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RACH Measurement Description
Last updated: May 8, 2002
This measurement is
not
applicable to GPRS.
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
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
-
Set operating mode to Active Cell.
-
Set the receiver control to manual.
-
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.
-
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.
Overview of Measurement Procedure in Test Mode
-
Set operating mode to Test.
-
Set the test function to either BCH, or, BCH + TCH.
-
Set the Broadcast Channel to the channel you wish to use.
-
Using your proprietary commands, initiate the mobile to generate a sequence of RACH bursts on the BCH.
-
Start the appropriate measurement.
Example Procedure
The following procedure details how to make a power versus time RACH measurement manually while in Active Cell mode.
-
Press the blue
SHIFT
key then the green
Preset
key. The "Call Setup Screen" is displayed.
-
Press the
More
key (which is positioned immediately below
F12
) two times. This displays screen
3 of 4
.
-
Press
Receiver Control
(F7) and set to
Manual
.
-
Press
Manual Channel
(F9) and change from 30 to 20. (This sets it to the same channel as the
Broadcast Chan
on screen
1 of 4
.)
-
Press
Measurement selection
. (This key is positioned below the display.)
-
Select
Power vs Time
.
-
Press
Power vs Time Setup
(
F1
).
-
Press
Measurement Setup
(
F1
).
-
Set
Trigger Arm
to
Single
, then select
Close Menu
(
F6
).
-
Press
START SINGLE
on the front panel of the test set. (Note, you are starting the measurement before originating a call. This is to ensure that it is the RACH burst that is measured.)
-
Connect the mobile, then originate a call from the mobile.
-
Immediately you press send on the mobile the power versus time measurement result is displayed. You can confirm that the measurement has occurred on the RACH by examining the measurement results. With a RACH measurement, since the burst is shorter than normal, the power drops off rapidly after 331 ms. To examine the results select
Return to PvT Control
(
F6
),
Change View
(
F2
), then select
Numeric 1 of 2
(
F2
), and
Numeric 2 of 2
(
F3
).
-
To do further measurements on the TCH ensure that the Receiver Control is returned to Auto.
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