GSM Test Failure: RxLevF and RXQUAL Out of Range

This document explains why a GSM test might fail due to RxLevF (Receive Level Full) and RXQUAL (Receive Quality) being outside the acceptable range, especially when using CMU200 equipment. Both RxLev and RXQUAL are critical measurements in GSM/GPRS systems, primarily used for power control and handover decisions.

Understanding RxLev and RXQUAL

Both RxLev and RXQUAL provide essential information about the signal strength and quality, enabling the Base Station Controller (BSC) to make informed decisions about power adjustments and handovers.

RXLEV (Receive Level)

RXLEV represents the received signal level, measured independently by both the Mobile Station (MS) and the Base Transceiver Station (BTS). This measurement occurs during an active connection. The receive levels of neighboring cells are broadcast in System Information message 2.

Both RXLEV and RXQUAL are transmitted by the Mobile Station within a MEASUREMENT REPORT to the BSC. The BSC then uses this data to decide whether to adjust power levels or initiate a handover to another cell. RXLEV has distinct values for active Discontinuous Transmission (DTX), i.e., SUBSET, and without DTX, i.e., FULL or ALL. The RXLEV field is 6 bits long.

This binary coded RXLEV value corresponds to a specific received signal level in dBm. The following table maps the RXLEV value to its corresponding signal level:

RXLEV vs. Receiving Level (dBm)

For example, an RXLEV value of 30 would represent a received signal level between -81 dBm and -80 dBm (calculated as -(111-30) and -(110-30) respectively).

RXQUAL (Receive Quality)

RXQUAL measures receiver quality, directly related to the Bit Error Rate (BER) of the system. It reflects the data’s accuracy received over the air interface.

The RXQUAL field is 3 bits long, allowing values from 0 to 7. Ideally, for tests to pass, RXQUAL should be 0, indicating a BER within acceptable limits. Any non-zero RXQUAL value will typically cause a test to fail. Refer to “GPRS RxQUAL vs BER” for a detailed mapping of RXQUAL values to BER.

Handover and Power Control Parameters

RXLEV and RXQUAL are crucial input parameters for handover and power control algorithms. Several parameters, as measured by both the MS and the BTS, contribute to these decisions:

Measurement Parameters for Handover and Power Control

These parameters provide a comprehensive view of the signal strength, quality, and timing between the MS and BTS, facilitating optimal network performance. Failure of a GSM test due to RxLevF or RXQUAL suggests a problem in signal strength, signal quality, or the synchronization within the network.