Channel quality estimation and impairment mitigation in 802.11 networks

摘要

Wireless communication has been boosted by the adoption of 802.11 as standard de facto for WLANudtransmission. Born as a niche technology for providing wireless connectivity in small office/enterpriseudenvironments, 802.11 has in fact become a common and cheap access solution to the Internet, thanks toudthe large availability of wireless gateways (home modems, public hot-spots, community networks, andudso on). Nowdays, the trend towards increasingly dense 802.11 wireless deployments is creating a realudneed for effective approaches for channel allocation/hopping, power control, etc. for interference mitigationudwhile new applications such mesh networks in outdoor contexts and media distribution withinudthe home are creating new quality of service demands that require more sophisticated approaches toudradio resource allocation.udThe new framework of WLAN deployments require a complete understanding of channel qualityudat PHY and MAC layer. Goal of this thesis is to assess the MAC/PHY channel quality and mitigateudthe different channel impairments in 802.11 networks, both in dense/controlled indoor scenariosudand emerging outdoor contexts. More specifically, chapter 1 deals with the necessary backgroundudmaterial and gives insight into the different channel impairments/quality it can be encountered inudWLAN networks. Then the thesis pursues a down/top approach: chapter 2, 3 and 4 aim at affordingudimpairments/quality at PHY level, while chapter 5 and 6 analyse channel impairments/quality fromuda MAC level perspective. An important contribution of this thesis is to undisclose that some PHY layer parameters, suchudas the transmission power, the antenna selection, and interference mitigation scheme, have a deepudimpact on network performance. Since the criteria for selecting these parameters is left to the vendorudspecific implementations, the performance spread of most experimental results about 802.11 WLANudcould be affected by vendor proprietary schemes. Particularly, in chapter 2 we find that switchingudtransmit diversity mechanisms implemented in off-the-shelf devices with two antenna connectors canuddramatically affect both performance and link quality probing mechanisms in outdoor medium-rangeudWLAN deployments, whenever one antenna deterministically works worse than the other one. A second physical algorithm with side-effects is shown in chapter 3. Particulary the chapter shows thatudinterference mitigation algorithms may play havoc with the link-level testbeds, since they may erroneouslyudlower the sensitivity threshold, and thus not detect the 802.11 transmit sources. Finally, onceuddisabled the interference mitigation algorithm — as well as any switching diversity scheme describedudin the previous chapter — link-level experimental assessment concludes that, unlike 802.11b, whichudappears a robust technology in most of the operational conditions, 802.11g may lead to inefficienciesudwhen employed in an outdoor scenario, due to the lower multi-path tolerance of 802.11g. Since multipathudis hard to predict, a novel mechanism to improve the link-distance estimation accuracy — basedudon CPU clock information — is outlined in chapter 4. The proposed methodology can not only beudapplied in localization context, but also for estimating the multi-path profile. The second part of the thesis moves the perspective to the MAC point of view and its impairments.udParticularly, chapter 5 provides the design of a MAC channel quality estimator to distinguish theuddifferent types of MAC impairments and gives separate quantitative measures of the severity of eachudone. Since the estimator takes advantage of the native characteristics of the 802.11 protocol, theudapproach is suited to implementation on commodity hardware and makes available new measuresudthat can be of direct use for rate adaptation, channel allocation, etc. Then, chapter 6 introduces audprevious unknown phenomenon, the Hidden ACK, that may cause frame losses into multiple WLANudnetworks when a node replies with an ACK frame. Again, a solution is provided without requiringudany modification to the 802.11 protocol. Whenever possible, the quantitative analysis has been led through experimental assessments withudimplementation on commodity hardware. This was the adopted methodology in chapter 2, 3, 4 and 5.udParticularly, this has required an accurate investigation of two brands of WLAN cards, particularlyudthe Atheros and Intel cards, and their driver/firmware, respectively MADWiFi and IPW2200, whichudare currently the most adopted, respectively, by researchers and layman users.