A Feedback-less Design for Wireless Transmission of Video


SoftCast is a communication system designed to transmit a single data stream to multiple multicast receivers that dynamically decode and scale the signal as a function of channel quality. This approach is applicable to video, audio, or sensor data communication with degradable content and is particularly useful to avoid broadcasting glitches in mobile TV, media sharing, and live broadcasting environments.

Problem Addressed

The growing demand for wireless video multicasting is challenged by the need to scalably serve multiple receivers who have different channel characteristics. Ideally, a source would broadcast a single video stream yet deliver to each of multiple receivers a video resolution that matches its channel. However, current wireless transmission schemes cannot support smooth degradation and therefore the source is conflicted with transmitting at a high bitrate but reaching only nearby receivers, or transmitting at a low bitrate and reducing everyone to the channel quality of the worst receiver. The Inventors have developed SoftCast, an alternative approach to wireless video multicast where a sender simply broadcasts its packets without specifying a bitrate or a video resolution, but each receiver sees a video resolution scaled to its channel resources.


SoftCast achieves this by encoding videos using fixed-precision real numbers and maintaining that representation throughout the video codec to the physical layer. This departs from traditional encoding methods in which transmitted video undergo repeated modulation schemes to convert pixel values to code-words lacking numerical properties of the original values. SoftCast pushes the real representation all the way to the channel by making the magnitude of the transmitted signal proportional to the pixel value such that channel noise directly translates to a perturbation in pixel values. This has a small impact on video quality and allows for smooth degradation as channel noise increases.

While traditional compression and error-correcting cannot be applied to real values, SoftCast achieves the analogous result of redistributing the bits in a data stream, removing redundant bits and allowing for correction, by directly redistributing power among the real values. In this way, a source can compute the optimal power allocation to produce a compressed and accurate representation of the video signal, subject to a hardware power budget. The source can perform this optimization purely based on the video data, without any need for receiver feedback. This allows SoftCast to achieve a smooth degradation property of real numbers, while maintaining compactness and resilience.


  • First system that exploits power allocation to achieve a compact and resilient representation of video using real numbers
  • System performance scales linearly with channel noise
  • For a diverse multicast group, SoftCast improves the average receiver’s peak signal-to-noise ratio by 3.6-3.7 dB over an MPEG baseline
  • Competitive performance for a single receiver with up to 4 dB improvement over MPEG