Red
MAX
™
Base Station
user manual
Doc. #70-00058-01-01-DRAFT
Proprietary Redline Communications © 2006
November
29,
2006
Page 81 of 106
lower modulation. A moderately high C/I ratio is required for the modulation to remain
unchanged.
In case 2 the signal is very strong (C/N is more than 10 dB above the CINR threshold)
and can experience more severe interference without being forced to change modulation.
The amount of interference required to force the system to go to the lower modulation
rate in these two extreme cases is very different, and in an actual deployment scenario a
full range of results are possible. Careful planning is necessary when frequency reuse is
required, and sufficient fade margin must be included to anticipate fluctuations of both
C/I and C/N.
6.2 Interference
Issues
6.2.1
Multipath Interference
The base station is designed with high immunity to interference and multipath signals. Its
core technology is Orthogonal Frequency Division Multiplexing (OFDM), capable of
reliable performance under multi-path and frequency selective fading known to have
severe signal fading and distortion effects in the sub-11 GHz frequencies.
Multipath interference is a significant problem in long-range links, and in near line-of-
sight, and non line-of-sight links. Multipath is a form of self-interference occurring when
signal reflections arrive slightly later than the primary signal. The result can be
destructive interference that can essentially null out the primary signal or overlap the
original signal such that it cannot be decoded. Multi-path interference is a problem with
long-range links where reflections off the ground, snow, and water frequently interfere
with the primary signal. It is also a problem in urban environments where the signal
reflects off buildings, trees, and roads.
OFDM breaks up the transmit signal into many smaller signals. For example, instead of
one single carrier carrying 70 Mbps of data (wireless interface rate), there are 192
separate carriers, each carrying about 364 Kbps of data (in the case of the Redline
product) in a 14 MHz bandwidth. If selective fading degrades one or two carriers, the
impact is minimal since the information is spread across the remaining carriers.
Figure 34: Op Notes:
OFDM Multiple Carriers
One key aspect of OFDM implementation is that the individual carriers overlap
significantly to preserve overall bandwidth. Normally, overlapping signals would
interfere with each other, however, through special signal processing, the carriers in an
OFDM waveform are orthogonal to each other.
Multipath interference may cause individual narrowband channels to be altogether lost.
This problem is addressed in the Redline implementation in three ways: