Hyukjune Chung

Rate adaptation for video telephony in 3G networks

In this paper, we address challenges to packet switched video telephony (PSVT) in 3G wireless networks such as High Speed Packet Access (HSPA), and propose a technique which can improve PSVT service in 3G wireless networks. In HSPA, temporary decrease in available bandwidth can occur based on the user’s location in the cell and/or system loading conditions. This can result in significant increase in observed packet latency, potentially resulting in dropped packets at the receiver. To overcome this problem we describe a feedback based rate adaptation scheme. The proposed scheme is capable of adapting the video encoder bitrate to the varying channel bandwidth and hence minimize the packet latency. The novelty of our proposed approach is that the algorithm matches encoded source rate to varying channel conditions without making any piecewise constant bitrate (CBR) assumptions on the channel. In the proposed scheme the observed throughput is monitored on the uplink (i.e. from sending mobile to the network) and on the downlink (i.e. network to the receiving mobile). The desired target data rate is determined based on these observed throughput. Based on the source variability and the desired target data rate, the algorithm controls the encoding time and packet size (e.g. bytes) of video frames scheduled for transmission. We present simulation results of the proposed approach, based on approved specifications and common simulation conditions adopted in 3GPP and ITU. We show that the one-way delay can be significantly reduced by providing timely feedback of observed throughput from receiving mobile to the sending mobile. By using such a feedback message once every 200 ms, we show that 95 percentile of the packets were delivered with a one-way delay under 250 ms. We also show that our proposed algorithm is capable of graceful degradation based on available system resources and user’s location in the cell.