Digital Signal Processing Reference
In-Depth Information
Fig. 2.6
802.11 and 802.15.4 channels in the 2.4 GHz ISM band
output power of 802.11g devices is typically 15 dBm or above. Also, 802.15.4 sen-
sor networks are designed to monitor the environment or buildings, and can be very
large, while 802.11 networks are mostly local hotspots organized around an Ac-
cess Point (AP). Finally, sensor network applications are not demanding in terms
of throughput, which means that their duty cycle is relatively low. Because these
networks are very different in nature, achieving smooth coexistence between those
networks is not trivially achieved since that would require the detection and chan-
nel access techniques of both technologies being harmonized. While their operating
characteristics are very different, both rely on a LBT channel access mechanism.
Nevertheless, the LBT mechanism cannot give any guarantee about the caused in-
terference, and more cognitive adaptation will be needed to improve the coexistence
performance.
2.3.1.1 IEEE 802.11h for Spectrum and Transmit Power Management
Extensions
Interest in wireless technology has experienced explosive growth over the past
decade. Due to the finalization of many standards, the development of wireless
applications has eased, which has contributed to increased spectrum use by a va-
riety of heterogeneous devices, standards, and applications. This holds especially
true for the Industrial, Scientific and Medical (ISM) bands that are unlicensed and
hence host the most heterogeneous mix of networks. To address the coexistence
problems in those bands, the IEEE has started the 802.11h Working Group to make
recommendations for better future coexistence [31]. It solves problems like interfer-
ence with satellites and radar using the same 5 GHz frequency band. It was origi-
nally designed to address European regulations but is now applicable in many other
countries. The standard provides Dynamic Frequency Selection (DFS) and Transmit
Power Control (TPC) to the 802.11a MAC that especially coexists with radar sys-
tems. Both DFS and TPC will be used further in this topic to improve coexistence
performance of a CR.
