Iterative Detection in a Multi-user Cooperative OFDM System with Carrier Frequency Offsets

The problem of multi-user cooperation to achieve cooperative diversity for single-antenna terminals is considered. A cooperative strategy is proposed which divides multiple users into groups with each group having two users as partners. The two users alternately transmit their own information first, then simultaneously amplify and forward the partner's data in the same channel. Comparing to the conventional multi-user cooperation where the two users relay the partner's data in two orthogonal channels, the spectral efficiency of the proposed scheme is increased. However, as the two users relay the partner's data in the same channel, multi-user detection problem arises. An iterative detection making use of the new scheme's property is proposed in the presence of frequency offsets. The proposed scheme only requires time and frequency synchronization but no channel state information at the user terminals. To estimate the channel at the destination, a training scheme is proposed. The simulation results show that the proposed scheme can achieve the same cooperative diversity as the conventional scheme, while the spectral efficiency is lifted by 4/3.