Rayleigh衰落信道下相位误差对相干跳频多址接入MRC分集接收系统的影响

该文研究一种新型混合通信系统,即基于多载波BPSK调制的相干跳频多址接入(FH/CDMA)系统,接收端采用最大比合并(MRC)分集接收。由于采用相位调制,接收端载波相位恢复的精确度直接影响系统性能。假设载波同步通过一阶锁相环电路(PLL)完成,由于载波恢复环路中存在高斯噪声,当系统达到同步稳态时不可避免地会产生稳态相位误差。该文采用Beaulieu级数方法和特征函数方法,得到理想相干和部分相干(存在稳态相位误差)时FH/CDMA系统误码率性能表达式。仿真结果表明,相对于单用户系统,多用户系统误码率受到相位误差影响较小,误码率性能降低的主要因素是多用户干扰。MRC分集接收能提高系统抵抗相位误差和信道衰落的能力。     

自干扰幅度及相位误差对全双工系统的影响

在加性白高斯噪声无线传播信道和自干扰信道中,分析了自干扰射频抵消幅度及载波相位估计误差对二进制相位调制同时同频全双工系统误码率的影响.分析结果表明:相同信干比条件下,误码率随幅度估计相对误差、载波相位估计数值误差的绝对值减小而减小;当载波频率为1.9 GHz、信干比为-120 dB、误码率为10-2,自干扰载波相位估计数值误差为10-5度时,同时同频全双工系统损失发射功率0.2 dB.     

TDD相位预均衡OFDM系统中的定时误差分析

研究了在多径瑞利衰落环境中，抽样定时误差对相位调制的时分双工(TDD)预均衡正交频分多路复用系统(OFDM)的误码率性能影响．给出了理想和非理想相位预均衡处理的误码率解析表达式，并进行了计算机仿真，仿真和理论分析相符．对于TDD相位预均衡OFDM系统，定时误差导致了解调数据的相位旋转，且系统性能随定时误差的增加呈加速恶化的趋势．与无定时误差的瑞利衰落性能上界相比，当归一化定时误差为1／8时，理想和非理想相位预均衡的性能分别损失了约1dB和3dB；归一化定时误差为1／4时，二者的性能均严重恶化且差距减小，这时候定时误差成为影响系统性能的主要因素．     

多径瑞利衰落信道条件下射频干扰抵消误差对CCFD系统中OFDM性能的影响

针对同时同频全双工无线通信系统,考虑远端到近端的无线信道为多径瑞利衰落信道,近端发射天线到接收天线的自干扰信道为加性白高斯噪声信道,分析了同时同频全双工传输场景中,自干扰射频抵消幅度及载波相位误差对OFDM误码率的影响。结果表明,在相同信干比和信噪比条件下,幅度和载波相位估计误差的绝对值越小,误码率越低;针对载波频率2.3 GHz, OFDM子载波个数4096,子载波间隔15 kHz的同时同频全双工传输方式,在信干比为-70 dB,误码率为10-2时,若期望信噪比损失小于0.8 dB,则需要射频干扰抵消的载波相位估计误差的绝对值小于610-6,幅度估计相对误差的绝对值小于310-5;若期望获得40 dB的射频自干扰抑制效果,则射频干扰抵消的载波相位估计误差的绝对值小于0.5,幅度估计相对误差绝对值小于1%。     

一种跳相发射分集技术的差分方案

将差分编码方案引入到跳相发射分集系统中,提出了跳相差分空时编码(PHDSTBC)方案,该方案有效地改善了整个系统中的差错性能和时延。同时,伪随机的相位跳变大大增强了系统的保密性及干扰对抗性,提高了系统抗突发错误和深度衰落的能力。仿真结果表明,在准静态Rayleigh信道下,误码率为10-3时,新方案相比TarokhJafarkhani差分空时方案(TJDSTBC)性能有1.5dB的信噪比增益。     

数字调相通信系统的性能

本文分析数字调相系统在高斯噪声下的性能,并确定无衰落情况下所需信噪比与分立的相数及误码率的关系,既考虑到由本机产生参考载波的相干检波又考虑到相位比较检波。计算表明,多相调制较之多电平调幅在带宽换取信噪比方面更为有效。同时发现除二进制调制外,相位比较检波的性能较相干检波约低3 dB。而二进制调相在误码率约不超过0.001时,性能下降小于1dB。     

一种基于APPA的码辅助载波同步算法

在低信噪比环境下,小的频偏和相偏的存在会使Turbo编码系统的译码性能恶化,所以必须结合迭代译码系统,对信号的残留频偏和相差进行估计。该文提出一种改进的残留频偏载波相位估计算法后验概率辅助(APPA)相位估计,该算法将译码器输出的外信息用于辅助迭代的相位估计。环路滤波器将相位误差信号转换成控制信号,控制数控振荡器的输出,这样可以得到待估计的相位误差。仿真表明,在极低信噪比下(比如,SNR-7.8 dB)该算法在同时存在频偏和相偏的时候能正常工作,其性能非常接近理想同步条件下的性能。     

基于FRFT的多普勒频率跟踪算法

高动态环境下卫星通信终端面临高动态和低信噪比问题,传统载波跟踪方法难以在高动态应力和跟踪精度两方面取得较好折中,在信号设计中考虑插入分散导频用于跟踪载波,可在低信噪比下实现高机动用户多普勒频移捕获及多普勒频率变化率的跟踪。给出了一种基于FRFT的频率变化率估计方法,利用估计值辅助载波跟踪环路。理论分析与计算机仿真表明,在典型高动态参数条件下,基于分散导频的多普勒频率跟踪算法和传统载波跟踪方法相比,灵敏度提高2 dB以上,Es/N0工作点可低至0 dB以下,跟踪范围可达符号速率的2倍,频率跟踪的均方根误差小于符号速率的1%,性能损失小于0.1 dB。     

基于Hilbert鉴相算法的全数字BPSK解调方法研究

数字通信系统中,调制解调效果的好坏直接关系着其性能优劣。本文就BPSK调制信号,讨论了实现其全数字解调的几种方法,如平方环法,Costas环法,并提出一种以Hilbert鉴相算法为基础构建的环路解调方法。针对此法,利用MATLAB进行仿真,仿真结果表明,此种环路解调方法之于锁相环路解调而言,能处理载波频率较高的调制信号,并更快的解调出基带数据,且此法在信噪比较低的情况下,本文取信噪比为12dB,经过软件仿真,解调的结果达到接近于10-4的误码率。     

面向自由空间光通信的PC-STBC方法研究

针对大气湍流对自由空间光通信系统误码率影响较大的问题,提出了一种在多输入多输出系统中将极化编码与空时编码直接级联的方案。在弱湍流信道中,使用高斯近似的方式构造出编码序列并进行极化编码,并使用二进制相移键控对编码序列进行调制,再与空时编码进行直接级联。仿真结果表明:在多输入多输出系统中,码字长度N=1 024时,系统的误码率可下降至10-6级以下;若码字长度N=2 048时,信噪比高于2 dB时,误码率可趋近于0。接收天线数越多,系统的分集阶数越高,系统误码率下降速度变慢,系统整体性能变差。因而该级联方案能够更好地改善系统误码率性能。并且在不同的湍流强度下,表现出对强湍流具有更好的抑制效果,误码率下降更明显。     

Partially coherent DS-SS performance in frequency selectivemultipath fading

The bit-error rate (BER) performance of a direct sequence spread spectrum (DS-SS) signal, operating over a multipath Rayleigh fading channel, is investigated when corrupted by phase noise as well as additive white Gaussian noise (AWGN). The phase noise arises from phase locked loop (PLL) dynamics and results in imperfect receiver phase estimates whereby the phase errors assume Tikhonov densities. The phase estimates are used by a multipath-combining RAKE receiver for demodulation. Approximate upper-bounds on the bit error probability are obtained and evaluated for different combinations of channel parameters and for various values of the average loop signal-to-noise ratio (SNR). Results indicate that for a PLL with loop SNR 10 dB above the system E _b/η₀, the degradation is less than 3 dB, and for a loop SNR of 20 dB above E_b/η₀, the degradation is less than 1 dB     

Performance of a Modified Multicarrier Direct Sequence CDMA System

In this paper, we present an improved multicarrier direct sequence (DS) code division multiple access (CDMA) scheme by modifying the system originally proposed by Kondo and Milstein [13]. In this modified system, different spreading sequences multiplied by a data sequence modulate different carriers. This is to prevent the multiple access capability from reducing when the fading characteristics of different carrier frequencies are highly correlated. We have derived a formula which determines the mean values of the relative received signal strength in a single carrier DS CDMA rake system and in a multicarrier DS-CDMA system. We present results on the comparison of the bit error rate (BER) performance of the two systems including the effect of correlation between fading characteristics of different frequencies under various multipath fading conditions. The results indicate that with 50 users the modified multicarrier DS CDMA system can achieve an uncoded irreducible BER of 1.7 × 10^??3 with an average received signal-to-noise ratio per bit of 10 dB, which is better than 3.0 × 10^??3 achieved by the single carrier DS CDMA rake system, and also show that if multicarrier CDMA system is used with respect to single carrier CDMA system, the SNR gain is up to 4.5 dB for the uncode BER of 10^??3 being achieved.     

Adaptive mobile spot diffusing angle diversity MC-CDMA optical wireless system in a real indoor environment

In this paper, we introduce a mobile optical wireless (OW) multicarrier - code division multiple access (MC-CDMA) system that employs a new adaptive line strip multibeam system (ALSMS) with diversity detection. Our results indicate that a significant improvement in the bit error rate (BER) can be obtained in the presence of very directive noise, multipath propagation and shadowing typical in a real indoor environment. With transmitter and/or receiver mobility, ALSMS can improve the BER performance by almost 10^-5 with 2 active users, and increase the signal-to-noise ratio (SNR) by more than 13 dB compared to the unadaptive LSMS.     

Performance of broadband multicarrier DS-CDMA using space-time spreading-assisted transmit diversity

In this contribution multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS)-assisted transmit diversity is investigated in the context of broadband communications over frequency-selective Rayleigh-fading channels. We consider the issue of parameter design for the sake of achieving high-efficiency communications in various dispersive environments. Furthermore, in contrast to conventional MC DS-CDMA schemes employing time (T)-domain spreading only, in this contribution we also investigate broadband MC DS-CDMA schemes employing both T-domain and frequency (F)-domain spreading, i.e., employing TF-domain spreading. The bit-error rate (BER) performance of STS-assisted broadband MC DS-CDMA is investigated for downlink transmissions associated with the correlation based single-user detector and the decorrelating multiuser detector. Our study demonstrated that when appropriately selecting the system parameters, broadband MC DS-CDMA using STS-assisted transmit diversity constitutes a promising downlink transmission scheme. This scheme is capable of supporting ubiquitous communications over diverse communication environments without BER performance degradation.     

Performance of generalized multicarrier DS-CDMA over Nakagami-mfading channels

A class of generalized multicarrier direct sequence code-division multiple-access (MC DS-CDMA) schemes is defined and its performance is considered over multipath Nakagami-m fading channels. The spacing between two adjacent subcarriers of the generalized MC DS-CDMA is a variable, allowing us to gain insight into the effects of the spacing on the bit error rate (BER) performance of MC DS-CDMA systems. This generalized MC DS-CDMA scheme includes the subclasses of multitone DS-CDMA and orthogonal MC DS-CDMA as special cases. We present a unified analytical framework for determining the exact average BER of the generalized MC DS-CDMA system over generalized multipath Nakagami-m (1960) fading channels. The optimum spacing of the MC DS-CDMA system required for achieving the minimum BER is investigated and the BER performance of the system having optimum spacing is evaluated. The resultant BER is compared with that of both multitone DS-CDMA and orthogonal MC DS-CDMA     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

Performance Analysis of Multicarrier DS-CDMA with Antenna Array in a Fading Channel

In this paper, an antenna array based base station receiver structure for multicarrier direct sequence code-division-multiple-access (Multicarrier DS-CDMA) system with binary phase shift keying (BPSK) modulation is proposed. One of the main advantages of the receiver structure is that the spatial diversity is obtained by combining signals at different array elements. Based on the detailed analysis of multiple access interference (MAI) and additive white Gaussian noise (AWGN) characteristics, the uplink bit error rate (BER) performance of the proposed antenna array Multicarrier DS-CDMA is provided. With regard to spatial domain combining, the optimum spatial combining (OSC) and suboptimum spatial combination (SOSC) weights is derived while the suboptimum set of weights is simplified in the sense that only the knowledge of array vector of desired user alone is sufficient for the combining. Simulation results verify the analysis, and it is shown that MAI is mitigated and subsequently the system performance is improved significantly by incorporating antenna array at the base station.     

A novel MC DS-CDMA system scheme with high spectral efficiency

1 Introduction Recently, some studies [1, 2] have shown that the Multi- Carrier code-division multiple-access (CDMA) systems can exhibit excellent performance in beyond 3G or 4G mobile communication systems. In general, multicarrier (MC) CDMA systems have…     

Cooperative communications with multilevel/AES-SD4-CPFSK in wireless sensor networks

In this paper, a new joint multilevel data encryption and channel coding mechanism is proposed, which is called ??multilevel/advanced encryption standard?Csystematic distance 4?Ccontinuous phase frequency shift keying?? (ML/AES-SD4-CPFSK). In the proposed scheme, we have not only taken advantage of spatial diversity gains but also optimally allocated energy and bandwidth resources among sensor nodes as well as providing high level of security and error protection for cooperative communications in wireless sensor networks. Relay protocols of cooperative communications, such as amplify-and-forward and decode-and-forward with/without adversary nodes, have been studied for 4CPFSK, 8CPFSK, and 16CPFSK of ML/AES-SD4-CPFSK. We have evaluated the error performances of multilevel AES for data encryption, multilevel SD-4 for channel coding, and various CPFSK types for modulation utilizing cooperative communications in wireless sensor networks. According to computer simulation results, significant diversity gain and coding gain have been achieved. As an example, bit error rate (BER) performance of 10^?5 value has been obtained at a signal-to-noise ratio (SNR) of ?6?dB for SD-4-CPFSK scheme in a compared related journal paper, whereas in our proposed system, we have reached the same BER value at a SNR of ?23?dB with amplify-and-forward with direct path signal protocol in 16-level AES, two-level SD-4 coded 16CPFSK, and at the same time, we have reached the same BER value at a SNR of ?22?dB with amplify-and-forward without direct path signal protocol in 16-level AES, two-level SD-4 coded 16CPFSK.