Product Info

System Description WipLL Radio Technology - Physical Layer
25030311-08 Airspan Networks Inc. 2-39
2.6.6.3.2. Inter-system Interference
The immunity of the WipLL system to external interference is due to its spread
spectrum system. Exposure to external interference is highest when operating in the
unlicensed band. Both FCC and ETSI set limits on the spreading level that is
required by a frequency hopping spread spectrum system.
 ETSI:
According to EN 300 328, FHSS modulation must use at least 20 well-defined,
non-overlapping channels separated by the channel bandwidth as measured at 20
dB below peak power.
Since WipLL’s carriers must be located at integer multiples of megahertz
(MHz), the above statement limits the channel spacing to 1 MHz for the 1 Msps
mode, and 2 MHz for the 1.33 Msps mode. This limits the number of possible
hops in the WipLL system when operating under ETSI regulations to between 20
and 80 for the 1 Msps mode, and between 20 and 40 for the 1.33 Msps mode.
 FCC:
Part 15 of the FCC sets the same requirement for non-overlapping channels.
For frequency hopping systems operating in the 902 MHz to 928 MHz band with
a channel bandwidth greater than 250 KHz, the system shall use at least 25
hopping frequencies.
For frequency hopping systems operating in the 5725 MHz to 5850 MHz band,
the system shall use at least 75 hopping frequencies.
For frequency hopping systems operating in the 2400 MHz to 2483.5 MHz band,
the system shall use at least 15 non-overlapping channels.
In general, since the number of different hops is substantially lower, the capability of
WipLL to overcome interferences is reduced. In practice, ETSI allows the operator
the flexibility not to use the entire spectrum, in case, for example, a constant
interference is identified in a portion of the spectrum. This makes the problem much
less critical than in the FCC case, where the entire spectrum must be used.