FDMA vs TDMA Underground: Why Amplifier Group Delay Favours FDMA on Leaky Feeder and DAS Cascades
On the surface, TDMA wins the spectrum argument: two calls on one 12.5 kHz carrier for DMR and P25 Phase 2, four on 25 kHz for TETRA. Underground, every signal reaches the radio through a cascade of cable, line amplifiers, repeaters and sometimes fibre, and every one of those elements delays it. FDMA has no slot clock and no guard time, so that delay costs it almost nothing. TDMA carries an inbound timing allowance of 1 ms for DMR and 389 µs for TETRA, and the delay tolerance where two copies of a signal meet scales with the symbol period, which is longest for the FDMA standards in common use. This article works the three delay questions with the numbers the ETSI and TIA material actually publishes, shows why a channelised amplifier at 12.5 kHz adds the delay of 16 km of cable in a single pass, and sets out the cases where TDMA underground is comfortable and the cases where it is not.
Read article →
