A.4 Scenario 3: PLMN and CTS-GFL have locally some common frequencies

03.523GPPLower layers of the GSM Cordless Telephony System (CTS) radio interfaceStage 2TS

In this section we consider the locations where the BA list of a covering cell and the GFL of a covering CTS-FP have at least one common frequency.

This clause typically corresponds to GSM and CTS operated in shared band where GFL is sub-optimal for that location. It could happen for example where almost all frequencies are used for BCCH, or at the border between GFL areas.

One AFA requirement is to avoid that BCCH frequencies are used by a CTS-FP. Then scenario 3 is equivalent to scenario 2.

However, if this is not achieved (just after a new frequency planning for example), impacts are described hereafter. Note that impacts are similar to those on a GSM-MS in a PLMN where two cells with same BCCH frequency overlap.

A.4.1 Cell selection

Due to the presence of the CTS-FP, the power measured by the GSM-MS on a BCCH may be higher than its actual value. The ranking of the BCCH power may be modified.

In addition, if the GSM-MS tries to synchronise to such a "double" frequency it may find the CTSBCH-FB before the FCH (probability is 1/5 due to frequency burst repetition schemes). In thus case, it will fail to find the SCH at the expected frame, then discard that BCCH.

The impact is that the GSM-MS could camp on a cell which is not the most powerful. This is as for a GSM-MS in a PLMN at a location where 2 different BCCH are transmitted on same frequency.

A.4.2 Cell reselection

Due to the presence of the CTS-FP, the power measured by the GSM-MS on a BCCH from the BA list may be higher than its actual value. The ranking of the 6 strongest BCCH carriers may be modified.

In addition, the first time the GSM-MS tries to decode the BSIC of such a "double" frequency it may find the CTSBCH-FB before the FCH (probability is 1/5 due to FCH and CTSBCH-FB repetition schemes). It will fail to find the SCH at the expected frame, then discard that BCCH candidate. The MS shall attempt a new BSIC decoding every 30 s if the BCCH is still in the 6 strongest surrounding cells, with same probability of failure. Note that if the BSIC is decoded once, the GSM-MS keeps track of the synchronisation and BSIC decoding will not fail anymore.

The impact is that the GSM-MS ranking of the 6 strongest surrounding cells may be modified, the GSM-MS may camp on a valid but not best cell, and a valid BCCH may be temporarily discarded from the 6 strongest surrounding cells. This could also happen for a GSM-MS in a PLMN at a location where 2 different BCCH are transmitted on same frequency.

A.4.3 Handover

Due to the presence of the CTS-FP, the power measured by the GSM-MS on a BCCH from the BA list may be higher than its actual value. The ranking of the 6 strongest BCCH carriers may be modified. It has an impact only if it affects the ranking of a candidate cell for handover.

In addition, the first time the GSM-MS tries to decode the BSIC of such a "double" frequency it may find the CTSBCH-FB before the FCH (probability is 1/5 due to FCH and CTSBCH-FB repetition schemes). It will fail to find the SCH at the expected frame, then discard that BCCH candidate. The MS shall attempt a new BSIC decoding every 10 s if the BCCH stays in the 6 strongest surrounding cells, with same probability of failure. Note that if the BSIC is decoded once, the GSM-MS keep track of the synchronisation and BSIC decoding will not fail anymore.

The impact is that the GSM-MS ranking of the 6 strongest surrounding cells may be modified, the GSM-MS may handover on a valid but not best cell, and a valid BCCH may be temporarily discarded from the 6 strongest surrounding cells. This could also happen for a GSM-MS in a PLMN at a location where 2 different BCCH are transmitted on same frequency.