Feedback Quantization Errors Research Articles

Multi-User Analog Beamforming in Millimeter Wave MIMO Systems Based on Path Angle Information

We aim to design an analog-only beamforming scheme for downlink multi-user mm-wave systems to optimize the beamforming gain and the inter-user interference at the same time. Traditional analog beamforming schemes, such as the beam selection method, use the array response vector corresponding to the strongest path of the channel to generate a beam pointing to the user. In multi-user systems, such schemes will lead to large inter-user interference, especially when the users are closely located. In this paper, we formulate a multi-objective problem to strike a balance between the beamforming gain and the inter-user interference. To solve the problem, we first use the weighted-sum method to transform the multi-objective problem into a single-objective problem. Then, we use the semi-definite programing technique to make the analog beamforming with constant-magnitude constraints tractable. Furthermore, to alleviate the effects of the channel estimation and feedback quantization errors, we design a robust beamforming scheme to provide robustness against imperfect channel information. We first develop a channel error model for the scattering clustered channel model, which can serve as a general channel error model for the mm-wave channels. Then, we formulate a multi-objective problem using the stochastic approach to suppress the interference and enhance the beamforming gain at the same time. The simulation results show that our proposed non-robust multi-user analog beamformer outperforms the traditional analog beamforming method when the SNR is high and our proposed robust beamformer can provide up to 109% improvement in the sum-rate compared with the beam selection method.

Performance Analysis of Maximal-Ratio Transmission/Receive Antenna Selection in Nakagami- $m$ Fading Channels With Channel Estimation Errors and Feedback Delay

This paper focuses on multi-antenna systems that employ both maximal-ratio transmission and receive antenna selection (MRT&RAS) in independent and identically distributed Nakagami-m flat-fading channels with channel estimation errors (CEE) [or feedback quantization errors (FQE)] and feedback delay (FD). Useful statistics of the postprocessing signal-to-noise ratio (SNR) such as the probability density function, cumulative distribution function, moment-generating function, and th-order moments are presented. To examine the capacity and the error performances of the MRT&RAS scheme, ergodic capacity, outage probability, and bit error rates/symbol error rates (BERs/SERs) for binary and M-ary modulations are analyzed. Exact analytical expressions for all the performance metrics are separately derived for practical situations that consist of CEE (or FQE) and FD, as well as ideal estimation and feedback conditions. By considering approximations for the statistics of the postprocessing SNR, it is shown that, at high SNRs, the investigated system achieves full-diversity orders in the ideal case, whereas it keeps maintaining receive diversity at practical impairments. The performance superiority of the MRT&RAS scheme to other diversity schemes is also shown by numerical comparisons. In addition, analytical performance results related to the outage probability and BERs/SERs are validated by Monte Carlo simulations.

Performance analysis of hybrid transmit antenna selection/maximal-ratio transmission in Nakagami-mfading channels

Hybrid diversity systems have been of great importance because they provide better diversity orders and robustness to the fading effects of wireless communication systems. This paper focuses on the performance analysis of multiple-input gle-output systems that employ combined transmit antenna selection (TAS)/maximal-ratio transmission (MRT) techniques (i.e., hybrid TAS/MRT). The probability density function, the moment generating function and the nth order moments of the output signal-to-noise ratio of the investigated diversity scheme are derived for independent identically distributed flat Nakagami-m fading channels. The system capacity of the hybrid TAS/MRT scheme is examined from the outage probability perspective. Exact bit/symbol error rate (BER/SER) expressions for binary frequency shift keying, M-ary phase shift keying and square M-ary quadrature amplitude modulation signals are derived by using the moment generating function-based analysis method. By deriving the upper bounds for BER/SER expressions, it is also shown that the investigated systems achieve full diversity orders at high signal-to-noise ratios. Also, by Monte Carlo simulations, analytical performance results are validated and the effect of feedback delay, channel estimation error and feedback quantization error on BER/SER performances are examined. Copyright © 2011 John Wiley & Sons, Ltd.