With ever increasing demand on the radio spectrum and the limited number of frequencies available, the need for efficient technologies that maximise the utility of those frequencies has never been greater.
Spectrum Efficiency
FDMA, true 6.25 kHz Spectrum Efficiency in all Modes
Both TDMA and FDMA technologies achieve the same 6.25kHz narrowband capability via different methods. The difference is that the FDMA system is a 'true' 6.25kHz channel and the TDMA system provides 6.25kHz channel 'equivalence' via the time slots in 12.5kHz band-width.
From the perspective that 12.5kHz is considered the current narrowband standard channel spacing, then both systems achieve so called "double capacity". The difference is that the FDMA system is ALWAYS double capacity whether it is used with or without infrastructure. For TDMA, double capacity is ONLY achieved when a repeater is synchronising the time slots and when two users are in the same geographical area, accessing the same repeater at the same time.
FDMA Digital Mode
TDMA - Digital
TDMA - peer to peer in digital mode
The difference between FDMA and TDMA
Without getting too technical, the basic difference between FDMA (Frequency Divided Multiple Access) and TDMA (Time Divided Multiple Access) is the definition of a channel and how it is used (accessed). In FDMA, a particular bandwidth (E.g. 6.25kHz) at a particular frequency (E.g. 150.000MHz) is used to define a channel in essentially the same way channels have been allocated for decades. In TDMA, the same principle applies regarding bandwidth and frequency but the signal is divided into time slots that allow the channel to have 'extra' capacity in the same bandwidth E.g. Two 6.25kHz 'equivalent' channels in a 12.5kHz channel. See the diagram below for a graphical explanation.
TDMA - peer to peer in digital mode
Until now, TDMA was more spectrum efficient at wider channel spacing like 25kHz as, for example, two or three users could access the same bandwidth as one FDMA channel user. However, in the case of the newly developed narrowband 6.25kHz FDMA technology like dPMR, both this and 2-slot 12.5kHz TDMA technology achieve the same result as far as spectrum efficiency is concerned.
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The dPMR Association was established in 2007 and provides a forum for interested parties to contribute their expertise.
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