Nortel 7430 Deployment Manual page 17

3.4.2 RSSI
The Radio Signal Strength Indicator value is a relative expression for the field strength of signal
from the Base Station. The RSSI-value is used for the choice of the alternative Base Station(s).
The wireless handsets will choose the Base Station from which the strongest RSSI signal is
received as the first alternative Base Station. Alternative Base Stations are listed according to
RSSI values. If the "Best alternative Base Station" disappears the next Base Station with the
highest RSSI value will become the "Best alternative Base Station".
3.4.3 Q-value and RSSI value as they relate to voice quality
There is and always will be a relationship between the coverage of the Base Station and the
quality of sound provided to the user through the handset. Sound quality is therefore, typically,
directly proportionate to the distance from the handset to the radio signal source. The amount and
density of any obstacles, walls, plants, people, etc. in between also impact the quality of service.
Because identifying quality of signal by distance from the base alone is difficult, the Q-value is
used as an indicator for the quality of the signal. The RSSI signal is used as an indicator for the
signal strength.
The user will have an excellent quality of signal as long as the Q-value is high (>52) and is not
significant fluctuating.
Under circumstances where there is no interference from other bases, other equipment or
reflections from surroundings there is a relation between the Q-value and the RSSI signal. High
RSSI – high and stable Q-value. Low RSSI – low and/or unstable Q-value. Be aware that a
situation can occur that a high RSSI value not necessarily means a high and stable Q-value.
Clicks, distortion, audio breaking up is to be seen as a result of bit failures in the communication
between the handset and the Base Station.
As a guideline an RSSI signal where the loss of signal is not higher than 10dB relative to the
signal measured near the Base Station is a very good signal where only some minor clicks will be
heard. A RSSI signal where the loss of signal is equal to a loss of 20dB is an acceptable signal
where some clicking and popping may occur. A RSSI signal where the loss of signal is higher
than 30dB relative to 100% near the Base Station is not considered acceptable signal strength.
When locating mounting locations for Repeaters the signal quality must be equal to a signal
where it is possible to obtain a good connection between a handset and a Base Station. The
RSSI signal, at a location for a Repeater, should normally be no less than a loss of 25dB relative
to the signal measured near to the Base Station.
Related to the position of repeaters a loss equal to 25dB can be used as a guideline only.
At the position where the Repeater is mounted the signal quality most be 'ok' with reference to
the Q-value. Place a handset at the repeater position locked to the Base Station to which the
repeater is connected. Go off-hook. Here the Q-value must be high and stable.
If the Q-value is not high and stable the link between the base station and Repeater is generating
bit failures. If this happen the bit failures measured in the link between base station and repeater
is transferred into the connection between the repeater and handset giving poor sound quality as
result.
Please note that the RSSI value reported by the handset is a relative expression of the signal
strength and can not be used alone as an indicator for the quality of the signal.
The Q-value must be taken into consideration. As an example: when the handset is placed right
next to the base station the signal is 100%. However, the RSSI value in the display can be
showing 95. When moving away from the base station the RSSI value drops to 85 you will have a
db loss of 10 db. If moving further away the RSSI value changes to 75 the total db loss is 20 db.
Digital Mobility Deployment Guide
17