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S:\Hp8960\E1960A GSM Mobile Test Application\A.04 Release\Reference_Manual\Chapters\meas_rach_desc.fm
RACH Measurement Description
RACH Measurement Description
What is a RACH?
When a mobile first attempts to originate a call it sends a RACH (Random Access Channel) burst. The RACH
is transmitted on the uplink frequency of the channel number used by the Broadcast channel (BCH). The
RACH is the first burst sent by the mobile. This burst is short, only 312.2 ms, as opposed to the normal GSM
burst of 542.8 ms. The RACH is used by the base station to determine the timing advance which it then sends
back to the mobile. Once the mobile receives this information it starts to transmit normal bursts.
Measurements that can be performed on a RACH
The test set can perform the following three measurements on a RACH in Active Cell or Test mode:
• Power versus Time
• Transmit Power
• Phase and Frequency Error
NOTE
Only one measurement at a time can be made on the RACH even if two measurements are
initiated.
Triggering
The type of triggering used is dependent on whether you are in Active Cell or Test mode:
Active Cell mode:
The default triggering of Auto is acceptable for most signals. (In Active Cell mode Auto is equivalent to
Protocol.) However, if the mobile’s RACH timing is outside the specified limits you may need to use RF Rise
triggering.
Test mode:
The default triggering of Auto should be used. (In Test mode Auto is equivalent to RF Rise.)
Overview of Measurement Procedure in Active Cell Mode
1. Set operating mode to Active Cell.
2. Set the receiver control to manual.
3. Set the test set’s measurement receiver to the frequency the RACH will arrive on. The simplest way to do
this is to set the manual channel (that is, the expected ARFCN) to the ARFCN of the BCH. Alternatively
you could set the expected frequency to the uplink frequency of the BCH ARFCN.
4. Ensure trigger mode is set to Auto.
Once the RACH measurement is completed, in order to make further measurements on the TCH, ensure you
reset the receiver control to Auto.
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