MixCast modulation for layered video multicast over WLANs
The major challenge in wireless multicast is the heterogeneous channel conditions of multiple users. In video multicast, the combination of a layered video coding scheme and a layered transmission scheme can gracefully accommodate user heterogeneity. This paper presents MixCast, a novel physical layer scheme for layered transmission. The key innovation in MixCast is the rateless Euclidean symbol mapping which mixes base layer and enhancement layer bits into arbitrary number of wireless symbols using arithmetic weighted sum operation. This design brings two benefits when compared with the state-of-the-art physical layer technique known as hierarchical modulation (HM). First, MixCast uses a fixed modulation constellation, avoiding the complexity in adaptive modulation and coding when channel condition varies. Second, the rateless symbol mapping allows MixCast to achieve much smoother rates than the stair-shaped rates in HM. We implemented MixCast for typical video multicast over OFDM physical layer, and evaluate its performance against HM through both simulations and software radio testbed. In simulations, MixCast shows consistent gain of 3dB to 5dB over HM under various rate combinations. In the testbed experiments, MixCast achieves significant gain up to 15dB in video PSNR over HM for football sequence.