Get the most out of your system

Another advantage of the DMR TDMA approach is that you get two communications channels with one repeater, one antenna and a simple duplexer. Compared to FDMA solutions, two-slot TDMA allows you to achieve 6.25 kHz efficiency while minimising investments in repeaters and combining equipment. The required equipment of the two approaches for a simple system is shown in figure 6 below.
 
 
Figure 6: Equipment required for two channel FDMA and TDMA system

 

FDMA requires a dedicated repeater for each channel, plus expensive combining equipment to enable multiple frequencies to share a single base-station antenna. It can be particularly expensive to make combining equipment work with 6.25 kHz signals, and there’s typically a loss in signal quality and range when it’s used in this way, hence the need for power amplifiers shown in figure 6. With FDMA 6.25 kHz systems there is also lower tolerance for errors introduced by the phenomenon of oscillator ageing and resulting signal drift away from the desired centre frequency by the transmitting radio. This results in less robust adjacent channel protection, making the system vulnerable to interference. This can be offset using a specialised piece of equipment, called a high stability oscillator, but at a cost. In contrast, two-slot TDMA achieves stable two-channel equivalency using single-channel equipment. No extra repeaters or combining equipment are required (and there is lower drain on air conditioning and less back up power supplies needed at a repeater site). This means lower cost and simpler site planning for DMR users