Effect Of Multiple Access Interference Research Articles

Influence of different codes on the performance of SAC‐OCDMA over FSO link

SummaryThis paper evaluates the performance of a spectral‐amplitude‐coding optical code division multiple access (SAC‐OCDMA) system over a free space optical (FSO) communication link under various weather conditions such as fog, snow, and rain. The system uses different code families, including modified quadrature congruence (MQC), random diagonal (RD), and diagonal eigenvalue unity (DEU), to address user codes and reduce the effects of multiple access interference (MAI). The system's bit error rate (BER) performance has been analyzed taking into account receiver noises and MAI, and calculates the required optical power for a BER value of 10−9 under different weather conditions. The results show that fog has the most significant impact on the system's performance, while rainy conditions require the lowest optical power. The performance of the system also declines with increasing transmission length and concurrent user count. Finally, the paper compares the performance of different code families under unfavorable weather conditions and concludes that DEU performs better than MQC and RD in bad weather.

Interference Mitigation-Enabled Signal Detection in Diffusive Molecular Communications Systems With Molecular-Type Spreading

To accomplish complex tasks, several nano-machines may need to communicate via multiple-access channel with one access point, where information fusion is carried out. However, multiple-access Diffusive Molecular Communications (DMC) systems suffer from severe Multiple-Access Interference (MAI) and Inter-Symbol Interference (ISI), which should be effectively mitigated at receiver in order to achieve acceptable performance. Built on two fundamental single-user detection schemes, namely Threshold assisted Majority Vote Detection (TMVD) and Equal Gain Combination Detection (EGCD), we first propose three low-complexity interference cancellation schemes, which are the TMVD-assisted Iterative Interference Cancellation (TMVD-IIC), TMVD-based Minimum-Distance Decoding assisted Interference Cancellation (TMVD-MDDIC) and the EGCD-assisted N-order Iterative Interference Cancellation (EGCD-NIIC), for operation in the Molecular Type Spread assisted Molecular Shift Keying (MTS-MoSK) DMC systems. Then, following the principle of maximum likelihood detection, we propose a Simplified Approximate Maximum Likelihood (SAML) detection scheme. The error performance of the MTS-MoSK DMC systems employing respectively the considered detection schemes is comprehensively investigated and compared. Furthermore, the complexities of the detection schemes are analyzed and discussed in terms of the complexity-performance trade-off. Our studies and results show that, compared with the single-user TMVD and EGCD schemes, the proposed interference cancellation schemes are capable of mitigating efficiently the effect of MAI and enabling significant performance improvement at the slightly increased complexity.

Adequate spreading codes to reduce MAI in quasi‐synchronous MC‐DS‐CDMA system

In this study, the authors consider the effect of multiple access interference (MAI) on the performance of a quasi-synchronous multicarrier direct-sequence code-division multiple access (QS-MC-DS-CDMA) system. An analytical expression of the bit error rate (BER) is derived for a QS-MC-DS-CDMA system over additive white Gaussian noise channel. The BER performance degradation is proportional to the squares of the even correlation function (ECF) and odd correlation function (OCF) of the spreading codes. The obtained results show that the adequate spreading code is a set of binary zero correlation zone sequences with both zero ECF and OCF. The construction of the latter codes remains a challenge for researchers in the field of code design.

An Efficient Carrier Frequency Offset Tracking for OFDMA Systems Using Normalized Least-Mean-Square Algorithm

In orthogonal frequency-division multiple access (OFDMA), carrier frequency offset (CFO) destroys the orthogonality between subcarriers due to Doppler effect of the channel and also causes inter-carrier interference (ICI) as well as multiple access interference (MAI). In this paper, CFO tracking algorithm for OFDMA system is presented to overcome the effect of ICI as well as MAI. The proposed algorithm employs normalized least-mean-square (NLMS) algorithm which tracks the residual carrier frequency offset in OFDMA system. The proposed NLMS-based tracking algorithm uses average of all residual carrier frequency offset (RCFO) for improving the mean square error (MSE) performance. The proposed NLMS-based frequency tracking along with frequency offset compensator improves the error performance by reducing the effect of ICI and MAI. Simulation results indicate that the proposed algorithm achieves better tracking performance in the presence of frequency offset. From the results, it is observed that the MSE of the proposed estimator is improved about 3.8 dB at SNR = 20 dB with the frequency offset.

MMSE based beamformer in massive MIMO-IDMA downlink systems

Abstract Now-a-days Massive MIMO (mMIMO) become an attractive technology due to its spectral and energy efficiency by the means of simple signal processing. However, in overloaded scenario, wherein the number of users equipments (UEs) are larger than number of antennas, the spectral effciency (SE) suffers and hence error rate performance, it has been shown recently that use of code domain NOMA in mMIMO can improve the SE performance. Further, interleave division multiple access (IDMA) has been drawn much attention as a suitable code domain non-orthogonal multiple access (NOMA) for future communication standards. This paper proposes the work in two folds, first a massive multiple input and multiple output (MIMO) and IDMA communication system is jointly proposed in which antennas on the base station serves users simultaneously in the same frequency band. Both and are large in numbers. Secondly, the minimum mean square error (MMSE) based beamformer is suggested to combat the propagation loss and effect of multiple access interference (MAI), for massive MIMO-IDMA system under downlink communication constraints. With the help of simulation results, the performance of the proposed system with MMSE beamforming have been investigated in terms of BER vs SNR curve plot.

Generation of Spreading Codes with Minimum Correlation using Sorting Genetic Algorithm-II

In a code division multiple access (CDMA) system, multiple access interference (MAI) and Inter-symbol interference (ISI) appears if generated spreading codes are not maintained orthogonally and the communication channel is taken as multi-path communication channel. When generated spreading codes are multi-path spread and then channel delay occurs, it shows that ortho-gonality of the spreading codes is not maintained. The effect of MAI can be mitigated by maintaining low cross-correlation values as much as low between the large numbers of spreading codes. The code division multiple technique spreading codes must maintain absolutely impulsive autocorrelation at origin and very low cross correlation other than origin to avoid false synchronisation. i.e autocorrelation must be maximum at origin and cross correlation must be minimum at non origin point. In this paper, we propose multi-objective Genetic Algorithm approach –Genetic Algorithm-II (NSGA-II) to reduce the out-of-phase average mean-square aperiodic autocorrelation and average mean-square aperiodic cross-correlation value of randomly initialized binary spreading code set

Packet Switching Strategy and Node Architecture of Extended Spectral-Amplitude-Coding Labels in GMPLS Networks

We present packet switching applications based on extended spectral-amplitude-coding (SAC) labels in generalized multi-protocol label switching (GMPLS) networks. The proposed approach combines the advantages of wavelength-division multiplexing (WDM) and optical code-division multiple access (OCDMA). The extended SAC labels preserve the orthogonal property to avoid the effect of multiple access interference (MAI) shown at the decoder. We investigate the node architecture of label generation/recognition based on arrayed waveguide grating (AWG). Combining cyclic-shifted maximal length sequence (MLS) codes with the wavelength routed property of AWG simplifies the node structure. The simulation results show that the proposed labels achieve good performances against receiver noise due to the low average cross-correlation values. Under a given bit-error-rate (BER), the switching efficiency of the extended SAC labels outperforms the previous OCDMA schemes, as the network nodes are capable of processing a large number of labels simultaneously.

Successive interference cancellation in asynchronous CC-CDMA systems under Rician fading channels

The unprecedented frequency diversity gain offered by complementary coded code-division multiple access (CC-CDMA) makes it more attractive than conventional CDMA systems in frequency selective fading channels. CC-CDMA loses its interference-resilient capability due to non-ideal correlation characteristics of complementary codes (CC) under uplink asynchronous communications. The existence of multiple access interference (MAI) in asynchronous systems results in drastic reduction in error rate performance in CC-CDMA systems. In this paper, the analysis of successive interference cancellation (SIC) is proposed for CC-CDMA systems as an alternative to existing non-interference cancellation schemes in uplink communication. The average bit error probability expressions for single branch and M-branch CC-CDMA systems operating under Rician fading channels are derived using alternate integral expression of Q-function. The accuracy of obtained closed form error probability expressions are confirmed through the proposed joint SIC with maximal ratio combining and minimum mean square error combining techniques in uplink CC-CDMA systems. The significance of soft-decision compared to hard decision SIC for CC-CDMA systems is also analyzed for both equal power and extreme near-far conditions. Furthermore, the simulation results exposed that significant performance improvement is obtained under near-far situations in uplink asynchronous CC-CDMA systems overcoming the effects of MAI and achieving improved system capacity.

Performance Evaluation of Sub-carrier Multiplexed SAC-OCDMA System Using Optimal Modulation Index

Abstract In this paper, we investigate the impact of modulation index on the performances of SCM SAC-OCDMA system. Knowing that zero cross-correlation (ZCC) codes remove the effect of multiple access interference and of phase-induced intensity noise, SCM SAC-OCDM has been investigated using ZCC codes with two types of direct detection, one and all wavelengths of the code. In this hybrid system, the SNR depends on the modulation index for a given effective power of broadband source. In order to maximize the SNR, the optimum values of the modulation index, for different number of subcarriers, are determined analytically for two types of detection. The performance of the optimized system is evaluated based up on BER. The obtained numerical results show that, by optimizing the modulation index, the number of subcarriers (number of active users per ZCC code) is increased and the performance is significantly improved.

DWDM混合光学系统中帧间和信道内四波混频效应的抑制

In this study, a hybrid system of optical Code Division Multiple Access(optical CDMA) and Dense Wavelength Division Multiplexing(DWDM) is proposed with a comprehensive investigation into the effect of four-wave mixing(FWM). In such system, two major FWM problems exist, inter and intra-channel FWM, including multiple access interference(MAI) and inter-symbol interference(ISI). Results show that the optimum transmitted power is 18dBm in order to control the trade-off between inter and intra-channel FWM, where an increase in the BER of the hybrid system at transmitted power above 18 dBm is indicated. Hence, an electro-optic phase modulator(EOPM) module is proposed and placed after the WDM multiplexer to simultaneously modulate the phase of all wavelengths signals to increase the nonlinear tolerance in the hybrid system by suppressing the impact of intra-channel FWM, which is shown to greatly improve the performance of the optical CDMA-DWDM hybrid system based OOK transmission. In addition, the effect of MAI can be reduced by the use of multi-diagonal(MD) identification sequence code, due to the zero cross-correlation property of MD. The results also reveal that the CDMA technology in conjunction with chromatic dispersion helps to reduce the effect of inter-channel FWM. Moreover, the identification sequence code interval plays crucial role in the mitigating of ISI as the results expose that the best performance of the proposed hybrid system can be achieved when the identification sequence code interval squeezed into 25% of bit duration where the avoidance of ISI is guaranteed.

Performance Enhancement of Optical PPM-CDMA Using Successive Interference Cancellation Scheme

A comprehensive mathematical model of Successive Interference Cancellation (SIC) using Direct Sequence (DS) and Spectral Amplitude Coding (SAC), which is employed in incoherent optical CDMA using On-Off Keying (OOK) Pulse Position Modulation (PPM) and On-Off Keying (OOK) considering the presence of Phase–Induced Intensity Noise (PIIN), shot noise and thermal noise is presented in this work. SIC scheme has the potential to suppress Multiple Access Interference (MAI). The concept of SIC is the following: detection is taken place in successive stages where the most reliable symbols are chosen at each stage. Where the interference contributed by the already detected symbols will be removed. It has been found out that, theoretically, the size of M-ary on optical CDMA system limits the MAI effect. Thus, the system performance improves with the increasing of M-ary PPM. Hence, the system using PPM has better performances if compared to the one using OOK, due to a lower average number of interfering optical pulses. In addition, a lot of users can be accommodated with SIC/SAC-optical CDMA based PPM signaling system instead of the SIC/SAC-optical CDMA based OOK one.

Performance Analysis of Spectral Amplitude Coding Optical Code Division Multiple Access System using Modified Double Weight Codes with Adomian Decomposition Method

Abstract In this manuscript, we have analyzed a Spectral Amplitude Coding-Optical Code Division Multiple Access (SAC-OCDMA) System. The system performance is enhanced by reducing the effect of Multiple Access Interference (MAI) using uniform Fiber Bragg Gratings (FBGs) encoders and decoders at central office and subscriber’s end. The results are verified through a mathematical model and Modified Double Weight (MDW) codes for the proposed system using Adomian Decomposition Method (ADM). Further, we have demonstrated SAC-OCDMA system for transmission of 40 Gbps data rate up to a distance of 80 km by increasing the number of FBGs. The proposed system has been analyzed in terms of distance, bit error rate (BER) and Quality Factor.

Suitable Spreading Sequences for Asynchronous MC-CDMA Systems

In order to meet the demand of high data rate transmission with good quality maintained, the multi-carrier code division multiple access (MC-CDMA) technology is considered for the next generation wireless communication systems. However, their high crest factor (CF) is one of the major drawbacks of multi-carrier transmission systems. Thus, CF reduction is one of the most important research areas in MC-CDMA systems. In addition, asynchronous MC-CDMA suffers from the effect of multiple access interference (MAI), caused by all users active in the system. Degradation of the system’s bit error rate (BER) caused by MAI must be taken into consideration as well. The aim of this paper is to provide a comparative study on the enhancement of performance of an MC-CDMA system. The spreading sequences used in CDMA play an important role in CF and interference reduction. Hence, spreading sequences should be selected to simultaneously ensure low CF and low BER values. Therefore, the effect that correlation properties of sequences exert on CF values is investigated in this study. Furthermore, a numerical BER evaluation, as a function of the signal-to-noise ratio (SNR) and the number of users, is provided. The results obtained indicate that a trade-off between the two criteria is necessary to ensure good performance. It was concluded that zero correlation zone (ZCZ) sequences are the most suitable spreading sequences as far as the satisfaction of the above criteria is concerned.

Cooperative relay beamforming in IDMA communication networks

Abstract In this paper, a new combination of Interleave division multiple access (IDMA) and spatial diversity offered by cooperative relay aided distributed beam forming is proposed. In the offered scheme communication strategy consists two steps. All users broadcast their message to relays in the first step and then relays amplifies and forward the information to the desired destination. IDMA, which is popular non-orthogonal multiple access (NOMA) technique is used to combat the effect of multiple access interference (MAI) at relay as well as destination nodes. Each relay processed the signal to maintain the QoS of destination. The goal of this work is to find the appropriate beam forming weights by minimising the transmit power and without compromising the QoS in terms of SINR. However power minimization is not the convex problem, so semi-definite relaxation is used to modify the problem in to semi-definite programming (SDP) problem and the conventional SDP problem solver CVX is used for solution. The numerical explanation and simulation experiment of the proposed scheme shows the performance improvements in terms of bit error rate.

Cardinality improvement of Zero Cross Correlation (ZCC) code for OCDMA visible light communication system utilizing catenated-OFDM modulation scheme

In this paper, we proposed a new OCDMA-VLC system design utilizing new modulation scheme known as catenated-OFDM (orthogonal frequency division multiplexing) technique. The catenated-OFDM-OCDMA-VLC system is demonstrated by implementing Zero Cross Correlation (ZCC) code. ZCC code is the preferred code generation in OCDMA system as the effect of multiple access interference (MAI) can be cancelled out due to reduction in Phase Intensity Induced Noise (PIIN). In particular, the use of catenated-OFDM and its application in intensity modulation and direct detection fiber optic visible light communication link is theoretically investigated by taking into account all the noises contribute in the visible light spectrum. The theoretical results demonstrated that cardinality enhancement with doublefold, triplefold and fourfold increase in number of user for (Band = 2), (Band = 5) and (Band = 8) respectively compared to previous ZCC-VLC system designs. The results also indicates a good performance with high data rate up 10Gbps using proposed system which is predicted to be new considerable for upcoming OCDMA-VLC mobile communication system design.

On the Performance Evaluation of a MIMO-WCDMA Transmission Architecture for Building Management Systems.

The goal of this study was to investigate the performance of a realistic wireless sensor nodes deployment in order to support modern building management systems (BMSs). A three-floor building orientation is taken into account, where each node is equipped with a multi-antenna system while a central base station (BS) collects and processes all received information. The BS is also equipped with multiple antennas; hence, a multiple input–multiple output (MIMO) system is formulated. Due to the multiple reflections during transmission in the inner of the building, a wideband code division multiple access (WCDMA) physical layer protocol has been considered, which has already been adopted for third-generation (3G) mobile networks. Results are presented for various MIMO orientations, where the mean transmission power per node is considered as an output metric for a specific signal-to-noise ratio (SNR) requirement and number of resolvable multipath components. In the first set of presented results, the effects of multiple access interference on overall transmission power are highlighted. As the number of mobile nodes per floor or the requested transmission rate increases, MIMO systems of a higher order should be deployed in order to maintain transmission power at adequate levels. In the second set of results, a comparison is performed among transmission in diversity combining and spatial multiplexing mode, which clearly indicate that the first case is the most appropriate solution for indoor communications.

On the effects of spreading sequence pairs over MIMO-STS systems

Previous studies have found that the Multiple Access Interference (MAI) effect is strongly associated with which sequence pairs are spread over Multiple-Input Multiple-Output Space-Time Spreading (MIMO-STS) systems. An improved method of determining which sequence pairs are to be spread is proposed. The choice of these pairs from the selected spreading code families results in a number of ways to mitigate the MAI effects. A Software Defined Radio (SDR) test bed system has been developed to evaluate the selected spreading sequences, in an attempt to mitigate the MAI effects over 2 × 1 MIMO-STS systems. The SDR test bed results reveal that, for a number of the spreading sequence families, improved transmission properties for asynchronous transmission can be achieved.

Utilization of spatial ternary coding scheme in optical CDMA system

In this paper, we propose a spatial ternary coding scheme for optical code-division multiple access systems. Since the multiple access interference (MAI) and beat noise leads to the error floor effect, using error correction mechanism is a preferable manner to further enhance the system performance. The proposed spatial ternary coding scheme possesses larger free distance, which is an effective way to relieve such deteriorating effect. It is expected that more users accommodated in the network can obtain higher throughput, while the side effects of MAI and beat noise will turn out to be more serious. To further reduce such adverse impact, the optical hard-limiter is also proposed. Once the channel condition is improved, it will give rise to the more powerful utilization of error correction code. The countermeasures utilized in the proposed system can effectively fight against the MAI and beat noise, and can allow more simultaneous users accommodated in the network. Therefore, it is a promising way for future high-speed optical access networks.

Differential Characteristics Based Iterative Multiuser Detection for Wireless Sensor Networks.

High throughput, low latency and reliable communication has always been a hot topic for wireless sensor networks (WSNs) in various applications. Multiuser detection is widely used to suppress the bad effect of multiple access interference in WSNs. In this paper, a novel multiuser detection method based on differential characteristics is proposed to suppress multiple access interference. The proposed iterative receive method consists of three stages. Firstly, a differential characteristics function is presented based on the optimal multiuser detection decision function; then on the basis of differential characteristics, a preliminary threshold detection is utilized to find the potential wrongly received bits; after that an error bit corrector is employed to correct the wrong bits. In order to further lower the bit error ratio (BER), the differential characteristics calculation, threshold detection and error bit correction process described above are iteratively executed. Simulation results show that after only a few iterations the proposed multiuser detection method can achieve satisfactory BER performance. Besides, BER and near far resistance performance are much better than traditional suboptimal multiuser detection methods. Furthermore, the proposed iterative multiuser detection method also has a large system capacity.

Comparison between Conventional OCDMA and Subcarrier Multiplexing SAC OCDMA System Based on Single Photodiode Detection

This paper demonstrates the comparison between conventional OCDMA system and subcarrier multiplexing (SCM) SAC-OCDMA system by applying Recursive Combinatorial (RC) code based on single photodiode detection (SPD). SPD is used in the receiver part to reduce the effect of multiple access interference (MAI) which contributes as a dominant noise in incoherent SAC-OCDMA systems. From this analysis, the performance of SCM OCDMA network could be improved by using lower data rates and higher received power. The hybrid SCM OCDMA system shows better performance compare to conventional OCDMA system although the number of users involved is very high. This is because, for hybrid SCM OCDMA system, the number of users can be increased by increasing the number of subcarriers without affect the number of code length and optical codes. Increasing the number of subcarriers will enhance the power consumption by applying hybrid SCM system in OCDMA compared to the conventional OCDMA system. This is because increasing the number of users for hybrid SCM system does not affects the number of code length and the number of optical codes but only increase the number of subcarriers. Thus, hybrid SCM OCDMA system has to increase spectral efficiency and produce better performance compared to conventional of OCDMA system.