Control traffic overhead for VoIP over LTE

Abstract: With increasing technological advancements more sophisticated mobile devices are being used by end-users. Third generation (3G) mobile communication systems such as Universal Mobile Telecommunication System (UMTS) are not able to satisfy the rising demand for higher throughputs and low latencies. New standards based on Orthogonal Frequency Division Multiplexing (OFDM), such as Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX), have been proposed and are currently being integrated into existing mobile networks all over the world. LTE specifications are being finalized within the 3rd Generation Partnership Project (3GPP) with the ambitious goals of increased spectral efficiency and end user throughput. Despite the introduction of several high data rate services, voice communication is still an essential part of the overall wireless wide area cellular communication market. In LTE, the core network is purely packet switched, thus voice is transmitted entirely using a Voice over Internet Protocol (VoIP). Like its predecessor standards it is desired that a large number of simultaneous VoIP calls be supported in LTE, while satisfying the desired Quality of Service (QoS) demands. This thesis examines issues related to VoIP capacity for LTE. One of the key challenges is the limited number of schedulable voice packets per sub frame. The main goal of this thesis is to quantify the impact of this limitation. After describing basic LTE concepts, a detailed description of the control channel resource limitations for the scheduling of voice packets is presented. Consequences of these limitations are explained systematically by presenting the problem in a wider context. Simulation results were obtained using the openWNS Simulator, an event driven system level simulation platform developed at the Communication Networks Research Group (ComNets), RWTH Aachen University Germany. Results are presented showing the impact of different scheduling strategies on VoIP capacity. These results illustrate how the limited control channel resources, specifically the Physical Downlink Control Channel (PDCCH) resources, affect the total number of schedulable VoIP user audio media streams.