Enhancing Wireless local area Networks by leveraging Diverse Frequency Resources
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In this thesis, signal propagation variations, that are experience over the frequency resources of IEEE 802.11 Wireless Local Area Networks (WLANs) are studied. It is found that exploitation of these variations can improve several aspects of wireless communication systems. To this aim, frequency varying behavior is addressed at two different levels. First, the intra-channel scale is considered, i.e. variations over the continuous frequency block that a device uses for a cohesive transmission. Variations at this level are well known but cur- rent wireless systems restrict to basic equalization techniques to balance the received signal. In contrast, this work shows that more ﬁne grained adaptation to these differences can accomplish throughput and connection range gains. Second, multi-frequency band enabled devices that access widely differing frequency re- sources in the millimeter wave range as well as in the microwave range are analyzed. These devices that are expected to follow the IEEE 802.11ad speciﬁcation experience intense propaga- tion variations over their frequency resources. Thus, a part of this thesis revises, the theoretical speciﬁcation of the IEEE 802.11ad standard and complements it by a measurement study of ﬁrst generation millimeter wave devices. This study reveals deﬁciencies of ﬁrst generation millime- ter wave systems, whose improvement will pose new challenges to the protocol design of future generation systems. These challenges are than addressed by novel methods that leverage from frequency varying propagation characteristics. The ﬁrst method, improves the beam training process of millimeter wave networks, that need highly directional, though electronically steered, transmissions to overcome increased free space attenuation. By leveraging from omni-directional signal propagation at the microwave bands, efﬁcient direction interference is utilized to provide information to millimeter wave interfaces and replace brute force direction testing. Second, deafness effects at the millimeter wave band, which impact IEEE 802.11 channel access methods are addressed. As directional communication on these bands complicates sensing the medium to be busy or idle, inefﬁciencies and unfairness are implied. By using coordination message exchange on the legacy Wi-Fi frequencies with omni- directional communication properties, these effects are countered. The millimeter wave bands can thus unfold their full potential, being exclusively used for high speed data frame transmission.