GSM Burst Measurement Failure: Timing Advance Error Explained

As we know, GSM is an FDMA/TDMA system where each carrier frequency has eight time slots on the time axis. Time synchronization between the mobile and the base station is crucial.

To achieve perfect synchronization, the GSM base station (BS) transmits timing advance information to the mobile subscriber (MS). This timing advance value, in bits, instructs the GSM mobile subscriber to advance the timing of its transmit frame. This is necessary to compensate for the propagation delay between the MS and the BS. A timing advance value of zero is used when the distance between the BS and MS is effectively zero under static channel conditions. This should be within a tolerance of +/- 1 bit.

The timing advance is reported once per SACCH period or after the RACH channel. If we consider the round-trip propagation delay to be P bits and the timing advance to be T bits, the timing offset reported by the base station to the GSM mobile will be the nearest quantized value of (P - T). The timing advance value can range from 0 to 63.

A value of 0 indicates no timing advance, meaning the GSM mobile’s transmissions to the base station are 468.75 bit periods behind. A value of 63 corresponds to the maximum timing advance, meaning the MS transmissions are 405.75 bit periods behind.

In tests, the GSM device under test (DUT) is subjected to various timing advance values by the test system (like a CRTU from Rohde & Schwarz). The mobile uses this information to adjust its uplink transmissions. The CRTU test system receives this transmission and compares it to the value it previously sent. The timing advance error is then determined.

If the error exceeds one bit, the test fails. If it is less than one bit, the test passes.

During tests, the Rohde & Schwarz TS8950G CRTU protocol test system measures the validity of the burst spectrum transmitted by the mobile across the time axis. It measures the timing advance error and determines the pass/fail verdict.

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