Digital Signal Processing Reference
In-Depth Information
Fig. 2.7
Spatial reuse of the TV white spaces requires large safety margins to ensure that the
receive contour of the primary transmitter is protected
In the US, the FCC adopted rules for unlicensed use of television white spaces
already in 2008. This announcement was a major endorsement for Cognitive Radio
and will softly introduce spectrum sensing technologies in the market in the coming
years. The FCC has proposed to use geo-location technology as a primary mea-
sure to combat interference: i.e. to check in a database whether a certain frequency
band is available or not at a certain location before authorizing the unlicensed use.
However, the FCC also stipulates that the geo-location based devices should be
complemented with sensing technology:
The Commission also has required that devices include the ability to listen to the
airwaves to sense wireless microphones as an additional measure of protection for
these devices
.
To implement reuse of the TV white spaces, from a platform point of view, the
main challenges are related to the sensing requirements. Indeed, as illustrated in
Fig.
2.7
, it is impossible to sense the exact impact of the secondary transmitter on
the primary receiver that should be protected. It is only possible to sense the channel
from the primary transmitter to the secondary transmitter, and huge safety margins
are added in order to accommodate unknown blocking in the sensed channel. In-
deed, if the sensed channel would be blocked, the primary transmitter would seem
to be further away than it actually is, and the secondary transmitter could falsely
assume that the channel is free. For the TV white spaces, sensing up to
−
116 dBm
is hence targeted, which is well below the noise floor in those bands.
The FCC's conclusion however stated that
devices do not consistently sense TV or
wireless microphone signals and the transmitter is capable of causing interference
to these signals
.
September 23, 2010, the FCC issued a Second Memorandum Opinion and Or-
der [32] determining the final rules for the use of WSD. The new rules remove
the mandatory requirement that WSD should include sensing technology to detect
the signals of TV stations and low-power auxiliary service stations (wireless mi-
crophones). The FCC states that the geo-location and database access method and
other provisions of the rules will provide adequate and reliable protection for in-
cumbent devices, thus making spectrum sensing not necessary since this mandatory
requirement would not best serve the public interest. However, the FCC recognized
the value of sensing for TVWS in the following statement:
We continue to believe
that spectrum sensing will continue to develop and improve
.
We anticipate that some
