Orthogonal Multiple Access Systems Research Articles

Timing and Frequency Synchronization and Doubly Selective Channel Estimation for OFDMA Uplink

In this brief, a novel algorithm is proposed for timing and frequency synchronization and doubly selective channel (DSC) estimation in the uplink of orthogonal frequency division multiple access systems. By using a basis expansion model (BEM) to represent a DSC, a maximum likelihood approach is proposed to jointly estimate the timing and carrier frequency offsets of different users as well as the BEM coefficients of the time-varying channels. A space-alternating generalized expectation-maximization algorithm is then employed to transform the maximization problem for all users into several simpler maximization problems for a single user. Implementation issues are investigated and, the computational complexity and mean square error of the proposed scheme are evaluated. Performance assessment of the new scheme shows a significant improvement as compared with previous methods.

Cooperative NOMA System With Virtual Full Duplex User Relaying

Cooperative non-orthogonal multiple access (NOMA) system employs user relaying to enhance performance. Although full-duplex (FD) user relaying scheme does not incur additional bandwidth for relaying, the imperfect self-interference cancellation degrades the performance significantly. Alternatively, a virtual full-duplex (VFD) user relaying scheme can be used. In this paper, we study a cooperative NOMA system in cognitive radio scenario using VFD relaying scheme, where two half-duplex (HD) secondary users take turn to relay message of a primary user with decode-and-forward (DF) strategy. First, we derive the closed-form and asymptotic expressions of the outage probability and ergodic rate of the users. Then, we demonstrate the outage probability, ergodic rate, and ergodic sum rate performance of the proposed scheme using Monte Carlo simulations. We also provide a comprehensive comparison between the proposed VFD user relaying and existing FD and HD user relaying in orthogonal multiple access and NOMA systems. The simulation results verify the analytical results and demonstrate the advantages of the proposed VFD user relaying over existing schemes.

Effects of frequency and timing offsets in DCT-based multiple access system for VLC

Abstract Visible light communication (VLC) has gained a significant momentum in the recent times due to its distinctive characteristics of imparting concurrent ‘illumination’ and ‘communication’ by exploiting sustainable and energy-efficient opto-electronic devices like light emitting diodes (LEDs). Consequently, by utilizing the already installed LED lighting fixtures a small scale cellular communication network can be created within an indoor environment imparting economic and ubiquitous data transmission services to multitude of roaming mobile stations within the proximity of LEDs. Therefore, it is vital to address the detrimental aspects like carrier frequency offset (CFO) and symbol time offset (STO) which emanates predominantly in an uplink scenario. This paper evaluates the performance of DC offset optical fast orthogonal frequency division multiple access (DCO-FOFDMA) system in the presence of both CFO and STO and accomplishes a thorough mathematical analysis emphasizing the different interferences like inter carrier (ICI) and multi user interferences (MUI) emanating due to different timing disparities and frequency discrepancies. The simulation results evidences that, with the increase in sensitivity of CFO and timing misalignments, the system performance drastically deteriorates thereby hindering the detection capability of multiple users operating at the same time. In order to estimate the STO, synchronization algorithms like Maximum Likelihood (ML) and Classen are imposed to the developed system model. Additionally, Cramer Rao Lower Bound (CRLB) is derived for the estimation of CFO and STO.

Downlink Power Allocation for CoMP-NOMA in Multi-Cell Networks

This paper considers the problem of dynamic power allocation in the downlink of multi-cell networks, where each cell utilizes non-orthogonal multiple access (NOMA)-based resource allocation. Also, coordinated multi-point (CoMP) transmission is utilized among multiple cells to serve users experiencing severe inter-cell interference (ICI). Under this CoMP- NOMA framework, CoMP transmission is applied to a user experiencing less distinctive channel gain with multiple base stations (BSs)/cells (i.e., severe ICI-prone user) and non-CoMP transmission (i.e., transmission without any coordination among multiple BSs) is applied to a user experiencing dominating channel gain with only one BS/cell, while NOMA is utilized at each BS to schedule CoMP and non-CoMP users over the same transmission resources, i.e., time, spectrum and space. After discussing various CoMP- NOMA models for downlink power allocation in multi-cell networks, we focus on a joint transmission CoMP- NOMA (JT-CoMP-NOMA) model. For the JT-CoMP-NOMA model, an optimal joint power allocation problem is formulated and the solution is derived for each CoMP- set consisting of multiple cooperating BSs (i.e., CoMP BSs). To avoid the huge computational complexity of the joint power optimization approach, we propose a distributed power optimization approach at each cooperating BS whose optimal solution is independent of the solution of other coordinating BSs. The distributed solution for the joint power optimization problem is validated and numerical performance evaluation is carried out for the proposed CoMP- NOMA models including JT-CoMP-NOMA and coordinated scheduling CoMP- NOMA (CS-CoMP-NOMA). The obtained results reveal significant gains in spectral and energy efficiency in comparison with conventional CoMP- orthogonal multiple access (CoMP-OMA) systems.

Robust and Secure Resource Allocation for Full-Duplex MISO Multicarrier NOMA Systems

In this paper, we study the resource allocation algorithm design for multiple-input single-output (MISO) multicarrier non-orthogonal multiple access (MC-NOMA) systems, in which a full-duplex base station serves multiple half-duplex uplink and downlink users on the same subcarrier simultaneously. The resource allocation is optimized for maximization of the weighted system throughput while the information leakage is constrained and artificial noise is injected to guarantee secure communication in the presence of multiple potential eavesdroppers. To this end, we formulate a robust non-convex optimization problem taking into account the imperfect channel state information of the eavesdropping channels and the quality-of-service requirements of the legitimate users. Despite the non-convexity of the optimization problem, we solve it optimally by applying monotonic optimization which yields the optimal beamforming, artificial noise design, subcarrier allocation, and power allocation policy. The optimal resource allocation policy serves as a performance benchmark since the corresponding monotonic optimization-based algorithm entails a high computational complexity. Hence, we also develop a low-complexity suboptimal resource allocation algorithm which converges to a locally optimal solution. Our simulation results reveal that the performance of the suboptimal algorithm closely approaches that of the optimal algorithm. Besides, the proposed optimal MISO NOMA system can not only ensure downlink and uplink communication security simultaneously but also provides a significant system secrecy rate improvement compared with the traditional MISO orthogonal multiple access systems and two other baseline schemes.

Performance Improvement of NOMA Scheme Based on Gain of Narrow and Wide Beams

In this paper, we propose a non orthogonal multiple access (NOMA) system using coexisting narrow and wide beams to increase total capacity. In our proposed system, a base station considers the state of user mobility and supports users with either a narrow or a wide beam according to the mobility state. The proposed NOMA system uses the beam gain between a narrow and a wide beam, as well as the power gain from the pathloss difference in NOMA scheme. The system is thus less influenced by user mobility at the point of NOMA pairing. In the proposed system, we consider two power allocation schemes, namely the max and fair power allocation methods, and propose a pairing algorithm to maximize the capacity by minimizing the interference between narrow and wide beam user. The result verifies that the proposed system improves sum throughput, compared with the OMA and conventional NOMA system.

Amplify-and-Forward Virtual Full-Duplex Relaying-Based Cooperative NOMA

This letter studies the performance of a virtual full-duplex relaying technique in cooperative non-orthogonal multiple access (NOMA)-based systems. In particular, two half-duplex amplify-and-forward relays imitate the operation of full-duplex relaying in cooperative NOMA systems using the successive relaying (SR) technique, in order to recover the loss of spectral efficiency due to the half-duplex constraint at the relays. The closed-form outage probability of the proposed scheme is derived. The simulation results show that the proposed SR-based cooperative NOMA system with an opportunistic power split factor achieves a lower outage probability and a higher ergodic rate than the existing SR-based orthogonal multiple access system and ideal full duplex relaying-based NOMA systems.

Improving the Performance of Cell-Edge Users in MISO-NOMA Systems Using TAS and SWIPT-Based Cooperative Transmissions

In this paper, we study the performance of a cell-edge user in a two-user multiple-input single-output non-orthogonal multiple access (MISO-NOMA) system. Since the outage performance and fairness data rate of cell-edge users are essential issues in NOMA systems, we focus on how to resolve such problems. To this end, we propose three cooperative downlink transmission schemes utilizing hybrid simultaneous wireless information and power transfer (SWIPT) and transmit antenna selection (TAS) protocols. Particularly, in each scheme, a cell-center user acts as a relay to assist the cell-edge user and its relaying operation is powered by a hybrid time-switching/power-splitting SWIPT protocol. Additionally, each scheme employs a different TAS criterion to exploit the spatial diversity gain of a multiple antennas base station. In particular, we derive tight closed-form approximate expressions for the outage probabilities (OPs) and the corresponding asymptotic OPs to provide significant insights into the impact of our proposed schemes on the system performance. Our numerical results demonstrate the achievable performance improvements of the proposed schemes in comparison to that of the orthogonal multiple access and non-cooperative NOMA systems. In addition, the proposed schemes achieve various level of diversity gains accompanying with different complexity requirements.

Optimal User Scheduling and Power Allocation for Millimeter Wave NOMA Systems

This paper investigates the application of non-orthogonal multiple access (NOMA) in millimeter wave (mm-Wave) communications by exploiting beamforming, user scheduling, and power allocation. Random beamforming is invoked for reducing the feedback overhead of the considered system. A non-convex optimization problem for maximizing the sum rate is formulated, which is proved to be NP-hard. The branch and bound approach is invoked to obtain the $\epsilon$ -optimal power allocation policy, which is proved to converge to a global optimal solution. To elaborate further, a low-complexity suboptimal approach is developed for striking a good computational complexity-optimality tradeoff, where the matching theory and successive convex approximation techniques are invoked for tackling the user scheduling and power allocation problems, respectively. Simulation results reveal that: 1) the proposed low complexity solution achieves a near-optimal performance and 2) the proposed mm-Wave NOMA system is capable of outperforming conventional mm-Wave orthogonal multiple access systems in terms of sum rate and the number of served users.

Exploiting Uplink NOMA to Improve Sum Secrecy Throughput in IoT Networks

This paper exploits nonorthogonal multiple access (NOMA) to enhance the uplink secure transmission in Internet of Things (IoT) networks. Considering the different intercept ability of eavesdroppers (Eve), secrecy performances of both strong and weak Eve wiretap scenarios have been investigated. In strong Eve wiretap scenario (SWS), Eve is assumed to be powerful enough to decode message without interference and, in weak Eve wiretap scenario (WWS), Eve is assumed to have significant demodulation capability constraint. The new closed‐form expressions of joint connection outage probability (JCOP), joint secrecy outage probability (JSOP), and sum secrecy throughput (SST) are derived in these two scenarios to indicate the impact of parameters, i.e., transmit power, codeword rate, and the placement of devices, on security performance. In order to demonstrate the superiority of NOMA, we also investigate the secrecy performance of orthogonal multiple access (OMA) system as a benchmark. Analysis results show that the performance in WWS is always better than that in SWS and, in low signal‐to‐noise ratio (SNR) or high codeword rate region, the performances of these two scenarios are close. In addition, we present the condition that NOMA outperforms OMA in terms of SST. Moreover, the placements of devices are significant to the SST performance of NOMA system. The suboptimal device placement scheme has been designed to maximize SST. Analysis results demonstrate that when Eve is far away from legal users, the suboptimal results tend to optimal.

Performance Analysis of Non-Orthogonal Multiple Access With Underlaid Device-to-Device Communications

Non-orthogonal multiple access (NOMA) and device-to-device (D2D) communications are promising technologies for 5G cellular networks. In this paper, we investigate the impact of the integration of D2D communications with a downlink NOMA system. In a cluster of two cellular users and a D2D pair, we extract power control strategies for the base station (BS), which allow for cellular users to achieve a higher sum rate and higher individual rates in NOMA compared to an orthogonal multiple access (OMA). The D2D link has the same rate in both OMA and NOMA systems in our proposed scheme. We further derive the probability that both users obtain higher rates in NOMA under a fixed power control strategy. We provide numerical results that illustrate the performance of the proposed power control policies and demonstrate the accuracy of the derived closed-form expression for the probability that both cellular users obtain higher rates in NOMA. We also study the effects of the BS transmit power level and the power portion assigned to each user on the probability that both users outperform in NOMA through numerical results.

Impact of Residual Hardware Impairments on Non-Orthogonal Multiple Access Based Amplify-and-Forward Relaying Networks

We quantify the impact of residual hardware impairments (RHI) on a non-orthogonal multiple access (NOMA)-based relaying network, where a source communications simultaneously with multiple users via an amplify-and-forward relay. Specifically, exact and asymptotic expressions for the outage probability are first derived in closed-form over Nakagami- $m$ fading channels, accounting for RHI at the source, relay, and all users. Our results show that the outage performance loss induced by RHI is small in the low signal-to-noise ratio (SNR) regime or/and at low target rates, however it is significant at high SNRs or/and target rates. Furthermore, we present new tight analytical approximated and asymptotic expressions for the system ergodic sum rate (ESR). For comparison, we also discuss the ESR of the conventional hardware-impaired relaying system employing orthogonal multiple access (OMA) transmission and obtain the asymptotic high-SNR expression of the ESR. The provided numerical results demonstrate that in the absence of RHI, the ESR in either the NOMA or the OMA system increases monotonically with the increase of SNR, whereas an unavoidable ESR ceiling is introduced in the hardware-impaired scenario for both systems. Notably, since the ESR ceiling value is only depended on the RHI levels, the NOMA-based and OMA-based systems achieve the same ESR performance in high SNRs.

Divide‐and‐allocate: An uplink successive bandwidth division NOMA system

AbstractThis paper investigates a new spectrum‐sharing strategy using non‐orthogonal multiple access (NOMA), a technique that is expected to lead toward the fifth generation of wireless networks. The proposed scheme, namely, successive bandwidth division (SBD), NOMA, is a hybrid approach exploiting the characteristics of both the conventional NOMA system and the orthogonal multiple access (OMA) system. Power allocation is being performed in successive bandwidth division NOMA to maximize the sum rate using a divide‐and‐allocate approach such that all users are allocated with optimal transmission power. Under Rayleigh fading, the probability density function (PDF) of the received signal‐to‐interference‐plus‐noise ratio (SINR) is derived, and closed‐form expressions for the outage probability are presented when channel state information (CSI) is available at the base station. The performance evaluation is carried out in terms of receiver complexity, average sum rate, and outage probability. Simulations results are provided to access and compare the performance of the proposed scheme with other contemporary approaches.

Transmitter diversity scheme for OFCDMA systems based on space‐time spreading with iterative detection receiver

Orthogonal frequency code division multiple access (OFCDMA) system is one of the most promising multi-user wireless communications systems. It outperforms orthogonal frequency division multiplexing (OFDM) and multi-carrier CDMA (MC-CDMA) through utilisation of two-dimensional spreading. This study proposes a downlink air interface that targets data rate increase and bit error rate (BER) performance enhancement with low complexity receiver. The proposed system is an integration of space-time spreading (STS) and OFCDMA that exploits transmit diversity needed for BER enhancing and data rate boosting with low complexity advantage. Further BER improvement was achieved using an effective iterative interference cancellation (IIC) algorithm at the receiver. Analytical and complexity analyses for the proposed system performance are presented in addition to simulation results. The proposed system attains better performance with lower complexity compared to the STS-aided direct sequence MC-CDMA that uses beamforming as a closed-loop transmit diversity. Moreover, the effect of frequency domain spreading factor with different number of iteration loops is investigated. The achieved BER performance was very close to maximal ratio receive combining system with 1Tx and 4Rx. A considerable improvement was obtained by increasing the number of IIC iteration loops. The system performance was enhanced significantly with the frequency domain spreading factor increase.

Power allocation scheme and spectral efficiency analysis for downlink non‐orthogonal multiple access systems

In this study, the authors investigate the power allocation (PA) problem and spectral efficiency (SE) analysis for the single antenna downlink non-orthogonal multiple access (NOMA) system under the sum rate maximising criteria with minimum rate constraints (SRMC-MRC). For the PA problem, they propose the duality scheme for SRMC-MRC which is considered as the optimal solution for the PA problem on one orthogonal subband in the single antenna NOMA system. They propose the specific user rate maximising criteria with minimum rate constraints (SURMC-MRC) scheme to decrease the computational complexity of SRMC-MRC. The PA problem on one subband under SURMC-MRC is proved to be equivalent to SRMC-MRC. Numerical results show that both the SE and fairness performance for SURMC-MRC can strictly approach to those of the duality scheme in the whole signal-to-noise ratio (SNR) region. They prove that NOMA under SRMC-MRC can obtain SE performance advantage in the high SNR region but accompany with some disadvantages in the low SNR region over the orthogonal multiple access system. This conclusion is verified through numerical results under different parameter conditions.

Optimal Joint Power and Subcarrier Allocation for Full-Duplex Multicarrier Non-Orthogonal Multiple Access Systems

In this paper, we investigate resource allocation algorithm design for multicarrier non-orthogonal multiple access (MC-NOMA) systems employing a full-duplex (FD) base station for serving multiple half-duplex (HD) downlink and uplink users simultaneously. The proposed algorithm is obtained from the solution of a non-convex optimization problem for the maximization of the weighted sum system throughput. We apply monotonic optimization to develop an optimal joint power and subcarrier allocation policy. The optimal resource allocation policy serves as a system performance benchmark due to its high computational complexity. Furthermore, a suboptimal iterative scheme based on successive convex approximation is proposed to strike a balance between computational complexity and optimality. Our simulation results reveal that the proposed suboptimal algorithm achieves a close-to-optimal performance. In addition, FD MC-NOMA systems employing the proposed resource allocation algorithms provide a substantial system throughput improvement compared with conventional HD multicarrier orthogonal multiple access (MC-OMA) systems and other baseline schemes. In addition, our results unveil that FD MC-NOMA systems enable a fairer resource allocation compared with traditional HD MC-OMA systems.

Error Vector Magnitude Analysis of Uplink Multiuser OFDMA and SC-FDMA Systems in the Presence of Nonlinear Distortion

We analyze the impact of nonlinear distortion on the performance of uplink multiuser orthogonal frequency division multiple access and single carrier-frequency division multiple access systems in terms of the error vector magnitude (EVM). Based on the power spectrum density of the distortion noise, we derive a unified framework of theoretical EVM for both systems. The obtained results provide us approaches to evaluation of both the self-distortion and the inter-distortion interference from other users. In addition, simulation results agree well with our theoretical analyses.

Non-orthogonal multiple access in a downlink multiuser beamforming system with limited CSI feedback

Non-orthogonal multiple access (NOMA) has been recognized as a promising multiple access technology for fifth generation (5G) mobile communication system. However, the advantage of NOMA is only verified under the ideal condition that the transmitter has the perfect knowledge of channel state information (CSI). In this paper, NOMA downlink multiuser system, in which the transmitter acquires the CSI through limited feedback channel, is studied. Two traditional beamforming technologies, zero-forcing beamforming and random beamforming, are investigated in the NOMA downlink multiuser system. Making use of the imperfect CSI feedback, channel direction information (CDI), and channel quality indicator (CQI), we propose the user selection scheme to reduce the interference between the NOMA users. Furthermore, a power allocation scheme is proposed to improve the sum-rate of NOMA system. Numerical results show that NOMA system with limited feedback channel still gains larger system rate than traditional orthogonal multiple access system. It is also shown that random beamforming is more suitable for the NOMA system with limited CSI feedback.

Precoding Design with Joint Dynamic Channel Assignment and Band Selection Techniques for Downlink CoMP OFDMA Systems

Multi cell transmission can prevent the multi-cell interference by localizing the signals at different sub-band for the downlink (DL) localized orthogonal frequency division multiple access (OFDMA) systems. However, the synchronized errors, such as carrier frequency offset (CFO), will induce the challenging problems of interference due to the non-orthogonality between the subcarriers and cells. In this paper, we propose the novel precoding design with joint dynamic channel assignment and band selection techniques for DL coordinated multipoint (CoMP) OFDMA systems with CFO effect. Further, a two-stage pre-processing technique is proposed to eliminate the above interference problems and reduce the computational complexity of the cell-edge users for DL CoMP OFDMA systems. Moreover, the central unit can assign the resource dynamically to enhance the link quality. Therefore, the dynamic channel on/off assignment techniques and the dynamic band selection techniques are proposed to enhance the bit error rate (BER) performance. Simulation results confirm that the proposed precoding transceiver provides excellent BER and lower computational complexity of the cell-edge communication than the conventional block diagonalization technique.

Impact of Sub-Band Correlation on SFR and Comparison of FFR and SFR

In soft frequency reuse (SFR) when the user is classified as a cell-center user based on signal-to-interference-plus-noise-ratio in a sub-band, the user retains its sub-band. On the other hand, if the user is classified as a cell-edge user, a new sub-band is allocated. We analyze the impact of correlation between the cell-center sub-band and the cell-edge sub-band for a user when the SFR technique is used in a cellular orthogonal frequency division multiple access (OFDMA) system. The coverage probability (CP) and the average rate are derived for the following two cases: 1) when the sub-bands are independent and 2) when the sub-bands are completely correlated. We show that correlation significantly decreases the edge CP and the average rate of the SFR technique, and as the power control factor increases, the impact of correlation decreases. Fractional frequency reuse (FFR) and SFR techniques are compared and it is shown that the impact of correlation on the FFR is significantly lower. Furthermore, it is also shown that the FFR provides better edge CP and average rate when compared with the SFR in single-input single-output (SISO) networks, thereby suggesting that the FFR should be preferred over SFR in cellular SISO OFDMA systems. However, for single-input multiple-output networks, the SFR provides a better average rate when compared with the FFR, especially when the sub-band correlation is not significant. Finally, the impact of log-normal shadowing has been carefully studied.