Dirty paper coding (DPC) is a strategy to achieve the region capacity of multiple input multiple output (MIMO) downlink channels and a DPC scheduler is throughput optimal if users are selected according to their queue states and current rates. However, DPC is difficult to implement in practical systems. One solution, zero-forcing beamforming (ZFBF) strategy has been proposed to achieve the same asymptotic sum rate capacity as that of DPC with an exhaustive search over the entire user set. Some suboptimal user group selection schedulers with reduced complexity based on ZFBF strategy (ZFBF-SUS) and proportional fair (PF) scheduling algorithm (PF-ZFBF) have also been proposed to enhance the throughput and fairness among the users, respectively. However, they are not throughput optimal, fairness and throughput decrease if each user queue length is different due to different users channel quality. Therefore, we propose two different scheduling algorithms: a throughput optimal scheduling algorithm (ZFBF-TO) and a reduced complexity scheduling algorithm (ZFBF-RC). Both are based on ZFBF strategy and, at every time slot, the scheduling algorithms have to select some users based on user channel quality, user queue length and orthogonality among users. Moreover, the proposed algorithms have to produce the rate allocation and power allocation for the selected users based on a modified water filling method. We analyze the schedulers complexity and numerical results show that ZFBF-RC provides throughput and fairness improvements compared to the ZFBF-SUS and PF-ZFBF scheduling algorithms.

Multiple-Input Multiple-Output (MIMO) systems can achieve high data-rate and high capacity transmission. In MIMO systems, eigen-beam space division multiplexing (E-SDM) that achieves much higher capacity by weighting at the transmitter based on feedback channel state information (CSI) has been studied. Early studies for E-SDM have assumed perfect CSI at the transmitter. However, in practice, the CSI fed back to the transmitter from the receiver becomes outdated due to the time-varying nature of the channels and feedback delay. Therefore, an outdated E-SDM cannot achieve the full performance possible. In this paper, we evaluate the performance of E-SDM with methods for reducing performance degradation due to feedback delay. We use three methods: 1) method that predicts CSI at future times when it will be used and feeds the predicted CSI back to the transmitter (denoted hereafter as channel prediction); 2), 3) method that uses the receive weight based on zero-forcing (ZF) or minimum mean square error (MMSE) criterion instead of those based on singular value decomposition (SVD) criterion (denoted hereafter as ZF or MMSE-based receive weight). We also propose methods that combine channel prediction with ZF or MMSE-based receive weight. Simulation results show that bit error rate (BER) degradation of E-SDM in the presence of feedback delay is reduced by using methods for reducing performance degradation due to feedback delay. We also show that methods that combine channel prediction with ZF or MMSE-based receive weight can achieve good BER even when the large feedback delay exists.

The minimum mean square error (MMSE) multiple antenna transmission scheme for a code division multiple access (CDMA) system was recently developed by Choi and Perreau [1]. In this paper, we first show that the zero-forcing (ZF) transmit beamformer for multiple antenna CDMA system has the same form as the MMSE beamformer given by Choi and Perreau. We then develop an analytical method to obtain a closed-form expression of the bit error rate (BER) of the ZF transmit beamformer when there are channel estimation errors. The analytical and simulation results show good agreement, and confirm the importance of accurate channel state information (CSI) at the transmitter when using the ZF transmit beamformer.

This paper proposes a suboptimal algorithm for the maximum likelihood detection (MLD) in multiple-input multiple-output (MIMO) communications. The proposed algorithm regards transmitted signals as continuous variables in the same way as a common method for the discrete optimization problem, and then searches for candidates of the transmitted signals in the direction of a modified gradient vector of the metric. The vector is almost proportional to the direction of the noise enhancement, from which zero-forcing (ZF) or minimum mean square error (MMSE) algorithms suffer. This method sets the initial guess to the solution by ZF or MMSE algorithms, which can be recursively calculated. Also, the proposed algorithm has the same complexity order as that of conventional suboptimal algorithms. Computer simulations demonstrate that it is much superior in BER performance to the conventional ones.

In this letter, a low complexity ML detection technique for V-BLAST systems is proposed. In this proposed scheme, V probable streams are detected according to the first detected sub-stream of DFE detector and most probable stream is selected by likelihood test, since the performance of V-BLAST system depends on the first sub-stream detection capability. It has been shown that the proposed technique can detect the transmitted data more accurately than conventional DFE decoding scheme, and has very lower complexity than ML detector.

A powerful HARQ-based linear precoding scheme is proposed to utilize the flexibility of post-combining HARQ strategy in MIMO communications systems. The scheme selects the appropriate symbols and transmit powers for each eigen-mode to acquire more performance gains. Simulation results show that the proposed scheme achieves about 5.5 dB signal-to-noise ratio gains over original spatial multiplexing scheme at an average bit error rate of 10-4. Furthermore, the gap between the two schemes increases with the number of transmissions.

We propose a likelihood detection scheme that utilizes ordering and decision of partial bits in MIMO spatial multiplexing systems. We compute BER performance of the proposed detection scheme under Rayleigh fading channels in a 33 MIMO spatial multiplexing system and compare it with BER performance using MLD only and detection utilizing ZF or MMSE only. In addition, the computational complexity of the proposed detection scheme is compared with that of MLD and detection utilizing ZF or MMSE. The results of our investigation show that the proposed detection is a scheme achieves both good BER performance and low computational complexity.

We propose optical wireless multiple-input multiple-output (OMIMO) communications to achieve high speed transmission with a compact transmitter and receiver. In OMIMO, by using zero forcing (ZF), minimum mean square error (MMSE) or other detection techniques, we can eliminate the interference from the other optical transmit antennas. In this paper, we employ ZF as the detection technique. We analyze the signal-to-interference-plus-noise ratio (SINR) and the bit error rate (BER) of the proposed OMIMO with a linear array and a square array of optical transmit and receive antennas, where we employ subcarrier multiplexing (SCM) for each optical transmit antenna. Note that the proposed OMIMO is applicable to other arrangements of optical transmit and receive antennas. We show that the proposed OMIMO system can realize MIMO multiplexing and achieve high speed transmission by correctly aligning the optical transmit and receive antennas and the transmitter semiangle.

A multi-user space-time block coding (STBCa) system is a multi-access system where co-channel users employ space-time block codes (STBC). In this paper, we aimed at the design of efficient zero-forcing (ZF) receivers, especially ZF iterative interference cancellation (IC) receivers, for multi-user {G2, G3, G4} STBC systems with an arbitrary number of users, based on the identification of algebraic properties existing in the systems. First, we identify some algebraic properties for {G2, G3, G4} STBC systems. Then, utilizing these algebraic properties, we further expose two significative properties, called "ZF output uncorrelated property" and "ZF output equal Post-detection SNR property" respectively, for least-squares (LS) ZF receivers in multi-user {G2, G3, G4} STBC systems by detailed proofs. Based on the two properties, a novel LS ZF user-ordered successive interference cancellation (ZF UOSIC) detection algorithm is proposed subsequently. Finally, simulation results show that ZF UOSIC is superior to the conventional ZF IC and maximum-likelihood (ML) algorithms and the non-ordered ZF user-based SIC (ZF USIC) algorithm due to adopting iterative IC and optimal ordering among users, and has very close performance to the ZF symbol-ordered SIC but with lower complexity due to the fewer iterative times.

This paper presents a reduced-complexity maximum likelihood detection (MLD) scheme for orthogonal frequency division multiplexing with space division multiplexing (OFDM-SDM) systems. Original MLD is known to be an optimal scheme for detecting the spatially multiplexed signals. However, MLD suffers from an exponentially computational complexity because it involves an exhaustive search for the optimal result. In this paper, we propose a novel detection scheme, which drastically reduce the complexity of MLD while keeping performance losses small. The proposed scheme decouples the spatially multiplexed signals in two stages. In stage one, the estimated symbols obtained from zero-forcing (ZF) are used to limit the candidate symbol vectors. In stage two, to form a final estimate of the transmitted symbol vector, the Euclidean or original defined likelihood metric is examined over all symbol vectors obtained from stage 1. Both the bit error rate (BER) and packet error rate (PER) performances are evaluated over a temporally and spatially uncorrelated frequency selective channel through the computer simulations. For a four-transmit and four-receive OFDM-SDM system transmitting data at 144 Mbit/s and 216 Mbit/ss i.e., employing 16 Quadrature Amplitude Modulation (16QAM) and 64QAM subcarrier modulation over 16.6 MHz bandwidth channel, the degradation in required SNR from MLD for PER = 1% are about 0.6 dB and 1.5 dB, respectively. However, the complexity of MLD is reduced to 0.51000% and 0.01562%.

This paper describes implementation and performance evaluation of simple SDM-COFDM (Space Division Multiplexed-Coded Orthogonal Frequency Division Multiplexing) prototype over fading MIMO (Multi-Input Multi-Output) channel in order to achieve higher frequency utilization efficiency. It employs ZF (Zero Forcing) type detection scheme for SDM transmission to reduce hardware implementation complexity, where ZF type detection scheme needs to only multiply the received data by the estimated inverse propagation coefficient matrix at each OFDM subcarrier. Moreover, in order to improve the performance degradation due to the increase of the transmitted data length per frame in fast fading environments, the inverse matrix tracking using STC (Space-Time Coded) pilot is proposed and implemented in the prototype. Experimental results show that the prototype with 22 antennas achieves about 90% increase of the frequency utilization efficiency compared to the SISO (Single-Input Single-Output) transmission.

In this paper, we consider a direct sequence spread spectrum (DS-SS) microphone system in 2.4 GHz ISM band under indoor environments. It can be modeled as the asynchronous DS-CDMA system in which multiple transmitters share a common channel to transmit information to a single receiver. However, this system has the two critical defects, one is that many burst errors are occurred because of the long-term deep fade in an indoor channel, and the other is that power control is impossible due to the unidirectional structure. Since the reliable performance of DS-CDMA systems can be guaranteed under a reliable bit error rate performance and perfect power control, the solution to these problems is imperative. Hence, in order to overcome these problems, we propose a zero forcing equalizer (ZFE) based multi-user detection (MUD) receiver for this system, since an MUD receiver doesn't require rigorous power control and is able to remove multiple access interference using linear equalization. We derive the modeling and the tap coefficients of the proposed ZFE, and also analyze the performance of the proposed receiver. Based on the performance comparison with the conventional receivers, such as single user detection and post-detection combining diversity receivers, it is shown that the proposed receiver can achieve a lower bit error rate performance.

This paper presents a high-throughput VLSI architecture for LZFG data compression and decompression. To reduce the hardware cost and maintain both of the interior node and the leaf node numbering systems, we modify the original LZFG data structure. Compared to the original LZFG tree, the number of characters in our modified LZFG data structure must be greater than one to establish one new interior node down the root node into the new node. Meanwhile, this architecture employs a series of encoding cells with content addressable memory (CAM) to search the longest match and maintain the LZFG data tree during the encoding and decoding processes. By using the parallel design, the compressor and decompressor can keep a constant high bit rate to encode and decode one character per clock cycle, that is, it is directly proportional to the operating clock rate, but independent of the sizes of the word dictionary and the input file. By using 0.25 µm CMOS silicon technology, the operating clock rate can be as high as 85 MHz. Some untargeted encoding cells will be disabled to reduce the power consumption during the comparison operation. Therefore, this architecture can be easily applied in the high-speed real-time communication and data storage systems.