User guide

TD 92422GB

18 February 2010 / Ver. E

System Planning

Ascom IP-DECT System

6.4 Reflective Environment

When providing coverage in a metal hall (for example, a production hall or storage

building), there are a number of issues which call for additional attention. The dimensions

of the hall and the material used (metal, concrete, brick etc.) are important deciding

factors in the hall’s radio reception. Every hall is different, and it is very difficult to predict

the radio reception. Check carefully, therefore, whether the walls are made of metal, what

the hall’s dimensions are, whether the roof is reflective, what is contained in the hall, and

whether objects in the hall are stationary or constantly moving.

In the case of poor speech quality in a metal hall, this can be attributed to time delay

spread and/or the actuation of the soft suppressor.

Time delay spread can be compared with dispersion in cables and fibres. This means that

the radio signal can travel by various paths to reach the user because the signal can reach

the user directly but also via reflections. These possibilities are illustrated in the figure

below.

Figure 3. Time delay spread

The base station at corner A reaches the cordless phone at corner B by means of a direct

signal (signal 1) and by means of a reflected indirect signal (signal 2). Generally there are

many reflected signals reaching the cordless phone. In general the paths travelled by these

signals are not equal which means in turn that they will arrive at the cordless phone at

different times.

A DECT signal consists of frames of 420 bits transmitted every 10 ms. The bit length for

DECT is 0.868 µs. Mutes

and clicks on the line will occur if the time difference between

the various received signals is of the order of 1/10 bit length. If this occurs, the receiver has

difficulty in distinguishing between the different transmitted bits. Therefore, the base

stations in a metal hall must be sited in a way that the time delay spread is minimized. This

means in turn that you must locate the base stations such that the number of reflections is

minimized.

1. Interruptions in your conversation

008

2009-01-01

System A

12345

Reflecting wall

AAA