Introduction to Wireless
Sensor Networks
Spread Spectrum and CDMA
24 February 2005
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Organizational
Class Website
www.engineering.uiowa.edu/~ece195/2005/
Class Time
Monday
4:30-5:20
Room 4511 SC
Thursday
12:30-1:20
Room 3220 SC
Please note that the room numbers are different for
Mondays and Thursdays.
Midterm Exam
Time: March 10, 2005
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Modulation Schemes
• Basic Question
– Why do we modulate at all?
•
•
•
•
•
AM – amplitude modulation
FM frequency modulation
OOK – On/Of Keying
PAM, M-PAM…
FSK, PSK, QPSK, OQPSK, M-PSK,
MSK, GMSK
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FSK
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OOK –On/Off Keying
1
0
1
0
1
Relationship with TDMA?
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Spread Spectrum
• Transmitted signal has much greater (20-200
times) the bandwidth that base band signal
• External function determines bandwidth
Frequency Hopping
Power
Direct Sequence
Frequency
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Frequency Hop Systems
• Bluetooth (PAN)
– 1,600 hops /second
– 79 randomly selected hop frequencies
– 2.402 to 2.480 GHz
– Dwell time 625 microsecond
– During dwell time data are transmitted at 1
Mbps
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Spread Spectrum Noise
Performance
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Spread Spectrum Pros & Cons
• Pros
– Better noise/interference performance
– Better multipath performance
– Better channel utilization (why)
– Security
• Cons
– More complex implementation
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CDMA
• Steps
– Apply orthogonal spreading codes to
spread baseband signals
– Transmit (wide spectrum)
– Receive
– Correlate to despread
• Unwanted codes/signals are discarded
– Narrowband filter
• Spreading codes are carefully designed
• Spreading code rate is chip rate
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CDMA (Direct Sequence)
• Orthogonal codes:
C1 = (1,1) and C2 = (1,-1)
C1.C1
= (1,1).(1,1)
= +1
C1.-C1 = (1,1).(-1,-1) = -1
C1.C2
= (1,1).(1,-1)
=
0
C1.-C2 = (1,1).(-1,1)
=
0
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CDMA
• C1 = (1,1) and C2 = (1,-1)
• Transceiver 1 uses code C1
– To send 1 use C1
– To send 0 use –C1
– => for 10 send 1 1 0 0
•
Transceiver 2 uses code C2
– To send 1 use C2
– To send 0 use –C2
– => for 11 send 1 -1 1 - 1
• Signals add up in air: 2 0 1 -1
• Receiver 1 decodes by taking dot product with C1
(2, 0).(1, 1) (1, -1).(1, 1) = 10
• Receiver 2 decodes by taking dot product with C2
(2,0).(1,-1) (1, -1).(1,-1) = 1 1
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FDMA vs TDMA vs CDMA
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Review Questions
• Explain in a paragraph what frequency-hopping
spread spectrum is.
• What is an orthogonal code? Give an example of an
orthogonal code.
• Explain why spread spectrum techniques can
provide better channel utilization than conventional
(e.g., AM, FM).
• List an briefly explain four advantages of spread
spectrum.
• What is a disadvantage of spread spectrum
modulation?
• True or false – CDMA can be seen as an example of
what is known a direct sequence spread spectrum?
• What is FSK and OOK? Use simple figures to
explain.
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Review Question
• Show that the following codes are
orthogonal
C1
C2
C3
C4
=
=
=
=
1
1
1
1
1
-1
1
-1
1
1
-1
-1
1
-1
-1
1
• The output from a CDMA receiver is “1 2 2 1
1” which contains messages from two
transmitters. The spreading codes are:
C1 = 1
C2 = 1
1
-1
Decode the two messages.
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