0.2 Examples of Signal Processing Applications 7different users of the radio spectrum (commercial radio and T V, amateur radio, police, etc.). Although
most bands have been allocated, implying a scarcity of spectrum for new users, it has been found that
locally at certain times of the day the allocated spectrum is not being fully utilized. Cognitive radio
takes advantage of this.
Conventional radio systems are composed mostly of hardware, and as such cannot be easily recon-
figured. The basic premise in SDR as a wireless communication system is its ability to reconfigure
by changing the software used to implement functions typically done by hardware in a conventional
radio. In an SDR transmitter, software is used to implement different types of modulation procedures,
while ADCs and DACs are used to change from one type of signal to another. Antennas, audio ampli-
fiers, and conventional radio hardware are used to process analog signals. Typically, an SDR receiver
uses an ADC to change the analog signals from the antenna into digital signals that are processed
using software on a general-purpose processor. See Figure 0.3.
Given the need for more efficient use of the radio spectrum, cognitive radio (CR) uses SDR technology
while attempting to dynamically manage the radio spectrum. A cognitive radio monitors locally the
radio spectrum to determine regions that are not occupied by their assigned users and transmits
in those bands. If the primary user of a frequency band recommences transmission, the CR either
moves to another frequency band, or stays in the same band but decreases its transmission power
level or modulation scheme to avoid interference with the assigned user. Moreover, a CR will search
Super-
heterodyne ADC Demodulator DACAntennaSpeakerTRANSMITTERRECEIVERADC Modulator DACMicrophoneAntennaFIGURE 0.3
Schematics of a voice SDR mobile two-way radio.Transmitter:The voice signal is inputted by means of
a microphone, amplified by an audio amplifier, converted into a digital signal by an ADC, and then modulated
using software, before being converted into analog by an DAC, amplified, and sent as a radio frequency signal
via an antenna.Receiver:The signal received by the antenna is processed by a superheterodyne front-end,
converted into a digital signal by an ADC before being demodulated and converted into an analog signal by a
DAC, amplified, and fed to a speaker. The modulator and demodulator blocks indicate software processing.