Introduction to Wireless Sensor NetworksOrganizationalModulation SchemesFSKOOK –On/Off KeyingSpread SpectrumFrequency Hop SystemsSpread Spectrum Noise PerformanceSpread Spectrum Pros & ConsCDMACDMA (Direct Sequence)Slide 12Slide 13FDMA vs TDMA vs CDMAReview QuestionsReview Question1The University of Iowa. Copyright© 2005A. KrugerIntroduction to Wireless Sensor Networks Spread Spectrum and CDMA24 February 20052The University of Iowa. Copyright© 2005A. KrugerOrganizationalMonday 4:30-5:20 Room 4511 SCThursday 12:30-1:20 Room 3220 SCPlease note that the room numbers are different for Mondays and Thursdays.Class Websitewww.engineering.uiowa.edu/~ece195/2005/Class TimeMidterm ExamTime: March 10, 20053The University of Iowa. Copyright© 2005A. KrugerModulation 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, GMSK4The University of Iowa. Copyright© 2005A. KrugerFSK5The University of Iowa. Copyright© 2005A. KrugerOOK –On/Off Keying1 1 100Relationship with TDMA?6The University of Iowa. Copyright© 2005A. KrugerSpread Spectrum•Transmitted signal has much greater (20-200 times) the bandwidth that base band signal •External function determines bandwidthFrequencyPowerDirect SequenceFrequency Hopping7The University of Iowa. Copyright© 2005A. KrugerFrequency 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 Mbps8The University of Iowa. Copyright© 2005A. KrugerSpread Spectrum Noise Performance9The University of Iowa. Copyright© 2005A. KrugerSpread Spectrum Pros & Cons•Pros–Better noise/interference performance–Better multipath performance–Better channel utilization (why)–Security•Cons–More complex implementation10The University of Iowa. Copyright© 2005A. KrugerCDMA•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 rate11The University of Iowa. Copyright© 2005A. KrugerCDMA (Direct Sequence)•Orthogonal codes: C1 = (1,1) and C2 = (1,-1) C1.C1 = (1,1).(1,1) = +1C1.-C1 = (1,1).(-1,-1) = -1C1.C2 = (1,1).(1,-1) = 0 C1.-C2 = (1,1).(-1,1) = 012The University of Iowa. Copyright© 2005A. KrugerCDMA•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 113The University of Iowa. Copyright© 2005A. Kruger14The University of Iowa. Copyright© 2005A. KrugerFDMA vs TDMA vs CDMA15The University of Iowa. Copyright© 2005A. KrugerReview 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.16The University of Iowa. Copyright© 2005A. KrugerReview 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 1C1 = 1 1C2 = 1