Uplink Performance Evaluation of CDMA Communication System with RAKE Receiver and Multiple Access Interference Cancellation

In CDMA communication systems, all the subscribers share the common channel. The limitation factor on the system’s capacity is not the bandwidth, but multiuser interference and the near far problem. This paper models CDMA system from the perspective of mobile radio channels corrupted by additive white noise generated by multipath and multiple access interferences. The system’s receiver is assisted using different combining diversity techniques. Performance analysis of the system with these detection techniques is presented. The paper demonstrates that combining diversity techniques in the system’s receivers markedly improve the performance of CDMA systems.

New acquisition decision algorithm for multiple access interference suppression in DSSS system

For decreasing the multiple access interference of weaker signal acquisition in direct sequence spread spectrum(DSSS) systems,a new single decision algorithm is presented.The maximum value of correlation results is conventionally detected.However,there may be not only one strong peak among correlation results when the cross-correlation noise is strong enough to affect the correlation results.The proposed algorithm decreases the false alarm probability through the decision of the ratio of the maximum value and the second maximum value of the correlation results.Theoretical analysis and simulation results indicate that the proposed algorithm effectively suppresses the acquisition problem of multiple access interference in DSSS system.

Intelligent reflecting surface-assisted cognitive radio-inspired rate-splitting multiple access systems

Intelligent reflecting surface(IRS)has been widely regarded as a promising technology for configuring wireless propagation environments.In this paper,we utilize IRS to assist transmission of a secondary user(SU)in a cognitive radio-inspired rate-splitting multiple access(CR-RSMA)system in which a primary user's(PU's)quality of service(QoS)requirements must be guaranteed.Without introducing intolerable interference to deteriorate the PU's outage performance,the SU conducts rate-splitting to transmit its signal to the base-station through the direct link and IRS reflecting channels.For the IRS-assisted CR-RSMA(IRS-CR-RSMA)scheme,we derive the optimal transmit power allocation,target rate allocation,and successive interference cancellation decoding order to enhance the outage performance of the SU.The closed-form expression for the SU's outage probability achieved by the IRS-CR-RSMA scheme is derived.Various simulation results are presented to clarify the enhanced outage performance achieved by the proposed IRS-CR-RSMA scheme over the CR-RSMA scheme.

A Survey of Downlink Non-Orthogonal Multiple Access for 5G Wireless Communication Networks

Non-orthogonal multiple access(NOMA) has been recognized as a promising multiple access technique for the next generation cellular communication networks. In this paper, we first discuss a simple NOMA model with two users served by a single-carrier simultaneously to illustrate its basic principles. Then, a more general model with multicarrier serving an arbitrary number of users on each subcarrier is also discussed. An overview of existing works on performance analysis, resource allocation, and multiple-in-put multiple-output NOMA are summarized and discussed. Furthermore, we discuss the key features of NOMA and its potential research challenges.

Non-Orthogonal Multiple Access in Cell-Free Massive MIMO Networks

In this paper, we investigate the downlink performance of cell-free massive multi-input multi-output non-orthogonal multiple access(CF-m MIMO-NOMA) system with conjugate beamforming precoder and compare against the orthogonal multiple access(OMA) counterpart. A novel achievable closed-form spectral efficiency(SE) expression is derived, which characterizes the effects of the channel estimation error, pilot contamination, imperfect successive interference cancellation(SIC) operation, and power optimization technique. Then, motivated by the closedform result, a sum-SE maximization algorithm with the sequential convex approximation(SCA) is proposed, subject to each AP power constraint and SIC power constraint. Numerical experiments indicate that the proposed sum-SE maximization algorithms have a fast converge rate, within about five iterations. In addition, compared with the full power control(FPC) scheme, our algorithms can significantly improve the achievable sum-SE. Moreover, NOMA outperforms OMA in many respects in the presence of the proposed algorithms.

Frameworks of Non-Orthogonal Multiple Access Techniques in Cognitive Radio Communication Systems

Recently,the increasing demand of radio spectrum for the next generation communication systems due to the explosive growth of applications appetite for bandwidths has led to the problem of spectrum scarcity.The potential approaches among the proposed solutions to resolve this issue are well explored cognitive radio(CR)technology and recently introduced non-orthogonal multiple access(NOMA)techniques.Both the techniques are employed for efficient spectrum utilization and assure the significant improvement in the spectral efficiency.Further,the significant improvement in spectral efficiency can be achieved by combining both the techniques.Since the CR is well-explored technique as compared to that of the NOMA in the field of communication,therefore it is worth and wise to implement this technique over the CR.In this article,we have presented the frameworks of NOMA implementation over CR as well as the feasibility of proposed frameworks.Further,the differences between proposed CR-NOMA and conventional CR frameworks are discussed.Finally,the potential issues regarding the implementation of CR-NOMA are explored.

Joint Non-Orthogonal Multiple Access (NOMA) & Walsh-Hadamard Transform: Enhancing the Receiver Performance

Non-orthogonal multiple access(NOMA) is a new access method to achieve high performance gains in terms of capacity and throughput, so it is currently under consideration as one of the candidates for fifth generation(5 G) technologies. NOMA utilizes power domain in order to superimpose signals of multiple users in a single transmitted signal. This creates a lot of interference at the receive side. Although the use of successive interference cancellation(SIC) technique reduces the interference, but to further improve the receiver performance, in this paper, we have proposed a joint Walsh-Hadamard transform(WHT) and NOMA approach for achieving better performance gains than the conventional NOMA. WHT is a well-known code used in communication systems and is used as an orthogonal variable spreading factor(OVSF) in communication systems. Application of WHT to NOMA results in low bit error rate(BER) and high throughput performance for both low and high channel gain users. Further, it also reduces peak to average power ratio(PAPR) of the user signal. The results are discussed in terms of comparison between the conventionalNOMA and the proposed technique, which shows that it offers high performance gains in terms of low BER at different SNR levels, reduced PAPR, high user throughput performance and better spectral efficiency.

BER Performance of Space-Time Coded MMSE DFE for Wideband Code Division Multiple Access (WCDMA)

In recent times, there has been growing interests in integration of voice, data and video traffic in wireless communication networks. With these growing interests, WCDMA has immerged as an attractive access technique. The performance of WCDMA system is deteriorated in presence of multipath fading environment. The paper presents space-time coded minimum mean square error (MMSE) Decision Feedback Equalizer (DFE) for wideband code division multiple access (WCDMA) in a frequency selective channel. The filter coefficients in MMSE DFE are optimized to suppress noise, intersymbol interference (ISI), and multiple access interference (MAI) with reasonable system complexity. For the above structure, we have presented the estimation of BER for a MMSE DFE using computer simulation experiments. The simulation includes the effects of additive white Gaussian noise, multipath fading and multiple access interference (MAI). Furthermore, the performance is compared with standard linear equalizer (LE) and RAKE receiver. Numerical and simulation results show that the MMSE DFE exhibits significant performance improvement over the standard linear equalizer (LE) and RAKE receiver.

Performance Analysis of Multiple User Optical Code Division Multiple Access

In this paper, we discuss and analyze an optical code division multiple access for multiple user system. Media access control implementation has been considered. For fulfilling the huge need of bandwidth services, technology tends to move to optical networks and three major optical systems come into existence. Code division of the optical network is most used and real concept interacted with users up to this time. Optical code division multiple access provides complete spectrum to each and every user for the time of accessing the channel. In the paper, we have proposed network architecture with optical encoder and decoder along with optical code translator which is supporting multiple user systems. We integrate the code translator with encoding and decoding of optical code to use the optical network at full extent and present the simulation validation results of 6 Gb/s 3-hop transmission by use of proposed architecture. Further, we have implemented experimentation with 6 users 3 Gb/s optical code division multiple access network. Through simulation structure, it is shown that the combination of encoding with EDFA in multiple user environment system provides improvement in bit error rate and also improves the multiple access interference. With increase of users in the network, MAI value increases and our proposed scheme controls interference in the network.

Routing Protocol in Underwater Wireless Acoustic Communication Using Non Orthogonal Multiple Access

The underwater wireless communication with the complexity of attenuation and low propagation speed makes resource constraints in networking sensor nodes and sink.Underwater Sensor Transmission with Attenuation Calculation using Non Orthogonal Multiple Access(UWSTAC-NOMA)protocol has been proposed.This protocol calculates channel gain along with attenuation in underwater channels and provides internetworking sensor for rate allocation minimizing interference.Successive Interference Cancellation has been used at the receiving sensor to decode the information sent.The network level performance of sensors and increasing the data rate improves the overall throughput.Simultaneously,connecting several sensors to sink based on its depth region of deployment has been achieved using Underwater Sensor Transmission with Attenuation Calculation using Non Orthogonal Multiple Access(UWSTAC-NOMA).The analytical background of attenuation never confuted the simulation results of the proposed protocol in NS2 simulator.Simulation results shows that the throughput,average bit error rate and residual energy of sink performance.

Evaluation of Multiusers’ Interference on Radiolocation in CDMA Cellular Networks

Radiolocation has been previously studied for CDMA networks, the effect of Multiple Access Interference has been ignored. In this paper we investigate the problem of Radiolocation in the presence of Multiple Access Interference. An extensive simulation technique was developed, which measures the error in location estimation for different network and user configurations. We include the effects of lognormal shadow and Rayleigh fading. Results that illustrate the effects of varying shadowing losses, number of base stations involved in position location, early-late discriminator offset and cell sizes in conjunction with the varying number of users per cell on the accuracy of radiolocation estimation was presented.

QPSK DS-CDMA System over Rayleigh Channel with a Randomly-Varying Frequency Narrow-Band Interference: Frequency Tracking Analysis

This paper analyses frequency tracking characteristics of a complex-coefficient adaptive infinite-impulse response (IIR) notch filter used for suppression of narrow-band interference (NBI) with a randomly-varying frequency in a quadriphase shift keying (QPSK) modulated direct-sequence code-division multiple-access (DS-CDMA) communication system. The QPSK DS-CDMA signals are transmitted over a frequency non-selective Rayleigh fading channel. The analysis is based on a first-order real-coefficient difference equation with respect to steady-state instantaneous frequency tracking error from which a closed-form expression that relates frequency tracking mean square error (MSE) with number of DS-CDMA active users and NBI power is obtained. Closed-form expressions for optimum notch bandwidth coefficient and step size constant that minimize the frequency tracking MSE are also derived. Computer simulations are included to substantiate the accuracy of the analyses.

Improved Space-Time Selective Interference Cancellation Scheme for CDMA Systems

Space-time selective parallel interference cancellation(ST-SPIC) is a computationally effective approach combining multiuser detection (MUD) with antenna array technology for CDMA systems. The exploitation of signal reliability is a key issue in ST-SPIC. In order to improve the reliability estimation, a pair of reliability thresholds are introduced. Then an improved selective interference cancellation algorithm is proposed to exploit the reliability accordingly. More practical space-time processing algorithms are also incorporated in the proposed ST-SPIC scheme to overcome the limitation caused by some idealised assumptions taken in the original ST-SPIC scheme. Numerical results show that the proposed ST-SPIC scheme outperforms its traditional counterpart in a CDMA microcell environment.

非理想干扰消除全双工协作NOMA系统性能分析及功率分配

随着非正交多址接入(Non-Orthogonal Multiple Access,NOMA)技术的不断发展,协作NOMA(Cooperative Non-Orthogonal Multiple Access,CNOMA)也受到广泛关注.该文研究了由一个基站(Base Station,BS)、一个远端用户和一个近端用户组成的全双工(Full-Duplex,FD)协作NOMA(FD-CNOMA)系统,其中近端用户作为一个FD解码转发中继以传输远端用户的信号.考虑实际情况中存在基站与远端用户有无直接链路两种情况,以及非理想连续干扰消除(Successive Interference Cancellation,SIC)对系统带来的残留干扰问题,该文提出并解决了在该模型下用户的功率分配问题.最后,该文基于此模型给出了中继用户和远端用户的中断概率闭式表达式和遍历速率的近似闭式表达式.理论分析与仿真结果表明,即便存在非理想SIC和自干扰,相对于半双工(Half-Duplex,HD)协作NOMA(HDCNOMA)和NOMA系统而言,FD-CNOMA系统表现出更好的系统性能.同时,非理想SIC和中继用户自干扰都会对用户性能产生负面影响,且非理想SIC在遍历速率上对HD机制的影响较FD机制更为严重.最后,文中提出的功率分配方法较随机功率分配和固定功率分配分别获得了13%和10%的速率提升.

Multiple access interference suppression for CDMA systems via l_(∞)-minimization

A key problem in code-division multiple access(CDMA)system is to mitigate the multiple access interference(MAI)from other users while detecting the desired user.The performance of the conventional minimum output energy(MOE)multiuser detector for CDMA system significantly degrades in the presence of signature waveform distortions induced by multipath propagation or timing asynchronism.In this paper,a robust linear programming(ROLP)algorithm for blind multiuser detection is proposed.Different from the existing MOE-based multiuser detection techniques,the proposed ROLP minimizes the l_∞-norm of the output to exploit the non-Gaussianity of the communication signals.To achieve robustness against signature waveform mismatch,the proposed method constrains the magnitude response of any signature vector within a specified uncertainty set to exceed unity.The uncertainty set is modeled as a rhombus,which differs from the spherical uncertainty region widely taken in the existing robust multiuser detectors.The resulting optimization problem is reformulated into a linear programming program and hence can be solved efficiently.The proposed ROLP is computationally simpler than its robust counterparts that requires solving a second-order cone programming.Simulation results demonstrate the superiority of the ROLP over several robust detectors,which indicate that its performance approaches the optimal performance bound.

An Acquisition Performance Analysis of DS-MC-CDMA System under Multiple Access Interference Conditions

Multi-Access Interference’s (MAI’s) effect on parallel acquisition performance of DS-Multi-carrier CDMA system under the typical indoor radio channel is discussed in the paper. Moreover, a comparison of acquisition performance impairment produced by the MAI in DS-MC-CDMA and DS-Single-carrier CDMA is also made. Results are given in terms of average acquisition time obtained by means of simulations and theory analysis.

不完美串行干扰消除下多RIS辅助的NOMA系统中断性能

考虑到实际通信应用中不能实现完美的串行干扰消除(SIC,serialinterferencecancellation),研究了不完美SIC情况下多个智能反射面(RIS,reconfigurableintelligentsurface)辅助的非正交多址接入(NOMA,non-orthogonal multiple access)系统的中断性能和吞吐量。联合考虑直射链路和反射链路,利用Gamma分布近似表征多RIS辅助NOMA系统的信道系数,推导出用户中断概率的近似闭合表达式,并通过Monte-Carlo仿真进行验证,分析系统中断性能与不完美SIC、RIS个数及反射单元数等因素的关系。仿真结果表明,与单RIS辅助的NOMA系统和传统NOMA系统相比,在给定中断概率要求下,多RIS辅助系统所需的信噪比(SNR,signal-to-noise ratio)降低了14.2 dBm,说明了采用多个RIS可以有效地提升用户的中断性能和系统吞吐量,从而补偿不完美SIC带来的性能损失。

辅助矢量接收机抗MAI性能的研究

多址干扰(MAI)是损害CDMA系统性能的主要因素之一。本文研究了辅助矢量接收机的抗MAI性能。与多用户检测(MUD)相比,辅助矢量接收机无需在相关器后边增加MUD模块、无需进行矩阵求逆运算、可以用于下行链路、便于盲实现。本文的仿真结果表明:辅助矢量(Auxiliary Vector,AV)的权系数随AV序号的增大而迅速减小,因此增加辅助矢量接收机中AV的数量而使系统得到的误比特性能的增益随AV数量的增加而迅速减小。当含有5个AV时,辅助矢量接收机就基本上达到了其性能界。根据此结论提出辅助矢量接收机中,AV的个数不应多于5个,并应根据通信业务对通信质量的具体要求来确定。较少的AV数使辅助矢量接收机有较低的计算复杂度,因此更有实用价值。随后的仿真结果表明辅助矢量接收机具有略优于解相关检测器和MMSE检测器的抗MAI性能。

两种OCDMA系统中OMAI的研究

光多址干扰是影响时域振幅编码和跳频扩时OCDMA系统误码率和用户数的主要原因。该文分别在时域振幅编码和跳频扩时两种OCDMA系统中对光多址干扰问题进行研究,提出通过改良码系采用色散管理的技术、选用适当的线路码及合理使用光限幅器等方法来减小光多址干扰。

SAC-OCDMA系统三分支信号检测方案

为了解决三分支信号检测(TBSD)方案的上界表达式不精确的问题,提出了一种频谱-振幅编码光码分多址(SAC-OCDMA)系统TBSD方案,建立了一个严谨的数学模型,并对该方案进行了数值评估和仿真。仿真结果表明,该方案在传输速率为500 Mb/s、1 Gb/s时能够完全消除多用户干扰(MUI),而在传输速率为5 Gb/s时则可以消除大部分MUI,证明了数学模型的精确性。