IEEE Circuits and Systems Magazine - Q1 2022 - 14

would be composed by three main building blocks: the
antenna, the A/D interfaces and the DSP. As will be discussed
later, this implementation is not realistic due to
the huge amount of power demanded by the A/D interfaces,
what requires at least some analog signal conditioning
circuitry (see Fig. 1), to really implement an efficient
interface between RF signals and the digital data.
SDRs are the base platform to implement CR technology
in order to make a more efficient use of the electromagnetic
spectrum [14], by dynamically modifying its
transmission and reception parameters according to the
information sensed from the environment-a technique
also referred to as spectrum sensing. Essentially, CR-based
technology enables wireless networks and handheld
terminals to use the RF spectrum in a dynamic manner.
As a result, a more efficient use of licensed/unlicensed
spectrum can be done, with reduced interferences and/or
at a lower power consumption. This way, SDR/CR-based
mobile terminals would be capable of dynamically sensing
their spectral environment and to exploit the captured information
to change their transmission/reception parameters
in order to improve the communication link and to
reduce the energy consumed on the fly.
Therefore, SDR/CR devices must be smart enough to
incorporate cognitive (also referred to as spectrum-sensing)
capabilities, and flexible enough to be dynamically
programmed according to the information obtained from
their interaction with the environment. As conceptually
illustrated in Fig. 4(b), SDR/CR systems can be divided
into two main building blocks: the SDR transceiver itself
and a spectrum-management subsystem. The latter will
do the cognitive tasks required to implement CR functionalities.
Note that this approach is completely different
from the fixed spectrum assignment policy followed by
today's mobile telecom systems, in which a large portion
(around
80 %90-
) of the assigned radio spectrum is used
sporadically while the remaining bands are truly busy at
any given time. Indeed, it is expected that the combined
use of SDR and CR approaches can be advantageous to
optimally selecting the most suitable parameters for the
communication systems, depending on the environment
conditions, interferences, battery level, etc. as well as optimizing
power consumption. It will also add flexibility in
a number of applications such as autonomous vehicles,
thanks to the incorporation of new protocols such as
Vehicle-to-Everything (V2X) [16], [30].
As will be discussed later, one of the direct consequences
of the physical implementation of SDR/CR-based terminals
is that the RF signal conditioning, the ASP and the
digitizer, i.e the circuits responsible for transforming the
signals from the analog/RF to the digital domain, should
be moved as close as possible to the antenna, so that most
of the hardware is digital and hence, it is easier to program
via software. This is however one of the main design challenges
and bottlenecks-specially from the circuit designer
perspective. But, before going down to the circuit level,
it is illustrative to get some insight about CR systems.
III. Background on Cognitive Radio
ADC
DSP
(a)
Power
ADC
DSP
DAC
Spectrum
Spectrum-Management
Subsystem
(b)
Figure 4. Conceptual diagram of SDR and CR transceivers:
(a) SDR transceiver, where signals are ideally transformed
from RF to digital domain-unfeasible in practice due to the
power-hungry A/D interfaces (ADC in the receiver path and
DAC in the transmitter path). (b) Ideal CR transceiver made
up of two main building blocks: a SDR and a spectrum-management
subsystem.
14
IEEE CIRCUITS AND SYSTEMS MAGAZINE
Frequency
Licensed Bands
(Primary Users)
" Holes " or Vacants
(CR Users)
DAC
The idea of CR was first proposed by Mitola more than
20 years ago [14]. As illustrated in Fig. 5, the core concept
behind CR systems consists of using free, vacant bands
or frequency holes, by CR users, also known as Secondary
Users (SUs), when those bands are not occupied by
the Primary Users (PUs). This way, those users (PUs)
who occupy frequency bands assigned or licensed to
commercial communication standards and protocols in
a given location, can allow CR users (SUs) to use their
bands when they are vacant. For that reason, sometimes
CR is also referred to as an opportunistic radio approach,
Time
Figure 5. Illustration of the concept of CR: frequency allocation
and occupancy over time by primary users (PU) versus
secondary users (SU).
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