7 Network pre-requisites

05.083GPPRadio subsystem link controlTS

7.1 BCCH carriers

The BCCH carrier shall be continuously transmitted on all timeslots and without variation of RF level. However, the RF power level may be ramped down between timeslots to facilitate switching between RF transmitters.

Furthermore, 8-PSK modulated timeslots on the BCCH carrier, with the exception of TN 7, may use a mean power which is at most 4 dB lower than the power used for GMSK modulated timeslots. The MS requirements on signal strength measurements are defined for the case when only GMSK modulation is used on the BCCH carrier. There are no defined signal strength measurement requirements for the MS if 8-PSK modulation is used on the BCCH carrier.

On the PCH the network shall send valid layer 3 messages according to 3GPP TS 04.18. Unused signalling blocks on the CCCH/BCCH shall contain L2 fill frames. Other unused timeslots shall transmit dummy bursts.

The number of neighbour cell BCCH carriers in the BCCH allocation shall not exceed 32.

NOTE 1: This BCCH organization enables MS to measure the received signal level from surrounding cells by tuning and listening to their BCCH carriers. Providing that an MS tunes to the list of BCCH carriers indicated by the network it will, providing the list is sufficiently complete, have listened to all possible surrounding cells, i.e. the surrounding cell list for handover purposes is effectively defined by the MS. Refer to 3GPP TS 03.22 for definitions of the BCCH carrier lists. This can be achieved without inter‑base station synchronization.

NOTE 2: If the operator decides to allow 8-PSK modulation on the BCCH carrier in certain cells, the cell selection, cell reselection and handover procedures involving these cells will be somewhat sub-optimal. This is due to the fact that the signal level measured by the MS at some instances in time will be affected by the possibly lower mean power level of the 8-PSK modulation and by the power fluctuation resulting from the 8-PSK modulation characteristics. The extent of the performance degradation is dependent upon the measurement schedule in each particular MS as well as upon the used average power decrease (APD) and the current 8-PSK load. By limiting the maximum number of 8-PSK slots simultaneously allowed on the BCCH carrier, and/or carefully selecting the values of involved network parameters, the impact on the above mentioned procedures may be minimised. Additionally, in areas with very low cell overlap, some coverage loss effects may have to be taken into account by the operator when selecting network parameters.

NOTE 3: In the case that 8-PSK modulation is allowed on the BCCH carrier and frequency hopping including the BCCH carrier is used, the reception quality in connected mode for some fast moving MS (meaning MS experiencing Doppler frequencies of 100 Hz or more) may be degraded. This may be seen as a backwards compatibility problem for some existing MS, most likely occurring if the used APD is larger than 2 dB.

7.2 Identification of surrounding BSS for handover measurements

It is essential for the MS to identify which surrounding BSS is being measured in order to ensure reliable handover. Because of frequency re‑use with small cluster sizes, the BCCH carrier frequency may not be sufficient to uniquely identify a surrounding cell, i.e. the cell in which the MS is situated may have more than one surrounding cell using the same BCCH frequency. Thus it is necessary for the MS to synchronize to and demodulate surrounding BCCH carriers and identify the base station identification code (BSIC). The MS shall be able to perform this task at levels down to the reference sensitivity level or reference interference levels as specified in 3GPP TS 05.05.

The MS shall use at least 4 spare frames per SACCH block period for the purpose of decoding the BSICs (e.g. in the case of TCH/F, the four idle frames per SACCH block period). These frames are termed "search" frames.

A 6 bit Base Station Identity Code (BSIC), as defined in 3GPP TS 03.03, shall be transmitted on each BCCH carrier. The PLMN part of the BSIC can be regarded as a "PLMN colour code".

The MS shall attempt to demodulate the SCH on the BCCH carrier of as many surrounding cells as possible, and decode the BSIC as often as possible, and as a minimum at least once every 10 seconds. A multi-RAT MS is allowed to extend this period to 13 seconds, if the neighbour cell list contains cells from other RATs and if indicated by the parameter 3G_SEARCH_PRIO. The MS shall give priority for synchronization attempts in signal strength order and considering the parameter MULTIBAND_REPORTING. A list containing information about the timing of the surrounding cells at the accuracy required for accessing a cell (see 3GPP TS 05.10) including the absolute times derived from the parameters T1, T2, T3 shall be kept by the MS. This information may be used to schedule the decoding of BSIC and shall be used in connection with handover in order to keep the switching time at a minimum. The network may provide Real Time Difference (RTD, see 3GPP TS 05.10) to assist the MS in neighbour cell synchronization attempts. This assistance data is included in the MEASUREMENT INFORMATION message (See 04.18). RTD is provided modulo one multiframe (51 TDMA frames). The resolution is either one TDMA frame, in which case the MS can assume that the cells are frame synchronised, or 1/64 TDMA frame. The MS may use other assistance data too, if received elsewhere, e.g. for position services received information. The actual number of carriers the MS is capable of synchronising to, depends on the Observed Time Difference (OTD, see 05.10) for each neighbour cell and the availability of the assistance information.

If, after averaging measurement results over 2 SACCH block periods, the MS detects one or more BCCH carriers, among the 6 strongest, whose BSICs are not currently being assessed, then the MS shall as a matter of priority attempt to decode their BSICs.

In the case of a multi band MS, the MS shall attempt to decode the BSIC, if any BCCH carrier with unknown BSIC is detected among the number of strongest BCCH carriers in each band as indicated by the parameter MULTIBAND_REPORTING.

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 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.

The MS shall report a new strongest GSM cell in the measurement report at the latest 5 s after a new strongest cell (which is part of the BA(SACCH)) 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 BCCH carrier is switched on at RXLEV= -60 dBm.

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

If either no BSIC can be demodulated on a surrounding cell BCCH carrier, or the BSIC is not allowed, then the received signal level measurements on that channel shall be discarded. The allowed BSIC is either the BSIC broadcast for that carrier in the neighbour cell list or, if no BSIC is included or if indicated by the parameter INVALID_BSIC_REPORTING, a BSIC with permitted NCC part. The permitted NCCs are defined by the NCC_PERMITTED parameter transmitted in the BCCH data. This is an 8 bit map that relates to the NCC part of BSIC (e.g. NCC_PERMITTED = 01101001, defines that only carriers having a BSIC with the NCC part = 000, 011, 101,110 shall be reported).

If a change of BSIC is detected on a carrier, then any existing received signal level measurement shall be discarded and a new averaging period commenced. This occurs when the MS moves away from one surrounding cell and closer to another co‑channel cell.

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.

If an MS receives a handover command towards a GSM cell to which it is not synchronised to, then the MS shall search for synchronisation information up to 300 ms. In case of failure, the MS shall refer to the handover failure procedure (see 3GPP TS 04.18).

If a multi-RAT MS receives a handover command towards a not known cell (see 3GPP TS 25.133 and 3GPP TS 25.123), then the multi-RAT MS shall search for synchronisation information up to 800 ms. In case of failure, the multi-RAT MS shall refer to the handover failure procedure (see 3GPP TS 04.18).

Details of the synchronization mechanisms appear in 3GPP TS 05.10. The procedure for monitoring surrounding BTS with respect to HO measurement shall begin at least at the time of assignment of a dedicated channel.

When a BCCH carrier is found to be no longer among the reported, timing and BSIC information shall be retained for at least 10 seconds. (This is in case a handover is commanded to this cell just after the MS stops reporting RXLEV and RXQUAL on this cell).

7.3 Handover measurements on other radio access technologies

For a multi-RAT MS, the network controls the identification and measurements of cells belonging to other radio access technologies by the parameter Qsearch_C sent on SACCH or, if Qsearch_C is not received, by Qsearch_C_Initial sent on BCCH. Qsearch_C defines a threshold and also indicates whether these tasks shall be performed when RXLEV (see subclause 8.1.3) of the serving cell is below or above the threshold. The MS may use the search frames, which are not required for BSIC decoding, for these measurements. If indicated by the parameter 3G_SEARCH_PRIO, the MS may use up to 25 search frames per 13 seconds without considering the need for BSIC decoding in these frames.

The MS shall report a new best UTRAN cell, which is part of the neighbour cell list, at the latest 5 seconds after it has been activated under the condition that there is only one UTRAN frequency in the neighbour cell list and that no new GSM cells are activated at the same time and under good radio conditions. For test purposes the following radio conditions can be used: Serving GSM cell at RXLEV= -70 dBm, with 6 GSM neighbours at RXLEV= -75 dBm. Then either an UTRAN FDD neighbour cell or an UTRAN TDD neighbour cell is switched on. The radio conditions for the UTRAN FDD cell are as follows (see TS 25.101 for definitions):

Parameter

Unit

UTRAN FDD Cell

CPICH_Ec/Ior

dB

-10

PCCPCH_Ec/Ior

dB

-12

SCH_Ec/Ior

dB

-12

PICH_Ec/Ior

dB

-15

DPCH_Ec/Ior

dB

-

OCNS

-0.94

dB

10

dBm/3.84 MHz

-70

CPICH_Ec/Io

dB

-10.4

CPICH RSCP

dBm

-70

FDD_MULTIRAT_
REPORTING

integer

1

Qsearch_C

integer

7 (search always)

3G_SEARCH_PRIO

integer

1

Propagation Condition

AWGN

The radio conditions for the UTRAN TDD cell (3.84 Mcps) are as follows (see 3GPP TS 25.123 for definitions and for the values of the remaining configuration parameters):

Parameter

Unit

UTRA TDD Cell

(3.84 Mcps)

Timeslot Number

0

8

P-CCPCH_Ec/Ior

dB

-3

SCH_Ec/Ior

dB

-9

-9

SCH_toffset

integer

0

0

PICH_Ec/Ior

dB

-3

OCNS_Ec/Ior

dB

-3.12

-3.12

PCCPCH RSCP

dBm

-70

-70

TDD_MULTIRAT_
REPORTING

integer

1

Qsearch_C

integer

7 (search always)

3G_SEARCH_PRIO

integer

1

Propagation Condition

AWGN

NOTE: On timeslot 8 the P-CCPCH is not transmitted; on that timeslot, the PCCPCH RSCP defines the power level of the beacon channel.

The allowed reporting time is increased by 5 seconds for each additional UTRAN frequency in the neighbour cell list and by the time required for BSIC decoding of new activated GSM cells. However, multiple UTRAN cells on the same frequency in the neighbour cell list does not increase the allowed reporting time.

When on TCH, identification of a TDD cell is guaranteed only in case of single slot operation and if the TDD cell uses synchronisation option 2 (see 3GPP TS 25.221). In all other cases, the MS may not be able to fulfil the requirement above. If after 5 seconds the MS has not been able to identify a TDD cell, the MS is allowed to stop searching for it in the current GSM cell.

When on SDCCH, the MS may use all TDMA frames, which are not part of the allocated channel or required for GSM signal strength measurements, for the above task. In this case the allowed reporting time is 1.7 seconds, with the same assumptions as above.

A multi-RAT MS shall be able to monitor 64 cells from other radio access technologies, divided into (depending on the MS capability):

– FDD cells on up to 3 FDD frequencies, with a maximum of 32 cells per frequency
– TDD cells on up to 3 TDD frequencies, with a maximum of 32 cells per frequency; and/or
– CDMA2000 cells.