User guide

March 2005 Understanding Canopy Networks
Through Software Release 6.1
Page 68 of 425 Issue 1
Canopy System User Guide
7.2 UNDERSTANDING SYNCHRONIZATION
Although Canopy modules are band selective, they are not channel selective. For this
reason, the receiver of a module can receive too much signal from unsynchronized
modules that are operating in the same spectrum (frequency band range, such as
5.2 GHz). This would overload the front end of the receiver, which would cancel the
carrier-to-interference (C/I) ratio advantage of Canopy modules.
Moreover, Canopy modules must be synchronized so that they transmit and receive in
the proper cycles. An unsynchronized module that transmits during the receive cycle of
another module can render the other module insensitive to the desired signal (desensed).
7.2.1 GPS Synchronization
The Navigation Satellite Timing and Ranging (NAVSTAR) Global Positioning System
(GPS) uses 24 satellites to relay information for precise derivation of position and time.
The Canopy Cluster Management Module (CMM) contains a Motorola Oncore GPS
Receiver. The CMM is a critical element in the operation of the Canopy system. At one
AP cluster site or throughout an entire wireless system, the CMM provides a GPS timing
pulse to each module, synchronizing the network transmission cycles.
The Oncore GPS Receiver tracks eight or more satellites. The CMM uses the signal from
at least four of these satellites to generate a one-second interval clock that has a rise
time of 100 nsec. This clock directly synchronizes APs and BHMs which, in turn,
synchronize all other modules in the Canopy network.
The Oncore GPS Receiver also provides
◦ the latitude and longitude of the GPS antenna (collocated with the CMM)
◦ the number of satellites that are being tracked
◦ the number of satellites that are available
◦ the date
◦ the time in Universal Coordinated Time (UCT)
◦ the altitude of the GPS antenna
◦ other information that can be used to diagnose network problems.
Alternative to GPS Sync
A Canopy link can operate without GPS sync, but cannot operate without sync. The
alternative to GPS sync is to configure the AP or BHM in the link to generate a sync pulse
to pass to the SM or BHS, respectively. Depending on the RF environment in which the
link operates, this latter alternative may or may not be plausible.
For example, in Figure 29, AP4
◦ is not synchronized with any of the other APs.
◦ is transmitting nearby the other APs while they are expecting to receive SM
transmissions from a maximum distance.