Generalised link-layer optimisation: Application and performance evaluation

A wireless communication system work more efficiently if link-layer parameters such as modulation order, symbol rate and packet size are (adaptively) optimised. A common criterion is to maximise spectral efficiency subject to a very low bit-error constraint. But for systems equipped with strong error detection and a selective packet re-transmission mechanism, a packet-oriented criterion is more appropriate. Recently we showed that the link configuration that maximises bits per second or bits per Joule can be identified by drawing a tangent from the origin to the scaled graphs of the corresponding packet-success rate functions: the steeper the tangent the better the configuration. We now consider a tight symbol-rate constraint that forces the terminal to switch its configuration from the ideal as channel quality improves, and report on analytically-grounded performance experiments. A terminal with a flexible and unconstrained symbol rate enjoys a growing and overwhelming performance advantage over a similarly-endowed fixed-rate adaptive terminal. And the rate-flexible terminal retains a significant performance edge (up to 2-to-1) even when its symbol rate cannot exceed that of the fixed-rate terminal.