Optimisation of orthogonal low-rate convolutional codes in a DS-CDMA system

The paper describes a novel approach to the use of direct-sequence spread-spectrum modulation for code division multiple access (DS-CDMA) applications by replacing the normal PN spreading code with an orthogonal low-rate convolutional code. This achieves both the spreading in bandwidth of the data and a powerful coding strategy within the output state of the convolutional shift register system, which maximises the number of simultaneous CDMA channels. In order to achieve orthogonality, Hadamard codes are incorporated within the convolutional coding strategy. The effects of using such a design within a DS-CDMA system are studied and the results show a direct increase in the capacity of the cellular system compared with a DS-CDMA using maximal length sequences. The importance of randomising the output symbols from the Hadamard encoder is also considered with respect to the spectral characteristics of the transmitted signal and the isolation between different simultaneously transmitting users