A novel transmitter-based selective-precoding technique for DS/CDMA systems

In this paper a new transmitter precoding technique is presented that outperforms conventional precoding by making use of a portion of the interference between the users in a CDMA system downlink. The proposed technique selectively pre-decorrelates users that are experiencing destructive interference while allowing interference to other users when it is expected to contribute to their signal. The existence and exploitation of constructive interference effectively spreads the signal constellation and enhances the SNR at the receiver. The SNR improvement happens by making use of energy that is already in the system so the performance improvement is attained with no additional power-per-user investment. This however comes with the trade-off of some extra processing at the transmitter for the measurement of the expected interference. The proposed technique applies to the downlink of cellular CDMA systems employing PSK modulation. Theoretical analysis supported by comparative simulations of this and other precoding methods are presented and discussed.