1. Wireless Networks (PHY): Design for Diversity Y. Richard Yang 9/20/2012

2. 2 Outline r Admin and recap r Design for diversity

3. 3 Admin r Assignment 1 questions r Assignment 1 office hours m Thursday 3-4 @ AKW 307A

4. 4 r Channel characteristics change over location, time, and frequency small-scale fading Large-scale fading time power Recap: Wireless Channels path loss log (distance) Received Signal Power (dB) frequency signal at receiver LOS pulse multipath pulses

5. 5 Outline r Recap r Wireless channels r Physical layer design m design for flat fading how bad is flat fading? diversity to handle flat fading

6. 6 Recap: Impact of Channel on Decisions

7. 7 Recap: Impact of Channel Averaged out over h, at high SNR. Assume h is Gaussian random:

8. 8 Recap: Impacts of Channel static channel flat fading channel

9. 9 Outline r Recap r Wireless channels r Physical layer design m design for flat fading how bad is flat fading? diversity to handle flat fading

10. 10 Main Storyline Today r Communication over a flat fading channel has poor performance due to significant probability that channel is in a deep fade r Reliability is increased by providing more resolvable signal paths that fade independently r Name of the game is how to find and efficiently exploit the paths

11. 11 Where to Find Diversity? r Time: when signal is bad at time t, it may not be bad at t+  t r Space: when one position is in deep fade, another position may be not r Frequency: when one frequency is in deep fade (or has large interference), another frequency may be in good shape

12. 12 Outline r Recap r Wireless channels r Physical layer design m design for flat fading how bad is flat fading? diversity to handle flat fading –time

13. 13 Time Diversity r Time diversity can be obtained by interleaving and coding over symbols across different coherent time periods interleave coherence time

14. 14 12 3 4 5 6 78 935-960 MHz 124 channels (200 kHz) downlink 890-915 MHz 124 channels (200 kHz) uplink frequency time GSM TDMA frame GSM time-slot (normal burst) 4.615 ms 546.5 µs 577 µs tailuser dataTrainingS guard space Suser datatail guard space 3 bits57 bits26 bits 57 bits1 13 Example: GSM Time Structure S: indicates data or control

15. 15 Example: GSM Bit Assignments r Amount of time diversity limited by delay constraint and how fast channel varies r In GSM, delay constraint is 40 ms (voice) r To get better diversity, needs faster moving vehicles !

16. 16 Simplest Code: Repetition After interleaving over L coherence time periods,

17. 17 Performance

18. 18 Beyond Repetition Coding r Repetition coding gets full diversity, but sends only one symbol every L symbol times r We can use other codes, e.g. Reed-Solomon code

19. 19 Outline r Recap r Wireless channels r Physical layer design m design for flat fading how bad is flat fading? diversity to handle flat fading –time –space

20. 20 Space Diversity: Antenna Receive TransmitBoth

21. 21 User Diversity: Cooperative Diversity r Different users can form a distributed antenna array to help each other in increasing diversity r Interesting characteristics: m users have to exchange information and this consumes bandwidth m broadcast nature of the wireless medium can be exploited m we will revisit the issue later in the course

22. 22 Outline r Recap r Wireless channels r Physical layer design m design for flat fading how bad is flat fading? diversity to handle flat fading –time –space –frequency

23. 23 r Discrete changes of carrier frequency m sequence of frequency changes determined via pseudo random number sequence m used in 802.11, GSM, etc r Co-inventor: Hedy Lamarr m patent# 2,292,387 issued on August 11, 1942 m intended to make radio-guided torpedoes harder for enemies to detect or jam m used a piano roll to change between 88 frequencies Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum) http://en.wikipedia.org/wiki/Hedy_Lamarr

24. 24 r Two versions m slow hopping: several user bits per frequency m fast hopping: several frequencies per user bit Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum) user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 01 tbtb 011t f f1f1 f2f2 f3f3 t tdtd f f1f1 f2f2 f3f3 t tdtd t b : bit periodt d : dwell time

25. 25 r Frequency selective fading and interference limited to short period r Simple implementation r what is a major issue in design? r Uses only small portion of spectrum at any time m explores frequency sequentially m used in simple devices such Bluetooth FHSS: Advantages

26. 26 Bluetooth Design Objective r Design objective: a cable replacement technology m 1 Mb/s m range 10+ meters m single chip radio + baseband (means digital part) low power low price point (target price $5 or lower)

27. 27 Bluetooth Architecture

28. 28 Bluetooth Radio Link r Bluetooth shares the same freq. range as 802.11 r Radio link is the most expensive part of a communication chip and hence chose simpler FHSS 2.402 GHz + k MHz, k=0, …, 78 1,600 hops per second m A type of FSK modulation 1 Mb/s symbol rate m transmit power: 1mW

29. 29 Bluetooth Physical Layer r Nodes form piconet: one master and upto 7 slaves m Each radio can function as a master or a slave r The slaves follow the pseudorandom jumping sequence of the master A piconet

30. 30 Piconet Formation r Master hopes at a universal frequency hopping sequence (32 frequencies) m announce the master and sends Inquiry msg r Joining slave: m jump at a much lower speed m after receiving an Inquiry message, wait for a random time, then send a request to the master r The master sends a paging message to the slave to join it

31. 31 Outline r Recap r Wireless channels r Physical layer design m design for flat fading how bad is flat fading? diversity to handle flat fading –time –space –frequency »sequential »parallel

32. 32 Direct Sequence Spread Spectrum (DSSS) r Basic idea: increase signaling function alternating rate to expand frequency spectrum (explores frequency in parallel) f c : carrier freq. R b : freq. of data 10dB = 10; 20dB =100

33. 33 Direct Sequence Spread Spectrum (DSSS) r Approach: One symbol is spread to multiple chips m the number of chips is called the expansion factor m examples 802.11: 11 Mcps; 1 Msps –how may chips per symbol? IS-95 CDMA: 1.25 Mcps; 4,800 sps –how may chips per symbol? WCDMA: 3.84 Mcps; suppose 7,500 sps –how many chips per symbol?

34. 34 dP/df f f sender Effects of Spreading un-spread signal spread signal BbBb BbBb BsBs BsBs BsBs : num. of bits in the chip * B b

35. 35 DSSS Encoding/Decoding: An Operating View X user data chipping sequence modulator radio carrier spread spectrum signal transmit signal transmitter demodulator received signal radio carrier X chipping sequence receiver low pass products decision data sampled sums correlator

36. 36 DSSS Encoding user data d(t) chipping sequence c(t) resulting signal 1 11 1 1 1 111 X = tbtb tctc t b : bit period t c : chip period 11 1 11 1 1

37. DSSS Encoding Data: [1 -1 ] 37 chip: 11 1 11 1 1 1 1

38. DSSS Decoding Data: [1 -1] 38 1 1 1 chip: 11 1 11 1 11 1 Trans chips 11 1 Chip seq: inner product: 6 decision:1 1 1 1 11 1 decoded chips -6

39. DSSS Decoding with noise Data: [1 -1] 39 1 1 1 chip: 11 1 11 1 11 1 Trans chips 11 1 Chip seq: inner product: 4 decision:1 1 1 1 11 decoded chips -2

40. DSSS Decoding (BPSK): Matched Filter 40 s: modulating sinoid compute correlation for each bit time c: chipping seq. y: received signal take N samples of a bit time sum = 0; for i =0; { sum += y[i] * c[i] * s[i] } if sum >= 0 return 1; else return -1; bit time

41. 41 Outline r Recap r Wireless channels r Physical layer design m design for flat fading how bad is flat fading? diversity to handle flat fading –time –space –frequency »DSSS: why it works?

42. Assume no DSSS r Consider narrowband interference r Consider BPSK with carrier frequency fc r A “worst-case” scenario m data to be sent x(t) = 1 m channel fades completely at fc (or a jam signal at fc) m then no data can be recovered 42

43. 43 Why Does DSSS Work: A Decoding Perspective r Assume BPSK modulation using carrier frequency f : m A: amplitude of signal m f : carrier frequency m x(t): data [+1, -1] m c(t): chipping [+1, -1] y(t) = A x(t)c(t) cos(2  ft)

44. 44 Add Noise/Jamming/Channel Loss r Assume noise at carrier frequency f: r Received signal: y(t) + w(t)

45. 45 DSSS/BPSK Decoding

46. 46 dP/df f i) dP/df f ii) sender user signal broadband interference narrowband interference dP/df f iii) dP/df f iv) receiver f v) dP/df Why Does DSSS Work: A Spectrum Perspective i) → ii): multiply data x(t) by chipping sequence c(t) spreads the spectrum ii) → iii): received signal: x(t) c(t) + w(t), where w(t) is noise iii) → iv): (x(t) c(t) + w(t)) c(t) = x(t) + w(t) c(t) iv) → v) : low pass filtering

47. Backup Slides

48. 48 Inquiry Hopping

49. 49 The Bluetooth Link Establishment Protocol FS: Frequency Synchronization DAC: Device Access Code IAC: Inquiry Access Code

50. 50 Bluetooth Links

51. 51 Bluetooth Packet Format Header

52. 52 Multiple-Slot Packet