Cell Reselection

5.3 Cell Reselection

The cell reselection can be controlled either autonomously by the mobile or by the network. It is based on measurements performed by the mobile. The network can order that these measurements be reported periodically.

Three cell reselection modes NC0, NC1, and NC2 have been defined:

  • NC0. In this mode, the GPRS mobile performs autonomous cell reselection without sending measurement reports to the network.

  • NC1. In this mode, the GPRS mobile performs autonomous cell reselection and periodically sends measurement reports to the network.

  • NC2. In this mode, the network controls the cell reselection. The mobile sends measurement reports to the network.

When the mobile performs autonomous cell reselection, it chooses a new cell and triggers a cell reselection on its own. The NC2 mode allows the network to control the mobility of GPRS users within the network.

The GPRS cell reselection mode for a GPRS-attached MS is given by the network control mode (NETWORK_CONTROL_ORDER parameter), which is broadcast on the BCCH or PBCCH. The mobile behavior is determined by both its GMM state and the network control mode. Whatever the value of the network control mode, when the MS is in GMM STANDBY state, it performs autonomous cell reselection and does not send measurement reports to the network. In GMM READY state, the mobile performs cell reselection according to the network control mode.

Two criteria are defined for autonomous cell reselection: one is based on the (C1, C2) criteria. It corresponds to the GSM cell-reselection criteria. The other, based on the (C'1, C31, C32) criteria, has been introduced for GPRS. All these criteria are based on received signal level (RXLEV) measurements in the serving cell and in the neighbor cells.

In a cell where PBCCH does not exist (GPRS cell reselection parameters are not broadcast), the MS performs cell reselection based on (C1, C2) criteria; except if the cell reselection parameters are sent explicitly to the mobile in one control message, in which case the mobile performs autonomous cell reselection based on criteria (C'1, C31, C32). In a cell where PBCCH exists, the MS performs cell reselection based on (C'1, C31, C32) criteria.

Table 5.8 summarizes the different reselection modes and the criteria used.

Table 5.8: Mode of Reselection and Criteria for Cell Reselection

NETWORK_CONTROL_ORDER Value

GMM State of the MS

Mode of Cell Reselection

Criteria If PBCCH Exists

Criteria If PBCCH Does Not Exist

NC0

Standby/Ready

Autonomous cell reselection

C'1, C31, C32

C1, C2 [*]

NC1

Standby

Autonomous cell reselection

C'1, C31, C32

C1, C2[*]

 

Ready

Autonomous cell reselection with measurement reports

C'1, C31, C32

C1, C2[*]

NC2

Standby

Autonomous cell reselection

C'1, C31, C32

C1, C2[*]

 

Ready

Network-controlled cell reselection with measurement reports

-

-

[*]Except if the GPRS cell reselection parameters are sent to the MS in an RLC/MAC control message.

5.3.1 Measurements

When the mobile camps on a cell, it receives from the network the list of the neighbor cells identified by their BCCH frequencies and their BSICs. The mobile measures periodically the RXLEV on these neighbor BCCHs and checks the BSIC of the BCCH carriers.

These RXLEV measurements are used in autonomous cell reselection mode and reported to the network in case of cell reselection controlled by the network and NC1 mode.

5.3.1.1 In Packet Idle Mode

In packet idle mode, the mobile measures the RXLEV on the BCCH carrier of the serving cell and on each BCCH carrier of the neighboring cells. It calculates an average RXLEV on each carrier, with at least five measurements during a maximum period corresponding to the maximum of 5 seconds or five consecutive paging blocks of the mobile. The mobile must perform a minimum of one measurement on a cell every 4 seconds or a maximum of 20 measurements every second. The mobile maintains a list of the six strongest received BCCH carriers, which is updated every averaging period.

The mobile must decode the BSIC on the BCCHs of the six most powerful cells every 14 paging block periods or at least every 10 seconds.

5.3.1.2 In Packet Transfer Mode

In packet transfer mode, the mobile measures the RXLEV on the BCCH carrier of the serving cell and on each BCCH carrier of the neighboring cells. It calculates an average RXLEV on each carrier during a period of 5 seconds, with at least five measurements.

The mobile performs a measurement on each TDMA frame for at least one of the given BCCH carriers, one after the other. The mobile must decode the BSIC on the BCCHs of the six most powerful cells at least every 10 seconds. The mobile does this decoding during the two idle frames of the 52-multiframe.

5.3.2 Criteria

In this section, the criteria C1, C'1, C2, C31, and C32 that were introduced in the previous section are described in detail. The way these parameters are used in the cell reselection process will be described in the next section.

5.3.2.1 C1 Criterion

The path loss criterion parameter C1 is used as a minimum signal level criterion for cell reselection when there is no PBCCH in the cell.

C1 is defined by the following formula:

(5.3) 
(5.4) 
(5.5) 

RXLEV_ACCESS_MIN represents the minimum RXLEV at the MS to access the cell. MS_TXPWR_MAX_CCH represents the maximum transmit power level allowed to the MS when accessing the cell. P is the maximum RF output power of the MS (parameter specific to the MS).

A represents the reception margin. The MS is allowed to access the cell if its RXLEV is higher than RXLEV_ACCESS_MIN; that is, A > 0. If A > 0, the mobile is in the cell coverage and the downlink is good enough. If A < 0, the mobile is outside the cell coverage.

B represents the mobile transmission capability margin. If B < 0, the transmission capabilities of the mobile are sufficient; if the mobile is in the cell coverage A > 0, the cell can be selected. If B > 0 and A - B > 0, the mobile transmission capabilities are compensated by the reception margin and the cell can be selected. If B > 0 and A - B < 0, the mobile transmission capabilities are not compensated, and the mobile cannot select the cell.

Thus the path loss criterion is satisfied if C1 > 0.

5.3.2.2 C'1 Criterion

The path loss criterion parameter C'1 is used as a minimum signal level criterion for GPRS cell reselection in the same way as for GSM idle mode. C'1 is the same as C1, except that GPRS-specific parameters are used instead.

C' 1 is defined by the following formula:

(5.6) 

where

(5.7) 

and

(5.8) 

5.3.2.3 C2 Criterion

The C2 criterion is used for cell ranking in the GSM cell-reselection process. It is computed with the following formulas:

(5.9)  Click To expand
(5.10)  Click To expand

T is a timer that is started from 0 at the time the cell enters in the list of strongest carriers. CELL_RESELECT_OFFSET is a parameter that is used to prioritize one cell in relation to the others. TEMPORARY_OFFSET is a parameter used to penalize during PENALTY_TIME when this cell just enters the list of strongest carriers.

5.3.2.4 C31 Criterion

The signal level threshold criterion parameter C31 for hierarchical cell structures (HCS) is used to determine whether prioritized hierarchical GPRS cell reselection applies.

The C31 criterion allows cells for GPRS-attached mobiles to be prioritized during autonomous cell reselection. A GPRS-attached MS will preferably select the cell having the highest priority as indicated by the parameter PRIORITY_CLASS. However, a sufficient RXLEV in the cell (HCS_THR parameter) is required for it to belong within the highest-priority class. If the signal level becomes too low, a determination of lowest priority is made.

The C31 criterion contains a time-based offset. This offset can be used to penalize a cell, belonging to another priority level as the serving cell, during GPRS_PENALTY_TIME.

The C31 criteria for the serving and neighbor cells are evaluated with the following formula:

(5.11) 
(5.12)  Click To expand
(5.13)  Click To expand
(5.14) 

where HCS_THR is the signal threshold for applying HCS GPRS reselection, and GPRS_PENALTY_TIME gives the duration for which the temporary offset GPRS_TEMPORARY_OFFSET is applied. T is a timer that is started from 0 at the time the cell enters in the list of strongest carriers.

5.3.2.5 C32 Criterion

The cell-ranking criterion parameter (C32) is used to select cells among those with the same priority and is defined for the serving cell and the neighbor cells by:

(5.15) 
(5.16)  Click To expand
(5.17)  Click To expand
(5.18)  Click To expand

The C32 criterion also contains a time-based offset that can be used to penalize a cell belonging to the same priority level during a certain time. It could be use to perform speed discrimination. The GPRS_RESELECT_OFFSET parameter allows a neighbor cell to be penalized.

When the MS is in GMM READY state (a transfer can be ongoing or about to start), in order to avoid unnecessary cell reselection or to be sure that it is really needed, an hysteresis GPRS_CELL_RESELECT_HYSTERESIS is subtracted from the C32 criteria of the neighbor cells. This hysteresis is also subtracted from the C31 criterion if requested by the network.

When the neighbor cell does not belong to the same RA as the serving cell, an hysteresis RA_RESELECT_HYSTERESIS is subtracted from the C32 criterion of the neighbor cell. This is done to avoid changing from one RA to the other and so triggering an RA update procedure, increasing the signaling within the network.

In case cell reselection occurred within the previous 15 seconds, all neighbor cells are penalized by 5 dB in order to avoid ping-pong between cells.

All the GPRS cell-reselection parameters described in this section are broadcast on the PBCCH carrier of the serving cell. The cell-reselection parameters used for calculation of the (C1, C2) criteria are broadcast on the BCCH carrier of the serving cell and the BCCH carriers of the neighbor cells.

5.3.3 Autonomous Cell-Reselection Process

The process of cell reselection is either based on the (C1, C2) criteria or (C'1, C31, C32) criteria. (C1, C2) criteria are used when there is no PBCCH in the serving cell and the cell-reselection parameters needed for (C'1, C31, C32) criteria evaluation have not been explicitly transmitted to the mobile.

(C'1, C31, C32) criteria are used when the PBCCH is transmitted in the serving cell, or when the cell reselection parameters needed for (C'1, C31, C32) criteria evaluation have been explicitly transmitted to the mobile and there is no PBCCH.

5.3.3.1 Procedure for (C1, C2)

The mobile must compute the C1 and C2 values for the serving cell and the neighbor cells at least every 5 seconds. A cell-reselection is triggered if one of the following events occurs:

  • The path loss criterion parameter C1 indicates that the path loss in the serving cell has become too high (i.e., C1 < 0 ).

  • Downlink signaling failure is experienced (see Section 5.4.5).

  • The serving cell has become barred.

  • A neighbor cell for which the C2 criterion is higher than the serving cell for a period of 5 seconds is detected. When the neighbor cell is not in the same RA or if the mobile is in GMM READY state, the C2 value for the neighbor cell must exceed the C2 value for the serving cell by at least CELL_RESELECT_HYSTERESYS. This allows for the avoidance of unnecessary cell reselection when the mobile is in READY state or between two different RAs. For a cell reselection occurring in the previous 15 seconds, the C2 value for the neighbor cell must exceed the C2 value for the serving cell by at least 5 dB for a period of 5 seconds.

  • A random access attempt is still unsuccessful after "Max retrans" repetitions, Max retrans being a parameter broadcast on the control channel.

5.3.3.2 Procedure for (C'1, C31, C32)

The mobile computes the values of the C'1, C31, and C32 criteria for the serving cell and the nonserving cells at least every second or at every new measurement, whichever is the greatest. A cell reselection is then triggered if one of the following events occurs:

  • The path loss criterion parameter C'1 indicates that the path loss to the cell has become too high (i.e., C'1 < 0).

  • A nonserving suitable cell is evaluated to be better than the serving cell. The best cell is the cell with the highest C32 value among: (a) the cells that have the highest PRIORITY_CLASS among the cells that fulfill C31 >=0, or (b) all cells, if no cell fulfills the criterion C31 >= 0.

  • Downlink signaling failure is experienced (see Section 5.4.5).

  • The cell on which the mobile is camping has become barred.

  • A random access attempt is still unsuccessful after MAX_RETRANS_RATE attempt.

The cell that the mobile was camped on must not be returned to within 5 seconds.

5.3.4 Measurement Report Sending

When the cell reselection mode given by the NETWORK_CONTROL_ORDER parameter indicates NC1 or NC2 and the MS is in GMM READY state, the MS reports measurements in the PACKET MEASUREMENT REPORT message to the network. This message contains:

  • The measured RXLEV for the serving cell;

  • In packet idle mode, the average interference level γch for the serving cell;

  • The measured RXLEV from the nonserving cells.

The reporting period is defined as follows:

  • In packet idle mode, the reporting period is indicated by NC_REPORTING_PERIOD_I parameter;

  • In packet transfer mode, the reporting period is indicated by the NC_REPORTING_PERIOD_T parameter.

NC_REPORTING_PERIOD_I and NC_REPORTING_PERIOD_T parameters are broadcast on PBCCH. They can also be broadcast on PACCH in case there is no PBCCH.

5.3.5 Network-Controlled Cell-Reselection Process

When the cell-reselection mode indicates NC2 and the mobile is in GMM READY state, the network controls the cell-reselection process. Based on the measurements reported by the mobile (RXLEV and RXQUAL only during packet transfer) or by the BTS during packet transfer mode, the network decides whether a cell reselection must be triggered.

When the decision to trigger a cell reselection is made by the network, it sends a PACKET CELL CHANGE ORDER message. When the MS receives the command, it reselects the cell according to the included cell description (BCCH frequency and BSIC) and changes the network control mode according to the NETWORK_CONTROL_ORDER parameter included in the command.

Note that in order to reduce the cell-reselection delay, the MS may continue its transfer in the old cell until it decodes the BCCH or PBCCH information necessary to reestablish a TBF in the new neighbor cell.



 
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