Joint signal precoding in the downlink of spread-spectrum systems

The downlink capacity tends to be the limiting factor in future communications systems. We propose two schemes for the downlink of code-division multiple-access systems that help reduce the multiuser interference by jointly precoding the transmitted signal based on knowledge of the downlink channel. Methods that operate on blocks of bits are first developed and a bitwise simplification of these for systems with large processing gains are derived. The use of multiple transmit antennas are also considered. These methods provide a substantial capacity increase without requiring complex multiuser detectors at the mobile terminals, and can be applied for instance to time-division duplex systems, which have reciprocal channels.