Coordinated Multipoint Transmission Research Articles

Mobility in the Sky: Performance and Mobility Analysis for Cellular-Connected UAVs

Providing connectivity to unmanned aerial vehicle-user equipment (UAV-UE), such as drones or flying taxis, is a major challenge for tomorrow’s cellular systems. In this paper, the use of coordinated multi-point (CoMP) transmission for providing seamless connectivity to UAV-UEs is investigated. In particular, a network of clustered ground base stations (BSs) that cooperatively serve a number of UAV-UEs is considered. Two scenarios are studied: scenarios with static, hovering UAV-UEs and scenarios with mobile UAV-UEs. Under a maximum ratio transmission, a novel framework is developed and leveraged to derive upper and lower bounds on the UAV-UE coverage probability for both scenarios. Using the derived results, the effects of various system parameters such as collaboration distance, UAV-UE altitude, and UAV-UE speed on the achievable performance are studied. Results reveal that, for both static and mobile UAV-UEs, when the BS antennas are tilted downwards, the coverage probability of a high-altitude UAV-UE is upper bounded by that of ground user equipments (UEs) regardless of the transmission scheme. Moreover, for low signal-to-interference-ratio thresholds, it is shown that CoMP transmission can improve the coverage probability of UAV-UEs, e.g., from 28% under the nearest association scheme to 60% for an average of 2.5 collaborating BSs. Meanwhile, key results on mobile UAV-UEs unveil that not only the spatial displacements of UAV-UEs but also their vertical motions affect their handover rate and coverage probability. In particular, UAV-UEs that have frequent vertical movements and high direction switch rates are expected to have low handover probability and handover rate. Finally, the effect of the UAV-UE vertical movements on its coverage probability is marginal if the UAV-UE retains the same mean altitude.

NOMA-Enabled CoMP Clustering and Power Control for Green Internet of Things Networks

In this paper, we investigate the clustering and power control problem for coordinated multipoint (CoMP) transmission in green non-orthogonal multiple access (NOMA) networks. NOMA is a promising multiple access technique to support massive connectivity in the case of a large number of Internet-of-things (IoT) devices. Recently, CoMP transmission is introduced into NOMA networks to improve network capacity, which enables multiple access points (APs) to simultaneously serve one device while not cutting off the association with other devices. However, non-orthogonal resource sharing brings interference in AP clusters, which further boosts up the transmit-power consumption. To tackle this issue, we propose a scheme to minimize the total transmit power of APs by jointly optimizing AP clustering and power control. Specifically, the AP clustering is modeled as a many-to-one matching problem with externalities. We design an exchange-matching based algorithm to iteratively update the AP clustering and finally achieve the matching with one-sided stability. Furthermore, we show that the power control with a given AP clustering can be solved by linear programming. Simulation results demonstrate that the proposed scheme can release the potential of CoMP in NOMA networks and effectively mitigate interference to save transmit power while ensuring rate requirements for all devices.

Precoding and Selection for Coordinated Multipoint Transmission in Fronthaul-Constrained Cloud-RAN

A joint transmission point selection and precoding scheme is proposed herein to mitigate interference in a C-RAN system with distributed fronthaul-constrained transmission points. Two methods are proposed to deal with the original NP-hard problem. First, reformulation into convex form is done using an upper bound method, where tightness of the bound and its implication on system scenario are easy to establish. The second one uses successive convex approximation to approximate the nonconvex constraint and is shown to maximize spectral efficiency at the expense of higher computational complexity compared to the first approach. Numerical results will show the efficacy of the proposed approaches.

An efficient two-stage user association scheme for green C-RAN systems

<span style="color: black; font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;">This paper addresses the energy efficiency (EE) based user-association problem in cloud radio-access network. In specific, a low-complexity 2-stage iterative algorithm is developed for this purpose by jointly employing the coordinated multi-point (CoMP) and user-clustering techniques. The first stage of the algorithm applies a greedy search to form user-cluster and activate corresponded remote radio heads (RRHs) for CoMP transmission, only an RRH j with good channel gain could be included in the serving cluster of user k. Next, in the second stage, a re-clustering procedure is utilized to mitigate inter-cluster interference (ICI) and assign optimal power for scheduled users. Our obtained findings confirm that the transmit power, RRH density, and the number of users served by these RRHs can significantly affect the EE. Also, the results show that under certain system settings the proposed scheme can enhance the EE by almost 39.6% compared to conventional CoMP without user clustering technique.</span>

Effective Service Architecture and Antenna Allocation Mechanism for CoMP Transmissions in Ultra Dense Networks

A practical and effective service deployment method is proposed for coordinated multi-point (CoMP) transmissions in ultra-dense networks (UDN). Despite the potential of UDN for throughput enhancement, the throughput of edge users is still considerably deteriorated by interferences from adjacent cells. Applying the CoMP transmissions among cells appears to be a feasible solution to resolve this issue. However, the nature of the almost randomly deployed locations of small cell base stations (BSs) and their limited numbers of antennas make it difficult to design an effective resource coordination mechanism for the CoMP services in UDN. Reconsidering this problem from an antenna allocation viewpoint, we present a simple yet effective approach to improve the cell-edge throughput under a centralized radio access network architecture. The first step is to partition the entire service area into multiple subareas, and the second step is to allocate the antenna degrees of freedom to each subarea. To improve the scalability of this service method, a two-tier service deployment method is further introduced based on the BS clustering and service area partitioning of the entire network. The simulation results show that the proposed service architecture can provide a significant improvement on the cell-edge throughput while still maintaining a good average throughput in the UDN.

Downlink Coordinated Multi-Point Transmission in Ultra-Dense Networks with Mobile Edge Computing

Small cell base stations (SBSs) in 5G and beyond networks will play multiple roles in addition to radio access, and will be deployed in a very high density. They can facilitate mobile edge computing (MEC), such that SBSs can store, process, and control network data signals, becoming important elements in ultra-dense networks (UDNs). However, intercell interference may jeopardize the gains from network densification, and thus coordinated multi-point (CoMP) transmission is important in UDNs. MEC and CoMP are different technologies, but they can work jointly to benefit UDNs. In this article, we aim to bridge the gap between MEC and CoMP based on three MEC functions. We will show that CoMP transmission reduces UDN complexity and transmission delay with collaboration among MEC servers in UNDs. In return, MEC-enabled UDNs can alleviate the backhaul pressure on CoMP transmission and make it possible to implement scalable CoMP.

CoMP Transmission for Safeguarding Dense Heterogeneous Networks with Imperfect CSI

To ensure reliable and secure communication in heterogeneous cellular network (HCN) with imperfect channel state information (CSI), we proposed a coordinated multipoint (CoMP) transmission scheme based on dual-threshold optimization, in which only base stations (BSs) with good channel conditions are selected for transmission. First, we present a candidate BSs formation policy to increase access efficiency, which provides a candidate region of serving BSs. Then, we design a CoMP networking strategy to select serving BSs from the set of candidate BSs, which degrades the influence of channel estimation errors and guarantees qualities of communication links. Finally, we analyze the performance of the proposed scheme, and present a dual-threshold optimization model to further support the performance. Numerical results are presented to verify our theoretical analysis, which draw a conclusion that the CoMP transmission scheme can ensure reliable and secure communication in dense HCNs with imperfect CSI.

Interference alignment based hybrid cooperative transmission strategy with limited backhaul

The performance of coordinated multi-point (CoMP) transmission system depends heavily on the quality of information exchange via backhaul links. However, the limited capacity of backhaul leads to quantisation error and delay of shared channel state information. In this study, the authors propose a novel hybrid CoMP transmission strategy to mitigate the impacts of the backhaul delay for data sharing and switch the transmission modes adaptively based on the current arrivals of the shared user data. Furthermore, they present analytical expressions for the average sum rate in an interference alignment network, in which the combining effects of quantisation from limited capacity and connection uncertainty introduced by finite delay are considered. With the analytical results, they evaluate the sum rate performance under limited backhaul for the conventional CoMP transmission modes and the proposed strategy. The simulation results demonstrate that the proposed strategy functions better on overcoming the impacts of capacity limited backhaul than the conventional ones.

Coordinated Multi-Point Downlink Transmission for Dense Small Cell Networks

The small cell network (SCN) is one of the emerging technologies for the next generation wireless systems. It can provide lots of advantages, such as enlargement of cell coverage, boost of peak data rate, improvement of link quality, flexibility of deployment, etc. In SCN, a specific scenario, where the density of small cell base stations (SBS) is much higher than that of user equipments (UE), becomes popular topic recently. Because of the high density of SBSs, it can offer diversity of SBS selection and, hence, improve the system performance. Besides, under this condition, each UE can be served by multiple SBSs, which also motivates the possibility of using coordinated communication schemes such as coordinated multi-point (CoMP) transmissions. Therefore, in this paper, we investigate the SCN with highly dense deployment of SBSs, and suggest a CoMP transmission structure to improve the transmission performance. In addition, for well performing CoMP transmissions, we also propose CoMP set formation method, which selects SBSs to form CoMP set according to the designed metric. We conduct simulations to validate the performance of the SCN with proposed CoMP method. The simulation results show that the proposed CoMP method can indeed improve the dense SCN corresponding to the designed metric. Specifically, the proposed CoMP method can significantly improve sum rate and prompt most of the transmissions to reach the quality of service requirement of the UEs. This method is flexible and easy to be adopted in many kinds of systems, including single-operator or inter-operator ones.

Assessment of Coordinated Multipoint Transmission Modes for Indoor and Outdoor Users at 28 GHz in Urban Macrocellular Environment

Assessment of Coordinated Multipoint Transmission Modes for Indoor and Outdoor Users at 28 GHz in Urban Macrocellular Environment

Joint CoMP Transmission for UAV-Aided Cognitive Satellite Terrestrial Networks

This paper investigates a novel architecture of coordinated multi-point (CoMP) transmission in the cognitive satellite-terrestrial network associated with an unmanned aerial vehicle (UAV). We consider the downlink communication where the UAV and base station (BS) cooperatively serve the terrestrial user by sharing the licensed satellite network spectrum. The goal of this paper is to maximize the achievable rate of the terrestrial user by jointly optimizing BS/UAV transmit power allocation and UAV trajectory, subjecting to the interference temperature threshold imposed on satellite network as well as UAV mobility constraint. The formulated problem is shown in a complicated non-convex form, which is hard to tackle. To this end, we decompose the original problem into two sub-problems, and the block coordinate descent method is employed to solve the two sub-problems alternately. Specifically, for the first sub-problem, the optimal BS/UAV transmit power is obtained with a given UAV trajectory by using the Lagrangian dual method. For the second sub-problem, the UAV trajectory is attained with a given BS/UAV power allocation by using the successive convex approximation technique. Simulation results show that our proposed joint CoMP transmission scheme significantly improves the terrestrial network throughput compared with other benchmark schemes.

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.

Optimization of 5G Virtual Cell Based Coordinated Multipoint Networks Using Deep Machine Learning

Providing seamless mobility and a uniform user experience, independent of location, is an important challenge for 5G wireless networks. The combination of Coordinated Multipoint (CoMP) networks and Virtual- Cells (VCs) are expected to play an important role in achieving high throughput independent of the mobile’s location by mitigating inter-cell interference and enhancing the cell-edge user throughput. Userspecific VCs will distinguish the physical cell from a broader area where the user can roam without the need for handoff, and may communicate with any Base Station (BS) in the VC area. However, this requires rapid decision making for the formation of VCs. In this paper, a novel algorithm based on a form of Recurrent Neural Networks (RNNs) called Gated Recurrent Units (GRUs) is used for predicting the triggering condition for forming VCs via enabling Coordinated Multipoint (CoMP) transmission. Simulation results, show that based on the sequences of Received Signal Strength (RSS) values of different mobile nodes used for training the RNN, the future RSS values from the closest three BSs can be accurately predicted using GRU, which is then used for making proactive decisions on enabling CoMP transmission and forming VCs.

Coordinated Multipoint Transmission in Downlink Multi-Cell NOMA Systems: Models and Spectral Efficiency Performance

We outline a general framework to use CoMP transmission technology in downlink multi-cell NOMA systems considering distributed power allocation at each cell. In this framework, CoMP transmission is used for users experiencing strong received signals from multiple cells, while each cell adopts NOMA for resource allocation to its active users. After a brief review of the working principles of different CoMP schemes, we investigate their applicability and necessary conditions for their use in a downlink multi-cell NOMA system. After that, we discuss various network scenarios with different spatial distributions of users and present the formula for achievable rate of users under each of the CoMP-NOMA scenarios. To this end, a numerical performance evaluation is carried out for the proposed CoMP-NOMA systems, and the results are compared with those for conventional orthogonal multiple access based CoMP systems. The numerical results quantify the spectral efficiency gain of the proposed CoMP-NOMA models over CoMP-OMA. Finally, we conclude this article by identifying the major challenges in implementing CoMP-NOMA in future cellular systems.

A Cache-Aided Communication Scheme for Downlink Coordinated Multipoint Transmission

Content caching at the base stations (BSs) is a promising strategy that can provide improved quality-of-service (QoS) provisioning for content-centric services, and at the same time reduce instantaneous backhaul payload. This paper focuses on the applications of the caching mechanism in the downlink of coordinated multipoint (CoMP) transmission, where the collaborating BSs cooperatively serve the user equipments (UEs). A cache-aided transmission scheme for downlink CoMP with limited local cache resources at the BSs is proposed to improve QoS provisioning in terms of outage performance. In order to cache files more efficiently while guaranteeing the QoS, the contents are divided into two sets, a popular set and a less popular set, according to their popularity profile following the Zipf model. The popular files are cached at all the BSs, while the less popular files are cached only at part of the BSs in a cache cluster subject to the strategy design. Furthermore, based on the popularity of the requested files, UEs are categorized into two groups and the notion of request competition between the two groups is introduced. The corresponding request competition model and admission probabilities for transmission requests are analyzed. Based on the request competition model, the best cache placement strategy for the popular and less popular sets that minimizes the average outage probability is derived. Numerical results show that the proposed cache-aided CoMP scheme outperforms the baseline traditional caching strategies in terms of the average transmission outage.

QoS priority‐based coordinated scheduling and hybrid spectrum access for femtocells in dense cooperative 5G cellular networks

AbstractA third‐generation partnership project long‐term evolution system uses the concept of a 2‐tier heterogeneous network, where low‐power and short‐range femtocells are laid under macrocells to fulfill the quality of service (QoS) requirements of users and to boost overall network throughput. However, cochannel deployment gives rise to the problem of cross‐tier interference that significantly deteriorates the performance of the cellular network. The coordinated multipoint (CoMP) transmission scheme is proposed as a promising solution to alleviate cross‐tier interference by doing cooperation among the neighbor base stations. In this paper, we propose a QoS priority‐based coordinated scheduling and hybrid spectrum access (QoS‐CSaHSA) scheme for downlink CoMP transmission in 2‐tier networks. The proposed QoS‐CSaHSA scheme dynamically reduces the femtocells' power requirements by considering the neighbor cell interference, and it also balances the macrocell load by switching the femtocells to HA mode. Moreover, it can reduce unnecessary muting of the base stations because it only enables the coordinated scheduling CoMP when there is no other possibility to reduce the cross‐tier interference. System‐level simulation results prove the validity of the proposed QoS‐CSaHSA scheme in a heterogeneous network environment. Compared with the existing schemes that operate the femtocells in a closed‐subscriber group without considering users' QoS requirements, the proposed scheme almost doubles the cell‐edge user throughput and reduces the packet loss rate and call‐blocking probability.

Optimal scheduling of coordinated multipoint transmissions in cellular networks

SummaryIn this paper, we study the optimal scheduling problem in coordinated multipoint (CoMP) transmission–based cellular networks. We consider joint transmission and coordinated scheduling together in CoMP transmission–based cellular networks and develop an optimization framework to compute the optimal max‐min throughput and the optimal scheduling of the transmissions to the users. The optimization problem is found to be a complex linear program with number of variables in for a cellular network of N users and K cells. We solve the optimization problem for several network instances using an optimization tool. The numerical results show that the optimal CoMP transmission provides a significant throughput gain over a traditional transmission. We find that in optimal scheduling the fraction time of coordinated scheduling is higher than that of joint transmission. To solve the optimization problem without any optimization tool, we propose a heuristic algorithm. The performance of the heuristic algorithm is evaluated and found to be provided throughput around 97% of the optimal throughput. Further, we extend the optimization framework to study joint scheduling and power allocation (JSPA) problem in CoMP transmission–based cellular networks. We numerically solve the JSPA problem for the network instances and demonstrate that the optimal power allocation at the base stations is not binary for a significant fraction of time of scheduling. However, the gain in max‐min throughput by the optimal JSPA technique over the optimal scheduling technique is not significant.

Reconfigurable Optical Mobile Fronthaul Networks for Coordinated Multipoint Transmission and Reception in 5G

The fifth generation of mobile communication is characterized by ultra-dense cellular networks, where massive small cells are deployed in hot spots to increase the network capacity. As a promising technology in the small-cell scenario, coordinated multipoint (CoMP) is proposed to improve cell-edge user throughput by converting inter-cell interference into useful signals. In this paper, we study how reconfigurable fronthaul enabled by optical networks can help improve the CoMP service. According to the baseband unit (BBU) that serves the purposes of coordination, we classify the CoMP service into intra-BBU CoMP and inter-BBU CoMP. Inter-BBU CoMP will cause data exchange between the BBUs, which requires backhaul to support a large bandwidth and low latency. Our proposal is to associate coordinated RRHs with a single BBU to reduce the data exchange. To achieve this goal, we propose two heuristic algorithms: a minimum-cut graph-based lightpath reconfiguration (MCG-LR) and a BBU weightbased lightpath reconfiguration (BW-LR). The two algorithms are emulated on a time and wavelength division multiplexing passive optical network-based fronthaul under different cell conditions. The simulation results show that the reconfigurability of fronthaul can improve the CoMP service through elastic radio resource allocation. In addition, we can observe that MCG-LR achieves a lower inter-BBU CoMP ratio and lower BBU migration than BW-LR after lightpath (wavelength) reconfiguration.

Performance analysis of relay‐assisted CoMP transmissions with non‐ideal transceiver

Coordinated multi-point (CoMP) transmission is a promising technique for improving spectral efficiency and overall system throughput in future cellular communication systems. This study investigates the downlink performance of relay-assisted CoMP transmissions. The non-ideal transceiver scenario that considers the effects of hardware impairments on the performance of CoMP transmission is presented. The performance of relay-assisted CoMP transmission using a hybrid opportunistic cooperative (HOC) protocol is evaluated and compared for two different CoMP transmission strategies: coordinated beam-forming and joint processing (JP). For each strategy, the performance in terms of outage probability, average sum rate and throughput is studied. The non-COMP strategy is also included for comparison. Both theoretical and simulation results are considered to test the performance of ideal and practical scenarios of both CoMP transmission using HOC protocol and other relaying protocols such as amplify-and-forward (AF) and decode-and-forward (DF) protocols. The obtained results show that the HOC protocol with ideal scenario, significantly improves the average sum rate, the net throughput and the outage probability of CoMP transmission especially when considering the JP strategy. The obtained results also indicate that, the performance degradation due to non-ideal transceiver with HOC protocol is small when compared with AF and DF protocols.

Signal Alignment for Secure Underwater Coordinated Multipoint Transmissions

Underwater acoustic networked systems often consist of geographically distributed antenna elements (DAEs) that are connected via cables or high-rate radio links (e.g., bottom-anchored nodes or surface buoys). This paper investigates countermeasures against eavesdropping attack in the coordinated multipoint (CoMP) transmission of DAEs to an underwater legitimate user. Exploiting the low sound speed in water and the spatial diversity of DAEs, we propose signal alignment for transmission secrecy, where a transmission strategy will be judiciously designed such that useful signals will collide at the eavesdropper while stay collision-free at the legitimate user. Specifically, the transmit DAE set, and the transmission schedule and transmission power of each active DAE are jointly optimized with a goal of minimizing the maximal received signal-to-interference-and-noise ratio (SINR) of useful signals at the eavesdropper, under a lower bound constraint of the received signal-to-noise ratio (SNR) at the legitimate user. Taking the orthogonal frequency-division multiplexing (OFDM) as the modulation technique, simulation and emulated experimental results demonstrate that the proposed method significantly degrades the eavesdropper's interception capability. We further investigate the secrecy capacity and the secure degrees of freedom (d.o.f.) of the signal alignment method from an information-theoretic perspective, which reveals that without external helpers, secure d.o.f. greater than 1/2 can be achieved.