Time and Frequency in Wireless Communications
Petar Popovski · 2020
This chapter makes a connection between, on the one side, the channel uses in a wireless communication channel and, on the other side the actual data carrying signals that occur in time. This is instrumental to create the physical basis for the notion of time used in medium access control protocols and other protocols running at the higher layers. The chapter discusses how a digital communication signal is mapped onto physical signals, and the notions of signal bandwidth and carrier frequency. It makes a paradoxical observation: all signals encountered in practice are band limited, but then the Fourier transform implies that they have to have an infinite duration. It shows under which statistical conditions the power spectrum is defined, and therefore, the actual band, where a certain signal resides. Finally, the chapter introduces the idea of spread spectrum and code division multiple access.