Multiuser MIMO Systems Research Articles

Hybrid precoding for multiuser massive MIMO systems based on MMSE-PSO

Hybrid Precoding has been adopted as a promising technology for 5th generation wireless communication systems. In this paper, we propose a hybrid precoding scheme based on minimum mean square error (MMSE) and particle swarm optimization (PSO) for multiuser massive multiple-input multiple-output systems. The closed-form solutions of baseband precoding and the combiner are solved by convex optimization method. Meanwhile, the MMSE between the transmitted signal and the received signal is considered as an objective function of PSO, and the radio frequency precoding is obtained by updating the global optimal positions of the particles. The simulation results show that the proposed hybrid MMSE-PSO precoding significantly improves achievable rate and the system reliability.

G-MultiSphere: Generalizing Massively Parallel Detection for Non-Orthogonal Signal Transmissions

The increasing demand for connectivity and throughput, despite the spectrum limitations, has triggered a paradigm shift towards non-orthogonal signal transmissions. However, the complexity requirements of near-optimal detection methods for such systems becomes impractical, due to the large number of mutually interfering streams and to the rank-deficient or ill-determined nature of the corresponding interference matrix. This work introduces g-MultiSphere; a generic massively parallel and near-optimal sphere-decoding-based approach that, in contrast to prior work, applies to both well- and ill-determined non-orthogonal systems. We show that g-MultiSphere is the first approach that can support large uplink multi-user MIMO systems with numbers of concurrently transmitting users that exceed the number of receive antennas by a factor of two or more, while attaining throughput gains of up to 60% and with reduced complexity requirements in comparison to known approaches. By eliminating the need for sparse signal transmissions for non-orthogonal multiple access (NOMA) schemes, g-MultiSphere can support more users than existing systems with better detection performance and practical complexity requirements. In comparison to state-of-the-art detectors for NOMA schemes and non-orthogonal signal waveforms (e.g., SEFDM) g-MultiSphere can be up to an order of magnitude less complex, and can provide throughput gains of up to 60%.

Effective user service strategies in a multi-user MIMO system

Introduction: One of the main advantages of multiple antenna systems is the support of multi-user connections, simultaneouslytransmitting parallel data streams. This can significantly increase the overall system throughput. However, the spectral efficiency can be additionally increased through the usage of various dynamic strategies which take into account the current state of the system.Here we propose a method to evaluate the effectiveness of such strategies with the help of appropriate mathematical models. Purpose:Development of mathematical models for a multi-user MIMO system which would allow you to evaluate the average delay of a filetransfer. Results: Mathematical models of the MIMO system are presented in the form of discrete/continuous time queuing systemsfor three user service strategies. We provide formulas for estimating the average file transfer delay in a closed form, which simplifynumerical analysis as compared to the well-known methods using mathematical models in continuous time. This method allows you tofind the most effective strategy with less computational complexity. Another advantage of the constructed models is the opportunityof further research on the effect of file length distributions on the file transfer delay. The conclusions we made about the effectivenessof each strategy are consistent with our earlier work, which is confirmed by a numerical example. Practical relevance: The developedmathematical models for a multi-user MIMO system can be used to set the operating parameters of a real wireless MIMO system.

Optimal cluster formation and power control for high connectivity wireless MIMO‐NOMA applications

The sum rate for uplink multiple-input multiple-output non-orthogonal multiple access (MIMO-NOMA) channel is derived over a Rayleigh fading environment. Then, an optimal cluster formation and power control technique is jointly designed to maximise the sum rate under power and user rate constraints. Besides, a near-optimal algorithm is proposed to reduce the computational efforts significantly. Simulation results validate the effectiveness of presented schemes compared with the reference MIMO-NOMA and multiuser MIMO systems.

What is the Value of Limited Feedback for Next Generation of Cellular Systems?

The demand for higher data traffic is exponentially growing. Since last 2 years or so, new research movements have emerged towards LTE-B (Release 13 and beyond) and 5th generation (5G) networks. As the next generation of cellular networks are going to be an integration of different radio access technologies and denser in term of space and frequency, the problem of interference management becomes more vital and challenging for academic and industrial units. One of the main technical challenges of interference management in the frequency division duplex system is the issue of channel state information (CSI) feedback overhead. Accordingly, this article briefly overviews the key candidate technologies for LTE-B and 5G and as well as to addresses their features and problems from the interference management perspective. Then, the problem of limited feedback in the multi-user MIMO systems is formulated and discussed. Afterward, the performance of distributed antenna systems is investigated under imperfect CSI and different phases of limited feedback technique. It is shown that accuracy of CSI feedback plays a vital role towards the realization of next generation of cellular systems. Finally, the future research directions are addressed in this domain.

A decentralized beam selection for mmWave beamspace multi-user MIMO system

A millimeter beamspace multi-user multi-input-multi-output system is a key enabling technology of 5G to fulfil the demand for high data rate. But, it suffers from high RF complexity. However, RF Complexity can be reduced by the beam selection algorithm. Hence, few beam selection algorithms were proposed with a centralized approach. In this paper, a decentralized beam selection algorithm for millimeter wave beamspace multi-user MIMO is discussed. Unlike the centralized beam selection algorithm, the decentralized beam selection algorithm doesn’t require a central unit or access point to select beams. The users themselves play an active role to select beams by sharing the local information with the neighbour users. This is also the requirement of a dynamic network. The performance thoroughly confides on the network cooperation to select beams in an efficient manner. The proposed decentralized beam selection algorithm enables the access point to be free from handling of a large channel matrix. Therefore, this low complexity beam selection algorithm is a better alternative. The performance and complexity of the proposed beam selection algorithm are examined in the following section.

Coordinated Hybrid Precoding for Interference Exploitation in Heterogeneous Networks

We consider a downlink multiuser massive MIMO system comprising multiple heterogeneous base stations with hybrid precoding architectures. To enhance the energy efficiency of the network, we propose a novel coordinated hybrid precoding technique, where the coordination between the base stations is aimed at exploiting interference as opposed to mitigating it as per conventional approaches. We formulate an optimization problem to compute the coordinated hybrid precoders that minimize the total transmit power while fulfilling the required quality of service at each user. Furthermore, we devise a low-complexity suboptimal precoding scheme to compute approximate solutions of the precoding problem. The simulation results reveal that the proposed coordinated hybrid precoding yields superior performance when compared to the conventional hybrid precoding schemes.

Scheduling for Multi-User Multi-Input Multi-Output Wireless Networks with Priorities and Deadlines

The spectral efficiency of wireless networks can be significantly improved by exploiting spatial multiplexing techniques known as multi-user MIMO. These techniques enable the allocation of multiple users to the same time-frequency block, thus reducing the interference between users. There is ample evidence that user groupings can have a significant impact on the performance of spatial multiplexing. The situation is even more complex when the data packets have priority and deadlines for delivery. Hence, combining packet queue management and beamforming would considerably enhance the overall system performance. In this paper, we propose a combination of beamforming and scheduling to improve the overall performance of multi-user MIMO systems in realistic conditions where data packets have both priority and deadlines beyond which they become obsolete. This method dubbed Reward Per Second (RPS), combines advanced matrix factorization at the physical layer with recently-developed queue management techniques. We demonstrate the merits of the this technique compared to other state-of-the-art scheduling methods through simulations.

Low-Complexity OFDM-Based Hybrid Precoding for Multiuser Massive MIMO Systems

A low-complexity fully connected hybrid precoding design is proposed for multiuser massive MIMO systems over frequency-selective fading channels. Digital precoding and analog precoding are adaptively designed based on instantaneous channel state information (CSI) and statistical CSI to maximize transmission efficiency. Furthermore, a dynamic partially connected structure and a novel beamforming algorithm are proposed to reduce the power consumption. Our simulation shows a satisfying performance compared with other proposed schemes.

Performance of ZF and CB Precoding Techniques for Large-Scale MU-MIMO System

Large Scale Multi User-MIMO (MU-MIMO) is a key technology with reference to 5G to achieve higher spectrum as well as energy efficiency. The new technology refers to the use of a large number of antennas at the base station serving many user terminals in the same time and frequency resource allowing the channel vectors nearly orthogonal as a result, there is a reduction in inter-user interference and users may be served with the significant data rate. The linear precoding techniques play a vital role in the reduction of interference among users and cells. In this paper, we have derived, analyzed and compared two important precoding techniques i.e. Zero-forcing (ZF) and Conjugate beamforming (CB) for large-scale multiuser-MIMO. We analyze these precoding techniques with respect to spectral efficiency and downlink power with imperfect channel state information (CSI) as well as with perfect CSI. It is shown that ZF performs better as compared to CB precoding for achieving higher spectral efficiency and requires lower downlink power. CB outperforms the ZF in terms of downlink transmit power when there is a requirement to achieve low spectral efficiency and also for cell- edge users, hence energy efficient in these cases. It is shown from simulation results that ZF precoding is the better choice for attaining higher spectral and energy efficiency for a large scale multiuser-MIMO communication system.

On Hybrid Pilot for Channel Estimation in Massive MIMO Uplink

This paper introduces a hybrid pilot-aided channel estimation technique for mitigating the effect of pilot contamination for the uplink of multi-cell multiuser massive MIMO systems. The proposed hybrid pilot is designed such that it enjoys the complementary advantages between time-multiplexed (TM) pilot and time-superimposed (TS) pilot, and thereby, allows superior solution to the conventional pilot schemes. We mathematically characterize the impact of hybrid pilot on the massive MIMO uplink by deriving a closed-form approximation for the uplink achievable rate. In large-number-of-antennas regime, we obtain the asymptotically optimal solution for hybrid pilot by jointly designing the TM pilot and the TS pilot. It is shown that either TM pilot or TS pilot has the advantages for large frame-size and limited frame-size transmission, respectively, while the hybrid pilot scheme can offer a superior performance to that employing either TM pilot or TS pilot. Numerical results demonstrate the effectiveness of the proposed design.

Code-aided non-orthogonal multiple access in downlink multiuser MIMO system

Conventional downlink non-orthogonal multiple access (NOMA) uses the power domain for multiple access and applies successive interference cancellation (SIC) to mitigate intra-cluster interference. It requires sophisticated user terminals (UTs) clustering, moreover, imperfect SIC results in error propagation that severely degrades system capacity. In this paper, a novel code-aided multiuser multiple-input multiple-output NOMA (MIMONOMA) framework for downlink transmission is proposed. Since the total number of UTs in a cell is in general more than the number of transmit antennas at the base station (BS), purely zero-forcing (ZF) downlink beamforming is unable to mitigate multiuser interference (MUI). We first propose a simple grouping algorithm that separate all UTs in a cell into several clusters, with cluster number less than or equal to BS antennas. ZF-based multiuser beamforming is then employed to remove the inter-cluster interference. To mitigate the intra-cluster interference, we propose to modulate the downlink data for each UT within the same cluster by orthogonal codes, thereby, instead of SIC, matched-filtering can be employed at each UT for interference suppression. Computer simulation results quantify the capacity gain of the proposed coded-aided MIMONOMA method over conventional MIMONOMA schemes.

Sum-Rate of Multi-User MIMO Systems with Multi-Cell Pilot Contamination in Correlated Rayleigh Fading Channel.

This paper presents some exact results on the sum-rate of multi-user multiple-input multiple-output (MU-MIMO) systems subject to multi-cell pilot contamination under correlated Rayleigh fading. With multi-cell multi-user channel estimator, we give the lower bound of the sum-rate. We derive the moment generating function (MGF) of the sum-rate and then obtain the closed-form approximations of the mean and variance of the sum-rate. Then, with Gaussian approximation, we study the outage performance of the sum-rate. Furthermore, considering the number of antennas at base station becomes infinite, we investigate the asymptotic performance of the sum-rate. Theoretical results show that compared to MU-MIMO system with perfect channel estimation and no pilot contamination, the variance of the sum-rate of the considered system decreases very quickly as the number of antennas increases.

Energy Efficient Precoder in Multi-User MIMO Systems With Imperfect Channel State Information

This letter is on the energy efficient precoder design in multi-user multiple-input multiple-output systems which is also robust with respect to the imperfect channel state information at the transmitters. In other words, we design the precoder matrix associated with each transmitter to maximize the general energy efficiency of the network. We investigate the problem in two conventional cases. The first case considers the statistical characterization for the channel estimation error that leads to a quadratically constrained quadratic program with a semi-closed-form solution. Then, we turn our attention to the case which considers only the uncertainty region for the channel estimation error; this case eventually results in a semi-definite program.

Secure Massive MIMO Under Imperfect CSI: Performance Analysis and Channel Prediction

In recent years, physical layer security has been regarded as a promising technique to facilitate secure communications in next generation mobile systems, where theoretically massive MIMO can significantly enhance the system secrecy performance under its advantage in shaping the transmitted signals to null the interference or leakage. However, in practical systems, the achievable secrecy performance under imperfect channel state information (CSI) deserves further investigation. In this paper, we give a detailed analysis about the physical layer security problem in a multi-user massive MIMO system with imperfect CSI. The considered imperfect CSI includes both the outdated CSI due to the transmission and processing delay, and the channel estimation error. We first derive a tight asymptotic lower bound of the ergodic system secrecy capacity under imperfect CSI, and then analyze how imperfect CSI affects the system secrecy performance. Moreover, we propose a channel prediction scheme that can result in more accurate CSI, in order to alleviate the negative effect on the achievable system secrecy capacity caused by imperfect CSI. Simulation results reveal that the imperfect CSI greatly reduces the system secrecy capacity, while our designed channel prediction scheme can effectively mitigate the harmful impact of imperfect CSI and thus improve the system secrecy performance.

다중 사용자 다중 안테나 시스템을 위한 유사 최적 무선 충전 정보 전송 기법

본 논문에서는 다중사용자 다중안테나 (MU-MIMO; Multiuser Multi-Input Multi-Output) 채널에서 유사 최적무선 충전 정보 전송 기법을 제안한다. 공유기는 하향링크 빔포밍을 통해 다수의 단말에 전력을 전송하고, 단말은 충전된 전력을 이용하여 ZF (Zero-forcing) 기반의 공간 다중화 기법을 이용하여 상향링크로 정보를 전송한다. 이때 최대-최소 전송률을 최적화하기 위해 Gradient 알고리즘 기반 하향링크 에너지 빔포밍 기법을 유도하고, 단말별 데이터 스트림 수 선정 기법을 적용한 상향링크 정보 전송 기법을 제안한다. 모의실험을 통해 제안된 기법이 기존 부최적 기법에 비해 성능이 우수하고, 최적 전송 기법과 유사한 성능을 보이면서 복잡도가 현저히 낮음을 보인다.

BER and Capacity Improvement for Multiuser MIMO System by MRT Precoding Technique

In recent years several of endeavors have been conducted in various papers to improve capacity and bit error rate performance of multiuser MIMO system. Every effort tried to increase gain of the individual stream between base station and users’ antennas while reducing the interference among multiple users and interference between streams of single user itself. All these papers adopted precoding techniques for beam forming of individual stream. Various papers have compared precoding techniques such as DPC, BD-ZF, BD-MMSE, BD-QR-MRL, and BD-QR-LRL. Block diagonalization technique is capable to reduce multiuser interference, but not able to combat interference between antennas of single user itself. BD combines with other techniques to overcome this problem. This paper also proposes a precoding technique called modified maximum ratio transmission, which is able to deal with the problem without much complexity. Simulation is done to compare all precoding technique with MRT. The paper tests the precoding techniques on the parameters of BER and capacity, to achieve optimum receiver performance. BER curve and capacity curve are plotted to conclude the paper.

Zero-Forcing Precoding in the Measured Massive MIMO Downlink: How Many Antennas Are Needed?

In order to understand how many antennas are needed in a multiuser massive MIMO system, theoretical derivation and channel measurements are conducted; the effect of a finite number of base station (BS) antennas on the performance capability of Zero-forcing (ZF) precoding in a rich scattering channel is quantified. Through the theoretical analysis, the needed number of the transmit antennas for ZF precoder to achieve a certain percentage of the broadcast channel (BC) capacity will monotonically decrease with the increase of the transmit signal-to-noise ratio (SNR), and the lower bound of the needed transmit antennas is derived with a simple expression. Then the theoretical derivation is verified by simulation results, and the transmission performance is evaluated by channel measurements in urban microcell (UMi) scenario with frequencies of 3.5 and 6 GHz. From the measurement results, the ZF capability can be enhanced by improving the SNR and enlarging the antenna array spacing when the massive MIMO channel does not under a favorable propagation condition. Furthermore, because of the lower spatial correlation, the performance of ZF precoding at 6 GHz is closer to the theoretical derivation than 3.5 GHz.

Pilot decontamination in massive multiuser MIMO systems based on low‐rank matrix approximation

In multi-cell massive multiuser multi-input multi-output (MU-MIMO) systems, the channel estimation performance is degraded by the pilot contamination problem. This effect occurs when non-orthogonal pilot sequences are reused by other users in the adjacent cells. In this study, two channel estimation methods based on low-rank matrix approximation technique are proposed to mitigate pilot contamination problem in time division duplex multi-cell massive MU-MIMO systems. In the first method, the massive MU-MIMO channel estimation is formulated as the nuclear norm (NN) optimisation problem and solved by using a novel estimation algorithm proposed in this study. In the second method, the iterative weighted NN (IWNN) is proposed to improve the NN estimation performance. Consequently, the regularisation parameter of both optimisation methods is selected based on the cross-validation curve method. The simulation results show that both proposed methods outperform the traditional least square method in terms of the normalised mean square error. Moreover, the IWNN estimation method demonstrates substantial improvement over the NN estimation method in the presence of strong pilot contamination problem.

Performance Analysis of Block MSN Algorithm with Pseudo-Noise Control in Multi-User MIMO System

Performance Analysis of Block MSN Algorithm with Pseudo-Noise Control in Multi-User MIMO System