User Guide

PMP 450 Planning Guide Configuration options for TDD synchronization
pmp-0047 (December 2012)
2-39
Configuration options for TDD synchronization
The PMP 450 system uses Time Division Duplexing (TDD) - one channel alternately transmits and receives - rather
than using one channel for transmitting and a second channel for receiving. To accomplish TDD, the AP must
provide sync to its SMs – it must keep them in sync. Furthermore, collocated APs must be synced together - an
unsynchronized AP that transmits during the receive cycle of a collocated AP can prevent that second AP from
being able to decode the signals from its SMs. In addition, across a geographical area, APs that can “hear” each
other benefit from using a common sync to further reduce self-interference within the network.
The configuration options available for synchronization on the PMP 450 Access Point are:
• Generate Sync Signal: This option may be used when the AP is not receiving GPS synchronization pulses
from either a CMM or UGPS module, and there are no other APs active within the link range. Using this
option will not synchronize transmission of APs that can “hear” each other, it will only generate a sync signal
for the local AP and its associated SMs. See Advantage of GPS synchronization on page 2-40.
• Sync to Received Signal (Timing Port / UGPS): This option may be used to set the AP to receive GPS
synchronization pulses from a connected CMM, an AP in the cluster, or a UGPS (Universal Global Positioning
System) module via the RJ11 sync port.
• Sync to Received Signal (Power Port): This option may be used to set the AP to receive GPS
synchronization pulses from a connected CMM via the RJ45 port.
• Sync to iGPS: This options may be used to set the AP to receive GPS synchronization pulses from its internal
GPS module.
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 cluster management module (CMM) contains a Cambium GPS Receiver. The CMM is a critical element in the
operation of the 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 NAVSTAR 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 which, in turn, synchronize the SMs in the 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.