Laurence Eugene Mailaender

CDMA downlink transmission with transmit antenna arrays and power control in multipath fading channels

Wireless code-division multiple-access (CDMA) cellular downlink communications with transmit antenna arrays in multipath fading channels is studied. Various array signal processing techniques at the transmit end are investigated and compared under various settings, in conjunction with power control. No instant downlink channel information is assumed; however, the obtained results are also compared with results assuming ideal feedback. The study is carried out for both circuit-switched and packet-switched systems, where different goals are pursued and different conclusions are drawn. In particular, it is found that the traffic type impacts the algorithm choice in downlink transmission, and that there is no need to seek optimum power control/allocation schemes, which are either too complex or infeasible in practice. Another interesting conclusion is that, even though feedback does not help much for packet-switched systems, it does help for circuit-switched systems, the gain of which increases with the number of antennas.

Indoor Network MIMO Performance with Regularized Zero-Forcing Transmission

Network MIMO uses the coherent coordination of the transmitted signals from a cluster of bases to greatly reduce other-cell interference and improve spectral efficiency. Zero-forcing transmission is a simple and readily implemented form of Network MIMO that can give good performance gains over conventional systems. In this paper, we consider regularized ZF as a simple modification and explore the potential benefits for an indoor wireless system comprising 8 access points. The ZF solution is scaled to obey either sum-power or per-antenna power constraints. In the context of this simplified evaluation we find that regularization gives no gain in indoor systems, but the results hint that gain may be achieved in outdoor systems.

Adaptive multi-user detection of fading CDMA channels using antenna arrays

We consider the problem of detecting a number of short-code CDMA signals that impinge (bit-) synchronously on the antenna array of a base station (reverse link). The signals are assumed to be corrupted by both multipath fading and multiple access interference (MAI). To boost performance (measured either in BER for each user or in user capacity), we are interested in using jointly for the signal detection three key signal detection technologies: (1) RAKE processing, (2) array processing, and (3) multi-user detection. The resulting space-time multi-user detectors (ST-MUDs) have been shown in previous works to offer significant improvements over previously existing techniques. In this paper we focus on a specific type of linear detector presented by Huang and Papadias (see Proc. 1998 PIMRC Conference, Boston, 1998), which has been shown to combine good performance with implementation advantages. We propose to update this linear detector with adaptive filtering algorithms, and in particular with stochastic-gradient type trained or blind algorithms. It is shown that, such simple techniques allow the receiver to approach the ST-MUD MMSE solution and achieve significant capacity gains with respect to conventional techniques.

A unified derivation and cutoff rate analysis of transmit diversity

We present a framework for systematically studying transmit diversity schemes with various levels of feedback about the channel coefficients from the receiver to the transmitter. We define a simple optimality criterion with respect to which we show that traditional schemes designed by conventional wisdom are optimal and also show that when only the channel gains are fed back the switched-time scheme is optimal. We then determine the channel cutoff rates for the different transmit diversity schemes.

CDMA cellular downlink transmission with transmit arrays and power control: circuit-switched and packet-switched systems

Wireless CDMA cellular downlink communications with transmit antenna arrays in multipath fading channels is studied. Transmit diversity and various beamforming techniques are investigated and compared, in conjunction with power control. No instantaneous downlink channel information is assumed; however, the obtained results are also compared with results assuming ideal feedback. The study is carried out for both circuit-switched and packet-switched systems, for which different conclusions are drawn.