2. Transmitter Receiver Techniques

The transmission over a wireless channel is restricted to a certain range of frequencies around the some central carrier frequency. The wire is a low-pass filter and hence the carrier frequency for the wireline channel is \(f_c= 0\). This restriction immediately poses some questions about the design of the wireless communication systems. The foremost question is how is reliable communication related to the carrier frequency? Is the communication strategy and hence the transmitter–receiver design particular to the specific carrier frequency? Do we have to design the system based on \(f_c\)? It turns out that we can always work in with the baseband signal (i.e., the signal with \(f_c = 0\)) even for the wireless communication and then convert the baseband signal into the passband signal (a signal that is centered around some nonzero carrier frequency) with the desired carrier frequency. This makes the design of the transmitter and receiver transparent to the carrier frequency. Thus, only the front end of the system needs to be changed if we change \(f_c\). Also, since the bandwidth of the signal \(W\) (typically in KHz) is smaller than the carrier frequency \(f_c\) (typically in MHz), the design of DAC and ADC becomes much easier and modular. The focus of this chapter is on the conversion of the baseband signal to the passband signal and vice versa.