TDMA advanages of TETRA
This page describes the advantages of TETRA related to TDMA (Time Division Multiple Access) focused on higher data rates, improved data throughput, bandwidth on demand, spectrum efficiency, etc.
Four time slot TDMA technology was adopted in TETRA as it offered the optimum solution to balance the cost of equipment with that of supporting the services and facilities required by user organisations for a medium to high capacity network providing single site local RF coverage and/or multiple site wide area RF coverage.
RF Spectrum efficiency is a combination of three main factors being the occupied bandwidth per communication channel, the frequency re-use factor determined by the Carrier to Interference protection ratio C/I in dB’s and the trunking technology used. As previously mentioned TETRA utilises the latest in trunking technology. Also, as can be seen in the diagram, the TDMA technology used in TETRA provides 4 independent communications channels in a 25 kHz RF bandwidth Channel, making it twice as efficient in occupied bandwidth terms as a traditional 12.5 kHz RF bandwidth FDMA channel. Although FDMA technologies tend to have a better C/I performance than TDMA TETRA, the overall spectrum efficiency advantage lies with TETRA, especially for medium to high capacity networks.
Because trunking is employed to increase network capacity and/or RF spectrum efficiency (for a given Grade of Service) the cost and equipment space at base station site can be significantly reduced compared with traditional FDMA technology trunking solutions.
From the base station equipment configuration in figure 2 it can be seen that the FDMA solution requires 4 separate transceivers where as the TDMA solution only requires 1 transceiver. As a consequence, the FDMA solution requires a transmitter antenna combining and receiver splitting network to enable single transmit and receive antenna working. Also, the RF power output of the FDMA transmitters will need to be higher in order to compensate for transmission losses in the transmit antenna combining network. Because 4 slot TDMA already supports four independent communication paths, no antenna combining equipment is required, thereby saving space as well as cost.
Another advantage of TDMA technology is that it enables new services and facilities to be supported with minimum cost.
Some of these examples are:
Higher Data Rates
The ‘laws of physics’ limits the maximum data rate in a given RF channel bandwidth. Assuming the same modulation scheme, the wider the channel bandwidth the higher the data rate. Because TDMA uses wider channels than FDMA, the combined data rate on a single RF carrier is greater.
Improved Data Throughput in Poor RF Signal Conditions
The net data rate in TDMA is better than FDMA in poor RF propagation conditions. This is because Automatic Repeat Requests (ARQ’s) are required when received data is corrupted as a result of RF fading. As TDMA terminal devices effectively
operate in full duplex ARQ’s can be sent efficiently after each time slot transmission if required. As FDMA terminals operate mainly in simplex (either transmit or receive), any ARQ scheme employed will require a transmission interrupt to allow ARQ’s to be received, which dependent on the number of interrupts employed can be very inefficient.
Bandwidth on Demand
In TDMA any number of time slots up to the maximum limit of the technology being employed can be combined to increase data throughput as required for specific applications.
Concurrent Voice and Data
Because of the TDMA time slot structure it is possible to assign one time slot to support voice and the next time slot to support data in a two slot transmission from radio terminals. This capability effectively allows a single radio terminal to concurrently transmit or receiver voice and data at the same time.
Full duplex Voice Communications
TDMA technology inherently supports full duplex communications. Although full duplex voice communications can be supported on FDMA systems the need for duplex operation requires RF screening between the transmitter and receiver and also a duplexer to allow single antenna working. Because of this, duplex FDMA radio terminals are usually bulkier and more costly to produce than TDMA terminals, which do not need RF screening or antenna duplexers.
In summary, TETRA’s 4 time slot TDMA technology offers the optimum solution, which balances the cost of equipment with that of supporting the services and facilities required by user organisations for a medium to high capacity network providing single site local RF coverage and/or multiple site wide area RF coverage.
Source: TETRA Association