Product manual

System description, DECT air interface

Technical Product Manual - DCT1800-GAP

TD 92093 (1/LZBNB 103 108 R4D) / 2006-03-09/ Ver.C

6.2.6 Antenna diversity

Antenna diversity is a function that continuously selects the best antenna of a base station. It is used

against fading effects. In DECT a base station usually has two antennas that can be used for

transmitting and receiving.

In radio environments there may be fast changing fluctuations of the field strength due to reflections.

Radio signals may have peaks and dips, especially in buildings (see figure

28). When the signal

strength for one antenna has a dip in a certain position there is a good chance that the signal strength

for the other antenna has a peak. Antenna diversity tries to follow the peaks of both antennas.

Antenna diversity can operate on a forecast or measurement base. In the case of forecast, a base

station has one receive circuit and two antennas. A diversity algorithm switches between the two

antennas. In the case of measurement, each antenna has its own receive circuit. The signals from

both circuits are continuously compared and the best signals are used.

Fig. 28 Simplified representation of antenna diversity

6.3 TDMA frames and packets

A string of 24 successive time slots on one carrier is referred to as a TDMA frame. One TDMA frame

takes 10 ms. For speech applications, one packet is transmitted in each time slot (see figure

29). A

packet contains 320 speech data bits and 64 signalling bits. Since one packet is transmitted each 10

ms, the following sub-channels are created:

• 32 kbit/s for speech data

• 6.4 kbit/s for signalling

Sync field

The synchronization field is 32 bits long. The first 16 bits (preamble) are used for clock synchronization

and the last 16 bits represent the packet synchronization word. The 32 bits represent a code that is

the same for all DECT systems.

Signalling field

This field contains messages to control the connection, for instance to set-up a call, to request a

handover or to clear a call. Amongst others, a message may contain the following parameters:

Antenna A

Antenna B

Without

reflections

Distance from base station

Radio

signal

strength

015