METHOD OF ENHANCING FREQUENCY DIVERSITY IN BLOCK CDMA SYSTEMS

Abstract

Code division multiple access (CDMA) is a popular multiple access technique that is used to support multiple users simultaneously in a network. Many variants of CDMA exist, including direct sequence (DS) CDMA, multi-carrier (MC) CDMA, cyclic prefixed (CP) CDMA, and chip interleaved block spread (CIBS) CDMA. In addition to these variations, many receiver architectures are often available for implementation in CDMA systems, such as the well-known RAKE receiver, interference cancellation receivers, and receivers that rely on channel equalisation.

Description

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

FIG. 1 illustrates one user's set of frequency tones in a GMC-CDMA or an SC-FDMA system.

FIG. 2 depicts a block diagram of a transmitter/receiver structure of block CDMA.

FIG. 3 illustrates the spreading operation.

FIG. 4 shows an example of users' signals interfering with each other in the channel and combining at the receiver.

FIG. 5 illustrates the bandwidth utilisation when DFT codes are used as spreading codes and each user cycles through the codes with each transmitted block.

FIG. 6 illustrates an example of maximally exploiting frequency diversity through cycling of spreading codes (R=4).

FIG. 7 illustrates the frequency diversity exploitation for DFT spreading codes and self-shift-orthogonal spreading codes.

FIG. 8 illustrates user separation and subsequent MIMO detection.

FIG. 9 illustrates joint user and multiple stream separation.

FIG. 10 illustrates the SINR improvement that occurs when DFT spreading codes are used adaptively.

FIG. 11 depicts a block diagram of a MIMO system that incorporates the invention.

Claims

1. A method of enhancing the frequency diversity in the channel in multiple access transmission based on block CDMA for any number of users whose data to be transmitted is separated into a number of blocks, wherein each block is spread using any one of a plurality of predetermined spreading codes prior to transmission.

2. The method of claim 1, wherein the plurality of predetermined spreading codes comprises DFT spreading codes.

3. The method of claim 1, wherein the spreading code is selected in a pseudorandom fashion from the plurality of predetermined spreading codes.

4. The method of claim 1, wherein the order in which the spreading codes are used is such that the frequency diversity is enhanced for a given number of blocks.

5. The method of claim 4, wherein the selection of the spreading codes follows a cyclic fashion and comprises: selecting a first spreading code for the first block to be transmitted from the plurality of predetermined spreading codes in a random manner; andselecting each subsequent spreading code for each subsequent block to be transmitted among those of the plurality of predetermined spreading codes not previously used during the present cycle, according to the highest frequency shift induced.

6. The method of claim 1, further comprising employing a channel code or error correction code to enhance the frequency diversity in the channel.

7. The method of claim 1, wherein the spreading code for a given user can be temporarily altered so as to transmit over the full bandwidth and thus maximally exploit the frequency diversity in the channel for a given period of time.

8. The method of claim 1, wherein a plurality of transmit antennas are employed.

9. A signal in multiple access transmission based on block CDMA, as generated by a method of any one of the preceding claims.

10. A method of receiving a signal in multiple access transmission based on block CDMA enhancing the frequency diversity in the channel for any number of users whose transmitted data is separated into a number of blocks, wherein each received block is de-spread using each one of a plurality of predetermined de-spreading codes.

11. The method of claim 10, wherein the plurality of predetermined spreading codes comprises DFT de-spreading codes.

12. The method of claim 10, wherein the de-spreading code is selected in a pseudorandom fashion from the plurality of predetermined de-spreading codes.

13. The method of claim 10, wherein the order in which the de-spreading codes are used is such that the frequency diversity is enhanced for a given number of blocks.

14. The method of claim 13, wherein the selection of the de-spreading codes follows a cyclic fashion and comprises: selecting a first de-spreading code for the first block to be transmitted from the plurality of predetermined de-spreading codes in a random manner; andselecting each subsequent de-spreading code for each subsequent block to be transmitted among those of the plurality of predetermined spreading codes not previously used during the present cycle, according to the highest frequency shift induced.

15. The method of claim 10, further comprising employing a channel code or error correction code in decoding the user's signal.

16. The method of claim 10, wherein the de-spreading code for a given user can be temporarily altered so as to receive over the full bandwidth for a given period of time.

17. The method of claim 10, wherein a plurality of receive antennas is employed.