GB2445033A - HARQ for OFDM with bit or symbol rearrangement for retransmissions - Google Patents

Abstract

A method of retransmission comprises providing a series of bits, allocating subsets of the series of bits to symbols, ordering the symbols into a sequence and transmitting the sequence of symbols on a channel. In one embodiment the sequence of symbols is reordered for each transmission. In the case of OFDM, this has the effect of allocating symbols to different sub-carriers to those which they were allocated to at initial transmission. Symbols may also be reallocated to a different time position within a burst at retransmission. The reordering of symbols can be achieved by cyclic shifting. Pseudo-random reordering is also proposed. In an alternative embodiment at retransmission bits are reordered within the series of bits before allocation to symbols so that retransmitted symbols contain different bits from their respective originally transmitted symbols. These methods ensure that if a particular frequency band of the signal is consistently impaired, for example by deep fading of the same sub-carriers, different bits are affected at each retransmission.

Description

A METHOD OF RETRANSMISSION

This invention relates to a method of retransmission, in particular one which improves the operation of hybrid automatic repeat request (1-IARQ) protocols employed e.g. in mobile communication systems, although not limited to operation with HARQ protocols only. The invention is particularly applicable, but not limited to existing and currently developed wideband systems such as the universal mobile telecommunications system (UMTS) terrestrial radio access network (UTRAN), including long term evolution (LTE), or IEEE 802.16.

In a retransmission method where some or all symbols that have been sent in a first transmission, are subsequently resent in a retransmission, the energy received at the receiver from each of'the transmission and retransnhissions is recombined to improve the chance of successfully receiving the message. Better results are achieved if the reliability of individual symbols in a single transmission or accumulated over multiple transmissions is the same, which can be measured, for example by the symbols having equivalent quality, or signal to noise ratio. Where a sequence of symbols is transniitted on a channel, they are often subject to different channel or interference conditions. if the reason for failure of the original transmission is due to the varying channel or interference conditions, then the probability of a successful transmission is not fully improved by the straight forward retransmission.

One example ola retransmission method involving the combining of multiple transmissions is UTRAN downlink power control (DPC)_MODE = 1, where the power control command is derived by accumulating three separate transmissions from the mobile terminal.

Another example is the UTRAN retransmission of identical multicast-broadcast control channel (MCCH) contents or identical broadcast channel (BCH) contents in consecutive transmission time intervals (TTI), which can be combined in the receiver.

Another example is the possibility in UTRAN to configure and combine repeated uplink transmissions oithe channel quality indicator or the acknowledgement command for HSDPA.

Another example is the retransmission method known as the HARQ protocol employed by the UTRAN physical layer since Rclease-5. HARQ has a number of advantages, such as extending coverage, or compensating for the lack of fast power control in high speed downlink packet access (HSDPA).

An orthogonal frequency division multiplexing (OFDM)-based UTRAN LTE radio interlace, which is currently under standardization, introduces some new aspects that can be taken into account when designing HARQ operation. The main difference between the UTRAN radio interface, based on wideband code division multiple access (WCDMA) and the UTRAN LTE radio interface based on OFDM, is that with WCDMA, the spreading and scrambling operations result in the energy of each modulation symbol being transmitted in the entire transmission bandwidth, but with OFDM, the transmission of each modulation symbol is essentially narrowband. Thus, lbr WCDMA each transmitted symbol is subjected to roughly the same channel conditions in frequency domain. This is true if the time-domain changes to propagation conditions are ignored. In practice, a time-constant channel (compared to the payload transmission time) occurs frequently. This is referred to as quasi time invariant propagation conditions. However, the narrowband transmission of OFDM introduces an extra dimension, frequency selectivity, into consideration. Unlike with WCDMA, symbols within the same transmission time interval (TTI) will, in general, be exposed to different channel conditions due to frequency selectivity.

Further, each transmission attempt may be exposed to similar yet non-uniform interlërence, when interfering transmissions occur on partly overlapping time or frequency resources. For example, a single dominant interferer, operating according to the same retransmission protocol as the desired transmission, may affect the same or similar part of the time, or frequency resource allocated for each transmission, or retransmission burst, while not affecting the remaining part of that burst. Thus, the synibols within the same TTI may be exposed to different interference conditions due to different interference in time, or frequency. Since both the desired and interfering transmissions are controlled by the same protocol, the interference characteristics in each transmission attempt will vary in the same manner.

In accordance with the present invention, a method of retransmission comprises providing a series of bits; allocating subsets of the series of bits to symbols; ordering the symbols into a sequence: and transmitting the sequence of symbols on a channel; wherein, for each subsequent transmission, at least one of the series of bits, or the sequence of symbols, is reordered, such that at least two symbols are allocated a position in the sequence which is different from their position in at least one of the preceding transmissions; or at least two symbols comprise subsets of bits which are different from the subsets of bits in the preceding transmission.

The present invention improves the likelihood of a successful retransmission, by reordering a series or sequence in order to expose each bit or symbol to different channel and interference conditions, so that the channel and interference conditions for each are similar when averaged out over several retransnhissions.

The mechanism for detemining which bits in symbols are retransmitted uses a retransmission protocol which niay vary according to the application, but preferably, the actual subsets of bits in each symbol in the sequence for retransmission are determined by a hybrid automatic repeat request protocol.

Although the HARQ protocol nieans that not necessarily all bits or symbols are retransmitted, those which arc, are retransmitted in the positions detemiined by the reordering.

It is desirable that the distance between symbols with the same index in succeeding transnhissions is maximised, so preferably, each bit position in the series is cyclically shifted by a fraction of the total number of bits in the series, or each symbol position in the sequence is cyclically shifted by a fraction of the total nuniber of symbols in the sequence.

This fraction can be varied for each subsequent retransmission, e.g. /2, ", 3/4 Preferably, the position of a symbol within the sequence is defined by a frequency location within a frequency band of operation of the channel.

Preferably, the position of the symbol within the sequence is further defined by a section of a transmission burst in which that symbol at that frequency location is transmitted.

Preferably, a change in position ol'a symbol in the sequence is made within the group of symbols in all sections of the transmission burst.

Alternatively, a change in position of a symbol is made only within the group of symbols in the sanie section of the burst.

Preferably, each symbol position is cyclically shifted by a fraction of the average number of symbols in a section.

Preferably, the fraction is varied in successive retransmissions.

In one embodiment, the change in position of a symbol is based on application of a pseudo randoni calculation.

In one embodiment, the position of a symbol within the sequence is delined by a time location within a transmission burst; or by a frequency and time location within a frequency band of operation of the channel and the transmission burst.

In one embodiment, allocation of position of symbols within the sequence for a retransmission takes place during physical channel mapping.

An implementation of change of mapping in the transmitter is to have a physical channel mapping niodule. In this case, the reordering is applied to symbols in a sequence.

Alternatively, allocation of position of bits in the series takes place during retransmission control operation, or interleaving.

In this case the reordering is applied to bits in a series. An example ola retransmission control operation is a HARQ processing operation.

Preferably, allocation of position of bits in the series, or the allocation of position of symbols within the sequence, is dynamically controlled by a transmitter for at least one retransmission; and wherein a receiver is informed about the allocation of position of bits, or symbols, applied for that transmission by dynamic signalling.

Alternatively, allocation of position of bits in the series, or the allocation of position of symbols within the sequence, is fixed according to slowly changing, or pre-configured rules for at least one retransnhission; and wherein the receiver is informed about the allocation of position of bits, or symbols, applied for that transmission by slowly changing, or pre-configured rules.

The method can be applied in the frequency domain, or in the tinie domain. In the latter case, prelrably, the symbol represents a part of a signal at a specific time location.

An example of a method of retransmission in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure Ia illustrates the effect of a frequency selective channel on WCDMA; Figure lb illustrates the effect of a frequency selective channel on OFDM; Figure 2a is a graph of link throughput versus signal to noise ratio (SNR) for 1/3 quadrature phase shift keying (QPSK) with Chase combining for a GSM typical urban propagation channel at 3km/h, using localised physical resource mapping; Figure 2b is a graph of link throughput versus signal to noise ratio for 1/3 quadrature phase shift keying (QPSK) with Chase combining for a GSM typical urban propagation channel at 3km/h, using distributed physical resource mapping; Figure 3a is a graph of throughput versus signal to noise ratio for V2 16 quadrature aniplitude modulation (QAM) with partial incremental redundancy for a GSM typical urban propagation channel at 3km/h, with localised physical resource mapping; Figure 3h is a graph of throughput versus signal to noise ratio for V2 16 QAM with partial incremental redundancy for a GSM typical urban propagation channel at 3km/h, using distributed physical resource mapping; Figure 4 illustrates examples where selection of a given mapping order may take place in a method according to the present invention; Figure 5 illustrates a lirst example of different physical channel mapping orders for different values of k; Figure 6 illustrates a second example of different physical channel mapping orders for different values of k; Figure 7 illustrates a third example of different physical channel mapping orders for different values of k; Figure 8 illustrates a fourth example of different physical channel mapping orders for different values of k; Figure 9 illustrates a fifth example of different physical channel mapping orders for different values of k; and, Figure 10 illustrates a sixth example of different physical channel mapping orders for different values of k.

Fig. Ia illustrates the efThct of the frequency selective channel on WCDMA and Fig. lb illustrates its effect on OFDM. In Fig Ia, received symbol energy is accumulated over the whole bandwidth and does not depend on symbol index. In Fig. Ib, received symbol energy does depend on the symbol index in the presence of the frequency selective channel, as each symbol takes a specific position along the frequency range and unless the channel conditions change from one transmission to the next at that frequency position, that symbol e.g. s4 will always suffer from poor conditions. An analogous example can be constructed to illustrate the effect of frequency or time selective interferencc on OFDM, or time selective interference on WCDMA or single-carrier frequency division multiplexing (SC-FDM) Depending on application, the retransnhission protocol can be configured to include the transmission, followed by multiple re-transmissions of either entirely, or at least partly the same content, which is reiirred to as Chase combining and partial incremental redundancy (IR), respectively. Full lR may also be conligured for high coding rates, but the lower the coding rate, the niore information is repeated. For a fixed overall received power budget, forward error correction (FEC) decoding performance is maxiniized if all bits arc received with the same reliability, or log-likelihood ratio (LLR).

Constellation re-arrangement, employed for l6Qam in HSDPA, aims to provide a level of such LLR equaliLation.

Thus, in the case of WCDMA, shown in Fig. I a, under the quasi time invariant propagation conditions, a!! transmitted symbols are exposed to the same radio channel, hence the relative LLR values, associated with different transmitted bits are independent of the channel. Whereas, for OFDM, as shown in Fig. 2b, transmitted symbols are exposed to different channel conditions due to frequency doniain selectivity. Hence, the relative LLR values, associated with diflerent transmitted bits depend upon the channel (i.e. local channel conditions in frequency). That is, the channel introduces an LLR variation, even under the quasi tinie invariant conditions. Moreover, in the case of retransmission with Chase combining or partial IR, when the same data are consistently mapped onto the same frequency location, the LLR variation is retained through the retransmissions, which has an adverse effect onto link throughput.

Further, in the presence of a strong interferer operating according to the same protocol and utilizing partly overlapping time or frequency resources, the interference characteristics in each transmission attempt will vary in the same manner. In the case of retransmission with Chase conibining, or partial IR, when the same data are consistently mapped onto the same time and frequency location, the LLR variation is retained through the retransniissions, which has an adverse efiect onto link throughput.

In the present invention, instead of' leaving the symbols in the same position in the channel for each retransmission, they are allocated different positions, or different bits are allocated to the same symbol, so that an equivalent effect is achieved, thereby exposing data bits to different channel or interference conditions in subsequent retransmissions, so improving reliability.

Where the retransmission protocol is used, the link performance of the retransmission protocol is improved by varying the order in which transmitted bits are mapped onto the time-frequency resource. For example, the I transmission attempt employs mapping MI, the 2' attempt employs mapping M2 (different from MI in general), the 3d attempt employs mapping M3 (different from M I and M2 in general), etc. The benefit of changing the mapping order for each attempt is that LLR averaging is achieved over retransmissions, even under the quasi time invariant channel. For fast time varying channels, the channel variation itself will provide some averaging. In such a case the benefit of changing the mapping order is reduced, but the link throughput is never penalized by this invention.

An advantage of the present invention is an increase of link throughput 1'or a constant SNR, or alternatively a reduction of required SNR Ibr a constant link throughput.

Figs. 2 and 3 illustrate the benefits Ibr both Chase combining (1/3 rate coded QPSK) and partial JR (1/2 rate coded 16QAM). The graphs 21, 31 show a lixed random interleaver, referring to using the same mapping order for each transmission attempt, while the graphs 22, 32 show a variable random interleaver referring to using a different mapping order for each transnhission attempt. The computational cost associated with the present invention is extremely small and can be considered negligible in the context of other physical layer processing, both in the transmitter and receiver.

The concept of purposefully varying the order in which transmitted symbols are mapped onto the time-frequency resource across retransmissions can be achieved in a number of ways at the transmitter. Some examples of this include controlling the retransmission control operation (which in the case of the HARQ protocol includes rate matching, HARQ process, retransmission and JR control) with the mapping order parameter, k, as shown in Fig. 4a; controlling the interleaver with the mapping order parameter, k, as shown in Fig. 4b (different attempts use different inter1cavers) and controlling the physical channel mapper with the mapping order parameter, k, as shown in Fig. 4c (different attempts use different physical channel mapping order).

Furthermore, it is also possible to purposefully vary the order in which transmitted symbols are mapped onto the time-frequency resource across retransmissions via a hybrid of these mechanisms.

Fig. 4 illustrates the basic procedure for transmitting data using the retransmission protocol. FEC coding 40 is applied to a layer 2 data block, then the retransmission control 41 is applied to the coded bits to determine which bits are for retransmission.

These bits then pass through an interleaving stage 42 and QAM modulation 43, forming the bits into groups as symbols. A physical channel mapping step 44 takes place, followed by further layer I processing 45.

In the examples shown in Fig. 4, a mapping order index, k is applied at different stages of the process. Generally, symbols are not created until the QAM phase, so a change in mapping order is applied to individual bits when it occurs before this step and the bits then form symbols with diffrent content for the same index, as compared with earlier transmissions. So in Fig. 4a, the mapping order index, k is applied to bits and those bits which the retransmission control has chosen for retransmission are forwarded to the interleaver 42 in their revised order. In Fig. 4b, the mapping order index is applied at the interleaving stage 42, so the selection of bits are reordered at this point, before being formed into symbols. In Fig. 4c, the mapping order index, k is applied at the physical channel mapping stage 45, so it is symbols rather than bits which are reordered.

The mapping order parameter, k, must be made known to the receiver. There are a number of ways of achieving this. For example, if the HARQ protocol is employed to control the retransmissions: 1. Implicitly, by inferring k from the TTI counter, C11., , on the given radio link, e.g. k =LC7,,/N,,1j10jmodK,where NFJ4RQ is the number of HARQ processes and K is the number of niapping orders so that kE 0,1,...K -i}. This is particularly applicable to the so-called synchronous HARQ, where the timing of HARQ retransmissions is deterministic, and thus avoids the explicit signalling overhead. This method is also applicable to asynchronous HARQ, where the timing of HARQ retransmissions is scheduler dependent. However, in this case there is some danger of undesirable repetition of the same values of k.

2. Explicitly, by signalling the parameter k for each HARQ attempt.

3. Implicitly, by inferring k from the HARQ attempt number, which is not signalled.

4. By linking k to the explicitly signalled HARQ RSN (retransmission sequence number). E.g. k=0 for the jt HARQ attempt, k=l for the 2' HARQ attempt, etc. 2-bit RSN signalling is employed e.g. in E-DPCCH in UTRAN FDD Rel-6.

5. By linking k to the HARQ ND! (new data indicator). E.g. k=0 when new data, k=l if otherwise. I-bit NDI signalling is employed e.g. in HS-SCCH in UTRAN FDD Rd-S.

6. By linking k to the redundancy and constellation rearrangement version index (RVJ).

For example, k=RVI or k=RVI mod 3, etc. Explicit 3-bit signalling of RVJ is employed on HS-SCCH in UTRAN FDD ReI-5: implicit 2-bit signalling of RVI is employed in E-DPCCH in UTRAN FDD Rel-6.

7. By a hybrid of(4) and (I), e.g. k=O for the l HARQ attempt, k1 for the 2 HARQ attempt, k=2 for the 3 HARQ attempt, k = [c,,, /N,, $/?Oj mod K for 4th or higher HARQ attempt.

8. By a hybrid of(S) and (1), e.g. k=0 when new data, k = Qc11, /N,, mod(K -l))+ 1 otherwise.

K mapping orders have to be pre-defined and known to both the transmitter and the receiver.

When the re-ordering is performed in the interleavcr (figure 4b), the mapping orders include, but are not limited to, the following: 9. A different pseudorandom interleaver for each value of'k.

10. A different row/column interleaver for each value of k, e.g. Cl columns for k0, C2 columns for k= 1, etc. (the number of rows is equal to R = [B/cl, where B is the length of the vector input into the interleaver.).

II. The same row/column interleaver but with different inter-column (or inter-row) permutation for each value of k.

12. A hybrid of(10) and (11): different row/column interleaver and different inter-column (or inter-row) permutation for each value of k.

13. Let the vector of bits b,, / = 0.. .B -I be input to the interlcaver. The interleaver is an operation that permutes the elements of b,, and is split into pre-processing (dependent on the mapping order parameter k) and reordering (which is independent of k). The pre-processing algorithm operates as follows: a. For k=0, no action.

b. For k1, perform a cyclic shill of b1 by B/2 to obtain (b/l,,bB, l,...,bB_,b,b,,...,hfl,_I) c. For k=2, perform a cyclic shift of b, by B/4 to obtain (b,14,h84+, hfi_1,b,b.

d. For k=3, perform a cyclic shift of b, by 3B/4 to obtain (h184,b184+i,...,i,_,i,i,,...,h3114_i) The algorithm can be extended to larger values of k, to generalize it for correct operation when B is not a multiple of4, or to extend it to other cyclic shifts, e.g. B/3, B/5 etc. 14. Let the vector olbits h,, / = 0... B -l be input to the interleaver. This vector is to he mapped onto the time-frequency resource, where S locations have been preallocated: B = NS, where N is the number of bits per modulation symbol. These locations can be structured into columns along the frequency axis, as exemplified in figure 5 or 6. A different number of modulation symbols, S( , can be niapped onto each column (including no symbols when S =0). In the example, S1=12, S2=8, S=l6. The parameter S. is introduced, which is related to the average number of allocated symbols in each column. For example, it can be the mean (or median) number of symbols, taken over those columns with S > 0. In our example, S = (12 + 8 + 16)/3 = 12. Further, we introduce the parameter B = NS.. The interleaver is an operation that permutes the elements of b,, and is split into cyclic-shifting (dependent on the mapping order parameter k) and reordering (which is independent olk). The cyclic-shifting may either precede the reordering, or vice versa. The cyclic shifting algorithm operates as follows: e. For k=0, no action.

1 For k= I, perform a cyclic shift of Li1 by B. /2 to obtain b8,,h11,...,h131_,b,h1,...,b8, ) g. For k=2, perlbrm a cyclic shift of h, by B1 /4 to obtain h8,4,h134,...,b,,h,b,,...,h,141) h. For k=3, perform a cyclic shill of Li, by 3B,/4 to obtain h34 Li38, 4+I' . , h1, Li,b2,. . . , b38 41) The above algorithm can be extended to larger values of k, to generalize it for correct operation when B, is not a multiple of 4, or to extend it to other cyclic shifts (including negative shifts), e.g. B/3, B/S etc. When the re-ordering is performed in the physical channel mapper, as shown in Fig. 4c, the mapping orders include, but arc not limited to, the following: 15. A different pseudorandom mapper for each value of k.

16. Let the vector of symbols s,, / = 0...S -l be input to the physical channel mapper.

This vector is to he mapped onto the time-frequency resource, where S locations have been pre-allocated. These locations can be structured into columns along the frequency axis, as exemplified in figureS. A different nuniher olsymbols, S,can be niapped onto each column (including no symbols when S =0). In the example, S1=12, S7=8, S3=16.

The location of the time-frequency resource may vary for each retransmission, as illustrated in Fig. 5. The mapping proceeds as follows: a. For k=0, the mapping proceeds in the natural order, i.e. the symbols s( are written into the 1st column in the increasing order, symbols sç VI+S2-I are written into the 2' column in the increasing order, symbols sç1ç. are written into the 3rd column in the increasing order etc. b. For k=1, the same subset olsymbols is mapped onto each column, however the mapping order is cyclically shifted for each column, compared to the mapping described in (a), where the shill is equal to Sj2 for column C (see figure 5b).

c. For k=2, the same subset of symbols is mapped onto each column, however the mapping order is cyclically shifted for each column, compared to the mapping described in (a), where the shift is equal to S /4 for column C (see figure Sc).

d. For k=3, the same subset of symbols is mapped onto each column, however the mapping order is cyclically shifted for each column, compared to the mapping described in (a), where the shill is equal to 3S/4 for column C. The algorithm can be extended to larger values of k, to generalize it for the case when S is not a multiple o14, or to extend itto other cyclic shifts, e.g. S(/3, S /5 etc. It is also straightforward to extend the algorithm to the case where S and S vary between attempts.

17. Let the vector of symbols s,, / = 0.. .S -1 be input to the physical channel mapper.

This vector is to be niapped onto the time-frequency resource, where S locations have been pre-allocated. These locations can be structured into columns along the frequency axis, as exemplified in Fig. 6. A different number of symbols, S, can be mapped onto each column (including no symbols when S = 0). In the example, S1=12, S2=8, S16.

The location of' the time-frequency resource may vary for each retransmission, as illustrated in Fig. 6. The parameter S is introduced, which is related to the average number of allocated symbols in each column. For example, it can be the mean (or median) nuniber of symbols, taken over those columns with S > 0. In our exaniple, = (12 + 8 + 1 6)/3 = 12. The mapping is split into pre-processing (dependent on the mapping order parameter k) and proper mapping' (which is independent of k). The proper mapping' simply maps its input onto the physical resource, e.g. as illustrated in figure 6a. The pre-processing algorithm operates as follows: e. For k=0, no action.

I For k=1, perform a cyclic shill of s, by S/2 to obtain 2,S 2+, g. For k=2, perform a cyclic shill of s, by S /4 to obtain ,...,Ssi4i) h. For k=3, perform a cyclic shift of s, by 3S( /4 to obtain S1 s-i' i 2 The operation of this algorithm is illustrated in Fig. 6. The above algorithm can be extended to larger values of k, to generalize it for the case when S1, is not a multiple of 4, to extend it to other cyclic shifts, e.g. S /3, S /5, or to express it in the equivalent form of negative shills such as -Se,, -S /2 etc. It is also straightforward to extend the algorithm to the case where S and S vary between attempts.

18. Let the vector of symbols s,, / = 0.. .S -1 be input to the physical channel mapper.

This vector is to be mapped onto physical resource, where S locations have been pre-allocated. The mapping is split into pre-processing (dependent on the mapping order parameter k) and proper mapping' (which is independent of k). The proper mapping' simply maps its input onto the physical resource. The pre-processing algorithm operates as follows: i. For k=0, no action.

j. For k=1, perform a cyclic shift of s, by S/2 to obtain s2_I) k. For k=2, perform a cyclic shift of s, by S/4 to obtain 4-I) I. For k=3, perform a cyclic shift of s1 by 3S/4 to obtain (sç4,s4+1,...,sç_,si,s2,...,s34_1) The algorithm can be extended to larger values of k, to generalize it for correct operation when S is not a multiple of 4, or to extend it to other cyclic shifts, e.g. S/3, S/5 etc. Fig. 5 illustrates different physical channel mapping orders lbr different values of k. In this example, k is linked to the retransmission number and K= 4. The shaded areas correspond to the time-frequency resource allocated to the physical channel. In the first transmission, a certain number of symbols are allocated

time frequency resource within each o13 time sections, A, B, C. In the next attempt, Fig.5b, the same time frequency resources have been allocated to different symbols by cyclically shifting the symbols in each section by a fraction of the number of symbols in each section, in this case V2. so by 6 symbols out of 12 in section A, by 4 symbols out of 8 in section B and by 8 symbols out of 16 in section C. In Fig. 5c, the cyclic shift is of the number of symbols in each section and the time frequency resources have changed. In Fig. Sd, the time frequency resources have been shilied as a whole in frequency and a further cyclic shift applied within each section.

Fig. 6 illustrates different physical channel mapping orders for different values of k. In this example, k is linked to the retransmission number and K 4. The shaded areas correspond to the time-frequency resource allocated to the physical channel. In the example of Fig.6, instead of applying the cyclic shift within each of the three time sections, the cyclic shill is applied across all three and is equal to a fraction (in this case , and %) of the average number of symbols per section. So for example, symbols 0 to in the first section in Fig. 6a are shifted into the 3rd section in Fig. 6b and the other symbols cycled accordingly. As with Fig.5, a shift and change of time frequency locations occurs in Fig. 6c and the time frequency locations are shifted as a whole in Fig. 6d.

Fig. 7 illustrates different physical channel mapping orders!br different values of k. In this example, k is linked to the retransmission nuniber. The shaded areas correspond to the time-frequency resource allocated to the physical channel. The cyclic shift applied is selected such that the distance in frequency and time (within the burst) between the symbols with the same index is maximized.

Fig. 8 illustrates different physical channel mapping orders for different values of k. In this example, k is linked to the retransmission number. The shaded areas correspond to the time-frequency resource allocated to the physical channel. The mapping variation is applied in the time domain, where the burst sections are transmitted in the order D, E, F in the I attempt, and in the order F, E, D in the 2' attempt.

Fig. 9 illustrates diIi'ereni physical channel mapping orders for different values of k. In this example, k is linked to the retransmission number. The shaded areas correspond to the time-frequency resource allocated to the physical channel. This example is applicable to radio interfaces with symbol energy spreading, such as WCDMA or single carrier frequency division multiplexing (SC-FDM). The mapping variation is applied in the time domain, where the burst sections are transmitted in the order G, H, I in the 1st attempt, and in the order I, H, G in the 2' attenipt.

Fig. 10 illustrates different physical channel mapping orders for different values of k. In this example, k is linked to the retransmission number. The shaded areas correspond to the time-frequency resource allocated to the physical channel. This example is applicable to narrowband radio interfaces. The mapping variation is applied in the time domain, where the transmission order changes pseudo-randomly between attempt I and 2.

Claims (16)

1. A method of retransniission, the method comprising providing a series of bits: allocating subsets of the series of bits to symbols: ordering the symbols into a sequence; and transmitting the sequence of symbols on a channel; wherein, for each subsequent transmission, at least one of the series of bits, or the sequence of symbols, is reordered, such that at least two symbols are allocated a position in the sequence which is different from their position in at least one of the preceding transmissions; or at least two symbols comprise subsets of bits which arc different from the subsets of bits in the preceding transmission.

2. A niethod according to claim I, wherein the actual subsets of bits in each symbol in the sequence for retransmission are determined by a hybrid automatic repeat request protocol.

3. A method according to claim 1 or claim 2, wherein each bit position in the series is cyclically shifted by a fraction of the total number of bits in the series or each symbol position in the sequence is cyclically shifted by a fraction of the total number of symbols in the sequence.

4. A method according to any preceding claim, wherein the position of a symbol within the sequence is defined by a frequency location within a frequency band of operation of the channel.

5. A method according to claim 4, wherein the position of the symbol within the sequence is further defined by a section of a transmission burst in which that symbol at that frequency location is transmitted.

6. A method according to claim 5, wherein a change in position of a symbol in the sequence is made within the group of symbols in all sections of the transmission burst.

7. A method according to claim 5, wherein a change in position of a symbol is niade only within the group of symbols in the same section of the burst.

8. A niethod according to at least claim 5, wherein each symbol position is cyclically shifted by a fraction of the average number of symbols in a section.

9. A method according to claim 8, wherein the fraction is varied in successive retransmissio ns.

10. A method according to any of claims Ito 5, wherein the change in position of a symbol is based on application of a pseudo random calculation.

11 A method according to any of claims I to 3, wherein the position of a symbol within the sequence is defined by a time location within a transmission burst; or by a frequency and time location within a frequency band of operation of the channel and the transmission burst.

12. A method according to any preceding claim, wherein allocation of position of symbols within the sequence for a retransmission takes place during physical channel mapping.

13. A method according to any of claims 1 to II, wherein allocation of position olbits in the series takes place during retransmission control operation, or interleaving.

14. A method according to any of claims I to 13, wherein allocation of position ofbits in the series, or the allocation of position of symbols within the sequence, is dynamically controlled by a transmitter for at least one retransmission; and wherein a receiver is informed about the allocation of position of bits, or symbols, applied for that transmission by dynamic signalling.

15. A method according to any of claims Ito 13, wherein allocation of position of bits in the series, or the allocation of position of symbols within the sequence, is fixed according to slowly changing, or pre-configured rules for at least one retransmission; and wherein the receiver is infornied about the allocation of position of bits, or symbols, applied for that transnhission by slowly changing, or pre-contigured rules.

16. A method according to claim 1, wherein the symbol represents a part of a signal at a specific time location.