Multiple Access Interference Research Articles

Performance analysis of NOMA uplink transmission over generalized‐K fading channels

SummaryNon‐orthogonal multiple access (NOMA) can serve multiple users in the same time slot and frequency band and thus eliminate the need for guard bands and guard intervals to provide significant capacity improvements. Therefore, the research on its performance has been increasing since it is a desirable candidate for next‐generation wireless networks. Compared with orthogonal multiple access (OMA) techniques, the reliability of NOMA is lower since multiple access interference increases as the number of users is increased. Most of the previous studies on NOMA systems only consider the impact of fading in the transmission medium; however, more realistic channel models include both fading and shadowing. In this paper, outage probability (OP), average channel capacity, and symbol error rate (SER) for uplink NOMA over generalized‐K fading are derived analytically for the arbitrary number of users and then verified by Monte Carlo simulations.

Equal-gain combining with interference mitigation for molecular type hopping assisted molecular shift keying systems

In multiple access molecular diffusive communications, many nano-machines exchange information and fuse data through a common Diffusive Molecular Communication (DMC) channel. Hence, there is Multiple-Access Interference (MAI), which should be sufficiently mitigated so as to achieve reliable communications. In this paper, we propose a novel low-complexity detection scheme, namely Equal-Gain Combining with Interference Mitigation (EGC-IM), for signal detection in the Molecular Type Hopping assisted Molecular Shift Keying (MTH-MoSK) DMC systems. By removing a number of entries from each row of the detection matrix formed during detection, the EGC-IM scheme shows its potential to significantly mitigate MAI and hence, outperform the conventional EGC scheme. Furthermore, the EGC-IM scheme has lower complexity than the conventional EGC scheme and therefore, it is beneficial for practical implementation.

HAP-aided two-way relaying for FSO access networks using optical CDMA

A free-space optical access network with multiple ground stations (GSs) connected by a high-altitude platform (HAP) is proposed. The HAP plays a role of a two-way relay node that supports all-optical amplify-and-forward and half-duplex transmission. In the network, optical code-division multiple-access is utilized to enable asynchronous and simultaneous data transmission among GSs. In further steps of the study, the mathematical expressions of two system metrics including bit-error rate (BER) and network throughput are derived. They consider many physical layer impairments, such as power loss, atmospheric turbulence, pointing error, multiple-access interference, and receiver noise. Eventually, numerical results of the system performance under consideration of the number of GSs, the bit rate, and the received power confirm the feasibility of the proposed system as well as provide useful information for network design.

Nonlinear Effect and MAI Impact on SAC-OCDMA System Based on 2D Multi-Diagonal Code and Laser Array

A new architecture for Spectral Amplitude Coding Optical Code Division Multiple Access (SAC-OCDMA) system based on two Dimensional Multi Diagonal (2D-MD) codes named 2D-MD SAC-OCDMA and utilizing a laser optical source is proposed for Long-Reach Passive Optical Network (LR-PON). In this work, a computer simulator tool is used, for the first time, as a SAC-OCDMA simulation set-up utilizing the unique combination of a coherent laser array and 2D-MD codes. In addition, the system performance is addressed numerically by taking into account Multiple Access Interference (MAI), optical coherent source noise, first, second and third order fiber dispersion, nonlinear effects and photo-detector noise. Simulation results indicate that for a single user (i.e., without considering MAI), the system can operate at a maximum bit rate of 55 Gb/s over 250 km of Single Mode Fiber (SMF), with a Bit Error Rate (BER) below 10−9 (Q-limit = 15.5 dB), when only first order fiber dispersion is considered. However, including the effects of second and third order fiber dispersion as frequency domain parameters, results in a reduction of the maximum bit rate to 40 Gb/s, while maintaining a Q-factor above the Q-limit under the same transmission distance. Furthermore, we demonstrate that the proposed architecture extends the SMF transmission reach up to 600 km and 480 km, when considering linear and nonlinear effects, respectively. Finally, we show that our proposed 2D-MD SAC-OCDMA system outperforms existing solutions presented in the literature for LR-PON configuration, in terms of both aggregate bit rate and transmission reach.

A performance investigation of SAC-OCDMA system based on a spectral efficient 2D cyclic shift code for next generation passive optical network

A two dimensional spectral/spatial cyclic shift (2D-CS) optical code division multiple access (OCDMA) systems is proposed for a potentially next generation passive optical network (NG-PON) implementation called (2D-CS NG-OCDMA-PON) system. The2D-CS proposed code is characterized by a high capacity and a zero cross correlation property leads to completely eliminating the multiple access interference (MAI) effect that is considered as the main OCDMA system drawback. Firstly, the 2D-CScode construction is investigated from 1D-CS code. Secondly, a system description is provided by exhibiting the transmitter and receiver architecture in the PON context. Analytical analysis reveals that our proposed 2D-CScode outperforms similar codes such as perfect difference (2D-PD) and dynamic cyclic shift (2D-DCS) codes in terms of spectral efficiency, simultaneous network subscribers and data bit rate. In addition, based on numerical analysis 2D-CS NG-OCDMA-PON system shows a good system performance by means of avery low BER and a high Q-factor values equal to $$10^{{ - 26}}$$ and 10.41 dB, respectively. Likewise, for four users and free-amplification the achievable reach ability distance of the NG-OCDMA-PON system is 63.21 km, 43.57 km and 33.2 km while Q-factor is equal to 6 dB at a bit rate of 622 Mb/s, 1 Gb/s and 1.5 Gb/s, respectively. On the other side, according to the system setup the number of single mode fiber (SMF) is reduced to the half compared to other 2D-OCDMA-PON systems based on enhanced multi diagonal (EMD) and single weight ZCC (SWZCC) codes.

Impact and Mitigation of Noise in Optical 2D Techniques Using (n, w, λa, λc) Optical Orthogonal Codes for Optical CDMA

The performance of two dimensional (2D) hybrid wavelength/time optical code division multiple access (OCDMA) system is affected severely by the presence of noise at its physical layer. This noise is present in the guise of multiple access interference (MAI) and beat noise in OCDMA systems and degrades its performance. In this paper an attempt has been made to analyze the impact of noise in OCDMA system using four different coding schemes. The mitigation of noise is then done by using either optical hard limiter (OHL) or by utilizing coherent detection for all these schemes. These codes use synchronized quadratic congruence sequences (SQC), synchronized prime procession (SPP), prime code sequences (PC) and synchronized prime sequences (SPS) for wavelength hopping and ( n, w, λ a , λ c ) one dimensional optical orthogonal codes for time spreading respectively. The code with heavier code weight or lower values of auto- and cross- correlation function performs the best with noise mitigation techniques. Investigations reveal that SQC/OOC coding scheme in combination with OHL and PC/OOC scheme in combination with coherent detection outperforms in comparison to other techniques.

Exploiting Capture Diversity for Performance Enhancement of ALOHA-Based Multi-Static Backscattering Systems in the 6G Perspective.

In this paper, we consider the emerging context of ALOHA-based multi-static backscattering communication systems. By assuming an architecture consisting of a set of passive backscattering nodes, an illuminator, and a set of spatially dislocated receivers, we firstly propose a cross-layer framework for performance analysis. The model jointly accounts for the shared wireless channel, including fading and capture effect, and channel contention strategy, which is regulated by a Framed Slotted ALOHA protocol. Furthermore, based on the inherent macroscopic diversity offered by the multi-static settings, we introduce the concept of capture diversity, which is shown to enable multiple packet detection in slots with multiple transmissions. In order to characterize the multiple access interference and approximate the capture probabilities, we enforce a log-normal approximation of the inverse Signal-to-Interference Ratio that relies on moment matching. Numerical results show the impact of deployment scenarios and the relative positions of illuminator, backscattering nodes, and receivers on the system normalized throughput. We show how the number of detection points impacts the system performance under various channel conditions. Moreover, the accuracy of the proposed approximation rationale is validated via Monte Carlo simulations. Finally, we analyze the optimal frame length in the presence of capture diversity.

Iterative channel estimation-based soft successive interference cancellation for multiuser underwater acoustic communications.

This paper presents a single-carrier multiuser receiver in underwater acoustic communications with strong multiple access interference (MAI) combining passive time-reversal (PTR) and direct-adaptation-based turbo equalization. The receiver works in an iterative block by block manner. In the first iteration, when the symbols of all users are detected by successive interference cancellation (SIC) in the block, the channel impulse response (CIR) of each user is then updated by post-SIC signal to achieve more accurate MAI reconstruction and PTR combination for the next block. In the following iterations, the a priori mean of symbol in the current block is available and used to further improve the accuracy of reconstructed MAI and the updated CIR. The proposed receiver is demonstrated using experimental data collected at Songhua Lake, China, in 2019. The results show that single-carrier nine-user communication data with quadrature phase-shift keying can be successfully recovered, with an average data rate of 1.67 kbps for each user.

Design of Bipolar Optical Code-Division Multiple-Access Techniques Using Phase Modulator for Polarization Coding in Wireless Optical Communication

In this study, a bipolar optical code-division multiple-access (Bi-OCDMA) technique based on spectral amplitude coding (SAC) was proposed by using a phase modulator to realize polarization coding through a free-space optical (FSO) channel. Various types of noise, such as amplified spontaneous emission (ASE) noise, thermal noise, and shot noise, were included in the simulation to approach the real application. The first simulation, utilizing a modified M-sequence as signature code, demonstrated that the proposed Bi-OCDMA system could be implemented in FSO communication. The proposed Bi-OCDMA scheme improves the transmission rate and power efficiency compared with the previous scheme. The structure of the proposed system alleviates multiple-access interference (MAI) with a simple and cost-effective design. The second simulation observed the performance of the proposed Bi-OCDMA for two users with several well-known SAC codes, i.e., multi-diagonal (MD) code, modified quadratic congruence (MQC) code, modified maximum length sequence (M-sequence) code, and Walsh–Hadamard code, in extreme weather conditions, both for additive white Gaussian noise (AWGN) and turbulence-induced fading channel. The simulation results indicated that the Walsh–Hadamard code has superior performance compared to other codes. The results show the MD code can be implemented in the proposed Bi-OCDMA scheme for a medium-distance FSO.

Multiple‐access interference suppression in massive multiple‐input multiple‐output‐non‐orthogonal multiple access based on space–time block coding

SummaryNon‐orthogonal multiple‐access (NOMA) scheme using massive multiple‐input multiple‐output (MIMO) transmission suffers from interference between users. In this paper, we propose a novel MIMO‐NOMA framework based on space–time block code (STBC) to decrease the multiple‐access interference (MAI), when the number of users is increasing. In this paper, besides giving an analysis of network, cluster, group, and users, we first present the system model. Then, an efficient precoder and detector are designed based on interference analysis. In particular, the proposed scheme can decompose a massive MIMO‐NOMA system into multiple separated single‐input single‐output NOMA channels. Also, analytical results are developed to evaluate the performance of the system. To address the dramatic increment of power level, with the aid of the proposed algorithm, the optimal solution is achieved. Mathematical analyses for both MIMO‐NOMA and STBC are presented to design efficient precoding and detection. Finally, simulation results, based on STBC, are provided to verify the theoretical analysis to demonstrate significant capacity improvements of the massive MIMO‐NOMA.

SPARCs for Unsourced Random Access

Unsourced random-access (U-RA) is a type of grant-free random access with a virtually unlimited number of users, of which only a certain number $K_a$ are active on the same time slot. Users employ exactly the same codebook, and the task of the receiver is to decode the list of transmitted messages. We present a concatenated coding construction for U-RA on the AWGN channel, in which a sparse regression code (SPARC) is used as an inner code to create an effective outer OR-channel. Then an outer code is used to resolve the multiple-access interference in the OR-MAC. We propose a modified version of the approximate message passing (AMP) algorithm as an inner decoder and give a precise asymptotic analysis of the error probabilities of the AMP decoder and of a hypothetical optimal inner MAP decoder. This analysis shows that the concatenated construction can achieve a vanishing per-user error probability in the limit of large blocklength and a large number of active users at sum-rates up to the symmetric Shannon capacity, i.e. as long as $K_aR < 0.5\log_2(1+K_a\SNR)$. This extends previous point-to-point optimality results about SPARCs to the unsourced multiuser scenario. Furthermore, we give an optimization algorithm to find the power allocation for the inner SPARC code that minimizes the required $\SNR$.

Nonlinear Quasi-Synchronous Multi User Chirp Spread Spectrum Signaling

Multi user orthogonal chirp spread spectrum (OCSS) can improve the spectral inefficiency of chirp spread spectrum (CSS) but is only feasible with perfect synchronism and without any channel dispersion. Asynchronism, channel dispersion, or unexpectedly large Doppler shifts can cause multiple access interference (MAI), which degrades performance. Conditions with small timing offsets we term quasi-synchronous (QS). In this paper, we propose two new sets of nonlinear chirps to improve CSS system performance in QS conditions. We analytically and numerically evaluate cross-correlation distributions. We also derive the bit error probability for Binary CSS analytically and validate our theoretical result with both numerical and simulation results; our error probability expression is applicable to any binary time-frequency (TF) chirp waveform. Finally, we show that in QS conditions our two new nonlinear chirp designs outperform the classical linear chirp and all existing nonlinear chirps from the literature. To complete our analysis, we demonstrate that our nonlinear CSS designs outperform existing chirps in two realistic (empirically modeled) dispersive air to ground channels.

Performance analysis of polar codes for one-coincidence-coded OCDM networks

A polar code over One-Coincidence-coded optical code-division multiplexing (OCDM) network is proposed, which is suitable for the multi-access optical fiber network. The excellent error correction ability of Polar code was applied to build a low error rate performance of network system. The proposed architecture not only supports lots of users by OCDM technology with spectral amplitude coding scheme to eliminate multiple-access interference, but also support that burst traffic at the same time of data transmitting by few users. The commutation channels will divide into two opposing channel quality, information channel and frozen channel by channel polarization of polar code. The buffer channel is created to support burst user, and it is build-in in information channel. Simulation results show that the proposed architecture can improve the system performance, compared with uncoded network. The proposed algorithm has better bit error rate performance.

Human opinion dynamics optimization for ICI mitigation in MC-CDMA systems

This paper presents a novel social impact based approach named as Human Opinion Dynamics Optimization (HODO) to detect signals of Multi-Carrier Code Division Multiple Access (MC-CDMA) systems in the presence of Inter Subcarrier Interference (ICI). The ICI occurs because of various factors, such as Carrier Frequency Offset (CFO), Sampling Frequency Offset (SFO) and channel mobility. The ICI degrades orthogonality between different subcarriers in MC-CDMA system due to which Multiple Access Interference (MAI) occurs. The HODO is based on the socio-psychological behavior of humans in the society. Opinion dynamics leads to human consensus and consensus builds up due to human interactions in a society. The proposed approach could be used to introduce human opinion based intelligent communication networks. Our method is novel in the sense that with this algorithm, social sciences could be made compatible with wireless communication technology, which will go a long way to bridge the gap between humans and the communication technology. This new method works with very few number of control parameters and reduces the bit error rate (BER) better as compared with other conventional stochastic methods like Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). Numerical simulation results show that the proposed detector also reduces the computational complexity manifold.

A Systematic review of Multi-Mode Fiber based on Dimensional Code in Optical-CDMA

This paper deals with optical code division multiple access (optical-CDMA) and provides a general analysis of multi-mode fiber (MMF). The purpose of this work is to classify the works in the literature that are related to Optical-CDMA based on MMF and to find the limitation of the researches done in this field. Several challenges that occur in the medium such as multiple-access interference (MAI), pulse dispersion (PD) and nonlinearity in optical fibers deteriorate system performances and become a major performance-limiting factor.

Resource Allocation for Open-Loop Ultra-Reliable and Low-Latency Uplink Communications in Vehicular Networks

To support ultra-reliable and low-latency communication (URLLC) in vehicular networks, the virtual cells, where multiple access points (APs) cooperatively serve one mobile node, have been proposed to reduce the end-to-end latency in the downlink. The latency can be further reduced by eliminating the need for retransmission and feedback control, i.e., open-loop communications. However, it is difficult to achieve a high reliability of the uplink via virtual cells, because of multiple access interference and collisions from other virtual cells nearby. In this paper, we formulate the proactive radio resource allocation in the open-loop uplink of vehicular networks as a stochastic optimization problem with the objective to maximize the uplink reliability while ensuring the network stability. The optimal resource allocation policies are obtained solving the optimization problem using the Lyapunov optimization technique in a distributed manner. To reduce the computational and to improve the challenges of the Lyapunov optimization, we propose a virtual resource slicing algorithm that maps radio resource units to virtual resource blocks. Simulation results exhibit that both ultra-low latency and ultra high reliability are guaranteed in the open-loop uplink of vehicular networks. Based on the theoretical performance analysis and simulations, the uplink radio access procedure is summarized for the URLLC in vehicular networks.

On Trading the Spreading Gain With the Coding Rate and Its Application to GNSS Data Component Design

The ubiquity of global navigation satellite system (GNSS)-based positioning and timing services is often frustrated by the necessity to operate in harsh environments, where the carrier-to-noise ratio is low, and hence, decoding of navigation data and even tracking of an acquired symbol are difficult. We consider the possibility of improving the decoding performance of the GNSS data component by trading the spreading gain against the coding rate. The rationale is that spreading codes can be seen as a form of repetition coding that can be (at least partially) replaced by more robust coding forms to improve robustness to (any form of) noise. This is true both for the classical additive white Gaussian noise channel case and for more realistic GNSS channel formats severely degraded by multiple access interference and near-far effects. By bringing results on finite-block-length channel capacity and coding rates from information/communication theory to the GNSS domain, we are able to establish and discuss the expected performance gap, as well as the limits of such tradeoff.

Delay Guaranteed Joint User Association and Channel Allocation for Fog Radio Access Networks

In the Fog Radio Access Networks (F-RANs), the local storage and computing capability of Fog Access Points (FAPs) provide new communication resources to address the latency and computing constraints for delay-sensitive applications. To achieve the ultra-low latency, a novel joint user association and channel allocation scheme is proposed in this paper, where the FAPs are clustered from a user-centric perspective. The delay performance is improved regarding both the control signaling procedure and the data transmission procedure. Specifically, the multiple access interference (MAI) between users is analyzed, where the closed-form expression for the effective rate of a typical user with multiple FAP connections and arbitrary interfering users is obtained. With the consideration of MAI, the proposed distributed joint user association and channel allocation algorithm provides a guaranteed delay violation probability. Moreover, the distributed algorithm can be conducted on individual FAPs, whose calculation is simplified by look-up tables. Simulation results show that the proposed algorithm is capable of providing statistical delay performance guarantee including both average delay and delay bound violation probability, which demonstrates its superiority in supporting delay-sensitive applications in F-RANs.

Precoding Matrix Selection for LTE System

In the current 4G Long Term Evolution (LTE) system, it adopts the orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO) schemes to improve the spectral efficiency and increase the data rate for the system. Besides, precoding is an effective technique in MIMO transmission for the transmitter (Tx) to use a suitable precoding matrix in accordance with the change of the channel. With the proper selected precoding matrix, MIMO system can reduce the multiple access interference and increase the received signal to noise ratio (SNR) at the receiver (Rx). In this paper, two precoding matrix selection methods are proposed via using the features of the beam patterns of the adopted precoding matrices for 3GPP LTE system. With a little performance degradation, the first proposed method has the advantage of low computational complexity and the second method can have the zero latency property compared with the conventional full search methods.

Resource Allocation Based on User Pairing and Subcarrier Matching for Downlink Non-Orthogonal Multiple Access Networks

The traditional orthogonal multiple access (OMA) is unable to satisfy the needs of large number of smart devices. To increase the transmission rate in the limited spectrum resource, implementation of both non-orthogonal multiple access (NOMA) and successive interference cancelation (SIC) is essential. In this paper, an optimal resource allocation algorithm in NOMA is proposed to maximize the total system rate in a multi-sector multi-subcarrier relay-assisted communication network. Since the original problem is a non-convex problem with mixed integer programming which is non-deterministic polynomial-time (NP)-hard, a three-step solution is proposed to solve the primal problem. Firstly, we determine the optimal power allocation of the outer users by using the approach of monotonic discrimination, and then the optimal user pairing is determined. Secondly, the successive convex approximation (SCA) method is introduced to transform the non-convex problem involving central users into convex one, and the Lagrangian dual method is used to determine the optimal solution. Finally, the standard Hungarian algorithm is utilized to determine the optimal subcarrier matching. The simulation results show that resource allocation algorithm is able to meet the user performance requirements with NOMA, and the total system rate is improved compared to the existing algorithms.