EEC173B/ECS152C, Winter Fundamentals of Wireless Communications

Transcription

1 EEC173B/ECS152C, Wnter 2006 Fundamentals of Wreless Communcatons #1: Frequences #2: Rado Propagaton Model #3: Modulaton Acknowledgment: Selected sldes from Prof. Schller & Prof. Goldsmth Characterstcs of Wreless Medum Comparson to wred meda Unguded lnk Unrelable Low bandwdth Untethered: supports moblty Broadcast nature Shared medum Capacty lmtaton Frequency of operaton and legalty of access dfferentates a varety of alternatves for wreless networkng Chuah, Wnter 06 2 Page 1

2 Frequences for Communcaton twsted par coax cable optcal transmsson 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 μm 3 THz 1 μm 300 THz VLF LF MF HF VHF UHF SHF EHF nfrared vsble lght UV VLF = Very Low Frequency UHF = Ultra Hgh Frequency LF = Low Frequency SHF = Super Hgh Frequency MF = Medum Frequency EHF = Extra Hgh Frequency HF = Hgh Frequency UV = Ultravolet Lght VHF = Very Hgh Frequency Frequency and wave length: λ = c/f - Wave length λ, speed of lght c 3x10 8 m/s, frequency f Chuah, Wnter 06 3 Frequences for Moble Communcaton VHF /UHF ranges for moble rado Smple, small antenna for cars Determnstc propagaton characterstcs, relable connectons SHF and hgher for drected rado lnks, satellte communcaton Small antenna, focusng Large bandwdth avalable Wreless LANs use frequences n UHF to SHF spectrum Some systems planned up to EHF Lmtatons due to absorpton by water and oxygen molecules (resonance frequences) Weather dependent fadng, sgnal loss caused by heavy ranfall etc. Chuah, Wnter 06 4 Page 2

3 Operatonal Ranges 1 GHz (cellular) 2 GHz (PCS and WLAN) 5 GHz (WLANs) GHz (local multpont dstrbuton servces (LMDS) and pont to pont base staton connectons) IR frequences for optcal communcatons Chuah, Wnter 06 5 Lcensed and Unlcensed Bands Lcensed: Cellular/PCS Expensve (PCS bands n US were sold for around $20B) Tme consumng to deploy new applcatons rapdly at low costs Unlcensed: Industral, Medcal, and Scentfc (ISM) Bands Free, component costs are also low New applcatons such as WLAN, Bluetooth are easly developed Wth the ncrease n frequency and data rate, the hardware cost ncreases, and the ablty to penetrate walls also decreases Chuah, Wnter 06 6 Page 3

4 Frequences and regulatons Cellular Phones Europe USA Japan ITU R GSM , holds 479- auctons AMPS, for TDMA, new CDMA frequences, PDC 486/ , , , manages 496, /935- frequency bands worldwde , (WRC, 960, TDMA, CDMA, GSM World Rado Conferences) Cordless Phones Wreless LANs Others / UMTS (FDD) , UMTS (TDD) , CT , CT DECT IEEE HIPERLAN , RF-Control 27, 128, 418, 433, , PACS , PACS-UB IEEE , RF-Control 315, , PHS JCT IEEE RF-Control 426, 868 Chuah, Wnter 06 7 Rado Propagaton Three most mportant rado propagaton characterstcs used n the desgn, analyss, and nstallaton of wreless networks are: Achevable sgnal coverage Maxmum data rate that can be supported by the channel Rate of fluctuatons n the channel Chuah, Wnter 06 8 Page 4

5 Sgnals 1 Physcal representaton of data Functon of tme and locaton Sgnal parameters: parameters representng the value of data Classfcaton Contnuous tme/dscrete tme Contnuous values/dscrete values Analog sgnal = contnuous tme and contnuous values Dgtal sgnal = dscrete tme and dscrete values Sgnal parameters of perodc sgnals: perod T, frequency f=1/t, ampltude A, phase shft ϕ sne wave as specal perodc sgnal for a carrer: s(t) = A t sn(2 π f t t + ϕ t ) Chuah, Wnter 06 9 Fourer representaton of perodc sgnals 1 g( t) = c + 2 n= 1 a sn(2πnft) + n n n= 1 b cos(2πnft) deal perodc sgnal t 0 real composton (based on harmoncs) t Chuah, Wnter Page 5

6 Sgnals 2 Dfferent representatons of sgnals Ampltude (ampltude doman) Frequency spectrum (frequency doman) Phase state dagram (ampltude M and phase ϕ n polar coordnates) A [V] A [V] Q = M sn ϕ t[s] ϕ I= M cos ϕ ϕ f [Hz] Chuah, Wnter Sgnal 3 Composed sgnals transferred nto frequency doman usng Fourer transformaton Dgtal sgnals need nfnte frequences for perfect transmsson modulaton wth a carrer frequency for transmsson (analog sgnal!) Chuah, Wnter Page 6

7 Sgnal propagaton ranges Transmsson range Communcaton possble Low error rate Detecton range Detecton of the sgnal possble No communcaton possble Interference range Sgnal may not be detected Sgnal adds to the background nose sender transmsson detecton nterference dstance Chuah, Wnter Rado Envronment Weak Strong Shadowng Path Loss Lmt the Bt Rate and/or Coverage Mult-path Fadng Chuah, Wnter Page 7

8 A. Path Loss of Rado Sgnal Sgnal propagaton n free space always lke lght (straght lne) Recevng power proportonal to 1/d² (d = dstance between sender and recever) Recevng power addtonally nfluenced by Fadng (frequency dependent) Shadowng Reflecton at large obstacles Refracton dependng on the densty of a medum Scatterng at small obstacles Dffracton at edges shadowng reflecton refracton scatterng dffracton Chuah, Wnter Path Loss Model (1) Many path loss models Analytcal, emprcal (fttng curves to measured data), or combnaton. A general model for path loss (or sometmes referred to as path gan), L, s: Pr 1 L = = GtGr 2 2 α P k(4π ) f d t where P r s the local mean receved sgnal power P t s the transmtted power d s the transmtter-recever dstance, f s frequency G t, G r are transmtter and recever antennae gans k s a loss factor not related to propagaton The path loss exponent: 2 α 4 (α = 2 n free space) Chuah, Wnter Page 8

9 Path Loss Model (2) We can smplfy thngs by lumpng the constant together: 1 L = K( ) 2 α f d In practce, one can measure the power receved at a reference pont, d o from the transmtted and estmate P r as: α d P r = Po do P = r ( dbm) Po ( dbm) 10α log Free space propagaton model: α = 2 Used when transmtter and recever has clear, unobstructed, lne ofsght (LOS) path For shadow urban, α = 4 (Example lnk budget calculaton: sldes # 35-36) Chuah, Wnter d d o Rado Propagaton Mechansms Reflecton and Transmsson: Upon reflecton or transmsson, the rado sgnals attenuates by factors that depend on the frequency, angle of ncdence, and the nature of medum Dffracton: Dffracted felds are generated by secondary wave sources formed at the edges of the buldngs, walls, and other large objects. Dffracton facltates the reachablty of sgnals that are not n lne of sght of the transmtter. However, the losses are more than that of reflecton and transmsson Scatterng: Irregular surfaces scatter sgnals n all drectons n the form of sphercal waves. Propagaton n many drectons results n reduced power levels. Chuah, Wnter Page 9

10 Real World Example Chuah, Wnter B. Shadow fadng Receved sgnal s shadowed by obstructons such as hlls and buldngs. Dependng on the envronment and the surroundngs, and the locaton of objects, the receved sgnal strength for the same dstance from the transmtter wll be dfferent. Ths varaton of sgnal strength due to locaton s referred to as shadow fadng Ths results n varatons n the local mean receved sgnal power P r (db) = P r (db) + G s 2 where G s ~ N(0, σ s ), 4 σ s 10 db. Chuah, Wnter Page 10

11 B. Shadow Fadng Implcatons Non unform coverage Increases the requred transmt power To overcome the shadow fadng effects, a fade margn s added to the path loss or receved sgnal strength. The fade margn s the addtonal sgnal power that can provde a certan fracton of the locatons wth the requred sgnal strength R P = P r0 Chuah, Wnter C. Multpath propagaton Sgnal can take many dfferent paths between sender and recever due to reflecton, scatterng, dffracton Lne-of-Sght (LOS) pulses multpath pulses sgnal at sender sgnal at recever Chuah, Wnter Page 11

12 Multpath propagaton Cont d Tme dsperson: sgnal s dspersed over tme Interference wth neghbor symbols Inter Symbol Interference (ISI) The sgnal reaches a recever drectly and phase shfted Dstorted sgnal dependng on the phases of the dfferent parts Chuah, Wnter Delay Spread Recev ed Power 2τ Two-ray model τ = rms delay spread Channel Input Delay τ small T Channel Output 0 1 T 1 2T 0 T 2T τ T τ T τ large T small neglgble ntersymbol nterference large 0 T 2T sgnfcant ntersymbol nterference, whch causes an rreducble error floor Chuah, Wnter C Cmn-7/98 Page 12

13 Multpath Propagaton Cont d Receved Power Delay Spread t h(t) = Σ a e jθ δ(t-t ) Chuah, Wnter Multpath Example Constructve and destructve nterference of arrvng rays 0.5λ db Wth Respect to RMS Value t, n seconds x, n wavelength Chuah, Wnter Page 13

14 Multpath Fadng Fluctuatons of the sgnal ampltude because of the addton of sgnals arrvng n dfferent phases (paths) s called multpath fadng Multpath fadng results n hgh BER, and can be mtgated by FEC, dversty schemes, and usng drectonal antennae Chuah, Wnter Effects of Moblty Channel characterstcs change over tme and locaton Sgnal paths change Dfferent delay varatons of dfferent sgnal parts Dfferent phases of sgnal parts Quck changes n the power receved (short term fadng) power Addtonal changes n Dstance to sender Obstacles further away Slow changes n the average power receved (long term fadng) short term fadng long term fadng t Chuah, Wnter Page 14

15 Doppler Shft Doppler Shft, f d Apparent change n frequency due to movement f d 1 Δφ ν = = cosθ 2π Δt λ If moble s movng toward the drecton of arrval of the wave, the Doppler shft s postve If the moble s movng away, the Doppler shft s negatve Max shft when angle = 0 (movng drectly toward/away transmtter) S X v θ θ Chuah, Wnter Y Tme varyng Channel Condtons Due to users moblty and varablty n the propagaton envronment, both desred sgnal and nterference are tmevaryng and locaton dependent A measure of channel qualty: SNR (Sgnal to Nose Rato) SNR = Desred Sgnal Power Nose power Pr = N Desred sgnal power = receved power = P r We know how to estmate ths from slde #17 Background nose, e.g., thermal nose Smple model: Nose power = ηw, where η s the average power per Hertz of the thermal nose, and W s the sgnal bandwdth Chuah, Wnter Page 15

16 Tme varyng Channel Condtons A more complete measure of channel qualty: SINR (Sgnal to Interference plus Nose Rato) In the mpact of nterference s much more than nose, another measure s carrer to nterference (C/I) rato,.e., assumng nose power s close to zero We wll talk about how to estmate Interference n the next lecture Chuah, Wnter Illustraton of Channel Condtons Chuah, Wnter Page 16

17 Physcal Layer Issues Practcal Lnk Performance Measures Probablty of Bt Error (BER) Effcency Modulaton Tradeoffs Flat Fadng Countermeasures Delay Spread Countermeasures EEC165, EEC166 EEC265, EEC266 Chuah, Wnter Lnk Performance Measure (1): BER The probablty of bt error, P b, n a rado envronment s a random varable Average P b (P b ) P r [P b > P btarget ] Δ outage (Pout) = Bt error rate s a functon of SNR (sgnal tonose rato), or C/I (carrer to nterference rato), at the recever The functon tself depends on the modulaton Chuah, Wnter Page 17

18 Calculate Lnk Budget usng Path Loss Models Lnk budget calculaton requres Estmate of power receved from transmtted at a recever Estmate of nose & power receved from nterferers For example, SNR (db) = P r (n dbm) N (n dbm) Recall on slde #17, P = r Po d d α Typcal approxmaton o P = r ( dbm) Po ( dbm) 10α log d s the dstance between transmtted and recever measured relatve to the reference pont d 0 αs the path loss exponent P 0 s a constant that accounts for antenna gans, carrer frequency, and reference pont d 0 d d o Chuah, Wnter Example Lnk Budget calculaton Maxmum separaton dstance vs. transmtted power (wth fxed BW) Gven: Cellular phone wth 0.6 W transmt power Unty gan antenna, 900 MHz carrer frequency SNR must be at least 25dB for proper recepton Nose = 119 dbm Assume path loss constant, α=2, and P 0 (at d 0 =1km) = 63.5 dbm What wll be the maxmum dstance? Chuah, Wnter Page 18

19 Lnk Performance Measure (2): Effcency Spectral Effcency: a measure of the data rate per unt bandwdth for a gven bt error probablty and transmtted power Power Effcency: a measure of the requred receved power to acheve a gven data rate for a gven bt error probablty and bandwdth Throughput/Delay Chuah, Wnter Modulaton 1 Dgtal modulaton Dgtal data s translated nto an analog sgnal (baseband) Analog modulaton Shfts center frequency of baseband sgnal up to the rado carrer Basc schemes Ampltude Modulaton (AM) Frequency Modulaton (FM) Phase Modulaton (PM) Chuah, Wnter Page 19

20 Modulaton and Demodulaton analog baseband dgtal sgnal data dgtal analog modulaton modulaton rado transmtter rado carrer analog demodulaton analog baseband sgnal synchronzaton decson dgtal data rado recever rado carrer Chuah, Wnter Dgtal modulaton Modulaton of dgtal sgnals known as Shft Keyng Ampltude Shft Keyng (ASK): Very smple Low bandwdth requrements Very susceptble to nterference Frequency Shft Keyng (FSK): Needs larger bandwdth t t Phase Shft Keyng (PSK): More complex Robust aganst nterference t Chuah, Wnter Page 20

21 Groupng the Informaton Bts nto Symbols b bts/symbol = M possble waveforms 1 bt/symbol bts/symbol T b T b T S If M the performance goes up, but at a cost of complexty (Shannon lmt) Chuah, Wnter Sgnal Space Representaton The basc dea s that we can transmt nformaton n parallel over a set of orthogonal waveforms wth respect to the symbol nterval T. The nverse of ths nterval s called the symbol rate: Rs = 1/T. T s1 ( t) s2( t) t =0 dt = δ j s 1 (t) s 2 (t) Chuah, Wnter Page 21

22 Detecton of the Symbols Correlaton or matched flter detector (bascally equvalent) s 1 (t) Sample at t = T s 2 (t) = T t 0 dt = T t 0 dt T s1 ( t) s2( t) t =0 dt = δ j => Look at example waveform n HW 1 Chuah, Wnter Dgtal Modulaton Any modulated sgnal can be represented as s(t) = A(t) cos [ω c t+ φ(t)] ampltude phase or frequency s(t) = A(t) cos φ(t) cos ω c t - A(t) sn φ(t) sn ω c t n-phase quadrature Lnear versus nonlnear modulaton Impact on spectral effcency Constant envelope versus non-constant envelope hardware mplcatons wth mpact on power effcency Chuah, Wnter Page 22

23 Frequency Doman a T tme 1/T frequency a g(t) Baseband BW (bandwdth) a g( t) cos(2π f t) c Passband f c BW (bandwdth) Chuah, Wnter Alternatve Interpretaton s ( t) = a g( t) cos(2π f t) b g( t).sn(2π f t) s ( t) = Re s ( t) = Re [ ( ) ] j 2π f c g( t) a + j b e t [ ( ) ] j θ j 2π f c g( t) r e e t c Q c r θ a + j b s ( t) = Re ( ) [ ] j 2π f c g t r e t +θ ( ) I s ( t) = g( t) r cos 2 ( π f t + θ ) c Chuah, Wnter Page 23

24 Advanced Phase Shft Keyng Q BPSK (Bnary Phase Shft Keyng): Bt value 0: sne wave Bt value 1: nverted sne wave Very smple PSK Low spectral effcency Robust, used e.g. n satellte systems QPSK (Quadrature Phase Shft Keyng): 2 bts coded as one symbol Symbol determnes shft of sne wave Needs less bandwdth compared to BPSK More complex A Q 0 I 11 I 01 t Chuah, Wnter Quadrature Ampltude Modulaton Quadrature Ampltude Modulaton (QAM): combnes ampltude and phase modulaton It s possble to code n bts usng one symbol 2 n dscrete levels, n=2 dentcal to QPSK bt error rate ncreases wth n, but less errors compared to comparable PSK schemes Q Example: 16 QAM (4 bts = 1 symbol) φ Symbols 0011 and 0001 have the a I same phase φ, but dfferent 1000 ampltude a and 1000 have dfferent phase, but same ampltude. used n standard 9600 bt/s modems Chuah, Wnter Page 24

25 QAM and PSK QAM (Quadrature Ampltude Modulaton) 4-QAM 16-QAM 64-QAM PSK (Phase Shft Keyng) 4-PSK 8-PSK 16-PSK Chuah, Wnter Symbol Error SER SNR E r 2 2 S av av = = = EN σ R + σ I 2 σ r SNR The demodulator chooses the symbol that s closest to the receved one (maxmum lkelhood decodng) If the nose (and dstortons) s such that we are closer to another symbol than the correct one, a symbol error occurs. Each symbol error results n a number of bt errors. By carefully choosng the mappng from bts to symbols (Gray encodng), one symbol error typcally results n just one bt error. Chuah, Wnter Page 25

26 Advanced Frequency Shft Keyng Bandwdth needed for FSK depends on the dstance between the carrer frequences Specal pre computaton avods sudden phase shfts MSK (Mnmum Shft Keyng) Bt separated nto even and odd bts, the duraton of each bt s doubled Dependng on the bt values (even, odd) the hgher or lower frequency, orgnal or nverted s chosen The frequency of one carrer s twce the frequency of the other Equvalent to offset QPSK Even hgher bandwdth effcency usng a Gaussan low pass flter GMSK (Gaussan MSK), used n GSM Chuah, Wnter Example of MSK data even bts odd bts bt even odd sgnal h n n h value low frequency hgh frequency h: hgh frequency n: low frequency +: orgnal sgnal -: nverted sgnal MSK sgnal t No phase shfts! Chuah, Wnter Page 26

27 Selectng a Modulaton Scheme (1) Hgh Bt Rate Robustness to Imparments Provdes low bt error rates (BER) at low sgnal to nose ratos (SNR) Performs well n multpath fadng Performs well n tme varyng channels (symbol tmng jtter) Hgh Spectral Effcency: occupes mnmal bandwdth Hgh Power Effcency Low cost and easy to mplement Low carrer to cochannel nterference rato Low out of band radaton Constant or near constant envelope constant: only phase s modulated may use effcent non lnear amplfers non constant: phase and ampltude modulated may need neffcent lnear amplfers Chuah, Wnter Selectng a Modulaton Scheme (2) Other desgn ratonale Smaller antennas (e.g., λ/4) Frequency Dvson Multplexng Medum characterstcs No perfect modulaton scheme a matter of trade offs! Two metrcs: Energy effcency E b /N 0 for a certan BER and Bandwdth effcency R/B Chuah, Wnter Page 27

28 Recever Performance Chuah, Wnter Energy Bandwdth Trade off Chuah, Wnter Page 28