Coordinated Multipoint Systems Research Articles

On the Application of Quasi-Degradation to Network NOMA in Downlink CoMP Systems

The application of network non-orthogonal multiple access (N-NOMA) technique to coordinated multi-point (CoMP) systems has attracted significant attention due to its superior capability to improve connectivity and maintain reliable transmission for CoMP users simultaneously. Based on the concept of quasi-degraded channel for N-NOMA, this paper studies the precoding design for downlink N-NOMA scenarios with two base stations (BSs) equipped with multiple antennas. In specific, under quasi-degraded channels, simple linear precoding based N-NOMA can achieve the same minimal total transmission power as theoretically optimal but complicated dirty paper coding (DPC) scheme, when the users’ target rates and minimal transmission power of each BS are given. In this paper, the channel quasi-degradation (QD) condition is first rigorously derived for the scenario with single CoMP user and two NOMA users. The closed-form optimal precoders for N-NOMA under quasi-degraded channels are also provided. Then, based on QD condition, a novel hybrid N-NOMA (H-N-NOMA) scheme is proposed, which is a mixture of N-NOMA and conventional zero-forcing beamforming (ZFBF) scheme. Further, for the scenarios with more users, a low-complexity QD based user pairing (QDUP) algorithm is proposed, which is then combined with H-N-NOMA to make a novel scheme termed H-N-NOMA/QDUP into being. Numerical results are presented to reveal the impact factors on QD channels, and also demonstrate the superior performance of the proposed H-N-NOMA/QDUP scheme. It is shown that the proposed H-N-NOMA/QDUP scheme can effectively exploit the benefit of multi user diversity.

OTFS Signaling for SCMA With Coordinated Multi-Point Vehicle Communications

This paper investigates an uplink coordinated multi-point (CoMP) coverage scenario, in which multiple mobile users are grouped for sparse code multiple access (SCMA), and served by the remote radio head (RRH) in front of them and the RRH behind them simultaneously. We apply orthogonal time frequency space (OTFS) modulation for each user to exploit the degrees of freedom arising from both the delay and Doppler domains. As the signals received by the RRHs in front of and behind the users experience respectively positive and negative Doppler frequency shifts, our proposed OTFS-based SCMA (OBSCMA) with CoMP system can effectively harvest extra Doppler and spatial diversity for better performance. Based on maximum likelihood (ML) detector, we analyze the single-user average bit error rate (ABER) bound as the benchmark of the ABER performance for our proposed OBSCMA with CoMP system. We also develop a customized Gaussian approximation with expectation propagation (GAEP) algorithm for multi-user detection and propose efficient algorithm structures for centralized and decentralized detectors. Our proposed OBSCMA with CoMP system leads to stronger performance than the existing solutions. The proposed centralized and decentralized detectors exhibit effective reception and robustness under channel state information uncertainty.

Impact of Decoding Order on User Pairing in NOMA-CoMP Joint Transmission

This correspondence investigates the optimal decoding order and its impact on user pairing in non-orthogonal multiple access (NOMA)-enabled coordinated multipoint (CoMP) joint transmission (JT) to maximize the sum rate of users. First, the optimal solution of power control for a user pair is obtained under different decoding orders, where closed-form expressions and a low-complexity algorithm are given by monotonic optimization. Then, a criterion for finding the optimal decoding order is derived for any given user pair in the considered scenario. Results show that the optimal decoding order is distinct for different user pairs, which can be achieved separately. This indicates that the decoding order is not a necessary prerequisite for user pairing in NOMA-enabled CoMP systems. Moreover, the optimal decoding order is shown to depend on the differentials of channel gains between the user served by JT and coordinated base stations.

STAR-RIS-UAV-Aided Coordinated Multipoint Cellular System for Multi-User Networks

Different from conventional reconfigurable intelligent surfaces (RIS), simultaneous transmitting and reflecting RIS (STAR-RIS) can reflect and transmit signals to the receiver. In this paper, to serve more ground users and increase deployment flexibility, we investigate an unmanned aerial vehicle (UAV) equipped with STAR-RIS (STAR-RIS-UAV)-aided wireless communications for multi-user networks. Energy splitting (ES) and mode switching (MS) protocols are considered to control the reflection and transmission coefficients of STAR-RIS elements. To maximize the sum rate of the STAR-RIS-UAV-aided coordinated multipoint (CoMP) cellular system for multi-user networks, the corresponding beamforming vectors as well as transmitted and reflected coefficient matrices are optimized. Specifically, instead of adopting the alternating optimization, we design an iteration method to optimize all variables for both the ES and MS protocols at the same time. Simulation results reveal that the STAR-RIS-UAV-aided CoMP system has a much higher sum rate than systems with conventional RIS or without RIS. Furthermore, the proposed structure is more flexible than fixed STAR-RIS and could greatly promote the sum rate.

Energy Efficient Resource Allocation for IRS Assisted CoMP Systems

A novel intelligent reconfigurable surface (IRS) assisted coordinated multi-point (CoMP) system is proposed. Our objective is to maximize the energy efficiency (EE) of this system by jointly optimizing base station (BS) clustering, user association, sub-carrier assignment, power allocation, and optimal design of the IRS, while satisfying the users’ quality of service requirements. Considering the amplitude and phase shift characteristics, both ideal and non-ideal IRS are investigated. The formulated problem is proved to be NP-hard. By analyzing its structure, we decouple it into the power allocation sub-problem, the BS clustering, UE association, and sub-carrier assignment sub-problem, and the reflection coefficients design sub-problem. For the power allocation sub-problem, we invoke the fractional programming to find the optimal solution. For the reflection coefficients design sub-problem of ideal IRS, the optimal solution is derived with the Lagrangian dual method. Whereas quantization-based method is employed to find the discrete phase shifts for non-ideal IRS. We finally propose a genetic algorithm (GA) to represent the potential solutions of sub-carrier assignment, and combine the other two optimization algorithms as fitness estimator in GA. Numerical results validate the feasibility, fast convergence, and the flexibility of the proposed algorithm. It shows that the proposed scheme outperform the system without IRS and that with a random initialized IRS.

A Proposed Method to Coordinate mmWave Beams Based on Coordinated Multi-Point in 5G Networks

5G overcame the severe shortage of cellular spectrum used by its predecessors by using millimeter-wave (mmWave) packets that provide broadband to 5G wireless networks. The problem of Inter User Interference (IUI) is also one of the most important and dangerous problems facing telecommunication networks. IUI is exacerbated, especially in the areas near the Tower as the narrow coverage area compared to the areas farthest from the Tower. As a result of this effect, our paper proposes a new model by taking a matrix of antennas at different heights and coordinating mmWave beams. Coordinated Multi-Point (CoMP) is the essential solution in 5G to improve throughput and coverage performance. As a result, millimeter wave technology was used in the proposed CoMP system. The new proposed system works to study the effect of the height of the antenna matrix on reducing IUI by calculating the peak capacity value on the user with different distances. In addition to the impact of the azimuth angle of arrival AoA (∅) on capacity when implementing each scenario. Simulation results show that the proposed system scheme with different heights of the antenna matrix can get a large capacity than a traditional system. Choosing the best antenna matrix height reduces interference and improves capacity. The highest improvement obtained is 53%.

Analysis of Drone Placement Strategies for Complete Interference Cancellation in Two-Cell NOMA CoMP Systems

The use of unmanned aerial vehicles (UAV) as flying base stations is rapidly growing in the field of wireless communications to leverage the capacity of congested cells. This study considers a two-cell system where one of the cells is saturated, i.e. can no longer serve its users, and is supported by a UAV. The UAV positioning problem is investigated specifically to benefit from the interference cancellation properties available through the introduction of power-domain non-orthogonal multiple access (NOMA) techniques in coordinated multipoint (CoMP) systems. Indeed, adequate placement of the UAV can enable triple mutual successive interference cancellation (TMSIC) between a triplet of users, including a cell-edge and a cell-center user in each cell, to maximize system throughput or a mixture of throughput and TMSIC probability. The random line-of-sight/non-line-of-sight realizations of air-to-ground links between users and UAV are taken into account in the problem modeling, showing a significant improvement in performance compared to the conventional mean path loss model. The performance evaluation highlights the existing trade-offs between system capacity, fairness, and computational complexity of the investigated approaches.

Mutual Successive Interference Cancellation Strategies in NOMA for Enhancing the Spectral Efficiency of CoMP Systems

The densification of mobile networks should enable the fifth generation (5G) mobile networks to cope with the ever increasing demand for higher rate traffic, reduced latency, and improved reliability. The large scale deployment of small cells and distributed antenna systems in heterogeneous environments will require more elaborate interference mitigating techniques to increase spectral efficiency and to help unlock the expected performance leaps from the new network topologies. Coordinated multi-point (CoMP) is the most advanced framework for interference management enabling the cooperation between base stations to mitigate inter-cell interference and boost cell-edge user performance. In this paper, we study the combination of CoMP with mutual SIC, an interference cancellation technique based on power-domain non-orthogonal multiple access (NOMA) that enables multiplexed users to simultaneously cancel their corresponding interfering signals. A highly efficient inter-cell interference cancellation scheme is then devised, that can encompass several deployment configurations and coordination techniques. The obtained results prove the superiority of this approach compared to conventional NOMA-CoMP systems.

Intelligent Scheduling and Power Control for Multimedia Transmission in 5G CoMP Systems: A Dynamic Bargaining Game

Intelligent terminals support a large number of multimedia such as picture, audio, video, and so on. The coexistence of various multimedia makes it necessary to provide service for different requests. In this paper, we consider interference-aware coordinated multi-point (CoMP) to mitigate inter-cell interference and improve total throughput in the fifth-generation (5G) mobile networks. To select the scheduled edge users, cluster the cooperative base stations (BSs), and determine the transmitting power, a novel dynamic bargaining approach is proposed. Based on affinity propagation, we first select the users to be scheduled and the cooperative BSs serving them. Then, based on the Nash bargaining solution (NBS), we develop a power control scheme considering the transmission delay, which guarantees a generalized proportional fairness among users. Simulation results demonstrate the superiority of the user-centric scheduling and power control methods in 5G CoMP systems.

Interference Analysis and Mitigation for Time-Asynchronous OFDM CoMP Systems

One of the major challenges, which prevents the coordinated multipoint (CoMP) communications concept from being widely deployed in new cellular systems, is timing synchronization. In order to achieve the gains promised by CoMP systems, the user equipment’s signals in uplink (UL) or the base stations’ signals in downlink should be synchronized, such that the time difference of arrivals do not exceed the cyclic prefix length of the transmitted signals; otherwise, the system suffers from unavoidable integer time offsets. These offsets lead to asynchronous interference in terms of inter-carrier, inter-block, and inter-symbol interference. This limits the percentage of area within a cell which can be covered by cooperation and imposes an upper bound on this area. In this paper, we investigate this problem in the UL, and by using geometrical and semi-analytical approaches, we define this upper bound. Also, by characterizing an accurate mathematical model for the asynchronous interference in Rayleigh fading channels, we were able to employ a standard MMSE-based receiver that mitigates this interference. Furthermore, a typical joint channel and delay estimation block is incorporated into the receiver to examine its performance with estimation errors.

Two-Scale Stochastic Control for Integrated Multipoint Communication Systems With Renewables

Increasing threats of global warming and climate changes call for an energy-efficient and sustainable design of future wireless communication systems. To this end, a novel two-scale stochastic control framework is put forth for smart-grid powered coordinated multi-point (CoMP) systems. Taking into account renewable energy sources, dynamic pricing, two-way energy trading facilities, and imperfect energy storage devices, the energy management task is formulated as an infinite-horizon optimization problem minimizing the time-averaged energy transaction cost, subject to the users’ quality of service requirements. Leveraging the Lyapunov optimization approach as well as the stochastic subgradient method, a two-scale online control (TS-OC) approach is developed for the resultant smart-grid powered CoMP systems. Using only historical data, the proposed TS-OC makes online control decisions at two timescales, and features a provably feasible and asymptotically near-optimal solution. Numerical tests further corroborate the theoretical analysis, and demonstrate the merits of the proposed approach.

Variable rate and power M‐QAM for DPS/DPB CoMP with adaptive modulation

AbstractDynamic point selection/dynamic point blanking (DPS/DPB)–based coordinated multipoint (CoMP) has attracted lots of interest because of the significant performance improvement with a particular low complexity. In this paper, we first investigate the average spectral efficiency (ASE) of M‐ary quadrature amplitude modulation (M‐QAM) satisfying instantaneous bit error rate constraint for DPS/DPB CoMP system with 3 different adaptive modulation strategies. Closed‐form expressions of the ASE for continuous‐rate M‐QAM are obtained in which calculus of variations is used to obtain the optimal power allocation policy. We also analyze the ASE for discrete‐rate M‐QAM which is more useful for the practical DPS/DPB CoMP wireless communication systems. Average bit error rate for DPS/DPB CoMP discrete‐rate M‐QAM is also investigated in this paper. Monte Carlo simulations are used to verify the correctness of our analysis. The results reveal that in M‐QAM DPS/DPB CoMP systems, when the channel condition is unfavorable, the transmit power is the crucial parameter to enhance the ASE, otherwise the transmit rate is the key parameter. We can also notice that by using adaptive modulation, DPS/DPB CoMP has an obvious channel capacity improvement compared to the case without coordination among different transmission points.

Auction-based spectrum allocation for CoMP systems utilizing hybrid division duplex

The coordinated multi-point (CoMP) transmission is a well-recognized promising technique for achieving high spectral efficiency. In this paper, we study the performance of the spectrum allocation and the system utility of the CoMP systems. First, we combine the hybrid division duplex (HDD) with a CoMP system to form a CoMP–HDD system, for the purpose of guaranteeing the quality demand of the channel state information (CSI) feedback. Second, in order to improve the system utility and the spectrum allocation efficiency of the CoMP–HDD system, we utilize the auction theory for the spectrum allocation. A system utility maximization problem is formulated as an NP-hard problem. Finally, we propose a multi-band multi-winner (MBMW) greedy algorithm to optimize the system utility and the spectrum allocation efficiency. Our simulation results demonstrate the effectiveness of the proposed algorithm.

Capacity Analysis of Joint Transmission CoMP With Adaptive Modulation

Joint transmission (JT)-based Coordinated Multipoint (CoMP) systems achieve high performance gains by allowing full coordination among multiple cells, transforming unwanted intercell interference into useful signal power. In this paper, we present an analytical model to perform adaptive modulation for a typical JT CoMP system, consisting of three transmission points, under a target bit error rate (BER) constraint. Probability density functions of the signal-to-interference-plus-noise ratio (SINR) are derived for different JT CoMP schemes, and based on them, closed-form expressions for the average spectral efficiency (ASE) are obtained when adopting continuous-rate adaptive modulation. The study of ASE is also extended for the case of discrete-rate modulations, where the performance comparison of different practical quantized modulation schemes is carried out.

An Optimized CoMP Transmission for a Heterogeneous Network Using eICIC Approach

Heterogeneous network deployment is considered to be among the most promising approaches to meet the demand for increasing communication capacity. The concept is that of increasing the number of cells while reducing their size to provide different layers of coverage. This paper proposes a coordinated method to face the interlayer and intralayer interference caused by the overlapping of heterogeneous cells. The aim is to exploit the benefits of both enhanced intercell interference coordination (eICIC) and coordinated multipoint (CoMP) approaches and to limit, at the same time, their drawbacks by means of their optimized joint use. The idea devised here is to build an ad hoc CoMP system on top of a basic eICIC mechanism. Therefore, the analysis focuses on the dynamic selection of users and cells involved in CoMP operations, with the goal of minimizing the unfulfilled data rate requests (UDRRs). Due to the computational complexity of the optimal solution, we propose a heuristic procedure and validate its accuracy by providing a comparison with the optimal numerical solution. In particular, our results show that the proposed method outperforms other benchmark solutions in terms of UDRRs and signaling overhead.

Power imbalance induced BER performance loss under limited-feedback CoMP techniques

Coordinated multipoint (CoMP) technology utilizes simultaneous transmission/reception from/to different access points, and it is considered as an important feature to exploit and/or mitigate intercell interference in fourth-generation mobile networks. Yet, channel power imbalance at the receiver is experienced in CoMP systems due to, e.g., spatially distributed transmissions. Traditional co-located multi-antenna systems may also experience power imbalance among antenna branches due to inaccurate antenna calibration. This paper presents a bit error rate (BER) analysis and derives asymptotic and approximate BER expressions for some practical CoMP transmission techniques under channel power imbalance. Besides the analytical results, numerical analysis is made to thoroughly capture the performance impact of channel power imbalance on the performance gain of the CoMP methods. The results demonstrate that power imbalance considerably affects BER performance and applying long-term amplitude information with fast phase feedback has insignificant benefit to effectively compensate the detrimental effect of large channel power imbalance when base stations use a single antenna. In this case, exploiting both short-term amplitude and phase information is a very good choice. On the contrary, for a large number of diversity antennas in base stations, using long-term amplitude information with a sparsely quantized phase shows BER performance close to the case where full channel state information is applied.

On Fair Resource Sharing in Downlink Coordinated Multi-Point Systems

An opportunistic scheduling scheme is proposed for the downlink of a coordinated multi-point (CoMP) system, which exploits both spatial multiplexing and multiuser diversity gains while maintaining fair resource sharing among users. The proposed scheduling scheme consists of two steps in each time slot: selecting a base station (BS) at each user, and then selecting a user at each BS. Although the proposed scheme selects users in a distributed manner, the channels among the selected users are proved to be orthogonal when the number of users approaches infinity. Simulation results demonstrate that the proposed scheme yields a better throughput than the existing scheduling schemes designed for fair resource sharing in CoMP systems.

Dynamic Energy Management for Smart-Grid-Powered Coordinated Multipoint Systems

Due to increasing threats of global warming and climate change concerns, green wireless communications have recently drawn intense attention toward reducing carbon emissions. Aligned with this goal, the present paper deals with dynamic energy management for smart-grid powered coordinated multipoint (CoMP) transmissions. To address the intrinsic variability of renewable energy sources, a novel energy transaction mechanism is introduced for grid-connected base stations that are also equipped with an energy storage unit. Aiming to minimize the expected energy transaction cost while guaranteeing the worst-case users’ quality of service, an infinite-horizon optimization problem is formulated to obtain the optimal downlink transmit beamformers that are robust to channel uncertainties. Capitalizing on the virtual-queue-based relaxation technique and the stochastic dual-subgradient method, an efficient online algorithm is developed yielding a feasible and asymptotically optimal solution. Numerical tests with synthetic and real data corroborate the analytical performance claims and highlight the merits of the novel approach.

Cooperative Energy Trading in CoMP Systems Powered by Smart Grids

This paper studies the energy management in coordinated multipoint (CoMP) systems powered by smart grids, where each base station (BS) with local renewable energy generation is allowed to implement two-way energy trading with the grid. Due to the uneven renewable energy supply and communication energy demand over distributed BSs, as well as the difference in the prices for their buying/selling energy from/to the grid, it is beneficial for the cooperative BSs to jointly manage their energy trading with the grid and energy consumption in CoMP-based communication for reducing the total energy cost. Specifically, we consider the downlink transmission in one CoMP cluster by jointly optimizing the BSs' purchased/sold energy units from/to the grid and their cooperative transmit precoding to minimize the total energy cost subject to the given quality-of-service (QoS) constraints for the users. First, we obtain the optimal solution to this problem by developing an algorithm based on techniques from convex optimization and uplink–downlink duality. Next, we propose a suboptimal solution of lower complexity than the optimal solution, where zero-forcing (ZF)-based precoding is implemented at the BSs. Finally, through extensive simulations, we show the performance gain achieved by our proposed joint energy trading and communication cooperation schemes in terms of energy cost reduction, as compared with conventional schemes that separately design communication cooperation and energy trading.

On the Performance of Opportunistic NOMA in Downlink CoMP Networks

In this letter, an opportunistic NOMA (ONOMA) scheme is proposed in a coordinated multipoint (CoMP) system. Compared with the conventional joint-transmission (JT) NOMA in CoMP, the main purpose of the ONOMA scheme is to advance the capability of the CoMP system and the effectiveness of the successive-interference-cancellation (SIC) decoding process. The relationship between the topology of the ONOMA cells and the sum-rate of CoMP network is analyzed. Meanwhile, the outage probability of ONOMA system is derived and evaluated in an ideal case and nonideal case, respectively. In the numerical results, the sum-rate and the outage performance of the proposed ONOMA CoMP are compared with the JT-NOMA CoMP and it is shown that the proposed ONOMA scheme outperforms the conventional JT-NOMA scheme in CoMP system.