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

ABSTRACT 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: We consider multi-input multi-output (MIMO) transmit beamforming under the uniform elemental power constraint. This is a non-convex optimization problem, and it is usually difficult to find the optimal transmit beamformer. First, we show that for the multi-input single-output (MISO) case, the optimal solution has a closed-form expression. Then we propose a cyclic algorithm for the MIMO case which uses the closed- form MISO optimal solution iteratively. The cyclic algorithm has a low computational complexity and is locally convergent under mild conditions. Moreover, we consider finite-rate feedback methods needed for transmit beamforming. We propose a novel vector quantization method, where the codebook is constructed under the uniform elemental power constraint and the method is referred as VQ-UEP. Numerical examples are provided to demonstrate the effectiveness of our proposed transmit beamformer designs and the finite-rate feedback technique.
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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.
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