Enhanced Throughput for Satellite Multicasting

Faithful information delivery in satellite multicasting requires appropriate error control. If multicast automatic-repeatrequest (ARQ) is employed, a retransmission does not benefit receivers which do not require it, and consequently the throughput suffers greatly as the number of receivers increases. This performance degradation might be alleviated substantially by conducting retransmissions through terrestrial paths from the transmitter to each receiver instead of through the multicast satellite link. By sending a retransmission directly to the receiver(s) which requires it, higher throughput can be provided in such a hybrid network than in a pure-satellite network. In this work, we examine the throughput improvement provided by the hybrid network. INTRODUCTION Satellites are excellently suited for distributing information simultaneously to multiple locations. As in nearly all communication systems, some sort of error control scheme is required in satellite multicasting to assure satisfactory fidelity of the information provided to each destination. Error control schemes may be broadly classified as forward error correction (FEC) or automatic-repeat-request (ARQ), and both can be applied for satellite communication. FEC has been used in satellite/space communication for decades, having grown from successful application by NASA for communication with interplanetary probes [1, 2]. However, satellite channel characteristics vary with time, and at any given time multiple receivers may perceive different channel qualities. Applying FEC for satellite multicast communication accordingly requires using an error-correcting code strong enough to protect data against the worstcase channel impairments. Unfortunately the error correction capability provided by powerful FEC code comes at the cost of sending many check symbols which constitute overhead in the communication. Further, this overhead penalty is exacted even at times of good channel quality, since FEC is not an adaptive error control technique. This is particularly troubling since good channel conditions will be experienced a majority of the time when using a well-designed satellite link [3]. ARQ protocols adapt to different channel qualities by retransmitting data only as needed. Also, an error-detecting code capable of detecting t or fewer errors in k information symbols requires fewer overhead symbols than would an FEC code designed to correct t errors in the same k symbols [1, 2]. Hence ARQ can provide high fidelity with less overhead than FEC during times of good channel quality, which tend to prevail as mentioned above. A drawback of ARQ not suffered by FEC is the need for a feedback channel, but this requirement is often an acceptable concession for achieving information transfer with excellent fidelity. A difficulty arises in applying ARQ in multicast settings. The typical problem in a multicast ARQ system is that since retransmissions are sent over the multicast channel, those retransmissions required by only a few receivers do not This work was supported in part by the Center for Satellite and Hybrid Communication Networks under NASA cooperative agreement NCC3528.