Optimized pulse shaping for OFDM multi-user communications over doubly dispersive channels

In multiple access OFDM systems, the division of the different users is achieved by exploiting the orthogonality of the sub-carriers. However, this orthogonality may be lost if pulse shapes used in the modulation and demodulation processes are badly localized in time and frequency and the channel is doubly dispersive. Hence a big amount of self and inter-user interference will arise and severely panelize system performance. This problem becomes more critical in the uplink, where users have different time offsets due to different propagation delays and different frequency offsets due to misalignment in carrier frequencies and different Doppler shifts. In this paper we present a pulse-shaped OFDM based analysis of inter-symbol/inter-carrier interference for synchronous downlink and asynchronous uplink of multi-user systems. We propose an optimization procedure for designing pulses maximizing the SIR. We show that system performances can visibly be improved by inserting frequency guard intervals between adjacent users, while making a convenient trade-off with spectral efficiency.