Cambium Networks PTP 250, User Manual

Page: 44 / 228

Wireless operation Chapter 1: Product description
1-16 phn-2510_002v000 (Mar 2012)
Wireless operation
This section describes how PTP 250 wireless links are operated, including modulation
modes, power control and security.
Wireless Transmissions
The PTP 250 uses Time Division Duplexing (TDD) transmission, which means that a single
frequency channel is used for both Transmit and Receive. This is handled automatically by
the radio.
The PTP 250 transmits using Orthogonal Frequency Division Multiplexing (OFDM). This
wideband signal consists of many equally spaced sub-carriers. Although each sub-carrier is
modulated at a low rate using conventional modulation schemes, the resultant data rate
from all the sub-carriers is high.
The channel width of the OFDM signal is configurable to one of two values: 20 MHz or
40 MHz. The higher channel width provides greater link capacity at the expense of using
more spectrum. The lower channel width provides better receiver sensitivity and can also
be appropriate where the amount of free spectrum is limited.
Each channel is offset in center frequency from its neighboring channel by 20 MHz.
Spectrum management
At system start-up, the spectrum management feature of the PTP 250 monitors the
available wireless spectrum and directs both ends of the wireless link to operate on a
channel with a minimum level of co-channel and adjacent channel interference.
Alternatively, the user can configure the master unit to use a single channel or a sub-set of
channels.
Adaptive modulation
The PTP 250 can transport data over the wireless link using different modulation modes.
For a given channel width, each modulation mode transports data at a fixed rate. Although
the high order modulations will transport data at a much higher rate than the lower order
modulation modes, the receiver requires a much higher signal to noise ratio.
The system 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.