Adaptive Modulation - Cambium Networks PTP 650 Series User Manual

Chapter 1: Product description Wireless operation
Measurement analysis
Spectrum Management uses statistical analysis to process the received peak and mean
measurement. The statistical analysis is based on a fixed, one minute, measurement quantization
period. Spectrum Management collects data for the specified quantization period and only at the
end of the period is the statistical analysis performed.
Statistical summary
The display of statistical measurement on the spectrum management page always shows a
statistical summary of all channel measurement. The statistical summary is controlled by the
Statistics Window attribute. This attribute defaults to a value of twenty minutes, which means that
the mean and percentile values displayed for each channel are calculated over the 20 minute
period. All channel decisions are made using the values computed over the statistics window
period.
Spectrum management in fixed frequency mode
The transmit and receive frequencies can be fixed in a PTP 650 wireless link. Once fixed frequency
mode is configured, the spectrum management software will not attempt to move the wireless link
to a channel with lower co-channel and adjacent-channel interference. Therefore this mode of
operation is only recommended for deployments where the installer has a good understanding of
the prevailing interference environment. Care must also be taken to ensure that the frequency
allocations at each end of the link are compatible.
Fixed frequency mode is not available in regions where radar detection is required by the
regulations.

Adaptive modulation

The PTP 650 series can transport data over the wireless link using a number of different
modulation modes ranging from 256QAM 0.81 to BPSK 0.63. For a given channel bandwidth and
TDD frame structure, each modulation mode transports data at a fixed rate. Also, the receiver
requires a minimum signal to noise ratio in order to successfully demodulate a given modulation
mode. Although the more complex modulations such as 256QAM 0.81 will transport data at a
much higher rate than the less complex modulation modes, the receiver requires a much higher
signal to noise ratio.
The PTP 650 series provides an adaptive modulation scheme where the receiver constantly
monitors the quality of the received signal and notifies the far end of the link of the optimum
modulation mode with which to transmit. In this way, optimum capacity is achieved at all times.
This is one of a number of features which allows the PTP 650 to operate in challenging non-line of
sight radio channels.
Page
1-10