Improving Performance for Streaming Video Services over CDMA-Based Wireless Networks

In video streaming services, the playout begins when the queue length of the receiver buffer is above a threshold. This threshold must be large to reduce the buffer underflow probability and absorb the bit rate variations caused by the wireless channel. On the other hand, it is important to reduce this threshold in order to reduce the initial playout delay and also the size of the receiver buffer. In this paper, a video streaming service is considered, where the last link is a wireless CDMA-based link and it has been shown how a truncated power control allows to reduce the pre-roll delay without increasing the average transmission power and without degrading the video quality. The paper presents the proposed truncated power control and an analytical model to evaluate the achievable pre-roll delay reduction. Also the advantages in terms of video quality are shown. Keywords— ARQ mechanism, code division multiple access, truncated power control, video quality, wireless video streaming. I. I NTRODUCTION AND MOTIVATION Market research finds that mobile commerce for 3G wireless systems and beyond will be dominated by basic human communication such as messaging, voice, and video communication [1]. Because of its typically large bandwidth requirements, video communication (as opposed to the lower rate voice and the elastic e-mail) is expected to emerge as the dominant type of service in 3G/4G wireless systems [2]. Video services are typically divided into real-time services and streaming. Our focus in this paper is on video streaming where the client may tolerate a small start up delay before the play-out commences. Video streaming schemes typically rely on the User Datagram Protocol (UDP) as the transport protocol. The use of the Transmission Control Protocol (TCP) as proposed in [3] has also some advantages, but is not addressed here. Using UDP implies that there is no reliability mechanism at transport level. The reliability of the communication over wireless channel is handled at lower layers through, for instance, the implementation of ARQ (Automatic Repeat reQuest) schemes at the data link layer. ARQ increases the reliability of the channel but it also introduces a longer and more variable delay at link layer. In order to limit the delay, a maximum number of retransmissions, called persistency of the ARQ protocol, are allowed. Moreover, the jitter can be combated by the introduction of a receiver buffer in combination with an initial playback delay, pre-roll delay, to smooth the bit rate variations caused by the transmission channel [4]. Designers of today’s commercial media streaming products find that buffering delays ranging from 5-15s typically strike a good balance between delay and playout reliability (low probability of playout interruption due to buffer underflow) [5]. In [6], the minimum initial delay for a deterministic receiver curve for a Variable Bit Rate (VBR) channel has been defined. In [7], an adaptive media playout is proposed to allow the client to buffer less data and thus reducing the pre-roll delay for a given buffer underflow probability over a wireless channel. However, those works either neglect the delay introduced by retransmissions or assume ARQ protocols with infinite persistency and hence no Fig. 1. System model of the video streaming. r sidual Frame Error Rate (FER) after retransmissions. In this paper, we first show that the persistency of the ARQ protocol can greatly impact the pre-roll delay, even for a Selective Repeat (SR) ARQ protocol. Therefore, in order to decrease the pre-roll delay, low persistency ARQ protocols should be used, thus resulting in less reliability. We then apply to the context of video streaming over CDMA-based wireless networks, a truncated power control that has been already shown to be efficient in reducing the average delay introduced by retransmissions at link layer for a given guaranteed reliability and power consumption [8]. Its exploitation results in two main benefits: • the pre-roll delay is reduced without degrading the video quality and without increasing the required transmit power; • once the pre-roll delay has been designed, better video quality can be achieved without increasing the buffer underflow probability (always at the same transmission power) In the rest of the paper, these two benefits will be shown. The remainder of the paper is organized as follows. Section II provides the system model under study. In Section III we introduce our power control policy. The performance of the proposed approach are shown in Section IV. Finally conclusions are drawn in Section V.