The design of efficient wireless fronthaul connections for future heterogeneous networks incorporating emerging paradigms such as cloud radio access network (C-RAN) has become a challenging task that requires the most effective utilization of fronthaul network resources. In this paper, we propose to use distributed compression to reduce the fronthaul traffic in uplink Coordinated Multi-Point (CoMP) for C-RAN. Unlike the conventional approach where each coordinating point quantizes and forwards its own observation to the processing centre, these observations are compressed before forwarding. At the processing centre, the decompression of the observations and the decoding of the user message are conducted in a successive manner. The essence of this approach is the optimization of the distributed compression using an iterative algorithm to achieve maximal user rate with a given fronthaul rate. In other words, for a target user rate the generated fronthaul traffic is minimized. Moreover, joint decompression and decoding is studied and an iterative optimization algorithm is devised accordingly. Finally, the analysis is extended to multi-user case and our results reveal that, in both dense and ultra-dense urban deployment scenarios, the usage of distributed compression can efficiently reduce the required fronthaul rate and a further reduction is obtained with joint operation. Heterogeneous network (HetNet), fronthaul, uplink Coordinated Multi-Point (CoMP), cloud radio access network (C-RAN), distributed compression. The astronomical growth in mobile data traffic volume is making it increasingly difficult for operators to meet the traffic demands with the available resources and network architecture. This remarkable growing momentum of improving the quality of experience of the users will most likely continue due to the emerging needs of connecting people, machines, and applications through mobile infrastructure. Extrapolation of current growth trend predicts that the future networks are required to be prepared to support up to a thousand fold increase in total mobile broadband traffic by 2020 [1] . In light of this trend, heterogeneous network (HetNet), where small cells (SCs) are deployed to provide improved coverage and capacity to highly concentrated users, is envisaged as one of the key solutions to economic and sustainable future networks [2]-[4]. However, each added small cell has a profound effect on overall network performance, both with the increased capacity it brings as well as the large amount of co-channel interference it generates when sharing the same frequency [5] . One of the ways to improve the spectral efficiency of the system and mitigate the adverse effect of the huge interference is the use of advanced Coordinated Multi-Point (CoMP) topologies among the SCs in order to combat the generated interference using interference management or interference alignment-based transmission solutions [6]-[8]. Figure. 1 A C-RAN Architecture with wireless front/fronthaul links As one of the most promising potential solutions to efficiently conduct inter-cell coordination, Cloud radio access network (C-RAN) recently attracted a great deal of attention [7]-[13]. C-RAN consists of a large number of spatially separated SCs and a centralized data processing centre as shown in Fig. 1 , where the SCs are simplified and activated wherever and whenever the demand arises and some or all other functionalities, e.g., digital baseband processing, are centralized [9]- [11] . In this paradigm, CoMP is the key to enable increased spectral resource usage as shown in Fig. 1 [12] - [13] . Nevertheless, this gain is expected to be harvested at the cost of high-capacity optical fibre fronthaul links (also Wireless fronthaul Interfering SC Coordinating Area C-RAN Coordinating SC called as fronthaul) connecting the SCs and the processing centre [9]-[10]. However, for those locations where existing fibre-access is unavailable and new fibre-access is difficult or expensive to be deployed, wired fronthaul links need to be replaced by wireless fronthaul solutions. However the wireless fronthaul links are usually with limited capacity, hence the design of fronthaul networks and efficient usage of fronthaul resources are of paramount importance to support ultra-dense SCs in a spectrum, cost, and energy efficient manner. While most of the previous research on uplink CoMP assumes lossless fronthaul links with infinite capacity and the obtained results are only of theoretical interest, some researchers have brought attention to realistic fronthaul links with capacity limit [14]-[17]. However, in these works the coordinating SCs are assumed to simply quantize their own received signals before forwarding them to the processing centre via the fronthaul links. Considering the fact that the observations of all coordinating cells are actually correlated because they are broadcasted from the same source, distributed Wyner-Ziv compression can be applied to exploit this correlation and reduce the required fronthaul link rate. Distributed Wyner-Ziv compression is proposed in [18]- [19] and further extended to multiple sources by Gastpar in [20] . In distributed compression, each coordinating SC compresses its own received signal and at the processing centre the correlation between the receptions of all coordinating SCs will be exploited to decompress and reconstruct their observations which will then be used to decode the user message. Although the information-theoretic fundamentals behind the associated schemes are well known, the formulation of the optimization problem to optimize compression at each SC has not been addressed. In [21] , distributed compression is investigated in a multi-relay scenario but the system assumptions are different since only one source is considered. In [22] , multiple users transmit independently to a singling destination via a relay node. However, only one relay node is assumed whereas in our work we focus on multiple relays nodes or SCs. Side information is also explored for data dissemination in [23] but the considered system model and applicable scenarios are different from ours in the sense that no relay nodes are involved. The main contributions of this work can be summarized as follows:  An uplink CoMP framework incorporating capacity-limited multiple fronthaul links is considered, which can be easily employed in the C-RAN architecture;  Under the proposed framework, distributed Wyner-Ziv compression is employed and optimized to maximize user rate with limited fronthaul capacity. On the other hand, the optimization can be interpreted as minimization of required fronthaul rate for a given target user rate. An iterative algorithm is designed to conduct the optimization task. Moreover, we consider joint operation of decompression and decoding and investigate its performance;  In addition to the single user case, the analysis is further extended to multi-user case. Another optimization problem is formed and decoupled into a simpler formation. A new iterative optimization algorithm is proposed accordingly;  Two typical uplink CoMP application scenarios with practical deployment and noise and interference assumptions are considered to yield insight into not only the efficiency of proposed schemes but also the applicability of uplink CoMP.
Finally, the impact of CoMP utilization and specifically of the required coordination traffic onto the C-RAN is investigated in REF , where Qi et al. evaluate the benefit of data compression in the context of wireless fronthauling.
Fronthaul data compression for Uplink CoMP in cloud radio access network C-RAN
Trans. Emerg. Telecommun. Technol.
Trans. Emerg. Telecommun. Technol.
Computer Science