Towards energy efficiency with uplink cooperation in heterogeneous networks

Wireless networks face big capacity challenges, struggling to meet ever increasing user data demands. Global mobile data traffic grew by 74% in 2015 and it is expected to grow 8-fold by 2020. Future wireless network will need to deploy massive number of small cells and improve spectral efficiency to cope with this increasing demand. Dense deployment of small cells will require advanced interference mitigation techniques to improve spectral efficiency and enhance much needed capacity. Coordinated multipoint (CoMP) is a key feature for mitigating inter-cell interference and to improve throughput and cell edge performance. In this chapter, we first provide the motivation for CoMP deployment from an operator perspective. Then, we discuss different types of CoMP schemes, their associated challenges and the third generation partnership project (3GPP) standardisation roadmap. Next, we provide insights into operational requirements for CoMP implementation and discuss potential solutions to enable cost-effective CoMP deployment. We then provide results for an intra-site uplink (UL) joint reception (JR) CoMP trial in a large Long-Term Evolution-Advanced (LTE-A) operator in the United Kingdom (UK) for three different deployment scenarios namely, dense, medium density and sparse deployment at macro layer. CoMP sets consist of co-located cells only with joint baseband processing units, and hence no backhaul is required for data exchange. On the other hand, interference between cells from different locations are not mitigated. Only two cells are allowed for coordination and interference rejection combining (IRC) is employed for joint processing. Trial performance is measured based on average network counters. Results show an average increase in signal-to-interference-plus-noise ratio (SINR) on physical uplink shared channel (PUSCH) by 5.56% which is then reflected in improved usage of higher modulation schemes and better UL user throughput. An average increase of 11.32% is observed on UL user throughput. Additionally, we discuss the limitations of the trialled CoMP scheme and suggest improvements for better CoMP gains. Furthermore, we review the evolution of CoMP into 5G and potential improvements CoMP can provide for some of the key 5G network objectives such as spectral efficiency, energy efficiency, load balancing and backhaul optimisation.

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 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.

In this paper, we consider a relative received link power (RRLP)-based coordinated multi-point (CoMP) joint transmission (JT) in the multi-tier ultra-dense networks (UDN). In this CoMP scheme, we identify the cooperating base stations (BSs) by comparing the average received link power (ARLP) of the neighbouring BSs with respect to the BS having the strongest ARLP (i.e., the main link BS) to a user. To analyze the performance of this CoMP scheme in the downlink multi-tier UDN, we first approximate the received signal power distribution, and derive the coverage probability using stochastic geometry. After revisiting the area spectral efficiency (ASE) to make it more suitable for CoMP transmission in UDN, we also analyze the ASE and the network energy efficiency (NEE). Using simulations, we validate the derived coverage probability, and investigate the CoMP performance in multi-tier UDN. Our simulations show that the RRLP-based CoMP scheme can outperform the fixed number of strongest BS-based CoMP scheme in the high BS density regime. Our study of the NEE performance reveals that not only the RRLP-based CoMP scheme is more efficient than conventional non-CoMP transmission scenario, but also its NEE performance improves with the average number of cooperating BSs.

An efficient direct feedback method from coordinated base stations (BSs) to a mobile station (MS) for an uplink (UL) coordinated multipoint (CoMP) reception with supporting hybrid automatic-repeat-request (H-ARQ) transmission is proposed. Acknowledgement (ACK)/negative acknowledgement (NACK) feedback based on local detection at each BS in the CoMP set is directly transmitted from the BSs using shared-feedback resources. Therefore, an MS can obtain global ACK/NACK information without experiencing extra backhaul delay with a small feedback overhead, and we can reduce the H-ARQ retransmission latency for the inter-cell UL-CoMP operation. To minimize the ACK/NACK detection error at the MS, we investigate the optimized design methods for shared-feedback resources. We derived the analytical detection error probability of the proposed feedback method and the optimum sub-block size to minimize misdetection probability for a fixed overhead and false alarm rate. Based on the analytical and computer simulation results, we have verified that the proposed feedback method can provide a reliable detection performance with a small feedback overhead. In addition, system level simulation results show the proposed fast feedback based inter-cell UL-CoMP system increases the system throughput compared to the conventional intra-cell UL-CoMP without increasing the feedback latency.

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.

Co-channel interference is considered as one of the most important issues limiting the performance of cellular systems. In LTE-A systems, the Coordinated Multi-Point (CoMP) transmission technology was introduced to effectively manage data traffic at the transmission nodes for reducing co-channel interference. CoMP transmission is realized by exchanging coordination information via backhaul links, typically available between the transmission nodes. However, in practical networks the backhaul links may have capacity constraints hindering the use of CoMP techniques with high-rate information exchange. In scenarios where ideal backhaul link is not available, coordinated scheduling and coordinated beamforming (CS/CB) CoMP schemes can be used. However, the conventional CS/CB CoMP scheme only assumes interference coordination between the transmission nodes which significantly limits the expected performance benefits that can be additionally achieved by using traffic management techniques. In this paper, we propose a CS/CB CoMP scheme with semi-static traffic offloading. Traffic management in the proposed scheme is achieved by using the cell range expansion concept that takes into consideration a limited set of the transmission points that can be simultaneously coordinated to reduce interference. For comparison, the LTE-A time-domain enhanced inter-cell interference coordination (eICIC) scheme with conventional cell range expansion is considered.

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.

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.

In this paper, we pursue a unified study on energy harvesting and coordinated multi-point (CoMP) enabled wireless communication by investigating a new joint energy and communication cooperation approach. We consider a practical CoMP system with clusters of multiple-antenna base stations (BSs) each powered by hybrid power supplies (including both the conventional grid and renewable energy sources) cooperatively transmitting to multiple single-antenna mobile terminals (MTs). We propose a new design paradigm termed energy cooperation among BSs within each cluster, which share energy for more efficient cooperative transmission via injecting/drawing power to/from the grid with a zero-sum net energy transfer. We maximize the downlink sum-rate for all MTs in one particular CoMP cluster with cooperative zero-forcing precoding at BSs subject to a new type of transmit power constraints featuring energy cooperation among BSs with a given loss ratio. To jointly optimize the precoders at BSs and the amount of energy transferred among them, we propose an efficient algorithm by applying the techniques from convex optimization. By simulations, we show that the proposed joint communication and energy cooperation solution substantially improves the downlink throughput for energy harvesting CoMP systems, as compared to suboptimal designs without communication and/or energy cooperation.

With the ever increasing usage of wireless devices, the power consumption and energy efficiency of cellular networks have become important performance indicators. In recent years, various types of energy saving schemes have been proposed for cellular networks. However, most of these schemes do not take advantage of the advanced features offered by the recent cellular standards, such as the 3GPP release 10 (also known as LTE-Advanced) and beyond. One of the recently proposed energy saving schemes in cellular networks is the cell switch off technique in which a lightly loaded cell is completely switched off and the traffic in that region is absorbed by the nearby cells whose transmit powers are increased to enable larger coverage areas. In this paper, we propose to use the cell switch off scheme without increasing the transmit powers of the active cells; instead, we propose to use the coordinated multipoint (CoMP) transmission radio technology, which is being standardized for the beyond-LTE cellular networks, to enable a sufficient received power level from a number of nearby cells whenever possible. Through simulations with realistic parameters, we demonstrate that the above described cell switch off + CoMP combination used with proper CoMP active set degree yields a more energy efficient solution with better received user experience in comparison to the traditional cell switch off schemes. In the proposed scheme, channel estimations are needed in order to determine the CoMP active set accurately. We analyze the performance by modeling the channel estimation error as a Gaussian random variable with 4, 8, and 12 dB standard deviation and observe that perfect channel estimation can provide up to 46% capacity and 24% energy efficiency increase in comparison to the scenarios with the channel estimation errors.

In this paper, we present a novel transmission and reception scheme in LTE heterogeneous networks (HetNets) that exploits the concept of Coordinated Multipoint (CoMP). In the proposed scheme, the uplink (UL) and downlink (DL) transmissions of a User Equipment (UE) are established with different cells assuming the existence of coordination between the cells. The scheme considers the link quality, cell load and Quality of Service (QoS) when deciding in the UL and DL's transmissions separation. The objectives of this scheme are to improve the performance at the cell edges through cell coordination and to increase the spectrum efficiency. The OPNET Modeler event-driven system level simulator was used to verify the feasibility of the scheme. The results show that the proposed scheme provides a significant improvement in the packet loss and delay.

A novel multi-stream carrier aggregation (MSCA) enabled coordinated multi-point (CoMP) framework in heterogeneous cellular networks (HetNets) is proposed. Our objective is to maximize the energy efficiency (EE) of the proposed framework by jointly optimizing base station (BS) clustering, user association, sub-carrier assignment and power allocation. The formulated problem is proved to be NP-hard. By analyzing the structure of the original problem, we decouple it into power allocation sub-problem and sub-carrier assignment sub-problem. For the power allocation sub-problem, we propose sequential minimization technique and Zoutendijk feasible direction method to find the optimal power allocation with fixed sub-carrier assignment. For the sub-carrier assignment sub-problem, we propose genetic algorithm (GA) to represent the BS clustering and sub-carrier assignment, and design two GA based joint optimization frameworks. One is using the power allocation algorithm for fitness evaluation and guiding the evolution of GA. Another is optimizing these two sub-problems in an iterative manner, whereas the optimal sub-carrier assignment is updated timely as any change of the power allocation. Numerical results validate the feasibility, fast convergence, and flexibility of the proposed algorithm, and is superior to conventional MSCA or CoMP enabled HetNets.

For coordinated multi-point (CoMP), sets of base stations (BSs) have to be selected to jointly serve user equipments (UEs). These sets are typically selected based on wireless characteristics only. However, using CoMP also poses strict capacity and latency requirements on the backhaul network. Hence, these requirements additionally need to be taken into account when selecting BSs for CoMP. We have developed a mixed integer linear program and a BSs selection heuristic for CoMP that takes into account both aspects: the wireless channels and the backhaul network status. This heuristic can also identify which bottlenecks in the backhaul network make a desired BSs selection infeasible. We exploit this to dynamically reconfigure the backhaul network to the wireless requirements. Our simulations show that the heuristic’s solution quality is close to the optimum while execution time and memory consumption are reduced by multiple orders of magnitude compared to solving the problem via mathematical optimization. In addition, we simulate the network reconfiguration in different backhaul network scenarios. The results illustrate how our approach helps to better exploit available backhaul resources.

Due to the increased energy consumption of cellular access networks, energy efficiency of the systems should be considered jointly with spectral efficiency to obtain the overall performance metrics and trade-offs. Downlink coordinated multipoint (CoMP) joint transmission aided cell switch off schemes can mitigate inter-cell interference and increase energy efficiency by using the active cells to serve the users in the switched off cell. However, the performance of this newly proposed scheme is heavily dependent on the accuracy of the selected CoMP joint transmission set. In this paper, we model the multi-point channel estimation enabled via channel state information reference symbols (CSI-RS) introduced in 3GPP release 10 systems and simulate possible scenarios that would lead to inaccurate transmission set clustering: multi-point channel estimation errors and possible CoMP system delays due to CSI transfers, node processing delays and network topology limitations. In order to mitigate the effects of channel estimation errors and system delay, we propose a framework for multi- point channel estimation in CoMP systems using a time-varying interpolation filter which tracks each multipath delay tap separately for every measured point. Possible performance gains with different filter lengths are demonstrated. Simulation results are presented to show the effectiveness of the proposed scheme. In addition, proof of concept is provided for CoMP adaptive time-varying multi-point estimation filter designs, where the UEs which are being served by higher cluster degrees need to enlarge the estimation filter memory spans only for the points which are more likely to be included in the joint transmission set.