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RF Channel & Modulation Fundamentals
Prof. Ryszard Struzak National Institute of Telecommunications, Poland
r.struzakATieee.org
The Abdus Salam International Centre for Theoretical Physics ICTP-ITU/BDT School on Sustainable Wireless ICT Solutions for Environmental Monitoring
6 – 22 February 2012, ICTP, Miramare-Trieste, Italy
(CC) R Struzak <[email protected]> 2
Objective
• Review of basic physical processes involved in modern communication systems that may be needed to understand better the further training/teaching activities at ICTP
(CC) R Struzak <[email protected]> 3
Topics for discussion
• Radio transmission • Modulation schemes • Constellation diagrams • Modulation spectra
(CC) R Struzak <[email protected]> 5 Source: URSI RSB No 339 Dec. 2011
(CC) R Struzak <[email protected]> 6
(CC) R Struzak <[email protected]> 8
Radio Transmission System • A message, from a source of messages, is delivered to a
distant destination via radio communication channel using energy of radio waves
» Message in its most general meaning is the object of communication. Depending on the context, the term may apply to both the information contents and its actual presentation, or signal.
» The baseband signal usually consist of a finite set of symbols. E.g. text message is composed of words that belong to a finite vocabulary of the language used. Each word in turn is composed by letters of a (finite) alphabet. (Analog-to-digital conversion)
» The channel consists of a transmitter node, radio propagation path and receiver node.
• The radio propagation process distorts the signal » Predictable vs unpredictable distortions
• The transmitter & receiver pair process the signal to repair the signal distortions following common communication protocol and communication policy
(CC) R Struzak <[email protected]> 9
Volume of information transmitted by isolated communication channel
C = B*log2{1 + [S/(No*B)]}
Bandwidth, Hz
Noise power density, W/Hz Received signal power, W
Capacity (maximum throughput), bit-per-second • The capacity to transfer error-free information is enhanced
with increased bandwidth B, even though the signal-to-noise ratio is decreased because of the increased bandwidth.
• C/B (bps/Hz) → bandwidth efficiency
• The actual throughput can only approach the channel capacity using an extremely complicated encoder. – The statistical properties of the signal approach those
of Gaussian noise and the coding time increases indefinitely.
– Rice (1950) has shown that in order to achieve a data rate which is 96 percent of the channel capacity with S/NoB = 10 and an error probability of 10-5 the number of block encoded channel signals required is 210,000. This is just as impractical as theinfinite coding delay required.
• Source: R. F. Linfield: Radio Channel Capacity Limitations; ITS NTIA
(CC) R Struzak <[email protected]> 10
(CC) R Struzak <[email protected]> 11
Time division headers; interactive & real-time applications that require acknowledgment!
V
Freq
uenc
y sl
ot B
Time slot T
V = B*T*log2{1 + [S/(No*B)]}
Frequency division
(CC) R Struzak <[email protected]> 12
FDD radio links • Two stations can talk and listen to each other at the
same time (time-sharing). • This requires separate synchronized frequency channels
– a technique known as Frequency Division Duplex (FDD)
Station 1 Station 2
FrequencyChannel 1
Frequency Channel 2 TX RX
TX RX
(CC) R Struzak <[email protected]> 13
TDD radio links • Two stations can talk and listen to each other using the
same frequency channel (frequency-sharing), one after another.
• This requires time synchronization/ handshaking – a technique known as Time Division Duplex (TDD)
Station 1 Station 2
Single Frequency Channel TX
RX
TX
RX
(CC) R Struzak <[email protected]> 14
Broadband (INTEL: 100+ Mbps)
INTEL opinion: 100 Mbps necessary! Ch. Legutko (Intel Corp. Wireless Standards & Regulations Mngr): „Regulatory & Spectrum”; presentation, Broadband Forum, Warsaw, July 2006
(CC) R Struzak <[email protected]> 15
!"#$%&'()*+)+#,-*.()&/$/-/01),*$$20/,#-/*0),3#00'&),#%#,/-4))
534(/,#&)),*0(-.#/0-()
6',30*&*1/,#&),*0(-.#/0-()
7'12&#-*.4),*0(-.#/0-()
!,*0*$/,)))))),*0(-.#/0-()
8#09:/9-3);*/(')!0</.*0$'0-#&)(/10#&()
=%2./*2()'$/((/*0))>).','%-/*0),3#00'&()?#.9:#.')0*0&/0'#./-/'()?#.9:#.')/0(-#@/&/-/'())
A.'B2'0,4)#((/10$'0-()5*:'.)&/$/-#-/*0()=/--/01).'(-./,-/*0()
C*(-))
)
• The fundamental physical limitations are bandwidth and noise.
• The bandwidth limits the number of signaling elements or symbols which can be transmitted per unit time.
• The noise limits the number of distinguishable features on each symbol.
• The combined effects limit the capacity of a communication system.
(CC) R Struzak <[email protected]> 16
(CC) R Struzak <[email protected]> 17
Envi
ronm
ent
Radio Channel Model
T-antenna
Propagation medium
R-antenna Noise, Fading,
Original message/ data
Reconstructed message/ data
EM waves: time- distance- direction- polarization
Receiver Time series Processing/ De-coding
Transmitter Coding/ Processing Time series
(CC) R Struzak <[email protected]> 18
SISO, SIMO, MISO, MIMO
Tx Rx SISO
Tx Rx SIMO
1
n
Tx Rx
1
m
MISO
Tx Rx
n
1 1
m
MIMO
Oestges C, Clerckx B, "MIMO Wireless Communications : From Real-world Propagation to Space-time Code Design," Academic, 2007 http://en.wikipedia.org/wiki/MIMO
(CC) R Struzak <[email protected]> 19
Radio transmitter
1. Generates a RF carrier 2. Combines it with the baseband signal into a
RF signal through modulation • Performs additional operations E.g. analog-to-digital
conversion, formatting, coding, spreading, adding additional messages/ characteristics such as error-control, authentication, or location information
3. Radiates the resultant signal in the form of modulated radio wave
- it maps the original message into the radio-wave signal launched at the transmitting antenna
(CC) R Struzak <[email protected]> 20
Propagation process
• Transforms (maps) the radio-wave launched by the transmitter into the incident radio wave at the receiver antenna
• Depends on extra variables (e.g. distance), additional radio waves (e.g. reflected wave, environmental waves), random uncertainty additive (noise), multiplicative (fading), etc.
• A separate talk
(CC) R Struzak <[email protected]> 21
Radio receiver: 1. Extracts the wanted signal from the
incident radio waves (a solid “window” in the signal hyperspace)
2. Recovers the original message through • reversing the transmitter operations
(demodulation, decoding, de-spreading, etc.),
• compensating propagation transformations, and
• correcting transmission distortions
• Maps the incident signals into the recovered message
• Coexistence - Separate talk!
Receiver +
Wanted signal
Unwanted signals
Noise
Unwanted signals and nonlinearities are often ignored!
(CC) R Struzak <[email protected]> 22
Modulation & Demodulation
• Modulation - translation the baseband message signal to radio frequencies
• Demodulation -- the reverse process) – Single carrier:
• continuous (sinusoidal), • Walsh functions (http://mathworld.wolfram.com/WalshFunction.html)
• Random carriers (http://en.wikipedia.org/wiki/Category:Radio_modulation_modes) – Multiple carriers (e.g. OFDM)
(CC) R Struzak <[email protected]> 23
Modulation Process
• Moves information from baseband to RF • Implies varying one or more characteristics
(modulation parameters a1, a2, … an) of a carrier f in accordance with the information-bearing (modulating) baseband signal.
( )1 2 3
1 2 3
, , ,... , (= carrier), , ,... (= modulation parameters)
(= time)
n
n
f f a a a a ta a a at
=
(CC) R Struzak <[email protected]> 24
Frequency translation
• Signal "at baseband" comprises all frequency components carrying information. Modulation shifts the signal up to RF frequencies to allow for radio transmission. Usually, the process increases the signal bandwidth. Steps are often taken to reduce this effect, such as filtering the RF signal prior to transmission.
Signal's baseband bandwidth is its bandwidth before modulation and multiplexing, or after demultiplexing and demodulation
(CC) R Struzak <[email protected]> 25
-50
-40
-30
-20
-10
0
-50 -40 -30 -20 -10 0 10 20 30 40 50
Frequency from centre, MHz
dB
802.11g
(CC) R Struzak <[email protected]> 26
802.11b/g channels
1. 2,412 2. 2,417 3. 2,422 4. 2,427 5. 2,432 6. 2,437 7. 2,442
8. 2,447 9. 2,452 10. 2,457 11. 2,462 12. 2,467 13. 2,472 14. 2,484
• Different subsets of these channels are made available in various countries • Spacing: ~5 (12) MHz • Occupied bandwidth: ~22 MHz (802.11b), ~16.6 MHz (802.11g)
(Center frequencies in GHz)
Source: R Morrow: Wireless network coexistence, p. 198
(CC) R Struzak <[email protected]> 27
Why translation?
• Effective radiation of EM energy requires antenna dimensions to be comparable with the wavelength: – 3 kHz: wavelength 100 km – 3 GHz: wavelength 10 cm
• Allows shared use of the radio transmission medium
(CC) R Struzak <[email protected]> 28
Continuous sinusoidal carrier
• Amplitude modulation (AM) – A = A(t) – ω = const – ϕ = const
Phase modulation (PM) – A = const – ω = const – ϕ = ϕ(t)
A sin[ωt +ϕ]; A, ω, or/and ϕ = var.
• Frequency modulation (FM) – A = const – ω = ω(t) – ϕ = const
(CC) R Struzak <[email protected]> 29
AM, FM, PM
The modulating signal superimposed on the carrier wave & the resulting modulated signal. [wikipedia]
Java simulation: http://www.home.agilent.com/upload/cmc_upload/All/Amplitude_Modulation.htm?cmpid=zzfindnw_amplitudemod
(CC) R Struzak <[email protected]> 30
Modulation Spectra • The Nyquist bandwidth is
the minimum bandwidth that can represent a signal (within an acceptable error)
• The spectrum occupied by a signal should be as close as practicable to that minimum, otherwise adjacent channel interference occur
• The spectrum occupied by a signal can be reduced by application of filters
Nyquist Minimum Bandwidth
Frequency
Rel
ativ
e M
agni
tude
(dB
) Adjacent Channel
(CC) R Struzak <[email protected]> 31
Digital modulation • Modulation that uses a finite number of distinct states (signals) to
represent data • Classic types:
– ASK (Amplitude-shift keying): uses a finite number of amplitudes; – FSK (Frequency-shift keying): uses a finite number of frequencies – PSK (Phase-shift keying):uses a finite number of phases – QAM (Quadrature amplitude modulation): uses at least 2 phases,
and at least 2 amplitudes » PSK can be regarded as a special case of QAM (magnitude of the signal is
a constant and the phase is varying. This can also be extended to FSK as a special case of phase modulation.
• Other: » Multiple-carrier modulation, » Carrierr-less modulation, » Walsh-type modulation » Differential modulation
• Combinations of the above.
(CC) R Struzak <[email protected]> 32
Effect of pulse width on spectral content. The shorter pulse width the wider the spectrum
(CC) R Struzak <[email protected]> 33
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Differential Modulation • At transmitter: each symbol is modulated
relative to the previous symbol and modulating data
• DPSK (Differential Phase-Shift Keying) example: 0 = no change; 1 = change phase by +1800
• At receiver: the current symbol is reproduced using the previous symbol as reference.
• The previous symbol serves as an estimate of the channel. A no-change condition causes the modulated signal to remain at the same 0 or 1 state as the previous symbol.
• Differential modulation has 2 sources of error: • a corrupted current symbol, and corrupted reference (the
previous symbol)
(CC) R Struzak <[email protected]> 35
ASK: Amplitude Shift Keying
• Pulse shaping can be employed to remove spectral spreading • ASK demonstrates poor performance, as it is heavily affected by
noise, fading, and interference during propagation • Can be expanded to M-ary scheme, with multiple amplitudes
Baseband Data
ASK (binary) signal
1 1 0 0 0
Acos(ωt) Acos(ωt)
(CC) R Struzak <[email protected]> 36
FSK: Frequency Shift Keying
• Example: The ITU-T V.21 modem standard uses FSK • BFSK can be expanded to M-ary scheme, with multiple frequencies
Baseband Data
BFSK (Binary Frequency
Shift Keying) signal
1 1 0 0
s0 =Acos(ωc-Δω)t; s1 =Acos(ωc+Δω)t
f0 f0 f1 f1
(CC) R Struzak <[email protected]> 37
PSK: Phase Shift Keying
• BPSK – better performance than ASK and BFSK • Drawback: rapid amplitude change between symbols due to phase
discontinuity requires infinite bandwidth. • BPSK can be expanded to M-ary scheme, with multiple phases
Baseband Data
BPSK (Binary Phase Shift Keying)
signal
1 1 0 0
s0 =-Acos(ωct); s1 =Acos(ωct) s0 s0 s1 s1
(CC) R Struzak <[email protected]> 38
• Phase Shift Keying (PSK) is often used as it provides efficient use of RF spectrum. π/4 QPSK (Quadrature PSK) reduces the envelope variation of the signal.
• High level M-array schemes (such as 64-QAM) are bandwidth-efficient but susceptible to noise and require linear amplification
• Constant envelope schemes (e.g. GMSK) allow for non-linear power-efficient amplifiers
• Coherent reception provides better performance but requires a more complex receiver
(CC) R Struzak <[email protected]> 39
Constellation diagram
= graphical representation of the amplitude and phase of each possible symbol state in time domain – The x-axis represents the in-phase
component and the y-axis the quadrature component of the complex envelope
– The distance between signal states on a constellation diagram indicates
• how different the modulation waveforms are • how easily a receiver can differentiate between
them.
(CC) R Struzak <[email protected]> 40
Example: BPSK Constellation Diagram
(|1|, -180o)
Q
I (|1|, 0o)
(CC) R Struzak <[email protected]> 41
QPSK
• Quadrature Phase Shift Keying (QPSK) can be interpreted as two independent BPSK systems (one on the I-channel and one on Q), and thus the same performance but twice the bandwidth efficiency
• Large envelope variations occur due to abrupt phase transitions, thus requiring linear amplification
(CC) R Struzak <[email protected]> 42
QPSK Constellation Diagram
• Quadrature Phase Shift Keying has twice the bandwidth efficiency of BPSK since 2 bits are transmitted in a single modulation symbol
Carrier phases {0, π/2, π, 3π/2}
Carrier phases {π/4, 3π/4, 5π/4, 7π/4}
Q
I I
Q
(CC) R Struzak <[email protected]> 43
QPSK examples
• All QPSK schemes require linear power amplifiers • Conventional QPSK has transitions through zero (i.e. 1800 phase transition)
which requires highly linear amplifiers. • In Offset QPSK, the phase transitions are limited to 900, the transitions
transitions through zero do not occur. • In π/4 QPSK transitions through zero do not occur. This scheme produces
the lowest envelope variations.
I
Q
I
Q
I
Q
Conventional QPSK π/4 QPSK Offset QPSK
(CC) R Struzak <[email protected]> 44
Multi-level (M-ary) Phase and Amplitude Modulation
• Amplitude and phase shift keying can be combined to transmit several bits per symbol.
– Often referred to as linear as they require linear amplification. – More bandwidth-efficient, but more susceptible to noise.
• For M=4, 16QAM has the largest distance between points, but requires very linear amplification. 16PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise.
16 QAM 16 APSK 16 PSK
(CC) R Struzak <[email protected]> 45
• Animations:
• http://www.home.agilent.com/upload/cmc_upload/All/IQ_Modulation.htm?cmpid=zzfindnw_iqmod
• http://www.home.agilent.com/agilent/editorial.jspx?cc=PL&lc=eng&ckey=1756523&nid=-34944.0&id=1756523
(CC) R Struzak <[email protected]> 46
Comparison
Modulation Format
C/B (bandwidth efficiency)
„Error-free” Eb/No
16 PSK 4 18 dB 16 QAM 4 15 dB 8 PSK 3 14.5 dB 4 PSK 2 10 dB 4 QAM 2 10 dB BFSK 1 13 dB BPSK 1 10.5 dB
(CC) R Struzak <[email protected]> 48
Carrier-less or Pulse-Carrier
• Carrier: A train of identical pulses regularly spaced in time
(CC) R Struzak <[email protected]> 49
(CC) R Struzak <[email protected]> 50
Ultra-Wideband (UWB) Systems • Radio systems that use time-domain processing
(e.g., pulse-position modulation) for communications, or sensing applications. – e.g. short-range radar; – Also called carrier-less or impulse systems; – Occupy frequency bandwidth >25% of the center
frequency or >1.5 GHz. – Use narrow pulses
• Of the order of 1 to 10 nanoseconds.
(CC) R Struzak <[email protected]> 51
Multiple-carrier systems example • OFDM = Orthogonal Frequency Division Multiplex • Basic idea:
– Using a large number of parallel narrow-band sub-carriers instead of a single wide-band carrier to transport information
• Advantages – Efficient in dealing with multi-path and selective fading – Robust again narrow-band interference
• Disadvantages – Sensitive to frequency offset and phase noise – Peak-to-average problem reduces the power efficiency of RF
amplifier at the transmitter • Adopted for various standards
• DSL, 802.11a, DAB, DVB
(CC) R Struzak <[email protected]> 52
OFDM
– Selective Fading – Very short pulses
pulse length ~1/B
– Flat Fading per carrier – N long pulses, each
of the length ~ N/B
N carriers
B
– Data are shared among several carriers and simultaneously transmitted
B
– Data are transmited using only one carrier
Frequency domain
(CC) R Struzak <[email protected]> 53
OFDM 2 Inverse Fourier
Transform IFFT
Parallel To Serial converter
Serial to Parallel
converter
Fourier Transform
FFT Transmission
Data coded in frequency domain one symbol at a time
Data in time domain one symbol at a time
Transmit time-domain samples of one symbol
Decode each
frequency bin
independently
More information: http://www.iss.rwth-aachen.de/Projekte/Theo/OFDM/node2.html
(CC) R Struzak <[email protected]> 54
SS communications basics
Original information
Propagation effects Transmission
Reconstructed information
Original signal Spread signal
Spread signal+ Reconstr. signal
Spreading
De-spreading
Unwanted signals + Noise
(CC) R Struzak <[email protected]> 55
Example: GPS • RF bandwidth Bc ~ 2MHz
Filter bw. Bm ~100 Hz
• Processing gain: (2Mhz) / (100Hz) = = 20’000 (~+43 dB)
• Input S/N = -20 dB (signal = 1% of noise [W])
• Output S/N = +23 dB (signal = 200 x noise [W])
(CC) R Struzak <[email protected]> 56
SS techniques
• FH: frequency hoping (frequency synthesizer controlled by pseudo-random sequence of numbers)
• DS: direct sequence (pseudo-random sequence of pulses used for spreading)
• TH: time hoping (spreading achieved by randomly spacing transmitted pulses)
• Random noise as carrier • Other techniques (radar and other applications)
• Combination of the above
(CC) R Struzak <[email protected]> 57
SS: basic characteristics
• Signal spread over a wide bandwidth >> minimum bandwidth necessary to transmit information
• Spreading by means of a code independent of the data
• Data recovered by de-spreading the signal with a synchronous replica of the reference code – TR: transmitted reference (separate data-channel and reference-channel, correlation
detector)
– SR: stored reference (independent generation at T & R pseudo-random identical waveforms, synchronization by signal received, correlation detector)
– Other (MT: T-signal generated by pulsing a matched filter having long, pseudo-randomly controlled impulse response. Signal detection at R by identical filter & correlation computation)
(CC) R Struzak <[email protected]> 58
SS Processing Gain =
= [(S/ I)in/ (S/ I)out ] = ~Bc/ Bm
Example: GPS signal RF bandwidth Bc ~ 2MHz Filter bandwidth Bm ~ 100 Hz Processing gain: (2Mhz)/(100Hz) = 20’000 (~+43 dB) Input S/N = -20 dB (signal power = 1% of noise power) Output S/N = +23 dB (signal power = 200 x noise power) (GPS = Global Positioning System)
(CC) R Struzak <r.struzakATieee.org> 59
• Beware of misprints!!! These materials are preliminary notes intended for my lectures only and may contain misprints. Feedback is welcome. If you notice serious faults, or you have an improvement suggestion, please describe it exactly and I will try to modify the materials.
• This work is licensed under the Creative Commons Attribution License (http://creativecommons.org/ licenbses/by/1.0) and may be used freely for individual study, research, and education in not-for-profit applications. Any other use requires the written author’s permission. If you cite these materials, please credit the author, title, and place.
• These materials and any part of them may not be published, copied to or issued from another Web server without the author's written permission.
• Copyright © 2012 Ryszard Struzak.