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192620010 Mobile & Wireless Networking Lecture 2: Wireless 192620010 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 Mobile and Wireless Networking 2013 / 2014 Outline of Lecture 2 q Wireless Transmission (2/2) q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing (OFDM) 2 Mobile and Wireless Networking 2013 / 2014 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 smaller antennas (e.g., λ/4) l Frequency Division Multiplexing l medium characteristics 3 Mobile and Wireless Networking 2013 / 2014 Modulation and demodulation analog baseband digital signal data digital analog 101101001 modulation modulation radio transmitter radio carrier analog baseband digital signal analog synchronization data demodulation decision 101101001 radio receiver radio carrier 4 Mobile and Wireless Networking 2013 / 2014 Modulation q Carrier s(t) = At sin(2 π ft t + ϕt) 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 Mobile and Wireless Networking 2013 / 2014 Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): 1 0 1 q very simple q low bandwidth requirements t q very susceptible to interference 1 0 1 Frequency Shift Keying (FSK): q binary FSK (BFSK) q continuous phase modulation (CPM) t q needs larger bandwidth 1 0 1 Phase Shift Keying (PSK): q Binary PSH (BPSK) q more complex t q robust against interference 6 Mobile and Wireless Networking 2013 / 2014 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 Mobile and Wireless Networking 2013 / 2014 Example of MSK 1 0 1 1 0 1 0 data bit even 0 1 0 1 even bits odd 0 0 1 1 odd bits signal h n n h value - - + + low h: high frequency frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal t No phase shifts! 8 Mobile and Wireless Networking 2013 / 2014 Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): Q q bit value 0: sine wave q bit value 1: inverted sine wave I q very simple PSK 1 0 q low spectral efficiency q robust, used e.g. in satellite systems 10 Q 11 QPSK (Quadrature Phase Shift Keying): q 2 bits coded as one symbol I q symbol determines shift of sine wave q needs less bandwidth compared to BPSK 00 01 q more complex A Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) t 11 10 00 01 9 Mobile and Wireless Networking 2013 / 2014 Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation q it is possible to code n bits using one symbol q 2n discrete levels, n=2 identical to QPSK q bit error rate increases with n, but less errors compared to comparable PSK schemes Q 0010 0001 Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, 0011 0000 φ but different amplitude a. 0000 and 1000 have a I different phase, but same amplitude. 1000 10 Mobile and Wireless Networking 2013 / 2014 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 q Example: 64QAM q good reception: resolve the entire 64QAM constellation q poor reception, mobile reception: 10 resolve only QPSK portion I q 6 bit per QAM symbol, 2 most significant determine QPSK q HP service coded in QPSK (2 bit), 00 LP uses remaining 4 bit 000010 010101 11 Mobile and Wireless Networking 2013 / 2014 Outline of Lecture 2 q Wireless Transmission (2/2) q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing (OFDM) 12 Mobile and Wireless Networking 2013 / 2014 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 power signal signal spread detection at interference receiver f f Side effects: q coexistence of several signals without dynamic coordination q tap-proof Alternatives: Direct Sequence, Frequency Hopping 13 Mobile and Wireless Networking 2013 / 2014 Effects of spreading and interference dP/df dP/df user signal i) ii) broadband interference narrowband interference f f sender dP/df dP/df dP/df iii) iv) v) f f f receiver 14 Mobile and Wireless Networking 2013 / 2014 Spreading and frequency selective fading channel quality narrowband channels 1 2 5 6 3 4 frequency narrow band guard space signal channel quality 2 2 spread spectrum channels 2 2 2 1 spread frequency spectrum 15 Mobile and Wireless Networking 2013 / 2014 Spread spectrum technology q Protection against narrow band interference q Tightly coupled to CDM q coexistence of several signals without dynamic coordination q High security q Military use q Overlay of new SS technologies on the same spectrum as old NB q Civil applications q IEEE802.11 q Bluetooth q UMTS q Disadvantages q High complexity q Large transmission bandwidth q Alternatives: Direct Sequence, Frequency Hopping 16 Mobile and Wireless Networking 2013 / 2014 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 tb fading user data q in cellular networks 0 1 XOR l base stations can use the same frequency range tc l several base stations can chipping detect and recover the signal sequence 0 1 1 0 1 0 1 0 1 1 0 1 0 1 l soft handover = Disadvantages resulting signal q precise power control necessary 0 1 1 0 1 0 1 1 0 0 1 0 1 0 tb: bit period tc: chip period 17 Mobile and Wireless Networking 2013 / 2014 DSSS (Direct Sequence Spread Spectrum) II spread spectrum transmit user data signal signal X modulator chipping radio sequence carrier transmitter correlator lowpass sampled received filtered products sums signal signal data demodulator X integrator decision radio chipping carrier sequence receiver 18 Mobile and Wireless Networking 2013 / 2014 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 Mobile and Wireless Networking 2013 / 2014 Time Dispersion – Rake receiver – Channel Estimation r(n) Channel τ2 τ1 h0 C(n) C(n) C(n) h2 h 1 g g g a2 a1 a0 τ1 τ2 Diversity Combination To Decoder Diversity Channel a0 a a a Combination Estimation 2 1 0 τ1 τ2 Selective Delay 0 0 1 a1 τ Equal gain Delay 1/3 1/3 1/3 2 a2 Maximum Delay and h * h * h * Ratio complex amplitudes 2 1 0 20 Mobile and Wireless Networking 2013 / 2014 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 Mobile and Wireless Networking 2013 / 2014 FHSS (Frequency Hopping Spread Spectrum) II tb user data 0 1 0 1 1 t f td f3 slow f2 hopping (3 bits/hop) f1 t f td f3 fast f2 hopping (3 hops/bit) f1 t tb: bit period td: dwell time 22 Mobile and Wireless Networking 2013 / 2014 FHSS (Frequency Hopping Spread Spectrum) III narrowband spread signal transmit user data signal modulator modulator frequency hopping synthesizer sequence transmitter narrowband received signal signal data demodulator demodulator hopping frequency sequence synthesizer receiver 23 Mobile and Wireless Networking 2013 / 2014 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 Mobile and Wireless Networking 2013 / 2014 Outline of Lecture 2 q Wireless Transmission (2/2) q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing (OFDM) 25 Mobile and Wireless Networking 2013 / 2014 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
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