Software Defined Radio
A Modular Approach
Jonathan Haws
Blair Leonard
Khemmer Porter
Joshua Templin
November 16, 2007
What is SDR?
• Software Defined Radio
– “A radio communication system which can potentially tune
to any frequency band and receive any modulation across
a large frequency spectrum by means of as few as
hardware possible and processing the signals through
software” (Wikipedia, “Software Defined Radio”)
– One device serves multiple purposes
– Significant utility in military and cellular markets
• Precursor to “Cognitive Radio”
– Radio will alter transmission and reception parameters
(modulation, frequency, and power) to avoid interference
and improve overall QoS
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History
• Many wireless devices are designed to
serve a single purpose
– Cell phone, wireless router, GPS receiver,
AM/FM radio, etc.
• Many current software radio groups exists
– GNURadio, HPSDR, SDR Forum, and others
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Problem
•
Design a software defined radio that has
the ability to:
1. Change modulation techniques “on-the-fly”
2. Avoid unwanted white noise
3. Provide a means to easily implement the
same software on other radios
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Solution
• Code and implement on a DSP various
algorithms that provide the desired
functionality of the radio
• Keep code modular (C++ classes)
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Block Diagram of a SDR
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Transmitter
• Coder Block
– Add bits to data stream to provide error protection
(data redundancy)
• Bit/Symbol
– Convert the data stream into transmission symbols for
transmission
• Transmit Filter
– Shape the symbols to the desired waveform
• Modulator
– Modulate the signal for transmission
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Channel
• Channel properties
– Model a wireless channel with an appropriate
transfer function
• White Noise
– WSS random process modeled as a process
with a constant power spectral density
• Interference
– Other interference modeled as normal
random variables
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Receiver
•
LNA/AGC
– Amplify signal to receiver circuit levels
•
Sampler
– Sample the received signal for digital processing
•
Demodulator
– Demodulate the received signal
•
Carrier Recovery
– Recover the phase of the carrier signal
•
Matched Filter
– Filter designed to match the transmitting filter
•
Timing Recovery
– Recover original clock of the transmitter
•
Equalizer
– Distortion compensation
•
Decoder
– Decode symbols into appropriate bit stream
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Software Algorithm
Development Process
Algorithms will be
1.
2.
3.
4.
5.
6.
Developed in MATLAB
Tested in SIMULINK
Converted to C/C++
Tested in C/C++ model
Compared to SIMULINK
Verified on DSP board
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Implementation Decisions
• C++
– Ease of transition between MatLab and C++
– C++ will provide modular classes and functions
• Inheritance and Virtual Functions
– DSP Boards can be programmed with C/C++
• Linux
– Documentation
– Cross-Platform
• Macintosh and IBM Compatible
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Modularity
Utilities Class
AM Modulation
Class
FM Modulation
Class
QA Modulation
Class
Transceiver
Transmitter
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Receiver
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Radio Software Design
…
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SIMULINK Model
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Modulation Techniques
• Software Defined Radios allow for multiple
modulation techniques
• NO additional hardware is needed
– Filtering can also be accomplished without
any additional hardware.
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Amplitude Modulation (AM)
• V(t) = Vocos(2ft + )
– For AM the value Vo is varied to change the
amplitude of the signal.
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Frequency Modulation (FM)
• V(t) = Vocos(2ft + )
– For FM f is varied.
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Quadrature AM
• V(t) = Vocos(2ft + ) +
Vosin(2ft + )
– For QAM, two signals that are out of phase by
90 degrees are used simultaneously.
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QAM Waveform
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Digital Modulation
• The techniques that were discussed are for analog
modulation.
– The process is similar for digital modulation
– The digital modulation types are: ASK, FSK,
and QASK
– Once these techniques are realized, they can
be adapted to achieve other modulation types
BPSK, QPSK, …
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Matched Filters
• Maximize the SNR to improve QoS
• Receiver filter is matched to the transmitter filter
by the relationship
P R f =C⋅ P T f
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Carrier and Timing Recovery
Data aided vs. Non-data aided
Data Aided
•Only applicable when data symbols
are detected reliably
•Requires phase and frequency
information of the carrier
•Based on decisions made by
receiver
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Non-Data Aided
•Independent of data symbols
•Can be used for both tracking and
acquisition
•Not as accurate as Data-aided
while tracking
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Spectrum Digital DSP Boards
SDI TMS320C6713 DSP Boards
• 225 MHz
• 512KB Flash
• 8MB SDRAM
• Microphone, Speaker,
Mono In/Out audio ports
• USB Interface
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Demonstration
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Preliminary Testing
MATLAB
simulation
using AM
modulation
– Receiver
rectifies signal
and detects
peaks
Raw Signal
Demodulated Signal
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Any Questions?
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Thank you!
We appreciate your time and
thank you for coming!
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