Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Wireless Local Area Networks 283MTBSbackbone networkMTMTBSMTBSMTFigure 5-49WLAN as an infrastructure network5.5 Wireless Local Area NetworksLocal area networks typically cover a story or building and their wireless realizationshould avoid complex installation of a wired infrastructure. WLANs are used in publicand private environments and support high data rates. They are less expensive than wirednetworks for the same data rate, are simple and fast to install, offer flexibility and mobility,and are cost efficient due to the possibility of license exempt operation.5.5.1 Network TopologyWLANs can be designed for infrastructure networks, ad hoc networks, or combinationsof both. The mobile terminals (MTs) in infrastructure networks communicate via the basestations (BSs), which control the multiple access. The base stations are linked to eachother by a wireless (e.g. FWA) or wired backbone network. Infrastructure networks haveaccess to other networks like the Internet. The principle of an infrastructure network isillustrated in Figure 5-49.In ad hoc networks, the mobile terminals communicate directly with each other. Thesenetworks are more flexible than infrastructure networks but require a higher complexityin the mobile terminals since they have to control the complete multiple access as in abase station. The communication within ad hoc networks is illustrated in Figure 5-50.5.5.2 Channel CharacteristicsWLAN systems often use the license exempt 2.4 GHz and 5 GHz frequency bands, whichhave strict limitations on the maximum transmit power since these frequency bands arealso used by many other communications systems. This versatile use of the frequencyband results in different types of narrowband and wideband interference, such as from amicrowave oven, which the WLAN system has to cope with.The WLAN cell size is up to several 100 m and multi-path propagation typically resultsin maximum delays of less than 1 µs. The mobility in WLAN cells is low and correspondsto a walking speed of about 1 m/s. The low Doppler spread in the order of 10–20 Hz makesOFDM very interesting for high rate WLAN systems.