Comp Transmission in Cloud Radio Access Networks

In 5G networks, mitigating blockage effects in millimeter-wave (mmWave) signals is a major challenge to overcome. Coordinated multipoint (CoMP) transmission is one of the inference mitigation schemes that can be used in a densified cellular network. CoMP transmission is usually implemented within a cloud radio access network (C-RAN) via fronthaul links which is connected to a control unit (CU). In this paper, a radio unit clustering scheme (RCS) was proposed based on CoMP transmission to mitigate blockage effect of mmWave links. Applying a clustering scheme in C-RAN can increase the network capacity by avoiding data transmission via non-line-of-sight (NLOS) links. Based on stochastic geometry, the coverage probability of the proposed RCS is derived into a closed-form expression by modeling the position of base stations as a Poisson point process. In addition, an enhanced cooperative handover scheme is proposed to increase mobility robustness of the proposed RCS. Simulation results show that the coverage probability is enhanced as the cluster size increases in mmWave C-RANs and a radio link failure (RLF) probability during handover procedure is reduced.

Coordinated multi-point (CoMP) transmission is considered as an efficient technique to improve cell-edge performance as well as system spectrum efficiency. In CoMP-enabled systems, a cluster of coordinated base stations (BSs) are typically assumed to be connected to a control unit (CU) via backhaul links, and the provided performance gain relies heavily on the quality of the channel state information (CSI) available at the CU side. In this paper, we consider the downlink of a CoMP cluster and compare three different CoMP transmission schemes: zero-forcing coherent joint transmission, non-coherent joint transmission and coordinated scheduling. Moreover, for each of the analyzed schemes, the performance in terms of average sum rate of the CoMP cluster is studied with predicted CSI, considering the effects of the feedback and backhaul latency, as well as the user mobility. Compared to zero-forcing coherent joint transmission, we show that non-coherent joint transmission and coordinated scheduling are more robust to channel uncertainty. In addition, depending on the latency, user mobility and user locations, different schemes would achieve the highest average sum rate performance. Hence, a system could switch between the transmission schemes to improve the sum rate.

Joint virtual edge-clustering and spectrum allocation scheme for uplink interference mitigation in C-RAN

Coordinated multi-point (CoMP) transmission is an effective means of improving network throughput in heterogeneous cellular networks (HetNets). However, its performance is seriously weakened if imperfect coordination happens between base stations (BSs). Many prior CoMP works do not consider inter-cell overhead message delays such that a seemingly astonishing CoMP throughput gain is attained. In this paper, the quantization error and delay that actually exist in overhead messages was modeled and we developed a much tractable SIR model based on the stochastic geometry framework. We proposed adaptive CoMP that is applied to downlink zero-forcing beamforming (ZFBF) and it can mitigate the interference from the coordinated cells with delayed overhead messages. The bounds on the complementary cumulative distribution function (CCDF) of the SIR of a user are characterized such that the average throughput of a user is able to be analytically evaluated. Numerical results show that the proposed adaptive CoMP scheme can make the throughput gain very robust to the overhead delay and thus significantly increase the throughput even when BSs are not perfectly coordinated.

In Heterogeneous Networks (HetNets) co-channel inter-cell interference (ICI) is a serious concern. Coordinated Multi-Point (CoMP) transmission has been emerged as a promising solution to downlink ICI in HetNets. But, the performance of CoMP transmission largely depends on the nature of backhaul network over which huge X2 signaling are executed among the coordinated cells. In this context, this article investigates the most advanced CoMP technique, called Joint Transmission CoMP (JT-CoMP), in LTE-Advanced HetNets under unreliable backhaul network and compares the results with perfect backhaul. The cluster throughput is measured by means of the Link Failure Probability (LFP) of the X2 interface. Here, we test three different types of JT-CoMP architectures, namely centralized JT-CoMP, semi-distributed JT-CoMP and fully-distributed JT-CoMP. In addition, this study considers only non-coherent JT-CoMP transmission, and explicit Channel State Information (CSI) feedback mechanism is utilized among the coordinating cells. The result shows that the centralized and semi-distributed architectures of JT-CoMP perform differently at different LFP condition, whereas the fully-distributed JT-CoMP remains constant regardless the backhaul LFP. Finally, we briefly discuss the practical challenges of the targeted JT-CoMP schemes under this investigation setup.

This paper demonstrates if the utilization of short-term channel information, i.e. channel state information (CSI) feedback, is beneficial in the coordination of multiple evolved Node Bs (eNBs) even with the practical non-ideal backhaul. Two CSI feedback delivery schemes, namely user equipment-specific and cell-specific CSI process configuration schemes, are addressed and compared. From the comparison, it is observed that the cell-specific CSI process configuration scheme exhibits acceptable performance with small overheads. Based on the cell-specific CSI process configuration, it is shown that the short-term channel information conveyance through the non-ideal backhaul is factually feasible. In addition, simulation results comparing performance of the inter-eNB coordinated multi point (CoMP) operations with short-term/long-term channel information and only with long-term channel information, verify that the use of the short-term channel information is advantageous. Finally, a signaling model for inter-eNB CoMP using short-term/long-term channel information is illustrated based on the X2 application protocol.

This article performs a unified analysis of three coordinated multipoint (CoMP) transmission strategies in the downlink of mmWave cellular networks, including the fixed-number base station (BS) cooperation (FNC), the fixed-region BS cooperation (FRC), and the interference-aware BS cooperation (IAC). We first develop a comprehensive framework for CoMP operation in cellular networks, and investigate the network performance under a Poisson point process (PPP) model together with mmWave spectrum. To show what fraction of users in the network achieve target reliability for a given signal to interference-plus-noise ratio (SINR)/signal-to-interference ratio (SIR), we derive the SINR/SIR meta distributions, and further obtain the coverage probability as well as mean local delay for the three cooperation strategies. A pivotal intermediate step to compute the performance metrics is the derivation of joint distributions of distances between a typical user and cooperative BSs. Our analysis demonstrates that parameters of blockage have a significant influence on the network performance for the three CoMP schemes. We find that the FRC scheme makes more users achieve the given link reliability for the scenario with a low density of BSs, while the IAC scheme provides better performance for the network with a high density of BSs. Moreover, the optimal CoMP scheme can be approximately selected by considering the nearest distance from the serving BS to user and the radius of the approximate line-of-sight (LoS) region in the cellular networks.

Introduction It is envisioned that the capacity demand in fifth generation (5G) wireless networks will increase by 1000 times before the year 2020, and this huge demand will be fueled by high-definition video and content streaming applications, which not only consume immense wireless bandwidth but also have stringent real-time quality of service (QoS) requirements. Small-cell dense wireless networks are regarded as a key candidate 5G technology to meet such aggressive demand. By dense deployment of small-cell base stations (BSs), the network becomes closer to mobile users, and thus the spectral efficiency per unit area can be significantly improved. Moreover, relay stations (RSs) without wired backhaul can also be deployed to enhance the coverage area and provide signal-to-noise ratio (SNR) gain. While there are many potential opportunities associated with dense wireless networks, the potential spectrum efficiency gain they can provide is significantly limited by increasingly severe interference due to the densification of BSs. There are several common schemes to mitigate interference. In the strong-interference case, channel orthogonalization, such as frequency division multiple access (FDMA) or time division multiple access (TDMA), is used to avoid interference [1], and advanced schemes, such as interference alignment (IA) [2, 3] and interference coordination [4, 5], have been proposed to increase the spectral efficiency of interference channels by jointly mitigating the interference using shared channel state information (CSI). However, these approaches may lead to inefficient use of channel resources because the transmitters do not share payload information and therefore are not fully cooperative. To improve the spectrum efficiency further, coordinated multipoint (CoMP) transmission has been proposed as one of the most important core technologies for Long Term Evolution-Advanced (LTE-A) and future 5G wireless networks [6]. By sharing both CSI and payload data among the BSs concerned, CoMP transmission can transform the wireless network from an unfavorable interference topology to a favorable broadcast topology, where the interference can be mitigated much more efficiently. However, the conventional CoMP scheme is quite costly because it requires a high-capacity backhaul for payload exchange between the BSs, and this poses a huge challenge for practical applications of CoMP, especially for small-cell dense wireless networks.

Coordinated Multipoint (CoMP) has been proposed as a promising way to enhance cell-edge performance in terms of throughput and coverage. Significant performance gain can be achieved in LTE-A through base station (eNB) coordination by exchanging information such as user data, channel state information (CSI) and scheduling decision. Multiple antennas technique (e.g., MIMO) on the other hand exploits antenna diversity which allows multiple simultaneous transmissions to a group of active users to enhance network performance. Therefore, user selection is important in maximizing system performance. Besides, system resources (e.g., power, bandwidth, transmission rate) in MIMO system must be managed efficiently. Optimal resource utilization should be used without performance compromise. In this paper we propose a joint approach of MIMO user selection strategy and optimal water filling (WF) power allocation in CoMP (JP) transmission. Simulation study shows that our proposed approach outperformed traditional CoMP with and without water filling power allocation.

Cloud radio access networks (C-RANs) have drawn considerable interests due to the significant improvements of spectral and energy efficiencies. Since most signal processing functions are moved to the centralized baseband unit (BBU), remote radio heads (RRHs) in C-RANs can be regarded as soft relays to transfer the received signals. The centralization characteristics in C-RANs make traditional channel estimation and training design approaches inefficient, and the requirements of perfect channel state information (CSI) would not be satisfied in turn. To solve this problem, a segment training based individual channel estimation scheme and the corresponding training design are proposed for C-RANs in this paper. Particularly, the channel estimator in terms of the sequential minimum mean-square-error (MMSE) is developed through a prior knowledge of long-term channel correlation statistics and previous channel estimates. The optimal training design for the developed estimator is derived by minimizing the estimation mean-square-error (MSE). Further, the optimal training design for the channel estimation of radio access links is computed by applying the eigenvalue decomposition (EVD). Numerical results show that performance gains of the proposed channel estimation and training design schemes are significant.

In cellular networks, a local uplink scheduler cannot compute good estimates of the inter-cell interference even with exact channel state information (CSI). Losses (due to decoding errors) will hence frequently occur. The maximum achievable performance (MAP) can only be achieved when all the cells are scheduled simultaneously centrally so as to manage interference and power optimally. In this paper, we focus on the performance of a practical system with local schedulers (i.e., each cell is scheduled independently) and compare it to MAP. We also study how to improve the performance of practical systems when a Cloud Radio Access Network (C-RAN) is present, with simple coordination schemes. To this end, to compute MAP, we first formulate an offline system-wide scheduling problem and transform it into a more tractable signomial problem that we solve quasi-optimally using an iterative algorithm. Since this problem requires all the channel information in the system and it has a high computational complexity, it is not suitable for a real-time system. Then, we show that a practical system using an efficient local scheduler (in each cell) yields a much lower performance than MAP. To decrease this performance gap, in a system with a C-RAN, we propose a very simple and fast scheme to coordinate the scheduling in all cells and show that it improves the performance significantly (in terms of throughput and losses) even when only partial CSI is available.

<p><strong> </strong><a href="https://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=7924448">2017 IEEE Wireless Communications and Networking Conference (WCNC)</a></p>

Chapter 2 - 5G Wireless technologies

Limited feedback techniques, that employ codebook-based quantisation, are used in coordinated multi-point (CoMP) transmission to convey the channel direction information between the mobile user and each base station in the cooperative set and also among the cooperating base stations. However, unlike single-cell multiple-user multiple input multiple output (MU-MIMO) systems, CoMP systems face additional challenges in this codebook design. Among these challenges are the huge feedback overhead and the need for dynamic codebook size. These problems are solved when CoMP uses the so-called per-cell codebook, in which the channel to each cell is quantised separately. However, this results in a further problem, that is having random phase values between the quantised per-cell channels of the same user. In this study, the phase ambiguity problem is explained and its effect on performance degradation is quantified. A new quantisation technique is proposed where the quantisation of each of the real and the imaginary parts is performed independently using the same real codebook. This novel solution prevents the phase ambiguity problem instead of trying to mitigate its effects, in addition to simplifying the codebook design. The proposed quantisation technique outperforms two recently introduced solutions in the literature. The first solution is to quantise phase ambiguity as part of channel state information (CSI), whereas the other avoids phase ambiguity by an iterative selection procedure. Mathematical validations and simulations are provided to verify these findings.

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