Trouble Shooting Guide, Advanced
4/00021-3/FEA 209 544/25 C
Ericsson Mobile Communications AB
34(78)
10 RSSI
10.1 What is RSSI
In the mobile phone, the received RF-signal strength is measured and indicated by a function
called RSSI, Received Signal Strength Indicator.
When switching the mobile phone ”ON” it starts searching the surrounding radio channels
ARFCN at the geographical site.
Receiving a lot of information on the broadcast channels it will be directed by the network to
“Camp on a cell”.
The surrounding base stations BSS are at the same time stored in the MS memory as Base
Tranceiver Stations Identity Codes, BSIC for further usage.
Up to 30 BSIC can be available, depending on the network. The MS memory capacity is 32
positions for BSIC.
Once the MS camped on the strongest available carrier it starts performing the RSSI
measurements on the surrounding cells from the list of identified BSS-carriers in its memory.
The measurements are averaging from all the base stations by sampling 5 times on each
carrier and then stored in the memory together with the corresponding BSIC.
This list of identified carriers and signalstrength is updated by continuously repeated
measurements.
Another list of carriers and signalstrength is monitored by the MS for Hand Over purposes
during a call in progress.
The serving cell is also measured during the cell.
The measurements results are equally stored in the MS memory.
The MS evaluates the RX levels from the measurements results on all carriers ordered by the
network and makes a “TOP 6”-ranking list. The list is recovered once a measurement result
differs from previous value.
The 6 STRONGEST RSSI levels from the surrounding non-serving carriers are then reported
uplink to the serving cell during a call in progress.
This report is repeated within 30 seconds if an RX level or BSIC has been changed.
The signal strength report is used in an evaluation process for Location and Handover, i.e.
when the switch evaluates the speech quality, signal strength and traffic parameters to be
outside the limit values of the current physical channel and chooses to start a new channel for
the connection.
A physical channel is the combination of a timeslot (TS) and a radio channel (ARFCN).
The physical channel (TS/ARFCN) can be allocated to the current base station or any of the
surrounding base stations at handover.
For the speech quality and the MS to Base distance it is important that the reported RF-signal
measurements are correct and calibrated towards known values.
If the reported value is too high it results in late handovers and bad readability due to the
limits are set out of reach for the MS.
The opposite, too low values, provokes the switch to make unnecessary handovers, increased
traffic load and perhaps dropped calls by forced release.
The received signal carries information both in the phase and the amplitude.
The phase contains the digital information about the message and it is detected later in a
phase digitizer for further processing in the main program as speech data and signalling.
The information about the amplitude corresponds to the strength of the received RF-signal,
i.e. the RF-level at the antenna input of the receiver.
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