交织OFDMA上行系统CFO估计算法的研究

文中对OFDMA系统中的子载波分配方法进行了研究,介绍了两种子载波分配方法的原理,对两种子载波分配方法进行了仿真,结果显示在频率选择性衰落信道中交织分配的OFDMA系统要比块分配的OFDMA系统获得更多的频率分集.给出了交织OFDMA系统上行链路的信号结构,由于发射机和接收机之间存在的载波频率偏移(CFO)将会影响子载波间的正交性,对多用户的CFO估计提出了一种估计算法,仿真的结果显示所提出的算法具有较好的性能.     

Multicarrier channels with non-identical subcarrier fading distributions-analysis and adaptation

In this letter, the subcarrier fading distribution of multicarrier modulation (MCM) systems is derived based on channel-impulse response (CIR) information. Frequency-selective channels with arbitrary Rician-Rayleigh fading paths and inter-path correlations are considered. It is discovered that certain MCM channels experience different fading severity among the subcarriers. Motivated by this observation, a novel bit-loading cum power-control scheme that can achieve better spectral and power efficiency by adapting the subcarrier modulation parameters based on the long-term subcarrier fading statistics, instead of instantaneous channel state information, is proposed and studied.     

Performance of Multicarrier CDMA Rake System over Rayleigh Fading Channel

Based on the theory of multicarrier (MC) technique and the Rake receiver, a multicarrier DS-CDMA Rake system is proposed, where a data sequence multiplied by a spreading sequence modulates multiple carriers. The receiver provides a Rake for each subcarrier, and the outputs of the Rakes are combined by a maximal-ratio combiner. The average probability of error of the system is derived from an uncorrelated subcarrier and frequency-selective fading channel model. The system performances are evaluated over Rayleigh fading channel with an exponential multipath intensity profile(MIP) and with a rectangular MIP, respectively, when multipath interference is present. It is found that this kind of model has larger superiority in an exponential MIP than in a rectangular MIP.     

Bounding performance and suppressing intercarrier interference in wireless mobile OFDM

While rapid variations of the fading channel cause intercarrier interference (ICI) in orthogonal frequency-division multiplexing (OFDM), thereby degrading its performance considerably, they also introduce temporal diversity, which can be exploited to improve performance. We first derive a matched-filter bound (MFB) for OFDM transmissions over doubly selective Rayleigh fading channels, which benchmarks the best possible performance if ICI is completely canceled without noise enhancement. We then derive universal performance bounds which show that the time-varying channel causes most of the symbol energy to be distributed over a few subcarriers, and that the ICI power on a subcarrier mainly comes from several neighboring subcarriers. Based on this fact, we develop low-complexity minimum mean-square error (MMSE) and decision-feedback equalizer (DFE) receivers for ICI suppression. Simulations show that the DFE receiver can collect significant gains of ICI-impaired OFDM with affordable complexity. In the relatively low Doppler frequency region, the bit-error rate of the DFE receiver is close to the MFB.     

Multidimensional Subcarrier Mapping for Bit-Interleaved Coded OFDM With Iterative Decoding

A power and bandwidth-efficient bit-interleaved coded modulation (BICM) with orthogonal frequency-division multiplexing (OFDM) and iterative decoding (BI-COFDM-ID) using combined multidimensional mapping and subcarrier grouping is proposed for broadband transmission in a frequency-selective fading environment. A tight bound on the asymptotic error performance is developed, which shows that subcarrier mapping and grouping have independent impacts on the overall error performance, and hence, they can be independently optimized. Specifically, it is demonstrated that the optimal subcarrier mapping is similar to the optimal multidimensional mapping for bit-interleaved coded modulation with iterative decoding (BICM-ID) in frequency-flat Rayleigh fading environment, whereas the optimal subcarrier grouping is the same with that of OFDM with linear constellation preceding (LCP). Furthermore, analytical and simulation results show that the proposed system with the combined optimal subcarrier mapping and grouping can achieve the full channel diversity without using LCP and provide significant coding gains as compared to the previously studied BI-COFDM-ID with the same power, bandwidth, and receiver complexity.     

A novel low‐complexity transmission power adaptation in MC‐CDMA systems with a‐MRC receiver over Nakagami‐m fading channels

In this paper, we propose a novel low‐complexity transmission power adaptation with good bit error rate (BER) performance for multicarrier code‐division multiple‐access (MC‐CDMA) systems over Nakagami‐m fading channels. We first propose a new receiver called ath‐order‐maximal‐ratio‐combining (a‐MRC) receiver with which the receiver power gain for the nth subcarrier is the ath (a?1) power of the corresponding channel gain. Incorporating the a‐MRC receiver, we then propose a new transmission power adaptation scheme where the transmission power is allocated over all the N subcarriers according to the subchannel gains and the transmitter adapts its power to maintain a constant signal‐to‐interference‐plus‐noise (SINR) at the receiver. The proposed scheme has a significant performance gain over the nonadaptive transmission scheme over both independent and correlated fading channels. Moreover, the proposed scheme keeps good BER performance while it is much simpler than the previous power control/adaptation schemes. Copyright © 2008 John Wiley & Sons, Ltd.     

A robust adaptive DFE receiver for DS-CDMA systems under multipathfading channels

This paper presents a novel receiver design from signal processing viewpoint for direct-sequence code-division multiple access (DS-CDMA) systems under multipath fading channels. A robust adaptive decision-feedback equalizer (DFE) is developed by using optimal filtering technique via minimizing the mean-square error (MSE). The multipath fading channels are modeled as tapped-delay-line filters, and the tap coefficients are described as Rayleigh distributions in order to imitate the frequency-selective fading channel. Then, a robust Kalman filtering algorithm is used to estimate the channel responses for the adaptation of the proposed DFE receiver under the situation of partially known channel statistics. The feedforward and feedback filters are designed by using not only the estimated channel responses but the uncertainties and error covariance of channel estimation as well. As shown in the computer simulations, the proposed adaptive DFE receiver is robust against the estimation errors and modeling dynamics of the channels. Hence, it is very suitable for receiver design in data transmissions through multipath fading channels encountered in most wireless communication systems     

Successive interference cancellation in multicarrier DS/CDMA

In this paper, we present a successive interference cancellation (SIC) scheme for a multicarrier (MC) direct-sequence code-division multiple-access (CDMA) system, using band-limited spreading waveforms to prevent self-interference. In every subband, the SIC receiver successively detects the interferers' signals and substracts them from the user-of-interest. A comparison is made among SIC, a minimum mean-squared error (MMSE) receiver, and matched filter (MF) detection with maximal-ratio combining (MRC). We also consider suboptimal combining using pilot symbol-assisted modulation (PSAM) to make the system more realistic. Analytic bit-error probabilities for SIC, MMSE, and MF in correlated Rayleigh fading channels are derived. The theoretical results for SIC, MMSE, and MF are shown to agree well with simulations. In particular, SIC and MMSE are shown to achieve better performance than MF with MRC; when the number of users is small, SIC provides better performance than does MMSE. Further, the correlation among different subcarriers is studied, and only large subcarrier correlation coefficients result in an obvious worsening of performance. Finally, we derive results for the performance degradation that an SIC scheme experiences in MC CDMA due to phase and timing tracking errors. It is shown that SIC can still retain a performance advantage over MF-MRC, although the advantage of SIC decreases with increasing tracking errors, especially when subcarrier correlation coefficients are small     

LDPC-based space-time coded OFDM systems over correlated fadingchannels: Performance analysis and receiver design

We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels     

On the performance of multicarrier RAKE systems

In this paper, we integrate the multicarrier signaling technique and the RAKE receiver to design a DS-CDMA system with the capability to increase the data rate, mitigate the effect of correlation among the various subcarriers, and suppress partial-band interference. In the proposed system, a data sequence is serial-to-parallel converted, and multicarrier DS-CDMA is used on each of the parallel data streams. The receiver provides a RAKE for each subcarrier, and the outputs of the RAKEs are combined by a maximal-ratio combiner. We employ a correlated subcarrier and frequency-selective fading channel model to derive the average probability of error of the system and compare the results with those of both a single carrier RAKE system and a multicarrier DS-CDMA system in a frequency selective Rayleigh fading channel with an exponential multipath intensity profile, when multiple access interference and partial-band interference are present     

Heterodyne video transmission with differentially encodedquadrature phase shift keying

Heterodyne transmission experiments of multiple video channels using differentially encoded quadrature phase shift keying (QPSK) are described. Combining the QPSK microwave modulation format with the subcarrier multiplexing technique results in a total of 20 video channels at 50 Mb/s being transmitted on 10 microwave subcarriers. Using a differential encoding approach, this system provides good receiver sensitivity and bandwidth efficiency, while maintaining a simple receiver design. No carrier recovery circuit or phase-locked loop is required for demodulation, which greatly reduces the receiver complexity. It is demonstrated that only a standard delay and multiply demodulator are necessary. With optical heterodyne detection, a receiver sensitivity of -37.5 dBm is achieved for the 20 channel system. The extension of this technique to intensity modulated direct detection subcarrier systems along with migration scenarios for digital video and broadband services is discussed     

自适应波束形成实现交织OFDMA上行链路的载波频偏补偿

在OFDMA系统中,采用交织子载波分配方法可以提高频率分集和系统容量,但是发射机和接收机之间的载波频率偏移会破坏子载波间的正交性,从而导致本用户的载波间干扰以及用户间干扰。该文分析了交织OFDMA上行链路的信号结构,提出了一种采用自适应波束形成器的载波频偏补偿方法,并给出了两种自适应波束形成器:匹配滤波器和MMSE方法。仿真结果验证了MMSE自适应波束形成器的误比特率性能优于匹配滤波器以及传统的循环卷积方法。     

一种改进的MC／CDMA系统的性能分析

多载波扩展频谱码分多址系统（MC／CDMA）是一种消除符号间干扰的有效技术。在多径传输信道下,考虑到各用户服务指标（QoS）的不同,本文提出多用户MC／CDMA系统可采用自适用最佳子载波分配方案。理论分析和仿结果表明,采用本文所提出的方案时,传统的MC／CDMA系统的性能将得到明显的改善。     

Capacity Analysis for Continuous Alphabet Channels With Side Information, Part II: MIMO Channels

In this second part of our two-part paper, we consider the capacity analysis for wireless mobile systems with multiple-antenna architectures. We apply the results of the first part to a commonly known baseband, discrete-time multiple-antenna system where both the transmitter and receiver know the channel's statistical law. We analyze the capacity for additive white Gaussian noise (AWGN) channels, fading channels with full channel state information (CSI) at the receiver, fading channels with no CSI, and fading channels with partial CSI at the receiver. For each type of channels, we study the capacity value as well as issues such as the existence, uniqueness, and characterization of the capacity-achieving measures for different types of moment constraints. The results are applicable to both Rayleigh and Rician fading channels in the presence of arbitrary line-of-sight and correlation profiles.     

Subcarrier multiplexed coherent lightwave systems for videodistribution

The application of subcarrier multiplexing techniques to coherent lightwave systems designed for video transmission is described. The theory of the intrinsic receiver sensitivity of multichannel coherent SCM (subcarrier multiplexing) systems is presented. Specific implementations of SCM coherent systems transmitting 60 FM video channels and 20 100 Mb/s FSK (frequency shift keying) channels are described. It is shown that a simple phase noise canceling circuit can be implemented, which allows these systems to be built with wide-linewidth DFB (distributed feedback) lasers. Several applications of multichannel coherent SCM systems to video distribution networks are proposed     

Finite-state Markov modeling of fading channels

Wireless communication systems that operate through fading channels have become more diverse and complex. In the last ten years, there has been a growing interest for research and development of advanced wireless communications systems that employ multicarrier (MC) techniques. So far, applications of FSMC models for fading channels has been mainly limited to single carrier (SC) communications with very few exceptions [24], [47]. FSMC models are particulary suitable to represent and estimate the relatively fast flat-fading channel gain in each subcarrier. An unexplored avenue for research is to find appropriate FSMC models to represent MC fading channels. However, the number of TV-FFC gains to be modeled in the MC system is often much higher than in a SC system and a major challenge would be to keep the number of FSMC states to computationally manageable levels.     

Performance of multicarrier DS/SSMA systems in frequency-selectivefading channels

Multicarrier direct-sequence spread-spectrum multiple-access systems in frequency-selective fading channels are investigated. A consistent channel model is used for each system. First, the tapped delay line (TDL) channel model with uniformly spaced, uncorrelated taps is investigated to model a complex Gaussian, wide-sense stationary, uncorrelated scattering channel. An approximate average bit-error-rate expression is obtained for the receiver with RAKE fingers on each subcarrier branch, whose outputs are combined according to the maximum signal-to-noise ratio criterion. Various system configurations are examined for the same TDL channel model, data rate, total system bandwidth, and excess bandwidth of the chip waveform. All the systems compared employ the same number of correlators. The numerical results show that, given a contiguous spectrum, the systems with more fractionally-spaced RAKE fingers per subchannel are more robust than the systems with more subcarriers     

An approximate analytical framework for performance analysis of equal gain combining technique over independent Nakagami,Rician and Weibull fading channels

In this paper, an approximate analytical method for performance analysis of equal gain combiner (EGC) receiver over independent Nakagami and Rician fading channels is presented. We use a convergent infinite series approach which makes it possible to describe the probability of error of EGC receiver in the form of an infinite series. In this paper, we develop a new approximation method for computation of the required coefficients in this series which lets us to derive simple analytical closed-form expressions with good accuracy compared with the exact results existing in the literatures. Our proposed approximation method only needs the mean and the variance of the fading envelope, which are known for various fading distributions, and hence, bypasses the required integration over the fading envelope distribution while computing the required coefficients. This feature lets us to extend our approximation method for performance analysis of EGC receiver over independent Weibull fading channels where the required integration has not any closed-form or tabulated solutions. To give an application of our developed method, we analyze the probability of error of an EGC receiver for binary, coherent PSK (CPSK) modulation over independent Nakagami, Rician and weibull fading channels and study the effect of the fading conditions on the system performance.