crosstalk (2)

GSM SYSTEM TECHNICAL NOTE 2/98 CROSSTALK

TELECOMMUNICATIONS NWS/MSG/NI

Crosstalk GSM System Technical Note 2/98

© COPYRIGHT Nokia Telecommunications Oy 1998 All rights reserved. No part of this document may be copied, distributed, transmitted, transcribed, stored in a retrieval system, or translated into any human or computer language without the prior written permission of Nokia Telecommunications Oy. The manufacturer has made every effort to ensure that the instructions contained in the documents are adequate and free of errors and omissions. The manufacturer will, if necessary, explain issues which may not be covered by the documents. The manufacturer's liability for any errors in the documents is limited to the correction of errors and the aforementioned advisory services. The documents have been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using them. The manufacturer welcomes customer comments as part of the process of continual development and improvement of the documentation in the best way possible from the user's viewpoint. Please submit your comments to the nearest Nokia sales representative. NOKIA is registered trademark of Nokia Corporation. Any other trademarks mentioned in the documents are the property of their respective owners.

Version history of the document Date

Version Notes

29.5.1998

1.0.0

Published version

No of Pages

Edited by

Checked by

Approved by

9

21-05-1998 Peter Bertels Marjatta Saha

21-05-1998 Marjatta Saha

25-05-1998 Sakari Niemi

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NOKIA CUSTOMER CONFIDENTIAL

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CONTENTS 1.

Executive summary

2.

REASON OF CROSSTALK

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3.

ACTIONS TO PREVENT CROSSTALK

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4.

Catalogue of published GSM System Technical Notes

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1.EXECUTIVE SUMMARY The Crosstalk phenomenon has been found out to happen within some of the GSM networks. One known Crosstalk situation happens in the following way: A mobile call is going on (a mobile originated or a mobile terminated call). A subscriber (A or B) hears suddenly a speech from some other call, and other end of the original call may be dropped. However the call is not always dropped, in some case the subscriber gets occasionally very short cuts of another unknown caller and the original call is going on. The known reason for this Crosstalk phenomenon is related to the GSM networks where Ciphering is not used and Frequency and Base Station Identity Code (BSIC) pairs are used too close each other in Radio Network planning. If a certain frequency and BSIC combination is used too tight in the network, this may occasionally cause interference and disturbance and in certain circumstances, two independent Handover messages may cause a short Crosstalk phenomenon. Whether Ciphering is in used then too tight use of Frequency and Base Station Identity Code (BSIC) pairs may cause interference and disturbance and it is heard as a short mute in speech and the call may be dropped. This System Technical Note explains the known reason and actions to prevent Crosstalk. There might occur more Crosstalk phenomenon in live GSM networks. We are interested in to receive any information related to other Crosstalk situations and will distribute the received information, if any to you in next System Technical Notes.

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2.REASON OF CROSSTALK Crosstalk is caused in simultaneous Handover (HO) phase within two separate calls. In the following example (ref. steps 1-5 in Figure 1.Crosstalk), the Crosstalk situation is explained more detailed. The used frequencies, BSIC values etc. are illustrated in this case. The subscriber is located in Cell 1 with the frequency 55. He is having a call on and it is going to make Handover to Cell 2 that has the frequency 78 and BSIC 8. Suddenly the mobile makes Handover to another Cell x that has the similar BSIC (8) and the similar frequency (78), but this Cell x is not the one the mobile was signalling concerning Handover attempt. At this point, the subscriber hears some unknown subscriber’s speech. Crosstalk occurs because there are two cells with the same combination of the frequency (78) and the BSIC (8) influencing to each other. It does not matter whether these cells were situated several kilometres from each other, but the antennas are pointing to the same direction causing the co-channel interference seen between these two cells. The area where the subscriber gets Crosstalk is situated in this interference area. When a mobile is making a handover it gets Handover Command from BSC. Handover Command includes among others the frequency, BSIC and the Handover_Reference. The Handover_Reference has information of a new timeslot. The mobile sends back Handover Access to BTS where it is going to make a handover. Handover Access includes the Handover_Reference. With the Handover_Reference BTS identifies the mobile that is making the handover. In this situation the subscriber was expecting to make a handover to Cell 2 with the frequency 78. At the same time there was another mobile going to make a handover to another Cell x that had the same frequency 78. Both mobiles had received Handover command (ref. Steps 1 and 3 in Figure 1.) from the BSC and were sending to BTS Handover access (ref. Steps 2 and 4 in Figure 1.). In this case both mobiles were defined to make Handover to a similar timeslot in different cells. Handover_Reference of Subscriber MS1 was noticed in Cell x with frequency 78 although Cell 2 (with the same frequency) was expecting it (ref. Step 5 in Figure 1.). The mismatch happened because of the same BSIC code and the same frequency. While Ciphering is not yet used in the network the subscriber is able to hear clear speech from an unknown person. It might happen that the call drops within Crosstalk situation. Then the MSC clear code for the drop is B14 (handover failure).

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Figure 1. Crosstalk Phenomenon

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Crosstalk GSM System Technical Note 2/98 Here are listed the steps 1-5 illustrated in Figure 1.

1 Handover Command to MS1 Signalling message: HO Command: (BSC1) Cell 1 -> MS1 ARCN=78 BSIC=8 Handover Reference: Timeslot=3

2 Handover Access from MS1 Signalling message: HO Access: MS1 uplink HO response -> (BSC1) Cell 2 ARCN=78 BSIC=8 Handover Reference: Timeslot=3

3 Handover Command to MSx Signalling message: HO Command: (BSC2) Cell x+1 -> MSx ARCN=78 BSIC=8 Handover Reference: Timeslot=3

4 Handover Response from MSx Signalling message: HO Access: MSx uplink HO response -> (BSC2) Cell x ARCN=78 BSIC=8 Handover Reference: Timeslot=3

5 CROSSTALK HAPPENS Cell x reacts first to MS1’s HO Access (Step 2) although Cell x should have reacted to MSx’s HO Access (Step 4).

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3.ACTIONS TO PREVENT CROSSTALK When there is any evidence of Crosstalk, then the network planning has to be checked, i.e. the following BSC related information has to be gathered and checked:

- BTS id - Frequency - Base Station Colour Code (BCC) - Network Colour Code (NCC)

The Base Station Identity Code (BSIC) consists of BCC and NCC. When using same frequencies in a network the meaning of BSIC is to separate cells with similar frequencies from each other. It is normally used one value for NCC in whole network. All frequencies have to be listed with the used BSIC values (i.e. BCC and NCC values). Same frequencies should have a different BSIC code as far as possible. For example if the frequency 50 is used 5 times in the area, these cells can use BCC values 0,1,2,3,4,5 (not for example 0,0,1,1,1). If the same frequency is needed to use more than eight times (BCC can vary between 0 –7), then the network planning has to be made carefully. This means that similar frequency & BCC pairs have to be located so far away that they do not interference to each other.

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4.CATALOGUE OF PUBLISHED GSM SYSTEM TECHNICAL NOTES 2/1998

Crosstalk

1/1998

Call Setup Time

2/1997

Reasons behind 3 Beeps SDCCH Congestion and Dropped Calls, Update Cell Outage, Update Mobile Terminated (MT) Call Failing in a Good Field, Update

1/1997

Power Consumption of GSM/DCS Mobiles

4/1996

Mobile Terminated (MT) Call Failing in a Good Field, update

3/1996

Trace function with Nokia BSC and Ericsson MSC only for Nokia customers with the above switch

2/1996

Fast Call Set-up Warning, Data Subscriber roaming problem, Cell Outage Status Update

1/1996

Mobile Terminated (MT) Call Failing in a Good Field

5/1995

Speech Quality and Echo in Nokia networks

4/1995

SDCCH Congestion & Dropped Calls, update

3/1995

Cell Outage Status Report, update

2/1995

SDCCH Congestion, Dropped Calls, Delay in Generic Fault Reporting & Test results for the compatibility between BTS SW 7.0 and mobile stations

1/1995

DL DTX activation with Ericsson or Nortel MSC's only for Nokia customers with the above switches

Oct. 10/1994

NCS policy towards fixing mobile terminal faults in our infrastructure NCS's policy towards Motorola terminal faults

Sep. 5/1994

SIEMENS S3 terminal problems in Nokia infrastructure downlink DTX with new BSC S3 software New Establishment Cause feature in the BSC S3 software

May 1/1994

Cell Outage Status Report - Update

Feb. 1/ 1994

"Pumping" problem with handsfree GSM terminals (only for Nokia MSC operators)

NOTE: Depending on the operator’s environment the listed STN’s may still be valid.

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crosstalk (2)

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