User's Manual Part 1

3: Detailed Description
APCD-LM043-8.0 (DRAFT C) 37
• Reduced power spectral density: Spreading over a wider bandwidth reduces the
spectral density (power per Hz of bandwidth) of the transmitted signal, allowing
simultaneous operation of many spread-spectrum systems in the same frequency
band and geographic area. The reduced spectral density also allows you to meet the
regulatory emissions requirements in the ISM frequency bands.
• Transmission security: It is technologically more difficult to recover (or interfere with)
spread-spectrum signals than it is to recover conventional narrowband signals.
• Interference suppression: The same mechanism that de-spreads the desired signal
in the receiver, spreads undesired signals, which then appear to the receiver as lower
levels of RF noise. This effect is illustrated in Figure 23.
Figure 23 Effect of Despreading
3.5.5 Data Rate
The raw channel bit rate is 2.75 Mbps. The maximum data rate presented to the MAC layer is
2.4 Mbps, which translates to a peak FTP rate of about 2 Mbps.
3.5.6 Co-located Channels
A maximum of four orthogonal (nonoverlapping) channels can be provisioned at a single CAP
but WaveRider recommends a maximum of three. To ensure adequate isolation between
channels, a minimum co-channel spacing of 6.6 MHz is recommended, as is the use of
channel filters and a properly engineered antenna system. A possible frequency set for a
three-channel CAP is
• 905.0 MHz
• 915.0 MHz
• 925.0 MHz
A separate CCU, channel filter, transmission line, lightning protector, and antenna are
required for each of the orthogonal channels.
3.5.7 Duplexing
The radio channel uses Time Division Duplexing (TDD), which means that the CCU or EUM is
in either receive or transmit mode, but does not transmit and receive at the same time.
Before De-spreading
Interferer
Desired
Signal
After De-spreading
Desired Signal
Inteferer
Becomes