Mobile & Wireless Networking. Lecture 2: Wireless Transmission (2/2)

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1 Mobile & Wireless Networking Lecture 2: Wireless Transmission (2/2) [Schiller, Section 2.6 & 2.7] [Reader Part 1: OFDM: An architecture for the fourth generation] Geert Heijenk

2 Outline of Lecture 2 q Wireless Transmission (2/2) q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing (OFDM) 2

3 Modulation Process of encoding information from a message source in a manner suitable for transmission Two major steps: 1. Digital modulation q digital data is translated into an analog signal (baseband) 2. Analog modulation q shifts center frequency of baseband signal up to the radio carrier q Motivation l l l smaller antennas (e.g., λ/4) Frequency Division Multiplexing medium characteristics 3

4 Modulation and demodulation analog baseband digital data signal digital analog modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data radio receiver radio carrier 4

5 Modulation q Carrier s(t) = A t sin(2 π f t t + ϕ t ) q Basic analog modulation schemes schemes q Amplitude Modulation (AM) q Frequency Modulation (FM) q Phase Modulation (PM) q Digital modulation q ASK, FSK, PSK - main focus here q differences in spectral efficiency, power efficiency, robustness 5

6 Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): q very simple q low bandwidth requirements q very susceptible to interference t Frequency Shift Keying (FSK): q binary FSK (BFSK) q continuous phase modulation (CPM) q needs larger bandwidth Phase Shift Keying (PSK): q Binary PSH (BPSK) q more complex q robust against interference t t 6

7 Advanced Frequency Shift Keying q bandwidth needed for FSK depends on the distance between the carrier frequencies (and bit rate of source signal) q special pre-computation avoids sudden phase shifts è MSK (Minimum Shift Keying) q bits separated into even and odd bits, the duration of each bit is doubled q depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen q the frequency of one carrier is twice the frequency of the other q even higher bandwidth efficiency using a Gaussian low-pass filter è GMSK (Gaussian MSK), used in GSM 7

8 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! 8

9 Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): q bit value 0: sine wave q bit value 1: inverted sine wave q very simple PSK q low spectral efficiency q robust, used e.g. in satellite systems QPSK (Quadrature Phase Shift Keying): q 2 bits coded as one symbol q symbol determines shift of sine wave q needs less bandwidth compared to BPSK q more complex Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) A Q Q 0 I 11 I 01 t

10 Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation q it is possible to code n bits using one symbol q 2 n discrete levels, n=2 identical to QPSK q bit error rate increases with n, but less errors compared to comparable PSK schemes Q φ a I 1000 Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, but different amplitude a and 1000 have different phase, but same amplitude. 10

11 Hierarchical Modulation DVB-T modulates two separate data streams onto a single DVB-T stream q High Priority (HP) embedded within a Low Priority (LP) stream q Multi carrier system, about 2000 or 8000 carriers q QPSK, 16 QAM, 64QAM q Example: 64QAM q good reception: resolve the entire 64QAM constellation q poor reception, mobile reception: resolve only QPSK portion q 6 bit per QAM symbol, 2 most significant determine QPSK q HP service coded in QPSK (2 bit), LP uses remaining 4 bit Q I 11

12 Outline of Lecture 2 q Wireless Transmission (2/2) q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing (OFDM) 12

13 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 f Side effects: q coexistence of several signals without dynamic coordination q tap-proof Alternatives: Direct Sequence, Frequency Hopping 13

14 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 14

15 Spreading and frequency selective fading channel quality narrowband channels frequency narrow band signal guard space channel quality spread spectrum channels spread spectrum frequency 15

16 Spread spectrum technology q q q q q q q Protection against narrow band interference Tightly coupled to CDM q coexistence of several signals without dynamic coordination q High security Military use Overlay of new SS technologies on the same spectrum as old NB Civil applications q IEEE q Bluetooth q UMTS Disadvantages q High complexity q Large transmission bandwidth Alternatives: Direct Sequence, Frequency Hopping 16

17 DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence) q many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages q reduces frequency selective fading q in cellular networks Disadvantages l base stations can use the same frequency range l several base stations can detect and recover the signal l soft handover q precise power control necessary t b 0 1 t c t b : bit period t c : chip period user data XOR chipping sequence = resulting signal 17

18 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 18

19 The Rake Receiver q Takes advantage of multipath propagation q Each multipath component is called a finger q Need to estimate delay, amplitude and phase for each finger q The Rake receiver combines multipath components with a separation in time one chip period Tchip Example: 3.84 Mcps Tchip = 0.26 µs 78 m 19

20 Time Dispersion Rake receiver Channel Estimation h 0 Channel r(n) C(n) τ 2 τ 1 C(n) C(n) h 2 h 1 g g g τ 1 τ 2 a 2 a 1 a 0 Diversity Combination To Decoder Diversity Combination Selective Channel Estimation Delay a 2 a 1 a τ 1 τ 2 a 0 a 1 Equal gain Delay 1/3 1/3 1/3 τ 2 Maximum Ratio Delay and complex amplitudes h 2 * h 1 * h 0 * a 2 20

21 FHSS (Frequency Hopping Spread Spectrum) I Discrete changes of carrier frequency q sequence of frequency changes determined via pseudo random number sequence Two versions q Fast Hopping: several frequencies per user bit q Slow Hopping: several user bits per frequency 21

22 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 t t slow hopping (3 bits/hop) fast hopping (3 hops/bit) t b : bit period t d : dwell time 22

23 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 23

24 FHSS (Frequency Hopping Spread Spectrum) IV Example: q Bluetooth (1600 hops/sec on 79 carriers) Advantages q frequency selective fading and interference limited to short period q simple implementation q uses only small portion of spectrum at any time Disadvantages q not as robust as DSSS q simpler to detect 24

25 Outline of Lecture 2 q Wireless Transmission (2/2) q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing (OFDM) 25

26 Multicarrier modulation q Target: increase data rate q Increase bandwidth? q Increase symbol rate? q Problem: increase of frequency selective fading and Inter Symbol Interference (ISI) q Solution: q High bit rate signal split into many low bit rate signals q Each low bit rate signal used to modulate a different carrier q Less vulnerable to ISI and frequency selective fading q But: How to make multicarrier systems efficient? 26

27 Spectrum of a rectangular pulse

28 Two-carrier pulse spectrum 28

29 Spectra for Three Orthogonal Carriers 29

30 OFDM(A) A multicarrier system based on orthogonal subcarriers: à Orthogonal Frequency-Division Multiplexing (OFDM) When different subcarriers can be used by different users: à Orthogonal Frequency-Division Multiple Access (OFDMA) Typically uses phase and amplitude modulation on each subcarrier: à QAM (BPSK, QPSK, 8PSK, 16QAM, 32QAM, 64QAM) Composite signal given by: N$1 f (t) = % A k cos(k" 0 t + # k ), for 0 & t & T p k= 0 where T p is symbol width, " 0 = 2'/T p, A k and # k are QAM amplitude and phase on carrier k à Can by computed by Inverse Fast Fourier Transform (IFFT) 30

31 OFDM(A) deployment OFDM is used in IEEE a and g 48 (+4 pilot) subcarriers of khz (total 20 MHz) à 3.2µs time 0.8µs guard space (ISI mitigation) à symbols/s 64QAM on 48 carriers results in 6 * 48 = 288 bits/symbol à 72 Mbit/s, with ¾ coding rate (error correction): 54 Mbit/s OFDM is also used in DAB, DVB OFDMA is used in WiMAX and LTE with up to 1200 subcarriers of 15 khz in 20 MHz. 31

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