Wireless Local Area Networks

This chapter gives an overview of the IEEE 802.11 (also known as WiFi) standard for wireless Local Area Networks (LANs). 802.11 actually is a family of standards, the (currently) most important ones of which are 802.11a, an OFDM-based system operating in the 5 GHz UNII band, 802.11g, a variant that operates in the 2.45 GHz ISM band, and 802.11n, the version using multiple antenna elements (MIMO) to achieve high throughput. The chapter first reviews the physical layer of 802.11a/g, which transmits packets of data employing OFDM modulation with cyclic prefix. Data are encoded with convolutional codes, possibly punctured to allow for different code rates. Adaptive modulation and coding is used to adapt to the quality of the link between transmitter and receiver, though the modulation format is the same on all OFDM subcarriers. For transmission, a preamble and a PLCP header are prepended to the encoded PSDU data that are received from the MAC layer, creating a PPDU. Synchronization is achieved by means of the PLCP preamble field. Channel estimation is enabled by a training sequence. In IEEE 802.11n, multiple-antenna techniques are used to increase the data rate. These include space-time coding including Alamouti coding and cyclic shift delay, antenna selection, spatial multiplexing and beamforming. Furthermore, advanced error correction coding, namely LDPC codes, can be used. Channel estimation is more complex due to the existence of multiple antenna elements; different forms are discussed. The chapter concludes with a discussion of multiple access in 802.11. It includes both contention-based and contention-free channel access methods, which are realized mostly through carrier sense multiple access and polling. The structure of the MAC frame formats is also described.