莱斯多径衰落信道下OFDM系统的容量方差分析

方差是系统容量的一个重要参数,可以用来估计通信系统的中断容量。该文研究了正交频分复用（OFDM）系统在莱斯衰落信道下的容量方差。首先该文建立了多径莱斯信道的模型并且定义了多径莱斯信道的莱斯因子,基于此信道模型推出了一个OFDM系统容量方差新的数学表达式,此表达式以OFDM系统的子载波数、信噪比、信道的多径时延等为参数。基于此表达式,计算机仿真和数值计算研究了信噪比、多径数目、莱斯因子对OFDM系统容量方差的影响。结果表明:计算机仿真和数值计算基本吻合,验证了所推导数学表达式的正确性;系统容量方差与信噪比成正比,与莱斯因子和信道的多径数目成反比。另外,该文以积分的形式给出了任意两个相关莱斯随机变量的联合概率密度函数。      

Effects of Power Amplifier Distortion and Carrier Frequency Offset on the Performance of WLAN System

Due to the efficiency of mitigation multipath delay spread, orthogonal frequency division multiplexing (OFDM) is extensively used in the wireless local area network (WLAN) domain, such as the IEEE 802.11a standard defined by the IEEE 802.11 standardization group and the HIPERLAN/2 defined by the European Telecommunication Standards Institute Project on Broadband Radio Access Networks. OFDM based WLAN system is very sensitive to the power amplifier distortion and the carrier frequency offset. The performance of the OFDM based WLAN system in the presence of the power amplifier distortion and the carrier frequency offset under practical fading channel is researched and analyzed in this paper. A closed form of bit error rate (BER) is derived for the OFDM based WLAN system in the presence of the power amplifier distortion and the carrier frequency offset under practical fading channel. The effects of the power amplifier distortion and the carrier frequency offset on the OFDM based WLAN system performance are comparatively studied by the theoretical method and by the simulation method under practical multipath fading channels. Studies show that the theoretical and simulated results match well.     

Simple ergodic and outage capacity expressions for correlated diversity ricean fading channels

Multiple-antenna systems have been shown to achieve very high spectral efficiencies. In this paper, we derive simple single-integral expressions for the ergodic and outage capacity of a diversity system in correlated Ricean fading channels, where the channel coefficients are assumed to be known to the receiver only. For illustration purpose, we present numerical results showing the effect of channel correlation, Ricean components, angular spread and multipath components in an orthogonal frequency-division multiplexing (OFDM) system     

Exploiting Multipath Diversity in Multiple Antenna OFDM Systems With Spatially Correlated Channels

Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only$0.44lambda$, the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results.     

On the capacity formula for multiple input-multiple output wirelesschannels: a geometric interpretation

The capacity of multiple input, multiple output (MIMO) wireless channels is computed for Ricean channels. The novelty is a geometrical (ray-tracing) interpretation of the MIMO channel capacity formula to find array geometries which greatly enhance channel capacity compared to single input-single output (SISO) systems     

信道老化时高速移动大规模MIMO上行系统的性能分析

针对高速移动场景下大规模多输入多输出(MIMO)上行通信系统,推导了时变莱斯衰落信道中信道老化时大规模MIMO上行通信系统的可达和速率表达式,并且分析了信道老化对系统功率缩放准则的影响.理论分析与仿真结果表明,在时变莱斯信道中,每个用户的发送功率按照1/M进行缩放,信道老化仅会引起系统可达和速率的降低,而莱斯因子对系统可达和速率及功率缩放准则均有影响,且信道老化对系统可达和速率的影响较莱斯因子的影响可以忽略不计.     

基于信道估计的限幅OFDM信号检测性能分析

大峰均比是正交频分复用(OFDM)系统的主要缺陷之一，对时域OFDM信号限幅是通常采用的一种抑制发送信号峰值功率的方法，但是限幅产生的非线性干扰，使系统的信道估计和信号检测性能降低。本文分析了限幅干扰对信道估计以及基于估计信道信息的OFDM数据信号检测性能的影响，推导出了基于信道估计信号检测的信噪比与限幅参数之间的关系。利用信噪比和高斯信道的误符号率及误比特率公式，本文对频率选择性瑞利衰落信道环境限幅OFDM系统中信号的误符号率和误比特率进行估算。计算机仿真和估算的结果相吻合。     

MIMO solution for performance improvements of OFDM–CDMA system with pilot tone

In this paper, MIMO OFDM–CDMA downlink scheme is proposed as a solution for improving performance of the OFDM–CDMA downlink system with pilot tone and threshold detection combining (optimum TDC). The new presented system with MIMO included uses space–time block coding applied to two, three and four transmit antennas and it has an arbitrary number of receive antennas. Bit error rate performance in the case of Ricean frequency selective fading is evaluated for the original system as well as for the one with MIMO included. For that reason an adequate simulation model is developed. The results show that the proposed system significantly outperforms the OFDM–CDMA downlink system with pilot tone and optimum TDC.     

Symbol error rate performance of nonlinear OFDM receiver with peak value threshold over frequency selective fading channel

Nonlinear blanking and clipping methods are widely used in Orthogonal Frequency Division Multiplexing (OFDM) receiver to mitigate impulse interference. To quantitatively analyze the reliability performance of nonlinear OFDM receivers with pulse blanking and clipping based on peak value threshold, the symbol error rate (SER) performance of nonlinear OFDM receiver over frequency selective Rayleigh and Ricean fading channels is presented. Firstly, the analytical expressions of instantaneous output signal-to-interference and noise ratio (SINR) for nonlinear OFDM receivers with regular method, peak value blanking and peak value clipping are derived. Then, the SER performance of nonlinear OFDM receiver over frequency selective Rayleigh and Ricean fading channels is given based on the SINR expressions. Finally, simulation results are demonstrated to show good agreement with theoretical results. It has been observed that the peak value blanking method has achieved the best SER performance, and the inter-carrier interference based on the peak value blanking and peak value clipping will lead to error floor.     

Analysis and Solutions for Amplitude Distortions in an OFDM Signal at Communication Receivers

This paper presents an analysis of amplitude distortions happening in an orthogonal frequency division multiplexing (OFDM) signal at communication receivers. It is shown in this paper that the amplitude distortion causes inter-carrier interference (ICI) in an OFDM system. Simple but efficient solutions are proposed for combating different amplitude distortions and subsequent ICI cancellation. Simulation results in typical urban channel and Gaussian channel show that extra gains are obtained by introducing these techniques into an existing OFDM receiver system.      

Symbol-level adaptive modulation for coded OFDM on block fading channels

It has recently been recognized by many researchers that adaptive modulation is most effective when the channel diversity order is small. In this letter, we propose a simple adaptive modulation scheme for orthogonal frequency division multiplexing (OFDM) systems on channels that provide a small order of diversity. The proposed adaptation algorithm is based on a novel and very simple analytical formula we derive for the performance of BICM on block fading channels. Simulation results show that the derived analytical formula is very tight when the targeted bit error probability is small. OFDM systems on indoor channels provide small orders of diversity and form an ideal scenario for adaptation. In order to keep system complexity and feedback requirements at a minimum, we will consider symbol-level adaptive modulation wherein all the subcarriers in an OFDM symbol use the same modulation.     

BER analysis of arbitrary QAM for MRC diversity with imperfect channel estimation in generalized ricean fading channels

Imperfect channel estimation (ICE) can severely degrade the bit error rate (BER) of digital modulations with maximum ratio combining (MRC) diversity reception. The resulting performance analysis problem in its most general setting has not been addressed before. In this paper, the effect of ICE on the BER of an arbitrary square/rectangular Gray-coded quadratic amplitude modulation (QAM) in generalized Ricean fading channels when MRC reception is employed is analyzed. A general expression for the bit error probability of an arbitrary square/rectangular QAM scheme is first derived. This general formula requires a number of conditional probabilities, which is derived in closed form for independent and nonidentically distributed (i.n.d.) Rayleigh-fading channels with MRC and ICE. An efficient numerical method is also presented to compute the conditional probabilities for i.n.d. and correlated Ricean fading. In addition, extensive Monte Carlo simulations that agree excellently with the analytical results are presented.     

Transforming the Statistical Distribution of Wireless OFDM Signal for PAPR Reduction

To reduce the high peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signal, a novel nonlinear companding transform (CT) scheme is proposed in this paper. This scheme can reallocate both the amplitude/power as well as statistical distribution of the companded signal more reasonably and flexibly than existing CT methods with low computational complexity. By choosing an appropriate companding parameter, it can provide more effective PAPR reduction but at the price of a minimal amount of bit-error-rate (BER) performance degradation caused by the companding distortion. The closed-form expressions including the achievable PAPR gain, signal attenuation factor, and corresponding selection criteria for the companding parameter were derived. Computer simulations demonstrate that the scheme significantly improves the overall performances of OFDM system in terms of PAPR, BER and bandwidth efficiency under the multipath fading channel or with the high power amplifier.

     

Level Crossing Rates of Ricean MIMO Channel Eigenvalues for Imperfect and Outdated CSI

In this paper, the exact closed-form expressions for the level crossing rates of the multiple-input multiple output (MIMO) channel ordered eigenvalues are derived for the Ricean propagation and arbitrary channel matrix dimensions. The novel expressions can be applied for the imperfect channel state information (CSI) and feedback delay, with or without prediction. The analytical results are validated by simulation.     

Improved transmit steering for MIMO-OFDM downlinks with distributed base station antenna arrays

Space-division multiple-access (SDMA) is a communication technique that enables a base station to communicate with several mobile users simultaneously. The ability of the base station to spatially separate several users depends on the pairwise cross correlations between the channel matrices of the users (the inter-user correlation). In this paper, we propose an improved steering downlink multiple-input-multiple-output-orthogonal frequency-division multiplexing (OFDM) system that reduces both the inter-user correlation and the near-far problem resulting in a significant enhancement in system performance. In this system, several base station multiantenna arrays are distributed in a given area. Each array communicates with the base station via optical fiber links, and all transmitter signal processing is performed at the base station. Multiantenna users are spatially separated such that only a subset of the users is served by each tone of the OFDM symbol. The served users are selected based on an algorithm that reduces the inter-user correlations. Distributing the arrays around the users also balances the channel matrix leading to significant reduction in the effect of the near-far problem. The channel matrix of each user is assumed correlated and Ricean distributed. Several data symbols can be spatially multiplexed to each user over each OFDM tone with high reliability and with good total system capacity.     

Coded modulation for a coherent DS-CDMA system employing an MMSE receiver in a fading channel

Previous results have shown that high rate codes tend to yield a lower average bit-error rate than low rate codes when employing a minimum mean-square error (MMSE) receiver for a direct-sequence code-division multiple-access (CDMA) system in either an additive white Gaussian noise channel or a flat Rayleigh fading channel. we consider the use of larger signal constellations with both trellis-coded modulation and bit-interleaved coded modulation (BICM) to determine if further gains can be achieved in either the Rayleigh or Ricean fading channel. The average bit-error probability is derived for both coding schemes using the general Ricean fading channel model, based upon the common assumptions of infinite interleaving, perfect channel state information, and optimal MMSE receiver coefficients. New bounds are presented for BICM with 8-PSK and 16-QAM symbols, which take advantage of the symmetries inherent in the signal constellations with Gray code mapping. In addition, simulation results are presented which show the important effect a finite interleaving delay constraint has on the comparison of various codes. The results show that there are cases when coded modulation does yield a significant improvement in performance for a CDMA system using an MMSE receiver, compared to standard convolutional coding. However, the best coding strategy depends upon several factors, including the nature of the fading process (Rayleigh or Ricean), the operating signal-to-noise ratio, the interleaving delay constraint, the time-variability of the channel, the number of users in the system, and the severity of the near-far problem.     

Ergodic capacity formula of MIMO-OFDM systems under correlated frequency selective Rayleigh fading channels

An explicit formula for the ergodic capacity of Orthogonal Frequency Division Multiplexing (OFDM)-based Multiple-Input Multiple-Output (MIMO) systems under correlated frequency selective Rayleigh channels is derived,by simplifying the channel response matrix in frequency domain into the so-called Kronecker model composed of three kinds of correlations,i.e.multipath tap gain correlation and spatial fading correlations at both transmitter and receiver.The derived formula is very simple and convenient for one to estimate the effects of all three kinds of correlations on MIMO-OFDM capacity.If taps are independent,there is a very simple expression for the ergodic capacity.In case of tap correlation,the capacity formula could be further given in an integral expression.The validity of the new formula is verified and the effects of correlations,delay spread as well as the number of subcarriers on the ergodic capacity are evaluated via Monte Carlo simulations.     

Parallel combinatory OFDM signaling

A parallel combinatory-orthogonal frequency division multiplexing (PC-OFDM) system is proposed and analyzed. The proposed system selects at each symbol interval a subset of the available subcarriers, and the selected subcarriers are modulated by points from an M-PSK signal constellation, PC-OFDM systems can be designed to have lower peak-to-average power ratio (PAPR), higher bandwidth efficiency, and lower bit error probability on Gaussian channels compared to ordinary OFDM systems. A bit mapping procedure using the Johnson (1993) association, together with a position algorithm for the PSK symbols, is proposed. Good analytical approximations of the BER for PC-OFDM systems are derived for AWGN and Ricean fading channels, and extensive simulation results are presented     

Performance of cross-layer design for orthogonal space-time block coded MIMO systems with imperfect CSI in Ricean fading channels

A cross-layer design (CLD) scheme for orthogonal space-time block coded MIMO systems with imperfect channel state information is presented by combining adaptive modulation and automatic repeat request, and the corresponding system performance is investigated over Ricean fading channel. The fading gain value is partitioned into a number of regions by which the modulation is adapted in terms of the region the fading gain falls in. The fading gain switching thresholds subject to a target packet error rate (PER) constraint are derived. According to these results, and using the generalized Marcum Q-function, we derive the theoretical formulae of average PER and spectrum efficiency (SE) of the system with CLD for both perfect and imperfect estimation in detail. As a result, closed-form expressions for average PER and SE are obtained. These expressions include some existing expressions in Rayleigh channel as special cases. With these expressions, the system performance in Ricean channel with perfect and imperfect estimation information can be evaluated effectively. Computer simulation for average PER and SE show that the theoretical analysis and simulation are consistent. The results show that the system performance will be effectively improved as Ricean factor increases, but it will be degraded as estimation errors increases.