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Introduction to Wireless Sensor Networks

Introduction to Wireless Sensor Networks. Spread Spectrum and CDMA 24 February 2005. Organizational. Class Website. www.engineering.uiowa.edu/~ece195/2005/. Class Time. Midterm Exam. Time: March 10, 2005. Modulation Schemes. Basic Question Why do we modulate at all?

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Introduction to Wireless Sensor Networks

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  1. Introduction to Wireless Sensor Networks Spread Spectrum and CDMA 24 February 2005

  2. Organizational Class Website www.engineering.uiowa.edu/~ece195/2005/ Class Time Midterm Exam Time: March 10, 2005

  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

  4. FSK

  5. OOK –On/Off Keying 0 1 0 1 1 Relationship with TDMA?

  6. Spread Spectrum • Transmitted signal has much greater (20-200 times) the bandwidth that base band signal • External function determines bandwidth Frequency Hopping Direct Sequence Power Frequency

  7. 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

  8. Spread Spectrum Noise Performance

  9. Spread Spectrum Pros & Cons • Pros • Better noise/interference performance • Better multipath performance • Better channel utilization (why) • Security • Cons • More complex implementation

  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

  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

  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 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

  13. FDMA vs TDMA vs CDMA

  14. 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.

  15. Review Question • Show that the following codes are orthogonal • The output from a CDMA receiver is “1 2 2 1 1” which contains messages from two transmitters. The spreading codes are: Decode the two messages. C1 = 1 1 1 1 C2 = 1 -1 1 -1 C3 = 1 1 -1 -1 C4 = 1 -1 -1 1 C1 = 1 1 C2 = 1 -1

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