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Software Defined Radio
Thanasis Korakis
University of Thessaly
[email protected]
In this lecture
● What is SDR?
● A look inside a software radio
● What do you need to know to develop on
SDR platform?
What is SDR?
● A number of definitions can be found in literature.
● Software-defined radio (SDR) is a radio
communication system where components that have
been typically implemented in hardware (e.g. mixers,
filters, amplifiers, modulators/demodulators, detectors,
etc.) are instead implemented by means of software on
a personal computer or embedded system.
Source: http://en.wikipedia.org/wiki/Software-defined_radio
Use in industry
● Pro: Software
radios can be
upgraded to new
communication
protocols over
the air
Image: NVIDIA Tegra 4i chip with “soft modem” SDR
Use in industry
● Con: Life of mobile handsets is anyways
short; hardware is obsolete before software
● Con: SDR chips are more expensive than
single-purpose ASICs for mass market
So, in practice: SDR seen more often in base
stations than mobile handsets
Use in research
● Pro: SDR devices are cheaper than singlepurpose ASICs for one-time prototypes
● Pro: Fast testing of new designs
● Pro: Use the same hardware for multiple
designs
Use in research
● Pro: You can do everything in software
● Con: You have to do everything in software
Ideal SDR
Image source: http://electronicdesign.com/communications/elusive-software-defined-radio
Real SDR
Image source: http://electronicdesign.com/communications/elusive-software-defined-radio
Actual SDR
The Ettus N210 / NI 2920
is a popular USRP
(Universal Software Radio
Peripheral) device
RF frontend
Functions of RF hardware (RX)
● Analog to digital conversion
● Signal downconversion to baseband
● Real signal into complex (I and Q
components
● Filtering
● Decimation
● Send to host over Ethernet
Not functions of RF hardware
●
●
●
●
No signal detection
No synchronization
No equalization
No demodulation
What do you need to know to
develop on SDR platform?
● There is no particular programming
language
● Typically we separate programming to:
○ low level functions programming (C++)
○ compose blocks programming (Python - XML)
Practical Hardware Limitations
People often approach USRP with the attitude:
“I can implement anything on this platform!”
Then they go to USRP mailing lists and
ask “Why doesn’t any of this work the
way I expect it to?”
Practical Hardware Limitations
Development on SDR platform requires basic
understanding of
● Platform limitations
● Hardware impairments
(more in the lab)
What is the bandwidth of my USRP?
Answer: Minimum of ADC sampling rate, RF daughterboard
(analog), FPGA processing, and host bandwidth
●
●
Example: A system that uses a USRP N200/N210 with a 1 GigE interface, a fully host-based
application that requires 16-bit samples, and a 40 MHz daughterboard will provide a useable
bandwidth of ~20 MHz. The limit is set by the host interface – 1 GigE can stream up to 25 MS/s,
which translates to ~20 MHz of usable bandwidth.
Example: A system that uses a USRP X300/X310 with a 1 GigE interface for command/control,
full FPGA processing of the rx/tx steams, and a 120 MHz daughterboard may provide usable
bandwidth up to 120 MHz. The FPGA can process samples at 200 MS/s. The daughterboard is
the limiting factor.
[Source: http://www.ettus.com/kb/detail/usrp-bandwidth]
Why don’t I get the sampling rate I
asked for?
● Interpolation/decimation must be integer {4,..,512}
● Minimum sample rate is 0.1953 MHz
100 MHz / ? = 8 MHz
100 MHz / 13 = 7.692 MHz
Why can’t I do more processing on
my host?
(Answer: processing depends on your host’s CPU power)
The UHD driver prints informational output
● ‘O’ = overrun. PC not consuming samples as fast as
the USRP is producing them.
● ‘U’ = underrun. PC not producing samples as fast the
USRP is consuming them
Why can’t I do more processing on
my host?
Note the underruns!
Why is my signal off-center?
Why is my signal off-center?
● Frequency offset on both sides of transmission due to crystal
oscillator
● varies from one USRP to the next, temperature, frequency,
vibrations, orientation of crystal relative to Earth’s gravity…
Example:
● TX at 2.5GHz
● N210 LO is a 100 MHz VCXO rated at 2.5PPM
● Max error: 2.5 x 100 000 000 / 1 000 000 = 250 Hz
● Multiplier: 2.5 GHz /100 MHz LO = 25
● Frequency offset: 250 Hz x 25 = 6.25 kHz
(In practice: often more)
Why is my signal off-center?
Why does increasing amplitude
cause more bit errors?
Why does increasing amplitude
cause more bit errors?
● Overflow ADC and/or DAC
● Drive amplifier (RX and/or TX) into nonlinear region
Programming USRP
Well-known softwares:
● UHD (Universal Hardware Driver)
● GNU Radio
● Iris
UHD
Manageme
nt of USRP
using C++
GNU Radio
GNU Radio
Creating
functionalities
(blocks) with
C++
GNU Radio
Visualizing
blocks and
connections
GNU Radio
Management
of USRP using
Python
Iris
Management
of USRP using
XML
In the lab
● Why does my spectrum “droop” at the
edges?
● Why is there always a spike in middle of my
FFT?
● Why do I see “phantom signals” on my FFT?