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The University of Iowa. Copyright© 2005 A. Kruger 1 Introduction to Wireless Sensor Networks Spread Spectrum and CDMA 24 February 2005
The University of Iowa. Copyright© 2005 A. Kruger 2 Organizational Monday 4:30-5:20Room 4511 SC Thursday12:30-1:20Room 3220 SC Please note that the room numbers are different for Mondays and Thursdays. Class Website Class Time Midterm Exam Time: March 10, 2005
The University of Iowa. Copyright© 2005 A. Kruger 3 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
The University of Iowa. Copyright© 2005 A. Kruger 4 FSK
The University of Iowa. Copyright© 2005 A. Kruger 5 OOK –On/Off Keying Relationship with TDMA?
The University of Iowa. Copyright© 2005 A. Kruger 6 Spread Spectrum Transmitted signal has much greater ( times) the bandwidth that base band signal External function determines bandwidth Frequency Power Direct Sequence Frequency Hopping
The University of Iowa. Copyright© 2005 A. Kruger 7 Frequency Hop Systems Bluetooth (PAN) –1,600 hops /second –79 randomly selected hop frequencies –2.402 to GHz –Dwell time 625 microsecond –During dwell time data are transmitted at 1 Mbps
The University of Iowa. Copyright© 2005 A. Kruger 8 Spread Spectrum Noise Performance
The University of Iowa. Copyright© 2005 A. Kruger 9 Spread Spectrum Pros & Cons Pros –Better noise/interference performance –Better multipath performance –Better channel utilization (why) –Security Cons –More complex implementation
The University of Iowa. Copyright© 2005 A. Kruger 10 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
The University of Iowa. Copyright© 2005 A. Kruger 11 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
The University of Iowa. Copyright© 2005 A. Kruger 12 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 Transceiver 2 uses code C2 –To send 1 use C2 –To send 0 use –C2 –=> for 11 send Signals add up in air: 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
The University of Iowa. Copyright© 2005 A. Kruger 13
The University of Iowa. Copyright© 2005 A. Kruger 14 FDMA vs TDMA vs CDMA
The University of Iowa. Copyright© 2005 A. Kruger 15 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.
The University of Iowa. Copyright© 2005 A. Kruger 16 Review Question Show that the following codes are orthogonal The output from a CDMA receiver is “ ” which contains messages from two transmitters. The spreading codes are: Decode the two messages. C1 = C2 = C3 = C4 = C1 = 1 1 C2 = 1 -1
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