Compliance with radio regulations
PTP 450 Configuration and User
pmp-0815 (August 2015)
6-39
the existing channel before attempting to initiate transmission. If the running average of
the energy sensed during the channel back-off time is below the detection threshold, the
BHM is allowed to initiate transmission.
LBT detection threshold calculation
The energy detection threshold used for LBT is proportional to the maximum transmit
power of the transmitter. For a 23 dBm EIRP transmitter the detection threshold is -73
dBm/MHz at the receiver’s input (assuming a 0 dBi receive antenna). At start up the
system monitors the channel frequency for 1 second before determining if the channel is
busy.
The detection threshold is proportional to the maximum transmit power of the
transmitter. The detection threshold is modified according to the following formula:
Detection Threshold (dBm) = -73 dBm/MHz + 10log
10
(B) + 23 – P
T
+ A
Where:
Is:
B
Monitored bandwidth in MHz
P
T
maximum transmit power in dBm
EIRP
A
receive antenna gain in dBi
The receive antenna gain A is set equal to the external antenna gain. As part of the radio
configuration for LBT the operator must enter the maximum desired conducted power
P
c
, the external antenna gain A and the channel bandwidth B. The PTP 450 BHM will
ensure that the sum of the actual conducted power and the external antenna gain used
to calculate P
T
does not exceed the regulatory EIRP limit.
LBT operation of PTP 450 BHS
Only the PTP 450 BHM (BHM) performs the threshold detection. The PTP 450 system
employs a proprietary media access layer that utilizes a TDD/TDMA scheduled
transmission which is synchronously framed. The client PTP 450 Backhaul Slave (BHS)
cannot transmit data until it is allocated bandwidth from the BHM. If the BHM detects
co-channel signals, then the BHM will not allocate any uplink data symbols to the BHS.
In this system, since permission to transmit is granted by the BHM, there is no hidden
node problem like that experienced by purely contention based protocols (e.g. Wi-Fi
using CBHSA/CA in the Distributed Coordination Function mode). The BHM is typically
installed in a high location where it is most likely to receive co-channel interference and
is most susceptible to detection.
