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The DECT physical layer / RF air interface was designed to enable costs to be kept within the realms of those acceptable for the mass domestic market, whilst also enabling a robust and reliable interface to be created.
The modulation format, multicarrier access system using the MC/TDMA/TDD principle (explained later), beacon transmission and carrier allocation all operate in an effective manner whilst not requiring high cost components.
The DECT physical layer / RF interface enables many features including continuous analysis of the available channels, and a multiple access scheme that allows multiple handsets to use a single base station. In this way, the DECT PHY provides a robust, flexible and cost effective format.
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.
The DECT TDMA structure enables a maximum of 12 simultaneous basic full duplex DECT voice connections per transceiver. The DECT protocol also provides the capability to use varying bandwidths by combining multiple channels into a single bearer. For data transmission purposes error protected net throughput rates of n x 24 kbit/s can be achieved, up to a maximum of 552 kbit/s with full security as applied by the basic DECT standard.
DECT RF spectrum usage
The MC/TDMA/TDD principle used for basic DECT provides for a total of 120 duplex channels for a DECT device at any instant at any location.
The use of the same channel by adjacent base stations or cells is limited by the interference between them. Carrier over Interference ratios of C/I = 10 dB can be achieved. This enables very high levels of spectrum usage and re-use of the different channels.
DECT RF channel broadcast
One key requirement for a DECT base station is that it continuously broadcasts a beacon which enables other portable devices to see the base station and access it if it is able.
The beacon transmission can be part of an active communication link when a portable is actually communicating and carrying a call or it can be a dummy bearer transmission.
The DECT base station RF beacon transmission carries broadcast information contained within a multi-frame multiplexed structure. The information includes:
- Base station identity
- System capabilities
- RFP status
- Paging information
When a portable device receives a beacon transmission it processes the information contained within the beacon data. From this it will be able to determine whether it is able to communicate with the base station. The DECT system has been set up so that only portables with access rights are allowed to set up a link. Typically this is achieved by pairing the portable with the base station.
The portable device also determines whether the system capabilities match with the services required by the portable, and also if the base station has sufficient capacity to handle communication to the portable device as it may already be at or near capacity.
Dynamic channel selection & allocation
One important aspect of the DECT RF interface / physical layer is the allocation of channels. In order to reduce interference, both to its own channels and to others, the DECT physical layer incorporates a comprehensive methodology for selecting the required channel and maintaining the best options.
To achieve this DECT features a capability called Dynamic Channel Selection and Allocation. Under this technique, DECT equipment scans the channels at least once every 30 seconds, but as a background process. It checks for signals on the channels and produces a list of free and occupied channels - one for each idle timeslot/carrier combination. An idle timeslot is one that is not in use for transmission or reception. With this information, the base station or portable device is able to select the optimal channel / timeslot combinations for a link.
The DECT RF interface / physical layer is relatively easy to implement so that costs can be kept to a minimum, yet it maintains a high level of functionality to enable it to maintain a trouble free reliable communications link between the base station and a number of portable devices.
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