Investigating Latency in gnu software Radio with usrp embedded Series sdr platform

TUN/TAP Virtual Network Interface
File descriptor
FPGA in
USRP
Tunnel GNU Radio
application
Ping utility
Send/receive
packets
rea
d()
, write
()
operations
UHD
Send/receive
samples
Send/receive samples
GPMC bus
read()
, writ
e()
ope
rat
ion
s
Send/receive
packets
User-space
Kernel space
Fig. 3. 
Ping
using 
Tunnel 
utility with TUN/TAP components in GNU 
Radio/USRP SDR Platform 
B.
Timestamp Approach to Measure Latency 
In the second method, we create some Tx/Rx programs 
(with different radio modules such as modulation schemes, 
amplifiers and filters) and timestamp at some places in the data 
flow from the GNU Radio programs to the FPGA on USRP 
E100. According to the analytical method in section III, we put 
timestamp at the beginning of Tx/Rx programs), at the GNU 
Radio-UHD bridge (gr-uhd source and sink C++ files), and at 
the GPMC driver in the Linux kernel. To do so, we add a nano-
timestamp function at the GNU Radio programs, at the GNU 
Radio source and sink block as well as GMPC driver module in 
the kernel. Generally, conducting the timestamp functions also 
increases latency as well as jitter since it takes time to get the 
clock tick and to switch between different processes in the 
Linux OS. Nonetheless, timestamp is a good method to 
estimate the latency because of its flexibility and simplicity. 
C. Approximation Approach to Estimate Latency 
The third method is to measure the transmission time from 
FPGA in USRP E100 to UHD buffer in the embedded CPU. 
Thus, this time is comprised of 
∆
bus and 
∆
hardware. The basic 
idea of this approach is to transmit samples in the FPGA buffer, 
to the UHD buffer, then, after some amount of time, check 
whether this transmission is successful or not. By doing so, we 
can estimate this transmission time. To conduct method, an 
UHD received stream
is issued for getting samples from the 
FPGA buffer in USRP to the UHD buffer at the embedded 
computer. After some time 
rt, 
we issue an 
UHD sending 
stream
for sending samples at the UHD buffer. We compare 
the received samples from FPGA buffer and the transmitted 
samples in UHD buffer to check the success of the 
transmission. To get the approximate 
rt
, a trial and fail method 
is used with upper and lower values (e.g 
rt1
,
rt2
in Fig.4). 
FPGA Buffer
UHD buffer
time
Issues received
streamer
UHD buffer is full
Receive successfully
t
rt1
rt2
rt
Issues transmit streamer
Fig. 4. Approximation method idea to measure the bus and hardware latency 
In order for samples to be generated and put into FPGA 
buffers, a small trick is used: the 
UHD received stream
is not 
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immediately issued. Instead, it is needed to wait for a while in 
order for USRP having enough time to generate samples then 
put into the FPGA buffer. UHD supports management and 
control messages such as Overflow, Stream command error 
(Rx side) and Underflow, sequence error (Tx side); and the 
success of a transmission is included in 
async_metadata
message, which is a type of ACK message. 
V.
R
ESULTS AND 
D
ISCUSSION
The USRP E100 in our experiments consists of a 720 MHz 
OMAP3 (Arm Cortex A8 processor & TI C64x+ DSP) 
microprocessor with 512Mb RAM and running a simplifier 
Linux. E100 image ver.3 (E1xx-003 image) provided by Ettus 
Research is used. This Linux kernel is integrated with GNU 
Radio ver. 4.5.3 and UHD ver. 3.4 (UHD_003.004.000). The 
USRP E100 is equipped with a RFX2400 daughterboard, 
operated in the 2.4GHz ISM band. 
In the first method, the distance between two SDR platforms 
is about 2m. Frequency calibration is also conducted to avoid 
the frequency offset, which frequently occurs during 
experiment on real hardware device. Amplitude/Gain 
adjustment is also applied to improve the successful rate of the 
transmission. To measure the RTT, we send out 5000 ICMP 
packets and 873 packets among them are successfully 
transmitted. Note that the successful rate is significantly 
decreased if we do not calibrate the frequency offset and adjust 
the amplitude and gain. We analyze these 873 successful 
transmissions for our report. The parameter UHD buffer size is
also put into account by changing two parameters