Abstract-This paper addresses the coordinated scheduling problem in cloud-enabled networks. Consider the downlink of a cloud-radio access network (CRAN), where the cloud is only responsible for the scheduling policy and the synchronization of the transmit frames across the connected base-stations (BS). The transmitted frame of every BS consists of several time/frequency blocks, called power-zones (PZ), maintained at fixed transmit power. The paper considers the problem of scheduling users to PZs and BSs in a coordinated fashion across the network, by maximizing a network-wide utility under the practical constraint that each user cannot be served by more than one base-station, but can be served by one or more power-zones within each base-station frame. The paper solves the problem using a graph theoretical approach by introducing the scheduling graph in which each vertex represents an association of users, PZs and BSs. The problem is formulated as a maximum weight clique, in which the weight of each vertex is the benefit of the association represented by that vertex. The paper further presents heuristic algorithms with low computational complexity. Simulation results show the performance of the proposed algorithms and suggest that the heuristics perform near optimal in low shadowing environments. Index Terms-Coordinated scheduling, maximum weight clique problem, optimal and near optimal scheduling. I. OVERVIEW The continuous increasing demand for high data rate services necessitates breakthroughs in network system architecture. With a progressive move towards full spectrum reuse and a positive trend in small-cell deployment, cloud-radio access networks (CRAN) become essential in large-scale interference management for next generation wireless systems (5G) [1] . Through its ability to allocate resources in a coordinated way across base-stations, cloud-enabled networks have the potential of mitigating inter-base-station interference through inter-basestation coordination. This paper investigates the coordinated scheduling problem in a cloud-radio access network, where base-stations are connected to a central processor (cloud) which is responsible for scheduling users to base-stations resource blocks. Recent literature on CRAN assumes signal-level coordination and allows joint signal processing in the cloud [2]- [4] . Such coordination, however, requires high-capacity links to share all data streams between all base-stations, and needs a substantial amount of backhaul communications. This paper considers the CRAN problem from a different perspective, as it only considers scheduling-level coordination at the cloud, which is more practical to implement, and at the same time allows base-stations to schedule users efficiently. Consider the downlink of cloud-radio access network comprising several base-stations connected to one central processor, which is only responsible for the scheduling policy and the synchronization of the transmit frames of all base-stations. The frame structure of every base-station consists of several resource blocks, maintained at fixed transmit power, called power-zones. Across the network, users are multiplexed across the power-zones under the constraint that each user cannot be connected to more than one base-station since, otherwise, signal-level coordination between base-stations is needed. Each user, however, can be connected to several power-zones belonging to the frame of one base-station. Further, each power-zone, which can be in practice seen as a generic term to denote a time/frequency resource block of every BS, serves one and only one user. The coordinated scheduling problem, under fixed transmission power, becomes that of optimally scheduling users to base-stations and their power-zones subject to the above practical constraints, as a means to mitigate interbase-station interference. The paper considers the scheduling problem with an objective of maximizing a generic networkwide utility, where scheduling decisions are carried out by the cloud and coordinated to the base-stations. In the past literature, scheduling is often performed on a per-base-station basis, given a pre-assigned association of users and base-stations, e.g., the classical proportionally fair scheduling [5]-[7]. Unlike the previous works where scheduling is performed with no inter-BS coordination, this paper considers the network-wide scheduling where coordination is carried by the cloud connecting the base-stations. The coordinated scheduling considered in this paper is particularly related to the concept developed in [8] in a soft-frequency reuse setup; however, the problem setup in [8] assumes an equal number of users and power-zones and boils down to a simple linear assignment problem, which can be solved using the classical auction methodology [9] . This paper main contribution is that it solves the coordinated scheduling for any number of users and power-zones by maximizing a generic network-wide utility subject to practical cloud-radio access network constraints. The paper solves the problem using a graph theory approach by introducing the corresponding scheduling graph and reformulating the problem as a maximum weight clique problem, which can be globally solved using efficient algorithms [10], [11] . The paper further proposes heuristic algorithms with low computational
The work in REF proposes a graph theoretic approach for coordinated scheduling problem.
Coordinated scheduling for the downlink of cloud radio-access networks
2015 IEEE International Conference on Communications (ICC)
2015 IEEE International Conference on Communications (ICC)
Mathematics Computer Science