Delivering Increased Capacity
MOTOTRBO uses a two-slot TDMA architecture. This architecture divides the channel into two alternating time slots, thereby creating two logical channels on one physical 12.5kHz channel. Each voice call utilizes only one of these logical channels, and each user accesses a time slot as if it is an independent channel. A transmitting radio transmits information only during its selected slot, and will be idle during the alternate slot. The receiving radio observes the transmissions in either time slot, and relies on the signaling information included in each time slot to determine which call it was meant to receive.
By comparison, analog radios operate on the concept of Frequency Division Multiple Access (FDMA). In FDMA, each transmitting radio transmits continuously on a designated channel, and the receiving radio receives the relevant transmission by tuning to the desired carrier frequency.
TDMA thereby offers a straightforward method for achieving 6.25kHz equivalency in 12.5kHz
repeater channels – a major benefit for users of increasingly crowded licensed bands. Instead of dividing channels into smaller slices of decreased bandwidth – which is what would be required to increase spectrum efficiency with FDMA methods, TDMA uses the full 12.5kHz channel bandwidth, but increases efficiency by dividing it into two alternating time slots. Additionally, this method preserves the well-known radio frequency (RF) performance characteristics of the 12.5kHz signal. From the perspective of RF physics – that is, actual transmitted power and radiated emissions – the 12.5kHz signal of two-slot TDMA occupies the channel, propagates, and
performs essentially in the same way as today’s 12.5kHz analog signals. With the added advantages of digital technology, TDMA-based radios can work within a single repeater channel to provide roughly twice the traffic capacity, while offering RF coverage performance equivalent to, or better than, today’s analog radio.
Two-Slot TDMA Reduces Infrastructure Equipment
As we have seen, two-slot TDMA essentially doubles repeater capacity. This means that one MOTOTRBO repeater does the work of two analog repeaters (a MOTOTRBO repeater supports two calls simultaneously).
This saves costs of repeater hardware and maintenance, and also saves on the cost and complexity of RF combining equipment necessary in multi-channel configurations.
Just as importantly, there is no need to obtain new licenses for the increase in repeater capacity, and compared to alternative technologies that may operate on different bandwidths, there is no comparative increase in the risk of interference with or from adjacent channels.
An Example below showing how three different DMR repeaters – one in Dundee, Edinburgh and Leeds can all be used at exactly the same time with three different Talkgroups in use.
Talkgroup 80 is a User Activated UK-Wide Talkgroup which unlike TG235 which opens all UK Repeaters only opens TG80 on the Repeater that a user keys up on TG80 to activate it, Talkgroup 850 is a Regional Scottish Talkgroup linking Dundee and Edinburgh Repeaters. Talkgroup 9 is a local chat Talkgroup.