A multiuser dual-hop relaying system over mixed radio frequency/free-space optical (RF/FSO) links is investigated. Specifically, the system consists of m single-antenna sources, a relay node equipped with n≥ m receive antennas and a single photo-aperture transmitter, and one destination equipped with a single photo-detector. RF links are used for the simultaneous data transmission from multiple sources to the relay. The relay operates under the decode-and-forward protocol and utilizes the popular V-BLAST technique by successively decoding each user's transmitted stream. Two common norm-based orderings are adopted, i.e., the streams are decoded in an ascending or a descending order. After V-BLAST, the relay retransmits the decoded information to the destination via a point-to-point FSO link in m consecutive timeslots. Analytical expressions for the end-to-end outage probability and average symbol error probability of each user are derived, while closed-form asymptotic expressions are also presented. Capitalizing on the derived results, some engineering insights are manifested, such as the coding and diversity gain of each user, the impact of the pointing error displacement on the FSO link and the V-BLAST ordering effectiveness at the relay.

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Multiuser Relaying over Mixed RF/FSO Links

Multi-user multiple input and multiple output (MU-MIMO) is one predominate approach to improve the wireless capacity. However, since the aggregate capacity of MU-MIMO heavily depends on the channel correlations among the mobile users in a beamforming group, unwisely selecting beamforming groups may result in reduced overall capacity, instead of increasing it. How to select users into a beamforming group becomes the bottleneck of realizing the MU-MIMO gain. The fundamental challenge for user selection is the large searching space, and hence there exists a tradeoff between search complexity and achievable capacity. Previous works have proposed several low complexity heuristic algorithms, but they suffer a significant capacity loss. In this paper, we present a novel MU-MIMO MAC, called SIEVE. The core of SIEVE design is its scalable multi-user selection module that provides a knob to control the aggressiveness in searching the best beamforming group. SIEVE maintains a central database to track the channel and the coherence time for each mobile user, and largely avoids unnecessary computing with a progressive update strategy. Our evaluation, via both small-scale testbed experiments and large-scale trace-driven simulations, shows that SIEVE can achieve around 90% of the capacity compared to exhaustive search.

SIEVE: Scalable user grouping for large MU-MIMO systems

The minimum-mean-square error (MMSE) plays a significant role in the signal detection process of massive multiple-input-multiple-output (MIMO) systems Matrix inversion, which is the major part of calculating the MMSE, suffers from high computing loads and low parallelism, especially in massive MIMO systems; as such, hardware implementation is difficult This paper proposes a user-level parallelism-based fully pipelined very large-scale integration (VLSI) architecture of an MMSE detector for an uplink $128\times 8~64$ -QAM massive MIMO system First, a diagonal-based systolic array with single-sided input is adopted; this array eliminates the throughput limitation Second, a weighted Jacobi-iteration-based architecture is proposed to iteratively achieve matrix inversion, thereby reducing the computational load and exploiting the potential parallelism of the matrix inversion Third, an approximated architecture is proposed to compute the log-likelihood ratio This architecture is verified on an FPGA and fabricated onto a 257 mm2 silicon with TSMC 65 nm CMOS technology, thereby achieving a 102 Gbps data rate at 680 MHz while dissipating 646 mW The results indicate an energy efficiency of 158 Gbps/W and an area efficiency of 040 Gbps/mm2, which are $293\times $ and $286\times $ that of state-of-the-art similar designs with CMOS technology, respectively

A 1.58 Gbps/W 0.40 Gbps/mm2 ASIC Implementation of MMSE Detection for $128\times 8~64$ -QAM Massive MIMO in 65 nm CMOS

In this paper, the modulation diversity is used to improve the performance of M-PSK modulation over fading channels. Modulation diversity can be achieved by rotating the signal constellation and using component interleaving. We derive symbol error probability expressions for rotated uncoded M-PSK over Ricean fading channels and obtain optimal rotation angles for M-PSK (M = 2, 4, 8). We show that rotated signal constellations with component interleaving improve the performance of M-PSK significantly as compared to the unrotated one over Rayleigh and Ricean fading channels. For example, when the ratio of the direct path power to the multipath signal power, K is 0 and 10, 8 and 1.5 dB gains are obtained, respectively, at a symbol error probability of 10?3 for 8PSK modulation. We also show that as K gets larger, the gain obtained by the rotation rapidly decreases. We develop a new asymmetric 8PSK signal constellation obtained from two QPSK signal constellations that are optimally rotated by different angles. This asymmetric 8PSK and also the rotated 8PSK signal constellation together with component interleaving are applied to four-state trellis-coded schemes. Simulation results show that these new schemes provide good performance improvements over the original TCM schemes and previous relevant works over Rayleigh and Ricean fading channels.

Error Performance of Rotated Phase Shift Keying Modulation over Fading Channels

We consider a multiuser system where a single transmitter equipped with multiple antennas (the base station) communicates with multiple users each with a single antenna. Regularized channel inversion is employed as the precoding strategy at the base station. Within this scenario we are interested in the problems of power allocation and user admission control so as to maximize the system throughput, i.e., which users should we communicate with and what power should we use for each of the admitted users so as to get the highest sum rate. This is in general a very difficult problem but we do two things to allow some progress to be made. Firstly we consider the large system regime where the number of antennas at the base station is large along with the number of users. Secondly we cluster the downlink path gains of users into a finite number of groups. By doing this we are able to show that the optimal power allocation under an average transmit power constraint follows the well-known water filling scheme. We also investigate the user admission problem which reduces in the large system regime to optimization of the user loading in the system.

Optimal Power Allocation and User Loading for Multiuser MISO Channels with Regularized Channel Inversion

We integrate zero-forcing beamforming (ZFBF) with signal space diversity (SSD) and investigate its performance. The beamforming is applied at the transmitter assuming a closed-loop scenario. First, a dual-stream transmission scheme with $M$ $(M\geq 2)$ transmit and two receive antennas is studied. Then, a generalization for any number of simultaneous substreams is established based on the previous results. A statistical analysis of the proposed scheme is provided and an exact closed-form expression on the average bit error probability is derived. We also present a high signal-to-noise ratio analysis for gaining further insight into diversity level and coding gain of the proposed system. It is shown that the error performance of a ZFBF system can be improved by making use of SSD with only a negligible increase in complexity and no extra use of bandwidth/time resources.

Zero-Forcing Beamforming With Signal Space Diversity

In this work, signal space diversity (SSD) is incorporated into a multiple-input multiple-output system with maximal ratio combining and transmit antenna selection. The performance of the proposed system with phase-shift keying (PSK) modulation is studied under a slow flat Rayleigh fading channel scenario with the correlated receive antennas. By adopting the realistic exponential correlation model, an exact closed-form expression is derived for pairwise error probability. The optimal rotation angles are obtained for binary and quadrature PSK signal constellations. The theoretical analysis is verified by the numerical simulations. It is shown that the error performance of the system can be improved with almost no extra complexity or cost. It is also demonstrated that the proposed scheme with SSD is more resistant against the studied correlation model as compared to the original scheme without SSD.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-com.2017.0402

Performance analysis of maximal ratio combining with transmit antenna selection and signal space diversity under exponential antenna correlation

Simultaneous multiuser beamforming in multiantenna downlink channels can entail dirty paper (DP) precoding (optimal and high complexity) or linear precoding (suboptimal and low complexity) approaches. The system performance is typically characterized by the sum capacity with homogenous users with perfect channel state information at the transmitter. The sum capacity performance analysis requires the exact probability distributions of the user signal-to-noise ratios (SNRs) or signal-to-interference plus noise ratios (SINRs). The standard techniques from order statistics can be sufficient to obtain the probability distributions of SNRs for DP precoding due to the removal of known interference at the transmitter. Derivation of such probability distributions for linear precoding techniques on the other hand is much more challenging. For example, orthogonal beamforming techniques do not completely cancel the interference at the user locations, thereby requiring the analysis with SINRs. In this paper, we derive the joint probability distributions of the user SINRs for two orthogonal beamforming methods combined with user scheduling: adaptive orthogonal beamforming and orthogonal linear beamforming. We obtain compact and unified solutions for the joint probability distributions of the scheduled users' SINRs. Our analytical results can be applied for similar algorithms and are verified by computer simulations.

Unified Performance Analysis of Orthogonal Transmit Beamforming Methods with User Selection

In this paper, an outage probability analysis is carried out on zero-forcing (ZF) vertical Bell Labs Layered Space-Time (V-BLAST) algorithm based on a greedy selection of decoding order, which is optimal in diversity-multiplexing tradeoff sense. We provide a mathematical framework to make the analysis tractable and derive the probability density function expressions of the squared substream gains. Especially, we obtain a compact expression on the joint pdf of squared substream gains for any number of transmit and receive antennas. Moreover, using the analytical results, we illustrate the optimum cutoff value under optimal power allocation that minimizes the total consumed power for a given target total rate. Finally, we provide numerical results and verify our analysis by means of simulations.

An exact outage analysis of zero-forcing V-BLAST with greedy ordering

Coordinate interleaving (CI) is applied to an orthogonal frequency division multiplexing system to exploit signal space diversity with binary phase-shift keying modulation. Two different CI strategies are introduced taking the correlation coefficients among the subcarrier channel gains into account. Accordingly, a number of different scenarios are investigated. Assuming Rayleigh fading, an exact closed-form expression on the bit error probability is derived. It is shown that the error performance of the system can be increased to a great extent with a feedback cost of a few bits and only a negligible increase in complexity. The proposed system is also shown to enhance the communication privacy under limited feedback.

Serdar Ozyurt

Papers

Zero-Forcing Beamforming With Signal Space Diversity

Performance analysis of maximal ratio combining with transmit antenna selection and signal space diversity under exponential antenna correlation

Unified Performance Analysis of Orthogonal Transmit Beamforming Methods with User Selection

An exact outage analysis of zero-forcing V-BLAST with greedy ordering

Performance of OFDM With Signal Space Diversity Using Subcarrier Coordinate Interleaving