Collaborative spreading for the downlink of overloaded CDMA

A new scheme called 'Collaborative Spreading' is proposed for the downlink of CDMA to allow the sharing of the same spreading, sequence by more than one user. In particular, it addresses the problem of overloading and maintain the use of the same set of available orthogonal sequences and simple receiver structure. In this scheme, a total of K users are divided into G groups each containing T users which are collaboratively coded to form uniquely decodable composite codewords. These codewords are spread using a single sequence to perform the CDMA function between the groups. At the receiver, a low complexity maximum likelihood (ML) joint detection and decoding is carried out over a small set of allowed composite codewords to recover the desired user's data. Theoretical and simulation performance analysis of the bit error rate (BER) and user capacity are presented in different channel conditions. It is shown that the proposed collaborative spreading is a simple and very effective means for extending the user capacity at the cost of a modest degradation in BER performance compared with non-overloaded fully orthogonal CDMA. It can achieve higher signal-to-interference plus noise ratio (SINR) and higher overloading ratio compared with orthogonal CDMA/orthogonal CDMA schemes and other group orthogonal CDMA schemes. Copyright (C) 2009 John Wiley & Sons, Ltd.