Wireless Networks (PHY): Design or Diversity Admin and recap Design or diversity Y. Richard Yang 9/2/212 2 Admin Assignment 1 questions Assignment 1 oice hours Thursday 3-4 @ AKW 37A Channel characteristics change over location, time, and requency Received Signal Power (db) Recap: Wireless Channels path loss power Large-scale ading log (distance) small-scale ading time requency LOS pulse multipath pulses 3 4 Recap: Impact o Channel on Decisions design or lat ading how bad is lat ading? diversity to handle lat ading 5 T y(t ) = x(t)g(t)dt = T T T = hg 2 (t)dt + w(t)g(t) dt = he + w [hg(t) + w(t)]g(t)dt y(t ) ~ N(hE,σ 2 w = N E 2 ) he P error {y[t ] > sends = } = Q( N E / 2 ) = Q( 2h2 SNR) 6 Page 1
Recap: Impact o Channel Assume h is Gaussian random: Recap: Impacts o Channel lat ading channel Averaged out over h, static channel at high SNR. 7 8 Main Storyline Today design or lat ading how bad is lat ading? diversity to handle lat ading 9 Communication over a lat ading channel has poor perormance due to signiicant probability that channel is in a deep ade Reliability is increased by providing more resolvable signal paths that ade independently Name o the game is how to ind and eiciently exploit the paths 1 Where to Find Diversity? Time: when signal is bad at time t, it may not be bad at t+δt Space: when one position is in deep ade, another position may be not Frequency: when one requency is in deep ade (or has large intererence), another requency may be in good shape design or lat ading how bad is lat ading? diversity to handle lat ading d π 1 d2 d1 d 2 + π = 2π 2 + π c λ 11 12 Page 2
Time Diversity Example: GSM Time Structure Time diversity can be obtained by interleaving and coding over symbols across dierent coherent time periods requency 935-96 MHz 124 channels (2 khz) downlink 89-915 MHz 124 channels (2 khz) uplink time coherence time GSM TDMA rame 1 2 3 4 5 6 7 8 4.615 ms interleave 13 GSM time-slot (normal burst) guard space tail user data S Training S user data tail 3 bits 57 bits 1 26 bits 1 57 bits 3 S: indicates data or control guard space 546.5 µs 577 µs 14 Example: GSM Bit Assignments Simplest Code: Repetition Ater interleaving over L coherence time periods, Amount o time diversity limited by delay constraint and how ast channel varies In GSM, delay constraint is 4 ms (voice) To get better diversity, needs aster moving vehicles! P e 1 SNR L 15 16 Perormance P e 1 SNR L Beyond Repetition Coding Repetition coding gets ull diversity, but sends only one symbol every L symbol times We can use other codes, e.g. Reed-Solomon code 17 18 Page 3
design or lat ading how bad is lat ading? diversity to handle lat ading space Space Diversity: Antenna Receive Transmit Both 19 2 User Diversity: Cooperative Diversity Dierent users can orm a distributed antenna array to help each other in increasing diversity Interesting characteristics: users have to exchange inormation and this consumes bandwidth broadcast nature o the wireless medium can be exploited we will revisit the issue later in the course design or lat ading how bad is lat ading? diversity to handle lat ading space 1 c requency ' = + 2 d d 1 2 21 22 Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum) Discrete changes o carrier requency sequence o requency changes determined via pseudo random number sequence used in 82.11, GSM, etc Co-inventor: Hedy Lamarr patent# 2,292,387 issued on August 11, 1942 intended to make radio-guided torpedoes harder or enemies to detect or jam used a piano roll to change between 88 requencies http://en.wikipedia.org/wiki/hedy_lamarr 23 Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum) Two versions slow hopping: several user bits per requency ast hopping: several requencies per user bit 3 2 1 3 2 1 t b 1 t d t d 1 1 t t b : bit period t d : dwell time t t user data slow hopping (3 bits/hop) ast hopping (3 hops/bit) 24 Page 4
FHSS: Advantages Bluetooth Design Objective Frequency selective ading and intererence limited to short period Simple implementation what is a major issue in design? Uses only small portion o spectrum at any time explores requency sequentially used in simple devices such Bluetooth Design objective: a cable replacement technology 1 Mb/s range 1+ meters single chip radio + baseband (means digital part) low power low price point (target price $5 or lower) 25 26 Bluetooth Architecture Bluetooth Radio Link Bluetooth shares the same req. range as 82.11 Radio link is the most expensive part o a communication chip and hence chose simpler FHSS 2.42 GHz + k MHz, k=,, 78 1,6 hops per second A type o FSK modulation 1 Mb/s symbol rate transmit power: 1mW 27 28 Bluetooth Physical Layer Piconet Formation Nodes orm piconet: one master and upto 7 slaves Each radio can unction as a master or a slave The slaves ollow the pseudorandom jumping sequence o the master Master hopes at a universal requency hopping sequence (32 requencies) announce the master and sends Inquiry msg A piconet Joining slave: jump at a much lower speed ater receiving an Inquiry message, wait or a random time, then send a request to the master 29 The master sends a paging message to the slave to join it 3 Page 5
Direct Sequence Spread Spectrum (DSSS) design or lat ading how bad is lat ading? diversity to handle lat ading space requency» sequential» parallel 31 Basic idea: increase signaling unction alternating rate to expand requency spectrum (explores requency in parallel) c : carrier req. R b : req. o data 1dB = 1; 2dB =1 32 Direct Sequence Spread Spectrum (DSSS) Eects o Spreading Approach: One symbol is spread to multiple the number o is called the expansion actor sender dp/d dp/d examples 82.11: 11 Mcps; 1 Msps how may per symbol? IS-95 CDMA: 1.25 Mcps; 4,8 sps how may per symbol? un-spread signal WCDMA: 3.84 Mcps; suppose 7,5 sps how many per symbol? spread signal B s B b B b B s 33 B s : num. o bits in the chip * B b 34 DSSS Encoding/Decoding: An Operating View DSSS Encoding user data X spread spectrum signal modulator transmit signal t b user data d(t) chipping sequence radio carrier 1-1 X received signal demodulator transmitter correlator sampled products sums data X low pass decision t c 1 1 chipping sequence c(t) = resulting signal radio carrier receiver chipping sequence 1 1-1 -1 1-1 1-1 t b : bit period t c : chip period 35 36 Page 6
DSSS Encoding chip: DSSS Decoding chip: Data: [1-1 ] Data: [1-1] 1-1 -1 1-1 1 Trans 1-1 -1 1-1 1 decoded 1-1 -1 1-1 1 Chip seq: inner product: 6-6 decision: 1-1 37 38 DSSS Decoding with noise chip: DSSS Decoding (BPSK): Matched Filter Data: [1-1] Trans decoded Chip seq: -1-1 1-1 1-1 inner product: 4 decision: 1 1-1 -1 1-1 1 1-1 -1-1 1 1-2 -1 compute correlation or each bit time take N samples o a bit time sum = ; or i =; { sum += y[i] * c[i] * s[i] } i sum >= return 1; else return -1; t b bit time y(t)c(t)cos( 2π t)dt y: received signal c: chipping seq. s: modulating sinoid 39 4 Assume no DSSS design or lat ading how bad is lat ading? diversity to handle lat ading space requency» DSSS: why it works? Consider narrowband intererence Consider BPSK with carrier requency c A worst-case scenario data to be sent x(t) = 1 channel ades completely at c (or a jam signal at c) then no data can be recovered 41 42 Page 7
Why Does DSSS Work: A Decoding Perspective Assume BPSK modulation using carrier requency : A: amplitude o signal : carrier requency x(t): data [+1, -1] c(t): chipping [+1, -1] y(t) = A x(t)c(t) cos(2π t) Add Noise/Jamming/Channel Loss Assume noise at carrier requency : w(t) = a(t)cos(2π t) Received signal: y(t) + w(t) = Ax(t)c(t)cos(2π t) + a cos(2π t) 43 44 DSSS/BPSK Decoding T sym [Ax(t)c(t)cos(2π t) + acos(2π t)]c(t)cos( 2π t)dt T sym = [Ax(t)cos(2π t) + ac(t)cos(2π t)]cos( 2π t)dt T sym = Ax(t)cos(2π t)cos( 2π t)dt T sym + ac(t)cos(2π t)cos( 2π t)dt Why Does DSSS Work: A Spectrum Perspective sender dp/d i) receiver iii) dp/d ii) iv) dp/d dp/d user signal broadband intererence narrowband intererence 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 iltering v) dp/d 45 46 Inquiry Hopping Backup Slides 48 Page 8
The Bluetooth Link Establishment Protocol FS: Frequency Synchronization DAC: Device Access Code IAC: Inquiry Access Code Bluetooth Links 49 5 Bluetooth Packet Format Multiple-Slot Packet Header 51 52 Page 9