lecture 10 digital i/q transceiverweb.mit.edu/6.02/www/f2006/handouts/lec10.pdf · 10/17/2006 l...
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10/17/2006
Digital I/Q Transceiver• Constellation Diagram• SNR• Eye DiagramLab 5• Transmitter• Analog Receiver • Digital Receiver
Lecture 10Digital I/Q Transceiver
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L10/17/2006 2Lecture 10 Fall 2006
Digital Modulation
• I/Q signals take on discrete values at discrete time instants corresponding to digital data– Receiver samples I/Q channels
• Uses decision boundaries to evaluate value of data at each time instant
• I/Q signals may be binary or multi-bit– Multi-bit shown above
Receiver Output
2cos(2πf1t)2sin(2πf1t)
Lowpassir(t)
Lowpassqr(t)
it(t)
qt(t)
2cos(2πf1t)2sin(2πf1t)
t
t
t
Baseband Input
tDecisionBoundaries Sample
Times
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L10/17/2006 3Lecture 10 Fall 2006
Constellation Diagram-16QAM
• We can view I/Q values at sample instants on a two-dimensional coordinate system
• Decision boundaries mark up regions corresponding to different data values
• Gray coding used to minimize number of bit errors that occur if wrong decisions made due to noise
DecisionBoundaries I
Q
DecisionBoundaries
00 01 11 10
00
01
11
10
Receiver Output
t
tSampleTimes
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L10/17/2006 4Lecture 10 Fall 2006
Impact of Noise on Constellation Diagram
Low PowerHigh Power
• Sampled data values no longer land in exact same location across all sample instants while decision boundaries remain fixed
• Significant noise causes bit errors to be made• Increasing signal power increases distance between decision boundaries
i.e., increased SNR
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L10/17/2006 5Lecture 10 Fall 2006
Transition Behavior Between Constellation Points
• Constellation diagrams provide us with a snapshot of I/Q signals at sample instants
• Transition behavior between sample points depends on modulation scheme and transmit filter
DecisionBoundaries I
Q
DecisionBoundaries
00 01 11 10
00
01
11
10
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L10/17/2006 6Lecture 10 Fall 2006
Need for Transmit Filter
• Steps in waveform x(t) have high frequency components. (Recall Fourier Series applet)
• We want spectral efficiency (i.e. narrow bandwidth signals) to conserve spectrum
t
Td
data(t)
t
x(t)
O-Order
Track & Hold
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L10/17/2006 7Lecture 10 Fall 2006
Transmit Filter
• Special low pass filtering (e.g. raised cosine filter) removes high-frequency content but preserves signal levels as sampling points.
• Trade-off bandwidth and signal integrity
t
Td
data(t)
t
x(t)|P(f)|2
f1/(2Td)0
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L10/17/2006 8Lecture 10 Fall 2006
Lab 5 Transmitter Block
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L10/17/2006 9Lecture 10 Fall 2006
Σ−Δ Modulator Pushes Quantization Noise to High Frequency
• Allows music to be encoded digitally• LPF at receiver is used to remove quantization noise while
preserving the signal. Tradeoff is noise vs. signal integrity
Time Domain Frequency Domain
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L10/17/2006 10Lecture 10 Fall 2006
Lab 5 Analog Receiver Block Diagram
Neglecting higher frequency termsrxa = it (t)cos Δω ct + φOFF (t)[ ]
Difference between transmitter and receiver modulation frequency
Difference in phase between transmitter and receiver. It is a function of time.
Where Δω c :φOFF (t) :
rxb = qt (t)cos Δω ct +φOFF (t)[ ]If we can measure Δωc and φOFF(t) we can remove both frequency and phase offset.
rx = rxa + jrxb
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L10/17/2006 11Lecture 10 Fall 2006
Complex ModulationWrite
in exponential form
rx = rxa + jrxb where rxa = RE rx( ); rxb = IM rx( )
rx = [it (t)+ jqt (t)]ej Δωct+θOFF (t )( )
rx by e− j Δωct+φOFF (t )( )
I = ir (t)Q = qr (t)
× ××××
Σ
Σ
I
Q
cos ω ct( )rxa
sin ω ct( )rxb
cos( )
sin( )
cos( )
+−
+ +
Where ( ) = Δω ct + φOFF (t)
Multiply
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L10/17/2006 12Lecture 10 Fall 2006
Lab 5 Digital Receiver Block Diagram
USRP
Receiverx_a (I)
rx_b (Q)
250 kSample/s
Complex
Mixer
250 kSample/s
cos(ωct)
sin(ωct)
rx_a (I)
rx_b (Q)
in
ωc = 2π(vco_freq + dco_freq)
freq
offset
phase
offset I
Q
listen to
song I
listen to
song Q
analog extract filter
analog extract filter
matchedfilter
matchedfilter
Digital
receiver
operations
i_sliced_data
q_sliced_data
i_raw
q_raw
Output i_raw & q_raw
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L10/17/2006 13Lecture 10 Fall 2006
Eye Diagram for 1 Gb/s Data Rate [2-level -it(t)]
• Wrap signal back onto itself every 2*Td seconds– Same as an oscilloscope would do
• Allows immediate assessment of the quality of the signal at the receiver (look at eye opening)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 10−9
−0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Time (seconds)
out
Eye Diagram
0 0.5 1 1.5 2 2.5
x 10−8
−0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
out
TIME
Snapshot in Time
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L10/17/2006 14Lecture 10 Fall 2006
Relationship of Eye to Sampling Time and Slice Level
• Horizontal portion of eye indicates sensitivity to timing jitter
• Vertical portion of eye indicates sensitivity to additional noise
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 109
0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Time (seconds)
out
Eye Diagram
SliceLevel
SamplingInstant
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L10/17/2006 15Lecture 10 Fall 2006
Realistic Eye Diagram
• Eye more closed due to amplitude noise and timing variation
• Line denotes best time to sample
0 2 4 6 8
x 10−11
−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
out
Eye Diagram
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L10/17/2006 16Lecture 10 Fall 2006
Multi-Level Signaling• Increase spectral efficiency by sending more than one bit
during a symbol interval• Example: 4-Level PAM on each channel I and Q
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
x 10−10
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
Time (seconds)
out
Eye Diagram