Handoff process in IEEE 802.11 wireless networks must be accomplished with as little interruption as possible to maintain the required quality of service (QoS). Previous research reported that channel scanning phase accounts for more than 90% of the overall link-layer handoff latency, ranging from 350 to 500 msec. In order to eliminate the time-consuming channel scanning latency we propose a stand-alone, shared beacon channel, where stations (STAs) can update information about neighboring access points (APs) via an extra receiver. Our proposed method contributes to completely eliminate the channel scanning phase, therefore, realizing fast handoff in IEEE 802.11 wireless networks. In this paper, we implement our proposed method on FreeBSD 6.1R kernel with Atheros AR5212-based 802.11 a/b/g chipsets. From experimental results, we find that it takes 6.063 msec delay on average during handoff from 802.11b AP to 802.11a AP while transmitting/receiving 480 byte ICMP frames with an interval of 10 msec.

The paper investigates and evaluates the application of log-likelihood ratio-based ordering in the decoding stage of amplify-and-forward cooperative spatial multiplexing system. The cooperative spatial multiplexing detects signals transmitted from the source with successive interference cancelation algorithm whose performance varies, depending on the ordering schemes applied. Log-likelihood ratio-based ordering uses and exploits the a posteriori information of the received signal at the destination to get a more accurate final estimation at the decoder output. Based on the evaluation, the authors demonstrate that with marginally extra complexity, log-likelihood ratio based-ordering offers better performance compared to signal-to-noise ratio-based ordering.

The current channel scanning phase in IEEE 802.11 Wireless LAN does not fit fast handoff due to the following two reasons. First, IEEE 802.11 Wireless LAN operates based on CSMA/CA. Unlike cellular system, STAs can not scan other channels while exchanging data frames with currently associated AP, because it would directly cause frame loss. To attack this problem, related works have tried to utilize Power Saving Mode (PSM) or multiple NICs. Second, beacon interval is too far apart for fast handoff (100 msec by default). Since an STA lacks knowledge about which channel its nearby APs are operating on, it should redundantly scan every single channel regardless whether any AP exists on that channel. Though shorter beacon interval or active scan enable an STA to locate its nearby BSSs more quickly, these approaches increase extra traffic to the limited bandwidth. Previous works tried to provide an STA with nearby AP information approximated by APs collaboratively.