Theoretical analysis and performance of OFDM signals in nonlinearAWGN channels

Orthogonal frequency-division multiplexing (OFDM) baseband signals may be modeled by complex Gaussian processes with Rayleigh envelope distribution and uniform phase distribution, if the number of carriers is sufficiently large. The output correlation function of instantaneous nonlinear amplifiers and the signal-to-distortion ratio can be derived and expressed in an easy way. As a consequence, the output spectrum and the bit-error rate (BER) performance of OFDM systems in nonlinear additive white Gaussian noise channels are predictable both for uncompensated amplitude modulation/amplitude modulation (AM/AM) and amplitude modulation/pulse modulation (AM/PM) distortions and for ideal predistortion. The aim of this work is to obtain the analytical expressions for the output correlation function of a nonlinear device and for the BER performance. The results in closed-form solutions are derived for AM/AM and AM/PM curves approximated by Bessel series expansion and for the ideal predistortion case     

Blind and semiblind detections of OFDM signals in fading channels

This paper considers the problem of blind joint channel estimation and data detection for orthogonal frequency-division multiplexing (OFDM) systems in a fading environment. Employing a regression model for a time-varying channel, we convert the problem into one that finds the data sequence x whose associated least-squares (LS) channel estimate z(x) is closest to the space of some regression curves (surfaces). We apply the branch-and-bound principle to solve the nonlinear integer programming problem associated with finding the curve that fits a subchannel in the LS sense. A recursive formula for fast metric update is obtained by exploiting the intrinsic characteristic of our objective function. The impacts of reordering the data sequence and selective detection are addressed. By employing a preferred order along with a selective detection method, we greatly reduce the detector complexity while giving up little performance loss. Both the complete and the reduced-complexity algorithms can be used for blind and semiblind detections of OFDM signals in a subchannel-by-subchannel manner. To further reduce the complexity and exploit the frequency-domain channel correlation, we suggest a two-stage approach that detects a few selected positions in some subchannels first, and then, treating the detected symbols as pilots, determines the remaining symbols within a properly chosen time-frequency block by a two-dimensional model-based pilot-assisted algorithm. The proposed methods do not require the information of the channel statistics like signal-to-noise ratio or channel correlation function. Performance of differential modulations like differential quaternary phase-shift keying and STAR 16-ary quadrature amplitude modulation are provided. Both blind and semiblind schemes yield satisfactory performance.     

Theoretical analysis and performance of the decorrelating detector for DS-CDMA signals in nonlinear channels

We analyze the effects of the nonlinear distortions, introduced by the transmitting high-power amplifier (HPA), on the performance of the linear decorrelating multiuser detector in direct-sequence code-division multiple-access (DS-CDMA) downlink systems. By assuming and motivating the Gaussian distribution of the nonlinear distortion noise, the symbol-error rate (SER) and the total degradation (TD) has been derived theoretically in additive white Gaussian noise (AWGN) and frequency-flat Rayleigh fading channels. Simulation results show that the analytical model is quite accurate in many scenarios.     

一种衰落信道下OFDM信号的半盲检测算法

本文提出了一种衰落信道下OFDM信号的半盲检测算法。采用文献[1]为时变OFDM信道建立的2维非线性递归模型,本文利用稀疏的导频符号估计信道响应,并运用最短路径搜索原理寻找原始的发送序列。计算机仿真结果证明,该算法对OFDM信号进行了快速、准确的半盲检测。     

Accurate error-rate performance analysis of OFDM on frequency-selective Nakagami-m fading channels

Error rates of orthogonal frequency-division multiplexing (OFDM) signals in multipath slow fading Nakagami-m fading channels are considered. The exact probability density function of a sum of Nakagami-m random phase vectors is used to derive a closed-form expression for the error rates of OFDM signals. The precise error-rate analysis is extended to a system using multichannel reception with maximal ratio combining. An asymptotic error-rate analysis is also provided. For a two-tap channel with finite values of Nakagami-m fading parameters, our analysis and numerical results show that the asymptotic error-rate performance of an OFDM signal is similar to that of a single carrier signal transmitted over a Rayleigh fading channel. On the other hand, our analysis further shows that a frequency-selective channel that can be represented by two constant taps has similar asymptotic error-rate performance to that of a one-sided Gaussian fading channel. It is observed that, depending on the number of channel taps, the error-rate performance does not necessarily improve with increasing Nakagami-m fading parameters.     

Maximum-likelihood frequency recovery for OFDM signals transmitted over multipath fading channels

In this paper, a novel feedback frequency synchronizer for orthogonal frequency-division multiplexing signals transmitted over multipath fading channels is described. Its derivation is based on maximum-likelihood estimation techniques and assumes an approximate statistical knowledge of the communication channel. The performance of the proposed algorithm is assessed by computer simulation, and is compared with that provided by other synchronization algorithms and with Cramer-Rao bounds.     

OFDM code-division multiplexing in fading channels

In this paper, orthogonal frequency-division multiplexing-code-division multiplexing (OFDM-CDM) is presented and investigated as alternative to conventional OFDM for high rate data transmission. An additional multipath diversity gain can be obtained with OFDM-CDM by spreading each data symbol in frequency and time. The optimum reliability information for the Viterbi (1979) decoder is derived for OFDM-CDM systems, and the tradeoff between spreading and channel coding in OFDM systems is presented. By using efficient interference cancellation or joint detection techniques, it can be shown that OFDM-CDM outperforms conventional OFDM with respect to bit error rate (BER) performance and bandwidth efficiency.     

Analysis and optimization of the performance of OFDM onfrequency-selective time-selective fading channels

In mobile radio communication, the fading channels generally exhibit both time-selectivity and frequency-selectivity. Orthogonal frequency-division multiplexing has been proposed to combat the frequency-selectivity, but its performance is also affected by the time-selectivity. We investigate how various parameters, such as the number of carriers, the guard time length, and the sampling offset between receiver and transmitter, affect the system performance. Further, we determine the optimum values of the above parameters, which minimize the degradation of the signal to-noise ratio at the input of the decision device     

BER performance of OFDM system over frequency nonselective fastRicean fading channels

The fast changing frequency nonselective Ricean fading channel introduces a complicated multiple distortion and an extra additive noise component for an OFDM system. The multiple distortion is the average of the sum of N(N⩾3) correlated Ricean random variables. We propose an approximate technique for calculating the probability density function (PDF) of the multiple distortion under the assumption that the channel response changes in a linear fashion during one OFDM symbol. As a result, the bit error rate (BER) formula of a BPSK modulated OFDM system is derived. The results obtained using the derived formula describe well the OFDM performance under the time variant channel and match very well with the simulation results     

适用于多径衰落信道的OFDM同步方法

针对正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)系统在多径衰落信道中容易受到同步误差的影响,本文提出了两种OFDM符号同步方法.该方法利用重复共轭对称的训练序列和改进的定时测度与判决方法完成符号定时;利用序列的一致性与频域差分编码进行频偏估计.分析与仿真结果表明,在多径衰落信道中与传统方法相比,本文提出的方法具有更好的同步性能和较低的计算复杂度.     

Simulated error performance of encoded MSK signals in Gaussian andRician fading channels

New trellis codes are constructed that are based on binary concordant convolutional codes and minimum shift keying. The codes are investigated in various communication channels by means of computer simulation     

Frames theoretic analysis of zero-padding OFDM over deep fading wireless channels

The performance of the orthogonal frequency division multiplexing (OFDM) systems may be severely deteriorated when passing through wireless channels with the spectral s. Zero-Padding (ZP) for the OFDM signal guarantees the symbol recovery regardless of the channel spectral s if complicated ZP-OFDM-MMSE equalizer is used. In this paper we first establish a connection between ZP-OFDM and DFT codes and then two novel signal reconstruction schemes, signal projection and low-complexity signal reconstruction, are presented to deal with the spectral s based on the frames theory. The simulation results demonstrate that significant performance improvement can be achieved using our proposed methods over the so-called ZP-OFDM overlap and add (ZP-OFDM OLA) scheme.     

OFDM channel estimation with dispersive fading channels

In this paper, we studied the channel estimation problem for the orthogonal frequency division multiplexing (OFDM) system when the statistics of the multi-path fading channel is not known or partial known. A channel estimation approach based on polynomial approximation of the channel response is proposed. The pilot symbols are periodically inserted the channel responses for entire OFDM data sequence for exploiting channel correlation in both time and frequency domain, which is obtained from a time-variant frequency-selective Rayleigh fading channel model. Simulation shows that the method is robust to different channel statistics. Moreover, a window dimension adaptation algorithm is proposed to adapt the channel estimator to the channel statistics which further improves the robustness of the system.     

Space-time-frequency coded OFDM over frequency-selective fading channels

This paper proposes novel space-time-frequency (STF) coding for multi-antenna orthogonal frequency-division multiplexing (OFDM) transmissions over frequency-selective Rayleigh fading channels. Incorporating subchannel grouping and choosing appropriate system parameters, we first convert our system into a set of group STF (GSTF) systems. This enables simplification of STF coding within each GSTF system. We derive design criteria for STF coding and exploit existing ST coding techniques to construct both STF block and trellis codes. The resulting codes are shown to be capable of achieving maximum diversity and coding gains, while affording low-complexity decoding. The performance merits of our design are confirmed by corroborating simulations and compared with existing alternatives.     

Better performance of OFDM using three special triangular constellations over AWGN and fading channels

Orthogonal frequency division multiplexing (OFDM) has been adopted as a major data transmission technique by many wireless communication standards. In this research, 3 new triangular constellations schemes, which are named as TRI1, TRI2, and TRI3, are introduced to replace for the well‐known rectangular quadrature amplitude modulation (QAM) constellation in OFDM modulation. In this study, it has been shown that these new schemes have 3 major advantages with respect to the QAM. The first advantage is their lower bit error rate, which results from the better usage of the constellation space with longer minimum distances. The 2 other advantages are a lower peak to average ratio and higher noise immunity. Both mathematical analysis and simulation results demonstrate that by applying high fading channels with additive white Gaussian noise and intersymbol interference impairment simultaneously, the proposed constellations exhibit a superior performance in criteria compared to the commonly used rectangular 16QAM and 64QAM constellations. As a result, they are good choice for high speed and real‐time multicarrier applications such as digital video broadcasting terrestrial at no extra cost.     

Model-based channel estimation for OFDM signals in Rayleigh fading

This paper proposes a robust pilot-assisted channel estimation method for orthogonal frequency division multiplexing (OFDM) signals in Rayleigh fading. Our estimation method is based on nonlinear regression channel models. Unlike the linear minimum mean-squared error (LMMSE) channel estimate, the method proposed does not have to know or estimate channel statistics like the channel correlation matrix and the average signal-to-noise ratio (SNR) per bit. Numerical results indicate that the performance of the proposed channel estimator is very close to the theoretical bit error propagation lower bound that is obtained by a receiver with perfect channel response information     

多径衰落信道下频域差分检测OFDM系统的误码率分析

在多径衰落信道条件下，分析了正交频分复用系统中频域差分检测的误码率性能，并进行了仿真验证。分析与仿真结果表明，信道相干时间和相干带宽均影响频域差分检测的误码率性能，信道相干带宽对频域差分检测的影响比信道相干时间的影响更大，频域差分检测适合于多径延迟扩展比较小的信道。     

Complex-field coding for OFDM over fading wireless channels

Orthogonal frequency-division multiplexing (OFDM) converts a time-dispersive channel into parallel subchannels, and thus facilitates equalization and (de)coding. But when the channel has s close to or on the fast Fourier transform (FFT) grid, uncoded OFDM faces serious symbol recovery problems. As an alternative to various error-control coding techniques that have been proposed to ameliorate the problem, we perform complex-field coding (CFC) before the symbols are multiplexed. We quantify the maximum achievable diversity order for independent and identically distributed (i.i.d.) or correlated Rayleigh-fading channels, and also provide design rules for achieving the maximum diversity order. The maximum coding gain is given, and the encoder enabling the maximum coding gain is also found. Simulated performance comparisons of CFC-OFDM with existing block and convolutionally coded OFDM alternatives favor CFC-OFDM for the code rates used in a HiperLAN2 experiment.     

Cyclostationarity-based OFDM signal sensing over doubly-selective fading channels

In this paper, using cyclostationarity-based sensing method to detect the presence of Or-thogonal Frequency Division Multiplexing (OFDM) signal over doubly-selective fading channels is studied. By approximating the channel with Basis Expansion Model (BEM), we derive the second-order cyclostationary statistics of the received OFDM signal over doubly-selective fading channels. Theo-retical analysis indicates that new cyclostationary signatures produced by Doppler spread and mul-tipath delay can be further exploited in the detecting process. Simulation examples demonstrate that the sensing methods using channel-induced cyclostationary features provide substantial improvements on detection performance.