Abstract-Orthogonal random beamforming (ORB) constitutes a mean to exploit spatial multiplexing and multi-user diversity (MUD) gains in multi-antenna broadcast channels. To do so, as many random beamformers as transmit antennas (M ) are generated and on each beam the user experiencing the most favorable channel conditions is scheduled. Whereas for a large number of users the sum-rate of ORB exhibits an identical growth rate as that of dirty paper coding, performance in sparse networks (or in networks with an uneven spatial distribution of users) is known to be severely impaired. To circumvent that, in this paper we modify the scheduling process in ORB in order to select a subset out of the M available beams. We propose several beam selection algorithms and assess their performance in terms of sum-rate and aggregated throughput (i.e., rate achieved with practical modulation and coding schemes), along with an analysis of their computational complexity. Since ORB schemes require partial channel state information (CSI) to be fed back to the transmitter, we finally investigate the impact of CSI quantization on system performance. More specifically, we prove that most of the MUD can be still exploited with very few quantization bits and we derive a beam selection approach trading-off system performance vs. feedback channel requirements.Index Terms-Orthogonal Random Beamforming (ORB), beam selection, sparse networks, opportunistic scheduling, Multiuser Diversity (MUD), broadcast channel, feedback quantization.

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A multicell WiMax system which supports orthogonal frequency division multiplexing (OFDM) and antenna arrays at the base stations is considered in this paper as conforming to the IEEE 802.16-2004 standard. Focusing on the uplink, we propose an analytical framework to assess the average error probability of the system over time-dispersive (or, equivalently, frequency selective) and space-dispersive (due to the antenna array) Rayleigh fading channels. The proposed method takes into account the effects of the correlation of the channel gains over the space-frequency domain, the power-angle structure of the inter-cell interference, the array processing at the base station and the interleaving scheme. , which prescribes the employment of OFDM modulation and supports antenna array technology. The pushing demand for broadband systems makes WiMax one of the most promising technologies for the near future in wireless communications. The system is expected to operate in heterogeneous propagation environments as applications range from the provisioning of wireless services in rural areas to intensive and real-time applications on notebooks and other mobile devices. Thereby, a thorough analysis of its performance under realistic and different propagation environments is mandatory.Several works have recently focused on the evaluation of the error rate of bit-interleaved coded OFDM systems over frequency selective fading channels [3]- [5]. To simplify the performance evaluation, the concept of effective signal-to-noise ratio (SNR) has been introduced in [3] as the SNR of an equivalent additive white Gaussian noise (AWGN) channel which would yield the same error probability as that of the considered frequency-selective channel. The effective SNR can be used to adapt modulation and coding to the instantaneous channel conditions (and the service requirements) or to assess the average error rate for a given transmission mode [4]. Average and outage system performances have also been derived in [5] for multiantenna combining techniques in Nakagami-fading propagation environments, while the effects of non-stationary intercell interference on the performance of multiantenna WiMax systems have been studied in [6] [7].

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