tc x Flu, 40 ms) (2) where T is the current time and Te is ~~e time at which the last correction ~~~as received.
j The MIN function means that the timing uncertainty cannot exceed the maximum uncertainty of 40 rr~s.
T'ne timing offset X is reduced by the timing uncertainrd~ to such that:
X = MIN(X~ - tu, 0) (3) where X~ is the initial value of X indicated in the last timing correction, the MIN function 1 Q ensuring that X car~~ot fall belov~ zero.
Figure 8b illustrates the transmission timing of one of the MAN's ~' with timing uncertainty. Steps ~0 to ~4 correspond to those shown in Figure ~a and their description will not be repeated. At step ~6, the MAN 2 calculates the MESF return schedule as starting ( 120 -°- X) rns after the beginni~ig of reception of the subfi°ame SF, using the value of X as reduced by the timing uncertainty tL,. As a result of the timing uncertainty tu, the MAN 2 must ignore the first I slots of a random access group, where I = II~TT[(ts - to + tu)~tsj (4) is is the slot duration of ~ ms and tG is the guard time GI, ~~hich is 6 symbol periods in this case.
20 In the example shown in Figure fib, there are four 5 ms slots at the start of the MESI~' return schedule. but to is 7 ms, s~ that the first two slots must be ignored. The MAN
2 can then only transmEt in the third anca fourth slots.
If the timing uncertain;y to is greater than a predetermined value, such as the value of the guard time, the MAN 2 reverts to the random accc,ss 'timing correction request 2~ process shown in Figure 7 and inhibits transmission in time slots allocated exclusively to itself, except where a sufficient number of these are concatenated so that their total length can accammodate bath the timing uncertainty and the burst itself, until a new timing correction offset has been received from the SAN C. I3owever, the protocol differs from that of Figure 7 in that the Mfi~N 2 uses its current timing offset X instead of returning to 30 the default value of 4~? rns in step '76. This protocol reduces the chance of interference between bursts in allocated slots.
In the above embodiment, the timing OffsE°$ ~ is reduced by the timing uncertainm tL for all transmissions by the :i~AN ~.'. In an alter~~ati~v embodiment, the timing offset ?~ is reduced by the timing uncertainty tU r3nly for transmissions by the MAST '~ in random access slots, while the original timing offset A~ received in the last timing correction j message from the SAN 6 is applied when transmitting in allocated slots. In this alternative embodiment, it is important to distinguish between timing correction messages initiated by the SAN 6, after detection of a transmission by the NIAN "~ in arr allocated slot too close to the slot boundary, and timing correction messages sent by the SAN E in response to a timing correction request by the MAN 2, which wall have a different timing offset from the transmissions in allocated slots. Therefore, the SAN 6 indicates in the timing correction message ~a~hether this is being sent in response to a request by the MAN 3, or was initiated by the SAN 6. The IRAN 2 then determines the new timing offset ?LO from the timing offset indicated in the timing correction message according to how the timing correction messaue was initiated.
1j Unique Word Structure As shorten in Figures 6a and 6b, each MESP burst includes an initial unique word UW 1 and a final unique word UW2. This format is particularly advantageous for TDMA
channels with short guard times between slots. By ~w°ay of comparison.
Figures 9a to 9c 2G show a conventional burst structure with initial uni~:lue word onlg-, respectively with no collision, two-burst collision and three~burst collision, ~avhile Figtcres l 0a to l Oc show the equivalent situations with both an initial and final U'~6% structure.
As shown in Figure gb, if burst ? transmitted in slot 2 is delayed because of timing error, the data contents of burst 2 interfere with the LTW of burst 3 in slot 3 and may both 2~ be corrupted, possibly preventing the data contents of burst 3 from being read correctly as a result of a failure to acquire the symbol timing of burst 3. I-Iowever, in the situation shown in Figure l Ob, the final UW of burst 2 interferes with the initial U W of burst 3, but in both bursts the data and one of the unique words is uncorrupted, giving: a good chance of reading both data bursts.
In the situation shown in Figure qe, burst 2 is delayed and burst 4. is advanced, both as a result of timing errors. The data content of bursts 2 and 3, and the unique words of bursts 3 and 4, are corrupted so that it will be diffic:alt to read any of the data contents of bursts ? to 4. In contrast. in the situation shown in figure l Oc, the final U~' of burst ?. both unique words ofbursi 3 and the initial unique wor~a ofburst 4 are comapted.
l~ievertheless, if the timing of bursts ? and 4 can be acquired from the uncorrupted unique words. the .
corrupted unique words of bursts ? and 4 can be synthesised and subtracted from the received signal of burst 3, allowing the corrupted uniqc~e words of burst 3 to be recovered and the data content of burst 3 to be read successfully.
The use of twc> unique words per burst also provides the advantages of time diversity: in the presence of fading or impulsive noise, the chance of two separate unique 10 words being comapted is less than that of one uniqv~ce word of the combined length. The two unique words can be detected independently ao.d the results crombined before a timing decision is made.
In order to der_zodulate a received burst, the SAh 6 needs to estimate the carrier amplitude, phase and frequency. The estimated channel state is also used by the decoder 1 f 38a/38b. Since there is an UW present at both the bcginning and end of each burst, the channel state at bath the beginning and end of the ~mrst can be determined.
and optionally the channel state tl~.roughout the burst can be interpolated from these. This may result in improved demodulation and decoding performance. ~ urthermore, timing slip between the beginning and the end of the data burst can be detected; this is advantageous where there is 20 considerable drift in the transmiver or receiver clock. commonly., the channel state cannot be estimated from the data burst itself, because the energy of the data portion is typically too low.
As a further advantage, tl~e proposed unique word structure gives improved performance with high-power amplifiers (IiPA). A common problem with HPA's is their 25 slow ramp-up/dow~n at the beginning and end of a bnurst. This may result in distortion or attenuation of symbols at the start and end of a burst. If these symbols were carrying encoded data, their distortion could lead to loss of tl-m whole encoded data in the burst.
With the proposed structure, only some of the UW symbols will be distorted, which is less likely to cause loss of the whole burst.
30 As a Iess advantageous alternative, additional fields may be transmitted in each burst either before the initial UW or after the final U~~I of the burst, or both. The additional fields may be additional data fields carryinz' addi~:aonal data or signalling, or may cam flxrther burst format signals designed to assist in the demodulation and.~'or decoding of the data content of the burst. I-lowever, such addition~.l fields are evalnerable to interference and preferably should not carry data or signalling esse~xztiai. for dcmcsdulation and/or decoding of the burst.
The above embodiments have been descrii~ed ~~ith reference to certain lnmarsatT"'' systems purely by ~~ay of example and aspects of ti°~e present invention are in no '~~ay limited thereto. For example, it will be readily understood to the skilled person that the problem of timing correction occurs in geo-stationary, geosyncrzronous and non-geostationazy satellite systems and aspects of the 1 aresent in'~enti.on are applicable to these systems. Moreover, timing errors can occur for reasons such as clock instability as well as relative movement between satellites, base stations and wireless transceivers, so that aspects of the present invention are also applicablf: to wireless communication systems not using satellites as relay stations. such as terrestrial communications systems or systems involving alternative relay stations such as balloons or other aircraft.
Although the above embodiments have beew described with reference to a TDMA
channel format, it u-ill be readily understood by the skilled person that the problem of interference as a result of timing error can occur with other chara,nel forr~aats, such as combined TDMA-CDMA, slotted Aloha and other time-divided formats and that aspects of the present invention are also applicable to such formats.
The description of the above embodiments iz~eludes a detailed description of the transmission formats of LESP and MESP channels. Aspects of these chef forrraats are particularly advantageous for packet data transmission via satellite, particularly via geostationary satellite and have been selected after considerable investigation of alternative formats, but may also be advantageous in different cor°~texts. Dr the other hand, it will be apparent that some aspects of the present invention are entirely independent of the specific channel formats used.
iRr'hile the apparatus of the specific embodiments has been described in terms of functional blocks, these blocks do riot necessarily correspond to discrete hardware or software objects. As is well known, most baseband functions may in practice be performed 19 . .
by° suitably programmed I~S~''s or ger~eraI purpose processors and the software may be opzimised for speed rather tha.~ str~czure.
Claims (7)
1. A wireless link signal comprising a data burst including in temporal sequence:
an initial predetermined synchronisation sequence;
a data field carrying the data content of the burst; and a final predetermined synchronisation sequence.
an initial predetermined synchronisation sequence;
a data field carrying the data content of the burst; and a final predetermined synchronisation sequence.
2. A wireless link signal comprising a data burst including in temporal sequence:
a first predetermined synchronisation sequence;
a data field carrying substantially alb of the data content of the burst; and a second predetermined synchronisation sequence.
a first predetermined synchronisation sequence;
a data field carrying substantially alb of the data content of the burst; and a second predetermined synchronisation sequence.
3. A signal as claimed in claim 1 or 2, wherein the burst includes an initial preamble preceding the first synchronisation sequence.
4. A signal as claimed in any one of claims 1 to 3, wherein the burst is transmitted in a time-slotted channel.
5. A signal as claimed in claim 4, wherein the channel comprises a plurality of slots sequentially separated by a guard band, wherein the length of the guard band is less than the maximum relative timing error between transmissions in adjacent time slots.
6. A method of transmitting a signal over a wireless link, comprising transmitting a signal as claimed in any one of claims 1 to 5.
7. A method of receiving a signal over a wireless link, comprising receiving a signal as claimed in any one of claims 1 to 5,
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9905182A GB2347828B (en) | 1999-03-05 | 1999-03-05 | Communication methods and apparatus |
| GB9905182.3 | 1999-03-05 | ||
| CA002289838A CA2289838A1 (en) | 1999-03-05 | 1999-11-15 | Communication methods and apparatus |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002289838A Division CA2289838A1 (en) | 1999-03-05 | 1999-11-15 | Communication methods and apparatus |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2463797A1 true CA2463797A1 (en) | 2000-09-05 |
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ID=32509172
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002463797A Abandoned CA2463797A1 (en) | 1999-03-05 | 1999-11-15 | Communication methods and apparatus |
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| Country | Link |
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| CA (1) | CA2463797A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US8457100B2 (en) | 2004-09-30 | 2013-06-04 | Kabushiki Kaisha Kenwood | Mobile wireless communication apparatus, wireless communication apparatus and communication processing method |
-
1999
- 1999-11-15 CA CA002463797A patent/CA2463797A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US8457100B2 (en) | 2004-09-30 | 2013-06-04 | Kabushiki Kaisha Kenwood | Mobile wireless communication apparatus, wireless communication apparatus and communication processing method |
| US8457102B2 (en) | 2004-09-30 | 2013-06-04 | Kabushiki Kaisha Kenwood | Mobile wireless communication apparatus, wireless communication apparatus and communication processing method |
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