From the moment users power up their radios they have an expectation of seamless roaming and instant communications.
All types of network offer this service and Tetra networks are no different. What is different is the way this service is managed and performed by the system and subscriber units.
The Tetra subscriber units are helped in their search for control channels by base station broadcast messages on their MCCH.
The control channel information of adjacent cells is broadcast by each base station and instead of wasting time doing a back ground hunt that covers all the frequencies held in its channel lists, the mobile stations are now told which channels to scan for the acquisition of adjacent cells.
34.1.
Cell Acquisition at Power Up
Once a subscriber unit is powered up it will go though several procedures to acquire a control channel, especially if it is being powered up for the first time on the system.
Assuming this is the case, then the radio will scan its list of control channels looking for a strong enough transmission from which it can synchronise and obtain adjacent cell information.
It will then register and send a Location Update which informs the network of its present position.
Once this occurs, it will continue to check the control channels of the adjacent cells for the best signal as part of the ongoing process.
34.2. Channel Selection
The radio must select the control channel that offers the best quality of service. This decision is based on the received RSSI and a calculation using defined cell selection parameters (C1) which are obtained from the system information transmitted on the Broadcast Channel.
Once the decision is made, the radio registers with the system by sending a location update message which tells the system of its activation and its cell location.
C1 is based on the following criteria: The RSSI from the received cell.
Pms – the maximum transmit power of the subscriber unit.
RxLev_Access_Min. – The minimum usable signal strength received by the
MS_TxPwr_Max_Cell – The maximum allowed subscriber unit transmit power. Max – Notation means take the highest of the expressions (0 or
MS_TxPwr_Max_Cell) used in the bracket.
Where C1 = (RSSI - RxLev_Access_Min) - (Max(0 or MS_TxPwr_Max_Cell - Pms))
34.2.1.
RSSI - RxLev_Access_Min = > 0
The RxLev_Access_Min entry field in the BSC configuration file is used to define the talk in/out path of a cell.
It is the lowest allowed received signal level at the subscriber.
As soon as the subscriber receives this level of signal it is a usable cell.
The entry level in this field is from 1 to 15 and represents signal levels from -125dBm to –55dBm in 5dBm steps. i.e. 1 = -125dBm & 15 = -55dBm
34.2.2. Radio Improvable
The quality of radio service becomes improvable when the service quality of a neighbouring cell exceeds that of the present cell by a certain amount.
When this occurs the radio may select the new cell.
If the radio is in the idle mode and the quality is Improvable it will reselect. If the radio is in a call, it may not reselect, but will wait until the present cell is Relinquishable.
34.2.3. Radio Relinquishable.
The service of a cell becomes Radio Relinquishable when it falls below a certain level and the quality of a neighbouring cell downlink exceeds that of the present cell by a specific amount.
When this occurs, the radio will use C2 to justify the switch.
C2 is based on the transmitted system information in the Broadcast Channel which is set as part of the system optimisation.
The transmitted information is the fast and slow values for both the Reselect Hysteresis and the Reselect Threshold.
34.3. Cell Optimisation
Ideal Cell Coverage.
34.4. Acquiring an Adjacent Cell
As the users move through the cell, their radios not only monitor the control channel down link for paging messages (call set up, SDS messages etc) and system information, but it also monitors the RSSI level.
The system information also provides the radio with adjacent cell control channel information and the radio uses this information for its back ground hunt.
As it scans this information it will become apparent that the signal strength of one channel will be increasing as the signal strength of its present control channel is decreasing.
There will then be a point in time, which has relevance to the radio’s position in its present cell, when it will switch to the control channel of the new cell.
To enable the radio to make the decision to switch, it continually uses both the C1 & C2 calculation to compare the control channel signals of its present and proposed cell. FAST_RESELECT_HYSTERESIS (-80dBm) Acts up to 20dBm above FAST_RESELECT_THRESHOLD RXLEV_ACCESS_MIN = 4 (-110dBm) Direction of Travel SLOW_RESELECT_ THRESHOLD _ABOVE_FAST Theoretical Point at which Service becomes Radio Improvable –90dBm (RSSI only). SLOW_RESELECT_HYSTERESIS setting acts either side of and is centered
on the SLOW_RESELECT_THRESHOLD_ABOVE_FAST setting FAST_RESELECT_THRESHOLD
Theoretical Point at which Service becomes Radio Relinquishable –100dBm (RSSI).
34.4.1.
C2 – Radio Improvable
Cell reselection is initiated when the serving cell becomes radio improvable.
(C2(NCell) > (C1(SCell)+SRH)) > 5sec (C1(SCell) < SRT) > 5sec
Note: The above conditions must be met simultaneously. * Note: These levels are defined in the bssfactoryparams.txt file.
34.4.2.
C2 – Radio Relinquishable
(C2(NCell) > (C1(SCell)+FRH)) > 5sec (C1(SCell) < FRT) > 5sec
Note: The above conditions must be met simultaneously.
34.4.3.
C2 - Radio Usable
Reselection can be delayed if the new cell does not meet the switch criteria and a situation may occur where the cell is Radio Relinquishable, but no neighbouring cell meets the radio usable criteria, in which case the cell with the highest C2 is selected.
(C2(NCell) > (FRT+FRH)) > 5sec