12 COMPACT Mode Tasks

05.083GPPRadio subsystem link controlTS

12.1 Introduction

COMPACT is a radio interface mode for inter cell synchronized systems. The mapping of control channels for up to four cells is done on the same carrier and control channel separation is achieved by transmitting control in different cells on different timeslots. The mapping of the control channels is specified in 3GPP TS 05.02.

The COMPACT Mode Tasks defined in this clause applies for COMPACT capable MSs in cells employing a CPBCCH carrier as defined in 3GPP TS 05.02. The COMPACT Mode Tasks also applies for MSs in any cell, where at least one CPBCCH is defined in a BA list.

Note: A mobile stations designed prior to release 99 will not find CPBCCH carriers when it is looking for a BCCH carrier due to the different structure of synchronization and frequency bursts.

12.2 Network Pre-requisites

12.2.1 CPBCCH carriers

The CPBCCH carrier shall be transmitted with constant RF output power on at least 4 radio blocks per multiframe on the serving time group (see 3GPP TS 05.02).

12.3 COMPACT Idle Mode Tasks

12.3.1 Introduction

Whilst in idle mode, a COMPACT capable MS shall implement cell selection and cell reselection procedures as described in 3GPP TS 03.22. These procedures make use of measurements and sub-procedures described in this clause.

The procedures ensure that the MS is camped on a cell from which it can reliably decode downlink data and with which it has a high probability of communications on the uplink. Once the MS is camped on a cell, access to the network is allowed.

The MS shall not use the discontinuous reception (DRX) mode of operation (i.e., powering itself down when it is not expecting paging messages from the network) while performing cell selection algorithms defined in 3GPP TS 03.22. However, use of powering down is permitted at all other times in idle mode.

12.3.2 Measurements for COMPACT Cell Selection

An MS shall in the COMPACT Cell selection procedure search for CPBCCH carriers. The MS shall search all RF channels in the system within its band of operation, take readings of received RF signal level of each RF channel, and calculate the RLA_ P for each. Each reading of received RF signal level shall be performed in such a way that it corresponds to the RF level at occurrences when the potential CPBCCH carrier is transmitting CPPCH, CSCH, CFCCH or CPBCCH blocks. Since the CPBCCH carrier is discontinuous, a single random measurement will not suffice. A single reading may instead consist of taking the maximum value out of multiple measurements. The succeeding averaging is based on at least five such measurement readings per RF carrier, where the readings are at least 1 s apart. This procedure is referred to as a CPBCCH scan.

NOTE: It is allowed to only take readings of RF signal level of CPBCCH carriers if identified.

CPBCCH carriers may be identified, for example, by searching for frequency correction bursts and then synchronizing to and reading the synchronization bursts at the prescribed offset from the PFCCH bursts (see 3GPP TS 05.02). On finding a CPBCCH carrier, the MS shall attempt to read the CPBCCH data, taking into account that the actual timeslot allocation of the CPBCCH will change from multiframe to multiframe.

The maximum time allowed for synchronization to a CPBCCH carrier is 2.5 seconds. The maximum time allowed to read each system information message, when being synchronized to the CPBCCH, is the time it takes to complete a broadcast cycle of that message.

12.3.3 Measurements for COMPACT Stored List Cell Selection

The MS may include storage of CPBCCH carrier information when switched off as detailed in 3GPP TS 03.22. The CPBCCH list may include CPBCCH carriers from more than one band in a multi band operation PLMN. A MS may also store CPBCCH carriers for more than one PLMN which it has selected previously (e.g. at national borders or when more than one PLMN serves a country), in which case the CPBCCH carrier lists must be kept quite separate. The stored BCCH carrier information used by the MS may be derived by a variety of different methods. As a minimum, the MS shall store the last used HPLMN CPBCCH carriers. A memory shall host at least the 24 last CPBCCH carriers from the HPLMN that the MS has camped on.

For a stored CPBCCH carrier list of the selected PLMN an MS shall perform the same measurements as in subclause 12.3.2 except that only the CPBCCH carriers in the list need to be measured.

NOTE: If the selected PLMN is equal to one of the equivalent PLMNs, then stored list cell selection applies to all equivalent PLMNs.

If stored list cell selection is not successful, then as defined in 3GPP TS 03.22, normal cell selection shall take place. Since information concerning a number of channels is already known to the MS, it may assign high priority to measurements on the strongest carriers from which it has not previously made attempts to obtain CPBCCH information, and omit repeated measurements on the known ones.

12.3.4 Criteria for COMPACT Cell Selection

The path loss criterion parameter C1 used for cell selection and reselection  when in a COMPACT cell is defined by:

C1 = (A ‑ Max(B,0))

where

A = RLA_P ‑ GPRS_RXLEV_ACCESS_MIN
B = GPRS_MS_TXPWR_MAX_CCH ‑ P

GPRS_RXLEV_ACCESS_MIN= Minimum received signal level at the MS required for access to the system.

GPRS_MS_TXPWR_MAX_CCH= Maximum TX power level an MS may use when accessing the system until otherwise commanded.

P= Maximum RF output power of the MS.

All values are expressed in dBm.

The path loss criterion (3GPP TS 03.22) is satisfied if C1 > 0.

12.3.5 Downlink Signalling Failure

As defined for GPRS in subclause 6.5.

12.4 COMPACT Cell Reselection

In GPRS Standby and Ready states, cell reselection is performed by the MS. The cell reselection procedures defined in this subclause apply when an MS is attached in a cell with a CPBCCH carrier or monitors a cell with a CPBCCH (or both). Otherwise, the MS shall perform cell re-selection according to subclause 10.1, or if PBCCH does not exist according to subclause 6.6.

The cells to be monitored for cell re-selection, referred to as neighbour cells, are defined in the BA(GPRS) list, which is broadcast on PBCCH or CPBCCH. If PBCCH or CPBCCH does not exist, BA(GPRS) is equal to BA(BCCH).

12.4.1 Monitoring the received signal level and CPBCCH data

The MS shall measure the received RF signal level on the CPBCCH or BCCH carriers of the serving cell and the surrounding cells as indicated in the BA(GPRS) list and optionally the NC_FREQUENCY_LIST, and calculate the received level average (RLA_P) for each carrier.

In addition the MS shall verify the BSIC of the neighbour cells. Only cells with allowed BSIC shall be considered for re-selection. The allowed BSIC is either a valid BSIC or, for cells in BA(BCCH) where no BSIC is broadcast, a BSIC with allowed NCC part, (see subclause 7.2). A valid BSIC is a BSIC broadcast for that carrier in the BA(GPRS) list.

12.4.1.1 Packet idle mode

Whilst in packet idle mode an MS shall continuously monitor all BCCH and CPBCCH carriers as indicated by the BA(GPRS) list in the system information of the serving cell. Note that both BCCH and CPBCCH carriers may be defined in the BA(GPRS) list. At least one received signal level measurement sample on each neighbour cell shall be taken for each paging block monitored by the MS according to its current DRX mode and its paging group. As the minimum MS shall take one measurement for each BCCH or CPBCCH carrier for every 4 second. As the maximum, the MS is however not required to take more than 1 samples per second for each neighbour cell. For CPBCCH carriers, only the TDMA frames where common control or broadcast blocks are transmitted are used for monitoring signal levels, see subclause 12.3.2.

RLA_P shall be a running average determined using samples collected over a period of 5 s to Max {5s, five consecutive paging blocks of that MS}, and shall be maintained for each BCCH or CPBCCH carrier. The same number of measurement samples shall be taken for all neighbour cells, and the samples allocated to each carrier shall as far as possible be uniformly distributed over the evaluation period. At least 5 received signal level measurement samples are required for a valid RLA_P value.

The list of the 6 strongest non‑serving cells shall be updated at a rate of at least once per running average period.

The MS shall attempt to check the BSIC for each of the 6 strongest non‑serving cells at least every 14 consecutive paging blocks of that MS or 10 seconds, whichever is greater. If a change of BSIC is detected then the cell shall be treated as a new cell.

When requested by the user, the MS shall determine which PLMNs are available as described in subclause 6.6.1. However, for MSs without DRX or with short DRX period (see 3GPP TS 05.02), considerable interruptions to the monitoring of PPCH can not be avoided.

12.4.1.2 Packet transfer mode

Whilst in packet transfer mode a MS shall continuously monitor all BCCH or CPBCCH carriers as indicated by the BA(GPRS) list and the broadcast carrier of the serving cell. In every TDMA frame possible, a received signal level measurement sample shall be taken on at least one of the BCCH carriers or CPBCCH time groups, one after another, as evenly distributed as possible among the neighbours. As an exception for CPBCCH carriers on multislot allocations the MS shall take at least 4 received signal level measurement samples for CPBCCH carriers from two different time groups (2 samples/time group) in every 52 multiframe, as evenly distributed as possible among the neighbours. For CPBCCH carriers, only the TDMA frames where common control or broadcast blocks are transmitted are used for monitoring signal levels, see subclause 12.3.2.

RLA_P shall be a running average determined using samples collected over a period of 5 s, and shall be maintained for each BCCH and CPBCCH carrier. The samples taken on each carrier shall as far as possible be uniformly distributed over the evaluation period. At least 5 received signal level measurement samples are required for a valid RLA_P value.

The MS shall attempt to check the BSIC for as many non‑serving cells as possible and as often as possible, and at least every 10 seconds. The MS shall use TDMA frame 51 of the PDCH multiframe for checking BSICs of CPBCCH carriers and TDMA frame 25 or 51 for checking BSIC of BCCH carriers. These frames are termed search frames. A list containing BSIC and timing information for these strongest carriers at the accuracy required for accessing a cell (see 3GPP TS 05.10) including the absolute times derived from the parameters T1, T2 T3 or R1, R2, TG shall be kept by the MS. This information may be used to schedule the decoding of BSIC and shall be used when re-selecting a new cell in order to keep the switching time at a minimum. When a BCCH or CPBCCH carrier is found to be no longer among the reported, BSIC and timing information shall be retained for 10 seconds. (This is in case a cell re-selection command to this cell is received just after the MS has stopped reporting that cell, see subclause 10.1.4.2).

If, after averaging measurement results over 4 PDCH multiframes (1 sec), the MS detects one or more BCCH or CPBCCH carrier, among the 6 strongest, whose BSICs are not currently being assessed, then the MS shall as a matter of priority decode their BSICs.

The MS shall be able to send the first packet random access (PRACH) at the latest 5+x seconds after a new strongest cell (which is part of the BA(GPRS)) has been activated under the following network conditions: Initial serving cell at RXLEV= -70 dBm, with 6 neighbours at RXLEV= -75 dBm. Then the new CPBCCH or BCCH carrier is switched on at RXLEV= -60 dBm. x is the longest time it may take to receive the necessary system information on CPBCCH or BCCH in the new cell.

NOTE: Because of test equipment limitations it is acceptable to activate the new carrier to replace one of the 6 neighbours.

In the case of a multiband MS, the MS shall attempt to decode the BSIC, if any BCCH or CPBCCH carrier with unknown BSIC is detected among the number of strongest CPBCCH carriers in each band as indicated by the Multiband Reporting parameter (see subclause 8.4.3).

Thus an MS shall, for a period of up to 5 seconds, devote all search frames to attempting to decode these BSICs. If this fails then the MS shall return to confirming existing BSICs. Having re‑confirmed existing BSICs, if there are still BCCH or CPBCCH carriers, among the six strongest, with unknown BSICs, then the decoding of these shall again be given priority for a further period of up to 5 seconds.

If either no BSIC can be decoded on a surrounding cells, or the BSIC is not allowed, then the received signal level measurements on that channel shall be discarded and the MS shall continue to monitor that channel.

If a change of BSIC is detected on a carrier, then any existing received signal level measurement shall be discarded and the carrier shall be treated as a new carrier.

If the BSIC cannot be decoded at the next available opportunities re‑attempts shall be made to decode this BSIC. If the BSIC is not decoded for more than three successive attempts it will be considered lost and any existing received signal level measurement shall be discarded and the MS shall continue to monitor that carrier.

12.4.2 COMPACT cell reselection criteria

The COMPACT GRPS mode cell reselection criteria follows the GPRS cell reselection criteria described in subclause 10.1.2.

12.4.3 COMPACT cell reselection algorithm

The cell reselection algorithm for COMPACT follows the procedures described for GPRS in subclause 10.1.3.

12.4.4 Network controlled Cell reselection

The network controlled cell reselection for COMPACT follows the procedures described for GPRS in subclause 10.1.4.

12.4.5 COMPACT cell reselection measurement opportunities

COMPACT utilizes a timeslot mapping of control channel in a rotating fashion as described in 05.02. With this timeslot rotation, a mobile can make COMPACT neighbour cell measurements of all four time-groups. The timeslot that an MS can use for measurements is dependent on the timeslot number used for traffic. During one 52-multiframe, the MS is able to measure one time-group on up to 4 frequencies once per control block. During 1 s time period (4 multiframes), the MS is able to measure all 4 time-groups.

The CPBCCH carrier shall be transmitted with constant RF output power as defined in subclause 12.2.1, during a minimum number of control blocks. The actual number of control blocks transmitted with constant RF output power in a neighbour cell is indicated in the neighbour cell parameter GUAR_CONST_PWR_BLKS, broadcast in with the neighbour cell description for an EGPRS neighbour cell.

Annex A (informative):
Definition of a basic GSM or DCS 1 800 handover and RF power control algorithm