Overview of Simulcast Optimization

The following subsystems require optimization/calibration:

Simulcast subsystems require precise control of outbound timing and modulation levels. The components that make up your system are carefully selected to have the appropriate specifications for stability and accuracy. Timing control in Motorola Solutions systems is automatic and no routine alignments for timing are required. Once your system is configured to accommodate the longest link delay, it does not require further adjustments or alignment for timing.

For analog modulated simulcast operation, it is required to carefully adjust the deviation from each individual base radio to tightly controlled levels. Also, the frequency response of each base radio should be verified so that there are minimum variations in modulation levels with frequency. For proper operation, analog modulation levels must be held to ± 0.15 dB or within a total range of variability of 0.3 dB. Achieving this level of accuracy requires the use of different techniques than used with non-simulcast systems.

The traditional FM deviation meter responds to the peak deviation level. This is used to ensure that deviation does not exceed the emission mask requirements. However, a disadvantage of the peak measurement is that any noise introduced in the measurement shows up as a high deviation reading. It takes little noise to exceed the ± 0.15 dB level tolerance. Noise is often an issue due to the requirements of performing the fine level check.

The fine level check works best when all the same test equipment is used throughout the system. This check helps to ensure that different test equipment does not cause variations. Controlling and performing adjustments of the simulcast subsystem is easiest to achieve at the simulcast prime site. The general nature of the test equipment required is:

The best and most efficient method for performing the fine level check is to use an optimization receiver located at the simulcast prime site. The other simulcast sites need to produce usable signal strength levels for the optimization receiver at the prime site. An external antenna mounted on a tower or high location should be used. If enough signal strength is present, you may be able to use a service monitor as the optimization receiver. The service monitor is typically 20 dB to 30 dB less sensitive than a subscriber and can be influenced by other carriers or signals as it has virtually no RF selectivity. If a sufficient RF level cannot be obtained to use a service monitor, then use a consolette or mobile in a tray subscriber radio connected to the external antenna. Occasionally, to obtain sufficient RF levels, a Yagi antenna on a rotator needs to be used. In extreme situations, an optimization receiver may have to be located elsewhere. In this case, the optimization receiver audio must be made available at the prime site. This is due to the need to be at the prime site for control of the system while performing the fine level check.

Since the optimization receiver is receiving RF signals over the air and it is likely that the RF levels produce different quieting in the optimization receiver, it is nearly impossible to use the FM deviation meter in the service monitor for the fine level check. This leads to the need for a different level meter.

By far the best solution for simulcast optimization is to use a Dynamic Signal Analyzer. The DSA is an audio spectrum analyzer and displays level versus frequency. The DSA can improve immunity to noise in our demodulated simulcast audio by upwards of 50 dB over the peak FM meter, which helps to ensure that the final system level requirement of ± 0.15 dB can be met.