Minimum BER transmission for TDD-CDMA in frequency-selective channels

A novel approach to multiuser transmission (MUT) for the CDMA downlink in frequency-selective channels is proposed. The key idea is to minimize the BER at the receivers directly by pre-distortion of the transmitted symbols. The BER can he predicted in the transmitter In calculating the signal at the receiver (including interference) exactly and treating only the additive noise at the receiver statistically. Using state-of-the-art nonlinear optimization methods like SQP (sequential quadratic programming), the BER minimizing pre-distortion coefficients can be found. Simulations compare the proposed scheme with the RAKE matched filter and linear transmitter preprocessing schemes, like joint transmission. The proposed MUT scheme can be applied for instance in the 3GPP-TDD and the TD-SCDMA standards, where the channel impulse response is known in the transmitter due to channel reciprocity.