CS647: Advanced Topics in Wireless Networks. Basics of Wireless Transmission Part II

Transcription

1 CS647: Advanced Topics in Wireless Networks Basics of Wireless Transmission Part II Drs. Baruch Awerbuch & Amitabh Mishra Computer Science Department Johns Hopkins University CS

2 Antenna Gain For a circular reflector antenna G = η ( π D / λ ) 2 η = net efficiency (depends on the electric field distribution over the antenna aperture, losses such as ohmic heating, typically 0.55) D = diameter, thus, Example: G = η (π D f /c ) 2, c = λ f (c is speed of light) Antenna with diameter = 2 m, frequency = 6 GHz, wavelength = 0.05 m G = 39.4 db Frequency = 14 GHz, same diameter, wavelength = m G = 46.9 db * Higher the frequency, higher the gain for the same size antenna CS

3 Path Loss (Free-space) Definition of path loss L P : Path Loss in Free-space: L = P Pt P r, L where f c is the carrier frequency L f = (4π d/λ) 2 = (4π f c d/c ) 2 ( db) = log10 fc( MHz) 20log10 d( km), PF + This shows greater the f c, more is the loss. CS

4 Example of Path Loss (Free-space) Path Loss in Free-space Path Loss L f (db) Distance d (km) f c =150MHz f c =200MHz f c =400MHz f c =800MHz f c =1000MHz f c =1500MHz CS

5 Land Propagation The received signal power: P r = GtGr Pt L L is the propagation loss in the channel, i.e., L = L P L S L F Fast fading Slow fading (Shadowing) Path loss CS

6 Propagation Loss Fast Fading (Short-term fading) Slow Fading (Long-term fading) Signal Strength (db) Path Loss Distance CS

7 Path Loss (Land Propagation) Simplest Formula: L p = A d -α where A and α: propagation constants d : distance between transmitter and receiver α : value of 3 ~ 4 in typical urban area CS

8 Path Loss (Urban, Suburban and Open areas) α L where Urban area: PU [ h ( m) ] m ( db) = log f ( MHz) log h ( m) α = + [ log h ( m) ] log d( km) b c [ 1.1log10 fc ( MHz) 0.7] hm ( m) [ 1.56 log10 fc ( MHz) 0.8 ], [ log h m ] for f MHz m( ) 1.1, c 200, for 2 3.2[ log 11.75h ( m) ] 4.97, for f 400MHz 10 m 10 c 10 b [ h ( m) ] m for l arg e city small & medium city Suburban area: L PS ( db) = L PU ( db) 2 log 2 fc 10 ( MHz) Open area: L PO 2 [ f ( MHz) ] log f ( ) ( db) = LPU ( db) 4.78 log c 10 c MHz 10 CS

9 Path Loss Path loss in decreasing order: Urban area (large city) Urban area (medium and small city) Suburban area Open area CS

10 Example of Path Loss (Urban Area: Large City) Path Loss in Urban Area in Large City Path Loss L pu (db) Distance d (km) f c =200MHz f c =400MHz f c =800MHz f c =1000MHz f c =1500MHz f c =150MHz CS

11 Example of Path Loss (Urban Area: Medium and Small Cities) Path Loss in Urban Area for Small & Medium Cities Path Loss Lpu (db) Distance d (km) fc=150mhz fc=200mhz fc=400mhz fc=800mhz fc=1000mhz fc=1500mhz CS

12 Example of Path Loss (Suburban Area) Path Loss in Suburban Area Path Loss Lps (db) fc=150mhz fc=200mhz fc=400mhz fc=800mhz fc=1000mhz fc=1500mhz Distance d (km) CS

13 Example of Path Loss (Open Area) Path Loss in Open Area 150 Path Loss L po (db) f c =150MHz f c =200MHz f c =400MHz f c =800MHz f c =1000MHz f c =1500MHz Distance d (km) CS

14 Slow Fading The long-term variation in the mean level is known as slow fading (shadowing or log-normal fading). This fading caused by shadowing. Log-normal distribution: - The pdf of the received signal level is given in decibels by ( M M) 2 1 ( ) 2 2σ pm = e 2 πσ where M is the true received signal level m in decibels, i.e., M=10log 10 m, M is the area average signal level, i.e., the mean of M, σ is the standard deviation in decibels CS

15 Log-normal Distribution 2σ p(m) M M The pdf of the received signal level CS

16 Fast Fading The signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles. When MS far from BS, the envelope distribution of received signal is Rayleigh distribution. The pdf is p r σ () 2 2σ r = e, r 0 where σ is the standard deviation and r is the envelope of fading signal. Middle value r m of envelope signal within sample range to be satisfied by We have r m = r 2 2 > P( r rm) = 0.5. σ CS

17 Rayleigh Distribution P(r) σ=1 0.4 σ=2 0.2 σ= The pdf of the envelope variation CS r

18 Fast Fading (Continued) When MS is close to BS, the envelope distribution of received signal is Rician distribution. The pdf is () r rα = e 2 2σ I, r 0 p r σ 2 r α 0 σ where σ is the standard deviation, I 0 (x) is the zero-order Bessel function of the first kind, α is the amplitude of the direct signal CS

19 Rician Distribution α= 0 (Rayleigh) α = 1 α = 2 α = 3 Pdf p(r) σ = The pdf of the envelope variation 4 r 6 8 r CS

20 Characteristics of Instantaneous Amplitude Level Crossing Rate: Average number of times per second that the signal envelope crosses the level in positive going direction. Fading Rate: Number of times signal envelope crosses middle value in positive going direction per unit time. Depth of Fading: Ratio of mean square value and minimum value of fading signal. Fading Duration: Time for which signal is below given threshold. CS

21 Doppler Shift Doppler Effect: When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source. When they are moving toward each other, the frequency of the received signal is higher than the source. When they are opposing each other, the frequency decreases. Thus, the frequency of the received signal is f R = where f C is the frequency of source carrier, & f D is the Doppler frequency. f C f D Doppler Shift in frequency: f D = v λ cos θ MS θ Moving speed v where v is the moving speed, λ is the wavelength of carrier. Signal CS

22 Delay Spread When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections. This forces signal to follow different paths. Each path has different path length, so the time of arrival for each path is different. This effect which spreads out the signal is called Delay Spread. CS

23 Moving Speed Effect V 1 V 2 V 3 V 4 Signal strength Time CS

24 Delay Spread The signals from close by reflectors Signal Strength The signals from intermediate reflectors The signals from far away reflectors Delay CS

25 Intersymbol Interference (ISI) Caused by time delayed multipath signals Has impact on burst error rate of channel Second multipath is delayed and is received during next symbol For low bit-error-rate (BER) R < 2τ d R (digital transmission rate) limited by delay spread τ d. 1 CS

26 Intersymbol Interference (ISI) Transmission signal 1 1 Time 0 Received signal (short delay) Time Propagation time Delayed signals Received signal (long delay) Time CS

27 Coherence Bandwidth Coherence bandwidth B c : Represents correlation between 2 fading signal envelopes at frequencies f 1 and f 2. Is a function of delay spread. Two frequencies that are larger than coherence bandwidth fade independently. Concept useful in diversity reception Multiple copies of same message are sent using different frequencies. CS

28 Cochannel Interference Cells having the same frequency interfere with each other. r d is the desired signal r u is the interfering undesired signal β is the protection ratio for which r d βr u (so that the signals interfere the least) If P(r d βr u ) is the probability that r d βr u, Cochannel probability P co = P(r d βr u ) CS

29 Multiplexing Multiplexing in 4 dimensions space (s i ) time (t) frequency (f) code (c) channels k i k 1 k 2 k 3 k 4 k 5 k 6 c t c t Goal: multiple use of a shared medium s 1 f s 2 f Important: guard spaces needed! c t s 3 f CS

30 Frequency multiplex Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages: no dynamic coordination necessary works also for analog signals Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces t c k 1 k 2 k 3 k 4 k 5 k 6 f CS

31 Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high even for many users k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: precise synchronization necessary c f t CS

32 Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: better protection against tapping protection against frequency selective interference c k 1 k 2 k 3 k 4 k 5 k 6 higher data rates compared to code multiplex f but: precise coordination required t CS

33 Code multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages: k 1 k 2 k 3 k 4 k 5 k 6 c bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping f Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology t CS

34 Modulation Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e.g., λ/4) Frequency Division Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) CS

35 Modulation and demodulation analog baseband digital signal data digital analog modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data radio receiver radio carrier CS

36 Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference t Frequency Shift Keying (FSK): needs larger bandwidth t Phase Shift Keying (PSK): more complex robust against interference t CS

37 Advanced Frequency Shift Keying bandwidth needed for FSK depends on the distance between the carrier frequencies special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying) bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen the frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used in GSM CS

38 Example of MSK data even bits odd bits bit even odd signal h n n h value low frequency high frequency h: high frequency n: low frequency +: original signal -: inverted signal MSK signal t No phase shifts! CS

39 Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): Q bit value 0: sine wave bit value 1: inverted sine wave very simple PSK 1 0 I low spectral efficiency robust, used e.g. in satellite systems 10 Q 11 QPSK (Quadrature Phase Shift Keying): I 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK A more complex Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) t CS

40 Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation it is possible to code n bits using one symbol 2 n discrete levels, n=2 identical to QPSK bit error rate increases with n, but less errors compared to comparable PSK schemes Q Example: 16-QAM (4 bits = 1 symbol) 0011 Symbols 0011 and 0001 have the same 0000 phase φ, φ but different amplitude a and 1000 have a I different phase, but same amplitude used in standard 9600 bit/s modems CS

41 Hierarchical Modulation DVB-T modulates two separate data streams onto a single DVB-T stream High Priority (HP) embedded within a Low Priority (LP) stream Multi carrier system, about 2000 or 8000 carriers QPSK, 16 QAM, 64QAM Example: 64QAM Q good reception: resolve the entire 64QAM constellation poor reception, mobile reception: 10 resolve only QPSK portion I 6 bit per QAM symbol, 2 most significant determine QPSK HP service coded in QPSK (2 bit), 00 LP uses remaining 4 bit CS

42 Spread spectrum technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference power interference spread signal power detection at receiver signal spread interference f protection against narrowband interference Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping f CS

43 Effects of spreading and interference dp/df dp/df i) ii) f sender f user signal broadband interference narrowband interference dp/df dp/df dp/df iii) iv) v) f receiver f f CS

44 Spreading and frequency selective fading channel quality narrowband channels frequency narrow band signal guard space channel quality spread spectrum channels spread spectrum frequency CS

45 DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages t b reduces frequency selective fading user data in cellular networks 0 1 XOR base stations can use the same frequency range several base stations can detect and recover the signal soft handover Disadvantages precise power control necessary t c t b : bit period t c : chip period chipping sequence = resulting signal CS

46 DSSS (Direct Sequence Spread Spectrum) II user data X spread spectrum signal modulator transmit signal chipping sequence radio carrier transmitter correlator received signal demodulator lowpass filtered signal X products integrator sampled sums decision data radio carrier chipping sequence receiver CS

47 FHSS (Frequency Hopping Spread Spectrum) I Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time Disadvantages not as robust as DSSS simpler to detect CS

48 FHSS (Frequency Hopping Spread Spectrum) II t b user data f f 3 f 2 f 1 f f 3 f 2 f t d t d t b : bit period t t t t d : dwell time slow hopping (3 bits/hop) fast hopping (3 hops/bit) CS

49 FHSS (Frequency Hopping Spread Spectrum) III user data modulator narrowband signal modulator spread transmit signal transmitter frequency synthesizer hopping sequence received signal demodulator narrowband signal demodulator data hopping sequence frequency synthesizer receiver CS