Energy Savings for Wireless Terminals through Smart Vertical Handover

This paper optimizes handover decisions between WLAN (802.11) and WiMAX (802.16e) standards, for both uplink and downlink data transmission. The handover becomes part of a cross-layer approach controlling several knobs (air interface parameters or platform settings) in order to reduce the terminal power consumption. The first step is to derive detailed power and performance models for both standards, in order to correctly evaluate the opportunity of a handover. This includes channel fading fluctuations, extraction of MAC-level behavior and packet error rates, and overall power consumption from the wireless platform. Such models enable first optimal single-standard power-throughput trade-offs, that will be used as reference points before adding the handover possibility in order to assess its specific gain. The second step is the design of a handover controller that selects the network with the lowest expected power for the required rate. The proposed mechanism is based on regular scanning of both networks. It computes the expected energy in order to send a given amount of data over each network, taking handover cost into account, and selects the most appropriate one. Based on a software-defined radio platform and a typical channel coherence time of one second, simulations demonstrate a power saving factor up to 2.5 as a function of the scenario, compared to a single-standard system that is already cross-layer optimized. This illustrates the large gain that is available, besides the handover advantage in terms of improved connectivity.