Technical Report PHD-2013-06

Title: Scheduling Algorithms for OFDMA Broadband Wireless Networks
Authors: Guy Grebla
Supervisors: Reuven Cohen
Abstract: In this thesis, we define and study problems related to scheduling in OFDMA wireless networks. In such networks, the BS (Base Station) receives packets destined for its mobile stations. Downlink bandwidth is used to transmit the packets, and since this bandwidth is a limited resource, a careful optimization is required.

We start by addressing the problem that arises when the BS wishes to multicast information to a large group of nodes and to guarantee a certain level of reliability. The problem is to determine which MCS (Modulation and Coding Scheme) should be used by the BS for each packet. We present several variants of this problem, which differ in the number of rounds during which the information delivery must be completed.

A crucial step in the evolution of broadband wireless (cellular) networks is reducing the size of the cells and increasing their number. This target is usually obtained using cell sectorization, where the omni-directional antenna at each BS is replaced by 3 or 6 directional antennas. While each sector can run its own scheduling algorithm, bandwidth utilization can be significantly increased if a joint scheduler makes these decisions for all the sectors. This gives rise to the “joint scheduling" problem, addressed in this thesis for the first time.

LTE-advanced and other 4G OFDMA standards allow relay nodes (RNs) to be deployed as a substitute for BSs. Each RN is associated with a donor BS, to which it is connected through the wireless link. In a network with RNs, packet scheduling decisions must be made in each cell not only for the BS, but also for the RNs. Because the scheduler in a network with RNs must take into account the transmission resources of the BS and the RNs, it needs to find a feasible schedule that does not exceed the resources of a multi-dimensional resource pool. This makes the scheduling problem computationally harder than in a network without RNs. In this thesis we define and study this scheduling problem for the first time.

Advanced OFDMA technologies such as MIMO require each mobile station to send many feedback messages to the BS. This feedback consumes much of the uplink bandwidth, mainly because it is sent periodically. Therefore, the uplink bandwidth to these indicators must be allocated very carefully, while achieving certain optimization objectives. We propose a framework for the allocation of periodic feedback channels to the nodes of a wireless network, and scheduling algorithms that allow the BS to optimize this allocation.

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