Cambium Networks PMP 450 Series, Installation Manual

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Chapter 3: System planning 151
Fr am e st ar t
GPS synchronization is a way of guaranteeing that the frame start is the same for all APs. This is what is
used by the Cambium PMP devices.
However, the timing between the GPS signal and the start of the TDD frame is not necessarily the same for
all systems. The Cambium PMP devices and the LTE devices have different start time for their frames.
The Cambium PMP Radio configuration page offers an LTE co-location option in the GUI Radio page, that
allows the user to co-locate Cambium PMP devices with LTE devices by shifting the start of the PMP frame
to match the start of the LTE frame.
This can be found under Configuration > Radio > Frame Configuration, as shown in below figure.
If the Co-located LTE Frame Configuration Option is selected as Disable, then the Cambium PMP frame
start is not shifted from its legacy timing.
If any of the three other options (LTE Frame Configuration 0, 1 or 2) is selected, the Cambium PMP frame
start is shifted to align to the LTE frame start. Additionally, each Frame Configuration has its frame start
shifted by an integer number of subframes.
The Cambium PMP frame always starts with the downlink portion of the frame, followed by the turnaround
time and then the uplink portion of the frame.
The LTE 10-ms interval however, always starts with subframes D, S and U as the first three subframes, but it
may have additional downlink subframes in the same frame.
For example, in Frame Configuration 1 the 10 ms interval is composed of two repetitions of the following
subframes: D, S, U, U, D. In order to align the Cambium PMP frame to this LTE frame, the downlink start has
to align to the beginning of subframe 4, not subframe 0. With this shift, the sequence of subframes in the
LTE frame becomes D, D, S, U, U, which is the same structure of the Cambium PMP frame (downlink,
turnaround time, uplink).
When selecting one of the LTE Frame Configuration options from the Co-located LTE Frame
Configuration Option drop-down menu, a shift is applied to the Cambium PMP frame in order to correctly
line up with the selected LTE frame.
Fr am e p ar am et er s
After following the steps described above, the Cambium PMP and the LTE APs are synchronized, and their
frame length is the same.
Next, the frame parameters have to be selected in order to avoid any overlap between one AP