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Complex (IQ) Signals
Qasim Chaudhari
Cyberspectrum Melbourne
Why DSP?
 Software in Software Defined Radio (SDR)
 Writing your own software
+ Knowledge of communication systems and DSP algorithms is required
 Can be understood with basic mathematical skills
Complex Numbers
 Rick Lyons wrote a tutorial with the title Quadrature signals 
Complex but not complicated
 The question is: how to deal with an ordered pair of real numbers on a
2-d plane
 We will avoid the use of j and e
Complex Numbers 
 Start with an ordered pair of real numbers
 A complex number can be considered as a
vector with initial point at (0,0)
 Problem with vectors: all arithmetic
operations cannot be applied
+ Addition of two vectors is another vector in (x,y)-
plane  thats good
Complex Numbers 
+ Dot product of two vectors is a scalar, not a vector  not good
+ Cross product of two vectors in a plane is a vector that is outside of that plane
 not good as well
 Product of complex numbers is a complex number -- an extremely
useful property
Complex Numbers 
 x-axis = I (inphase), y-axis = Q (quadrature)
 From a signal processing perspective, I and Q are just two real signals
that appear on two separate wires
Magnitude and Phase
 The magnitude of a complex number is
 Phase is given by adjusting to the correct quadrant
Addition
 implies
Concept of Frequency
 Wireless signals between two devices travel through the use of radio
waves
 A radio wave is an electromagnetic wave propagated by an antenna
 What is a frequency?
Concept of Frequency 
 Consider a complex number V in an IQ-plane with an angle  and
length A
 Now imagine V rotating with an angle continuously increasing with
time
 Then, V can be treated as a signal with time as independent variable
and we call it a complex sinusoid
Concept of Frequency 
 Just like velocity is the rate of change of displacement, frequency is the
rate of change in phase of a complex sinusoid
+ This rate of change of phase results in V
rotating in the time IQ-plane at an angular
velocity 2
Concept of Frequency
 As time passes, V is shown as coming out of the page
+ When its projection from a 3-dimensional plane to a 2-dimensional plane
formed by time and I-axis is drawn, we get  cos 2告 + 
+ Similarly, when the projection is drawn on a 2-dimensional plane formed by
time and Q-axis, it generates  sin 2告 + 
+ Randomly choosing cos() as our reference sinusoid,
 I component is called inphase because it is in phase with cos()
 Q component is called qaudrature because sin is at 90o with cos()
Concept of Frequency 
 In conclusion, a complex sinusoid with frequency F is composed of two
real sinusoids
  =  cos 2告 + 
  =  sin 2告 + 
where F is the continuous frequency with units of cycles/second or
Hertz (Hz)
Phase
Amplitude
What is a Negative Frequency?
 Negative frequencies can cause some confusion
+ It is hard to visualize a negative frequency viewed as inverse period of a
sinusoid
 Define it through the rate of rotation of a complex sinusoid V
+ A negative frequency simply implies rotation of V in a clockwise direction
+ Negative frequencies are real, just like negative numbers are real
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Complex (iq) signals

  • 1. Complex (IQ) Signals Qasim Chaudhari Cyberspectrum Melbourne
  • 2. Why DSP? Software in Software Defined Radio (SDR) Writing your own software + Knowledge of communication systems and DSP algorithms is required Can be understood with basic mathematical skills
  • 3. Complex Numbers Rick Lyons wrote a tutorial with the title Quadrature signals Complex but not complicated The question is: how to deal with an ordered pair of real numbers on a 2-d plane We will avoid the use of j and e
  • 4. Complex Numbers Start with an ordered pair of real numbers A complex number can be considered as a vector with initial point at (0,0) Problem with vectors: all arithmetic operations cannot be applied + Addition of two vectors is another vector in (x,y)- plane thats good
  • 5. Complex Numbers + Dot product of two vectors is a scalar, not a vector not good + Cross product of two vectors in a plane is a vector that is outside of that plane not good as well Product of complex numbers is a complex number -- an extremely useful property
  • 6. Complex Numbers x-axis = I (inphase), y-axis = Q (quadrature) From a signal processing perspective, I and Q are just two real signals that appear on two separate wires
  • 7. Magnitude and Phase The magnitude of a complex number is Phase is given by adjusting to the correct quadrant
  • 9. Concept of Frequency Wireless signals between two devices travel through the use of radio waves A radio wave is an electromagnetic wave propagated by an antenna What is a frequency?
  • 10. Concept of Frequency Consider a complex number V in an IQ-plane with an angle and length A Now imagine V rotating with an angle continuously increasing with time Then, V can be treated as a signal with time as independent variable and we call it a complex sinusoid
  • 11. Concept of Frequency Just like velocity is the rate of change of displacement, frequency is the rate of change in phase of a complex sinusoid + This rate of change of phase results in V rotating in the time IQ-plane at an angular velocity 2
  • 12. Concept of Frequency As time passes, V is shown as coming out of the page + When its projection from a 3-dimensional plane to a 2-dimensional plane formed by time and I-axis is drawn, we get cos 2告 + + Similarly, when the projection is drawn on a 2-dimensional plane formed by time and Q-axis, it generates sin 2告 + + Randomly choosing cos() as our reference sinusoid, I component is called inphase because it is in phase with cos() Q component is called qaudrature because sin is at 90o with cos()
  • 13. Concept of Frequency In conclusion, a complex sinusoid with frequency F is composed of two real sinusoids = cos 2告 + = sin 2告 + where F is the continuous frequency with units of cycles/second or Hertz (Hz) Phase Amplitude
  • 14. What is a Negative Frequency? Negative frequencies can cause some confusion + It is hard to visualize a negative frequency viewed as inverse period of a sinusoid Define it through the rate of rotation of a complex sinusoid V + A negative frequency simply implies rotation of V in a clockwise direction + Negative frequencies are real, just like negative numbers are real