118 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
TABLE I
M
ULTI -DIMENSIONAL RADIO SPECTRUM SPACE AND TRANS MISSION OPPORTUNITIES
Dimension What needs to be sensed? Comments Illustrations
Frequency Opportunity in the frequency domain.
Availability in part of the frequency spectrum. The
available spectrum is divided into narrower chunks
of bands. Spectrum opportunity in this dimension
means that all the bands are not used simultane-
ously at the same time, i.e. some bands might be
available for opportunistic usage.
Time Opportunity of a specific band in time.
This involves the availability of a specific part of
the spectrum in time. In other words, the band is
not continuously used. There will be times where
it will be available for opportunistic usage.
Geographical
space
Location (latitude, longitude, and elevation) and
distance of primary users.
The spectrum can be available in some parts of
the geographical area while it is occupied in some
other parts at a given time. This takes advantage of
the propagation loss (path loss) in space.
These measurements can be avoided by simply
looking at the interference level. No interference
means no primary user transmission in a local area.
However, one needs to be careful because of hidden
terminal problem.
Code
The spreading code, time hopping (TH), or fre-
quency hopping (FH) sequences used by the pri-
mary users. Also, the timing information is needed
so that secondary users can synchronize their trans-
missions w.r.t. primary users.
The synchronization estimation can be avoided
with long and random code usage. However, partial
interference in this case is una voidable.
The spectrum over a wideband might be used at a
given time through spread spectrum or frequency
hopping. This does not mean that there is no avail-
ability over this band. Simultaneous transmission
without interfering with primary users would be
possible in code domain with an orthogonal code
with respect to codes that primary users are using.
This requires the opportunity in code domain, i.e.
not only detecting the usage of the s pectrum,
but also determining the used codes, and possibly
multipath parameters as well.
Angle
Directions of primary users’ beam (azimuth and
elevation angle) and locations of primary users.
Along with the knowledge of the location/position
or direction of primary users, spectrum oppor-
tunities in angle dimension can be created. For
example, if a primary user is transmitting in a
specific direction, the secondary user can transmit
in other directions without creating interference on
the primary user.
The noise/interference estimation problem is easier for these
purposes as receivers are tuned to receive signals that are
transmitted over a desired bandwidth. Mor eover, receivers
are capable of processing the narrowband baseband signals
with reasonably low complexity and low power processors.
However, in cognitive radio, terminals are required to process
transmission over a much wider band for utilizing any oppor-
tunity. Hence, cognitive radio should be able to capture and
analyze a relatively larger band for identifying spectrum op-
portunities. The large operating bandwidths impose additional
requirements on the radio frequencies (RF) components such
as antennas and power amplifiers as well. These components
should be able to operate over a range of wide operating
frequencies. Furthermore, high speed processing units (DSPs
or FPGAs) are needed for performing computationally de-
manding signal processing tasks with relatively low delay.
Sensing can be performed via two different architectures:
single-radio and dual-radio [14], [15]. In the single-radio
architecture, only a specific time slot is allocated for spectrum
sensing. As a result of this limited sensing duration, only a
certain accuracy can be guaranteed for spectrum sensing re-
sults. Moreover, the spectrum efficiency is decreased as some
portion of the available time slot is used for sensing instead of
data transmission [16], [17]. The obvious advantage of single-
radio architecture is its simplicity and lower cost. I n the dual-
radio sensing architecture, one radio chain is dedicated for data
transmission and reception while the other chain is dedicated
for spectrum monitoring [18], [19]. The drawback of such an
approach is the increased power consumption and hardware
cost. Note that only one antenna would be sufficient for both
chains as suggested in [14]. A comparison of advantages and
disadvantages of single and dual-radio architectures is given