Bird Technologies 5019D-Manual - Manual (Page 26)

14
Low Repetition Rate Waveforms
Many of today's channel access methods involve the use of
low repetition frequency time division-multiple access
(TDMA) techniques. Examples of these relatively new TDMA
formats include Digital Mobile Radio(DMR), and TETRA sys-
tems. The DMR format uses an approach where two discrete
time slots provide access to a single 25 kHz communication
channel, providing the equivalent of 12.S kHz channel band-
width for the system. The modulation techniques used within
these systems vary, from relatively benign forms of frequency
shift keying, to more complex quadrature formats such as
Quadrature Phase Shift Keying (QPSK).When measuring the
power output of transmitters using the DMR format, the base
station transmitters will normally transmit with both time
slots active, even if there is no traffic on one of the time slots.
Under these conditions, it is a simple matter to measure the
average power of this continuous waveform. When measuring
the output power of subscriber units however, normal opera-
tion is to use only one time slot, approximately 30 ms in dura-
tion, followed by approximately 30 ms at zero power in the
interval where the other time slot would normally be located.
The remaining waveform resembles a 30 ms burst, occurring
at a 17 Hz repetition rate. Since the measurement of average
power under this condition would result in an unstable read-
ing, the correct measurement would be that of burst average
power. When making this measurement, the sensor will mea-
sure the duty cycle of the signal, as well as the average power,
and compute the burst average power of the waveform based
upon these parameters.