User's Manual

PTP 250 User Guide Wireless operation
phn-2182_002v000
May 2011
1-17
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 specify a single channel to use (or several to be chosen
from) at the master unit.
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. Also, the receiver requires a given signal to noise ratio in order to successfully
demodulate a given modulation mode. Although the more complex modulations will
transport data at a much higher rate than the less complex modulation modes, the
receiver requires a much higher signal to noise ratio.