Conference Paper

Equal Gain MIMO Beamforming in the RF Domain for OFDM-WLAN Systems.

DOI: 10.1007/978-3-642-16644-0_13 Conference: Mobile Lightweight Wireless Systems - Second International ICST Conference, MOBILIGHT 2010, Barcelona, Spain, May 10-12, 2010, Revised Selected Papers
Source: DBLP


Equal gain beamforming (EGB) schemes are typically ap- plied in the baseband domain and hence require complex RF transceivers. In order to simplify the circuitry and energy consumption of the MIMO transceiver, in this paper we consider an EGB scheme that operates in the RF domain by means of analog phase shifters. Under OFDM trans- missions, the design of the optimal phases is a complicated nonconvex problem with no closed-form solution. Building upon a previously pro- posed solution for at-fading MIMO channels, this paper describes an alternating minimization algorithm to nd an approximate (suboptimal) solution for the OFDM case. Monte-Carlo simulations are performed in order to demonstrate the eectiveness of this new analog beamforming scheme under coded and uncoded WLAN 802.11a transmissions.

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    ABSTRACT: In this paper, we study beamforming schemes for a novel MIMO transceiver, which performs adaptive signal combining in the radio-frequency domain. Assuming perfect channel knowledge at both the transmit and receive sides, we consider the problem of selecting the transmit and receive RF beamformers that maximize the capacity (MaxCAP criterion) of the system under orthogonal frequency division multiplexing (OFDM) transmissions. This problem is non-convex and has no closed-form solution, therefore the maximum capacity beamformers are found using a gradient search algorithm. Furthermore, it is shown in the paper that, for low signal-to-noise ratios (SNR), the MaxCAP criterion is equivalent to maximizing the received SNR (MaxSNR criterion). However, for moderate and high SNRs, the maximum capacity beamformers sacrifice part of the received SNR in order to improve the worst subcarriers and, in this way, they increase the overall capacity of the multicarrier channel. Finally, by means of numerical examples we show that the MaxCAP criterion significantly outperforms the MaxSNR criterion in terms of bit error rate and outage probability.
    Proceedings of IEEE International Conference on Communications, ICC 2009, Dresden, Germany, 14-18 June 2009; 01/2009
  • 01/2009; IEEE Computer Society.
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    ABSTRACT: Two arrays with M and N elements are connected via a scattering medium giving uncorrelated antenna signals. The link array gain relative to the case of one element at each end is treated for the situation where the channels are known at the transmitter and receiver. It is shown that the maximum mean gain achieved through adaptive processing at both the transmitter and the receiver is less than the free space gain, and cannot be expressed as a product of separate gains. First, by finding the singular values of the transmission matrix, fundamental limitations concerning the maximum gain and the diversity orders are given, indicating that the gain is upper bounded by (√M+√N) <sup>2</sup> and the diversity order is MN. Next an iterative technique for reciprocal channels which maximizes power at each stage transmitting back and forth is described. The capacity or spectral efficiency of the random channel is described, and it is indicated how the capacity is upper bounded by N parallel channels of gain M(N<M) for large values of N and M
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