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

In this letter, we offer a low-complexity multiple access method based on coordinate interleaving coined as quadrature non-orthogonal multiple access (NOMA). It is shown that by properly utilizing two quadrature carriers, both error performance enhancement and reduction in the number of successive interference cancellation (SIC) operations can be attained as compared to the standard power-domain NOMA approach. Specifically, in a $K$ -user network with single-antenna nodes, the reduction in the number of SIC operations is more than 50% as compared to the original NOMA scheme. When $K=2$ , the power gain obtained by the inspected technique can be as large as 2.5 dB without resorting to any SIC process at all. Further, as compared to the original NOMA idea, the considered approach requires fewer number of SIC stability conditions on power allocation and functions for any power allocation strategy (without an error floor) when $K=2$ . Some similar gains are also applicable for $K>2$ . The decrease in the number of SIC operations manifests itself by boosting the immunity of the system against possible failures during the SIC process.

Quadrature NOMA: A Low-Complexity Multiple Access Technique With Coordinate Interleaving

We derive the joint probability distribution of zero-forcing (ZF) V-BLAST gains under a greedy selection of decoding order and no error propagation. Unlike the previous approximated analyses, a mathematical framework is built by applying order statistics rules and an exact closed-form joint probability density function expression for squared layer gains is obtained. Our analysis relies on the fact that all orderings are equiprobable under independent and identical Rayleigh fading. Based on this idea, we determine the joint distribution of the ordered gains from the joint distribution of the unordered gains. Our results are applicable for any number of transmit and receive antennas. Although we present our analysis in a ZF V-BLAST setting, our analytical results can be directly applied for the dual cases of ZF V-BLAST. Under the assumption of a low rate feedback of decoding order to the transmitter, a benefit of having exact expressions is illustrated by the calculation of the cutoff value under optimal power allocation that maximizes the sum of the substream outage capacities under a given sum power constraint. We provide numerical results and verify our analysis by means of simulations.

Exact Joint Distribution Analysis of Zero-Forcing V-BLAST Gains with Greedy Ordering

In this letter, an exact performance analysis is presented on the sum rate of zero-forcing beamforming with a greedy user selection algorithm in a downlink system. Adopting water-filling power allocation, we derive a compact form for the joint probability density function of the scheduled users' squared subchannel gains when a transmitter with multiple antennas sends information to at most two scheduled users with each having a single antenna. The analysis is verified by numerical results.

Serdar Ozyurt

Papers

Error Performance of Rotated Phase Shift Keying Modulation over Fading Channels

Quadrature NOMA: A Low-Complexity Multiple Access Technique With Coordinate Interleaving

Exact Joint Distribution Analysis of Zero-Forcing V-BLAST Gains with Greedy Ordering

Exact Joint Distribution Analysis of Zero-Forcing V-BLAST Gains with Greedy Ordering

Performance Analysis of Optimum Zero-Forcing Beamforming with Greedy User Selection