There are several values that depend on your specific setup that you should calibrate.
eNB
The most important paramter to calibrate at the eNB is the maximum transmit power.
This power is signalled to the UE so that it can do its power control. The parameter
is PDSCH Energy Per Resource Element (EPRE), measured in dBm, and it is part of the
configuration file as pdsch_referenceSignalPower. It should be measured using a vector
signal analyzer that has the LTE option for the utilized frequency and then put in the configuration file.
UE
Power
At the UE, also the maximum transmit power has to be calibrated. Here the maximum transmit
power is measured (in dBm) over the whole (useable) bandwidth. If the same hardware is used
at the UE and the eNB the power can be related by max_ue_power = PDSCH_EPRE + 10*log10(12*N_PRB).
The UE power can also be calibrated by using the following procedure. Start the eNB and run
the UE using the option --calib-prach-tx. Once the UE is connected to the eNB, it will then
transmit the PRACH all the time using its maximum power. Measure the TX of the UE with a spectrum
analyzer at 0 span and measure the maximum power level.
The max UE power is passed to the UE using the command line argument --ue-max-power.
Timing advance
The timing advance is used to calibrate the difference in timing between the RX and TX paths. Today this paramter is hardcoded in lte-softmodem.c. It depends on the hardware and RF frontend used. For a USRP without any additional RF frontend, a paramter of 0 seems to be ok.
To calibrate this paramter, set up the eNB and run the UE with the option --calib-prach-tx.
It will transmit always a preamle 19 on the PRACH. Check the logs of the eNB and adjust the timing
advance until it receives this preamble with a close to 0 delay.
Tx-gain and Rx-gain
To calibrate the Tx and Rx gain parameter you need to follow this steps :
- 1 You have to first set the attenuation between the eNB and the UE:
The RSRP of the signal received on UE side should be between -75 and -95 dBm, so you have to set the attenuation to be in this range. (See the screen shot in the Stats windows : RSRP[0] -85.32 dBm)
-
2 The received signal on UE side should not saturate for this you have to play with Rx-gain (See the received signal in the LTE DL SCOPE UE of the scrren shot, also in the stats windows you can see that i am using Rx-gain 105 dB)
-
3 Same way as the Rx-gain you should calibrate the Tx-gain to not saturate the eNB.
This three values of attenuation, Rx and Tx gain depend from your setup so you need to tune them properly.
