A unified view of RAKE reception is proposed which models a RAKE receiver as an antenna array. This unified view provides valuable insight to the signal environment under RAKE reception. Based on this view, an optimum combining scheme for RAKE receivers is proposed for the downlink of multi-code W-CDMA systems. In multi-code scenarios, the presence of inter-code interference causes severe performance degradation. The antenna array model suggests that enhancement can be achieved by increasing the receiver's degrees of freedom which is defined as the number of RAKE fingers and employs an appropriate combining scheme. The conventional maximum-ratio combining scheme is excluded since it is not capable of exploiting the increased degrees of freedom. In contrast, the proposed combining scheme provides better interference suppression when the degrees of freedom are increased. Numerical results obtained show that the proposed scheme provides very promising performance.

It is well known that interpath interference (IPI) is a major factor that limits the performance of high data rate transmissions over a variable spreading factor wideband-CDMA (W-CDMA) link since the spreading factor is in general small. An optimum combining scheme suppressing IPI was recently proposed for RAKE reception in [1]. The main contribution of this letter is to present a theoretical model for the outage probability and bit error probability of a RAKE receiver utilizing the optimum combining scheme. Analytical and simulation results are closely matched and show that the optimum scheme provides significant performance improvement compared to the conventional maximum ratio combining (MRC) scheme.

We propose a new receiver structure to mitigate interpath interference (IPI) in W-CDMA systems. We model IPI in RAKE combining as intersymbol interference (ISI) and use a two-stage receiver structure. The first stage is a RAKE receiver and the second stage is an equalizer. In cases of multi-code transmission, interference among code channels causes extra impairments which can not be modelled as ISI. Under these circumstances, they are estimated by using decisions from the first stage and then subtracted from the input of the equalizer. The residual interference is equivalent to ISI and can be mitigated by the equalizer. Simulation results show that the proposed receiver provides very promising performance in low spreading factor W-CDMA.

This paper presents a novel threshold-based selection scheme to combine adaptive transmit antenna selection with an adaptive quadrature amplitude modulation (AQAM) for a spatial multiplexing (SM) multiple-input multiple-output (MIMO) system with linear receivers in practical uncorrelated and correlated channel conditions. The proposed scheme aims to maximize the average spectral efficiency (ASE) for a given bit error rate (BER) constraint and also to lower the hardware complexity. Our simulations are run on a general MIMO channel model, under the assumption that the channel state information (CSI) is known at the receiver and the adaptive control signaling can be perfectly fed back to the transmitter. We deploy the low rank-revealing QR (LRRQR) algorithm in transmit antenna subset selection. LRRQR is computationally less expensive than a singular value decomposition (SVD) based algorithm while the two algorithms achieve similar error rate performances. We show that both the conventional AQAM scheme (i.e., without adaptive transmit antenna selection) and the SM scheme perform poorly in a highly correlated channel environment. We demonstrate that our proposed scheme provides a well-behaved trade-off between the ASE and BER under various channel environments. The ASE (i.e., throughput) can be maximized with a proper choice of the channel quality threshold and AQAM mode switching threshold levels for a target BER.