Abstract
Hybrid precoding, a combination of radio frequency (RF) beamforming and digital precoding, has been investigated intensively these days for millimeter wave (mmWave) communication systems employing large antenna arrays. The key problem is constructing beamforming and precoding matrices for the RF beamformer and the digital baseband, respectively, based on the channel matrix decomposition result. This paper presents a new computing algorithm to achieve the matrix decomposition efficiently without compromising the performance. The algorithm computes beamforming (steering) and precoding matrices in separate phases to alleviate the computing overheads of iterative matrix updates. This measure also creates the computing parallelism to facilitate efficient hardware implementation. A novel computing scheme based on QR decomposition and blockwise inversion techniques is also developed to tackle the most critical least square solution module. This leads to a computing complexity reduction by a factor of $0.3~{N}$ when compared with the popular orthogonal matching pursuit (OMP) scheme, where ${N}$ is the antenna array size. The simulation results indicate the percentage of choosing correct steering vectors is 90%, which is as good as the OMP scheme can achieve. A hardware accelerator design of the proposed scheme is developed by using a TSMC 40 nm CLN40G technology. The design, with a gate count of 419.3 k, can operate up to 333 MHz with a power consumption of 267.1 mW. This suggests a throughput rate of processing 10.4 M channel matrices per second. The core size is merely 0.58 mm2 while the entire die size including I/O pads is 2.26 mm2 .
Highlights
Millimeter wave communication systems can provide multi-gigabit per second data rates in short distance applications [1]
We present a modified orthogonal matching pursuit (OMP) scheme, which decouples the constructions of FRF and FBB in order to reduce the computing complexity and to enhance the computing parallelism
The steering matrix FRF is computed first followed by solving a least square problem to obtain the precoding matrix FBB
Summary
Millimeter wave (mmWave) communication systems can provide multi-gigabit per second data rates in short distance applications [1]. Assume the channel state information (CSI) is known, multiple data streams can be transmitted with a precoder and a combiner employed in the transmitter and the receiver, respectively This creates an equivalent transmission channel consisting of parallel and independent communication links. The scheme in [17], by assuming the millimeter wave systems have low rank channel matrices and the correlations between channels (beams) are low, performs the orthogonal projection only after all basis vectors are selected This leads to an MP-like approach but can reduce the computing complexity significantly. Efficient hardware accelerator designs of hybrid precoding are essential to the implementations of millimeter wave massive MIMO systems It requires both delicate algorithm simplifications to curb the circuit complexity and extensive architecture optimizations to enhance the throughput.
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