Chongbin Xu

Precoder Design for Multiuser MIMO ISI Channels Based on Iterative LMMSE Detection

Precoding has been widely investigated for multiple-input multiple-output (MIMO) and inter-symbol interference (ISI) channels. Most related work so far has been on uncoded systems and the impact of forward-error-correction (FEC) codes has not been well studied. In this paper, we present an optimized precoding technique that takes into account iterative joint LMMSE detection and FEC decoding. The proposed scheme has some interesting features. First, the design problem is convex. Second, the proposed scheme achieves water-filling gain when channel state information at the transmitter (CSIT) is available and diversity gain when CSIT is not available, as well as multi-user gain in multi-user environments. Third, the water-filling gain is achieved using a single code (instead of multiple codes as in conventional approaches), which greatly simplifies the design and implementation problems. Both simulation and evolution analyses are provided to show that significant performance improvement can be achieved with the proposed scheme.

Decentralized Power Control for Random Access with Successive Interference Cancellation

This paper is concerned with the decentralized power allocation problem in random access systems. We propose a scheme that is especially suitable for systems requiring high throughput but with difficulty in establishing centralized control, such as cognitive radio environments. Specifically, we assume successive interference cancellation (SIC) at the receiver for multi-packet reception (MPR). We consider a decentralized random power transmission strategy where each user selects its transmitted power level randomly according to a power distribution conditioned on its own channel state. Our focus is on the design of this distribution such that the system packet throughput is maximized under rate and power constraints. We start from a two-user system. A main finding of this paper is that the supports of the optimal power distributions are of discrete nature. This finding greatly simplifies the distribution optimization problem. We also discuss a sub-optimal solution to systems with more than two users. Numerical results demonstrate that the proposed scheme can achieve noticeable performance improvement compared with conventional single-user detection (SUD) based ones and offer a flexible tradeoff between the system throughput and power consumption.

Coded Random Access with Distributed Power Control and Multiple-Packet Reception

In this letter, a spatially-coupled low-density parity- check (SC-LDPC) coded ALOHA system with multiuser detection (MUD)-based multiple packet reception (MPR) is studied. A distributed power control scheme based on multiple power levels is proposed for medium and high rate applications. Numerical results show that the proposed scheme can provide noticeable throughput improvement. This improvement is more significant in the cases of fading channels.

Achievable Rates of MIMO Systems With Linear Precoding and Iterative LMMSE Detection

We establish area theorems for iterative detection and decoding (or simply, iterative detection) over coded linear systems, including multiple-input multiple-output channels, intersymbol interference channels, and orthogonal frequency-division multiplexing systems. We propose a linear precoding technique that asymptotically ensures the Gaussianness of the messages passed in iterative detection, as the transmission block length tends to infinity. Area theorems are established to characterize the behavior of the iterative receiver. We show that, for unconstrained signaling, the proposed single-code scheme with linear precoding and iterative linear minimum mean-square error (LMMSE) detection is potentially information lossless, under various assumptions on the availability of the channel state information at the transmitter. We further show that, for constrained signaling, our proposed single-code scheme considerably outperforms the conventional multicode parallel transmission scheme based on singular value decomposition and water-filling power allocation. Numerical results are provided to verify our analysis.

Optimal throughput for 802.11 DCF with multiple packet reception

In this paper, we propose an analytical model for evaluating the MAC throughput in an unsaturated IEEE 802.11 wireless local area network (WLAN) where multiple packets reception (MPR) is possible using multiuser detection techniques. In particular, a recently proposed successive interference cancellation (SIC) scheme for MPR is considered where users can randomly choose the transmission power from a set of discrete power levels. We derive an explicit expression for throughput of the WLAN based on such an SIC scheme and validate the accuracy of the model via ns-2 simulation results. We show that the throughput is significantly improved compared to the conventional 802.11 MAC protocol just by resolving collisions between two packets with different transmission power levels. In addition, we provide the optimal power distribution to maximize the throughput achievable in an SIC-enabled WLAN.

Massive MIMO, Non-Orthogonal Multiple Access and Interleave Division Multiple Access

This paper provides an overview on the rationales in incorporating massive multiple-input multiple-output (MIMO), non-orthogonal multiple access (NOMA), and interleave division multiple access (IDMA) in a unified framework. Our emphasis is on multi-user gain that refers to the advantage of allowing multi-user transmission in massive MIMO. Such a gain can potentially offer tens or even hundreds of times of rate increase. The main difficulty in achieving multi-user gain is the reliance on accurate channel state information (CSI) in the existing schemes. With accurate CSI, both OMA and NOMA can deliver performance not far away from capacity. Without accurate CSI, however, most of the existing schemes do not work well. We outline a solution to this difficulty based on IDMA and iterative data-aided channel estimation (DACE). This scheme can offer very high throughput and is robust against the pilot contamination problem. The receiver cost is low, since only maximum ratio combining (MRC) is involved and there is no matrix inversion or decomposition. Under time division duplex, accurate CSI acquired in the up-link can be used to support low-cost down-link solutions, such as zero forcing. These findings offer useful design considerations for future systems.

Linear precoding in distributed MIMO systems with partial CSIT

We study the transmission problem in a distributed multiple-input multiple-output (MIMO) system consisting of several distributed transmitters and a common receiver. Assuming partial channel state information at the transmitter (CSIT), we propose a low-cost weighted channel matching and scattering (WCMS) linear precoding strategy. The proposed precoder can be decomposed into two parallel modules: channel matching (CM) and energy scattering. The signals generated by the CM modules from different transmitters provide a coherent gain with improved power efficiency. The use of the scattering modules provides robustness against CSIT uncertainty. By properly combining these two modules, WCMS can achieve coherent gain proportional to the accuracy of the available CSIT as well as robustness against CSIT error. WCMS is simple and fully decentralized and thus is highly suitable for a distributed MIMO system. Numerical results demonstrate that WCMS indeed achieves significant gains in distributed MIMO environments with partial CSIT.

Joint beamforming, water-filling, and diversity coding in MIMO systems with CSIT uncertainty

In this paper, we study the transmission problem in multiple-input multiple-output (MIMO) systems with imperfect channel state information at the transmitter (CSIT). A scheme based on joint FEC coding and linear precoding at the transmitter and iterative linear minimum mean squared error (LMMSE) detection at the receiver is considered. The new scheme combines the advantages of beamforming, water-filling, and diversity coding. Simulation results show that the proposed scheme can utilize the available CSIT efficiently and achieve significant performance gain.

Joint FEC coding and linear precoding for MIMO ISI channels

In this paper, we present a joint forward-error-correction (FEC) coding and linear precoding scheme for multiple-input multiple-output (MIMO) and inter-symbol interference (ISI) channels with imperfect channel state information at the transmitter (CSIT). We first study the performance of ideally coded systems. We focus on an average power gain (APG) method that can achieve capacity in the two extreme cases of no CSIT and perfect CSIT. In the more general case, the performance of the APG method improves progressively with the CSIT quality. We then consider the implementation of this APG method in a practically coded system. We propose a unified scheme involving beamforming, water-filling, and diversity coding. The core of the new scheme is a joint FEC coding and linear precoding strategy at the transmitter and an iterative detection process at the receiver. Simulation results demonstrate that the proposed scheme can achieve significant performance gain by efficiently utilizing the available CSIT.

Transmitter Design for Uplink MIMO Systems With Antenna Correlation

We study the uplink transmission in multiple-input multiple-output (MIMO) systems with antenna correlation. We focus on schemes that require only channel covariance information at the transmitter (CCIT), which involves lower cost than full channel state information at the transmitter (CSIT). We start from mutual information analysis and show that a simple CCIT-based scheme, referred to as statistical water-filling (SWF), can perform close to the optimal full CSIT-based one in MIMO systems with more receive antennas than transmit ones. We then focus on the implementation of SWF in practically coded systems. An iterative linear minimum mean squared error (LMMSE) receiver is assumed and an extrinsic information transfer (EXIT) chart type curve matching technique is developed based on Hadamard precoding techniques. Simulation results show that the proposed scheme can obtain significant performance improvement compared to the conventional equal power transmission. Finally, we show that the proposed scheme is also very efficient in multi-user uplink MIMO systems with distributed channel information.