User Guide

PMP 450 Planning Guide
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:
• AutoSync: The AP automatically receives sync from one of the following sources:
o GPS Sync over Timing Port (UGPS, co-located AP GPS sync output, or “Remote AP” feed from a
registered SM’s GPS sync output)
o GPS Sync over Power Port (CMM)
o On-board GPS (internal GPS)
Upon AP power on, the AP does not transmit until a valid synchronization pulse is received from one of
the sources above. When there are synchronization sources on both the timing port and the power port,
the power port GPS source will be chosen first.
If there is a loss of GPS synchronization pulse, within two seconds the AP automatically attempts to
source GPS signaling from another source. On-board GPS (internal GPS) is the last source checked for
GPS signaling if there is no receipt of signaling from the timing port or from the power port. AutoSync
will source timing from the on-board GPS if a valid synchronization pulse is detected (the on-board GPS
module should not be used as the primary timing source). If no valid GPS signal is received, the AP ceases
transmission and SM registration is lost until a valid GPS signal is received again on the AP.
• AutoSync + Free Run: This mode operates similarly to mode “AutoSync”, but if a previously received
synchronization signal is lost and no GPS signaling alternative is achieved (from the timing port, power
port, or on-board GPS), the AP automatically changes to synchronization mode “Generate Sync”. While
SM registration ins maintained, in this mode there is no synchronization of APs that can “hear” each
other; the AP will only generate a sync signal for the local AP and its associated SMs. Once a valid GPS
signal is obtained again, the AP automatically switches to receiving synchronization via the GPS source
and SM registration is maintained.
In mode AutoSync + Free Run, if a GPS signal is never achieved initially, the system will not switch to “Free
Run” mode, and SMs will not register to the AP. A valid GPS signal must be present initially for the AP to
switch into “Free Run” mode (and to begin self-generating a synchronization pulse).
• Generate Sync (factory default): 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 1-118.
1-116
pmp-0047 (March 2014)