Performance Analysis of Multiuser MIMO Systems with Scheduling and Antenna Selection

In this paper, we present a performance analysis for the user scheduling for the multiuser MRC MIMO systems exploiting the user and antenna diversities. We consider two scheduling schemes including absolute SNR-based scheduling and normalized SNR-based scheduling schemes. We propose the utilization of an antenna selection scheme to overcome the drawback of channel hardening in multiuser MIMO systems and to improve the system performance. We derive new closedform expressions for the average bit error rate of the presented schemes for two scenarios: Heterogeneous (independent nonidentically distributed SNRs- i.ni.d.) and Homogeneous (independent identically distributed SNRs- i.i.d.) cases. Using mathematical analysis and numerical simulations, we compare the presented schemes.

By combining adaptive modulation (AM) and automatic repeat request (ARQ) protocol, cross-layer design (CLD) scheme of multiuser MIMO system with antenna selection (AS) is presented. By means of exact expression of packet error rate (PER), the switching thresholds (STs) of fading gain subject to a target PER constraint are derived, which is more accurate than the conventional STs based on approximate PER expression. According to these results, and using mathematical manipulation, we obtain the tightly closed-form expressions of the average spectral efficiency (SE) and average PER. Simulation results show that the multiuser MIMO system with CLD-AS scheme using the derived STs provides higher SE than that using existing approximate STs while meeting the PER requirement. Besides, simulations for average SE and PER are in good agreement with the derived theoretical expressions, which demonstrates the validity of the theoretical analysis.

Recent studies have revealed that the capacity improvement resulting from an open-loop multiple-input-multiple-output (MIMO) spatial multiplexing system may come at the sacrifice of reducing link reliability. The tradeoff of antenna multiplexing gain against antenna diversity gain may lead to smaller cell coverage sizes. In This work, we propose a scheduling scheme, called the strongest-weakest-normalized-subchannel-first (SWNSF) scheduling, to replenish the diversity-deficient MIMO system with multiuser diversity. Our analysis and results indicate that the SWNSF scheduling can significantly increase both cell coverage and system capacity requiring only limited amount of feedback.

For the poor transmission reliability of data, this paper focuses on the precoding techniques of the multi-user MIMO system in the broadcast phase in, and the single-user MIMO and multi-user MIMO precoding techniques have been studied. In the multi-user MIMO system, BD-MMSE-VP coding algorithm is proposed to improve the BER performance of the system by using the MMSE criterion to optimize the perturbation vector, and the system uses QR technique to decompose BD channel matrix. Simulation results verify the effective of the proposed precoding algorithm.

Performance analysis of multiuser diversity with antenna selection in MIMO MRC systems

This paper investigates simultaneous transfer of information and energy over wireless link in the multiuser MIMO systems in which interference is aligned at the reduced signal subspace at each receiver. We propose to utilize interference that falls into the reduced subspace as an energy source, which was formerly discarded in the conventional interference alignment system. This makes the receive antenna selection achieve better rate and energy tradeoff. Therefore, the receive antenna selection is beneficial in order to partition the receive signal space into two orthogonal spaces for information and energy transfer.

The performance of a downlink multi-user MIMO system seriously degrades in time-varying environments because MIMO channels change during the time interval between transmission parameter determination and actual MIMO transmission. Channel prediction techniques have been proposed to improve the performance in dynamic environments. In this paper, based on indoor measurement data, we show the effect of the linear channel extrapolation, and evaluate the bit error rate performance of the multi-user MIMO system with the channel prediction technique.

In this paper, the system-level performance is evaluated for the feedback scheme on the Precoding Matrix Index (PMI) and Channel Quality Indication (CQI), which are required for user selection in the multi-user MIMO system. Our analysis demonstrates that the number of users, the number of selected users, and codebook size are the key factors that govern the performance of the Best Companion Grouping (BCG)-based user scheduling. Accordingly, we have confirmed that the probability of forming the co-scheduled user group is determined by these factors, implying that the number of PMI’s and codebook size can be dynamically determined to maximize the average system throughput subject to the given feedback overhead, as the number of users varies in the cell.

In Multi-user MIMO system, traditional user scheduling methods are usually aiming at maximizing the sum-rate capacity of the target system. Such methods select the users which have the best channel quality to service but ignore users' satisfaction. In this paper, we proposed a user satisfaction based judgment criteria, and designed a novel scheduling method in order to ensure users' satisfaction. We apply zero-forcing beam forming (ZFBF) precoding into downlink transmission, and assume perfect knowledge of CSI at the base station. Simulation results show that the algorithm achieved a higher system average user satisfaction degree while suffered a little performance loss compared with the classical SUS-ZFBF strategy.

The design procedure of hybrid beamforming is investigated considering multi-user MIMO communication systems taking into account the number of independent data streams for each user. Hybrid precoding and combining are utilized at the transmitter and receiver sides of millimeter-wave multi-user massive MIMO. The obtained results indicate the trade-off in performance based on the ratio of the number of antennas to the number of data streams at the transmitter as well as the receiver. The optimization of the number of parallel data streams per user allows the increase of transmitted data rate while keeping the levels of error vector magnitude (EVM) at acceptable levels.

In this paper, the outage performance of multiuser multiple-input single-output (MISO) systems exploiting joint spatial and multiuser diversities is investigated for Rayleigh fading channels with outdated feedback. First, we derive closed-form exact outage probabilities for the joint diversity schemes that combine user scheduling with different spatial diversity techniques, including: 1) transmit maximum-ratio combining (TMRC); 2) transmit antenna selection (TAS); and 3) orthogonal space-time block coding (OSTBC). Then the asymptotic outage probabilities are analyzed to gain more insights into the effect of feedback delay. It is observed that with outdated feedback, the asymptotic diversity order of the multiuser OSTBC (M-OSTBC) scheme is equal to the number of transmit antennas at the base station, while that of the multiuser TMRC (M-TMRC) and the multiuser TAS (M-TAS) schemes reduce to one. Further by comparing the asymptotic outage probabilities, it is found that the M-TMRC scheme outperforms the M-TAS scheme, and the M-OSTBC scheme can perform best in the outage regime of practical interest when the feedback delay is large. Theoretical analysis is verified by simulation results.

The combination of millimeter wave (mmW) with massive MIMO is a promising approach to achieve the multi Gb/s required by future wireless systems. Fully digital architectures are not feasible due to hardware limitations, and thus, the design of signal processing techniques for hybrid analog-digital architectures is of paramount importance. In this paper, we propose a new hybrid iterative block space-time receiver structure for multiuser mmW massive MIMO systems. We consider low-complexity user terminals employing analog-only random precoding and a single RF chain. At the base station, a hybrid analog-digital equalizer/detector is designed to efficiently remove the multiuser interference. The analog and digital parts of the equalizer are jointly optimized using as a metric the average bit-error-rate. The specificities of the analog domain impose several constraints in the joint optimization. To efficiently handle these constraints, the analog part is selected from a dictionary based on the array response vectors. We also propose a simple, yet an accurate semi-analytical approach for obtaining the performance of the proposed hybrid receiver structure. The results show that the performance of the hybrid iterative equalizer is close to the fully digital counterpart after only a few iterations. Moreover, it clearly outperforms the linear receivers recently considered for hybrid mmW massive MIMO architectures.

Hybrid precoding has been recognized as a promising technology for millimeter wave-based massive MIMO systems. As an improved scheme for conventional hybrid precoding structures, the adaptively-connected structure requires low hardware cost and has attractive system performance. In this paper, we propose an adaptive hybrid precoding (AHP) scheme for multi-user massive MIMO systems. The hybrid precoder design is formulated as minimizing the difference between the hybrid precoder and the full digital precoder, and a rough design and revision based AHP (RDR-AHP) algorithm is developed to seek for a near-optimal solution. The proposed RDR-AHP algorithm is achieved by a three-step process including rough design of baseband precoder based on singular value unification, stepwise design of radio frequency precoder and baseband precoder revision. Simulation results demonstrate that the proposed scheme achieves higher spectral efficiency than existing schemes with same hardware cost in millimeter wave channel.

To fully utilize the spatial multiplexing gains or array gains of massive MIMO, the channel state information must be obtained at the transmitter side (CSIT). However, conventional CSIT estimation approaches are not suitable for FDD massive MIMO systems because of the overwhelming training and feedback overhead. In this paper, we consider multi-user massive MIMO systems and deploy the compressive sensing (CS) technique to reduce the training as well as the feedback overhead in the CSIT estimation. The multi-user massive MIMO systems exhibits a hidden joint sparsity structure in the user channel matrices due to the shared local scatterers in the physical propagation environment. As such, instead of naively applying the conventional CS to the CSIT estimation, we propose a distributed compressive CSIT estimation scheme so that the compressed measurements are observed at the users locally, while the CSIT recovery is performed at the base station jointly. A joint orthogonal matching pursuit recovery algorithm is proposed to perform the CSIT recovery, with the capability of exploiting the hidden joint sparsity in the user channel matrices. We analyze the obtained CSIT quality in terms of the normalized mean absolute error, and through the closed-form expressions, we obtain simple insights into how the joint channel sparsity can be exploited to improve the CSIT recovery performance.

This paper presents downlink spatial scheduling which nullifies interference among multiplexed signals perfectly in multiuser MIMO systems. Under the base station's zero-forcing transmit beamforming, each terminal can receive packet of interest without interference from the other multiplexed packets. The scheduling algorithm successively selects appropriate terminals for packet transmission in the presence of already selected packets. We show that the downlink spatial scheduler is equivalent to the virtual uplink spatial scheduler in terms of received signal characteristics. Applying the uplink scheduling concept to downlink, the downlink spatial scheduler achieves much higher system throughput than the system without spatial scheduling. Also, it is shown that the spatial scheduler has similar system throughput in uplink and downlink