Mobile Networks and Applications 3 (1998) 49–60 49 Adaptive source rate control for real-time wireless video transmission Hang Liu * and Magda El Zarki Video Processing and Telecommunications Laboratory, Department of Electrical Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA Hybrid ARQ schemes can yield much better throughput and reliability than static FEC schemes for the transmission of data over time- varying wireless channels. However these schemes result in extra delay. They adapt to the varying channel conditions by retransmitting erroneous packets, this causes variable effective data rates for current PCS networks because the channel bandwidth is constant. Hybrid ARQ schemes are currently being proposed as the error control schemes for real-time video transmission. An important issue is how to ensure low delay while taking advantage of the high throughput and reliability that these schemes provide for. In this paper we propose an adaptive source rate control (ASRC) scheme which can work together with the hybrid ARQ error control schemes to achieve efficient transmission of real-time video with low delay and high reliability. The ASRC scheme adjusts the source rate based on the channel conditions, the transport buffer occupancy and the delay constraints. It achieves good video quality by dynamically changing both the number of the forced update (intracoded) macroblocks and the quantization scale used in a frame. The number of the forced update macroblocks used in a frame is first adjusted according to the allocated source rate. This reduces the fluctuation of the quantization scale with the change in the channel conditions during encoding so that the uniformity of the video quality is improved. The simulation results show that the proposed ASRC scheme performs very well for both slow fading and fast fading channels. 1. Introduction Real-time video services require high transmission re- liability and stringent end-to-end delay. Wireless links on the other hand are error-prone, bandlimited and time- varying. Error control schemes are necessary to obtain high transmission reliability required by video services. Tradi- tionally, forward error correction (FEC) codes have been used for real-time services because they maintain a con- stant throughput and a bounded delay. Wireless channels are time-varying. FEC codes can be chosen to guarantee certain error rate requirements for the worst channel con- ditions. However, this causes unnecessary overhead and wastes bandwidth when the channel is in a good state. Recently, for wireless environments, it has been shown that automatic repeat request (ARQ) and hybrid ARQ schemes can significantly improve the video transmission reliability and provide for much higher throughput than FEC schemes because they can effectively adapt to the varying channel conditions [10,12]. The new video confer- encing standard, H.324, will support hybrid ARQ schemes for wireless video communications [6,7]. The MPEG-4 standard committee is also considering to adopt a hybrid ARQ scheme for video transmission in error-prone envi- ronments [4]. Several ARQ-based or hybrid ARQ-based schemes have been proposed for wireless ATM networks to support real-time video transmission [18]. However, retransmissions in hybrid ARQ schemes cause delay. Long delays are intolerable for interactive real-time applications. If data cannot arrive at the receiver within * Current address: NEC USA, Inc., C&C Research Lab, 4 Independence Way, Princeton, NJ 08540, USA. the required delay bound, it is considered as being lost. For example, it has been suggested that for video confer- encing applications the value of the end-to-end delay must be less than 400 ms and it is preferable if the end-to-end delay is below 200 ms [1]. In the current personal commu- nication services (PCS) networks [14], the total bandwidth for a channel is constant. The wireless channel condition changes over time. Hybrid ARQ schemes adapt to the vary- ing channel conditions by retransmitting erroneous packets. When the channel is good, no retransmissions are required and the effective data rate can be high. When the channel becomes poor, the retransmissions use up bandwidth and thus reduce the effective data rate (the effective data rate is defined as the rate of the information that is correctly transmitted). This results in a varying effective data rate from the point of view of the video source. Especially for a slow fading channel, when the channel is in a state, poor or good, it will remain in that state for a long duration so that the effective data rate becomes very bursty due to retransmissions. An important issue therefore is how to guarantee low end-to-end delay while taking advantage of the high throughput and reliability provided by the hybrid ARQ schemes. In this paper we investigate the impact of hybrid ARQ on real-time video transmission. We focus on the trans- mission of H.263 coded low bitrate video over a constant bandwidth wireless channel (QCIF format and 15 frames/s over 32 kbit/s channels) which complies with the current PCS networks. We propose an adaptive source rate control (ASRC) scheme and demonstrate that it can work together with the hybrid ARQ error control schemes to achieve ef- ficient transmission of real-time video with low delay and  Baltzer Science Publishers BV