Wideband Orthogonal Frequency Division Multiplexing Systems Research Articles

Low-Cost Passive Beamforming for RIS-Aided Wideband OFDM Systems

Reconfigurable intelligent surface (RIS) has become a promising solution to improve the performance of future wireless systems with low energy consumptions. Concerning a RIS-aided orthogonal frequency division multiplexing (OFDM) system, we perform optimization with respect to the RIS passive beamforming such that the sum rate over all subcarriers is maximized. Different from an existing successive convex approximation (SCA) method, we propose a majorization-minimization (MM)-based iterative approach to optimize the RIS passive beamforming, where a closed-form solution is obtained in each iteration. Simulation results show that the proposed RIS passive beamforming achieves almost the same performance as the SCA-based method while with much lower computational complexity.

Quantized Massive MIMO Systems With Multicell Coordinated Beamforming and Power Control

In this paper, we investigate a coordinated multipoint (CoMP) beamforming and power control problem for base stations (BSs) with a massive number of antenna arrays under coarse quantization at low-resolution analog-to-digital converters (ADCs) and digital-to-analog converter (DACs). Unlike high-resolution ADC and DAC systems, non-negligible quantization noise that needs to be considered in CoMP design makes the problem more challenging. We first formulate total power minimization problems of both uplink (UL) and downlink (DL) systems subject to signal-to-interference-and-noise ratio (SINR) constraints. We then derive strong duality for the UL and DL problems under coarse quantization systems. Leveraging the duality, we propose a framework that is directed toward a twofold aim: to discover the optimal transmit powers in UL by developing iterative algorithm in a distributed manner and to obtain the optimal precoder in DL as a scaled instance of UL combiner. Under homogeneous transmit power and SINR constraints per cell, we further derive a deterministic solution for the UL CoMP problem by analyzing the lower bound of the SINR. Lastly, we extend the derived result to wideband orthogonal frequency-division multiplexing systems to optimize transmit power and beamformer for all subcarriers. Simulation results validate the theoretical results and proposed algorithms.

Base Station Antenna Selection for Low-Resolution ADC Systems

For low-resolution analog-to-digital converter (ADC) systems, only high-complexity receive antenna selection has been developed and transmit antenna selection has been limited to a single antenna selection in prior work. In this paper, we propose low-complexity receive antenna selection algorithms and analyze transmit antenna selection by considering antenna selection at a base station with large antenna arrays and low-resolution ADCs. For downlink antenna selection, we show a selection criterion with zero-forcing precoding equivalent to a perfect quantization system; sum rate increases with number of selected antennas; derivation of the sum rate loss function from using a antenna subset; and sum rate loss reaches a maximum at a point of total transmit power and decreases beyond that point to converge to zero. For wideband orthogonal-frequency-division-multiplexing (OFDM) systems, our results hold when entire subcarriers share a common subset of antennas. For uplink antenna selection, we generalize a greedy antenna selection criterion; propose a quantization-aware fast antenna selection algorithm using the criterion; and derive a lower bound on sum rate achieved by the proposed algorithm. For wideband OFDM systems, we extend our algorithm and derive a lower bound on its sum rate. Simulation results validate theoretical analyses and show increases in sum rate over conventional algorithms.

Pilot Design for Sparse Channel Estimation in Orthogonal Frequency Division Multiplexing Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a well-known technique used in modern wide band wireless communication systems. Coherent OFDM systems achieve its advantages over a multipath fading channel, if channel impulse response is estimated precisely at the receiver. Pilot-aided channel estimation in wide band OFDM systems adopts the recently explored compressive sensing technique to decrease the transmission overhead of pilot subcarriers, since it exploits the inherent sparsity of the wireless fading channel. The accuracy of compressive sensing techniques in sparse channel estimation is based on the location of pilots among OFDM subcarriers. A sufficient condition for the optimal pilot selection from Sylow subgroups is derived. A Sylow subgroup does not exist for most practical OFDM systems. Therefore, a deterministic pilot search algorithm is described to select pilot locations based on minimizing coherence, along with minimum variance. Simulation results reveal the effectiveness of the proposed algorithm in terms of bit error rate, compared to the existing solutions.

A Robust Residual Frequency Error Estimator for UWB Application

In orthogonal frequency division multiplexing (OFDM) systems, the loss of orthogonality between the subcarriers due to carrier frequency offset (CFO) can degrade the performance of the system. In this paper, a residual CFO synchronization method is proposed for ultra wideband OFDM (UWB-OFDM) systems. The proposed fine frequency synchronization scheme uses a channel estimation sequence and physical layer convergence protocol (PLCP) header information which are equipped with frequency-domain spreading. A UWB-OFDM receiver with algorithm for fine frequency error synchronization is simulated and is evaluated in terms of mean square error.