How to check UL/DL signal power and modify it ?
When LTE eNB cell is setup, synchronization and broadcast signals are transmitted on downlink RF ports on a specific frequency (configured EARFCN). Once the UE decodes the DL signals, it tries to attach to the cell by sending PRACH in UL. The PRACH is decoded by eNB and logged through "t" cmd as follows:
If PRACH is not decoded by eNB, this could be due to:
- UE does not support configured EARFCN
- DL signal is too weak. In this case, increasing tx_gain amplifies the signal strength in DL. For PCIe SDR device, the range of tx_gain is between 0 and 89.75 dB. For the USRP N2x0 device with the SBXd aughterboard, the range is 0 to 31.5 dB.
Once UE got attached to the cell, UL DL signal levels have a direct impact on UL DL decoding performances. In order to reach full troughput on both UL and DL links, UL DL signals should be neither too low nor too high(saturated). The command "t spl" under eNB terminal allows UL DL signals strength monitoring:
- RX columns provides information about received signal power.
- RX RMS: stands for transmitted Root-Mean-Square. This is the mean value in dB FS (full Scale)
- RX MAX: stands for means Maximum. Displays the maximum sample value during the measurement interval period. The target value should around -10. If RX MAX value equals to 0, it means that the received signal in UL is saturated. In this case, rx_gain should de decreased until RX MAX is below 0.
In Wireless test conditions, the recommended values for PCIe SDR cards are :
In RF cables test conditions, the recommended values for PCIe SDR cards are :