Efficient use of DS/CDMA signals for broadband communications over power lines

This paper investigates the use of direct‐sequence/code‐division multiple access (DS/CDMA) signals for broadband communications over power lines. Each user is assumed to utilize all available spreading codes for sending the information to the destination. The transmitter and the receiver are assumed to have perfect channel knowledge with the receiver employing a zero‐forcing multiuser detector. Based on channel knowledge we attempt to maximize the data throughput by suitable choice of the number of codes used and the power and the constellation size (bit‐load) assigned to the data modulating each spreading code. We employ Gold codes, in addition to special codes derived based on the channel knowledge for ISI minimization, termed ‘eigen codes’. In contrast to some earlier results concerning CDMA and OFDM, we show that DS/CDMA signals can be optimized to achieve an overall data throughput of approximately 80% of that achieved by OFDM systems. This result shows that DS/CDMA signaling can be a good candidate for broadband power line communications. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance analysis and design optimization of LDPC-coded MIMO OFDM systems

We consider the performance analysis and design optimization of low-density parity check (LDPC) coded multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems for high data rate wireless transmission. The tools of density evolution with mixture Gaussian approximations are used to optimize irregular LDPC codes and to compute minimum operational signal-to-noise ratios (SNRs) for ergodic MIMO OFDM channels. In particular, the optimization is done for various MIMO OFDM system configurations, which include a different number of antennas, different channel models, and different demodulation schemes; the optimized performance is compared with the corresponding channel capacity. It is shown that along with the optimized irregular LDPC codes, a turbo iterative receiver that consists of a soft maximum a posteriori (MAP) demodulator and a belief-propagation LDPC decoder can perform within 1 dB from the ergodic capacity of the MIMO OFDM systems under consideration. It is also shown that compared with the optimal MAP demodulator-based receivers, the receivers employing a low-complexity linear minimum mean-square-error soft-interference-cancellation (LMMSE-SIC) demodulator have a small performance loss (< 1dB) in spatially uncorrelated MIMO channels but suffer extra performance loss in MIMO channels with spatial correlation. Finally, from the LDPC profiles that already are optimized for ergodic channels, we heuristically construct small block-size irregular LDPC codes for outage MIMO OFDM channels; as shown from simulation results, the irregular LDPC codes constructed here are helpful in expediting the convergence of the iterative receivers.     

Channel estimation for OFDM systems with transmitter diversity inmobile wireless channels

Transmitter diversity is an effective technique to improve wireless communication performance. In this paper, we investigate transmitter diversity using space-time coding for orthogonal frequency division multiplexing (OFDM) systems in high-speed wireless data applications. We develop channel parameter estimation approaches, which are crucial for the decoding of the space-time codes, and we derive the MSE bounds of the estimators. The overall receiver performance using such a transmitter diversity scheme is demonstrated by extensive computer simulations. For an OFDM system with two transmitter antennas and two receiver antennas with transmission efficiency as high as 1.475 bits/s/Hz, the required signal-to-noise ratio is only about 7 dB for a 1% bit error rate and 9 dB for a 10% word error rate assuming channels with two-ray, typical urban, and hilly terrain delay profiles, and a 40-Hz Doppler frequency. In summary, with the proposed channel estimator, combining OPDM with transmitter diversity using space-time coding is a promising technique for highly efficient data transmission over mobile wireless channels     

联合正交投影与CLEAN的测距仪脉冲干扰抑制方法

针对L频段数字航空通信系统1(L-DACS1)以内嵌方式部署在航空无线电导航频段而产生的高强度测距仪脉冲信号干扰正交频分复用(OFDM)接收机问题,提出联合正交投影与CLEAN的测距仪脉冲干扰抑制方法。接收机首先通过将接收信号矢量投影到干扰信号正交补空间的方法消除高强度测距仪脉冲干扰,然后利用OFDM信号循环前缀的对称特性,采用CLEAN算法估计信号来向,然后通过波束成形提取OFDM直射径信号。计算机仿真表明:论文提出方法可有效克服测距仪脉冲及OFDM散射径信号的干扰,提高L频段数字航空通信系统1的链路传输的可靠性。      

采用基于互补序列分组编码的OFDM系统性能分析与仿真

为了减小正交频分复用（OFDM）信号的峰值-平均值功率之比（PAPR），本文利用互补序列和Reed-Muller码的关系，详细提出了一种构造互补序列的分组编码方法的具体实现方案。分析了其在AWGN和选频衰落信道中的性能，并做了相应的仿真。仿真结果表明，编码后每个OFDM信号的最大PAPR不超过3dB；采用该编码方法的OFDM在AWGN中当信噪比不到11dB时就可以实现BER为10^-6，在衰落信道中如果采用软判决译码，则当信噪比达到20dB左右时可以实现BER为10^-3。     

Performance analysis of deliberately clipped OFDM signals

We analyze the performance of the clipped orthogonal frequency division multiplexing (OFDM) system in terms of peak power reduction capability and degradation of channel capacity. The clipping is performed on the baseband OFDM signals with and without oversampling, followed by the ideal low-pass filter. First, the effect of the envelope clipping on the peak-to-average power ratio (PAPR) and the instantaneous power of the band-limited OFDM signal is studied. We then discuss the channel capacity of the oversampled and clipped OFDM signals over the additive white Gaussian noise and ideally interleaved Rayleigh fading channels. The capacity is calculated based on the assumption that the distortion terms caused by the clipping are Gaussian. It is shown that the SNR penalty due to the clipping can be considerably alleviated by using optimal coding and reducing the information data rate. The results are justified by the simulation results using near optimal turbo codes     

Differential modulation techniques for a 34 MBit/s radio channel using orthogonal frequency division multiplexing

The orthogonal frequency division multiplexing (OFDM) technique has been proposed for terrestrial digital transmission systems due to its high spectral efficiency, its robustness in different multipath propagation environments and the ability of avoiding intersymbol interference (ISI). Our studies consider a radio channel bandwidth of 8 MHz and a data rate of 34 Mbit/s.In the case of the OFDM transmission system a coherent 64-QAM requires a channel estimation process and a channel equalization in frequency-selective interference situations [4]. The equalization process can be realized by a multiplier bank at the FFT output in the receiver, a so-called frequency-domain equalizer. Alternatively, a multilevel differential modulation technique, the so-called differential amplitude and phase shift keying (64-DAPSK) considering the phase and simultaneously the amplitude for differential modulation, is proposed and presented in this paper. Differential modulation/demodulation techniques do not require any explicit knowledge about the radio channel properties in the differential channel equalization. It is therefore not necessary to implement a frequency-domain equalizer in an OFDM/64-DAPSK receiver, which reduces the computation complexity. The performance of both modulation techniques has been analysed in the uncoded and coded case referring to Gaussian and frequency-selective Rayleigh fading channels. Simulation results are presented in this paper.The OFDM signal has a non-constant envelope with large instantaneous power spikes possible primarily resulting in an overdriving of the high power amplifier (HPA) at the transmitter. This leads to nonlinear distortion causing intermodulation noise and spectral spreading. Both effects can be limited by introducing an appropriate input backoff (IBO). In this paper the performance of OFDM signals in the presence of nonlinearities is analysed quantitatively.     

Precoded OFDM with adaptive vector channel allocation for scalable video transmission over frequency-selective fading channels

Orthogonal frequency division multiplexing (OFDM) has been applied in broadband wireline and wireless systems for high data rate transmission where severe intersymbol interference (ISI) always occurs. The conventional OFDM system provides advantages through conversion of an ISI channel into ISI-free subchannels at multiple frequency bands. However, it may suffer from channel spectral s and heavy data rate overhead due to cyclic prefix insertion. Previously, a new OFDM framework, the precoded OFDM, has been proposed to mitigate the above two problems through precoding and conversion of an ISI channel into ISI-free vector channels. In this paper, we consider the application of the precoded OFDM system to efficient scalable video transmission. We propose to enhance the precoded OFDM system with adaptive vector channel allocation to provide stronger protection against errors to more important layers in the layered bit stream structure of scalable video. The more critical layers, or equivalently, the lower layers, are allocated vector channels of higher transmission quality. The channel quality is characterized by Frobenius norm metrics; based on channel estimation at the receiver. The channel allocation information is fed back periodically to the transmitter through a control channel. Simulation results have demonstrated the robustness of the proposed scheme to noise and fading inherent in wireless channels.     

An efficient space-frequency coded OFDM system for broadband wireless communications

We propose an efficient space-frequency coded orthogonal frequency-division multiplexing (OFDM) system for high-speed transmission over wireless links. The analytical expression for the pairwise probability of the proposed space-frequency coded OFDM system is derived in slow, space- and frequency-selective fading channels. The design criteria of trellis codes used in the proposed system are then developed and discussed. It is shown that the proposed space-frequency coded OFDM can efficiently achieve the full diversity provided by the fading channel with low trellis complexity, while for traditional space-frequency coded OFDM systems, we need to design space-time trellis codes with high trellis complexity to exploit the maximum achievable diversity order. The capacity properties of space-frequency coded OFDM over multipath fading channels are also studied. Numerical results are provided to demonstrate the significant performance improvement obtained by the proposed space-frequency coded OFDM scheme, as well as the excellent outage capacity properties.     

ZP-CI/OFDM: A Power Efficient Wireless Transmission Technology

Low power efficiency is a deficiency in traditional Orthogonal Frequency Division Multiplexing (OFDM) systems. To counter this problem, a new wireless transmission technology based on Zero-Padding Carrier Interferometry OFDM (ZP-CI/OFDM) is proposed. In a ZP-CI/OFDM system, transmission symbols are spread to all OFDM subcarriers via carrier interferometry codes. This reduces the Peak-to-Average Power Ratio (PAPR) that traditional OFDM suffers and also exploits frequency diversity gain. By zero-padding at the transmitter, advanced receiver technologies can be adopted for ZP-CI/OFDM so that frequency diversity gain can be further utilized and the power efficiency of the system is improved.     

ZP-CI/OFDM：一种高功率效率的无线传输技术

针对传统正交频分复用（OFDM）系统的功率效率问题,文章给出一种新的基于添零方式的载波干涉正交频分复用（ZP-CI/OFDM）无线传输技术。ZP-CI/OFDM通过载波干涉码将发射符号扩展到所有OFDM子载波上,在有效消除传统OFDM面临的峰值平均功率比问题的同时,充分利用多载波的频率分集增益;同时,ZP-CI/OFDM通过在发射端添零,采用先进的接收机技术进一步利用频率分集增益,提高系统的功率效率。     

Combined array processing and space-time coding

The information capacity of wireless communication systems may be increased dramatically by employing multiple transmit and receive antennas. The goal of system design is to exploit this capacity in a practical way. An effective approach to increasing data rate over wireless channels is to employ space-time coding techniques appropriate to multiple transmit antennas. These space-time codes introduce temporal and spatial correlation into signals transmitted from different antennas, so as to provide diversity at the receiver, and coding gain over an uncoded system. For large number of transmit antennas and at high bandwidth efficiencies, the receiver may become too complex whenever correlation across transmit antennas is introduced. This paper dramatically reduces encoding and decoding complexity by partitioning antennas at the transmitter into small groups, and using individual space-time codes, called the component codes, to transmit information from each group of antennas. At the receiver, an individual space-time code is decoded by a novel linear processing technique that suppresses signals transmitted by other groups of antennas by treating them as interference. A simple receiver structure is derived that provides diversity and coding gain over uncoded systems. This combination of array processing at the receiver and coding techniques for multiple transmit antennas can provide reliable and very high data rate communication over narrowband wireless channels. A refinement of this basic structure gives rise to a multilayered space-time architecture that both generalizes and improves upon the layered space-time architecture proposed by Foschini (see Bell Labs Tech. J., vol.1, no.2, 1996)     

Multiband OFDM for Covert Acoustic Communications

A multiband OFDM transmitter and receiver are presented for underwater communications at low SNR. Compared with a single-band OFDMscheme, the multiband approach leads to a considerable reduction in the receiver complexity. The proposed system has been tested at sea with 16 subbands covering a total bandwidth of 3.6 kHz, at user data rates of 4.2 and 78 bit/s, and over ranges up to 52 km. At the lower rate, successful message recovery is achieved on a single hydrophone at SNRs down to ?17 dB in a benign channel. In channels with a severe delay-Doppler spread the critical SNR rises by some 4 dB. At 78 bit/s the limits of the OFDM signaling scheme are clearly revealed, but at 4.2 bit/s the performance is limited by failure of signal detection and initial synchronization.     

Broadband MIMO-OFDM wireless communications

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate wireless transmission. OFDM may be combined with antenna arrays at the transmitter and receiver to increase the diversity gain and/or to enhance the system capacity on time-varying and frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) configuration. The paper explores various physical layer research challenges in MIMO-OFDM system design, including physical channel measurements and modeling, analog beam forming techniques using adaptive antenna arrays, space-time techniques for MIMO-OFDM, error control coding techniques, OFDM preamble and packet design, and signal processing algorithms used to perform time and frequency synchronization, channel estimation, and channel tracking in MIMO-OFDM systems. Finally, the paper considers a software radio implementation of MIMO-OFDM.     

基于时变衰落信道分解的OFDM信号接收技术

OFDM信号在时变衰落信道中传输时存在载波间干扰(ICI),降低了系统的性能。基于衰落信道的多普勒分解,OFDM信号在时变衰落信道中的传输可以等效成信息数据直接通过无衰落的离散白噪声滤波器模型信道传输,改进的Viterbi算法可用于接收这类信号。该接收技术可以有效去除传统的OFDM系统中的载波间干扰,消除误码的地板效应,提高了系统性能。     

基于DFT滤波器组的多载波调制系统设计

采用DFT滤波器组来代替OFDM系统中的IDFT/DFT模块,形成基于滤波器组的收发器。设计适于多经衰落信道的DFT滤波器组收发器。其均衡与OFDM系统一样,是在接收端采用单抽头的均衡器。实验采用随机多径信道,仿真结果表明所提出的滤波器组收发器可以获得较好的频谱特性,以及较满意的SIR（信号与干扰比值）。     

A distributed differentially encoded OFDM scheme for asynchronous cooperative systems with low probability of interception

Recently, Li, Hwu and Ratazzi have proposed a physical-layer security design to guarantee low probability of interception (LPI) for MIMO systems without relying on upperlayer data encryption. The proposed scheme utilizes antenna array redundancy to deliberately randomize the transmitted signals to prevent eavesdropping. Motivated by their idea, in this paper we design a physical-layer transmission scheme to achieve LPI in cooperative systems. There are two major differences in cooperative systems: 1) each relay node may have only one antenna that can not provide antenna array redundancy for signal randomization; 2) there may exist timing errors due to the asynchronous nature of cooperative systems. Considering the two differences, we propose a distributed differentially encoded OFDMtransmission scheme with deliberate signal randomization to prevent eavesdropping and exploit the available spatial and frequency diversities in asynchronous cooperative systems. We use diagonal unitary codes to perform the differential encoding in the frequency domain over subcarriers within each OFDM block, or we use general (not necessarily diagonal) unitary codes to perform the differential encoding in the frequency domain across several OFDM blocks. By some deliberate signal randomization, the eavesdropper can not detect the transmitted symbols, while the authorized receiver can perform differential decoding successfully without the knowledge of the channels or the timing errors.     

正交空时分组码在OFDM系统中的性能估计

在宽带OFDM系统中对正交空时分组码方案进行了研究,根据Almouti方案的译码原理给出了在正交空时分组码传输的频率选择性衰落信道条件下接收机输出瞬时信噪比的一般表达式,同时分两种情况进一步分析了其最小距离球界的符号差错性能。结果表明,在系统发送天线数、接收天线数及多径数目乘积较小的情形下,系统可以达到最大的分集增益。     

Pulse Shaping in OFDM Based Single Frequency Networks

In systems using Orthogonal Frequency Division Multiplexed (OFDM) all transmitters in a radio broadcasting network may transmit identical signals in the same frequency block, forming a so called Single Frequency Network (SFN). The simultaneous transmission creates a severe artificial multipath propagation causing interference at the receiver. This interference consists not only by intersymbol interference (ISI), but also interchannel interference (ICI) due to orthogonality losses between OFDM scheme's subcarriers. A method that is often used to counter the self-interference is to insert a guard interval between consecutive OFDM frames combined with an increased symbol duration. The receiver in an SFN is normally experiencing a diversity gain of receiving several signals. Doubts concerning the performance has been raised due to the OFDM scheme's sensitivity to frequency errors. If the received signals are exposed to different carrier offsets it will lead to ICI, resulting into reduced coverage. In this paper we apply pulse shaping to reduce the effects of ISI and ICI. Pulse shaping of the transmitted OFDM signal is shown to give significant reductions in self-interference. It is hereby possible to achieve the same coverage with a reduced amount of transmitters. The system also becomes less sensitive to time and frequency synchronisation errors. A method to analyse the performance for time limited pulse shapes is presented using weight functions describing the amount of useful and interfering powers. In the receiver analysis, generalised expressions are given using Fourier transforms and series.     

MIMO OFDM Receivers for Systems With IQ Imbalances

Orthogonal frequency division multiplexing (OFDM) is a widely recognized modulation scheme for high data rate communications. However, the implementation of OFDM-based systems suffers from in-phase and quadrature-phase (IQ) imbalances in the front-end analog processing. Such imbalances are caused by the analog processing of the received radio frequency (RF) signal, and they cannot be efficiently or entirely eliminated in the analog domain. The resulting IQ distortion limits the achievable operating SNR at the receiver and, consequently, the achievable data rates. The issue of IQ imbalances is even more severe at higher SNR and higher carrier frequencies. In this paper, the effect of IQ imbalances on multi-input multioutput (MIMO) OFDM systems is studied, and a framework for combating such distortions through digital signal processing is developed. An input–output relation governing MIMO OFDM systems is derived. The framework is used to design receiver algorithms with compensation for IQ imbalances. It is shown that the complexity of the system at the receiver grows from dimension$(n_Rtimes n_T)$for ideal IQ branches to$(2n_Rtimes 2n_T)$in the presence of IQ imbalances. However, by exploiting the structure of space-time block codes along with the distortion models, one can obtain efficient receivers that are robust to IQ imbalances. Simulation results show significant improvement in the achievable BER of the proposed MIMO receivers for space-time block-coded OFDM systems in the presence of IQ imbalances.