Pre-equalisation, transmitter precoding and joint transmission techniques for time division duplex CDMA

We consider options for moving receiver complexity from the terminal to the base station in a mobile communications system downlink. We compare pre-equalisation, transmitter precoding and joint transmission techniques carried out at the base station with equalisation and joint detection at the user equipment receiver for synchronous CDMA over frequency selective fading channels. All the transmitter based methods assume knowledge of the channel at the transmitter. This is feasible under the time division duplex (TDD) mode of the 3G UMTS system where the channel is assumed to be the same on uplink and downlink and can be estimated from the uplink at the base station. The advantage over the well known receiver-based equalisation and multiuser detection techniques is that reduction of the mobile receiver complexity is achieved.