- You can also reduce the number of LDPC decoder iterations, which will make the LDPC decoder take less time: `--L1s.[0].max_ldpc_iterations 4`.
## 6.3 Uplink issues related with noise on the DC carriers
- There is noise on the DC carriers on N300 and especially the X300 in UL. To avoid their use or shift them away from the center to use more UL spectrum, use the `--tune-offset <Hz>` command line switch, where `<Hz>` is ideally half the bandwidth, or possibly less.
With devices like the USRP N300 and especially the X300, there is noise in the DC carriers: this can cause uplink PRBs that overlap with the DC carrier to experience interference and increased noise.
There are two possible solution that can be enabled in OAI:
*`--tune-offset`: it consists in shifting away the operational bandwidth to avoid the center frequency
*`ul_prbblacklist`: can be used to define specific PRBs that should not be used for uplink scheduling
A spectrum clean from the noisy PRBs will eventually result in an enhanced UL throughput.
Using `--tune-offset`, `ul_prbblacklist` or none at all is depending on the combination of the USRP model, the operational bandwidth configuration and the bandwidth of the daughterboard.
There are two main points to keep in mind:
- USRPs come with various daughterboards, each with its own specific bandwidth (see datasheets).
- Using `--tune-offset` to shift the entire operational bandwidth to avoid the center frequency could exceed the device capabilities: if the shifted operational bandwidth falls entirely within the bandwidth of the daughterboard, than `--tune-offset` is sufficient, otherwise `--tune-offset` is ineffective and `ul_prbblacklist` is needed.
The option `ul_prbblacklist` is more relevant when using high-bandwidth configurations (e.g., 100 MHz) with devices like the USRP N310 or X310: in this scenarios, `--tune-offset` could not be sufficient to get rid of the noisy PRBs in the center frequency entirely, because it is not possible to shift the DC carriers out of the bandwidth (tune offset shall be smaller than half the bandwidth of the board).
## 6.3.1 Tune offset
The value passed to the command line option `--tune-offset <Hz>` will be calling an UHD API. It represents the LO offset frequency in Hz.
The API (tune_request_t class) will send a frequency tuning request (`tx_tune_req`, `rx_tune_req`) to the USRP device, in order to configure the target baseband tx/rx frequency, therefore shifting the tx/rx signal spectrum.
The value passed to this option should be ideally equal to half the operational bandwidth, or possibly less, depending on the bandwidth configuration, and also it shall be lower or equal than half the bandwidth of the board.
A visual representation of the impact of tune-offset with a 120 MHz bandwidth daughterboard:
## 6.3.2 UL PRBs Blacklist
To use this option, in the configuration file, e.g. 100 MHz bandwidth setup: `ul_prbblacklist = "135,136,137,138"`.
## 6.4 Lower latency on user plane
- To lower latency on the user plane, you can force the UE to be scheduled constantly in uplink: `--MACRLCs.[0].ulsch_max_frame_inactivity 0` .
