Zero-forcing Precoding Scheme Research Articles

Joint SIC-Based Precoding and Sub-Connected Architecture Design for MIMO VLC Systems

Densely deployed light emitting diodes (LEDs) typically lead to high spatial correlation in multiple-input multiple-output (MIMO) visible light communication (VLC) systems. Although precoding can effectively alleviate the spatial correlation issue, most existing precoding algorithms require a dedicated baseband chain for each LED, leading to high energy consumption and hardware complexity when a large number of LEDs are used. In this paper, a successive interference cancellation (SIC)-based precoding scheme with sub-connected architecture (SIC-SA) is proposed. In the considered model, each baseband chain is connected to an LED sub-array containing multiple LEDs to reduce the complexity. Since, in this case, SIC-based precoding can only determine the signal of each baseband chain for an LED sub-array, while its target is to mitigate the spatial correlation between individual LEDs, the electrical/optical power of each LED must also be jointly optimized to accomplish the target. This joint SIC-based precoding, power allocation, and direct current offset design problem is formulated as an achievable sum rate maximization problem under dimming control and electrical power constraints. A two-step iterative algorithm is proposed to solve this problem. In the first step, the SIC-based precoding is designed to alleviate the multi-user interference. In the second step, the power allocation of LEDs is optimized by matrix decomposition and convex optimization, and a closed-form solution of DC offset is derived. Furthermore, considering the dynamic scenarios, a SIC-based precoding scheme with dynamic sub-connected architecture (SIC-DSA) is proposed, in which a switch network is used to adaptively adjust the LED sub-array structure based on the channel state information. Simulation results show that the proposed SIC-SA and SIC-DSA respectively achieve 0.1340 bps/Hz/W and 0.1305 bps/Hz/W energy efficiency gains over the zero-forcing precoding scheme with SA, when the signal-to-noise ratio is 30 dB.

Lower energy consumption in cache-aided cell-free massive MIMO systems

Cell-free (CF) massive MIMO systems provide uniform and fast service to users by the cooperation of multiple distributed access points (APs) at the expense of high energy consumption (EC) due to the heavy use of fronthaul and backhaul links. To tackle this issue, we propose a cache-aided cell-free (CCF) system where all APs are deployed with edge cache to reduce the load on links. In this paper, a scalable total EC model of CCF systems is established with two parts: EC of APs (ECAP) and EC for cache transmission (ECCT). An improved partial zero-forcing precoding scheme is proposed, which can provide high spectral efficiency and transmit few control data to reduce ECAP greatly. Then a minimum ECCT strategy is formulated. We transform ECCT minimization problem into a series of geometric programming problems by the successive convex approximation and an ingenious equivalent transformation. The obtained optimal cache placement makes the ECCT of CCF systems lower than that obtained by maximum hit ratio, equal probability and most popular caching strategies. Simulation results show that the proposed CCF systems are much more energy efficient than CF systems.

Spectral Efficiency of Network-Assisted Full-Duplex for Cell-Free Massive MIMO System Under Pilot Contamination

A cell-free massive multiple-input multiple-output (MIMO) network-assisted full-duplex (NAFD) system has been proposed to satisfy the exploding demand of higher data transmission speed and more efficient communication. However, a growing number of users in a cell-free system inevitably leads to pilot contamination. In this paper, we analyze the ergodic spectral efficiency of cell-free massive MIMO NAFD system in the presence of pilot contamination. The cell-free massive MIMO NAFD system model has been investigated and both uplink and downlink channel state information (CSI) is estimated under spatially correlated channels. Under pilot contamination, the closed-form sum-rate expressions of the uplink with maximum ratio combination (MRC) receiver while downlink with maximum ratio transmission (MRT), and uplink with zero-forcing (ZF) receiver while downlink with ZF precoding schemes are derived based on large-scale random matrix theory. Numerical results show that under several environmental settings, the theoretical results match well with the simulated results and cell-free massive MIMO NAFD system has a better performance than time-division duplex (TDD) system. Moreover, simulation results show that the achievable sum-rate of using ZF/ZF could be more spectrally efficient compared to MRC/MRT because of its interference suppression capability.

Ray-Tracing-Based Numerical Assessment of the Spatiotemporal Duty Cycle of 5G Massive MIMO in an Outdoor Urban Environment

In the near future, wireless coverage will be provided by the base stations equipped with dynamically-controlled massive phased antenna arrays that direct the transmission towards the user. This contribution describes a computational method to estimate realistic maximum power levels produced by such base stations, in terms of the time-averaged normalized antenna array gain. The Ray-Tracing method is used to simulate the electromagnetic field (EMF) propagation in an urban outdoor macro-cell environment model. The model geometry entities are generated stochastically, which allowed generalization of the results through statistical analysis. Multiple modes of the base station operation are compared: from LTE multi-user codebook beamforming to the more advanced Maximum Ratio and Zero-Forcing precoding schemes foreseen to be implemented in the massive Multiple-Input Multiple-Output (MIMO) communication protocols. The influence of the antenna array size, from 4 up to 100 elements, in a square planar arrangement is studied. For a 64-element array, the 95th percentile of the maximum time-averaged array gain amounts to around 20% of the theoretical maximum, using the Maximum Ratio precoding with 5 simultaneously connected users, assuming a 10 s connection duration per user. Connection between the average array gain and actual EMF levels in the environment is drawn and its implications on the human exposure in the next generation networks are discussed.

Performance Improvement in Large-Scale MU-MIMO System with Multiple Antennas on User Side in a Single-Cell Downlink System

Large-scale multi-user multiple input multiple output (LS-MU-MIMO) is a promising technology to meet the demands for higher spectral and energy efficiency in the next-generation communication system. LS-MU-MIMO technology can provide 50 times or more network throughput by enhancing the spectral efficiency without cell densification and extra bandwidth requirements. This paper focuses on improvement in the spectral and energy efficiency for a single-cell LS-MU-MIMO system by considering multiple antennas at the user end with the linear precoding schemes. Spectral efficiency of zero-forcing (ZF), minimum mean square error (MMSE) and maximum ratio transmission (MRT) precoding schemes are derived, analyzed and compared for single- and double-antenna users with perfect and imperfect channel state information (CSI). MMSE precoding scheme shows the optimum spectral efficiency as compared to MRT and ZF precoding schemes for a LS-MU-MIMO system. We consider downlink and uplink transmission with various processing techniques at the base station and designed a system model for power consumption to evaluate the energy efficiency of the large MU-MIMO system. We derived energy efficiency expressions for maximal ratio, zero-forcing (ZF) and MMSE processing schemes. Numerical results reveal that the energy efficiency with double-antenna user equipments (UEs) has been enhanced as compared to the UEs having single antenna. Simulation results show that the proposal of adding multiple antennas at the user end can provide a substantial improvement in spectral and energy efficiency as compared to single-antenna users for perfect and imperfect CSI with various precoding schemes.

Diversity order of precoding‐aided spatial modulation using receive antenna selection

In this Letter, the authors analyse the diversity order of the precoding-aided spatial modulation (PSM) system with receive antenna subset (RAS) selection. It is shown that when there are N T transmit antennas, N R receive antennas, and N S selected receive antennas, the achievable diversity order is given by N T − N S + 1 N R − N S + 1 , where the product gain of N T − N S + 1 is achieved by the zero-forcing precoding scheme and another of N R − N S + 1 can be attained by the optimal RAS selection algorithm.

Multiuser precoding scheme and achievable rate analysis for massive MIMO system

We analyze the downlink multiuser precoding of massive multiple input multiple output (MIMO) system, where the base station (BS) has ideal channel state information (CSI) and adopts three types of different linear precoding schemes, i.e., maximum ratio transmission (MRT), zero-forcing (ZF), and minimum mean squared error (MMSE). Under a Rayleigh fading channel, we attain the exact expressions on the achievable rate for these three precoding schemes. Moreover, we provide several insights on the achievable rates and reveal the relation of the number of BS antennas, the number of users, and the input signal-to-noise ratio (SNR) with the achievable rates respectively. It is found in general that the achievable rate increases with the number of BS antennas and the input SNR. To be more specific, the MRT precoding scheme is much inferior to the ZF and MMSE precoding schemes and tends to be at a fixed rate at the high SNR case. On the contrary, the MRT precoding scheme outperforms ZF precoding schemes at the low SNR case. Moreover, the total achievable rate always does not increase with the number of users and the optimal number of users always exists for the ZF and MMSE precoding schemes.

Hybrid TH-VP Precoding for Multiuser MIMO

Vector perturbation (VP) is a nonlinear precoding technique that achieves near-capacity performance in multiuser multiple-input multiple-output systems at the expense of large complexity due to the search for the optimum perturbation vector. In this paper, we present the hybrid Tomlinson-Harashima VP (TH-VP) algorithm, a novel zero-forcing precoding scheme, which combines TH precoding to remove interuser interference, and VP precoding to equalize each user's multiple spatial streams. We show that the two nonlinear techniques can be integrated in a single optimization and that the proposed algorithm has lower computational requirements than any other. The performance of TH-VP is analyzed and simulation results show that TH-VP outperforms conventional zero-forcing VP and approaches the performance of dirty paper coding.

Equivalent ZF precoding scheme for downlink indoor MU-MIMO VLC systems

In indoor visible light communication (VLC) systems, the channels of photo detectors (PDs) at one user are highly correlated, which determines the choice of spatial diversity model for individual users. In a spatial diversity model, the signals received by PDs belonging to one user carry the same information, and can be combined directly. Based on the above, we propose an equivalent zero-forcing (ZF) precoding scheme for multiple-user multiple-input single-output (MU-MIMO) VLC systems by transforming an indoor MU-MIMO VLC system into an indoor multiple-user multiple-input single-output (MU-MISO) VLC system through simply processing. The power constraints of light emitting diodes (LEDs) are also taken into account. Comprehensive computer simulations in three scenarios indicate that our scheme can not only reduce the computational complexity, but also guarantee the system performance. Furthermore, the proposed scheme does not require noise information in the calculating of the precoding weights, and has no restrictions on the numbers of APs and PDs.

A power-efficient ZF precoding scheme for multi-user indoor visible light communication systems

In this study, we propose a power-efficient ZF precoding scheme for visible light communication (VLC) downlink multi-user multiple-input-single-output (MU-MISO) systems, which incorporates the zero-forcing (ZF) and the characteristics of VLC systems. The main idea of this scheme is that the channel matrix used to perform pseudoinverse comes from the set of optical Access Points (APs) shared by more than one user, instead of the set of all involved serving APs as the existing ZF precoding schemes often used. By doing this, the waste of power, which is caused by the transmission of one user's data in the un-serving APs, can be avoided. In addition, the size of the channel matrix needs to perform pseudoinverse becomes smaller, which helps to reduce the computation complexity. Simulation results in two scenarios show that the proposed ZF precoding scheme has higher power efficiency, better bit error rate (BER) performance and lower computation complexity compared with traditional ZF precoding schemes.

Pilot distribution optimization in multi-cellular large scale MIMO systems

Salient characteristics of a wireless communication system deploying a great number of antennas at the base station (BS), namely a massive MIMO system, are investigated in this work. The asymptotic performance of the linear zero-forcing precoding scheme is found, both in terms of signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), and shown to be equivalent to the matched-filter beamforming performance. Furthermore, analysis of the massive MIMO system downlink is carried out from the viewpoint of uncoded BER performance, including some realistic adverse effects, such as interference from neighbouring cells, channel estimation errors due to background thermal noise, and pilot contamination. The latter has been shown to be the only impairment that remains in the MIMO multicell system with infinite number of BS antennas. For such scenario, we derive expressions for the asymptotic BER, i.e. in the limit of infinite number of antennas at BS. A quite simple and efficient method for optimizing the massive MIMO system performance under different optimization metrics is proposed, which consists of simply distributing the pilot sequences among the users of the cell in an efficient manner. As a result, a user rate gain of six times regarding the random strategy has been achieved for the downlink with unitary reuse factor, while the user rate increases twice for reuse factor of three. These benefits are achieved by only knowing the powers and the long-term fading coefficients of users in adjacent cells, for each pilot sequence.

Achievable rates of SVD‐based codebooks for zero‐forcing and Tomlinson‐Harashima precoding schemes with limited feedback MU‐MIMO system

SummaryIn this manuscript, we developed a new SVD‐based codebook design criterion for linear zero‐forcing (ZF) precoding and Tomlinson‐Harashima (TH) precoding with limited feedback. In limited feedback systems where only quantized channel state information is available, precoding on transmitter causes sum‐rate degradation because of quantization errors. Thus, we propose more efficient codebooks that reduce the quantization error with lower feedback overheads. We also develop a closed‐form expression for sum‐rate of the proposed codebooks in case of TH and ZF precoding schemes in a multiuser MIMO system with limited feedback. The numerical consequences prove that TH precoding can achieve sum‐rate gain of 0.5–2.5 bps/Hz at 25 dB signal‐to‐noise ratio as compared with linear ZF precoding with 10 and 20 feedback bits per user, respectively. Moreover, the numerical results also prove that the performance of nonlinear TH precoding is better than linear ZF precoding for low signal‐to‐noise ratio value or for the large number of feedback bits. Copyright © 2016 John Wiley & Sons, Ltd.

Generic Optimization of Linear Precoding in Multibeam Satellite Systems

Multibeam satellite systems have been employed to provide interactive broadband services to geographical areas under-served by terrestrial infrastructure. In this context, this paper studies joint multiuser linear precoding design in the forward link of fixed multibeam satellite systems. We provide a generic optimization framework for linear precoding design to handle any objective functions of data rate with general linear and nonlinear power constraints. To achieve this, an iterative algorithm which optimizes the precoding vectors and power allocation alternatingly is proposed and most importantly, the proposed algorithm is proved to always converge. The proposed optimization algorithm is also applicable to nonlinear dirty paper coding. As a special case, a more efficient algorithm is devised to find the optimal solution to the problem of maximizing the proportional fairness among served users. In addition, the aforementioned problems and algorithms are extended to the case that each terminal has multiple co-polarization or dual-polarization antennas. Simulation results demonstrate substantial performance improvement of the proposed schemes over conventional multibeam satellite systems, zero-forcing and regularized zero-forcing precoding schemes in terms of meeting the traffic demand, e.g., using real beam patterns, over twice higher throughput can be achieved compared with the conventional scheme. The performance of the proposed linear precoding scheme is also shown to be very close to the dirty paper coding.

User Selection and Precoding Schemes Based on Partial Channel Information for Broadcast MIMO Systems

Due to the large number of users and the time-varying characteristics of wireless channels, it is very tough to inform the transmitter of full channel information in real multi-user MIMO broadcast systems. On the other hand, the capacity of multi-user systems greatly depends on the knowledge of the channel at the transmitter while this is not always the case in single-user MIMO systems. In this paper, we investigate combined user selection and zero-forcing precoding schemes that use partial channel information, i.e., very low amount of channel information at the base station. We show that while greatly reducing the complexity and channel knowledge feedback load, the proposed schemes preserve the optimality of zero-forcing scheme in term of achievable ergodic sum capacity in limit of large number of active users.