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Lecture 3EITN75 2018
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STRUCTURE OF A WIRELESSCOMMUNICATION LINK
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A simple structure
Speechencoder Encrypt.A/D Chann.
encoding Modulation
Speechdecoder Decrypt.D/A Chann.
decoding Demod.
Key
Speech
Speech
Data
Data
(Read Chapter 10 for more details)
Arc tranmsitter / coherer reciever
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Frequency spectrum
ISM bands
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Bandwidth
Shannon-Hartley Theorem
C = B x log2(1+S/N)
The capacity of a communication link is linearly dependent on the bandwidth(B), and logarithmically on the signal to noise level(S/N).
Classic modulation formats
Analog formats• On-Off keying• Amplitude modulation• Frequency modulation
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AM/FM
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Simple model of a radio signal
•A transmitted radio signal can be written
•By letting the transmitted information change the amplitude, the frequency, or the phase, we get the tree basic types of digital modulation techniques
–ASK (Amplitude Shift Keying)–FSK (Frequency Shift Keying)–PSK (Phase Shift Keying)
Amplitude PhaseFrequency
Constant envelope
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Example: Digital amplitude, phase and frequency modulation
4ASK
4PSK
4FSK
( ) ( ) ( )( )cos 2 cs t A t f t tp f= +( )A t ( )tf
00 01 11 00 10
00 01 11 00 10
00 01 11 00 10
- Amplitude carries information- Phase constant (arbitrary)
- Amplitude constant (arbitrary) - Phase carries information
- Amplitude constant (arbitrary)- Phase slope (frequency)
carries information
Comment:
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The IQ modulator
-90o
cf
I-channel
Q-channel
Transmited radio signal
Complex envelopeTake a step into the complex domain:
Carrier factor
(in-phase)
(quadrature)
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Interpreting the complex notation
Complex envelope (phasor)
Polarcoordinates:
Transmitted radio signal
BymanipulatingtheamplitudeA(t)andthephaseΦ(t)ofthecomplexenvelope(phasor),wecancreateanytypeofmodulation/radiosignal.
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IMPORTANT MODULATIONFORMATS
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Binary phase-shift keying (BPSK)Rectangular pulses
Radiosignal
Base-band
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Binary phase-shift keying (BPSK)Rectangular pulses
Complex representation Signal constellation diagram
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Binary phase-shift keying (BPSK)Rectangular pulses
Power spectraldensity for BPSK
Normalizedfreq.𝑓×𝑇$
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Binary phase-shift keying (BPSK)Raised-cosine pulses (roll-off 0.5)
Base-band
Radiosignal
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Binary phase-shift keying (BPSK)Raised-cosine pulses (roll-off 0.5)
Complex representation Signal constellation diagram
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Binary phase-shift keying (BPSK)Raised-cosine pulses (roll-off 0.5)
Power spectraldensity for BAM
Much higher spectral efficiency than BPSK
(withrectangular
pulses).
Normalizedfreq.𝑓×𝑇$
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Quaternary PSK (QPSK or 4-PSK)Rectangular pulses
Complex representation
Radiosignal
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Quaternary PSK (QPSK or 4-PSK)Rectangular pulses
Power spectraldensity for QPSK
Twice the spectrum efficiency of BPSK (with rect. pulses).
TWO bits/pulseinstead of one.
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Quadrature ampl.-modulation (QAM)Root raised-cos pulses (roll-off 0.5)
Complex representation
Much higher spectral efficiency than QPSK
(withrectangular
pulses).
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Amplitude variationsThe problem
Signals with high amplitude variations leads to less efficient amplifiers.
Complex representation of QPSK
It is a problem that the signalpasses through the origin, wherethe amplitude is ZERO.(Infinite amplitude variation.)
Can we solve this problem in a simpleway?
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Amplitude variationsA solution
Let’s rotate the signal constellation diagram for eachtransmitted symbol!
π 4⁄ 2×π 4⁄etc.
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Amplitude variationsA solution
Looking at the complex representation ...
QPSK without rotation QPSK with rotation
A “hole” is created in the center. No close to zero
amplitudes.
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- Differential QPSK (DQPSK)/ 4p
Complex representation
Still uses the same rectangular pulses as QPSK - the powerspectral density and the spectral efficiency are the same.
This modulation type is used in several standards for mobilecommunications (due to it’s low amplitude variations).
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Offset QPSK (OQPSK)Rectangular pulses
In-phasesignal
Quadraturesignal
There is one bit-time offset between the in-pase and the quadraturepart of the signal (a delay on the Q channel). This makes the transitionsbetween pulses take place at different times!
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Offset QPSKRectangular pulses
Complex representation
This method also creates a
hole in the center, giving less amplitude
variations.
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Offset QAM (OQAM)Raised-cosine pulses
Complex representation
This method also creates a
hole in the center, but has larger amplitude
variations than OQPSK.
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Phase
32π
π
2π
12π
−12π
−π
−32π
−2π
𝑇$ 𝑡
Continuous-phase modulation
Basic idea:- Keep amplitude constant- Change phase continuously
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1 1
1
0
0 0
0
0
In this particular examplewe change the phase ina piecewise linear fashionby +/- p/2, depending onthe data transmitted.
This type of modulationcan be interpreted both as phase and frequencymodulation. It is calledMSK (minimum shift keying) orFFSK (fast frequency shift keying).
MSK/FFSK
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Minimum shift keying (MSK)
Simple MSK implementation
Rectangularpulsefilter
01001
0 1 0 0 1
Voltagecontrolledoscillator(VCO)
MSK signal
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Minimum shift keying (MSK)
Power spectraldensity of MSK
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Gaussian filtered MSK (GMSK)
Further improvement of the phase: Remove ’corners’
MSK(Rectangular pulse filter)
Gaussian filtered MSK - GMSK(Gaussian pulse filter)
(Simplified figure)Phase
32ππ
2π
12π
−12π
−π
−32π
−2π
𝑇$ 𝑡
1 1 1
1 1
1
0 0
0
1 1 1
1 1
1
0 0
0
Phase
32ππ
2π
12π
−12π
−π
−32π
−2π
𝑇$ 𝑡
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Gaussian filtered MSK (GMSK)
Simple GMSK implementation
Gaussianpulsefilter
01001
0 1 0 0 1
Voltagecontrolledoscillator(VCO)
GMSK signal
When implemented this “simple” way, it is usually called Gaussianfiltered frequency shift keying (GFSK).
GSFK is used in e.g. Bluetooth.
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Gaussian filtered MSK (GMSK)
Digital GMSK implementation
-90o
cf( )cos 2 cf tp
( )sin 2 cf tp-
D/A
D/A
Digitalbaseband
GMSKmodulator
Data
AnalogDigital
This is a more precise implementation of GMSK, which is used ine.g. GSM.
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Gaussian filtered MSK (GMSK)
Power spectraldensity of GMSK.
BT = 0.5 here(0.3 in GSM)
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How do we use all these spectral efficiencies?Example: Assume that we want to use MSK to transmit 50 kbit/sec,
and want to know the required transmission bandwidth.
Take a look at the spectral efficiency table:
The 90% and 99% bandwidths become:
90% 50000 /1.29 38.8 kHzB = =
99% 50000 / 0.85 58.8 kHzB = =
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Summary
TABLE 11.1 in textbook.
BPSK withroot-raised
cosinepulses
Another aspect:
802.11ax• 4– 1201Mbit/s• BPSK/QPSK/16-QAM/64-QAM/256-QAM/1024-QAM• Codingrate½,¾,2/3,5/6,…• Guard interval 800,1600,3200ns• Symbolduration3.2,6.4,12.8us• OFDM• MuMIMO• Triggerbased random access,spatialfrequency reuse,• NAV,TWT,…