srsRAN adds suppport for discontinuous transmission mode

[joaquim-srs]

Hi there,

With the latest release, srsRAN adds support for discontinuous transmission mode using USRP devices. This feature stops the transmit burst when there are no transmission requests coming from the stack, and starts a new burst when required.

Background

By default, the UHD radio manager included in srsRAN transmits samples continuously during normal operation. This means that a single transmit burst is started during radio initialization, and samples are transmitted until the GNB is stopped or there is a real time radio event (lates or underflows).

This behavior is fine for most use cases. However, in some situations it may be required, or beneficial, to stop the transmit chain when there is no data to transmit. As pointed out by some of our users, some TDD RF front-ends require switching between PA / LNA operation, in which case it may be useful to use the GPIO pins provided by some USRPs to drive the switching. In addition to that, powering the transmit chain only when necessary may provide power efficiency gains even for FDD GNB configurations, and reduce bandwidth utilization for networked USRPs.

Design

When discontinuous transmission mode is enabled, metadata information is passed from the PHY to the radio management class, indicating if there is transmission data within the current time window. Detection of empty transmission slots is not constrained to the TDD pattern or even to TDD mode. If the resource grid belonging to a particular slot is empty, the transmit stream will be stopped during that slot.

Upon initial testing, it became apparent that power ramping of the transmit chain may be an issue when using discontinuous transmission mode. Because of that, we have added a configurable power ramping guard period. This is used by the radio manager to start the stream some time in advance, which moves the transient response of the transmit chain ahead of the samples carrying data.

Below, you can see an IQ magnitude measurement of a TDD signal using a B200 mini, with observable power ramping effects, Then, the same measurement is repeated adding a 200 microseconds power ramping guard time.

Configuring DTX mode

To enable discontinuous transmission mode, use the ru_sdr -> expert_cfg -> discontinuous_tx GNB configuration parameter. To set the power ramping guard time, use the ru_sdr -> expert_cfg -> power_ramping_time_us parameter. As usual, parameter descriptions can be accessed by calling the GNB configuration help, i.e., apps/gnb/gnb -h.

Please be aware that using long power ramping guard times may increase the likelihood of having lates, since the radio must start the stream in advance of the current time.

Further testing

Upon further testing, we have discovered some unexpected side-effects of using the discontinuous transmission mode. These findings have been reproduced using a B200 mini and a B210, while testing with X and N families of USRPs is still pending.

We have used the radio_util_sample example application to transmit a series of bursts with transmission gaps in between, using discontinuous transmission mode. By altering the size of the gaps, we have found that the transmission power of the USRP changes based on the percentage of time we are transmitting. This is illustrated by the captures below, where the USRP transmission gain has not been changed and there is a roughly 5 dB power drop when we only transmit 50% of the time as opposed to when transmitting 90% of the time.

Furthermore, we have also seen that this power decrease is not present upon radio start and settles in gradually over the first milliseconds of operation. The next image belongs to a longer IQ capture of 1 second, using a 50% transmission duty cycle and, as you can see, the power starts at the expected level, then drops over the first 100 milliseconds.

As always, we appreciate your feedback regarding this feature, and we are looking forward to seeing what you achieve with it!

[joaquim-srs]

N310 Testing update:

We have repeated the same experiment with the N310 USRP. We have used the radio_util_sample application to send a series of transmission bursts separated by gaps of varying lengths. In contrast with the B200, we see no power ramping on the N310 signal, even with the power ramping guard period set to 0, as you can see in the image below.

Also unlike the B200 results, we have observed no power differences while using different transmission gap sizes, or compared to the transmit power in continuous transmission mode.