Multiple Access Interference Mitigation Research Articles

Cell-Straddling Robust-Fast Cross-Correlation Mitigation via MMSE Filter for DSSS Signals

Existing cross-correlation mitigation algorithms based on the minimum mean square error (MMSE) criterion can effectively suppress multiple access interference (MAI) but suffer from high complexity and modeling grid mismatch. In this work, we propose a cell-straddling robust-fast cross-correlation mitigation (CSR-FCCM) algorithm with two improvements. First, CSRFCCM combines the two-dimensional joint iterative adaptive filtering with interference cancellation, which significantly reduces the number and computational cost of complex amplitude MMSE filters. Second, two discriminators are designed to estimate the straddling offset of delay and frequency for the direct sequence spread spectrum (DSSS) signal. The mismatch problem can be ameliorated by substituting estimated straddling offsets into the signal model, which further improves the MAI mitigation effect. The effectiveness of the CSR-FCCM is verified by simulations using 1023- and 63- length Gold codes. Simulation results show that CSR-FCCM has a better MAI mitigation performance and a lower complexity than the open-loop MAI mitigation algorithms, including 2D-JIAF, and RSR-APC.

Integration of Spatial Modulation Scheme With Code Division Multiple Access for VIVO Based Frequency Selective Nano Sensor Networks

Due to their suitability for human-body based communications, in-VIVO nano sensor networks are envisioned to use the promising Terahertz signals in order to ensure the forthcoming high-rate communication needs of the modern medicine. However, the propagation losses at these frequency bands are quite significant and dependent on the operation frequency as well as on the physiological characteristics, thus impeding the use of Terahertz rates to their utmost benefit. Using numerous emitting elements is likely to improve the quality of the received signal, but gives rise to multi-channel interference (MCI) emanating from multi-antenna signaling reception, which necessitates a relatively complex signal processing to mitigate it. When multiple physiological signals are of interest, detecting them necessitates to mitigate multiple-access interference (MAI). In this perspective, orthogonal frequency division multiplexing (OFDM) allows to combat the channel frequency selectivity, whereas code division multiple access (CDMA) scheme cancels MAI. In this paper, we propose to embed the novel spatial modulation technique with CDMA architecture in an OFDM framework to ensure a viable communication in in-VIVO frequency selective Nano channels. The immunity of our proposed solution to such an interference is confirmed, since less than −2 dB in SNR level is required to support 5 users simultaneously communicating with a BER which is inferior to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${10}^{-{3}}$ </tex-math></inline-formula> when the operating frequency is equal to 1THz. This hybrid scheme is shown to efficiently combat the MCI while enabling a safe retrieval of the useful signal at the very-high data rate communications.

Detection of Signals in MC–CDMA Using a Novel Iterative Block Decision Feedback Equalizer

This paper presents a technique to mitigate multiple access interference (MAI) in multicarrier code division multiple access (MC-CDMA) wireless communications systems. Although under normal circumstances the MC-CDMA system can achieve high spectral efficiency and resistance towards inter symbol interference (ISI) however when exposed to substantial nonlinear distortion the issue of MAI manifests. Such distortion results when the power amplifiers are driven into saturation or when the transmit signal experiences extreme adverse channel conditions. The proposed technique uses a modified iterative block decision feedback equalizer (IB-DFE) that uses a minimal mean square error (MMSE) receiver in the feed-forward path to nullify the residual interference from the IB-DFE receiver. The received signal is re-filtered in an iterative process to significantly improve the MC-CDMA system’s performance. The effectiveness of the proposed modified IB-DFE technique in MC-CDMA systems has been analysed under various harsh nonlinear conditions, and the results of this analysis presented here confirm the effectiveness of the proposed technique to outperform conventional methodologies in terms of the bit error rate (BER) and lesser computational complexity.

Simultaneous Excitation Systems for Ultrasonic Indoor Positioning

The ultrasonic technology is a tool for indoor positioning systems (IPSs) and has been extensively used in many applications. In ultrasonic IPSs (UIPSs), the use of a chirp signal (in which the frequency varies with time) is widespread due to its capability to obtain high-range resolution through its time-frequency characteristic. It also provides an opportunity to design effective waveform diversity which has always been the key to mitigating multiple-access interference (MAI) in multi-user UIPSs. To explore this, we analyze the chirp signal from the signal design perspective, with the goal of developing a precise and efficient UIPS for multi-user environments. To achieve this, three waveform diversity design schemes are proposed in which all the benefits of the classical chirp, such as high-range resolution, are retained while all the transmitters can transmit chirp signals simultaneously. In each scheme, a linear chirp is divided into two linear sub-chirps with diverse durations and/or bandwidths. This process is optimized by selecting the concatenated sub-chirps that generate a waveform which has a high-range resolution and relatively low interference in the same scheme. Initially, the effectiveness of the proposed schemes is evaluated for five simultaneous excitation signals using several metrics and experimental results are then presented for the ultrasonic indoor positioning.

Two-Stage ICI Suppression in the Downlink of Asynchronous URLLC

Applying the concept of virtual cell together with open-loop communications has been recently shown to achieve ultra-reliable and low-latency communication (uRLLC) in vehicular networks. Nevertheless, losing perfect synchronization due to proactive communications creates difficulty in mitigating multiple access interference (MAI). Multiple carrier frequency offsets (CFOs) resulting from different oscillators at different access points (APs) incur serious inter-carrier interference (ICI) to further complicate downlink MAI. Asynchronous multiuser detection (MUD) with ICI-Whitening was shown leading to satisfactory performance, but the whitening scheme needs the covariance matrix of ICI that is practically hard to obtain for downlink receivers. We develop a two-stage ICI suppression method to resolve this challenge. The first-stage processing is Pseudo-ICI-Whitening (P-ICI-W) or its simplified version Pseudo-Truncate ICI-W (PT-ICI-W), which does not rely on the estimation of ICI covariance and is suitable for asynchronous downlink. In terms of post-processing signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), our proposed mechanism can approach ICI-Whitening. The second-stage processing is based on the recently proposed generalized linear minimum mean square error (LMMSE) projection to further cancel some ICI terms. Moreover, our proposed mechanism is compatible with space-time-block-coded signals, namely Alamouti coding and Complex Interleaved Orthogonal Design (CIOD), to yield more reliable proactive wireless communications.

MAI Mitigation in MC-CDMA Systems Using Social Impact Based Wireless Communication Algorithm

In this paper a novel optimization technique i.e. Social Impact based Wireless Communication Algorithm (SIWCA) has been applied on multi-carrier code division multiple access (MC-CDMA) communication systems to mitigate multiple access interference (MAI). MC-CDMA is being researched as an alternate technology for fourth generation (4G) as well as fifth generation (5G) mobile systems. MAI has been a major concern for the CDMA based systems. MAI increases the bit error rate in a MC-CDMA system, which in turn degrades the system performance. The SIWCA is based on the social impact theory of human behavior in the society. The proposed approach combines the social sciences with the communication technology. The simulation results show that SIWCA based MC-CDMA detector is capable of significantly reducing the MAI and gives a near-optimum performance. Further SIWCA is compared with popular optimization techniques like genetic algorithm (GA) and binary particle swarm optimization (PSO) for different parameters. Simulation results show that SIWCA converges fast and works with lesser number of control parameters as compared to GA and PSO.

Computationally Efficient Chaotic Spreading Sequence Selection for Asynchronous DS-CDMA

Abstract The choice of the spreading sequence for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems plays a crucial role for the mitigation of multiple-access interference. Considering the rich dynamics of chaotic sequences, their use for spreading allows overcoming the limitations of the classical spreading sequences. However, to ensure low cross-correlation between the sequences, careful selection must be performed. This paper presents a novel exhaustive search algorithm, which allows finding sets of chaotic spreading sequences of required length with a particularly low mutual cross-correlation. The efficiency of the search is verified by simulations, which show a significant advantage compared to non-selected chaotic sequences. Moreover, the impact of sequence length on the efficiency of the selection is studied.

Multiple-Access Interference Mitigation for Acquisition of Code-Division Multiple-Access Continuous-Wave Signals

Multiple-access interference mitigation is important in code-division multiple-access (CDMA) systems for both information recovery and signal acquisition. Therefore, in this letter, we propose an open-loop adaptive filter based on the reiterative minimum mean-square error for the acquisition of CDMA signals. The method is based on the existing multistatic adaptive pulse compression method, appropriately adapted to the characteristics of CDMA continuous-wave signals. The efficacy of the proposed method in such systems is verified by simulation, and its superiority when compared with the existing least-mean-square method in terms of the number of reiterations is established.

Re-Transmission Diversity Multiple Access Based on SIC and HARQ-IR

We consider multiple access with re-transmission diversity (RTxD) based on a hybrid automatic request (HARQ) protocol with incremental redundancy (IR) in this paper. In order to mitigate multiple access interference (MAI), we employ successive interference cancellation (SIC) at a receiver and derive conditions that all the signals from active users can be successfully decoded within a certain number of re-transmissions of IR blocks. We consider two special cases, and based on them we derive two multichannel random access schemes that have subchannels in the power and rate domains. Through the analysis and simulations, we can show that the proposed RTxD random access schemes can outperform other existing similar RTxD random access schemes.

Robust Neural Network based Multiuser Detector in MC-CDMA for Multiple Access Interference Mitigation

The aim of this paper is to design a multiuser detector technique using artificial Neural Network under multiple access interference mitigation. To improve the performance of Multi Carrier Code Division Multiple Access (MC-CDMA) under multiple access interference, we have used a multilayered perceptron model of Neural Network that has three layers namely, input layer, hidden layer and output layer. The Neural Network is further trained by Levenberg-Marquardt algorithm. This algorithm uses the error function as key factor based on which the weights are adjusted to get the desired output. The Bit Error Rate (BER) performance of the system has been evaluated under Rayleigh and Stanford University Interim (SUI) channels for Binary Phase Shift Keying (BPSK) and Quadrature Amplitude Modulation (QAM) techniques. The proposed Neural Network based receiver is compared with Equal Gain Combining (EGC) and Maximal Ratio Combining (MRC) with varying number of users and Signal to Noise Ratio (SNR). Under SUI channel conditions and for a BER of 10 -3 , the Neural Network based receiver shows an improvement of 1 dB and 8 dB than EGC and MRC receivers, respectively.

Fast cross-correlation mitigation via minimum mean-square error estimation based on matched filter outputs for consecutive DSSS signals

In the direct sequence spread spectrum (DSSS) system, multiple-access interference mitigation is critical to improving the acquisition performance. Therefore, in this paper, a fast cross-correlation mitigation algorithm based on the minimum mean-square error criterion is proposed. The algorithm exploits the matched filter outputs; hence, it is applicable to existing DSSS receivers based on direct pseudorandom noise code correlation. Essentially, the proposed scheme is an adaptive filter that optimizes each individual uncertain code phase delay. The algorithm employs both the code sequence information of the desired signal and that of the multiple-access interference signal, which enables it to outperform the standard matched filter, conventional least-mean-squares algorithm, and the pulse compression repair method. Furthermore, the proposed algorithm decreases the computational load compared with the existing multistatic adaptive pulse compression method by reducing the stage-number of the adaptive filter; this results in a slight degradation in performance. Numerical results verify the validity of the proposed algorithm.

MAI Mitigation Using V-BLAST and PIC Based Binary Orthogonal Code Spreaded Multi Antenna MC-CDMA System

In this paper, the reduction of Multiple Access Interference (MAI) in Multi Carrier-Code Division Multiple Access (MC-CDMA) uplink system is analyzed. Simultaneous access over channels by numerous users results in MAI which leads to high Bit Error Rate (BER). Therefore the data received at the receiver is intended to numerous users. As a result, it is necessary to separate the data corresponding to different users efficiently. So in this paper, the Vertical-BELL Laboratories Layered Space-Time (V-BLAST) algorithm with Parallel Interference Cancellation (PIC) technique based MC-CDMA system is proposed. Due to the combination of V-BLAST and PIC in MC-CDMA system the MAI is reduced to a certain extent, which is shown in simulation result. Further the system performance is analyzed using Binary Orthogonal Codes (BOC) as spreading sequence. The result shows that Binary Orthogonal Coded V-BLAST and PIC based System provide better performance than conventional spreading sequence like Walsh Hadamard and Pseudorandom Noise (PN) sequence.

Applying probability control to cognitive CDMA communication system for anti‐interference

The technique of cognitive code division multiple access (CCDMA) communication is attracting more and more attention. The existing researches use power control to suppress the interference, but it is not always optimal. Probability control is a recently introduced method that allows better mitigation of multiple access interference in CCDMA networks. In this reported work, probability control is applied to CCDMA for anti-interference. It can be concluded that the performance reflected by the signal-to-noise-plus-interference ratio resulting from probability control is better than the one resulting from power control by numerical simulation results.

Mitigation of multiple access interference using two-dimensional modified double weight codes for optical code division multiple access systems

We proposed newly two-dimensional (2- D ) spectral amplitude coding optical code division multiple access (OCDMA) scheme using modified double weight (MDW) code capable of suppressing phase-induced intensity noise (PIIN). The architecture of the spectral/spatial MDW OCDMA system with the property of multi-access interference cancellation is presented. The proposed code exhibits good cross-correlation property. At the optimized data transmission rate of 0.745 Gbps, 2-D MDW, M = 63, N = 3, reaches maximum cardinality of 200% increases compared to 2- D perfect difference code, M = 57, N = 3. The performance is severely deteriorated if the data rate further increases above 0.745 Gbps. The proposed code meets the optical transmission requirements at 10−9 bit error rate error floor, with lowest effective transmitted power ( P sr), −17.5 dBm, in comparison to the others through minimizing interference noise that result in PIIN suppression. The proposed system reaches optimum requirements performance in terms of cardinality, data transmission rate, and low effective transmitted power.

Novel Scheme for Multiple-Access Interference Mitigation with Low Complexity

To mitigate multiple access interference and improve the system capacity, an effective multiple access scheme, called GCDMA, is proposed. Exploiting the IDMA iterative multiuser detector in each group, the intra-group interference can be effectively handled. Analysis and simulations demonstrate that the proposed technique outperforms the conventional CDMA in terms of both system capacity and bit error performance. Lower complexity of the proposed scheme can be achieved than CDMA and even IDMA.

A Fair Comparison Between Hybrid and Conventional Beamforming Receivers with Moderate Values of System Parameters

The main role of smart antennasis to mitigate Multiple Access Interference (MAI) by beamforming (Spatial filtering) operation. In addition to MAI, the performance of receivers is limited by fast fading. In this context, we propose in this paper a hybrid scheme of beamforming and diversity called HBF (Hierarchical Beamforming) and we propose a system model for the mathematical characterization of HBF for the performance evaluation.Moreover, we compare the performance of HBF receiver with conventional Beamforming (CBF) one. The proposed model conforms the benefits of adaptive antennas in reducing the overall interference level (intercell/intracell) and to find an accurate approximation of the error probability.

An EM approach to multiple-access interference mitigation in asynchronous slow FHSS systems

In this paper, we apply the EM algorithm for mitigation of multi-access interference (MAI) in asynchronous slow frequency-hop spread spectrum (FHSS) systems that employ binary frequency-shift keying (BFSK) modulation. MAI occurs if the hopping patterns of the users are not orthogonal. We show that when FSK signals arrive asynchronously, the time offset exposes portions of the desired and interfering signals in a way that can be exploited to improve the decoder performance. We develop an iterative detection, estimation, and decoding scheme to recover the desired signal in the presence of MAI. We compare the performance of this algorithm with that of a conventional noncoherent BFSK transceiver and show that the EM-based algorithm is particularly effective in the presence of strong interfering signals and allows more users in a FHSS system.

Adaptive Hybrid Genetic Algorithm Parallel Interference Cancellation High Rate Multi-User Detection for Dual Rate W-CDMA Mobile Communications

Adaptive hybrid genetic algorithm concatenated with improved parallel interference cancellation, i.e. adaptive hybrid genetic algorithm parallel interference cancellation (AHGAPIC) was proposed. A study is conducted on the application of AHGAPIC to soft decoding high rate multi-user detection with diversity reception for dual-rate wideband DS-CDMA spread spectrum communications, aiming to mitigate the effect of multiple access interference. The relevant research has revealed that the local search capability of hybrid genetic algorithm (HGA) is still not good enough. Therefore, first, two evolutionary operations, i.e. inversion and insertion are merged into HGA to constitute a novel algorithm. With its moderate local search capability, this new algorithm can search for the global optimum region according to the information entropy, and then it is made adaptively vary its probabilities of crossover and mutation depending on the fitness values of the solutions to form the adaptive hybrid genetic algorithm (AHGA). Second, AHGA is utilized to effectively identify the better and better binary string to maximize the log-likelihood function of dual-rate multi-user detection. As AHGA converges to the optimum region, the control factor of the improved parallel interference cancellation (IPIC) detector is set to be the ratio of the average fitness value to the maximum fitness value of the population of AHGA. Finally, equipped with both the control factor and the binary string with the maximum fitness value as the initial data, the IPIC detector can rapidly find out the approximately optimum soft decoding vector. Then, it can obtain the approximately global optimum estimate point on the basis of the soft decoding rule, corresponding to the transmitted data bits. A lower bound of computational complexity has been achieved through simulations and qualitative analyses. The property of the proposed algorithm to converge rapidly leads to lower computational complexity. Emulation results have shown that the AHGAPIC soft decoding high rate multi-user detector is superior to other suboptimum detectors considered in this paper in terms of two points. They are the mitigation of multiple access interference and the resistance to near-far effects. Its performance is close to the sequential group optimum multi-user detector but with a shorter time delay.

Separate linear receiver interfaces for MIMO multiple-access channels

We address the design of a receiver for the uplink of a direct sequence code division multiple access wireless communication system, endowed with multiple transmit and receive antennas and affected by multiple-access interference (MAI) for the presence of multiple transmitting users. The receiver is assumed to keep separate the tasks of mitigating the spatial and MAI in order to be implementable in two options: a baseline option accommodating the MAI mitigation interface only and an enhanced option accommodating the spatial interference mitigation interface as well. The MAI mitigating interface is based on standard linear multiuser detectors. The spatial interference mitigating interface is derived according to a linear minimum mean-square error criterion, which assumes the MAI mitigating interface to be given.

Reduced-rank interference suppression for DS-CDMA based on interpolated FIR filters

Reduced-rank receivers based on interpolated finite impulse response (FIR) filters for direct sequence code division multiple access (DS-CDMA) systems are proposed and a novel scheme where the interpolator is rendered time-varying is introduced. The interpolated minimum mean squared error (MMSE) and constrained minimum variance (CMV) solutions are derived for both receiver and interpolator to mitigate multiple access interference (MAI) and intersymbol interference (ISI) in a downlink scenario.