Cognitive radio basics
In the early days of modem development, designers were developing modems based on different applications and different frequency bands as per requirement. This leads to more time to deployment of the system and increase in cost due to various reasons. This development was purely based on hardware. After the incorporation of software defined radio (SDR), required RF transceiver hardware,baseband transmitter and baseband receiver algorithms are configured using software. The research after SDR concept is on cognitive radio in which intelligence is build to allow radio to sense environment in terms of radio spectrum and track changes accordingly and utilize unused RF spectrum.
As shown in the figure, there are three components radio frequency part, ADC/DAC and baseband processing part. Radio frequency part consists of mainly RF transceiver,LNA,PA,RF filters,LO and AGC. ADC/DAC is used to convert analog signal to digital signal and digital to analog respectively. Baseband processing part converts IF signal to baseband I/Q signals and vice versa. This baseband part varies based on wireless technologies such as WLAN,WiMAX,LTE,LTE advanced, DVB-T,IMT-A etc. In cognitive radio RF part,ADC/DAC clocks and baseband part is configured dynamically.
Let us see, configurations of RF hardware done by cognitive radio.
• RF filter is configured as per frequency band needed.
• LNA gain is configured based on RF link budget and input signal strength so that it will not overdrive ADC.
• Necessary RF signal generation required for mixing with input modulated RF signal to produce modulated IF signal.
LO for RF signal is generated as per band of operation.
• Channel selection filter is tuned as per selected channel need and to reject the undesired adjacent channel frequencies.
• AGC circuit is developed dynamically based on signal strength so that signal at the input of ADC always falls within dynamic range of the ADC.
• The clock required based on data rate and baseband requirement.
The biggest challenge in using cognitive radio is to design efficient spectrum sensing algorithm so that unutilized spectrum can be efficiently used by various wireless technologies.This is because spectrum is scarce resource. Information sharing between cognitive radios can be achieved efficiently.
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