GB2440979A

GB2440979A - Achieving coding gain in soft handover using incremental redundancy and bit mapping techniques

Info

Publication number: GB2440979A
Application number: GB0616390A
Authority: GB
Inventors: Przemyslaw Jan Czerepinski
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2006-08-18
Filing date: 2006-08-18
Publication date: 2008-02-20
Also published as: GB0616390D0

Abstract

A method of transmission between one or more base stations BS1, BS2 and a terminal 3 in soft handover. The method comprises transmitting from the base stations a first signal, comprising data bits and parity bits, and a second signal, which differs in bit content or bit mapping from the first signal. The second signal may comprise only parity bits and/or additional parity bits, or it may differ in bit mapping arrangement. Conventionally, the two base stations BS1, BS2 transmit the same content to the terminal 3 arriving in the form of different waveforms with powers P1, P2, together with interference I and noise N. A processing gain is achieved, but not a coding gain. The present invention addresses the absence of coding gain by using Incremental Redundancy (IR) and Bit Mapping (BM) techniques, transmitting different signals from each base station BS1, BS2 to get Forward Error Correction (FEC) coding gain as well.

Description

A METHOD OF TRANSMISSION BETWEEN A TERMINAL AN!) A BASE

STATION

This invention relates to a method of transmission between a terminal and a base station, in particular for a terminal in soft handover. Soft handover (SHO) is a method of improving coverage lbr intercell interference-limited radio communication systems. In iiiicrccll interference-limited radio communication systems, such as universal terrestrial radio access network (UTRAN) frequency division duplex (FDD) particularly, where for a given base station, all geographically neighbouring base stations also transmit and receive in the same frequency band, SHO can be employed to improve system coverage.

In soft handover, the mobile device receives in downlink from multiple transmitters and dr multiple transmissions are combined in the receiver. Incoming power to the receiver is made up of the power from each transmitter, interference and noise. The accumulated signal energy from the multiple transmissions gives a processing gain, but no coding gain.

In accordance with the present invention a method of transmission between one or more base stations and a terminal in soft handover comprises transmitting from the, or each base station a first signal and a second signal, wherein the first signal comprises data hits and parity bits; and wherein the second signal differs in bit content or bit mapping from the first signal.

There may be more than two signals from any base station, hut the benefits accrue provided that there are at least two signals. For the case of three signals, the bit mapping, or content is different again in the third signal.

The present invention deliberately changes the content of the signals sent from each transmitter, or base station, in order to achieve coding gain.

Preferably, the second signal comprises only parity bits.

Preferably, additional parity bits are provided in the second signal.

Alternatively, the second signal comprises the same data bits and parity bits as the first signal, differing in their bit mapping arrangement.

Preferably, the parity bits which were present in the first signal have different bit mapping in the second signal.

Exaniples of the difference in the second signal include a difference by virtue of omitting the data bits and filling the space with more parity bits, or by keeping the data bits and adding more parity bits, with a resulting change in length; or by having both all parity bits and more parity bits.

Preferably. a plurality of base stations transmit to a plurality of terminals.

An example of a method of transmitting between a terminal and a base station in soft handover according to the present invention will now be described with reference to the accompanying drawings in which:-Figure I illustrates the principles of soft handover; Figure 2 is a logical block diagram of an example of soft handover implementation in a terminal receiver; Figure 3 is a logical receiver block diagram of an example of soft handover implementation in a terminal receiver in accordance with the present invention, with increased redundancy; Figure 4 is a logical receiver block diagram of an example of soft handover implementation in a terminal receiver in accordance with the present invention, with bit mapping; Figure 5 is a logical receiver block diagram ol an example of soft handover implementation in a terminal receiver in accordance with the present invention, with both incremental redundancy and bit mapping; Figure 6 illustrates typical signal structures for transmission according to the present invention; Figure 7 illustrates the invention applied to a specific receiver; Figure 8 illustrates the invention as applied to multiple receivers; and, Figure 9 illustrates various arrangements of bits for the multiple receivers of Fig. 8.

The present invention describes an improvement to soft handover by using incremental redundancy (1R) and hit mapping (BM) techniques. The SHO principle is illustrated in Fig. 1. Fig. I shows a pair of base stations BS 1, BS2 and a single terminal 3. The two base stations BS 1 and BS2 transmit the same content to a mobile terminal 3. However, in general, the content arrives at the mobile in tie form of different waveforms with powers P1, P2. This maybe due to lack of synchronicity, or orthogonality, or for other reasorE. These different waveforms interfere with each other and in addition, the terminal suffers from the interference 1 from other base stations (not shown), as well as its own noise N. For illustration purposes, but without losing generality, it is assumed that the same signal power arrives at the terminal 3 from BS I and BS2, i.e. = P-, P. The signal-to-interference-and-noise ratio (SINR) for a single radio link can be written as: SINR = _______ = = P 1-I+N I-P,+N 1-P+N The S/NR that can be obtained by maximum ratio combining of the two radio links is equal to: I, P)J) SINR = I + 2 _______ I-+N 1-P,+N I-P+N Thus, a processing gain equal to G1, 2 = /SINRQ,,, = 2 is achieved. In general, when combining K equal-power radio links, the processing gain is equal to K: G,,=K Fig. 2 shows one possible example of a logical block diagram of a receiver chain supporting SHO. In this particular example, transmissions 4, 5 over radio link I and radio link 2 are combined 6, demodulated 7, dc-rate matched 8 and forward error correction (FEC) is applied 9. The soft handover technique is analogous to so-called Chase combining, where multiple transmissions, containing the same symbols, are made to the receiver. The receiver accumulates the energy of all the transmissions in order to achieve an SJNR gain, equal to SINR, . The received per bit' energy can be expressed as: Eh=_!_LE,I (I) where Eç is the received symbol energy, B is the number of bits transmitted per symbol, K is the number of transmissions, and C is the coding rate.

Through accumulating signal energy over multiple transmissions, the soft handover (and Chase combining) improves the received SINR i.e. it achieves a processing gain. However, it does not offer any coding gain. Every radio link (retransmission) is identical, so the coding rate after K-way combining remains equal to C. An overall gain figure G that combines processing gain and the coding rate is: cc The value G represents purely an energy gain, offered by the combination of FEC coding and soft combining. Of course, in practice, the benefit of coding exceeds the benefit of the processing gain.

The present invention addresses the absence of the coding gain by choosing to transmit different signals from each transmitter to get FEC coding gain as well. There are a number of ways in which the two signals can be made to he different. Although, these examples are described with respect to a iirst signal and a second signal which differs from this, they are not limited to occurring in thit particular order in time.

In Fig. 3 demodulation 10, 12 and de-rate matching II, 13 are applied for the signals from radio links I and 2. The resultant signals are combined 14 and FEC decoding applied 15.

In Fig.4, demodulation 10, 12 is applied to each signal on radio links I and 2 and then hit de-mapping 16, 17. The outputs are combined 14 and the combined outputs are dc-rate matched 18, then FEC decoding 15 is applied.

The example of Fig. 5 combines both the methods of Fig. 3 and Fig. 4, thus the originals from radio links I and 2 are demodulated 10. 12; bit dc-mapped 16, 17 and dc-rate matched 11, 13. The resulting signals are then combined 14 and FEC decoding applied.

The examples of how the invention could be used are described mainly with respect to incremental redundancy (IR), hut analogous examples apply for hit mapping (BM), or a combination of BM and IR. A typical signal is structured as shown in Fig. 6a. In Fig.7, a single terminal 3 receives signals from BSI, BS2. The signal from BS1, S(BS 1) is made up of systematic bits and parity bits. Conventionally, the signal from BS2 would have had the same content and format as the signal from BS I. However, in one example of applying incremental redundancy, the signal from BS2, S(BS2) is made up of only parity bits. This is illustrated in Fig. 6b and Fig.6c. For Fig. 6b, the total number of bits is the same, bitt in Fig. 6c, there are more parity bits than the total number of bits in S(BS 1).

For the example of Fig. 7, with the transmission addressed to a specific receiver, a single receiver combines multiple radio links. This is similar to a mobile terminal receiving a dedicated channel in SHO in Rel-99 FDI) UTRAN. Thus, only one set of systematic bits needs to be provided (e.g. on radio link I), while the remaining K- 1 radio links transmit parity bits only. The coding rate on RL1 is equal to C. Assuming that an identical number of bits are transmitted on each radio link, this leads to the following: G,, c,, =SL

K

That is, the processing gain is entirely converted into the FEC coding gain, representing a clear improvement.

The example of Fig. 8 illustrates Uansmission addressed to multiple receivers.

Groups of one or more terminals TI, 12, T3, T4 are positioned within range of one or two transmitters BS3, BS4, BSS, BS6, each transmitter sending a signal S3, S4, S5, S6.

The same higher layer data stream is transmitted to multiple receivers. This can be likened to point-to-multipoint MBMS with macrodiversity in Rel-6 FDD UTRAN. In the case of the present invention, each transmitter broadcasts the same systematic bits (and all systematic bits are present on each radio link), but the parity bits are different on different radio links. The coding rate on each radio link is identical and equal to C. Thus: #of_bits_on_RL1 +#ofbits_on_RL2 +...+#of_bits_on_RLK - (I,- # systematic _bits + total_# _of_parity _bits = (numerator unchanged) # of_bits_on _RL1 + #of_bits_on _RL, +... + #of_bits_on _RL K -(K -l systematic _bits

I I I K

-±+...±

-C C C -C _ K

)-+!+...+J-_(K_l) --(K_l) K-(K-l)C

C C C C

<p≥ #systematic _bits = e2 #systematic _hits + total_# _of_parity _bits = # systematic _bits #of_bits_on RL1 +#oLbils_on_RL,, +

.+#of_bits_on _RLA -(K -l)#systernatic _bits -1 -C -(K-I) K-(K-l.. DTD: C

It can be seen that T2 is within range of both S3 and S4 whereas TI is only within range of S3. Thus, it is not advisable to remove the systematic bits from any transmission because there are transmitters which could lose data because they are in only in range of one signal. Fig. 9 shows how for 9a, b and c the systematic bits are the same, hut the parity bits change from bits A to bits B or C. Fig. 9d is a bit mapping example where the same parity bits A are remapped to provide the redundancy. Thus the present invention gives some of the processing gain has been converted into FEC coding gain compared to the original SHO implementation The invention described gives an improvement to SHO by introducing incremental redundancy and/or bit mapping techniques across different radio links.

This means that differert radio links no longer transmit identical symbols. instead, they are allowed to contain different parity information or constellation symbols obtained through different bit mapping. The coding rates andlor constellation sizes on different radio links may differ, although they may also remain identical.

There are a number of advantages including the features that the overall energy gain. G, remains unchanged and the processing gain component, G, is reduced. In the case of IR, the coding gain component, I/C, is increased, with the benefit of improving the coding gain by an amount which will exceeds the processing gain loss. In the case of 13M, there is no improvement to the energy or coding gains, but reliability of the so-called,,soft bits", input into the FEC decoder is improved. In another embodiment IR and BM are combined, giving both a coding gain and improvement to soft bit reliability.

Although this is implementation dependent, the required content synchronization in the transmitter can typically be relaxed, compared to the original SHO. This is because the original SHO can be implemented at chip level, whereas the proposed method operates on symbol level.

Claims

CLAIMS

1. A method of transmission between one or more base stations and a terminal in soft handover, the method comprising trawmitting from the, or each base station a first signal and a second signal, wherein the first signal comprises data bits and parity bits; and wherein the second signal differs in bit content or bit mapping from the first signal.

2. A method of transmission according to claim 1, wherein the second signal comprises only parity bits. 1 0

3. A method of transmission according to claim I or claim
2. wherein additional parity bits are provided in the second signal.

4. A method of transmission according to claim I, wherein the second signal comprises the same data bits and parity bits as the first signal, differing in their bit mapping arrangement.

5. A method of transmission according to any preceding claim, wherein the parity bits which were present in the first signal have different bit mapping in the second signal.

6. A method of transmission according to any preceding claim, wherein a plurality of base stations transmit to a plurality of terminals.