GSM Handover/Cell Reselection Failures: CRC Error Analysis

This issue is commonly observed by drive test engineers and engineers working in lab environments.

CRC, or Cyclic Redundancy Check, is an error detection scheme. It’s closely related to the Carrier to Interference ratio (C/I) measured. If the C/I is below a certain threshold, the CRC is usually considered to have failed or is set to a value of zero. Conversely, if the C/I is positive, it’s typically interpreted as passed and set to a value of one.

Here’s a breakdown of the reasons behind CRC failures:

• Instantaneous Poor Channel Conditions: Primarily due to multi-path fading occurring at the moment of failure.

• Co-channel and Adjacent Channel Interference: Interference from other channels can disrupt the signal.

• Insufficient Transmitted Power: If the transmitted power is less than the interference power, it won’t meet the standard specifications.

• RFIC Gain Settings: Improperly applied RFIC (Radio Frequency Integrated Circuit) gain settings in the User Equipment (UE) or Base Station systems.

• Faulty Power Amplifier: A malfunctioning power amplifier on the transmit side.

• Faulty LNA: A malfunctioning Low Noise Amplifier (LNA) on the receive side.

• Higher Frequency Offset: A higher frequency offset can lead to false sample detection in the front-end part of the receiver. This results in incorrect RSSI (Received Signal Strength Indicator) calculation, leading to CRC failures.

• Path Loss: Excessive path loss, whether through an RF cable or due to the air distance between the transmitter and receiver.

• GPRS Time Slot Power Imbalance: In the case of GPRS (General Packet Radio Service), if the power for individual time slots isn’t set correctly as per the class 33 multi-slot configuration.

• Inefficient Front-End Algorithms: Limitations in the algorithms used in the front-end can prevent them from working efficiently.

• Instrument Calibration Issues: Sometimes, incorrect instrument calibration can lead to CRC failures in lab tests or simulation environments.

The above points represent a basic checklist to consider when analyzing handover or cell reselection failures in GSM/WCDMA/LTE based cellular networks, as well as in non-cellular networks like WLAN, Zigbee, etc. Any of these factors can lead to CRC failures, preventing the expected channel from being received or transmitted at a particular instant. This, in turn, results in Handover (HO) or Cell Reselection failure.

The following GSM interference limits are needed for the 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

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