Ralf Irmer, Wolfgang Rave and Gerhard Fettweis Minimum Ber Multiuser Transmission for Spread-spectrum Systems in Frequency- Selective Channels

Spread Spectrum CDMA systems suffer from interference in frequency-selective channels. For the uplink, Multiuser Detection (MUD) mitigates the interference. For the downlink, Multiuser Transmission (MUT) is possible, provided that an estimation of the channel impulse response is available. A novel MUT approach is proposed, which outperforms other MUT methods, like Joint Transmission (JT) and die Transmit Wiener filter. The Bit Error Probability (BER) at the receivers is minimized directly, by calculating appropriate transmit-symbol pre-processing coefficients. For this approach, nonlinear optimization algorithms are used. The focus of this paper is on the comparison of Minimum BER MUT in conjunction with Rake and Pre-RAKE. The complexity of the numerical optimization algorithm is also investigated.