Wireless Networks (PHY): Design for Diversity Y. Richard Yang 9/20/2012
Outline Admin and recap Design for diversity 2
Admin Assignment 1 questions Assignment 1 office hours Thursday 3-4 @ AKW 307A 3
Recap: Wireless Channels Channel characteristics change over location, time, and frequency Received Signal Power (db) path loss power Large-scale fading log (distance) small-scale fading time frequency LOS pulse multipath pulses 4
Outline Recap Wireless channels Physical layer design design for flat fading how bad is flat fading? diversity to handle flat fading 5
Recap: Impact of Channel on Decisions T 0 T 0 y(t ) = x(t)g(t)dt = T 0 T 0 = 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 0E 2 ) he P error0 {y[t ] > 0 sends = 0} = Q( N 0 E / 2 ) = Q( 2h2 SNR) 6
Recap: Impact of Channel Assume h is Gaussian random: Averaged out over h, at high SNR. 7
Recap: Impacts of Channel flat fading channel static channel 8
Outline Recap Wireless channels Physical layer design design for flat fading how bad is flat fading? diversity to handle flat fading 9
Main Storyline Today Communication over a flat fading channel has poor performance due to significant probability that channel is in a deep fade Reliability is increased by providing more resolvable signal paths that fade independently Name of the game is how to find and efficiently exploit the paths 10
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 fade, another position may be not Frequency: when one frequency is in deep fade (or has large interference), another frequency may be in good shape d π 1 d2 d1 d 2 f + π = 2π 2 + π c λ 11
Outline Recap Wireless channels Physical layer design design for flat fading how bad is flat fading? diversity to handle flat fading time 12
Time Diversity Time diversity can be obtained by interleaving and coding over symbols across different coherent time periods coherence time interleave 13
Example: GSM Time Structure 935-960 MHz 124 channels (200 khz) downlink 890-915 MHz 124 channels (200 khz) uplink time GSM TDMA frame 1 2 3 4 5 6 7 8 4.615 ms 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 Amount of time diversity limited by delay constraint and how fast channel varies In GSM, delay constraint is 40 ms (voice) To get better diversity, needs faster moving vehicles! 15
Simplest Code: Repetition After interleaving over L coherence time periods, 1 P e SNR L 16
1 Performance P e SNR L 17
Beyond Repetition Coding Repetition coding gets full diversity, but sends only one symbol every L symbol times We can use other codes, e.g. Reed-Solomon code 18
Outline Recap Wireless channels Physical layer design design for flat fading how bad is flat fading? diversity to handle flat fading time space 19
Space Diversity: Antenna Receive Transmit Both 20
User Diversity: Cooperative Diversity Different users can form a distributed antenna array to help each other in increasing diversity Interesting characteristics: users have to exchange information and this consumes bandwidth broadcast nature of the wireless medium can be exploited we will revisit the issue later in the course 21
Outline Recap Wireless channels Physical layer design design for flat fading how bad is flat fading? diversity to handle flat fading time space frequency f ' = f + 1 2 d 1 c d 2 22
Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum) Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence used in 802.11, GSM, etc Co-inventor: Hedy Lamarr patent# 2,292,387 issued on August 11, 1942 intended to make radio-guided torpedoes harder for enemies to detect or jam used a piano roll to change between 88 frequencies http://en.wikipedia.org/wiki/hedy_lamarr 23
Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum) Two versions slow hopping: several user bits per frequency fast hopping: several frequencies per user bit t b user data f f 3 f 2 f 1 f f 3 f 2 f 1 0 1 t d t d 0 1 1 t t slow hopping (3 bits/hop) fast hopping (3 hops/bit) t b : bit period t d : dwell time t 24
FHSS: Advantages Frequency selective fading and interference limited to short period Simple implementation what is a major issue in design? Uses only small portion of spectrum at any time explores frequency sequentially used in simple devices such Bluetooth 25
Bluetooth Design Objective Design objective: a cable replacement technology 1 Mb/s range 10+ meters single chip radio + baseband (means digital part) low power low price point (target price $5 or lower) 26
Bluetooth Architecture 27
Bluetooth Radio Link Bluetooth shares the same freq. range as 802.11 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 A type of FSK modulation 1 Mb/s symbol rate transmit power: 1mW 28
Bluetooth Physical Layer Nodes form piconet: one master and upto 7 slaves Each radio can function as a master or a slave The slaves follow the pseudorandom jumping sequence of the master A piconet 29
Piconet Formation Master hopes at a universal frequency hopping sequence (32 frequencies) announce the master and sends Inquiry msg Joining slave: jump at a much lower speed after receiving an Inquiry message, wait for a random time, then send a request to the master The master sends a paging message to the slave to join it 30
Outline Recap Wireless channels Physical layer design design for flat fading how bad is flat fading? diversity to handle flat fading time space frequency» sequential» parallel 31
Direct Sequence Spread Spectrum (DSSS) 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 32
Direct Sequence Spread Spectrum (DSSS) Approach: One symbol is spread to multiple chips the number of chips is called the expansion factor 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? 33
Effects of Spreading sender dp/df dp/df f f un-spread signal spread signal B s B b B b B s B s : num. of bits in the chip * B b 34
DSSS Encoding/Decoding: An Operating View user data X spread spectrum signal modulator transmit signal chipping sequence radio carrier transmitter correlator received signal demodulator X products low pass sampled sums decision data radio carrier chipping sequence receiver 35
DSSS Encoding t b user data d(t) 1-1 X t c chipping sequence c(t) -1 1 1-1 1-1 1-1 1 1-1 1-1 1 = resulting signal -1 1 1-1 1-1 1 1-1 -1 1-1 1-1 t b : bit period t c : chip period 36
DSSS Encoding chip: -1 1 1-1 1-1 Data: [1-1 ] -1 1 1-1 1-1 1-1 -1 1-1 1 37
DSSS Decoding chip: -1 1 1-1 1-1 Data: [1-1] Trans chips decoded chips Chip seq: -1 1 1-1 1-1 -1 1 1-1 1-1 -1 1 1-1 1-1 1-1 -1 1-1 1 1-1 -1 1-1 1-1 1 1-1 1-1 inner product: 6 decision: 1-6 -1 38
DSSS Decoding with noise chip: -1 1 1-1 1-1 Data: [1-1] Trans chips decoded chips Chip seq: -1 1 1-1 1-1 -1-1 1-1 1-1 -1 1 1-1 1-1 1-1 -1 1-1 1 1-1 -1-1 1 1-1 1 1-1 1-1 inner product: 4-2 decision: 1-1 39
DSSS Decoding (BPSK): Matched Filter compute correlation for each bit time 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 y: received signal c: chipping seq. s: modulating sinoid t b 0 y(t)c(t)cos( 2π ft)dt 40
Outline Recap Wireless channels Physical layer design design for flat fading how bad is flat fading? diversity to handle flat fading time space frequency» DSSS: why it works? 41
Assume no DSSS Consider narrowband interference Consider BPSK with carrier frequency fc A worst-case scenario data to be sent x(t) = 1 channel fades completely at fc (or a jam signal at fc) then no data can be recovered 42
Why Does DSSS Work: A Decoding Perspective Assume BPSK modulation using carrier frequency f : A: amplitude of signal f : carrier frequency x(t): data [+1, -1] c(t): chipping [+1, -1] y(t) = A x(t)c(t) cos(2π ft) 43
Add Noise/Jamming/Channel Loss Assume noise at carrier frequency f: w(t) = a(t)cos(2π ft) Received signal: y(t) + w(t) = Ax(t)c(t)cos(2π ft)+ acos(2π ft) 44
DSSS/BPSK Decoding T sym 0 [Ax(t)c(t)cos(2π ft) + a cos(2π ft)]c(t)cos( 2π ft)dt T sym 0 = [Ax(t)cos(2π ft) + ac(t)cos(2π ft)]cos( 2π ft)dt T sym 0 = Ax(t)cos(2π ft)cos( 2π ft)dt T sym 0 + ac(t)cos(2π ft)cos( 2π ft)dt 45
Why Does DSSS Work: A Spectrum Perspective sender i) dp/df ii) dp/df user signal broadband interference narrowband interference receiver dp/df f dp/df f dp/df iii) iv) f f f 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 v) 46
Backup Slides
Inquiry Hopping 48
The Bluetooth Link Establishment Protocol FS: Frequency Synchronization DAC: Device Access Code IAC: Inquiry Access Code 49
Bluetooth Links 50
Bluetooth Packet Format Header 51
Multiple-Slot Packet 52