This paper focuses on a multi-beam combining scheme for DS-CDMA systems, which has RAKE combiners in multiple overlapped beams, in order to increase the reverse link capacity of DS-CDMA. This scheme is a very attractive technique because the maximal ratio combining (MRC) is carried out in space and time domains. However, in a practical situation, since the terminals in own sector are not uniformly located, the interference levels in respective beams are different. Therefore, receivers at the base station do not achieve ideal combining. This paper proposes a multi-beam combining scheme for DS-CDMA systems using weighting factor based on interference level of each beam. A fast closed loop transmission power control (TPC) scheme for the multi-beam combining system is also proposed. It is confirmed by computer simulation that the proposed scheme has excellent performance in the reverse link even if terminals in own sector are not uniformly located.

Maximal-ratio combining (MRC), which maximizes the carrier to noise ratio (CNR) of the combined signal, generally requires envelope detection and multiplication having linear characteristic over a wide dynamic range to generate a weighting factor for each branch. In this paper, we propose a simplified two-branch diversity combining scheme without linear envelope detection. The proposed scheme, called "level comparison weighted combining (LCWC),"is simplified in a manner that its weighting factor for each branch is generated from hard-decision results of comparing signal envelopes between two branches. Performance of LCWC is evaluated by computer simulation and laboratory experiment, which shows that its diversity gain is almost identical to that of MRC in a Rayleigh fading channel.

A diversity strategy is efficient to reduce the fluctuation of communication quality caused by fading. In order to further maintain the communication quality and improve the communication capacity, this paper proposes a two-dimensional diversity approach by serially-concatenating spectral precoding and power normalized-differential space time block coding (PN-DSTBC). Spectral precoding is able to take benefit from a frequency diversity effect without loss in spectral efficiency. In addition, PN-DSTBC is robust against serious phase noise in an extremely high frequency (EHF) band by exploiting a spatial diversity effect. However, there is a problem that a naive concatenation degrades the performance due to the imbalance of equivalent noise variances over transmit frequencies. Thus, we examine an equalized PN-DSTBC decoder as a modified approach to uniform equivalent noise variances over frequencies. The performance evaluation using computer simulations shows that the proposed modified approach yields the performance improvement at any modulation schemes and at any number of transmit frequencies. Furthermore, in the case of 64QAM and two transmit frequencies, the performance gain of the modified approach is 4dB larger than that of PN-DSTBC only at uncoded BER=10-4.