Diagramming Network Layouts
ptp-450 (July 2014)
2-69
Diagramming Network Layouts
Avoiding Self Interference
The following section includes information maximizing tower performance by minimizing self-interference.
Physical Proximity
Two BHM clusters co-located on the same tower require a CMM. The CMM properly synchronizes the transmit
start times of all modules to prevent interference and de-sensing of the modules. At closer distances without GPS
synchronization, the frame structures cause self-interference. Non-synchronized deployments are highly
discouraged.
Furthermore, non-synchronized BHMs on the same tower require that the effects of their differing receive start
times be mitigated by either
100 vertical feet (30 meters) or more and as much spectral separation as possible within the same frequency
band range
the use of the frame calculator to tune the
Downlink Data
parameter in each, so that the receive start time in
each is the same
The constraints for collocated modules in the same frequency band range are to avoid self-interference that would
occur between them. Specifically, unless the uplink and downlink data percentages match, intervals exist when one
is transmitting while the other is receiving, such that the receiving module cannot receive the signal from the far
end.
The interference is less a problem during low throughput periods and intolerable during high. Typically, during low
throughput periods, sufficient time exists for the far end to retransmit packets lost because of interference from the
collocated module.
Spectrum Analysis
You can use an BHS as a spectrum analyzer. See
Mapping RF Neighbor Frequencies
on Page
. Through a
toggle of the
Device Type
parameter, you can temporarily transform an BHM into an BHS to use it as a spectrum
analyzer.
BHS Automatic Transmit Power Control
The PTP 450 BHM automatically sets the transmitter output power in its BHSs through a feature named Auto-TPC
(Transmit Power Control). The conceptual reason for this feature is OFDM reception in the BHM is sensitive to
large differences in power levels received from its BHSs, and by limiting power levels of close-in BHSs the overall
RF noise floor is lowered.
Summary of Contents for PTP 450 series
Page 1: ...Cambium PTP 450 User Guide System Release 13 2...
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