A semiclassical analysis of optical code division multiple access

A model noncoherent, optical, asynchronous, CDMA system is described. The error rate for a single-user matched-filter receiver that is valid for arbitrary photomultipliers and signature sequence sets, adheres to the semiclassical model of light, and does not depend on approximations for large user groups, strong received optical fields, or chip synchronism is analyzed. The exact minimum probability of error and optimal threshold are compared to those obtained with user-synchronism and multiple-access interference (MAI) distribution approximations. For the special case of unity-gain photodetectors and prime sequences, it is shown that the approximation of chip synchronism yields a weak upper bound on the exact error rate. It is demonstrated that the approximations of perfect optical-to-electrical conversion and Gaussian-distributed MAI yield a poor approximation to the minimum error rate and an underestimate of the optimal threshold. Arbitrarily tight bounds are developed on the error rate for unequal energies per bit. In the case when the signal energies coincide, these bounding expressions are considerably easier to compute than the exact error rate     

采用广义正交序列的多载波DS-CDMA系统在瑞利衰落信道中的性能分析

在CDMA系统中，采用正交序列可以有效地减小多址干扰的影响。然后，当由于多径衰落的影响使码片间的相对移位增加时，系统中用户扩频序列间的相互正交性减弱，导致性能大大恶化。本文基于新近提出的广义正交序列，分析了采用该序列的多载波直接序列码多分址系统在瑞利衰落信道中的误比特性能。数值结果表明，当扩频序列间相互移位在零相关区之内时，扩频序列仍可保持正交，因而大大提高了系统误比特性能。     

Performance analysis of MC‐CDMA by employing the MMSE‐PIC scheme in wireless communications

From the past decade, the multicarrier code division multiple access (MC‐CDMA) transmission schemes have placed major role in wireless communications. It is providing a secured wireless communication to the users with guaranteed performance. In many situations, the performance of the MC‐CDMA is restricted due to the interference caused by multiple access interference (MAI), which also influences the frameworks of CDMA. To overcome this issue, we concentrated on developing the efficient technique for data transmission with interference cancellation for downlink MC‐CDMA. In the proposed method, the interference cancellation procedure is done by using the regeneration and subtraction of MAI from the signal. The simulation results are evaluated using the MC‐CDMA system with different decision functions. Results proved that the proposed system is efficient in reducing the MAI along with an improved bit error rate (BER).     

An embedded transmission scheme using PPM signaling with symmetric error-correcting codes for optical CDMA

An embedded transmission (ET) scheme is proposed to easily apply error-correcting codes into optical code-division multiple-access (CDMA) systems for immunity from multiple-access interference (MAI). The ET scheme offers high transmission capability over the traditional scheme using pulse position modulation (PPM) signaling, because a 2/sup J/-ary symbol of each user is embedded in the signature sequence with 2/sup J/ weighted positions. Furthermore, the ET scheme with 2/sup J/-ary PPM signaling makes the optical CDMA system J parallel transmission systems, because J bits consisting of 2/sup J/-ary symbol are separately decided. Since such a separate decision is a comparison decision, the effect of MAI added in the optical channel is converted to symmetric errors in the individual parallel transmission systems. Using the symmetric error-correcting (SEC) code immunizes the individual parallel transmission systems against MAI more easily than the embedded-modulation scheme described in because the ET scheme avoids using the asymmetric error correcting code, which is difficult to implement. We analyze the bit error rate under Poisson photon counting channel and show that the ET scheme has an advantage of good energy efficiency over the traditional scheme in applying SEC codes.     

Channel coding for asynchronous fiberoptic CDMA communications

Several studies have explored the feasibility and systems performance aspects of a code division multiple access (CDMA) format for fiberoptic networks. Previously discussed CDMA architectures would either have to tolerate a high bit error rate or be forced to use long code sequences in networks with even a moderate number of simultaneous users. The use of long sequences lowers the maximum achievable bit rate or places unrealistic requirements on the system hardware. This paper examines some of the possible improvements to system performance that could be realized by combining the CDMA format with external error correcting codes (ECCs) or a PPM format. It is determined that ECCs can be highly effective in lowering the BER, and/or increasing the achievable information bit rate and number of network users. The results are sufficiently encouraging to conclude that one should seriously consider including ECCs in any practical fiberoptic CDMA network     

Performance Analysis of Internally Coded Time-Hopping Coherent Ultrashort Light Pulse CDMA Scheme in Fiber-Optic Communication Systems

We consider an internally coded time-hopping coherent ultrashort light pulse code division multiple access scheme (TH-CULP CDMA) and analyze its performance in the fiber-optic communication systems. This system combines the TH and CULP CDMA techniques and exploits the advantages of both. In our method, each bit time interval is divided into $N_{s}$ frames, and the spectral phase-coded pulse is transmitted in one of these frames. Two exclusive PN sequences are assigned to each user. One is added to a superorthogonal convolutional encoder output to select the transmission frame, and the other is used to encode the phase of the ultrashort pulse in the spectral domain. We evaluate the bit error rate (BER) of the system considering the effects of the multiple access interference (MAI) and thermal noise for both soft and hard decoders. The BER is evaluated using both the Chernoff bound and saddle point approximation. We compare the performance of our proposed system with that of the conventional CULP CDMA system. The numerical results indicate that for the same bit rate and processing gain, the performance of our proposed system (TH-CULP CDMA) is substantially better than that of the conventional CULP CDMA system. In addition, we observe that the hard decoder has a better performance at large number of users compared to the soft decoder.      

CDMA fiber-optic systems with optical hard limiters

The code-division multiple-access (CDMA) technique has recently received substantial attention for fiber-optic communication systems. Simultaneous users are allowed to send their data asynchronously in CDMA fiber-optic systems through the assignment of unique “signature codes.” The multiple-access interference (MAI), which increases with the number of simultaneous users, severely limits the user capacity of the systems. An optical hard limiter can be placed at the front end of the desired receiver to reduce the effects of the MAI. We obtain the bit error probability for the CDMA fiber-optic systems with optical hard limiters. The exact performance analysis is performed and the result is a generalization of a previous analysis. For λ_c=1 codes, the result is not restricted to the case for threshold Th=w, yielding a more general and useful result than that obtained previously. The improvement in the performance from using a hard limiter with prime code and extended prime code is shown. The effect on the choice of decision threshold Th is also examined     

Performance and optimization of multistage partial parallel interference cancellation for wideband CDMA systems in multipath fading channels

In this paper, a closed-form bit error rate (BER) of hard-decision multistage partial parallel interference cancellation (PPIC) having perfect and imperfect channel estimation for complex spreading code-division multiple access (CDMA) over multipath Rayleigh channels was derived. The effects of receiving BER, power ratio of pilot to data channels, multiple-access interference (MAI) and multipath interference, additive white Gaussian noise, and other factors affecting channel estimation error are analyzed. The expression determining the optimal interference cancellation weight at path level for PPIC is first derived. Then, the oscillatory behavior of user-level-optimized PIC and the convergence of perfect channel estimation PIC and path-level-optimized PIC are validated. Results indicate that all factors determining the optimal weight certainly affect the BER performance of PPIC and that the lower BER limit of multistage PPIC depends on the adjustable factors, which include power ratio, observation length, and signal-to-noise ratio, and an unchangeable factor, i.e., number of users. In addition, the variance of MAI for complex spreading sequence is obtained, and the analytical expression can also be employed to predict the performance of multicode CDMA.     

Analysis of adaptive multistage interference cancellation for CDMAusing an improved Gaussian approximation

We consider a simple model for adaptive multistage interference cancellation within a CDMA system, and seek to develop an accurate analytical expression for the performance of this system. Previous work on interference cancellation has relied heavily on simulation techniques or a simple Gaussian approximation (GA). The standard GA ran lead to bit-error rate (BER) results which are optimistic for the conventional receiver, and this also occurs when the approximation is applied to the interference cancellation problem. Additionally, this approximation does not allow the second order effects of the multiple access interference (MAI) to be included in the performance estimates. Several improvements on the standard GA have been suggested which result in accurate performance results for a standard CDMA receiver. This paper presents an analytical expression for the probability of bit error for an adaptive multistage interference canceller, using an improved Gaussian approximation (IGA) for MAI. The BER at any stage of interference cancellation can be recursively computed from the signal-to-noise ratio (SNR), the statistics of the random powers of users, and the processing gain of the CDMA system. The performance of the resulting EER expression is compared with simulation results. Since the second order effects of MAI can be included, the analytical framework presented here can also be used to evaluate the performance of multistage interference cancellation in arbitrary fading environments, and we present results for the performance of interference cancellation in lognormal fading environments     

A new modulation scheme using asymmetric erorr-correcting codesembedded in optical orthogonal codes for optical CDMA

An embedded-modulation scheme is proposed for optical code division multiple access (CDMA). In this scheme, codewords in an asymmetric error-correcting (AEC) code is embedded in a signature sequence in an optical orthogonal code (OOC) used for identification. We classify the codewords of the AEC code according to the number of "1"s in each codeword, and then derive the bit error rate (BER) of the embedded-modulation scheme. Numerical results show that although the performance has the error floor, for achieving the floor value, the embedded-modulation scheme requires less optical energy per bit than the traditional modulation scheme with the correlation and chip-level receivers. The error floor is primarily due to multiaccess interference (MAI), we furthermore apply Reed-Solomon (RS) coding to the embedded-modulation scheme. Consequently, RS coding reduces the floor value to be negligibly small     

Multiuser channel estimation and tracking for long-code CDMA systems

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next-generation wireless systems use long spreading codes whose periods are much larger than the symbol duration. We derive the maximum-likelihood channel estimate for long-code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be negligible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI-resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long-code CDMA systems over multipath fading channels.     

Communication systems with PPM signaling

Effect of imperfect slot synchronization between the transmitter and the receiver on optical synchronous code-division multiple-access (CDMA) systems using pulse position modulation as data modulation (PPM/CDMA) is investigated. Optical orthogonal codes (OOC's) are employed as signature sequences, and parallel optic-fiber delay line encoders and correlators are adopted in the transmitters and the receivers, respectively. The upper bound on the bit error probability of PPM/CDMA is derived under the condition that the receiver slot timing shifts from the transmitter timing clock. The bit error probability performance is evaluated for some values of the number of slots per frame, average signal photocount, and the number of simultaneous users. It is shown that as the number of slots per frame increases, the timing offset should be restricted to be smaller to achieve low bit error probability. Further, when the timing offset is small, the improvement of the bit error probability performance with the increase of the number of slots per frame under the photocount per second constraint is shown to be larger than that under the photocount per symbol constraint     

基于CDMA的无线传感器网络误码率分析

为了分析多址干扰和节点干扰对CDMA无线传感器网络的影响,在考虑传输距离、传输速率、阴影效应、节点密度等影响的基础上,通过Matlab对误码率(BER)进行了仿真.仿真结果表明,CDMA在物理层对抵抗干扰和节约能量具有良好的性能.     

BER Performance Analysis of Spectral Phase-Encoded Optical Atmospheric PPM-CDMA Communication Systems

We propose spectral phase-encoded optical atmospheric (OA) code-division multiple-access (CDMA) systems with pulse-position modulation (PPM) and theoretically analyze the bit error rates (BERs). We compare the BERs of the proposed system to those of time-encoded OA-PPM-CDMA systems at the same bit rate and chip rate. We show that, even if the proposed spectral phase-encoded OA-PPM-CDMA system is equipped with no mitigation techniques of multiple access interference (MAI) and scintillation, the proposed system achieves lower BERs than both time-encoded OA-PPM-CDMA systems, both with and without a mitigation technique of MAI/scintillation, due to its higher code spreading gain and higher tolerance to noise.     

一种新的估计CDMA系统误码率的方法

本文提出一种新的估计ＣＤＭＡ系统的误码率的方法，得出了较高斯函数更精确的多址干扰的密度函数，我们得出的解析结果首次可解释扩频增益变大、传统高斯估计法不准的原因。     

平坦瑞利衰落环境下CDMA系统的多用户检测性能

在采用传统信号检测方式的CDMA无线通信网中,多址干扰限制了系统的容量和性能。多用户信号检测利用扩频码互相关信息在高斯信道中可以减小或消除多址干扰,缓解远近效应,增加系统容量。本文将多用户检测器直接用于平坦瑞利衰落的CDMA环境中,用分析与仿真相结合的方法讨论了典型多用户算法的误码性能。结果表明,平坦瑞利衰落信道中多用户检测算法的性能仍然优于传统检测算法。     

CDMA通信系统仿真及抑制多址干扰的技术研究

CDMA作为第三代无线通信系统,具有频谱利用率高、保密性强、可多址复用等优点。在移动通信领域,越来越受到人们的普遍关注和重视。文章阐述了多址干扰产生的原因及其对系统性能的影响。通过MATLAB/Simulink建立仿真平台,设计并实现了直接序列扩频通信中信号的收发,采用误码率这一指标,重点对比分析传统单用户检测器、解相关线性检测器和最小均方误差检测器对系统性能的影响。实验结果表明,解相关线性多用户检测器在抑制多址干扰方面占有明显的优势。     

Performance Analysis of Subspace Based Downlink Channel Estimation for W-CDMA Systems Using Chaotic Codes

This paper presents blind channel estimation for downlink W-CDMA system that employs chaotic codes and Walsh codes for spreading information bits of the multiple users. In a W-CDMA system, while transmitting over multipath channels, both intersymbol interference (ISI) as a result of Inter Chip Interference and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users’ signature waveform. Earlier work focused on a subspace based channel estimation algorithm for asynchronous CDMA systems to estimate the multiple users’ symbols, where only AWGN channel was considered. In our work, we study a similar subspace-based signature waveform estimation algorithm for downlink W-CDMA systems, which use chaotic codes instead of pseudo random codes, that provide estimates of the multiuser channel by exploiting structural information of the data output at the base station. In particular, we show that the subspace of the (data+noise) matrix contains sufficient information for unique determination of channels, and hence, the signature waveforms and signal constellation. We consider Rayleigh and Rician fading channel model to quantify the multipath channel effects. Performance measures like bit error rate and root mean square error are plotted for both chaotic codes and Walsh codes under Rayleigh and Rician fading channels.     

Bit error rate performance of Haar wavelet based scale‐code division multiple access (HW/S‐CDMA) over the asynchronous AWGN channel

In this paper, we study a recently proposed multirate system, called wavelet based scale‐code division multiple access (W/S‐CDMA). W/S‐CDMA depends on the code, time and scale orthogonality introduced by pseudo‐noise (PN) sequences, and wavelets. In this system, the channel is partitioned into different scales, and each scale into time slots. In addition, the PN sequences are used in each scale to identify multiple users. In W/S‐CDMA, each user is assigned a specific scale and PN sequence, and transmits its successive information symbols with its PN sequence and the wavelets in that scale. More symbols are transmitted in finer scales. We analyse the bit error rate performance of Haar wavelet based S‐CDMA (HW/S‐CDMA) over an asynchronous additive white Gaussian noise (AWGN) channel by using a conventional detector for deterministic PN sequences. The performance of the system is compared to that of an equivalent multirate CDMA (MR‐CDMA) system for Gold and Kasami PN sequences. Results show that HW/S‐CDMA outperforms MR‐CDMA. In addition, because of its suitable format HW/S‐CDMA is also capable of employing the optimal PN sequence families with limited number of sequences such as Kasami, Bent, etc. repeatedly in different scales. Copyright © 2006 John Wiley & Sons, Ltd.     

STBC MIMO CDMA系统CMVDR接收机研究

提出了STBC MIMO CDMA系统中一种新的基于多用户ML估计的带制约的最小方差无失真响应(CMVDR)接收机，能有效抑制多址干扰(MAD和符号间干扰(ISI)。给出了多用户、多径的STBC MIMO CDMA系统信道模型及ML参数估计，并给出了分部加权和整体加权两种CMVDR接收机，本接收机也适用于SIMO CDMA系统。进行了系统模拟，并对分部加权和整体加权两种CMVDR接收机的性能以及自适应CMMSE接收机的性能进行了比较分析。