DECT RF / Air Interface

- summary of the DECT RF or Air Interface /physical layer including the DECT modulation format..

The DECT RF air interface was designed to be relatively straightforward to keep costs within reasonable bounds, while still maintaining a robust RF link.

Apart from the DECT modulation format, the DECT RF air interface incorporates a number of features including continuous analysis of the available channels, and a multiple access scheme that allows multiple handsets to use a single base station.

DECT modulation

The signal is modulated using a form of modulation called Gaussian Frequency Shift Keying (GFSK) and has a BT of 0.5. This provides the optimum spectral usage for the system.

The system uses dynamic channel allocation and is thereby able to reduce the levels of interference, and ensure that links are set up on the least interfered channels. All DECT equipment scans the frequency allocation at least every 30 seconds as a background activity. This produces a list of free and occupied channels along with the available timeslots to be used for the channel selection, should this be required.

Additionally the DECT portable continuously analyses the signals to ensure that the signals originate from the base station to which it is connected and has access rights. The portable locks onto the strongest base station and checks it can access the base station as detailed in the DECT standard, and the channels with the best signal strength (RSSI - Receive Signal Strength Indication) are used for the radio link as required. This Dynamic Channel Selection and Allocation mechanism guarantees that radio links are always set-up on the least interfered channel available and hence the best performance is obtained.

DECT MC/TDMA/TDD principle

The DECT radio interface employs a number of techniques in its access methodology. The scheme uses Multi-Carrier, Time Division Multiple Access, Time Division Duplex (MC/TDMA/TDD).

The basic DECT system has a total of ten possible carrier frequencies between 1880 and 1900 MHz, i.e. it is a Multi-Carrier (MC) system.

In addition to this the time dimensions for each carrier is divided to provide timeframes repeating every 10 ms. Each frame consists of 24 timeslots, each of which is individually accessible and may be used for either transmission or reception. For the basic DECT speech service two timeslots - with 5 ms separation - are paired to provide bearer capacity for typically 32 kbps (ADPCM G.726 coded speech) full duplex connections.

In order to simplify the way DECT can be used when only basic implementations are needed, the allocations of timeslots within the 10 ms timeframe are restricted. The first 12 timeslots are used for downlink transmissions and the remaining 12 are used for the uplink. This reduces the level of complexity, and as this is not needed for basic implementations, it can provide some cost savings.

By Ian Poole

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