Adaptive frame length control for improving wireless link throughput, range, and energy efficiency

Wireless network links are characterized by rapidly time varying channel conditions and battery energy limitations at the wireless mobile user nodes. Therefore static link control techniques that make sense in comparatively well behaved wired links do not necessarily apply to wireless links. New adaptive link layer control techniques are needed to provide robust and energy efficient operation even in the presence of orders of magnitude variations in bit error rates and other radio channel conditions. For example, research has advocated adaptive link layer techniques such as adaptive error control, channel state dependent protocols, and variable spreading gain. We explore dynamic sizing of the MAC layer frame, the atomic unit that is sent through the radio channel. A trade-off exists between the desire to reduce the header and physical layer overhead by making frames large, and the need to reduce frame error rates in the noisy channel by using small frame lengths. Clearly the optimum depends on the channel conditions. Through analysis supported by physical measurements with Lucent's WaveLAN radio we show that adaptive sizing of the MAC layer frame in the presence of varying channel noise indeed has a large impact on the user seen throughput (goodput). In addition, we show how that adaptive frame length control can be exploited to improve the energy efficiency for a desired level of goodput, and to extend the usable radio range with graceful throughput degradation. We describe the implementation of the adaptive MAC frame length control mechanism in combination with adaptive hybrid FEC/ARQ error control in a reconfigurable wireless link layer packet processing architecture for a low-power adaptive wireless multimedia node.