GSM Issue-Burst measurement failed due to timing advance error out of range

As we know that GSM is FTDMA system, where in each carrier frequency has eight time slots on the time axis. It is very important to have time synchronization of the transmission from both the mobile and base station side. To have perfect synchronization in the air, GSM base station(BS) transmit timing advance information to the mobile subscriber. This timing advance value is in bits and is used by the GSM mobile subscriber(MS) to advance the timings of the transmit frame. This is required for compensation of the propagation delay from MS to BS.

The value of zero for the timing advance is taken for the zero distance between BS and MS under static channel condition environment. This should be with tolerance of about +/- 1 bit. It is reported once in SACCH period or after RACH channel.

If we consider the round-trip propagation delay equal to P bits and timing advance of about T bits, then timing offset reported by base station to gsm mobile will be equal to nearest value of quantization of (P-T) .

The timing advance is reported with any value from 0 to 63. Here value of 0 refers to no timing advance i.e. GSM mobile transmissions to the base station are 468.75 bits periods behind. The value of 63 corresponds to the maximum timing advance which refers to MS transmissions are 405.75 bit periods behind.

As a requirement in the tests, GSM DUT is subjected to various timing advance values by test system (CRTU from R&S) and the same is used by mobile to adjust its uplink transmissions. This transmission is received by the CRTU test system and is compared with the value it has directed previously. Based on this timing advance error is determined in the test system. If it exceeds more than one bit then the test is declared as failed. If it is less than one bit than the test is declared as pass.

While performing the tests TS8950G CRTU protocol test system from R&S measures the validity of burst spectrum transmitted by the mobile across time axis and measures timing advance error and determines the verdict (pass or fail).

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