Configuration options for TDD synchronization
PTP 450 Configuration and User
pmp-0815 (August 2015)
4-29
Configuration options for TDD synchronization
The PTP 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 BHM must provide sync to its BHS – it must
keep it in sync. Furthermore, collocated BHMs must be synced together - an
unsynchronized BHM that transmits during the receive cycle of a collocated BHM can
prevent that second BHM from being able to decode the signals from its BHS. In addition,
across a geographical area, BHMs 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 PTP 450 BHM are:
AutoSync
: The BHM automatically receives sync from one of the following sources:
o
GPS Sync over Timing Port (UGPS, co-located BHM GPS sync output or “Remote
BHM” feed from a registered BHS’s GPS sync output)
Upon BHM power on, the BHM does not transmit until a valid synchronization pulse is
received from one of the sources above.
If there is a loss of GPS synchronization pulse, within two seconds the BHM
automatically attempts to source GPS signaling from another source.
Au 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), the BHM automatically changes to synchronization
mode “Generate Sync”. While BHS registration is maintained, in this mode there is no
synchronization of BHMs that can “hear” each other; the BHM only generates a sync
signal for the local BHM and its associated BHSs. Once a valid GPS signal is obtained
again, the BHM automatically switches to receiving synchronization via the GPS source
and BHS registration is maintained.
In mode Au Free Run, if a GPS signal is never achieved initially, the system will
not switch to “Free Run” mode, and BHSs does not register to the BHM. A valid GPS
signal must be present initially for the BHM 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 BHM is not
receiving GPS synchronization pulses from UGPS module, and there are no other
BHMs active within the link range. Using this option does not synchronize transmission
of BHMs that can “hear” each other; it only generates a sync signal for the local BHM
and its associated BHSs. See
Advantage of GPS synchronization
on page
Alternative to GPS synchronization
A link can operate without GPS sync, but cannot operate without sync. The alternative to
GPS sync is to configure the BHM in the link to generate a sync pulse to pass to the BHS.
Depending on the RF environment in which the link operates, this latter alternative may or
may not be plausible.
For example, in
, BHM:
