E-mail: luda_rose@ukrtop.com
The members of the European Telecommunications Standards Institute (ETSI) have developed the DECT standard. The Digital Enhanced Cordless Telecommunications (DECT) standard provides a general radio access technology for wireless tele-communications, operating in the preferred 1880 to 1900 MHz band using GFSK (BT = 0.5) modulation.
| Frequency Band | 1880-1900 MHz |
| RF Carrier Spacing | 1,728 MHz |
| Access Technology | MC/TDMA/TDD |
| Number of RF Carriers | 12 duplex (on 32 kbit/s) |
| Speed of Transfer in the Channel | 1152 kbit/s |
| Modulation | GFSK (BT=0,5) |
| TDMA Frame Length | 10 ms |
| Speed of Data Transmission (ISDN) | 144 kbit/s |
| Delay | 16 s |
| Required E/No | 12 |
| Speed of the Speech Codec (ADPCM) | 32 kbit/s |
| TRX Power of MS | 10 mW (average), 240 mW (peak) |
The MC/TDMA/TDD principle
The DECT radio interface is based on the Multi Carrier, Time Division Multiple Access, Time Division Duplex (MC/TDMA/TDD) radio access methodology. Basic DECT frequency allocation uses 10 carrier frequencies (MC) in the 1880 to 1900 MHz range. The time spectrum for DECT is subdivided into timeframes repeating every 10 ms. Each frame consists of 24 timeslots each individually accessible (TDMA) that 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 kbit/s (ADPCM G.726 coded speech) full duplex connections. To simplify implementations for basic DECT the 10 ms timeframe has been split in two halves (TDD); where the first 12 timeslots are used for FP transmissions (downlink) and the other 12 are used for PP transmissions (uplink).
The TDMA structure allows up to 12 simultaneous basic DECT (full duplex) voice connections per transceiver providing a significant cost benefit when compared with technologies that can have only one link per transceiver. Due to the advanced radio protocol, DECT is able to offer widely 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.
Dynamic Channel Selection and Allocation
DECT features continuous Dynamic Chan-nel Selection and Allocation. All DECT equipment is obliged to regularly scan - its local radio environment - at least once every 30 seconds. Scanning means receiving and measuring local RF signal strength on all idle channels. Scanning is done as a background process and produces a list of free and occu-pied channels (RSSI list; RSSI = Received Signal Strength Indication), one for each idle timeslot/carrier combination, to be used in the channel selection process. An idle time-slot is (temporarily) not in use for transmis-sion or reception. Within the RSSI list, low signal strength values represent free and non-interfered channels, while high values repre-sent busy or interfered channels. With the aid of the RSSI information, a DECT PP or FP is capable of selecting the most optimal (least interfered) channel to set-up a new commu-nication link.
In a DECT portable part, the channels with highest RSSI values are continuously ana-lysed to check if the transmission originates from a base station to which the portable has access-rights. The portable will lock onto the strongest base station, as mandated by the DECT standard. Channels with lowest RSSI value are used to set-up a radio link with the base station if the portable user decides to establish a communication or when an in-coming call is signalled to the portable through the reception of a paging message. In a DECT base station the channels with low RSSI values are used when selecting a channel to set-up a beacon transmission (dummy bearer). The Dynamic Channel Selection and Alloca-tion mechanism guarantees that radio links are always set-up on the least interfered channel available.
Portable user originated call set-up
The initiative to set-up radio links in basic DECT applications is always taken by the portable part. The portable selects (using its Dynamic Channel Selection) the best channel available for set-up, and accesses the fixed part on this channel. To be able to detect the PP’s set-up attempts the fixed part must be receiving on the channel when the PP transmits its access request. To allow portables to use all 10 DECT RF carriers, the fixed part continuously scans its idle receive channels for portable setup attempts in a sequential way. Portables synchronise to this sequence by means of the information transmitted through the FP continuous broadcast service. From this information portables can deter-mine the exact moment when successful ac-cess the FP is possible on the selected chan-nel.
Network originated call set-up
When a call comes in for a DECT portable, the access network will page the portable by sending a page message - containing the PP’s identity - through its continuous broadcast service. A portable receiving a paging mes-sage with its identity included will set-up a radio link - to serve the incoming call - using the same procedure as used for the PP origi-nated link set-up.
Handover
Due to the powerful Dynamic Channel Se-lection and Allocation and seamless handover capabilities of DECT, portables can escape from an interfered radio connection by estab-lishing a second radio link - on a newly se-lected channel - to either the same or to another base station. The two radio links are temporarily maintained in parallel with identical speech information being carried across while the quality of the links is being analysed. After some time the base station determines which radio link has the best quality and releases the other link.
If the DECT portable is moving from one cell area into another, the received signal strength - as measured by the portable’s Dynamic Channel Selection and Allocation functions - of the base station will reduce gradually. The signal strength of the base station serving the cell towards which the portable is moving will gradually increase. At the moment the new base station’s signal becomes stronger than the signal from the old base station, a seamless handover (as described above) will be performed to the new base station. The seamless handover is a fully autonomous initiative from the DECT portable part, which the user will not notice. Although a handover is always initiated by the DECT portable part, it may also be the uplink (from PP to FP) that suffers from poor quality. For this case, DECT has signalling protocols that enable the fixed part to signal the perceived link quality to the PP, that can subsequently initiate the handover.
Application profiles contain additional specifications defining how the DECT air interface should be used in specific applications. Standard message and protocol subsets have been derived from the base standard’s tool-box tailored for specific applications with the aim to achieve maximum interoperability between DECT equipment from different manufacturers. In addition to the profiles, profile conformance test specifications have been developed by ETSI that enable harmonised testing of DECT equipment designed to meet the profile requirements. Future developments of the DECT standard will be initiated by the market for DECT equipment and services as encountered by those involved in the standardisation process. Evolution of DECT due to new applications may lead to additions to the existing toolbox or new standard subsets (pro-files).