Performance of Haar wavelet based scale-code division multiple access (HW/S-CDMA) using decorrelating multi-user detector

Recently, we have proposed a new multi-rate/multi-media system called wavelet based scale-code division multiple access (W/S-CDMA) that depends on the code, time and scale orthogonality introduced by pseudo-noise (PN) sequences, and wavelets. Wavelets are used as an orthogonal set of symbols for signaling, and their orthogonality is exploited over scale and time. In this system, the channel is divided into different scales, and each scale into time slots. In addition, the PN sequences are used in each scale to accommodate multiple users. In other words, each user encodes its successive information symbols with time-shifted replicas of the same basic wavelet in a specific scale and spreads its scaled and translated wavelets with its PN sequence. The finer the scale used, more symbols are transmitted. In this paper, we analyze the performance of Haar wavelet based S-CDMA (HW/S-CDMA) over a synchronous additive white Gaussian noise (AWGN) channel using a decorrelating multi-user detector. Results reveal that HW/S-CDMA holds promise since the users of HW/S-CDMA can achieve variable and higher data rates than those of direct sequence (DS)-CDMA for a similar bit error rate performance when real-valued PN sequences are used. However, HW/S-CDMA achieves better performance than DS-CDMA when complex-valued PN sequences are used. In addition, for all rates the same performance is achieved in HW/S-CDMA and the multi-user detector has no processing delay for different rates and has the same features as the standard decorrelating detector. Because of the reuse capability introduced by the scales, HW/S-CDMA is also capable of employing many available PN sequences from the optimal PN sequence families with limited number of sequences such as Kasami, Bent, etc.     

Performance of multi-processing gain QS-CDMA over AWGN and Rayleigh fading channels

In this paper, we analyze the bit error rate performance of multi-processing gain quasi-synchronous (QS) code division multiple access (CDMA). Analysis is carried out for deterministic spreading sequences over additive white Gaussian noise (AWGN) and frequency selective slowly Rayleigh fading channels. We also find the bit error rate performance of multi-processing gain QS-CDMA for random spreading sequences and compare to that of deterministic spreading sequences. Results show that deterministic Gold codes outperform random codes considerably and perform differently for different rates while random codes give almost the same performance for all rates.     

衰落信道下SCMA性能仿真研究

本文在一种上行SCMA（稀疏码分多址）系统模型的基础上,研究了衰落信道下SCMA通信系统的误码性能。首先比较了AWGN（高斯）信道以及衰落信道下SCMA和SC-FDMA（单载波频分多址）两种系统的性能;接着分析了快衰落信道、慢衰落信道、平坦衰落信道和频率选择性衰落信道下SCMA系统性能。仿真结果表明,与传统正交多址SC-FDMA相比,衰落信道下SCMA在用户连接数和误码性能上有一定优势。与AWGN信道相比,衰落信道使得SCMA系统误码性能迅速下降。在瑞利平坦衰落信道下,信道衰落的快慢变化导致SCMA系统误码性能最大产生约5dB损失;在频率选择性衰落信道下,多径时延扩散致使SCMA系统误码性能最差。     

基于正交混沌多用户混沌通信及其误码率

为了解决多用户混沌通信系统中由于通信用户数目的增多以及混沌序列间的准正交性所引起的信道干扰问题,对不同混沌信号采用施密特(Schmidt)正交化方法产生其对应的正交混沌序列作为扩频码在加性高斯白噪声(AWGN)和瑞利(Rayleigh)衰落信道下研究多用户混沌通信系统中的误码率(BER)。首先将其统计自相关和互相关特性作为刻画正交混沌扩频码序列性能的重要指标,测试并分析各正交混沌序列的统计相关特性,同时也对其互相关特性曲线的均值和方差进行了测试与分析;然后通过两种信道仿真实验获取各正交混沌扩频码的BER,并将其与对应统计相关特性作对比揭示其内在关联性;最后比较分析两种不同信道下多用户混沌通信系统的BER,并揭示信道对其的影响。仿真实验表明,两种信道下相空间混沌信号中的空时正交混沌序列都能获得较低BER,尤其是在Rayleigh衰落信道下的空时正交混沌序列能够获得更低的BER,说明所提方法能更有效地降低多用户通信中信道间的干扰,更有利于多用户通信需要。     

Resource Allocation for TDMA Video Commtmmunication Over AWGN Using Cross-Layer Optimization Approach

Cross-layer optimization approach is applied to allocate channel times of time-division multiple access (TDMA) to utility functions which send different video streams with different rate-distortion characteristics. Given a channel time, each utility function solves an optimization problem to obtain the optimal source code rate, channel code rate and media access control (MAC) frame size. In this paper, we derive mathematical models to represent end-to-end distortion of video streams and 802.11a-like MAC and PHY with automatic repeat reQuest (ARQ) and rate compatible punctured convolutional code (RCPC) over additive white Gaussian noise (AWGN) channel. These models are formulated as a convex optimization problem, and it is solved by the primal-dual decomposition method. In addition, coexistence among the proposed utility functions and conventional utility functions is discussed. Finally, numerical examples show that significant reduction of overall distortion of utility functions can be achieved.      

On the effect of Multiple Access Interference in a Space Time Spreading Time Hopping PPM UWB system

This study investigates the effect of multiple sources using STS across a Time-Hopping-PPM Modulation that is over an UWB Saleh-Valenzuela channel with each source using two transmit antennas. We model and analyze this system, showing that the presence of such MAI results in a significant degradation in BER performance. Simulation results show that for strong signal strength of interfering MAI signals degradation in BER performance is observed for the cases where we have five and 10 distinct MAI sources present when using Walsh-Hadamard thirty two chip spreading sequences. Using simulation, we investigate the use of other sets of orthogonal spreading sequences with improved cross correlation properties and find that for some sequences a significant improvement is seen compared to Walsh-Hadamard sequences when they are similarly misaligned. We show that MAI degrades the BER performance of such systems and that selection of improved cross correlation spreading code sequence sets improves BER.     

基于码分多址接入的无线移动自组网吞吐量性能分析

窄带移动自组网在同一时隙内只能有一个节点发送数据,否则会导致碰撞,降低网络吞吐量性能。移动自组网中引入码分多址技术可以显著提高吞吐量,将网络状态建模为马尔可夫链,提出一种多址干扰和噪声共同影响下的吞吐量性能分析方法,综合考虑了扩频码同步和解调误码率的影响。数值仿真表明提高信噪比、编码纠错能力,以及增加扩频码长度均能提升网络吞吐量性能,低信噪比时编码纠错能力对性能提升比高信噪比时明显。编码纠错能力和扩频码长度均存在最佳的取值范围,超过此范围继续提高则对吞吐量性能的提升作用不再明显,因此需要根据实际环境优化网络参数。提出的新的分析方法具有更加普遍的适用性,更符合网络的真实情况,仿真实验对网络设计具有参考价值,可以用作网络参数选取的理论依据。     

ICMIC扩频序列对CDMA系统性能改善的分析

将混沌映射ICMIC应用于CDMA系统,产生相应的ICMIC二元扩频序列,并对系统的抗干扰性能以及系统容量进行了分析,分析表明,与Logistic周期混沌扩频序列、传统的EOE－Gold序列相比,该扩频序列具有良好的自相关与互相关特性,可以有效地抑制多径干扰和多址干扰,并使扩频通信系统的信道容量得到提高,对CDMA系统性能的改善起到重要的作用。     

Performance of punctured convolutional codes in asynchronous CDMA communications under perfect phase-tracking conditions

In this paper, the performance of punctured convolutional codes of short constraint lengths is discussed. The punctured codes are used to provide error protection to a particular user in an asynchronous code division multiple access (A-CDMA) system. Perfect channel estimation is assumed at the receiver. A slow fading Rician or Rayleigh channel is assumed. Maximum likelihood decoding through a Viterbi algorithm is used to decode the received symbols. Soft decision decoding for perfect phase tracking of the received signal is considered. Analytical bounds, which are useful in predicting the performance of the A-CDMA system are derived and plotted for the cases of infinite and finite channel memory. The upper bounds with Viterbi decoding are derived and plotted for the various punctured codes considered. The simulated results are found to agree very well with their upper bounds and predicted results.     

WCDMA系统中的差分最小均方误差多用户检测算法

宽带码分多址(Wideband code division multiple access,WCDMA)系统中现有的自适应多用户检测算法需要训练比特,不能适应快衰落信道。本文根据WCDMA系统中特有的二级扩频体制的特点,提出基于差分最小均方误差(Differential minimum mean square error,DMMSE)准则的自适应多用户检测算法。该算法根据相邻接收符号幅度变化的比率变化自适应调整横向滤波器的权系数,从而不需要通过训练比特开销跟踪信道状态信息的方式来抑制多址干扰。仿真结果表明,在存在多普勒频移的传播环境下,DMMSE算法的误码性能优于现有的自适应最小均方误差(Minimum mean square error,MMSE)算法。     

Achievable throughput enhancement based on modified carrier interferometry for MIMO/OFDM

In MIMO systems, channel estimation is important to distinguish the transmitted signals from multiple transmit antennas. When MIMO systems are introduced in cellular systems, we have to measure the received power from all the connectable base station (BS), as well as to distinguish all the channel state information (CSI) for the combination of transmitter and receiver antenna elements. One of the most typical channel estimation schemes for MIMO in a cellular system is to employ a code division multiplexing (CDM) scheme in which a unique spreading code is assigned to distinguish both BS and MS antenna elements. However, by increasing the number of transmit antenna elements, large spreading codes and pilot symbols are required to estimate an accurate CSI. To reduce this problem, in this paper, we propose a high time resolution carrier interferometry (HTRCI) for MIMO/OFDM to achieve an accurate CSI without increasing the number of pilot symbols.     

A high performance parallel Radon based OFDM transceiver design and simulation

A major goal of the next-generation wireless communication systems is the development of a reliable high-speed wireless communication system that supports high user mobility. They must focus on increasing the link throughput and the network capacity. In this paper a novel, spectral efficient system is proposed for generating and transmitting two-dimensional (2-D) orthogonal frequency division multiplexing (OFDM) symbols through 2-D inter-symbol interference (ISI) channel. Instead of conventional data mapping techniques, discrete finite Radon transform (FRAT) is used as a data mapping technique due to the increased orthogonality offered. As a result, the proposed structure gives a significant improvement in bit error rate (BER) performance. The new structure was tested and a comparison of performance for serial one-dimensional (1-D) Radon based OFDM and parallel 2-D Radon based OFDM is made under additive white Gaussian noise (AWGN), flat, and multi-path selective fading channels conditions. It is found that Radon based parallel 2-D OFDM has better speed and performance than serial 1-D Radon based OFDM.     

基于POMDP的信道感知接入算法

在认知无线电中,为了最大化次用户的吞吐量,同时对主用户的干扰低于预定值,提出一种基于POMDP的信道感知接入算法。次用户将主用户信道在时间轴上细分成等间隔的时隙,在每个时隙开始时,次用户从频谱感知、以较高的功率接入信道和以较低的功率接入信道三种可选策略中选择最优的策略。将次用户的选择过程建模成一个POMDP问题,并采用一些相应的最优策略求解。计算机仿真结果验证了算法的有效性。     

Performance evaluation of cyclic prefix CDMA systems with frequency domain interference cancellation

Direct-sequence code division multiple access (DS/CDMA) systems transmitting over multipath channels suffer from intersymbol interference (ISI) and multiple access interference (MAI). Recently, DS/CDMA with frequency domain equalization (FDE) has attracted much attention for its ability to obtain an excellent performance even in strong frequency selective fading channels. However, the presence of residual interference after the FDE degrades the orthogonality among the spreading codes and hence the BER performance deteriorates. Parallel interference cancellation (PIC) is an effective method to suppress the MAI and improve the CDMA system capacity. In this paper, we propose three interference cancellation architectures for downlink cyclic prefix CDMA systems. The common thread between these three architectures is their implementation in frequency domain. A comparison between such architectures is presented. Simulation results show that the combination of PIC and FDE provides an efficient solution to suppress the MAI in downlink CDMA systems over frequency selective fading channels.     

具有减少码间干扰的多用户混沌非相干通信系统

该文用混沌载波信号代替传统载波信号,设计了一个多用户通信系统。混沌序列具有良好的相关的特性,但是不足以克服多用户通信中普遍存在的不同用户信号之间的相互干扰。利用沃尔什正交函数系优秀的正交特性,把混沌振荡器产生的混沌序列和沃尔什矩阵行向量组合,生成正交的基函数,进而产生正交的时间序列。对应不同的符号、不同的用户,会有不同的正交序列与其对应。理论推导证明改进的混沌多用户非相干通信系统可以极大地减少不同用户间信号的干扰,计算机仿真了系统的误码率性能,表明系统的性能可以得到很大提高。     

Blind information-theoretic multiuser detection algorithms for DS-CDMA and WCDMA downlink systems

Code division multiple access (CDMA) is based on the spread-spectrum technology and is a dominant air interface for 2.5G, 3G, and future wireless networks. For the CDMA downlink, the transmitted CDMA signals from the base station (BS) propagate through a noisy multipath fading communication channel before arriving at the receiver of the user equipment/mobile station (UE/MS). Classical CDMA single-user detection (SUD) algorithms implemented in the UE/MS receiver do not provide the required performance for modern high data-rate applications. In contrast, multi-user detection (MUD) approaches require a lot of a priori information not available to the UE/MS. In this paper, three promising adaptive Riemannian contra-variant (or natural) gradient based user detection approaches, capable of handling the highly dynamic wireless environments, are proposed. The first approach, blind multiuser detection (BMUD), is the process of simultaneously estimating multiple symbol sequences associated with all the users in the downlink of a CDMA communication system using only the received wireless data and without any knowledge of the user spreading codes. This approach is applicable to CDMA systems with relatively short spreading codes but becomes impractical for systems using long spreading codes. We also propose two other adaptive approaches, namely, RAKE-blind source recovery (RAKE-BSR) and RAKE-principal component analysis (RAKE-PCA) that fuse an adaptive stage into a standard RAKE receiver. This adaptation results in robust user detection algorithms with performance exceeding the linear minimum mean squared error (LMMSE) detectors for both Direct Sequence CDMA (DS-CDMA) and wide-band CDMA (WCDMA) systems under conditions of congestion, imprecise channel estimation and unmodeled multiple access interference (MAI).     

DS-CDMA downlink systems with fading channel employing the generalized receiver

In this paper, we investigate a generalized receiver (GR) constructed on the basis of the generalized approach to signal processing (GASP) in noise employing by direct-sequence code-division multiple access (DS-CDMA) downlink wireless communication system with multipath fading. Transmitted signaling technique is based on using the orthogonal unified complex Hadamard transform spreading sequences. The use of GR allows us to maintain the orthogonality between users and reduce the multipath fading effect and interference from other users. A general multipath-fading model is assumed. Bit-error rate (BER) performance of system is evaluated by means of the signal-to-interference-plus-noise ratio (SINR) at the output of GR employed by DS-CDMA downlink wireless communication system. Using the orthogonal unified complex Hadamard transform spreading sequences as the transmitted signaling technique, we obtain that SINR at the GR output is independent of the phase offsets between different paths. If the Walsh–Hadamard (WH) spreading sequences are used as the transmitted signaling technique, the SINR at the output of GR employed by the same system is a function of squared cosine of path phase offsets. As a result, the BER performance of the last DS-CDMA downlink wireless communication system is worse in comparison with that of the first system. Comparative analysis between the BER performance of DS-CDMA downlink wireless communication systems employing both the GR and the Rake receiver, which consists of a bank of correlation receivers, with each individual receiver correlating with a different arriving multipath component, shows us a superiority of the first system over the second one both at high and low SINRs.     

低速率矿山透地通信扩频信号的快速捕获方法研究

针对弹性波透地通信系统低频、低码速率的特点,提出了一种时分多通道分段滑动相关捕获方法。该方法对到达的码元进行分段接收,利用接收下一段码的时间处理之前接收的码段,即1个PN码周期分成若干个通道时隙,每个时隙对之前接收的PN序列段与通道的本地PN码进行快速滑动相关搜索。理论分析结果表明,在所需硬件开销与串并行搜索相同的情况下,使用时分多通道分段滑动相关捕获能获得与并行捕获相同的搜索速度;仿真结果进一步验证了该捕获方法的有效性。     

基于部分信息的SLT-LT联合码防窃听方案设计

近年来, 信息的安全传输备受人们关注, 现有的物理层安全技术从信息论的角度出发, 将物理层安全编码与传输信道的动态物理特性进行结合, 实现信息的保密传输。作为一种纠删码, LT(Luby transform)码由于其编码随机性、码率不固定等特性,使得窃听者不能直接从泄露的编码符号中得到有用信息, 只要合法用户在窃听者之前接收到足够数量的编码符号, 便可实现信息的安全传输。而作为一种转移 LT(Shifted LT, SLT)码, SLT 码能高效恢复信息的同时具有更小的译码开销。因此, 我们将 SLT码应用于 Wyner 降阶窃听信道模型进行研究, 提出一种基于部分信息转移的 SLT-LT 联合码防窃听方案, 信源利用接收者已知的部分信息对度分布进行调整, 并对信源符号进行 SLT-LT 码级联编码。由于窃听信道是合法信道的降阶信道, 因此外在的窃听者截获到的消息符号是合法接收者的降阶版本, 在相同时间内, 合法接收者能够收到更多消息符号, 随着编解码过程的不断进行, 合法接收者的优势不断累积, 能够优先完成解码, 而度分布的调整以及级联编码方案使得编码符号的平均度进一步增大,窃听者难以完成解码, 进一步降低了窃听者译出率; 之后, 对所提方案的编解码性能以及安全性进行理论分析, 并通过实验仿真进行验证, 仿真结果表明, 与其他防窃听 LT 方案相比, 本文所提方案仅增加少量的译码开销但具有更好的安全性能。     

Cross-layer analysis of the random access mechanism in Universal Terrestrial Radio Access

In 3GPP Universal Terrestrial Radio Access, a user equipment uses the Random Access Channel (RACH) for uplink transmission if it does not have a pre-established dedicated channel. RACH is a contention-based transport channel in which access control and open-loop power control functionalities are provided under the interactions between Medium Access Control (MAC) and physical layers. This paper develops a Markov chain model to analyze complex cross-layer interactions in RACH procedures, which is crucial to understanding the RACH performance. Besides, the Markov chain accurately describes the time-synchronized RACH behavior in slot level. The presentation of this time-synchronized behavior is simplified by merging multiple sub-states, each of which corresponds to a slot delay, into a single state. Finally, the proposed model analytically evaluates the RACH performance in Rayleigh fading channels in terms of throughput and delay.