Cambium PTP 450 series, Руководство пользователя

Страница: 96 / 180

PTP 450 User Guide Configuration options for TDD synchronization
2-76
ptp-450 (July 2014)
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 BHSs – it must keep them 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 BHSs. 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. 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 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 ins maintained, in
this mode there is no synchronization of BHMs that can “hear” each other; the BHM will only generate 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 will 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 either a CMM or UGPS module, and there are no other BHMs active within the
link range. Using this option will not synchronize transmission of BHMs that can “hear” each other; it will only
generate a sync signal for the local BHM and its associated BHSs. See Advantage of GPS synchronization on
page 2-77.
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 Figure 22 , BHM

is not synchronized with any of the other BHMs.

is transmitting nearby the other BHMs while they are expecting to receive BHS transmissions from a maximum
distance.