Interference cancellation schemes for MIMO MC-CDM in fading channels

A novel interference cancellation (IC) scheme for MIMO MC-CDM systems is proposed. It is shown that the existing IC schemes are suboptimum and their performance can be improved by utilising some special properties of the residual interference after interference cancellation.     

A class of signal-processing schemes for reducing the envelope fluctuations of CDMA signals

In this paper, we present a class of nonlinear signal-processing schemes to reduce the envelope fluctuations in the downlink transmission within code-division multiple-access (CDMA) systems. These schemes are compatible with conventional CDMA receivers since they just require some modifications at the transmitter side. The proposed transmitter structure includes a nonlinear operation in the time-domain followed by a linear frequency-domain filtering operation. We also include a semi-analytical statistical characterization of the transmitted signals, which takes advantage of the Gaussian-like nature of CDMA signals with a high number of users. This characterization is used for an analytical performance evaluation of the proposed techniques. A set of performance results is presented showing that we can reduce significantly the envelope fluctuations of the transmitted signals, while maintaining the spectral occupation of the corresponding conventional CDMA signals. Moreover, the performance degradation due to the nonlinear distortion effects on the transmitted signals can be kept relatively low.     

New Blind Block Synchronization for Transceivers Using Redundant Precoders

This paper studies the blind block synchronization problem in block transmission systems using linear redundant precoders (LRP). Two commonly used LRP systems, namely, zero padding (ZP) and cyclic prefix (CP) systems, are considered in this paper. In particular, the block synchronization problem in CP systems is a broader version of timing synchronization problem in the popular orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithms exploit the rank deficiency property of the matrix composed of received blocks when the block synchronization is perfect and use a parameter called repetition index which can be chosen as any positive integer. Theoretical results suggest advantages in blind block synchronization performances when using a large repetition index. Furthermore, unlike previously reported algorithms, which require a large amount of received data, the proposed methods, with properly chosen repetition indices, guarantee correct block synchronization in absence of noise using only two received blocks in ZP systems and three in CP systems. Computer simulations are conducted to evaluate the performances of the proposed algorithms and compare them with previously reported algorithms. Simulation results not only verify the capability of the proposed algorithms to work with limited received data but also show significant improvements in the block synchronization error rate performance of the proposed algorithms over previously reported algorithms.      

OFDM or single-carrier block transmissions?

We compare two block transmission systems over frequency-selective fading channels: orthogonal frequency-division multiplexing (OFDM) versus single-carrier modulated blocks with zero padding (ZP). We first compare their peak-to-average power ratio (PAR) and the corresponding power amplifier backoff for phase-shift keying or quadrature amplitude modulation. Then, we study the effects of carrier frequency offset on their performance and throughput. We further compare the performance and complexity of uncoded and coded transmissions over random dispersive channels, including Rayleigh fading channels, as well as practical HIPERLAN/2 indoor and outdoor channels. We establish that unlike OFDM, uncoded block transmissions with ZP enjoy maximum diversity and coding gains within the class of linearly precoded block transmissions. Analysis and computer simulations confirm the considerable edge of ZP-only in terms of PAR, robustness to carrier frequency offset, and uncoded performance, at the price of slightly increased complexity. In the coded case, ZP is preferable when the code rate is high (e.g., 3/4), while coded OFDM is to be preferred in terms of both performance and complexity when the code rate is low (e.g., 1/2) and the error-correcting capability is enhanced. As ZP block transmissions can approximate serial single-carrier systems as well, the scope of the present comparison is broader.     

Performance of Single-Carrier Block Transmissions Over Multipath Fading Channels With Linear Equalization

We study uncoded bit-error-rate (BER) performances of single-carrier block transmissions, zero-padded (ZP), and cyclic-prefixed (CP) transmission, when linear equalizers are applied and the BERs are averaged over one block. We show analytically that the BER of ZP transmission with linear equalization degrades as the bandwidth efficiency increases, i.e., there is a tradeoff between BER and bandwidth efficiency in ZP transmission. It is also proven that when minimum mean-squared-error (MMSE) equalization is adopted, ZP transmission outperforms CP transmission and uncoded orthogonal frequency-division multiplexing (OFDM) on the average over random channels. However, the difference between the ZP and the CP transmission becomes smaller as the block size gets larger, since the average BER performance of the ZP transmission degrades, while the average BER performance of CP transmission improves, as a function of the block size. Numerical examples are provided to validate our theoretical findings and to compare the block transmission systems.     

Blind-Channel Estimation and Interference Suppression for Single-Carrier and Multicarrier Block Transmission Systems

Block transmission has recently been considered as an alternative to the conventional continuous transmission technique. In particular, block transmission techniques with zero padding (ZP) and cyclic prefix (CP) are becoming attractive procedures for their ability to eliminate both intersymbol interference (ISI) and interblock interference (IBI). In this paper, we present a unified approach to blind-channel estimation and interference suppression for block transmission using ZP or CP in both single-carrier (SC) and multicarrier (MC) systems. Our approach uses a generalized multichannel minimum variance principle to design an equalizing filterbank. The channel estimate is then obtained from an asymptotically tight lower bound of the filterbank output power. Through an asymptotic analysis of the subspace of the received signal, we determine an upper bound for the number of interfering tones that can be handled by the proposed schemes. As a performance measure, we derive an unconditional CramÉr–Rao bound (CRB) that, similar to the proposed blind channel estimators, does not assume knowledge of the transmitted information symbols. Numerical examples show that the proposed schemes approach the CRB as the signal-to-noise ratio (SNR) increases. Additionally, they exhibit low sensitivity to unknown narrowband interference and favorably compare with subspace blind-channel estimators.      

Weighted-Quadrature-Amplitude Modulation

A family of weighted-quadrature-amplitude-modulation (WQAM) schemes is introduced and analyzed. One of the staggered WQAM (SWQAM) schemes has low spectral sidelobes, even after hardlimiting, and its performance in an adjacent-channel interference (ACI) environment is superior to that of the QPSK; SQPSK, MSK, and SQORC. The subscriber networks and the highest data-rate systems operating in a nonlinear channel are preferable applications.     

CFO estimation schemes for differential OFDM systems

This paper proposes two blind carrier frequency offset (CFO) estimation schemes for differentially modulated orthogonal frequency division multiplexing (OFDM) systems. The proposed schemes estimate the fractional part of the CFO with only two consecutive OFDM blocks, and they exploit two implicit properties associated with differentially modulated OFDM (DOFDM) systems, i.e., the channel keeps constant over two consecutive OFDM blocks, and the DOFDM systems employ an M-ary phase-shift keying constellation. One of the schemes is based on the finite alphabet (FA) constraint and the other one is based on the constant modulus (CM) constraint. They provide a trade-off between the performance and computational complexity. The constrained Cramer-Rao lower bound is also derived. Several numerical examples are presented to validate the efficacy of the proposed schemes.     

A class of nonlinear signal-processing schemes for bandwidth-efficient OFDM transmission with low envelope fluctuation

This work presents a wide class of digital signal-processing schemes for orthogonal frequency-division multiplexing (OFDM) transmission which combine a nonlinear operation in the time domain and a linear filtering operation in the frequency domain. The ultimate goal of these schemes is to reduce the envelope fluctuation of ordinary OFDM, while keeping its high spectral efficiency and allowing a low-cost, power-efficient implementation. An appropriate statistical model concerning the transmitted frequency-domain blocks is developed, which is derived from well-established results on Gaussian stochastic processes distorted by memoryless nonlinearities. This model can be employed for performance evaluation by analytical means, with highly accurate results whenever the corresponding conventional OFDM signals exhibit quasi-Gaussian characteristics. Cases where the signal-processing scheme is repeatedly used, in an iterative way, are treated through an extension of the proposed statistical modeling. A set of numerical results is presented and discussed so as to show the practical interest of both the proposed schemes and the analytical methods for evaluation of their performance. For the sake of comparisons, this paper includes numerical results concerning the partial transmit sequence technique, which is an alternative peak-to-mean envelope power ratio-reducing technique of higher complexity, often recommended due to its distortionless nature. The superior performance/complexity tradeoffs through the proposed class of nonlinear signal-processing schemes is emphasized.     

Interleaved orthogonal frequency division multiplexing (IOFDM) system

In orthogonal frequency division multiplexing (OFDM) systems, for every block of K data samples, an overhead of L samples of cyclic prefix (CP) or zero padding (ZP) is added to combat frequency selective channels. The code rate, which is defined as the ratio K/(K+L), is a measure of the efficiency of transmitting user information. In this paper, a new system is proposed to increase the code rate without increasing the number of subcarriers and without increasing the bandwidth. The proposed system considers appending the L zeros (ZP) once for every P blocks of data samples, which would increase the code rate to PK/(PK+L). It is assumed that the channel is not varying over the transmission of P consecutive data blocks. In order to recover the P data blocks in a computationally efficient manner, an interleaving scheme is proposed, and the proposed system is called the interleaved OFDM (IOFDM) system. Various issues such as computational complexity, peak-to-average power ratio (PAPR), and the effect of synchronization errors on the performance of the IOFDM system are also presented. Based on a numerical simulation study, the average bit-error-rate (BER) performance of the IOFDM system is shown to be very close to that of the OFDM system for a moderate increase in computational complexity and delay.     

Nonlinear harmonic generation in free-electron lasers

A three-dimensional nonlinear simulation code to treat multiple frequencies simultaneously is described and used to study nonlinear harmonic generation in free-electron lasers (FELs). Strong nonlinear harmonic gain is found where the gain length varies inversely with the harmonic number. Substantial power levels are found in the harmonics. The odd harmonics are favored with generally higher power levels since a planar wiggler geometry is employed; however, the second harmonic exhibits substantial power as well. The analysis is relevant to the emission expected from self-amplified spontaneous emission (SASE) free-electron laser schemes     

The effects of time-delay spread on unequalized TCM in a portableradio environment

This paper studies the effects of time-delay spread on trellis-coded modulation (TCM) in portable radio channels, where equalization is not employed to mitigate frequency-selective fading. The average irreducible bit error rate (BER) of three different TCM schemes are analytically formulated first and then numerically evaluated by simulation. The results for a delay spread lower than 0.2 of the symbol period indicate that the performance of TCM schemes with interleaving/deinterleaving is much better than that of QPSK, and better TCM schemes for flat fading also give better performance under low delay spread. Analytical results indicate that a good TCM scheme in frequency-selective fading channels should have both a large Euclidean distance and a high degree of built-in time diversity. If higher time-delay spread is encountered, TCM does not have advantages over QPSK. We also compare TCM performance with and without diversity. It is found that diversity greatly improves the performance under low delay spread, while the diversity gain quickly diminishes as the RMS delay spread approaches 0.2 of the symbol period     

Efficient Routing in Intermittently Connected Mobile Networks: The Single-Copy Case

Intermittently connected mobile networks are wireless networks where most of the time there does not exist a complete path from the source to the destination. There are many real networks that follow this model, for example, wildlife tracking sensor networks, military networks, vehicular ad hoc networks (VANETs), etc. In this context, conventional routing schemes would fail, because they try to establish complete end-to-end paths, before any data is sent. To deal with such networks researchers have suggested to use flooding-based routing schemes. While flooding-based schemes have a high probability of delivery, they waste a lot of energy and suffer from severe contention which can significantly degrade their performance. With this in mind, we look into a number of ldquosingle-copyrdquo routing schemes that use only one copy per message, and hence significantly reduce the resource requirements of flooding-based algorithms. We perform a detailed exploration of the single-copy routing space in order to identify efficient single-copy solutions that (i) can be employed when low resource usage is critical, and (ii) can help improve the design of general routing schemes that use multiple copies. We also propose a theoretical framework that we use to analyze the performance of all single-copy schemes presented, and to derive upper and lower bounds on the delay of any scheme.     

UWB-OFDM系统的符号盲同步方法

该文提出一种适合于UWB-OFDM系统的符号盲同步方法。该方法是基于UWB-OFDM符号中的ZP功率出现规律性的分布特性,利用滑动窗能量检测的办法来进行符号同步。研究了高斯信道和UWB信道下的符号同步方法,并详细分析了UWB信道下接收信号的能量分布特性。为了提高同步性能,设计了多滑动窗方法代替双滑动窗方法。计算机仿真分析表明该方法无论在高斯还是UWB信道下都具有优异的性能。     

Adaptive bit-interleaved coded modulation

Adaptive coded modulation is a powerful method for achieving a high spectral efficiency over fading channels. Previously proposed adaptive schemes have employed set-partitioned trellis-coded modulation (TCM) and have adapted the number of uncoded bits on a given symbol based on the corresponding channel estimate. However, these adaptive TCM schemes do not perform well in systems where channel estimates are unreliable, since uncoded bits are not protected from unexpected finding. In this paper, adaptive bit-interleaved coded modulation (BICM) is introduced. Adaptive BICM schemes remove the need for parallel branches in the trellis-even when adapting the constellation size, thus making these schemes robust to errors made in the estimation of the current channel fading value. This motivates the design of adaptive BICM schemes, which will lead to adaptive systems that can support users with higher mobility than those considered in previous work. In such systems, numerical results demonstrate that the proposed schemes achieve a moderate bandwidth efficiency gain over previously proposed adaptive schemes and conventional (nonadaptive) schemes of similar complexity     

Comparison of nonlinear pulse interactions in 160-Gb/s quasi-linear and dispersion managed soliton systems

The understanding and development of 160-Gb/s transmission systems requires the study of the impact of different dispersion compensation schemes on pulse propagation in nonlinear fiber. In this paper, we present an investigation of 160-Gb/s optical transmission systems, focusing on optimal propagation regimes, and in particular, we analyze different transmission limitations and dominant nonlinear effects by comparing quasi-linear and dispersion managed soliton systems. Two quasi-linear systems, one using nonzero dispersion-shifted fiber (NZDSF) and the other single-mode fiber (SMF), and one short-period (1 km) dispersion managed soliton (DMS) system are studied, both for single-channel and wavelength-division-multiplexed (WDM) transmission. First, the performance of the two quasi-linear systems in single-channel transmission are compared and it is shown that the NZDSF and SMF systems allow similar error-free transmission distances with only small differences in the intrachannel four-wave mixing (IFWM) induced amplitude jitter. The effect of pulsewidth on transmission performance in this regime was investigated and the use of shorter pulses was found to result in lower amplitude jitter. We analyzed the behavior of the DMS system and showed that the reduced pulse broadening during transmission allowed a significantly longer single-channel transmission distance with a smaller impact of nonlinearities compared to quasi-linear propagation. The sensitivity of the DMS system performance to statistical fluctuations in the fiber dispersion was studied and the results show the level of accuracy in the dispersion management map which must be ensured in these systems. Finally, the performance of the DMS in WDM transmission was investigated and it was found that it was subject to very large penalties increasing the minimum channel spacing possible because of the strong impact of interchannel cross-phase modulation (XPM).     

A new polyphase sequence with perfect even and good odd cross-correlation functions for DS/CDMA systems

A class of polyphase signature sequences for direct-sequence code-division multiple-access (DS/CDMA) systems is proposed. The proposed class has zero periodic (=even) cross-correlation (CC) function and approximate maximum magnitude N//spl pi/ of odd cross-correlation (OCC), where N is the length of sequences. Although the maximum magnitude is relatively large, it is observed that the maximum magnitude has little effect on the performance of the DS/CDMA system, since its frequency is very low. The performance of the proposed sequence in DS/CDMA systems is investigated and shown to be better than that of other sequences for an asynchronous additive white Gaussian noise channel environment with and without Rayleigh multipath fading.     

On the binary quadratic residue system with noncoprime moduli

The residue number system (RNS) appropriate for implementing fast digital signal processors since it can support parallel, carry-free, high-speed arithmetic. A development in residue arithmetic is the quadratic residue number system (QRNS), which can perform complex multiplications with only two integer multiplications instead of four. An RNS/QRNS is defined by a set of relatively prime integers, called the moduli set, where the choice of this set is one of the most important design considerations for RNS/QRNS systems. In order to maintain simple QRNS arithmetic, moduli sets with numbers of forms 2ⁿ+1 (n is even) have been considered. An efficient such set is the three-moduli set (2^2k-2+1.2^2k+1.2^2k+2+1) for odd k. However, if large dynamic ranges are desirable, QRNS systems with more than three relatively prime moduli must be considered. It is shown that if a QRNS set consists of more than four relatively prime moduli of forms 2ⁿ+1, the moduli selection process becomes inflexible and the arithmetic gets very unbalanced. The above problem can be solved if nonrelatively prime moduli are used. New multimoduli QRNS systems are presented that are based on nonrelatively prime moduli of forms 2ⁿ +1 (n even). The new systems allow flexible moduli selection process, very balanced arithmetic, and are appropriate for large dynamic ranges. For a given dynamic range, these new systems exhibit better speed performance than that of the three-moduli QRNS system     

A novel approach to modeling of OQPSK-type digital transmissionover nonlinear radio channels

Offset quaternary PSK-type (OQPSK) digital modulation schemes are often recommended for radio transmission systems employing grossly nonlinear amplifiers, and have found widespread use in land mobile and/or satellite communications. Most of the recommended OQPSK-type schemes-also called MSK-type schemes-jointly exhibit the following features: a constant envelope or, at least, a low envelope fluctuation; a compact spectrum after a power-efficient, saturated amplifier, and a high detection efficiency obtained with simple, low-cost receivers. We introduce a novel approach to computer-aided modeling of OQPSK-type digital transmission over nonlinear radio channels. By taking into account the specific nature of OQPSK-type modulations, when using the Volterra approach as proposed by Benedetto et al. (1987), we conclude that two signal representations (“parallel” and “serial”) can be achieved which are similar to the conventional Laurent (1986) representation of binary CPM signals with h=1/2. With the proposed approach, the OQPSK-type signals have an invariant structure along the transmission path (which is supposed to include, at least, one nonlinearity) in the sense that they are all made of several linear components of the same type. Some examples are given, including signal representation and power spectrum results. The proposed unified signal representations can be very useful for design and performance evaluation of radio communication systems (mobile and/or satellite applications). They combine flexibility, accuracy, and simplicity, and allow high computational efficiencies     

Kasami Code-Shift-Keying Modulation for Ultra-Wideband Communication Systems

Ultra-wideband (UWB) communication systems are generally applied to short-range wireless communications. In order to achieve higher rates or to support multiple access capabilities, direct sequence spread spectrum (DSSS) techniques have been introduced to UWB systems, and multiple pulses corresponding to a certain pseudo-noise (PN) code are transmitted to represent a symbol. In addition, the concept of M-ary code shift keying (M-CSK) was introduced into DSSS systems to achieve higher rates. In this work, we propose an M-CSK modulation technique based on the large set of Kasami sequences since it possesses good code properties, including a large code set size and low cross correlations. The modulation and demodulation schemes are developed, and the system performance in additive white Gaussian noise (AWGN) and UWB channels exposed to multiple user interference is investigated thoroughly. It was found that the Kasami M-CSK modulation is superior to other M-CSK modulation schemes in the bandwidth efficiency, and therefore a higher data rate can be achieved. Furthermore, based on our proposed demodulation scheme, the hardware complexity of receivers can be greatly reduced to O(M^1/3), and the implementation of receivers for a very large M becomes feasible.