Multiuser MIMO-OFDM Systems Research Articles

Alternating Optimization Based Hybrid Transceiver Designs for Wideband Millimeter-Wave Massive Multiuser MIMO-OFDM Systems

Alternating Optimization Based Hybrid Transceiver Designs for Wideband Millimeter-Wave Massive Multiuser MIMO-OFDM Systems

Deep Learning for Super-Resolution Channel Estimation in Reconfigurable Intelligent Surface Aided Systems

Reconfigurable intelligent surface (RIS) enables the configuration of the propagation environment. Channel estimation is an essential task in realizing the RIS-aided communication system. A RIS-aided multi-user multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication system involves cascaded channels with high dimensions and sophisticated statistics. Thus, implementing the optimal minimum mean square error (MMSE) with the integration computation is infeasible in practice. To accurately estimate channels with high accuracy in a RIS-aided multi-user MIMO-OFDM system, we model the channel state information (CSI) estimation as an image super-resolution (SR) problem to recover and denoise the channel matrix. Particularly, a convolutional neural network based on a super-resolution convolutional neural network (SRCNN) and denoising convolutional neural network (DnCNN), named SRDnNet, is then proposed. By taking estimated channels at pilot positions as a low-resolution image, the enhanced SRCNN can fully exploit the features of inputs to learn a suitable interpolation method and generate the coarse estimation of the channel matrix. The denoising model DnCNN with an element-wise subtraction structure can exploit features of the additive noise and recover channel coefficients from the coarse channel matrix. The simulation results demonstrate the effectiveness and excellent performance of the proposed SRDnNet.

A Phased Array Antenna with Novel Composite Right/Left-Handed (CRLH) Phase Shifters for Wi-Fi 6 Communication Systems

A linear phased array antenna excited with a novel composite right/left-handed (CRLH) phase shifters structure is proposed. The phase of the conventional CRLH transmission line is controlled with magnetically aligned micron-sized particles embedded inside the unit cell of the CRLH transmission line. The cascading of unit cells produces the desired phase shifts for the main beam scanning of the linear antenna array operating at a 5.5 GHz center frequency for Wi-Fi 6 applications. The proposed phase shifter design has a very low insertion loss (0.5–2 dB), excellent matching characteristics with the antenna array (less than −10 dB) and a small phase error (1–2 degrees). A 1 × 4 linear patch antenna phased array operating at a 5.5 GHz center frequency of the Wi-Fi 6 band is simulated using the Method of Moments (MoM) simulator platform. Then, the array is driven with the proposed novel CRLH phase shifters for the main beam at broadside and the main beam steered at 15- and 30-degree scan angles toward the desired users. For experimental validation, multiple unit cells of the proposed phase shifters are fabricated, and the 1 × 4 patch antenna array is fed with these fabricated unit cells of the phase shifters. The phased array radiation patterns are measured using an in-house fully calibrated anechoic chamber and were compared with simulated phased array patterns. The measured phased array patterns are in good agreement with the simulated patterns. As compared with commercially available phase shifters, the proposed novel CRLH phase shifters do not need external complex biasing circuitry, which is a major advantage in space constraint limitations at the router side of multi-user MIMO-OFDM systems.

Hybrid Beamforming and Data Stream Allocation Algorithms for Power Minimization in Multi-User Massive MIMO-OFDM Systems

A hybrid beamforming structure is suggested as one of the solutions to reduce implementation costs and energy consumption in millimeter-wave massive multiple-input, multiple-output (MIMO) systems. In this study, an optimization problem of resource allocation was formulated to minimize the total system transmission power on downlink under a certain quality-of-service (QoS), such as bit/block error rates and data rates for each user, and the solution was proposed therein. Our proposed stream incremental algorithm can dynamically adjust the number of data streams for each user according to the channel state information. Precoding and combining schemes need to be developed to solve the formulated problem and also are proposed in this paper to be paired up with the stream incremental algorithm. Our proposed algorithms consider the practical modulation and coding scheme for transmission in various data stream allocation and beamforming designs. The proposed algorithms provide beamforming solutions for millimeter-wave massive MIMO systems, which achieve comparable performance to that of a fully digital block-diagonalization (BD) algorithm with a lower implementation cost and outperform those of modified existing hybrid beamforming algorithms. Simulation results demonstrate the efficacy of the proposed schemes by allocating the different numbers of data streams for each user according to the channel state information. The simulation results verified that the proposed method can achieve a good trade-off between complexity and performance on comparison with the modified existing schemes and the full digital solutions.

Performance Evaluation of Uplink Multiuser MIMO-OFDM System With Single RF Chain Receiver

In response to the recent increase in wireless-traffic demand in various wireless communication networks, the number of spatial multiplexings, represented by multiple input multiple output (MIMO) systems, has been increasing. Assuming a realistic hardware configuration achieving an increase in the number of spatial multiplexings depends on increasing the amount of hardware, such as analog/digital (A/D) and D/A converters, and radio frequency (RF) chains. Simplifying the hardware configuration while maintaining large channel capacity was the challenging issue to further improve the channel capacity in Beyond-5G and 6G. This paper proposes a novel system configuration based on an uplink multiuser MIMO - orthogonal frequency division multiplexing (OFDM) system with a single RF chain receiver. By oversampling OFDM signals with a single RF chain receiver, our approach provides an additional spatial multiplexing by switching the antenna directivity pattern in synchronization with the received sampling frequency. Oversampled OFDM signals are typically used for noise reduction, in the proposed configuration, they are used for adding spatial multiplexings. We clarify the potential of the proposed configuration for improving the channel capacity by applying it to the uplink multiuser MIMO-OFDM system. To demonstrate its feasibility, we developed a testbed and analyzed its effectiveness in experiment in an actual indoor office environment.

Precoder and Postcoder Design for Wireless Video Streaming with Overloaded Multiuser MIMO-OFDM Systems

Precoder and Postcoder Design for Wireless Video Streaming with Overloaded Multiuser MIMO-OFDM Systems

Analyzing Impacts of Spatial Correlation for Multi-user Environment with Robust Concatenation of Advanced FEC Schemes

In recent days, the enhancement of MIMO in the area of wireless communications with spatial correlation characteristics by means of the forward-error-correction (FEC) techniques has been introduced as a vital technology. To overcome the limitations of the FEC in MIMO–OFDM systems, this paper introduces the effect of spatial correlation in FEC for multi-user MIMO–OFDM system. Here, the error correction codes (ECC) namely low-density parity check (LDPC-CC), turbo code CC, convolutional code (CC) and Reed–Solomon code (RSC-CC), and polar code CC (PC-CC) are authenticated for three channel representations known as Rayleigh, Rician and additive white Gaussian noise for multi-user (MU) with 8 users and single user MIMO–OFDM system. The investigation is carried out by means of four modulation techniques like, binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM)-16 and QAM-64. It is done under three antenna configurations like 2 × 2, 2 × 4, and 4 × 4. In addition, the peak signal to noise ratio is deployed to recognize the image transmission and the bit error rate, is employed to recognize the data transmission. Furthermore, from the experimental results, the PC-CC in MU system is found to offer better performance than the other configurations.

Rotman lens-based two-tier hybrid beamforming for wideband mmWave MIMO-OFDM system with beam squint

In this paper, we study the hybrid beamforming for the wideband mmWave multiuser MIMO-OFDM system. We characterize the frequency-dependent beam angle problem, i.e., beam squint effect. Firstly, we extend the mmWave channel model from the Saleh-Valenzuela representation with the consideration of beam squint. Secondly, a two-tier hybrid beamforming is proposed to achieve the maximum spectral efficiency in wideband mmWave multiuser MIMO-OFDM system. In the first tier, the Rotman lens array is adopted as the analog precoder to provide true-time-delay (TTD) and reduce the beam squint impairment. Moreover, the first-tier baseband precoder is designed based on block diagonalization (BD) to cancel inter-user interference. We further consider the finite spatial resolution and power leakage problem of the lens array. The second-tier hybrid precoder is proposed to collect the leak power with the phase shifter array. The optimal second-tier hybrid precoder design is formulated as the matrix factorization problem, and the two-tier hybrid precoding algorithm is based on the alternative minimization. It is found that both the beam squint and frequency-selective effect should be considered in the wideband mmWave system. Simulation results show that the proposed two-tier hybrid precoder could reduce beam squint impairment and inter-user interference, and could approximate to the optimal full-digital precoder with much fewer RF chains.

Analytical Characterization of Dual-Band Multi-User MIMO-OFDM System with Nonlinear Transmitter Constraints

Recent studies have shown that carrier-aggregated multi-user (MU) multi-input-multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems can deliver unprecedented improvements in network throughput and capacity when compared with existing systems. However, most of these studies fail to take into account the nonlinear constraints that limit the performance of such systems. This paper proposes an analytical framework to evaluate the end-to-end performance of a carrier-aggregated dual-band (DB) MU-MIMO-OFDM system in the presence of transmitter nonlinearity. Specifically, we derive the closed-form expression for signal-to-interference-plus-noise ratio (SINR) and further utilize this expression to evaluate the average capacity and symbol error rate of a nonlinear DB MU-MIMO-OFDM system in a multi-path Rayleigh fading channel. We show that the transmit preprocessing technique conventionally used to mitigate inter-user interference in a linear DB MU-MIMO-OFDM system fails to eliminate inter-user interference in a nonlinear DB MU-MIMO-OFDM system. As a consequence, there is an irreducible error floor which is function of number of aggregated bands as well as number of users present in the system. The irreducible error floor has been shown to considerably deteriorate the performance of nonlinear DB MU-MIMO-OFDM system. Simulation results are also reported to validate the efficacy of the proposed analysis.

Joint Transmission and Coding Scheme for High-Resolution Video Streams over Multiuser MIMO-OFDM Systems

Joint Transmission and Coding Scheme for High-Resolution Video Streams over Multiuser MIMO-OFDM Systems

A Successive SLNR Based Pre-processing Matrix for Downlink Multi-user MIMO Systems

Aiming to co-channel interference (CCI) for LTE downlink multi-user MIMO-OFDM system, maximizing the successive signal to leakage plus noise ratio (SSLNR) interference cancellation algorithm based on Pre-processing is proposed. The algorithm utilizes SSLNR based on precoding matrix scheme to process emission signal and the known leakages is canceled by dirty paper coding (DPC) algorithm at the transmitter. At the receiver, the received signal are treated by preconditioned matrix, the interference between different users are eliminated in further, the sum-capacity is improved by pre-processing Matrix. Meanwhile, the algorithm is not limited by the number of transmit and receive antennas. The performance improvement is verified by simulation, when BER is 10-4, signal noise ratio (SNR) is improved about 4dB.

A Novel Adaptive Tone Reservation Scheme for PAPR Reduction in Large-Scale Multi-User MIMO-OFDM Systems

In this letter, a novel adaptive tone reservation (ATR) scheme is proposed for the orthogonal frequency division multiplexing-based large-scale multiuser multiple-input multiple-output systems to deal with the inherent peak-to-average power ratio (PAPR) problem. The key idea of the proposed ATR scheme is to iteratively perform the tone reservation (TR) scheme on the antenna with the maximum PAPR. Simulation results validate that the ATR scheme offers better PAPR reduction performance and lower computational complexity than the conventional TR scheme.

Performance evaluation of chaotic spreading sequences in a multi-user MIMO-OFDM system

A transmission with an acceptable performance is an important issue and is a topic of immense interest in wireless communications. In this paper, we propose an innovative solution which hires chaos theory in a “multiple-input-multiple-output, orthogonal frequency division multiplexing, code-division multiple-access (MIMO OFDM-CDMA)” system. The main goal is to improve performance by using chaotic systems as spreading codes. Since autocorrelation is an important criterion in selecting spreading codes, we investigate the autocorrelations of chaotic codes so as to choose proper sequences for spreading. Simulation results have shown that this method has better performance in comparison with other spreading codes such as the Walsh–Hadamard (WH) codes. In other word, bit-error rate has been refined and more possible code sequences are available.

Main-Branch Structure Iterative Detection Using Approximate Message Passing for Uplink Large-Scale Multiuser MIMO Systems

The emerging large-scale/massive multi-input multioutput (MIMO) system combined with orthogonal frequency division multiplexing (OFDM) is considered a key technology for its advantage of improving the spectral efficiency. In this paper, we introduce an iterative detection algorithm for uplink large-scale multiuser MIMO-OFDM communication systems. We design a Main-Branch structure iterative turbo detector using the Approximate Message Passing algorithm simplified by linear approximation (AMP-LA) and using the Mean Square Error (MSE) criterion to calculate the correlation coefficients between main detector and branch detector for the given iteration. The complexity of our method is compared with other detection algorithms. The simulation results show that our scheme can achieve better performance than the conventional detection methods and have the acceptable complexity.

Performance Analysis of SNR Based Scheduling Scheme for MIMO OSFBC-OFDM Systems in η–µ Fading Channels

High performance of MIMO-OFDM wireless systems is very much desirable. In this paper, we study the performance analysis of scheduling schemes for multiuser MIMO-OFDM systems employing Orthogonal Space-Frequency Block Coding in η–µ fading scenario over frequency selective fading channels. We analyze the advantages of user scheduling in MIMO-OFDM systems employing OSFBC and adaptive modulation schemes. It can be noted that η–µ distribution includes Nakagami-m and Hoyt distributions as special cases. Analytical expressions for average spectral efficiency (ASE), average bit error rate (BER), and outage probability for scheduling schemes are derived. Both full-feedback and limited-feedback channel information scenarios are considered. Using the results obtained from both mathematical expressions and numerical simulations, the ASE and BER are compared and discussed.

Weighted-Combining Calibration on Multiuser MIMO Systems with Implicit Feedback

Implicit feedback is an approach that utilizes uplink channel state information (CSI) for downlink transmit beamforming on multiple-input multiple-output (MIMO) systems, relying on over-the-air channel reciprocity. The implicit feedback improves throughput efficiency because overhead of CSI feedback for change of over-the-air channel responses is omitted. However, it is necessary for the implicit feedback to calibrate circuitry responses that uplink CSI includes, because actual downlink and uplink channel responses do not match due to different transmit and receive circuitry chains. This paper presents our proposed calibration scheme, weighted-combining calibration (WCC); it offers improved calibration accuracy. In WCC, an access point (AP) calculates multiple calibration coefficients from ratios of downlink and uplink CSI, and then combines coefficients with minimum mean square error (MMSE) weights. The weights are derived using a linear approximation in the high signal to noise power ratio (SNR) regime. Analytical mean square error (MSE) of calibration coefficients with WCC and calibration schemes for comparison is expressed based on the linear approximation. Computer simulations show that the analytical MSE matches simulated one if the linear approximation holds, and that WCC improves the MSE and signal to interference plus noise power ratio (SINR). Indoor experiments are performed on a multiuser MIMO system with implicit feedback based on orthogonal frequency division multiplexing (OFDM), built using measurement hardware. Experimental results verify that the channel reciprocity can be exploited on the developed multiuser MIMO-OFDM system and that WCC is also effective in indoor environments. key words: multiple-input multiple-output (MIMO), channel reciprocity, implicit feedback, calibration, minimum mean square error (MMSE)

Low complexity multiuser detection with recursively successive zero-forcing and SIC based on nullspace for multiuser MIMO-OFDM system

To improve the spectrum efficiency, this paper considers the multiuser detection with the MU-MIMO technology for multiuser MIMO-OFDM system uplink with the same subcarrier shared by multiple users. A low complexity multiuser detection algorithm with recursively successive zero-forcing and successive interference cancellation (RSZF-SIC) based on nullspace is proposed. The RSZF process based on the block diagonalization (BD) technique eliminates the co-channel interference (CCI) by a recursive method based on the nullspace orthogonal theorem. The SIC process detects the user signals respectively with the reasonable user detection sequence based on the results of the RSZF process. The computational complexity of the proposed algorithm is effectively reduced by reducing the total number of singular value decomposition (SVD) operations and the dimension of the SVD matrix in the recursive procedure. The performance of the proposed algorithm is improved in terms of bit error rate and sum capacity of the system, especially in the high-SNR regime.

Grouped Channel Quantization and Antenna Combining for Multiuser MIMO OFDM Systems

Grouped Channel Quantization and Antenna Combining for Multiuser MIMO OFDM Systems

Erratum to: Adaptive Resource Allocation Algorithms for Multi-user MIMO-OFDM Systems

Erratum to: Adaptive Resource Allocation Algorithms for Multi-user MIMO-OFDM Systems

Low-Complexity Iterative Detection for Large-Scale Multiuser MIMO-OFDM Systems Using Approximate Message Passing

One of the challenges in the design of large-scale multiuser MIMO-OFDM systems is developing low-complexity detection algorithms. To achieve this goal, we leverage message passing algorithms over the factor graph that represents the multiuser MIMO-OFDM systems and approximate the original discrete messages with continuous Gaussian messages through the use of the minimum Kullback-Leibler (KL) divergence criterion. Several signal processing techniques are then proposed to achieve near-optimal performance at low complexity. First, the principle of expectation propagation is employed to compute the approximate Gaussian messages, where the symbol belief is approximated by a Gaussian distribution and then the approximate message is calculated from the Gaussian approximate belief. In addition, the approximate symbol belief can be computed by the a posteriori probabilities fed back from channel decoders, which reduces the complexity dramatically. Second, the first-order approximation of the message is utilized to further simplify the message updating, leading to an algorithm that is equivalent to the AMP algorithm proposed by Donoho <etal xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"/> Finally, the message updating is further simplified using the central-limit theorem. Compared with the single tree search sphere decoder (STS-SD) and the iterative (turbo) minimum mean-square error based soft interference cancellation (MMSE-SIC) in the literature through extensive simulations, the proposed message passing algorithms can achieve a near-optimal performance while the complexity is decreased by tens of times for a 64 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\times$</tex></formula> 64 MIMO system. In addition, it is shown that the proposed message passing algorithms exhibit desirable tradeoffs between performance and complexity for a low-dimensional MIMO system.