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Mobile and Wireless Networking 2013 / 2014
Mobile and Wireless Networking 2013 / 2014
192620010 Mobile & Wireless Networking Lecture 2: Wireless - - PowerPoint PPT Presentation
192620010 Mobile & Wireless Networking Lecture 2: Wireless - - PowerPoint PPT Presentation
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
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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
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Modulation and demodulation
synchronization decision digital data analog demodulation radio carrier analog baseband signal 101101001 radio receiver digital modulation digital data analog modulation radio carrier analog baseband signal 101101001 radio transmitter
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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
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1 1
t
1 1
t
1 1
t
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
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
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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
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Example of MSK
data even bits
- dd bits
1 1 1 1 t low frequency high frequency MSK signal bit even 0 1 0 1
- dd
0 0 1 1 signal h n n h value
- - + +
h: high frequency n: low frequency +: original signal
- : inverted signal
No phase shifts!
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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)
11 10 00 01 Q I 1 Q I 11 01 10 00 A t
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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
Example: 16-QAM (4 bits = 1 symbol)
Symbols 0011 and 0001 have the same phase φ,
but different amplitude a. 0000 and 1000 have different phase, but same amplitude.
0000 0001 0011 1000 Q I 0010
φ a
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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 00 10 000010 010101
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Outline of Lecture 2
q Wireless Transmission (2/2)
q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing
(OFDM)
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13 detection at receiver interference spread signal signal spread interference f f power power
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 Side effects:
q coexistence of several signals without dynamic coordination q tap-proof
Alternatives: Direct Sequence, Frequency Hopping
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Effects of spreading and interference
dP/df f i) dP/df f ii) sender dP/df f iii) dP/df f iv) receiver f v) user signal broadband interference narrowband interference dP/df
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Spreading and frequency selective fading
frequency channel quality 1 2 3 4 5 6 narrow band signal guard space 2 2 2 2 2 frequency channel quality 1 spread spectrum
narrowband channels spread spectrum channels
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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
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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
l base stations can use the
same frequency range
l several base stations can
detect and recover the signal
l soft handover
Disadvantages
q precise power control necessary user data chipping sequence resulting signal 1 1 1 0 1 0 1 0 1 1 1 1 XOR 1 1 0 1 0 1 1 1 1 = tb tc
tb: bit period tc: chip period
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DSSS (Direct Sequence Spread Spectrum) II
X user data chipping sequence modulator radio carrier spread spectrum signal transmit signal transmitter demodulator received signal radio carrier X chipping sequence lowpass filtered signal receiver integrator products decision data sampled sums correlator
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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
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Time Dispersion – Rake receiver – Channel Estimation
τ1 τ2 h1 h0 h2
Channel
Diversity Combination Selective Equal gain Maximum Ratio Channel Estimation Delay Delay Delay and complex amplitudes a2 a1 a0 1 1/3 1/3 1/3 h2* h1* h0*
g g g
C(n)
τ2 τ1
C(n) C(n) a2 a1 a0 r(n) Diversity Combination
a0 a1 a2
To Decoder
τ1 τ2 τ2
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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
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FHSS (Frequency Hopping Spread Spectrum) II
user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 1 tb 1 1 t f f1 f2 f3 t td f f1 f2 f3 t td
tb: bit period td: dwell time
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FHSS (Frequency Hopping Spread Spectrum) III
modulator user data hopping sequence modulator narrowband signal spread transmit signal transmitter received signal receiver demodulator data frequency synthesizer hopping sequence demodulator frequency synthesizer narrowband signal
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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
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Outline of Lecture 2
q Wireless Transmission (2/2)
q Modulation q Spread Spectrum q Orthogonal Frequency Division Multiplexing
(OFDM)
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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?
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Spectrum of a rectangular pulse
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2 2
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Two-carrier pulse spectrum
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Spectra for Three Orthogonal Carriers
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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:
à Can by computed by Inverse Fast Fourier Transform (IFFT)
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f (t) = Ak cos(k"0t + #k) ,
k= 0 N$1
%
for 0 & t & Tp where Tp is symbol width, "0 = 2'/Tp , Ak and #k are QAM amplitude and phase on carrier k
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