Minimum BER block precoders for zero-forcing equalization

We determine the linear precoder that minimizes the bit error rate (BER) at moderate-to-high signal-to-noise ratios (SNRs) for block transmission systems with zero-forcing (ZF) equalization and threshold detection. The design is developed for the two standard schemes for eliminating inter-block interference, viz, zero padding (ZP) and cyclic prefix (CP). We show that both the ZP minimum BER precoder and the CP minimum BER precoder provide substantially lower error rates than standard block transmission schemes, such as orthogonal frequency division multiplexing (OFDM). The corresponding SNR gains can be on the order of several decibels. We also show that the CP minimum BER precoder can be obtained by a two-stage modification of the water-filling discrete multitone modulation (DMT) scheme in which the diagonal water-filling power loading is replaced by a full matrix consisting of a diagonal minimum mean square error power loading matrix post multiplied by a discrete Fourier transform (DFT) matrix.     

Optimal minimum distance-based precoder for MIMO spatial multiplexing systems

We describe a new precoder based on optimization of the minimum Euclidean distance d/sub min/ between signal points at the receiver side and for use in multiple-input multiple-output (MIMO) spatial multiplexing systems. Assuming that channel state information (CSI) can be made available at the transmitter, the three steps ( noise whitening, channel diagonalization and dimension reduction), which are currently used in investigations on MIMO systems, are performed. Thanks to this representation, an optimal d/sub min/ precoder is derived in the case of two different transmitted data streams. For quadrature phase-shift keying (QPSK) modulation, a numerical approach shows that the precoder design depends on the channel characteristics. Comparisons with maximum signal-to-noise ratio (SNR) strategy and other precoders based on criteria, such as water-filling (WF), minimum mean square error (MMSE), and maximization of the minimum singular value of the global channel matrix, are performed to illustrate the significant bit-error-rate (BER) improvement of the proposed precoder.     

Optimum mode-switching-assisted constant-power single- and multicarrier adaptive modulation

A set of optimum mode-switching levels is derived for a generic constant-power adaptive-modulation scheme based on a closed-form expression of the average bit error ratio (BER) and the average bits-per-symbol (BPS) throughput of the adaptive-modulation scheme. This results in a constant BER, variable-throughput arrangement. The corresponding BPS throughput performance and the achievable signal-to-noise ratio (SNR) gain are investigated for the optimum mode-switching assisted constant-power adaptive-modulation schemes employing various diversity schemes, including maximal ratio combining (MRC) receive-antenna diversity, a two-dimensional RAKE receiver, as well as transmit-diversity aided space-time (ST) coding, when communicating over various fading scenarios. The BPS throughput of our constant-power adaptive quadrature amplitude modulation (AQAM) scheme approaches the throughput of variable-power variable-rate AQAM within 1 dB. However, the achievable throughput gain of the adaptive-modulation scheme, in comparison to conventional fixed-mode modems, is substantially reduced as the diversity order of the receiver is increased. Hence, adaptive modulation constitutes a lower complexity alternative to multiple-transmitter and receiver-based systems when considering the range of techniques that can be used for mitigating the effects of the channel-quality fluctuations imposed by wireless channels.     

Minimum BER Block-Based Precoder Design for Zero-Forcing Equalization: An Oblique Projection Framework

This work devises a minimum bit error rate (BER) block-based precoder used in block transmission systems with the proposed cascaded zero-forcing (ZF) equalizer. The study framework is developed as follows. For a block-based precoder, a received signal model is formulated for the two redundancy schemes, viz., trailing-zeros (TZ) and cyclic-prefix (CP). By exploiting the property of oblique projection, a cascaded equalizer for block transmission systems is proposed and implemented with a scheme, in which the inter-block interference (IBI) is completely eliminated by the oblique projection and followed by a matrix degree-of-freedom for inter-symbol interference (ISI) equalization. With the available channel state information at the transmitter side, the matrix for ISI equalization of the cascaded equalizer is utilized to design an optimum block-based precoder, such that the BER is minimized, subject to the ISI-free and the transmission power constraints. Accordingly, the cascaded equalizer with the ISI-free constraint yields a cascaded ZF equalizer. Theoretical derivations and simulation results confirm that the proposed framework not only retains identical BER performance to previous works for cases with sufficient redundancy, but also allows their results to be extended to the cases of insufficient redundancy.     

异步V-BLAST中基于功率扩展的迭代并行干扰消除

在平坦瑞利衰落信道下,异步V-BLAST系统中,现有检测算法随信噪比提高误码率性能改善缓慢。为此,该文提出一种基于预处理矩阵的迭代检测算法:在发射端,通过预处理矩阵将发射信号扩展到整个数据帧上,以获取空时分集度;在接收端,采用低复杂的迭代并行干扰消除方法,由于在迭代过程中干扰重建基于预处理矩阵,所以上次迭代的检测误差被扩展,降低了迭代过程中的误差传播。仿真验证了所提方法的有效性,在8发4收场景下,误码率为10-3时,与现有串行干扰消除方法相比,带来了约7 dB信噪比增益。     

Hidden Pilot Based Precoder Design for MIMO-OFDM Systems

In order to improve bandwidth efficiency and bit error rate (BER) performance, a new hidden pilot scheme using a precoder is proposed for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. We show that the inevitable data interference of the hidden pilot, which degrades the performance of channel estimation, can be reduced successfully by the precoder design for each antenna with the aid of an iterative scheme. We also show that frequency diversity gain can be achieved due to the spreading effect of the precoder. Computer simulations are presented in which the proposed scheme is compared with conventional methods with respect to channel estimation, BER and bandwidth efficiency.     

Uniform channel decomposition for MIMO communications

Assuming the availability of the channel state information at the transmitter (CSIT) and receiver (CSIR), we consider the joint optimal transceiver design for multi-input multi-output (MIMO) communication systems. Using the geometric mean decomposition (GMD), we propose a transceiver design that can decompose, in a strictly capacity lossless manner, a MIMO channel into multiple subchannels with identical capacities. This uniform channel decomposition (UCD) scheme has two implementation forms. One is the combination of a linear precoder and a minimum mean-squared-error VBLAST (MMSE-VBLAST) detector, which is referred to as UCD-VBLAST, and the other includes a dirty paper (DP) precoder and a linear equalizer followed by a DP decoder, which we refer to as UCD-DP. The UCD scheme can provide much convenience for the modulation/demodulation and coding/decoding procedures due to obviating the need for bit allocation. We also show that UCD can achieve the maximal diversity gain. The simulation results show that the UCD scheme exhibits excellent performance, even without the use of any error correcting codes.     

异步垂直空时结构中的功率扩展技术

在平坦衰落信道中,针对异步垂直贝尔实验室空时结构(V-BLAST)信号模型下,现有线性最佳检测算法误码率性能随信噪比提高改善缓慢的问题,提出一种基于功率扩展的迭代检测方法:发射端用功率扩展将发射信号扩展到整个空时信号块上,接收端进行基于功率扩展的迭代检测。同时,分析了所提方法在每次迭代检测后的误码率性能。分析和仿真验证了误码率性能的改进。在4发4收场景下,误码率为10-5时,相比于线性最优最小均方误差(MMSE)方法,获得了约6dB信噪比增益。     

基于线性预编码的异步MIMOOFDM迭代检测算法

In asynchronous Multiple-Input-Multiple-Out-put Orthogonal Frequency Division Multiplexing(MIMO-OFDM) over the selective Rayleigh fading channel, the performance of the existing linear detection algorithms improves slowly as the Signal Noise Ratio (SNR) increases. To improve the performance of asynchronous MIMO-OFDM, a low complexity iterative detection algorithm based on linear precoding is proposed in this paper. At the transmitter, the transmitted signals are spread by precoding matrix to achieve the space-frequency diversity gain, and low complexity iterative Interference Cancellation(IC) algorithm is used at the receiver, which relieves the error propagation by the precoding matrix. The performance improvement is verified by simulations. Under the condition of 4 transmitting antennas and 4 receiving antennas at the BER of 10-4 , about 6 dB gain is obtained by using our proposed algorithm compared with traditional algorithm.     

BER minimized OFDM systems with channel independent precoders

We consider the minimization of uncoded bit error rate (BER) for the orthogonal frequency division multiplexing (OFDM) system with an orthogonal precoder. We analyze the BER performance of precoded OFDM systems with zero forcing and minimum mean squared error (MMSE) receivers. In the case of MMSE receivers, we show that for quadrature phase shift keying (QPSK), there exists a class of optimal precoders that are channel independent. Examples of this class include the discrete Fourier transform (DFT) matrix and the Hadamard matrix. When the precoder is the DFT matrix, the resulting optimal transceiver becomes the single carrier system with cyclic prefix (SC-CP) system. We also show that the worst solution corresponds to the conventional OFDM system; the conventional OFDM system has the largest BER. In the case of zero forcing receivers, the design of optimal transceiver depends on the signal-to-noise ratio (SNR). For higher SNR, solutions of optimal precoders are the same as those of MMSE receivers.     

Optimised secure transmission through untrusted AF relays using link adaptation

A new transmission scheme is presented for a two-hop relay network including two AF relays, considering physical layer security where relays are not able to detect signal with an acceptable bit error rate (BER) but the combined received signal is detected with an acceptable BER at the final receiver. It is assumed that there is no direct path between the transmitter and the receiver (relay network without diversity). Adaptive modulation and coding is utilised at the transmitter and transmission powers of the transmitter and of the relays are continuously adapted provisioning individual average power constraint for each node. Numerical evaluations show that an acceptable performance degradation is seen by the proposed secure relaying scheme compared to the optimum relay selection scheme without security constraint.     

一种联合迫零和独立信道预编码OFDM系统

提出了一种联合迫零和独立信道预编码OFDM系统。该系统用发送端迫零顸编码和哈达玛矩阵预编码对发送数据进行联合预处理,简化了接收端,提高了系统的分集增益,仿真结果表明该方法提高了系统的误码率性能。     

Tomlinson-Harashima precoding with partial channel knowledge

We consider minimum mean-square error Tomlinson-Harashima (MMSE-TH) precoding for time-varying frequency-selective channels. We assume that the receiver estimates the channel and sends the channel state information (CSI) estimate to the transmitter through a lossless feedback channel that introduces a certain delay. Thus, the CSI mismatch at the receiver is due to estimation errors, while the CSI mismatch at the transmitter is due to both estimation errors and channel time variations. We exploit a priori statistical channel knowledge, and we derive an optimal TH precoder, adopting a Bayesian approach. We use simulations to compare the performance of the so-derived TH precoder with that of the same-complexity MMSE decision-feedback equalizer (DFE). We observe that for low signal-to-noise ratios (SNRs) and sufficiently slow channel time variations, the optimal TH precoder outperforms the DFE, while at high SNR, the opposite happens.     

A Technique for Multiuser and Intercarrier Interference Reduction in Multiple-Antenna Multiuser OFDM Downlink

We propose a two-stage precoder/equalizer to suppress intercarrier interference (ICI) and multiuser interference (MUI) in downlink multiuser OFDM with multiple transmit antennas. The first stage, non-linear Tomlinson-Harashima preceding (THP) at the base station (BS) transmitter, mitigates the effect of the spatial inter-stream interference caused by transmission from multiple transmit antennas to decentralized users. In the second stage, each user's receiver employs low- complexity iterative linear minimum mean-square error (MMSE) equalization to suppress the ICI due to frequency offset. Our proposed technique virtually eliminates the bit error rate (BER) degradation due to normalized frequency offsets as high as 10%.     

具有同步跟踪功能的单用户TH-SS PPM超宽带系统性能分析

目前超宽带系统均假设已知发射机与接收机之间准确的传播时延,收发双方完全同步,而任何系统收发双方必然存在同步误差.该文针对这一点,提出了一种具有同步跟踪功能的单用户TH-SS PPM超宽带系统,设计了同步跟踪方法,推导出了系统中存在同步误差时,解调器输出端信噪比计算公式和系统BER表达式.理论分析与仿真结果表明,系统BER与同步误差大小有很大关系,同步误差越小,系统BER越小.     

多用户中继系统中联合收发机的设计方案

在多用户中继系统中,根据最小均方误差(MMSE)准则,提出了一种考虑直接路径的联合收发机设计方案.给出了迭代算法,对中继发射机和用户接收机进行了联合优化,其中用户接收机是用来接收合并源端发送信号和中继发送信号.数据仿真结果表明,提出的方案与不考虑直接路径的收发机方案以及其他已知的LMMSE方案相比,在均方误差(MSE)和误码率(BER)性能方面,有较大的提高.     

基于准正交空时码的最优线性预编码矩阵设计

为了充分利用信道信息改善应用准正交空时分组码的MIMO系统的性能,该文提出了将预编码矩阵和准正交空时分组码相结合的发射机制。信息在经过准正交空时分组码编码后,要经过线性预编码矩阵进行处理,才可以从发射天线发送。文中的预编码矩阵设计利用了MIMO信道的非零均值矩阵和相关矩阵,并采用成对差错概率作为其最优设计准则。仿真结果表明,相对于未经过预编码矩阵的系统,新系统的误码率性能有明显改善。     

Partially-coherent receivers architectures for QAM communications in the presence of non-constant phase estimation error

In this paper, we present a new partially coherent receiver architecture motivated by optimum detection of quadrature amplitude modulation (QAM) signals in the presence of time-varying Tikhonov-distributed residual phase estimation error due to phase-locked loop (PLL)-aided phase tracking scheme. Performance is established in terms of bit error rate (BER). In this paper, an approximate performance measure motivated by union bound is presented for the proposed receiver architecture for 8- and 16-QAM constellations. The performance measures are assessed via simulation and analytical means for additive white Gaussian noise (AWGN) as well as for Rayleigh and Rician fading channels. The performance measures are shown to follow those of the optimum receiver over a wide range of signal-to-noise ratio (SNR), while outperforming a standard coherent receiver operating in the presence of residual phase error by as much as 2 dB.     

The Amplify-and-Forward Half-Duplex Cooperative System: Pairwise Error Probability and Precoder Design

In this paper, an exact asymptotic pairwise error probability (PEP) is derived for a half-duplex cooperative system employing an amplify-and-forward (AF) protocol. When compared with the PEP of a traditional multiple-input multiple-output (MIMO) system, the "diversity gain" for the cooperative system is no longer just a simple exponential function of the signal-to-noise ratio (SNR), rather, it involves the logarithm of the SNR. The term diversity gain function is used to designate this characteristic of the PEP. The coding gain, on the other hand, is found similar to that for the MIMO system and is proportional to the determinant of the autocorrelation of the error matrix. Based on our analysis and observations, we propose a design of unitary precoder for the cooperative system to achieve the full diversity gain function. For the case of a 4-QAM signal being transmitted, we further optimize the coding gain and arrive at a closed-form optimum precoder. Simulations indicate that our proposed precoder designs greatly improve the performance of the cooperative system     

Multiuser MIMO Transceiver Design for Uplink and Downlink with Imperfect CSI

This paper considers a joint linear transmitter and receiver design for multi-user multiple-input multiple-output (MU-MIMO) systems using total mean square error (TMSE) criterion, subject to a total transmit power constraint assuming imperfect channel state information. Both the uplink and downlink MU-MIMO systems, which is employed with improper constellations such as binary phase shift-keying and $M$ -ary amplitude shift-keying are considered. A minimum TMSE design is formulated as a nonconvex optimization problem under a total transmit power constraint and the closed-form optimum linear precoder and decoder for both the downlink and uplink MU-MIMO systems with improper modulation are determined by solving this nonconvex optimization problem. A novel contribution in this paper is to derive a closed-form optimum linear precoder and decoder for both the downlink and uplink MU-MIMO systems with improper modulation by solving the nonconvex optimization problem under total power constraint. The simulation results show that the performance of the proposed design is improved over the previous design.