Page 4 - Introduction
Page 4
Compact DECT - Installation Manual
Compact DECT in Detail
38DHB0002UKFH – Issue 5 (27th October 2003)
Compact DECT in Detail
The Compact DECT (Digital Enhanced Cordless Telecommunications*) uses
cellular radio methods to handle multiple handsets on the same system at the
same time. It is designed for high density local area usage.
The Compact DECT system uses the frequency band of 1.88 to 1.9GHz. It
employs FDMA (Frequency Division Multiple Access) to divide each band
into ten separate carrier frequencies. TDMA (Time Division Multiple Access )
further sub-divides each carrier frequency into 24 time slots. 12 slots are
used for send and 12 for receive, i.e. two slots are required per conversation
on the carrier frequency, this is called TDD (Time Division Duplex).
The above method gives 120 call channels available to a DECT system (10 x
24 channels, 2 channels per call).
During a conversation, a handset continuously compares the speech quality
obtained in its time slot to that in the same time slot on another carrier
frequency. If the current signal falls below acceptable limits and the other
band is better, the handset will change over to that band.
The Compact DECT systems use DCS/DCA (Dynamic Channel
Selection/Allocation) to resolve availability of channels between handsets
and base stations. These processes allow a high density of handsets (on the
same or different DECT systems) to operate in the same area (provided
sufficient base station channels are available).
Speech within DECT is digitised using ADPCM (Adaptive Differential Pulse
Code Modulation). Traditional PCM digitises speech by sampling at time
intervals and assigning a value to the amplitude at each time interval. This
sequence of amplitude values is then sent as a digital signal. In ADPCM
information is only sent when their is a difference in amplitude. This requires
more processing electronics but allows lower signal rates to be used for the
same speech quality.
*Originally DECT stood for "Digital European Cordless Telephony" but this
was changed as use of the standard spread beyond the European market.
