User's Manual

INSTALLATION AND MAINTENANCE MANUAL
LYNX.sc E1 FAMILY
SPREAD SPECTRUM RADIOS
SEPTEMBER 1999
SECTION 4: TROUBLESHOOTING PAGE 4-19
4.8 Interference Countermeasures
The recommended interference countermeasures available to the LYNX.sc operator are as follows:
1. Short Paths
The single most effective countermeasure against interference is to maintain "short path" length.
This may be achieved by dividing long paths into multiple small paths by cascading hops.
Intermediate repeaters may be formed using back-to-back LYNX.sc terminals and transmit output
power reduced, if required.
By definition, "short path" is defined as a path where fades are extremely rare and signal levels vary
by no more than ±3 dB during fades. This distance will vary with the RF frequency. Typically a
"short path" is defined as any path length shorter than 5 miles at 5.8 GHz or 7 miles at 2.4 GHz.
2. Narrow Beam Antennas (high gain)
This is the next most effective countermeasure. Narrow beam antennas ensure that the transmitted
power is sent in a single direction and this minimizes the possibility of causing interference
inadvertently to other users. Narrow beam antennas also reject off-azimuth signals being received
from potential sources of interference and have high gain which boosts desired receive levels and
improves the carrier to interference ratio. When selecting narrow beam antennas, it is helpful to
know that larger antennas generally outperform smaller antennas. Another important antenna
specification is the front-to-back ratio which ensures rejection of unwanted signals from azimuth
angles behind the antenna.
3. Frequency Selection
This is another very effective countermeasure. The LYNX.sc radio offers several distinct non-
overlapping frequency channel plans (see Sections 3.5 and 4.2) and the radio’s RF filter is able to
reject interference more than 10 MHz away from the receive frequency. Offset frequencies combined
with other countermeasures may enable several receive channels to operate at a single hub site.
Because of the limited spreading ratio used, frequency selection is more efficient than code
selection for interference rejection when operating multiple LYNX.sc terminals at a single site.
Interference can often be overcome by exchanging frequencies of both-ends of the radio link (e.g.
change your A1 terminal to an A2 and change the other end from an A2 to an A1). Also, changing
channel plans (e.g. from A to B) can be very effective. (See Section 4.2).