Chapter 7
OFDM
Wideband wireless technologies, such as HSPA and EVDO, have difficulty in providing high data throughput over large distances due to multipath interference. There was clearly a need for another technology that could live with large multipath. Orthogonal Frequency Division Multiplex (OFDM) was the natural candidate, as although nominally broadband, it is subdivided into narrowband sub-carriers. Simultaneously the time was right for OFDM introduction as the hardware performance required to implement it became available, through powerful DSPs (Digital Signal Processor) and fast ADCs (Analog to Digital Converter). This chapter describes the basic principles of OFDM, which apply to all technology implementations described later in this book.
7.1 Multiplexing
Multiplexing is a technique that allows a medium to carry different streams of information at the same time. Multiplexing can be done in frequency (FDM, Frequency Division Multiplex), time (TDM, Time Domain Multiplex) or code (CDM, Code Division Multiplex) domains.
In FDM, different frequencies, called carriers, are used to carry information, and each frequency is assigned to a different user. It is necessary that the interference between carriers, the Inter Symbol Interference (ISI), be kept within values smaller than the signal to noise levels required to recover the information (between few dB to 30+ dB). To carry the maximum possible amount of information per Hz, each carrier spectrum has to be limited by filtering, ...
