Single-input Multiple-output Orthogonal Frequency-division Multiplexing Research Articles

Joint beamforming optimization for reconfigurable intelligent surface-enabled MISO-OFDM systems

In this paper, we investigate the reconfigurable intelligent surface (RIS)-enabled multiple-input-single-output orthogonal frequency division multiplexing (MISO-OFDM) system under frequency-selective channels, and propose a low-complexity alternating optimization (AO) based joint beamforming and RIS phase shifts optimization algorithm to maximize the achievable rate. First, with fixed RIS phase shifts, we devise the optimal closed- form transmit beamforming vectors corresponding to different subcarriers. Then, with given active beamforming vectors, near-optimal RIS reflection coefficients can be determined efficiently leveraging fractional programming (FP) combined with manifold optimization (MO) or majorization-minimization (MM) framework. Additionally, we also propose a heuristic RIS phase shifts design approach based on the sum of subcarrier gain maximization (SSGM) criterion requiring lower complexity. Numerical results indicate that the proposed MO/MM algorithm can achieve almost the same rate as the upper bound achieved by the semidefinite relaxation (SDR) algorithm, and the proposed SSGM based scheme is only slightly inferior to the upper bound while has much lower complexity. These results demonstrate the effectiveness of the proposed algorithms.

Focusing gain analysis of time-reversal precoding in MISO OFDM communication systems

Emerging communication systems can benefit from time-reversal (TR) technology thanks to its good spatio-temporal signal focusing effect. The recent advances in low-cost wideband devices fabrication further leverage the use of TR wideband communication systems. The TR is generally carried out in the time domain and the focusing effect of TR comes from the use of a high transmit rate back-off factor (BOF), which is the signal up-sampling rate. In spite of the widely-used orthogonal frequency-division multiplexing (OFDM) modulation, few works have investigated the frequency-domain (FD) TR precoding in combination with OFDM communication systems. Furthermore, most existing works on FD/TR precoding rely on multiple-antenna technology at the transmitter to create the focusing effect. In this paper, we investigate the focusing gain provided by TR precoding in multiple-input single-output (MISO) OFDM systems. In particular, we compare the communication performance of such system at the intended position and that at the unintended position. Based on the analysis, we demonstrate that increasing the BOF and/or the number of transmit antennas significantly improves the focusing effect at the intended position. In contrast, the unintended positions receive less useful power. We derive approximated mean-square-error (MSE) expressions of equalized received signals at both intended and unintended positions. The subsequent focusing gain is presented as a function of the BOF and the number of antennas, enabling us to gain insights in the contributions of each parameter to the system performance. Numerical simulations with multi-path Rayleigh fading channels are carried out to validate the MSE expressions.

ML-Type EM-Based Estimation of Fast Time-Varying Frequency-Selective Channels Over SIMO OFDM Transmissions

This paper investigates the problem of fast time-varying frequency-selective (i.e., multipath) channel estimation over single-input multiple-output orthogonal frequency-division multiplexing (SIMO OFDM)-type transmissions. We do so by tracking the variations of each complex gain coefficient using a polynomial-in-time expansion. To that end, we derive the log-likelihood function (LLF) both in the data-aided (DA) and non-data-aided (NDA) cases. The DA maximum likelihood (ML) estimates over fast SIMO OFDM channels are derived here for the first time in closed-form expressions and hereby shown to be limited to applying over each receive antenna the DA least squares (LS) estimator tailored in [1] to fast SISO OFDM channels. This DA ML is used to initialize periodically, over a relatively large number of data blocks (i.e., with further reduced and relatively close-to-negligible pilot overhead compared to DA ML), a new expectation maximization (EM) ML-type solution we developed here in the NDA case to iteratively maximize the LLF. We also introduce an alternative regularized DA ML (RDM) initialization solution no longer requesting - in contrast to DA ML - more per-carrier pilot frames than the number of paths to further reduce overhead without incurring significant performance losses. Simulation results show that the proposed hybrid ML-EM estimator (i.e., combines all new NDA ML-EM and DA ML or RDM versions) converges within few iterations, thereby providing very accurate estimates of all multipath channel gains. Most importantly, this increased estimation accuracy translates into very significant BER and link-level per-carrier throughput gains over the best representative benchmark solution available so far for the problem at hand, the SISO DA LS technique in [1] with its new generalization here to SIMO systems.

An Improved Selective Mapping Technique to Reduce Peak-to-Average Power Ratio in SISO and SIMO OFDM Systems Without Side Information

Selective mapping (SLM) is an attractive method to reduce the peak-to-average power ratio (PAPR) without causing any signal distortion in orthogonal frequency division multiplexing (OFDM) systems. The main drawback of this technique is the requirement of sending side information (SI) to the receiver for each data block which results in huge bandwidth and power overheads. Also, it may degrade the bit error rate (BER) performance of the system due to the corruption of SI over the channel. In this paper, we propose a novel technique to reduce PAPR in single input single output and single input multiple output OFDM systems using Hadamard code-based SLM. The proposed method eliminates the need for explicit SI resulting in better bandwidth and power efficiency. The basic idea is to embed the SI of each data block within the data block itself at some specific locations using special multiplying sequences. Furthermore, the proposed scheme yields superior PAPR reduction when compared to that of the conventional SLM for an equivalent system complexity. Simulation studies show that the receiver achieves a better BER performance with the proposed technique as compared to the conventional SLM.

Post-FFT Beamforming in a Pilot-Assisted MIMO-OFDM System

It has been shown that by employing beamforming in the receiver of a Single-Input Multiple-Output-orthogonal frequency division multiplexing (SIMO-OFDM) system its performance has been greatly improved by suppressing co-channel interferences and mitigating fading effects of the channel. To get even greater performance applying a similar beamforming technique to the transmitter side of a Multiple-Input Multiple-Output (MIMO)-OFDM system was further employed. However, while in SIMO-OFDM systems both blind and reference-based methods have been investigated, in MIMO-OFDM systems only algorithms based on blind methods have been proposed so far. In this article we develop a reference-based joint transmit---receive beamforming technique, based on pilot symbols, in a MIMO-OFDM system. Post-FFT scheme is used in both sides as the beamforming method. Appropriate adaptive algorithms are developed to obtain the joint optimal beamforming weights in the receiver by minimizing the error signal between the estimated pilot symbols and their actual values. Receiver weights are used locally whereas transmitter weights are sent to the transmitter through a low-rate feedback channel similar to blind methods. The effect of various factors on the performance of the proposed beamforming structure is considered and it is shown that the joint transmit---receive beamforming is more effective than the sole receive beamforming although the improvement due to transmit beamforming is limited by the total transmit power constraint.

Cross‐relation‐based frequency‐domain blind channel estimation with multiple antennas in orthogonal frequency division multiplexing systems

Frequency domain blind channel estimation methods for orthogonal frequency division multiplexing (OFDM) systems with multiple antennas are proposed, in which the cross relation between the channel gains and a single snapshot of the received signal on each subcarrier is utilised. Necessary and sufficient conditions for the identifiability of a single-input multiple-output OFDM channel estimator are derived and, furthermore, using a first-order perturbation analysis, the approximation of mean-squared-error (MSE) is analytically obtained, which shows that the proposed estimator is approximately unbiased at high signal-to-noise ratio. The proposed estimator exhibits lower MSEs than those of both the existing subspace-based and deterministic blind channel estimators and is less sensitive to the estimation error of the number of multipaths. Finally, the extension of the proposed channel estimation to multiple-input-multiple-output OFDM is presented.

Pilot-assisted data recovery for OFDM systems with blind selected mapping scheme and space-frequency block coding

Pilot-assisted blind selected mapping (PB-SLM) scheme for multiple input single output (MISO) orthogonal frequency division multiplexing (OFDM) systems is proposed. At the receiver, individual values of autocorrelation of two consecutive pilot sub-channel responses are evaluated for all the phase sequences available at the transmitter. The phase sequence index corresponding to the highest value of autocorrelation is considered as the side information (SI) and used for data recovery. Due to the robust phase sequence selection based on pilot symbols, data decoding performance of the PB-SLM scheme is substantially better than that of the recent blind SLM (B-SLM) scheme for MISO OFDM systems with space-frequency block coding, which is based on sub-optimal maximum likelihood of data symbols, and almost identical with that by the simplified SLM scheme with perfect SI. Moreover, computational complexity is significantly lower than that of the B-SLM scheme.

Optimized Joint Timing Synchronization and Channel Estimation for OFDM Systems

This paper addresses training-sequence-based joint timing synchronization and channel estimation for orthogonal frequency division multiplexing (OFDM) systems. The proposed approach consists of three stages. First, a coarse timing offset estimate is obtained. Then an advanced timing, relative timing indices, and channel impulse response estimates are obtained by maximum-likelihood estimation based on a sliding observation vector. Finally, the fine time adjustment based on the minimum mean squared error criterion is performed. The simulation results show that the proposed approach has excellent performance of timing synchronization in several channel models at low signal-to-noise ratio (SNR) which is smaller than 1dB. Moreover, for a low-density parity-check coded 1x2 single-input multiple-output OFDM system with maximum ratio combining, zero bit-error-rate is achievable using our proposed approach when SNR exceeds 1dB.

An Approximation Method for Evaluating SIMO-OFDM MRC System in Multipath Environment Where Delay Profile Exceeds the Guard Interval

Equivalent Transmission Path (ETP) model-based BER map method was proved to be used in fast estimating Bit Error Rate (BER) of Single-Input Single-Output Orthogonal Frequency Division Multiplexing (SISO-OFDM) system, where the delay spreading exceeds the guard interval. In this paper, we propose a method for evaluating transmission characteristics of Single-Input Multiple-Output Orthogonal Frequency Division Multiplexing (SIMO-OFDM) system with application to maximum ratio combining diversity at the reception. The simulation results show that the proposed approximation method is able to be used in estimating trasmission characteristics with high accuracy not only for SISO-OFDM system but also for SIMO-OFDM system.

Robust Precoding Schemes for Multi-User MISO-OFDM Downlink with Limited Time-Domain Feedback

In multi-user multiple-input single-output orthogonal frequency-division multiplexing (MISO-OFDM) downlinks with limited feedback, both linear precoders (LP) and Tomlinson-Harashima precoders (THP) experience performance degradation due to inaccurate channel state information at the transmitter (CSIT). This analysis treats the downlink channels as random quantities and exploits their second order statistics in robust precoding schemes to correct the errors introduced in the feedback procedure. The time-domain channel vectors are found to reduce the feedback overhead more than the frequency-domain vectors. A compression and restoration method and a codebook design are also given to obtain compact feedback quantities. Simulations show that the robust LP and THP are superior to the previous methods with tradeoffs possible between performance and feedback overhead.

Channel-aware multi-user uplink transmission scheme for SIMO-OFDM systems

The problem of medium access control (MAC) in wireless single-input multiple-output-orthogonal frequency division multiplexing (SIMO-OFDM) systems is addressed. Traditional random access protocols have low overheads and inferior performance. Centralized methods have superior performance and high overheads. To achieve the tradeoff between overhead and performance, we propose a channel-aware uplink transmission (CaUT) scheme for SIMO-OFDM systems. In CaUT, users transmit request-to-send (RTS) at some subcarriers whose channel gains are above a predetermined threshold. Using the channel state information provided by RTS, access point performs user selection with receive beamforming to decide which users can access and then broadcasts the selection results via clear-to-send (CTS) to users. We present a distributed power control scheme by using a simple fixed modulation mode. We optimize the modulation order and channel gain thresholds to maximize the separable packets subject to the bit-error-rate (BER) and temporal fairness requirements and the individual average transmit power constraints. The performance of CaUT scheme is analyzed analytically and evaluated by simulations. Simulation results show that CaUT can achieve more significant throughput performance than traditional random access protocols.

Equalisation of SIMO-OFDM systems with insufficient cyclic prefix in doubly selective channels

Time variations of a doubly selective wireless channel and insufficient cyclic prefix (CP) length of an orthogonal frequency division multiplexing (OFDM) transmission system cause intercarrier interference (ICI) and interblock interference (IBI) as significant limitations. This paper investigates the problem of joint ICI and IBI mitigation in single-input multiple-output OFDM (SIMO-OFDM) systems. It is assumed, unlike most existing literature, that the channel delay spread is larger than the CP, and also the channel varies on each OFDM block. First, doubly selective channel is modelled using basis expansion model (BEM) and a closed-form expression for signal-to-interference-plus-noise ratio (SINR) is derived. Then, a time-domain equaliser is developed, which maximises the SINR for all subcarriers. Moreover, a frequency-domain equalisation approach is proposed which is based on the MSE minimisation per tone. A low-complexity implementation of the per-tone equaliser is also derived. An important feature of the proposed equalisers is that no bandwidth expansion or redundancy insertion is required except for the CP. Finally, complexity comparison and simulation results over Rayleigh fading channel are provided to illustrate the effectiveness of the proposed approaches. Since both equalisers are designed in the frequency domain, they provide significant interference cancellation.

Performance evaluation of multicast MISO–OFDM systems

In this paper, we analyze the performance of multicast orthogonal frequency division multiplexing (OFDM) systems with single and multiple transmit antennas. We show that the resource allocation that includes the subcarrier allocation, bit loading, and the precoding vector selection in the multiple-input single-output (MISO) case is a difficult optimization problem. Consequently, we propose suboptimal algorithms based on the maximization of the sum data rate and the maximization of the minimum user data rate criteria. For practical application, we consider a complete transmission chain by combining powerful erasure codes with the proposed algorithms. Using this scheme, we guarantee that each user receives the same amount of information to decode the same data. Simulation results show that, for both single-input single-output (SISO)–OFDM and MISO–OFDM cases, the proposed multicast OFDM systems achieve gains over the worst user case algorithm.