Phase ambiguity mitigation for per-cell codebook-based limited feedback coordinated multi-point transmission systems

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.

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.

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.

In heterogeneous network several types of base station (BSs) are deployed to satisfying user traffic demands of future communication system and improve network performance. The coordinated multi point (CoMP) transmission technique is considered a key feature in future wireless network to improve both cell edge users throughput by exploiting interference. However, to provide CoMP transmission several BSs need to be active, which eventually increases network energy consumption. Therefore, there is need to investigate the effectiveness of CoMP technique for energy efficient communication, especially in heterogeneous network scenario, where different BSs have different characteristics. In this paper, we investigate the performance of wireless system in heterogeneous environment without and with CoMP. In particular, we analysed network outage with its energy consumption efficiency with varied coverage range. Mobiles are assumed to be connected to nearest BSs. Furthermore, we have also investigated energy efficiency vs. outage probability in terms of number of coordinating BSs for joint-transmission enabling sleep mode mechanism.

To further increase spectral efficiency and reliability of wireless communications systems, base station cooperation, or "coordinated multipoint (CoMP)", has become a major trend in physical layer research. We revisit, in a tutorial style, channel estimation algorithms for CoMP schemes in the uplink, i.e., when performing diversity combining using distributed antennas at the base station side. We focus on a single serving base station, and a geometric ring arrangement of a number of supporting base stations. Obviously, the channel estimator has to operate at very low SNR to make best use of CoMP by benefltting from many weak signal contributions. We show that conventional estimators based on least squares approaches, which are reasonable solutions for a single cell set-up, can cause severe degradation in the CoMP scenario. Rather, estimators are required that implicitly de-weight ("pinch-off") unreliable signal contributions. For those MMSE-type estimators, the challenge is practical parameter estimation of the auto - and cross-covariance.

Coordinated multi-point (CoMP) transmission has been considered in the multi-cell, multi-user system in the downlink where each remote radio equipment (RRE) unit transmits data stream(s) to all the user(s) in the CoMP system. CoMP transmission has been shown to improve the performance of the cell-edge users but the associated problem of high data and feedback overhead as well as complexity is still an issue for researchers. The main contribution in this paper is in reducing the data and feedback overhead as well as the complexity involved in designing the precoders at the transmitter or receiver through the use of multiple cooperating sets (CSs) in the CoMP system. We are able to show with our proposed cooperating set scheduling (CSS) algorithm, that we can select a subset of RREs to form multiple CSs in the system and still achieve good performance (as opposed to using the full set of RREs for CoMP transmission).

Coordinated multipoint (CoMP) transmission provides high theoretic gains in spectral efficiency with coherent joint transmission (JT) to multiple users. However, this requires accurate channel state information at the transmitter (CSIT) and also user groups with spatially compatible users. The aim of this paper is to use measured channels to investigate if significant CoMP gains can still be obtained with channel estimation errors. This turns out to be the case, but requires the combination of several techniques. We here focus on coherent downlink JT CoMP to multiple users within a cluster of cooperating base stations. The use of Kalman predictors is investigated to estimate the complex channel gains at the moment of transmission. It is shown that this can provide sufficient CSIT quality for JT CoMP even for long (>20 ms) system delays at 2.66 GHz at pedestrian velocities or, for lower delays, at 500 MHz, at vehicular velocities. A user grouping and resource allocation scheme that provides appropriate groups for CoMP is also suggested. It provides performance close to that obtained by exhaustive search at very low complexity, low feedback cost and very low backhaul cost. Finally, a robust linear precoder that takes channel uncertainties into account when designing the precoding matrix is considered. We show that, in challenging scenarios, this provides large gains compared with zero-forcing precoding. Evaluations of these design elements are based on measured channels with realistic noise and intercluster interference assumptions. These show that high JT CoMP gains can be expected, on average over large sets of user positions, when the above techniques are combined - especially in severely intracluster interference limited scenarios.

In this paper, we propose a novel bit allocation strategy in coordinated multi-cell systems with limited feedback. Multi-cell cooperation technique is an effective way to improve the system throughput and especially the performance of the cell edge users. In a Coordinated Multi-Point (CoMP) system, the rate loss caused by quantization error of Channel State Information (CSI) is minimized considering two aspects of the channels. Firstly, as the physical conditions and constraints of different users are not identical, appropriately allocating feedback bits to users based on their channels can achieve higher throughput. Secondly, in a Coordinated Beamforming/Coordinated Scheduling (CB/CS) system, each user has a serving channel to the serving base station and an interfering channel to the coordinated base station. The user needs to partition its available bits between the two channels. Thus, we study a global bit allocation strategy that can minimize the rate loss from that of the full CSI circumstance. From our simulation results, we prove that the proposed bit allocation strategy achieves higher per-cell sum-rate compared with the equal bit allocation strategy.

On-grid energy consumption of base stations (BSs) contributes up a significant fraction of the total carbon dioxide (CO2) emissions of cellular networks, among which remote radio units (RRUs) absorb most of the energy consumption. To eliminate the on-grid energy consumption and the corresponding CO2 emission, we propose a new transmission framework, in which all RRUs and associated power amplifiers (PAs) are powered by hybrid energy sources including on-grid energy source and off-grid renewable energy source. Based on the framework, we pursue a systematic study on the joint transmission and harvested energy scheduling algorithm for the hybrid energy powered cellular transmission system under coordinated multi-point (CoMP) transmission. Firstly, we formulate an optimal offline transmission scheduling problem with a priori knowledge about channel state information (CSI), under constraint of available amount of harvested energy and stored energy at each transmission time interval. Considering a practical constraint of limited pre-knowledge about CSI, we further transform the offline problem into an energy-aware energy efficient transmission problem. To solve the proposed problems, we undertake a convex optimization method to the optimal offline transmission scheduling problem and design corresponding optimal offline joint transmission and energy scheduling algorithm, which provides the upper bound on actual system performance. Then, we extend the non-linear fractional programming to the transmission scheduling problem with limited pre-knowledge about CSI and design corresponding joint transmission and energy scheduling algorithm, named as online algorithm. Numerical results show that the performance of the proposed online algorithm is close to that of the obtained upper bound and outperforms the existing algorithm. We also find that at each transmission time interval during the finite transmission period, the transmit power of each RRU is proportional to the weighted channel-gain-to-noise ratio (CNR) of each sub-channel.

Switching underutilized base stations (BSs) to sleep mode is recognized as a promising approach to reduce energy consumption of cellular networks, but it may increase the transmit power of remaining active BSs to guarantee service coverage. Coordinated multi-point (CoMP) can effectively reduce transmit power of BSs through BS cooperation but requiring extra power consumption due to extra signal processing and backhaul traffic. In this paper, we investigate the energy efficiency of BS sleeping combined with CoMP. A joint power and subcarrier allocation algorithm is proposed to minimize the overall network power consumption with minimum data rate constraints, which can be implemented distributedly across multiple clusters. Simulation results show that BS sleeping combined with CoMP can improve network energy efficiency for high data rate users compared with Non-CoMP systems without BS sleeping.

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.

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.

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

In this paper, we consider the transmission coordination among multiple cell sites, namely the Coordinated MultiPoint (CoMP) transmission scheme, where several Base Stations (BSs) cooperate to serve a set of Mobile Stations (MSs) and both BSs and MSs are equipped with multiple antennas. We model this multicell cooperative system under the presence of channel estimation errors that can significantly influence the obtained results. We derive the exact sum rate expression of the multicell cooperative system in the presence of channel estimation errors and we evaluate the performance of the Block-Diagonalization (BD) and Zero-forcing (ZF) beamforming approaches. Furthermore, we investigate a power allocation policy that maximizes the system sum rate in the presence of channel estimation errors. We show that in the multicell cooperative system, the performance of the BD or ZF beamforming approaches in terms of the maximization of the system sum rate, depends essentially on the system configuration parameters (i.e., number of antennas per BS and per MS). Simulation results are given to support our claims.