Digital Signal Processing Reference
In-Depth Information
802.22 only focuses on fixed devices, while the IEEE 802.22a is to add mobile and
portable device functionality to IEEE 802.22. First, the application domain is dis-
cussed briefly, to better understand the ultimate goal of the standard and hence the
decisions made for spectrum sensing, analysis and decision. The main application
target for 802.22 systems is wireless broadband access in rural and remote areas.
Because of this specific rural application domain targeting lowly populated areas,
the success of the standard was low. However, the sensing features for opportunistic
spectrum usage introduced in the standard are interesting and hence briefly summa-
rized in this chapter.
2.3.2.2 System Overview
The use of the lower frequency bands are particularly useful for rural access because
of the favorable propagation conditions encountered for those lower frequencies. Al-
though the population density is often very small in rural areas, large coverage areas
might render the deployment of 802.22 Base Stations (BSs) a profitable business.
These lower frequency bands are licensed for TV broadcasting and Wireless Mi-
crophones. However, many TV channels are largely unoccupied in many parts of
the United States, and often TV is delivered through cable access or satellite. As a
result, opening up those bands for WRAN systems could make a good case, both
from business and technical points of view. Next to the main WRAN application
domain, 802.22 networks can also be used for smaller markets such as small busi-
nesses or home offices. But the main goal is delivering broadband to households in
rural areas.
An example of a deployed 802.22 network is given in Fig.
2.8
. The 802.22 net-
works operate in a fixed point-to-multi-point topology where a BS controls a cell
consisting of a number of Consumer Premise Equipments (CPEs). The BS is an en-
tity installed by an operator, and controls the cell strictly. Next to more traditional
medium access control, that addresses when to transmit, it decides on how CPEs
should access the spectrum. Moreover, the BS maintains control of a distributed
sensing strategy to keep track of potential primary users (TV or wireless micro-
phone signals). Clearly, it possible to have multiple 802.22 cells that interfere. This
is aggravated because of the very large transmission area of those systems. Coexis-
tence issues of 802.22 cells are hence also addressed in the 802.22 standard.
2.3.2.3 Spectrum Sensing
One of the important components of the IEEE 802.22 document to achieve the re-
quired cognitive capability is related to spectrum measurements. The spectrum mea-
surement in 802.22 is primarily based on transmitter detection. In order to check the
presence of primary signals, 802.22 devices need to be able to detect signals at very
low Signal-to-Noise Ratio (SNR) levels. Since the detection is done at low SNR, it
is assumed that the detection of TV signals is done in a non-coherent manner, which
means that no synchronization is needed [34].
