What is the best power monitoring option for TDMA, P25, or DMR?

It depends on where you are measuring the power. Base stations and repeaters will use all available slots of these digital formats, so the APM-16, 4480A, or one of the 5000 Series sensors are great options for monitoring average power. 

Many of today's channel access methods involve the use of low repetition frequency time division-multiple access (TDMA) techniques, such as Project 25 (P25) and DMR.

The P25 or DMR format uses an approach where two discrete time slots provide access to a single 12.5 kHz communication channel, providing the equivalent of 6.25 kHz channel bandwidth for the system. When measuring the power output of transmitters using the either of these formats, the base station and repeater transmitters will normally transmit with both their 30 ms 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 portable and mobile subscriber units however, normal operation is to use only one time slot (whichever is assigned by the channel controller), approximately 30 ms in duration, followed by approximately 30 ms at zero power in the interval where the other time slot would normally occur.

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 reading, the correct measurement would be that of burst average power. When making this measurement, the sensor will measure 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. 

Burst width is the duration of a pulse. Period is the time from the start of one pulse burst to the start of the next pulse burst. Duty cycle is the percentage of time that the transmitter is on. To calculate the duty cycle simply divide the burst width by the period.

Low duty cycles mean that the burst width is much less than the period; a large amount of dead time surrounds each burst. For low duty cycles, the burst average power will be much larger than the average power.

After peak power is measured, a threshold of ½ the peak is set. The sampled power crosses that threshold at the beginning and end of each burst. The time between crossings is used to calculate the duty cycle. Burst Average Power (PBavg) is calculated by dividing the Average Power (Pavg) by the Duty Cycle.

Burst power measurements provide accurate, stable measurements in bursting applications such as DMR and P25 as well as radar.

Devices such as the Bird 5000 Series wideband sensor can perform the computations for both duty cycle and burst power.