We present two interference alignment techniques such that an opportunistic point-to-point multiple input multiple output (MIMO) link can reuse, without generating any additional interference, the same frequency band of a similar pre-existing primary link. In this scenario, we exploit the fact that under power constraints, although each radio maximizes independently its rate by water-filling on their channel transfer matrix singular values, frequently, not all of them are used. Therefore, by aligning the interference of the opportunistic radio it is possible to transmit at a significant rate while insuring zero-interference on the pre-existing link. We propose a linear pre-coder for a perfect interference alignment and a power allocation scheme which maximizes the individual data rate of the secondary link. Our numerical results show that significant data rates are achieved even for a reduced number of antennas.

The increase of hardware processing capabilities together with the forthcoming introduction of several receive antennas on handsets offer mobile terminals the ability to set up various receivers within the same architecture, each receiver being matched to specific radio conditions and/or a particular power consumption level. This paper addresses the problem of selecting the most appropriate receiver in a dynamic and autonomous way, according to the current radio conditions and possibly to a performance/complexity trade-off. To this end, we propose a low-complexity method allowing the terminal to predict the output SINR of various linear receivers without activating any of them. This feature allows the terminal to select the most appropriate receiver among a high number of candidate receivers, based on a reliable estimation of their respective performance in the current environment. Moreover, we introduce a simple method allowing the terminal to select the receiver according to a performance/complexity trade-off. The relevance and the efficiency of the proposed method are demonstrated by means of Monte-Carlo simulations in realistic WCDMA mobile network radio environments.

This paper introduces a low-complexity criterion enabling a reconfigurable receiver to select dynamically the most appropriate configuration between a Rake receiver and a chip-level equalizer in the downlink of a DS-CDMA system. To this end, the proposed criterion accounts for the frequency selectivity of the channel and the signal to noise ratio in a natural and integrated fashion, so that the criterion value provides a rough estimate of the minimum performance gain the equalizer can achieve with respect to the Rake receiver. This latter feature allows the receiver selection to be performed according to a performance/complexity trade-off, without activating any of the receivers. Simulation results in the context of a WCDMA downlink show the practical efficiency of the proposed approach

This paper deals with the cancellation of the interference caused by the HSDPA multicode and multiuser access on a frequency selective channel. Since HSDPA spreading codes are constant on one frame, the unknown codes can be estimated. A hybrid PIC/SIC receiver using two different soft interference cancellation methods is proposed. It outperforms the PIC and the LMMSE receivers for several scenarios

In this paper, we investigate the performance of two linear receivers for code-division multiple-access (CDMA) downlink transmissions over frequency-selective channels, the users having possibly different powers. The optimum minimum mean-square error (MMSE) receiver is first considered. Because this receiver requires the knowledge of the code vectors attributed to all the users within the cell when these vectors are time varying, its use may be unrealistic in the forward link. A classical suboptimum receiver, consisting in a chip rate equalizer followed by a despreading with the code of the user of interest, is therefore studied and compared to the optimum MMSE receiver. Performance of both receivers is assessed through the signal-to-interference-plus-noise ratio (SINR) at their outputs. The analytical expressions of these SINRs depend in a rather nonexplicit way on the codes allocated to the users of the cell, and are therefore not informative. This difficulty is dealt with by modeling the users code matrix by a random matrix. Because the code matrices used in the forward link are usually isometric, the code matrix is assumed to be extracted from a Haar-distributed random unitary matrix. The behavior of the SINRs is studied when the spreading factor and the number of users converge to /spl infin/ at the same rate. Using certain results of the free probability theory, we establish the fact that the SINRs converge almost surely toward quantities that depend only on the complex amplitudes of propagation channel paths. We then use the expressions of these SINR limits to discuss the influence of the various parameters on the performance of the receivers.

We address the problem of second order statistics estimation of a selective Rayleigh channel in the context of the wideband CDMA mode of the UMTS. The data to be transmitted are sent over slots over which the channel is assumed to remain constant. Each slot contains a pilot symbol sequence from which the channel can be estimated. The covariance matrix of the channel is usually estimated by a denoised version of the empirical covariance matrix of the trained channel estimate. However, this estimate is not consistent in the UMTS context. We propose a new consistent estimate of the channel covariance matrix, and evaluate the performances of two Wiener-like channel estimation schemes based on the proposed estimate.