When is wide area coverage too much coverage



Some manufacturers are able to support DMR Tier 3 simulcast by using PTP (Precision Time Protocol) on the IP backbone and GPS at the Base Stations. PTP ensures that a voice packet arrives at each Base Station at exactly the same time, even though the latency to all the RF Sites is different. Frequency and phase stability is achieved via GPS (or some other high stability clock source).

DMR requires that both the Base Station transmit frequency and time (phase) be perfectly synchronized - hence the need for GPS and PTP. For the Base Station receive signal (i.e. uplink), these systems uses voting and there is typically comparator somewhere in the network. The comparator looks at the incoming signal from all the Base Stations and chooses the signal (receiver) with the lowest BER (Bit Error Rate). There would be one comparator for each frequency pair - in addition to the MSO. The comparator also becomes the single point of failure since all voice and data traffic needs to pass through it - which may neccesitate geographically redundant comparators.

The advantage here is that using simulcast, one is able to cover a large area, using multiple Base Stations (RF Sites) and only one frequency pair. The disadvantage is that the whole system is reliant on GPS and the cost of the Base Stations would be more costly than a standard repeater (extra hardware, economy of scale). On a DMR Tier 3 system, having a single site, covering a large area can often be a disadvantage, as all the users in that coverage area will all the registered to the same site (remember that although there are multiple base stations, they are all seen as one channel at the radio and system level, much like IPSC) and this could lead to an overload and consequently a poor GoS (grade of service).

Capacity Max obviously does not support simulcast and this can be seen by some as a disadvantage, if there is a requirement for low user density, wide area coverage sites.

The advantage that Capacity offers here, is that no GPS synchronization or comparator is needed thus reducing the overall infrastructure cost.

When designing a trunked system, it sometimes makes sense to not have too much coverage.
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