GSM Issue-Handover or cell reselection failed due to CRC failures
This issue is most commonly observed by drive test engineers and engineers working in the lab environment. CRC or cyclic redundancy check is the error detection scheme. It is related to carrier to interference ratio (C/I) measured.
If C/I is below the threshold CRC is usually interpreted as failed or set as value zero. If C/I is positive then it is interpreted as passed or and set as value one.
Following are the reasons of CRC failures:
• Instantaneous poor channel conditions (mainly due multi-path fading) at the time of the failure.
• Due to co-channel as well as adjacent channel interference.
• Transmitted power is less than the interference power as required by the standard specifications.
• RFIC Gain settings have not been applied properly in the UE/Base Station systems.
• Power amplifier might have gone faulty on the transmit side.
• LNA might have gone faulty on the receive side.
• Higher frequency offset also results into false sample detection in the front end part of the receiver. This results into wrong RSSI calculation and results into CRC failures.
• Due to path loss either through RF cable or through air distance between transmitter and receiver.
• Power for the individual time slots in the case of GPRS is not properly set as mentioned in the class 33 multi slot configuration.
• Front end algorithms are not efficiently working as required due to limitation of the algorithms.
• Sometimes instrument calibration also results into CRC failure in the lab test or lab simulation environment.
Above are the basic checklists need to be considered to analyze the failure of handover or cell reselection in GSM/WCDMA/LTE based cellular and also in non cellular networks such as WLAN, Zigbee etc. Any of these factors result into CRC failures and due to which channel expected might not be received or transmitted at that instant. This results into HO or Cell reselection failure.
Following are GSM interference limit as needed for system to work efficiently.
Co-channel interference (C/I) = 9dB
First adjacent channel interference (200 kHz) (C/I) = -9dB
Second adjacent channel interference (400 kHz) (C/I) = -41 dB
Third adjacent channel interference (600 kHz) (C/I) = -49 dB