A generic architecture of a cognitive radio transceiver is shown in Fig. 4(a) . The main components of a cognitive radio transceiver are the radio front-end and the baseband processing unit.
Each. component can be reconfigured via a control busto adapt to the time-varying RF environment. In
the RF front-end, the received signal is amplified,
mixed and A/D converted. In the baseband processing
unit, the signal is modulated/demodulated and
encoded/decoded. The baseband processing unit of
a cognitive radio is essentially similar to existing
transceivers. However, the novelty of the cognitive
radio is the RF front-end. Hence, next, we focus
on the RF front-end of the cognitive radios.
The novel characteristic of cognitive radio transceiver
is a wideband sensing capability of the RF
front-end. This function is mainly related to RF
hardware technologies such as wideband antenna,
power amplifier, and adaptive filter. RF hardware
for the cognitive radio should be capable of tuning
to any part of a large range of frequency spectrum.
Also such spectrum sensing enables real-time
measurements of spectrum information from radio
environment. Generally, a wideband front-end architecture
for the cognitive radio has the following structure
as shown in Fig. 4(b) .
The components of a cognitive radio RF front-end are as follows:
• RF filter: The RF filter selects the desired band
by bandpass filtering the received RF signal.
• Low noise amplifier (LNA): The LNA amplifies the desired signal while simultaneously minimizing noise component.
• Mixer: In the mixer, the received signal is mixed with locally generated RF frequency and converted to the baseband or the intermediate frequency (IF).
• Voltage-controlled oscillator (VCO): The VCO generates a signal at a specific frequency for a given voltage to mix with the incoming signal. This procedure converts the incoming signal to baseband or an intermediate frequency.
• Phase locked loop (PLL): The PLL ensures that a signal is locked on a specific frequency and can also be used to generate precise frequencies with fine resolution.
• Channel selection filter: The channel selection filter is used to select the desired channel and to reject the adjacent channels. There are two types of channel selection filters. The direct conversion receiver uses a low-pass filter for the channel selection. On the other hand, the superheterodyne receiver adopts a bandpass filter.
• Automatic gain control (AGC): The AGC maintains
the gain or output power level of an amplifier
constant over a wide range of input signal
levels.
In this architecture, a wideband signal is received
through the RF front-end, sampled by the high
speed analog-to-digital (A/D) converter, and measurements
are performed for the detection of the
licensed user signal. However, there exist some limitations
on developing the cognitive radio front-end.
The wideband RF antenna receives signals from various
transmitters operating at different power levels,
bandwidths, and locations. As a result, the RF frontend
should have the capability to detect a weak signal
in a large dynamic range. However, this capability
requires a multi-GHz speed A/D converter with
high resolution, which might be infeasible .
The requirement of a multi-GHz speed A/D converter
necessitates the dynamic range of the signal to
be reduced before A/D conversion. This reduction
can be achieved by filtering strong signals. Since
strong signals can be located anywhere in the wide
spectrum range, tunable notch filters are required
for the reduction . Another approach is to use
multiple antennas such that signal filtering is performed
in the spatial domain rather than in the frequency
domain. Multiple antennas can receive
signals selectively using beamforming techniques
.
As explained previously, the key challenge of the
physical architecture of the cognitive radio is an
accurate detection of weak signals of licensed users
over a wide spectrum range. Hence, the implementation
of RF wideband front-end and A/D converter
are critical issues in xG networks.
