Precoded BICM design for MIMO transmit beamforming and associated low-complexity algebraic receivers

Abstract-In this paper, we design bit interleaved coded modulations including a partial algebraic precoder in order to transmit several spatial streams with full diversity over a transmit beamformed MIMO channel. The achievable diversity orders with the coded modulation are derived, and allow for efficiently choosing the system parameters for optimizing the performance. Additionnally, a very efficient low complexity soft output detector based on algebraic reduction is presented. With a low complexity at the receiver, the precoded BICM system allows achieving high performance at very high efficiencies, e.g., 12 bits per second per hertz. I. INTRODUCTION The new standards of wireless communications, e.g., 802.11n and 3GPP-LTE, include a variety of techniques for transmitting over mutliple antenna channels: Spatial-division multiplexing, space-time block coding, transmit beamforming, and so on. A trade-off between data rate, range, mobility and receiver complexity can be achieved by a relevant choice of the transmission technique. In this paper, we focus on low mobility users and transmit beamforming techniques. For low and average spectral efficiencies, the MIMO channel capacity can be maximized by transmitting on the best eigenvalue of the channel realization and can be further improved by applying water-filling techniques In section II, we present the channel model and notations. In section III, the equivalent channels as seen by the decoder input and including a partial precoder are described, while achievable bounds on the diversity for these channels are derived in section IV. These bounds are used to choose the optimal size of the partial algebraic precoder. In section V, w

On Reduced Complexity Soft-Output MIMO ML detection

In multiple-input multiple-output (MIMO) fading channels maximum likelihood (ML) detection is desirable to achieve high performance, but its complexity grows exponentially with the spectral efficiency. The current state of the art in MIMO detection is list decoding and lattice decoding. This paper proposes a new class of lattice detectors that combines some of the principles of both list and lattice decoding, thus resulting in an efficient parallelizable implementation and near optimal soft-ouput ML performance. The novel detector is called layered orthogonal lattice detector (LORD), because it adopts a new lattice formulation and relies on a channel orthogonalization process. It should be noted that the algorithm achieves optimal hard-output ML performance in case of two transmit antennas. For two transmit antennas max-log bit soft-output information can be generated and for greater than two antennas approximate max-log detection is achieved. Simulation results show that LORD, in MIMO system employing orthogonal frequency division multiplexing (OFDM) and bit interleaved coded modulation (BICM) is able to achieve very high signal-to-noise ratio (SNR) gains compared to practical soft-output detectors such as minimum-mean square error (MMSE), in either linear or nonlinear iterative scheme. Besides, the performance comparison with hard-output decoded algebraic space time codes shows the fundamental importance of soft-output generation capability for practical wireless applications