Improving end system performance using a robust link layer
A software-based modem design offers a new opportunity to integrate processing at various levels, to enhance performance (TCP throughput) and robustness (to jamming). There are several key problems that need to be addressed that impact link ARQ performance. ACKs are needed for link sender to retransmit or adapt and an imperfection of ACK feedback from receiver prevents a link responding to loss and may cause the forwarding to be suspended. Another problem identified is that although ARQ may efficiently retransmit corrupted frames resulting from Partial Band Noise Jamming, it is inefficient when operating in environments with significant levels of AWGN. At the same time, a single fixed FEC scheme is insufficient for a wide range of AWGN and would be lead to a high overhead at low AWGN. The approach presented in the thesis therefore provides features to make sure robustness is achieved over a wide range of error conditions. At the physical layer, the design sends individual frames in hops (rather than spreading errors with interleaving) and employs a tailbiting FEC code. At the link, ARQ is used in combination with diversity coding to ensure acceptable TCP performance. Robust framing is provided by status slot replication and an effective synchronisation algorithm. A range of ARQ methods are used to support multiple applications including (a) No ARQ link class option (for packets that do not require ARQ) and (b) ARQ option (3-level retransmission scheme, with increasing level of redundancy after each RTT). To achieve a good throughput with high levels of AWGN uses a novel application of diversity coding. Analysis shows this approach gives good performance. In comparison with HDLC, APRIL performance is comparable to HDLC at low FER (less than 0.1) over a range of jamming, while it outperforms this at high FER (0.1-0.5). Diversity in combination with ARQ significantly improves performance at high AWGN (or a combination of AWGN and PBNJ), but does so in an adaptive way without introducing significant overhead at low AWGN. A key feature of the approach is that it does not require a separate estimate of the link conditions and there is no need to explicitly inform the receiver of either the ARQ mode or the diversity level. The scheme is therefore well suited to the unpredictable characteristics of the military satellite environment.