SDR

GNURadio Period transmisstion

Introduction

Period transmission in GNURadio/USRP platform is the basic function to support time-related systems, such as

The period transmission is based on burst transmission with “tx_time” tag. All the functions have been tested on USRP N310 with GNURadio 3.7.13 and UHD 3.15.

GNURadio Tag

GNURadio tag is an isosynchronous data that runs parallel to the main data stream.

uhd_sink block supports two tags “tx_sob” and “tx_eob” for burst transmission. “tx_sob” tag means “start of burst” that should be added into the first sample of a frame burst. “tx_eob” means “end of burst” that should be added into the last sample. If UHD receives “tx_eob”, the TX mode of USRP will be done, until it receives a “tx_sob” tag. The “tx_sob” and “tx_eob” value should simply be PMT_T. As an example, user can use add_item_tag provided by GNURadio to add the “tx_sob” tag in C++.

add_item_tag(0, // Port number
         nitems_written(0) + i, // Offset
         pmt::mp("tx_sob"), // Key
         pmt::PMT_T // Value
    );

In the above code, the “tx_sob” tag will be added at port 0 with nitems_written(0) + i offset. Note that add tag can also in python.

Please refer to this page for a detail description about GNURadio tag.

The third important tag is “tx_time” which is a PMT encoded tuple with two components. The first item in the tuple is the PMT encoded uint64 epoch time in seconds. The second item in the tuple is the PMT encoded double fractional epoch time. The following example can be used to transfer the time from double to a PMT tuple in C++.

double d_next_time = 1.0;
uint64_t t_int = (uint64_t)d_next_time;
double t_double = d_next_time - t_int;
pmt::pmt_t tstamp_tuple = pmt::make_tuple(pmt::from_uint64(t_int), pmt::from_double(t_double));

The “tx_time” tag will be added at port 0 with nitems_written(0) + i offset.

add_item_tag(0, // Port number
         nitems_written(0) + i, // Offset
         pmt::mp("tx_time"), // Key
         tstamp_tuple // Value
    );

Note that if the user directly add the “tx_time” tag with the offset is zero, the USRP will have a bug with unknown reason that the first two burst frames are not sent with the predefined time by “tx_time”.

With the help of the above tags, the user can control the transmit time of a burst frame with a fix length, so it can be used to implement period transmission.

USRP Time Control

The last things is to set the time of USRP with gr-uhd, if the time has error, you may get a “L” error in the transmitter. “L” means a late packet in the TX chain. More details please refer to System Configuration for USRP X3x0 Series.

The following code snapshot is used for time setting for both the USRP source and sink in python API.

# Set time to 0 for each USRP
self.uhd_usrp_sink_0.set_time_next_pps(uhd.time_spec(0.0))
self.uhd_usrp_source_0.set_time_next_pps(uhd.time_spec(0.0))

time.sleep(1) # Wait for the PPS signal

# USRP sink sends at time 2.0
# USRP source receives at time 1.0
# This time can be set to any other times
self.uhd_usrp_sink_0.set_start_time(uhd.time_spec(2.0))
self.uhd_usrp_source_0.set_start_time(uhd.time_spec(1.0))