Multiuser Interference Research Articles

A Comprehensive Investigation on Multi-User Interference Effects in Vehicular Visible Light Communications.

Vehicular visible light communications (VLC) are considered a suitable technology for vehicular platooning applications. Nevertheless, this domain imposes strict performance requirements. Although numerous works have shown that VLC technology is compatible with platooning applications, existing studies are mainly focused on the physical layer performances, mostly ignoring the disruptive effects generated by neighboring vehicular VLC links. Nevertheless, the 5.9 GHz Dedicated Short Range Communications (DSRC) experience has shown that mutual interference can significantly affect the packed delivery ratio, pointing out that these effects should be analyzed for vehicular VLC networks as well. In this context, this article provides a comprehensive investigation focused on the effects of mutual interference generated by neighboring vehicle-to-vehicle (V2V) VLC links. Therefore, this work provides an intensive analytical investigation based on simulation and also on experimental results that demonstrate that although ignored, the influence of mutual interference is highly disruptive in vehicular VLC applications. Hence, it has been shown that without preventive measures, the Packet Delivery Ratio (PDR) can decrease below the imposed 90% limit for almost the entire service area. The results have also shown that although less aggressive, multi-user interference affects V2V links even in short-distance conditions. Therefore, this article has the merit of emphasizing a new challenge for vehicular VLC links and points out the importance of multiple-access techniques integration.

Multiple Beam Selection and Near-Optimal Digital Precoding for Multiuser Millimeter-Wave Massive MIMO Systems

Beamspace massive multiple-input multiple-output (MIMO) can reduce the number of required radio-frequency (RF) chains without obvious performance loss. However, the joint power leakage and multiuser interferences (MI) problem in beamspace channels degrades the system performance seriously, which is not considered in most recent works. To solve this problem, we propose a hybrid analog/digital precoding scheme with multiple beam selection. First, all the energy focusing beams are selected unshared to collect the channel power and alleviate the MI. Then, the analog precoding is determined by maximizing the array gains for each user. The digital precoding is designed by maximizing the system sum rate under the power constraint. However, the logarithms and fractions in the objective function make the optimization problem nonconvex. Hence, we use the Lagrangian transform to eliminate the logarithms, as well as fractional programming to remove the fractions. Finally, the digital precoding matrix is obtained via convex optimization. Simulation results verify that the proposed scheme can obtain the considerable sum rate performance, while achieving a higher energy efficiency than those of existing schemes.

A data-driven deep learning network for massive MIMO detection with high-order QAM

Massive multiple-input multiple-output (MIMO) can provide higher spectral efficiency and energy efficiency compared to conventional MIMO systems. Unfortunately, as the numbers of modulation orders and antennas increase, the computational complexity of conventional symbol detection algorithms becomes unaffordable and their performance deteriorates. However, deep learning (DL) techniques can provide flexibility, nonlinearity and computational parallelism for massive MIMO detection to address these challenges. We propose an efficient data-driven detection network, i.e., accelerated multiuser interference cancellation network (AMIC-Net), for uplink massive MIMO systems. Specifically, we first introduce an extrapolation factor regarded as a learnable parameter into the multiuser interference cancellation (MIC) algorithm for iterative sequential detection (ISD) detector through extrapolation technique to enhance the convergence performance. Then we unfold the above accelerated iterative algorithm and adopt a sparsely connected approach, instead of fully connected, to obtain a relatively simple deep neural network (DNN) structure to enhance the detection performance through the data-driven DL approach. Furthermore, in order to accommodate communication scenarios with higher-order modulation, a novel activation function is proposed, which is composed of multiple softsign activation functions with additional learnable parameters to implement a multi-segment mapping of the set of constellation points with different modulations. Numerical results show that the proposed DL network can bring significant performance gain to ISD detector with various massive antenna settings and outperform the existing detectors with the same or lower computational complexity, especially in high-order QAM modulation scenarios.

Joint SIC-Based Precoding and Sub-Connected Architecture Design for MIMO VLC Systems

Densely deployed light emitting diodes (LEDs) typically lead to high spatial correlation in multiple-input multiple-output (MIMO) visible light communication (VLC) systems. Although precoding can effectively alleviate the spatial correlation issue, most existing precoding algorithms require a dedicated baseband chain for each LED, leading to high energy consumption and hardware complexity when a large number of LEDs are used. In this paper, a successive interference cancellation (SIC)-based precoding scheme with sub-connected architecture (SIC-SA) is proposed. In the considered model, each baseband chain is connected to an LED sub-array containing multiple LEDs to reduce the complexity. Since, in this case, SIC-based precoding can only determine the signal of each baseband chain for an LED sub-array, while its target is to mitigate the spatial correlation between individual LEDs, the electrical/optical power of each LED must also be jointly optimized to accomplish the target. This joint SIC-based precoding, power allocation, and direct current offset design problem is formulated as an achievable sum rate maximization problem under dimming control and electrical power constraints. A two-step iterative algorithm is proposed to solve this problem. In the first step, the SIC-based precoding is designed to alleviate the multi-user interference. In the second step, the power allocation of LEDs is optimized by matrix decomposition and convex optimization, and a closed-form solution of DC offset is derived. Furthermore, considering the dynamic scenarios, a SIC-based precoding scheme with dynamic sub-connected architecture (SIC-DSA) is proposed, in which a switch network is used to adaptively adjust the LED sub-array structure based on the channel state information. Simulation results show that the proposed SIC-SA and SIC-DSA respectively achieve 0.1340 bps/Hz/W and 0.1305 bps/Hz/W energy efficiency gains over the zero-forcing precoding scheme with SA, when the signal-to-noise ratio is 30 dB.

Two code keying using optical CDMA system with 2D multi-method codes

We propose a novel family of two-dimensional multi-methods with two codes keying (2D MM with TCK) in spectral amplitude coding optical code division multiple access (OCDMA) networks. The idea transfers one code keying (OCK) of one-dimensional (1D) MM codes into two code keying (TCK) of 2D MM codes. The rationale has OCK transferred into TCK, 2D matrix formulations using the 2D MM codes, and modified cross-correlation. The research solution resolves the first item using two OCKs of 2D MM codes to become TCK of 2D MM codes to call 2D MM codes with TCK, the second item using two sets of 1D MM codes to produce 2D matrix formulations using 2D MM codes, and the third item using the cross-correlation to transfer into a modified cross-correlation. The modified cross-correlation eliminates multiuser interference. The proposed analysis utilizes a modified photocurrent to suppress phase-induced intensity noise. In the numerical results, the number of simultaneous users, data transmission rate, and effective source power using 2D MM codes with TCK are 294, 4.72 Gbps, and − 12 dBm, respectively.

LADMM-Based PAPR-Aware Precoding for Massive MIMO-OFDM Downlink Systems

We tackle the peak to average power ratio (PAPR) reduction of an orthogonal frequency division multiplexing (OFDM)-based massive multi-user (MU) multiple-input multiple-output (MIMO) downlink systems in this paper. Utilizing the redundant degree of freedom (DOF) provided by a large number of antennas equipped at the base station, we first formulate OFDM modulation, MU precoding, and PAPR reduction into a non-convex optimization problem. By using the linearized alternating direction method of multipliers (LADMM), we directly address the non-convex PAPR-aware precoding problem without any relaxation in terms of PAPR and multi-user interference (MUI), which is completely different from the existing convex optimization methods based on relaxations. At the same time, the paper is also the first work to directly solve the non-convex PAPR-aware precoding optimization. The experimental results demonstrate that the LADMM method gets the comparable PAPR reduction and symbol error rate (SER), faster convergence, and lower complexity than the existing relaxed methods.

On the Coexistence of eMBB and URLLC in Multi-Cell Massive MIMO

The non-orthogonal coexistence between the enhanced mobile broadband (eMBB) and the ultra-reliable low-latency communication (URLLC) in the downlink of a multi-cell massive MIMO system is rigorously analyzed in this work. We provide a unified information-theoretic framework blending an infinite-blocklength analysis of the eMBB spectral efficiency (SE) in the ergodic regime with a finite-blocklength analysis of the URLLC error probability relying on the use of mismatched decoding, and of the so-called saddlepoint approximation. Puncturing (PUNC) and superposition coding (SPC) are considered as alternative downlink coexistence strategies to deal with the inter-service interference, under the assumption of only statistical channel state information (CSI) knowledge at the users. eMBB and URLLC performances are then evaluated over different precoding techniques and power control schemes, by accounting for imperfect CSI knowledge at the base stations, pilot-based estimation overhead, pilot contamination, spatially correlated channels, the structure of the radio frame, and the characteristics of the URLLC activation pattern. Simulation results reveal that SPC is, in many operating regimes, superior to PUNC in providing higher SE for the eMBB yet achieving the target reliability for the URLLC with high probability. Moreover, PUNC might cause eMBB service outage in presence of high URLLC traffic loads. However, PUNC turns to be necessary to preserve the URLLC performance in scenarios where the multi-user interference cannot be satisfactorily alleviated.

Mixed Coded RF/FSO EH-Based Communication System Subject to Multiuser Interference and Residual Hardware Impairments

Mixed Coded RF/FSO EH-Based Communication System Subject to Multiuser Interference and Residual Hardware Impairments

Rethinking WMMSE: Can Its Complexity Scale Linearly With the Number of BS Antennas?

Precoding design for maximizing weighted sum-rate (WSR) is a fundamental problem for downlink of massive multi-user multiple-input multiple-output (MU-MIMO) systems. It is well-known that this problem is generally NP-hard due to the presence of multi-user interference. The weighted minimum mean-square error (WMMSE) algorithm is a popular approach for WSR maximization. However, its computational complexity is cubic in the number of base station (BS) antennas, which is unaffordable when the BS is equipped with a large antenna array. In this paper, we consider the WSR maximization problem with either a sum-power constraint (SPC) or per-antenna power constraints (PAPCs). For the former, we prove that any nontrivial stationary point must have a low-dimensional subspace structure, and then propose a reduced-WMMSE (R-WMMSE) with linear complexity by exploiting the solution structure. For the latter, we propose a linear-complexity WMMSE approach, named PAPC-WMMSE, by using a novel recursive design of the algorithm. Both R-WMMSE and PAPC-WMMSE have simple closed-form updates and guaranteed convergence to stationary points. Simulation results verify the efficacy of the proposed designs, especially the much lower complexity as compared to the state-of-the-art approaches for massive MU-MIMO systems.

Joint Waveform and Beamforming Design for RIS-Aided ISAC Systems

In this paper, we consider the problem of joint waveform and passive beamforming design for MIMO integrated sensing and communication (ISAC) systems, where a reconfigurable intelligent surface (RIS) is deployed to assist the downlink communication. The objective is to maximize the signal-to-interference-and-noise-ratio (SINR) for radar and meanwhile minimizing the multi-user interference for communication. To address this problem, a block coordinate descent (BCD) method is proposed. Specifically, given the fixed waveform, an Element-wise Closed-Form (ECF) technique is derived for the reflection coefficients optimization, where the high-dimensional problem is divided into multiple one-dimensional problems with closed-form solution. Then, given fixed reflection coefficients, a dinkelbatchs-ECF algorithm is developed to optimize the waveform. Simulation results show that the proposed method achieves joint sensing and communication improvement and presents a clear performance advantage over existing methods.

Interference Alignment for Physical Layer Security in Multi-User Networks With Passive Eavesdroppers

We investigate the physical layer security (PLS) in multi-user interference networks. In particular, we consider secure transmission from a legitimate source (Alice) to a legitimate destination (Bob), coexisting with multiple passive eavesdroppers (Eves), as well as multiple legitimate transceivers. By assuming that only statistical channel state informations (CSIs) of Eves and local CSIs of legitimate users are available, we propose an artificial noise (AN) assisted interference alignment (IA) for security enhancement. Unlike traditional IA based security approaches which may result in secret signal cancellation, we design a modified alternating minimization (AM) scheme to overcome this threat, by incorporating the max-eigenmode beamforming (MEB) for secure transmission. Moreover, by partitioning the IA equation into three independent subsets and their combinations, a much tighter necessary condition for IA feasibility is established. We also provide guiding insights into the practical system designs, including useful guidelines for the selection of the dimension of AN. Furthermore, the power allocation ratio between the secret signal and the AN signal is optimized to minimize the secrecy outage probability (SOP), subject to a minimum secrecy rate constraint. Numerical results demonstrate that our design can enhance both quality and security of the secret signal, and thus is suitable and reliable for PLS in interference networks.

Sensing Using Coded Communications Signals

A key challenge for common waveforms for Integrated Sensing and Communications – widely regarded as a resource-efficient way to achieve high performance for both functionalities – lies in leveraging information-bearing channel-coded communications signal(s) (c.c.s) for sensing. In this paper, we investigate the range-Doppler sensing performance of c.c.s in multi-user interference-limited scenarios, and show that it is affected by sidelobes whose form depends on whether the c.c.s modulates a single-carrier or OFDM waveform. While uncoded signals give rise to asymptotically zero sidelobes due to the law of large numbers, it is not obvious that the same holds for c.c.s, as structured codes (e.g., linear block codes) induce dependence across codeword symbols. In this paper, we show that c.c.s also give rise to asymptotically zero sidelobes – for both single-carrier and OFDM waveforms – by deriving upper bounds for the tail probabilities of the sidelobe magnitudes that decay as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\exp ( - O(\text {code rate} \times \text {block length}))$ </tex-math></inline-formula> . Consequently, for any code rate, c.c.s are effective sensing signals that are robust to multi-user interference at sufficiently large block lengths, with negligible difference in performance based on whether they modulate a single-carrier or OFDM waveform. We verify the latter implication through simulations, where we observe the sensing performance (i.e., the detection and false-alarm probabilities) of a QPSK-modulated c.c.s (code rate = 120/1024, block length = 1024 symbols) to match that of a comparable interference-free FMCW waveform even at high interference levels (SIR of −11dB), for both single-carrier and OFDM waveforms.

UNJUK KERJA KODE ORTHOGONAL M-SEQUENCE DALAM SISTEM DIRECT SEQUENCE SPREAD SPECTRUM MELALUI KANAL MULTIPATH FA

Multipath and multi-user interference are the main challenges in wireless communication systems. This has become a driving force for the development of data transmission technology that is focused to overcome these problems, one of which is spread spectrum. Spread spectrum is applied to CDMA technology which has the limitations of MAI, so a spreading code is required with specifications that have a very low cross-correlation value as in the orthogonal m-sequence code. In this study, simulate the orthogonal m-sequence code with a length of 8 chips on a multipath fading channel using the Direct Sequence Spread Spectrum system with aims of assessing the performance of the orthogonal m-sequence code on a multipath fading channel, based on the effect of changes in the number of multipath components. The research method used is a Monte Carlo simulation on simulink with the output data in the form of a graph of the comparison value of BER with Eb/No. The results show that on the flat fading channel with the number of components multipath 1, the BER value of the simulation results is in accordance with the BER theory so that the simulation is valid and can be applied to frequency selective fading channels. In the frequency selective fading channel with the number of multipath components 4,6, and 8, the best performance was shown in the multipath 4 component with the smallest BER value, this was influenced by the ACF and CCF values so that the multipath 4 component had the lowest ISI and interference between users.

Hybrid NOMA/OFDMA visible light communication system with coordinated multiple point transmission.

In this study, a hybrid non-orthogonal multiple access / orthogonal frequency division multiple access (NOMA/OFDMA) scheme is proposed for multi-cell multi-user visible light communication (VLC) systems. Each cell is divided into several sub-cells that are multiplexed using OFDMA. Users within the same sub-cell are multiplexed by NOMA. Thus, the hybrid NOMA/OFDMA scheme takes advantage of both NOMA and OFDMA, which not only improves the spectral efficiency of OFDMA but also avoids the co-channel interference of NOMA. Moreover, coordinated multiple-point transmission based on repeated coding is introduced to eliminate multiuser interference, which also improves the received signal-to-noise ratio of edge users. In this manner, spectrum resources are fully utilized, where the frequency reuse factor is equal to 1. Furthermore, we propose a two-dimensional power-allocation algorithm for the proposed hybrid NOMA/OFDMA VLC system. Based on the fixed power allocation strategy, power is allocated jointly among sub-cells and users within sub-cells to minimize the average symbol error rate (SER). The performance of the proposed system was investigated in detail by simulation, where the SERs were evaluated under different power ratios. Simulation results also show that the SER performance of the proposed hybrid NOMA/OFDMA VLC system is significantly improved compared to the traditional NOMA and OFDMA VLC systems in different VLC networks. Finally, the proof-of-concept experiment was set up, clearly validating the superiority of the proposed system further.

Precoded Large Scale Multi‐User‐MIMO System Using Likelihood Ascent Search for Signal Detection

AbstractMultiple antennas at each user equipment (UE) and/or thousands of antennas at the base station (BS) comprise the extremely spectrum efficient large scale multi‐user multiple input multiple output system (BS). Due to space constraints, the closely spaced numerous antennas at each UE may cause inter antenna interference (IAI). Furthermore, when one UE comes into contact with another UE in the same cellular network, multi‐user interference (MUI) may be introduced to the received signal. To mitigate IAI, efficient precoding pre‐coding is necessary at each UE, and the MUI present at the BS can be canceled by efficient Multi‐user Detection (MUD) techniques. The majority of earlier literature deal with one or more of these interferences. This paper implements a joint pre‐coding and MUD, Lenstra‐Lovasz (LLL) based Lattice Reduction (LR) assisted likelihood accent search (LAS) (LLL‐LR‐LAS), to mitigate IAI and MUI simultaneously LLL‐based LR pre‐coding mitigates IAI at each UE, and the LAS algorithm is a neighborhood search‐based MUD that cancels BS MUI. The proposed approaches' performance was evaluated using Bit Error Rate analysis, and their complexity were determined using multiplication and addition.

A Joint Equalization and Decoding Technique for Multiuser Massive MIMO Uplink System With Transmitter and Receiver RF Impairments Under Doubly Selective Channels

The impairments due to in-phase and quadrature-phase imbalance at the transmitter and/or receiver, carrier frequency offset, and doubly selective channel (DSC) result in multiuser interference (MUI) and self-intercarrier interference (S-ICI) in multiuser massive multiple-input–multiple-output (MIMO) uplink systems. The MUI and S-ICI significantly deteriorate the performance of the receiver and make data detection quite a challenging task. To tackle this issue, we developed a novel crisscross banded effective channel matrix and a joint equalization and detection technique for multiuser massive MIMO single-carrier frequency division multiple access/orthogonal frequency division multiple access uplink systems with RF impairments under DSC. During each iteration of the proposed method, the MUI and S-ICI are calculated using the soft information from the decoder section, and then the interference canceled signal is fed back to the equalizer section for the joint equalization of the mirror subcarrier pairs. Since the proposed method does not put any restriction on the allocation of mirror subcarrier pairs, it is equally applicable to all types of carrier assignment schemes. Further, the extrinsic information transfer chart of the proposed technique is developed for analyzing the convergence. Extensive simulation studies are conducted to validate the performance of the proposed technique.

Spectral Efficiency of OTFS Based Orthogonal Multiple Access With Rectangular Pulses

In this paper, we consider Orthogonal Time Frequency Space (OTFS) modulation based multiple-access (MA). We specifically consider Orthogonal MA methods (OMA) where the user terminals (UTs) are allocated non-overlapping physical resource in the delay-Doppler (DD) and/or time-frequency (TF) domain. Well known OMA methods include, i) Guard Band (GB) based MA (GBMA), ii) Interleaved Delay Doppler MA (IDDMA), iii) Interleaved Time Frequency MA (ITFMA). The spectral efficiency (SE) performance of these OMA methods has <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">only</i> been studied for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ideal</i> transmit and receive pulses. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">With ideal pulses</i> , IDDMA and ITFMA are free from multi-user interference (MUI) and unlike GBMA they <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">do not</i> use guard bands in the DD/TF domain. Since ideal pulses are not realizable, in this paper we study the SE performance of these OMA methods with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">practical rectangular pulses</i> . For these OMA methods, we derive the expression for the received DD/TF domain symbols at the base station (BS) receiver and the effective DD domain channel matrix when rectangular pulses are used. Study of these expressions reveals the presence of MUI when rectangular pulses are used, although the amount of MUI in IDDMA is observed to be significantly smaller than that in ITFMA. We also derive the expression for the achievable sum SE. Through simulations, for practical values of the received signal-to-noise ratio, we observe that with rectangular pulses the sum SE achieved by the IDDMA method is significantly higher than that achieved by the ITFMA and GBMA methods.

Multi-IRS-Aided Multi-User MIMO in mmWave/THz Communications: A Space-Orthogonal Scheme

Multiple-input multiple-output (MIMO) and intelligent reflecting surface (IRS) are two appealing technologies in millimeter-wave (mmWave) and terahertz (THz) communications. The challenge of combining these two technologies lies in joint design for active beamforming (at the base-station (BS)/users) and passive beamforming (at the IRSs). In this paper, we consider a multi-IRS-aided multi-user MIMO scenario and propose a novel space-orthogonal scheme by applying zero-forcing techniques. Specifically, we first propose a multi-IRS-based zero-interference criterion, under which multi-user interference can be eliminated regardless of the IRS’s phase shifts. Based on this criterion, we decompose the precoder/decoder matrix into a product of two matrices, with one of them devised for interference cancellation and the other one of them devised for achievable rate maximization. Next, an approximate space-orthogonal technique referred to as partial zero-forcing (IRS-PZF) is proposed for proposed for devising the former matrix whose objective is to cancel the multi-user interference; while two efficient phase-shift schemes are proposed for the IRS passive beamforming, namely, water-filling segment matching (WSM) and phase iterative evolution (PIE), which balance between performance and complexity. Finally, we calculate the latter matrix of the precoder/decoder by applying the singular value decomposition (SVD) for the effective BS-user channels, so as to maximize the users’ achievable rates. Numerical results demonstrate the effectiveness and superiority of our proposed scheme compared with the benchmarks.

Hybrid Multiuser Equalizer With Nonlinearity Compensation for Wideband mmWave Massive MIMO Uplink Systems

The use of efficient high-power amplifiers (HPAs) is critical for future millimeter-wave (mmWave) systems. HPAs can become highly nonlinear when operated near saturation, distorting the transmission signal. Therefore, the design of efficient solutions to mitigate nonlinear distortion (NLD) is of paramount importance. This article proposes a hybrid multiuser equalizer for the uplink of wideband mmWave massive MIMO systems. The equalizer is designed to mitigate both the multiuser interference (MUI) and inter-symbol interference (ISI), introduced by the multiple access channel, and the NLD, introduced by the user terminals HPA. The digital part of the hybrid equalizer is iterative and computed on a per subcarrier basis, to minimize the mean squared error between the transmitted and detected data. In the iterative processing, a feedback loop estimates and removes the distortion. The analog part is frequency-flat and its design assumes that the digital part efficiently removes the MUI, ISI and NLD. Performance results show a huge gain of the proposed hybrid nonlinear equalizer, when compared with linear ones. The nonlinear effects are almost completely removed, without significant error propagation effects, allowing a more efficient operation of the HPAs.

Joint beampattern and symbol-level waveform design for integrated sensing and communication

Tackling interference is an essential issue for the wireless system, which benefits the quality of service (QoS). In this paper, we consider an integrated sensing and communication (ISAC) system and focus on suppressing the signal-dependent interferences (SDIs) toward the radar and exploiting the constructive part of multi-user interference (MUIs) in communication. A novel optimization problem to minimize the transmitting power considering the channel fluctuations is formulated. We propose a tailored semidefinite relax (SDR) and successive convex approximation (SCA) based algorithm to simultaneously design the beampattern and symbol-level waveform. Simulations show that the proposed algorithm outperforms the conventional robust design method and a heuristic algorithm based on waveform scaling, reducing power consumption.