Wideband CDMA signal propagation with application to smart antennas

There is interest in increasing the data rates of terrestrial wireless systems by increasing the chip rates in code division multiple access (CDMA) systems, as well as by employing base stations equipped with smart antennas. We have constructed an experimental facility to assess certain key propagation effects in wideband CDMA (at 7 Mcps) employing digital multi-beamforming at a base station receiver antenna array. One of the unique features of our system is that we employ multiple transmitters to assess experimentally the impact of multiuser interference. After characterizing the resolvable multipath, including the temporal dynamics, we study the spatial signatures received from four simultaneous users. We show that by processing the received data in blocks of 25 bits, it is possible to achieve a significant array gain (over 3 dB; on average from 4 antennas) if the multiuser interference is taken into account via maximum signal-to-noise-plus-interference (SINR) beamforming. If the spatial signatures of users are too similar, however, these gains are not realized. Moreover, spatial signatures were not found to be predictable from users' physical locations.