radiocommunication channel and digital modulation:...
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ICTP-ITU/BDT-URSI School on Radio-Based Computer Networking for Research and Training in Developing Countries The Abdus Salam International Centre for Theoretical Physics ICTP, Trieste (Italy), 7th February - 4th March 2005
Radiocommunication Channel and Digital Modulation: Basics
Prof. Dr. R. Struzakr.struzak@ieee.org
Note: These are preliminary notes, intended only for distribution to participants. Beware of misprints!
Outline
• Radiocommunication channel• Modulation • Spreading spectrum • Nonlinearities & intermodulation• Summary
Property of R. Struzak 2
Microwave radio link
Property of R. Struzak 3
Property of R. Struzak 4
Wireless Local Loop
Property of R. Struzak 5
BSS
PTP, PMP, Mesh• A point-to-point (PTP) link is one station (node)
communicating with another one • A point-to-multipoint (PMP) network is one node
(base station) communicating with more than one other nodes
• Mesh network (fully connected): A network topology in which there is a direct communication path between any two nodes
• Mesh and PMP topologies share communication resources (media)• In a fully connected network with n nodes, there are n(n-1)/2 direct paths, i.e.,
branches.
Property of R. Struzak 6
FDD radio links• Two stations can talk and listen to each other at the same
time (time-sharing). • This requires separate (static) frequency channels – a
technique known as Frequency Division Duplex (FDD)
Property of R. Struzak 7
Station 1TX RX
TXRX
Frequency Channel 2
Station 2
FrequencyChannel 1
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 ChannelTX
RX
TX
RX
Property of R. Struzak 8
Radio Link model
Envi
ronm
ent
T-antenna
Propagation medium
R-antenna
Noise
Original message/ data
Reconstructed message/ data
EM waves:time-distance-direction-polarization
Receiver Time seriesProcessing/De-coding
TransmitterCoding/ProcessingTime series
Property of R. Struzak 9
Transmitting station
Electrical signal is represented by a function of time.Radio wave transmitted is represented by a function of time, distance, direction, and polarization.
Radio wave
Transmittingantenna
RF cable (signal attenuation)
Original signal
Transmitter (signal processing)
Electrical current EM wave
Focus of the school
Property of R. Struzak 10
Receiving station
Radio wave
Receivingantenna
RF cable (signal attenuation)
Recovered signal
Receiver (signal processing)
Electrical currentEM wave
Focus of the school
Radio wave received is represented by a function of time, distance, direction, and polarization that depends on signal-path environmentElectrical signal is represented by a function of time.
Property of R. Struzak 11
Modern radio: details
BASEBAND PROCESSING& PC INTERFACEDAC
CRYSTALREFERENCE
FILTER
SYNTHETIZER
ADC
FILTER
IFFILTER
SYNTHETIZER
RF FILTER RF FILTER
ANTENNA
RF A
MP
LIFI
CA
TIO
NR
F U
P/ D
OW
NC
ON
VE
RS
ION
IF G
AIN
& S
ELE
CTI
VIT
Y
SWITCH
TRANSMITTER COMMON PART RECEIVER
MO
DU
LATI
ON
& D
EM
OD
ULA
TIO
N
Property of R. Struzak 12
• Modern radio = combination of radio and computer hardware & software– Software-defined
radio• Systems with
most functions defined by software
• Automatically and/or at distance
RADIO WAVE PROPAGATION PATH
Beamforming
Freq. spread
Modulation
Multiplex
Format
Encryption
Encoding
Analog/Digital
Information source
Beamforming
Freq. despread
Demodulation
Demultiplex
Format
Decryption
Decoding
Digital/ Analog
Information sink
From
oth
er s
ourc
es
To o
ther
des
tinat
ions
Property of R. Struzak 13
Outline
• Radiocommunication channel• Modulation • Spreading spectrum • Nonlinearities & intermodulation• Summary
Property of R. Struzak 14
Modulation• = process of translation
the message from baseband signal to bandpass (modulated carrier) signal at frequencies that are very high compared to the baseband frequencies.
• Demodulation is the reverse process – Note: An information-bearing
signal is non-deterministic, i.e. it changes in an unpredictable manner.
Modulator/Signal Processing
Carrier Generator
m(t)
s(t)
f(t)
s'(t)
Demodulator/Signal Processing
m'(t)
RADIO ENVIRONMENT
TRANSMITTER RECEIVER
Property of R. Struzak 15
Communication Channel
Original messagem(t)
Transmitters(t) = U(m, f) m(t) = message (information, data)
s(t) = signal carrying the messagef = f(a,b,c,…, t) (carrier function)a,b,c, … = modulation parametersU, V, W = operatorsξ = noise, fading, perturbationsx(t) = perturbed signal at the receiver inputy(t) = reproduced message
Task: make y ≈ m (within an acceptable error)
Transport mediumx(t) = V(s, ξ)
Receivery(t) = W(x)
Reproduced (received) message
y = W{V[ξ,U(m,f)]}Property of R. Struzak 16
Modulation Process( )1 2 3
1 2 3
, , ,... , (= carrier), , ,... (= modulation parameters)
(= time)
n
n
f f a a a a ta a a at
=
• Modulation implies varying one or more characteristics (modulation parameters a1, a2, … an) of a carrier f in accordance with the information-bearing(modulating) baseband signal
• Each of the parameters a, b, c... carrying information can be modulated independently, increasing communication capacity at a cost of complexity.
Property of R. Struzak 17
• The carrier is generated in the transmitter • It may be a continuous (e.g. sinusoidal) current of
radio frequency, a sequence of short pulses, or noise– Systems using pulse sequences are also called
carrierless or impulse systems• It may also be a number of carriers, such as in
Orthogonal Frequency Division Multiplexing (OFDM) systems.
• For instance, one of standards Wireless Local Area Networks (WLANs) foresees 52 carriers spaced 312.5 kHz apart
Property of R. Struzak 18
Why Carrier?
• To radiate EM waves effectively– Radiation efficiency requires antenna dimensions to be
comparable with the radiated wavelength• Antenna for 30 kHz would be 10 km long• Antenna for 3 GHz carrier is 10 cm long
• To assure signal orthogonality (avoiding mutual interference by using orthogonal frequencies) – Note: There are also other methods of avoiding
interference (e.g. time- or code-orthogonality)• Standards and RR impose limitations on carrier
frequencies (interference, intercommunications)
Property of R. Struzak 19
Property of R. Struzak 20
Property of R. Struzak 21
Continuous carrier• In the case of sinusoidal carrier, three modulation
parameters can be varied: the amplitude, the frequency, and the phase of the sinusoid (+ polarization). This generates three distinct modulation types: the amplitude modulation (AM), the frequency modulation (FM) and the phase modulation (PM)
• Each of these may be continuous, when the instantaneous amplitude, frequency and phase of the sinusoid are continuous functions of time, or may be pulsed, when the variations occur instantaneously
Property of R. Struzak 22
• Amplitude modulation (AM)– A = A(t) – ω = const – ϕ = const
• Frequency modulation (FM)– A = const– ω = ω(t) – ϕ = const
• Phase modulation (PM)– A = const– ω = const – ϕ = ϕ(t)
• Polarization modulation – Not used in radio
communications(used in some optical communications)
Carrier: A sin[ωt +ϕ] + polarztn.; A, ω, ϕ, polarztn. = const
Property of R. Struzak 23
Amplitude Shift Keying (ASK)
Property of R. Struzak 24
1 10
Baseband Data
0 0ASK
modulated signal
Acos(ωt) Acos(ωt)
• Pulse shaping can be employed to remove spectral spreading• ASK demonstrates poor performance, as it is heavily affected by noise,
fading, and interference
Frequency Shift Keying (FSK)
Property of R. Struzak 25
1 10 0
Baseband Data
BFSK modulated
signalf0 f0f1 f1
where f0 =Acos(ωc-∆ω)t and f1 =Acos(ωc+∆ω)t
• Example: The ITU-T V.21 modem standard uses FSK • FSK can be expanded to a M-ary scheme, employing multiple
frequencies as different states
Phase Shift Keying (PSK)
Property of R. Struzak 26
1 10 0
s0 s0s1 s1
Baseband Data
BPSK modulated
signal
where s0 =Acos(ωct) and s1 =Acos(ωct + π) • Major drawback – rapid amplitude change between symbols due to phase
discontinuity, which requires infinite bandwidth. Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and BFSK
• BPSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states
PSK graphic representationThe two signals
s0 =Acos(ωt) s1 =Acos(ωt + π) can be represented by two vectors (or points) in the signal plane [Re(s), Im(s)]
• Noise & interference can change positions of the points and modify decision: 0 or 1
A-A
Im(s)
Re(s)
Decision: s = s0 Decision: s = s1
10
Property of R. Struzak 27
Differential Modulation
• In the transmitter, each symbol is modulated relative to the previous symbol and modulating signal, for instance in BPSK 0 = no change,
1 = +1800
• In the receiver, the current symbol is demodulated using the previous symbol as a 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 of the previous symbol.
Property of R. Struzak 28
• DPSK = Differential phase-shift keying: In the transmitter, each symbol is modulated relative to the phase of the immediately preceding signal element transmitted
• Differential modulation is theoretically 3dB poorer than coherent. This is because the differential system has 2 sources of error: a corrupted symbol, and a corrupted reference (the previous symbol)
Property of R. Struzak 29
Pulse trains as ‘Carrier’• A ‘carrier” = a train of identical
pulses regularly spaced in time• Example 2003 : Ultra Wideband
(UWB) systems– Systems that use time-domain
modulation and signal processing methods (e.g., pulse-position modulation)
– Used for sensing, short-range radar, and telecommunication applications
– Employ short pulses (duration of ~1 to 10 ns), occupying the bandwidth of more than 1.5 GHz (or more than 25% of the center frequency)
Property of R. Struzak 30
• In pulse-frequency modulation (PFM), the pulse repetition rate is varied in accordance with the modulating signal; in Pulse-Amplitude Modulation (PAM), the amplitude of individual pulses in the pulse train is varied
• In pulse-time modulation (PTM) generic class, the time of occurrence of some characteristic of the pulsed carrier is varied, eg. Duration (PDM) or position (PPM)
Property of R. Struzak 31
• In pulse-position modulation (PPM), the temporal positions of individual pulses are varied in relation to the reference positions, in accordance the modulating signal
Property of R. Struzak 32
• Noise (random processes), and pseudo-random processes can also be used as ‘carriers’– Example: spread-spectrum systems – In some systems, the carrier and modulation format
change during the transmission
• Independently of the modulation type, spectra of signals used in radiocommunications are, contained between 9 kHz and 275 GHz, as defined in ITU Radio Regulations
Property of R. Struzak 33
Demodulation & Detection
• Demodulation– Is process of removing the carrier signal to
obtain the original signal waveform• Detection – extracts the symbols from the
waveform– Coherent detection– Non-coherent detection
Property of R. Struzak 34
Coherent (synchronous) Detection
• Signal change introduced by the channel (phase and attenuation) is estimated. It is then possible to reproduce the transmitted signal and demodulate.
• Requires a replica carrier wave of the same frequency and phase to be delivered at the receiver.
• The received signal and replica carrier are cross-correlated using information contained in their amplitudes and phases.
Property of R. Struzak 35
• Carrier recovery methods include– Pilot Tone (such as Transparent Tone in Band)
• Less power in the information bearing signal, High peak-to-mean power ratio
– Carrier recovery from the information signal • E.g. Costas loop
• Applicable to – Phase Shift Keying (PSK)– Frequency Shift Keying (FSK)– Amplitude Shift Keying (ASK)
Property of R. Struzak 36
Non-Coherent Detection
• Requires no reference wave; does not exploit phase reference information (envelope detection)
• Applicable to– Differential Phase Shift Keying (DPSK)– Frequency Shift Keying (FSK)– Amplitude Shift Keying (ASK)
• Non coherent detection is less complex than coherent detection (easier to implement), but has worse performance.
Property of R. Struzak 37
Geometric Representation
• Digital modulation involves choosing a particular signal si(t) form a finite set S of possible signals.
• For binary modulation schemes a binary information bit is mapped directly to a signal and S contains only 2 signals, representing 0 and 1.
• For M-ary keying S contains more than 2 signals and each represents more than a single bit of information. With a signal set of size M, it is possible to transmit up to log2M bits per signal.
Property of R. Struzak 38
• Any element of set S can be represented as a point in a vector space whose coordinates are basis signals φj(t) such that
( ) ( )
( )
( ) ( )
2
1
0, ; (= are orthogonal)
1; ( normalization)
Then
i j
i
N
i ij jj
t t dt i j
E t dt
s t s t
φ φ
φ
φ
∞
−∞
∞
−∞
=
= ≠
⎡ ⎤= = =⎣ ⎦
=
∫
∫
∑Property of R. Struzak 39
Example: BPSK Constellation Diagram
( ) ( ) ( ) ( )
( ) ( )
1 2
1
2 2cos 2 , cos 2 ; ; 0
energy per bit; bit period For this signal set, there is a single basic signal
2 cos 2 ; 0
b bBPSK c c b
b b
b b
c bb
BPSK
E ES s t f t s t f t t TT T
E T
t f t t TT
S E
π π
φ π
⎧ ⎫⎡ ⎤ ⎡ ⎤⎪ ⎪= = = − ≤ ≤⎢ ⎥ ⎢ ⎥⎨ ⎬⎢ ⎥ ⎢ ⎥⎪ ⎪⎣ ⎦ ⎣ ⎦⎩ ⎭
= =
= ≤ ≤
= ( ) ( ){ }1 1,b bt E tφ φ⎡ ⎤ ⎡ ⎤−⎣ ⎦ ⎣ ⎦ -√Eb √Eb
Q
I
Constellation diagram
Property of R. Struzak 40
Constellation diagram
= graphical representation of the complex envelope of each possible symbol state – The x-axis represents the in-phase component
and the y-axis the quadrature component of the complex envelope
– The distance between signals on a constellation diagram relates to how different the modulation waveforms are and how easily a receiver can differentiate between them.
Property of R. Struzak 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
Property of R. Struzak 42
QPSK Constellation Diagram
Carrier phases {0, π/2, π, 3π/2}
Q
Carrier phases {π/4, 3π/4, 5π/4, 7π/4}
I
Q
I
• Quadrature Phase Shift Keying has twice the bandwidth efficiency of BPSK since 2 bits are transmitted in a single modulation symbol
Property of R. Struzak 43
Types of QPSK
I
Q
Property of R. Struzak 44
• Conventional QPSK has transitions through zero (i.e. 1800 phase transition). Highly linear amplifiers required.
• In Offset QPSK, the phase transitions are limited to 900, the transitions on the I and Q channels are staggered.
• In π/4 QPSK the set of constellation points are toggled each symbol, so transitions through zero cannot occur. This scheme produces the lowest envelope variations.
• All QPSK schemes require linear power amplifiers
I
I
Conventional QPSK Offset QPSK π/4 QPSK
Multi-level (M-ary) Phase and Amplitude Modulation
16 QAM 16 PSK 16 APSK
• 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.
• Simulation: http://www.educatorscorner.com/index.cgi?CONTENT_ID=2478Property of R. Struzak 45
Decision region
Property of R. Struzak 46
DistortionsDecision region
Perfect channel White noise Phase jitter
Property of R. Struzak 47
Eye Diagram• Eye pattern is an oscilloscope
display in which digital datasignal from a receiver is repetitively superimposed on itself many times
– (sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep).
• It is so called because the pattern looks like a series of eyes between a pair of rails.
Time (symbols)
Mag
nitu
de
•If the “eye” is not open at the sample point, errors will occur due to signal corruption
Property of R. Struzak 48
GMSK• Gaussian Minimum Shift Keying (GMSK) is a form of
continuous-phase FSK in which the phase change is changed between symbols to provide a constant envelope. Consequently it is a popular alternative to QPSK
• The RF bandwidth is controlled by the Gaussian low-pass filter bandwidth. The degree of filtering is expressed by multiplying the filter 3dB bandwidth (B) by the bit period of the transmission (T), i.e. by BT
• GMSK allows efficient class C non-linear amplifiers to be used
Property of R. Struzak 49
Outline
• Radiocommunication channel• Modulation • Spreading spectrum • Nonlinearities & intermodulation• Summary
Property of R. Struzak 50
Modulation Spectra• The Nyquist bandwidth is the
minimum bandwidth that can carry a given volume of information
• The spectrum occupied by a signal is usually larger and spill over adjacent channels causing interference
• The spectrum occupied by a signal can be reduced by application of filters
• Technical standards and RR impose limits on spectral masks
Nyquist MinimumBandwidth
Frequency
Rel
ativ
e M
agni
tude
(dB
) AdjacentChannel
Property of R. Struzak 51
Capacity of communication system
C = B*log2{1 + [S/(No*B)]}
Bandwidth, Hz
Noise density, W/HzReceived signal power, W
Capacity, bit/s
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.
Property of R. Struzak 52
SS communications basics
Originalinformation
Propagation effects Transmission
Reconstructedinformation
Original signal Spread signal
Spread signal+ Reconstr. signal
Spreading
De-spreading
Unwanted signals + Noise
Property of R. Struzak 53
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)
Property of R. Struzak 54
SS communication 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• Hybrid combination of the above• Other techniques (radar and other applications)
Property of R. Struzak 55
Multiple-access techniques
• FDMA: frequency-division multiple access• TDMA: time-division multiple access• CDMA: code-division multiple access• Other (e.g. OFDM)
Property of R. Struzak 56
FDMA
FrequencyTime
Power densityFDMA
Freq
uenc
y
Property of R. Struzak 57
TimeFrequency channel
Bm
Bc
Transmission is organizedin frequency channels. Each link is assigned a separate channel.
Example: Telephony Bm = 3-9 kHz
TDMA
Time
Power densityTDMA
Frequency
Property of R. Struzak 58
Freq
uenc
y
TimeTime slot
Time-frame
Transmission is organized in repetitive “time-frames”. Each frame consists of groups of pulses - time slots. Each user/ link is assigned a separate time-slot.
Example: DECT (Digital enhanced cordless phone) Frame lasts 10 ms, consists of 24 time slots (each 417µs)
FH SS (CDMA)
Property of R. Struzak 59
Freq
uenc
y
CDMA
Time
Power density
Frequency
Time-frequency slot
BmBc
Transmission is organized in time-frequency “slots”. Each link is assigned a sequence of the slots, according to a specific code.
Time
DS SS: transmitter
Modulator X Antenna
[A(t), ϕ(t)]Information
[g1(t)]
Modulated signalS1(t) = A(t) cos(ω0t + ϕ(t))band Bm Hz
Spread signal g1(t)S1(t)band Bc HzBc >> Bm
Carriercos(ω0t)
gi(t): pseudo-random noise (PN) spreading functions that spreads the energy of S1(t) over a bandwidth considerably wider than that of S1(t): ideally gi(t) gj(t) = 1 if i = j and gi(t) gj(t) = 0 if i ≠ j
Property of R. Struzak 60
DS SS-receiver
Spreading function
[g1(t)]
Correlator &
bandpassfilter
Xantenna
To d
emod
ulat
or
Linearcombinationg1(t)S1(t)g2(t)S2(t)…….gn(t)Sn(t)N(t) (noise)S’(t)
g1(t) g1(t)S1(t)g1(t) g2(t)S2(t)…….g1(t) gn(t)Sn(t)g1(t) N(t)g1(t) S’(t)
S1(t)
Property of R. Struzak 61
SS-receiver’s InputUnwanted signalsSS s.: g2(t)S2(t); …; gn(t)Sn(t)Other s. : S’(t)Noise: N(t)
W/Hz
Wanted (spread) signal: g1(t)S1(t)
BcHz
Signal-to-interference ratio (S/ I)in = S/ [I(ω)*Bc]
Property of R. Struzak 62
Bc = Input correlator bandwidthI(ω) = Average spectral power density of unwanted signals in BcS = Power of the wanted signal
SS-correlator/ filter output
Bm
Bc
Property of R. Struzak 63
(S/ I)out = S/ [I(ω)*Bm]Bc = Input correlator bandwidth Bm = Output filter bandwidthI(ω) = Average spectral power density of unwanted signals & noise in BmS = power of the wanted signal at the correlator output
Wanted (correlated) signal: de-spread to its original bandwidthas g1(t) g1(t)S1(t) = S1(t) with g1(t) g1(t) = 1
Uncorrelated (unwanted) signalsspread & rejected by correlator + noiseg1(t) S’(t); g1(t) N(t); g1(t) gj(t)Sj(t) = 0
as gi(t) gj(t) = 0 for i ≠ j
Signal-to-interference ratio
Spreading = reducing spectral power density
SS Processing Gain =
= [(S/ I)in/ (S/ I)out ] = ~Bc/ Bm
Example: GPS signalRF bandwidth Bc ~ 2MHz Filter bandwidth Bm ~ 100 Hz
Processing gain ~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)Property of R. Struzak 64
Outline
• Radiocommunication channel• Modulation • Spreading spectrum • Nonlinearities & intermodulation• Summary
Property of R. Struzak 65
• Sources of undesired nonlinear effects:– Receiver RF input/ mixing stage– Transmitter output stages – Vicinity of the equipment (usually of the
transmitter)
Property of R. Struzak 66
• Types of undesired nonlinear effects:– Receiver blocking – Transmitter spurious radiations & intermodulation– Receiver spurious responses & intermodulation
• Note: Several nonlinear interactions may occur simultaneously
» (Source: ITU/ CCIR Rep. 524-1, Vol. 1, p. 30, 1986)
Property of R. Struzak 67
• Non-ideal wideband memory-less linear devices are often treated by expressing the output (Y) of the system as a power series of the total input signal X:Y a a X a X a X a Xn
n= + + + + + +0 1 22
33 ... ...
X(t) Y(t)
X(t) = A1sin(w1t) + A2sin(w2t) + …The coefficients a are presumed to be real and independent on X.
Property of R. Struzak 68
• In practice, one focus only on those impairments that fall in the desired frequency channel
• The lowest from these (and often the dominant one) is the third-order nonlinearity
• The second-order nonlinearity may also be a critical performance parameter for a receiver. The approach presented here can easily be extended to the evaluation of the second-order and other nonlinearities of a receiver.
30 1 3Y a a X a X= + +
Property of R. Struzak 69
Blocking dynamic range (BDR)2 3
0 1 2 3 ...y a a x a x a x= + + + +
Noise Floor
Out
put p
ower
(dB
m)
Input power (dBm)
P1dB-inMDS
1dBP1dB-out
BDR (Blocking Dynamic Range)
MDS = MinimumDetectable Signal (Output Noise Floor)
Property of R. Struzak 70
IP1dB_b
• Two-signal Input Referred Blocking 1 dB Gain Compression Point – Refers to the condition when there are 2 single-
frequency input signals x(t) = [A1 cos(ω1t) + A2 cos(ω2t) and one of them (the blocker) is significantly stronger than the other, that is A2 >> A1.
( )20 1 3 2 1
3( ) cos 1 ...2
y t a a a A A tω⎛ ⎞= + + +⎜ ⎟⎝ ⎠
Property of R. Struzak 71
• If a3 < 0, the weaker signal A1cos(ω1t) experiences progressively less gain as the stronger signal A2cos(ω2t) gets stronger. The gain drops by 1 dB from its ideal value when the blocker amplitude reaches
11 _
3
0.0725IP dB b
aA
a=
Property of R. Struzak 72
• Assume a small signal test, i.e. A small enough such that
• The input level for which the output components at ω1 and ω2 have the same amplitude as those at (2ω1–ω2) and (2ω2–ω1) is the input referred third-order intercept point:
21 3
94
a a A
133
3
43IIP
aA
a=
Property of R. Struzak 73
Intermodulation
• Intermodulation spurious signals can be generated when two or more RF signals are applied to a non-linear device
• They could produce interference • The magnitude of the spurious signals depends on
the power of the original signals and on the degree of device nonlinearity
• Technical standards and RR impose limits on out-of-band (spurious) radiations
Property of R. Struzak 74
Intermodulation products
• The frequency (Fi) of an intermodulation product Fi = C1*F1+C2*F2+ .. +Cn*Fn
• {C1, C2, ...,Cn} are positive or negative integers or zero, and
• {F1, F2, ..., Fn} are the frequencies of the signals applied to the device
• The order of the intermodulation product is the sum: {|C1| + |C2| + ... + |Cn|}
Property of R. Struzak 75
IIP3
• Two-signal Input-Referred Third-order Intercept Point
• A third-order intermodulation component (IM3) due to two sinusoidal input signals of equal amplitude (Acosω1t, Acosω2t) [ ] [ ]
[ ] ( ) ( )
30 1 1 2 3 1 2
2 3 30 1 3 1 2 2 3 1 2 3 2 1
( ) cos( ) cos( ) cos( ) cos( )
9 3 3cos( ) cos( ) cos 2 cos 24 4 4
y t a a A t A t a A t A t
a a a A A t A t a A a A
ω ω ω ω
ω ω ω ω ω ω
= + + + + =
⎛ ⎞= + + + + − + −⎜ ⎟⎝ ⎠
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Example: 3rd order intermodulation
• Two ‘real’ signals of frequencies F1 and F2 when applied to a nonlinearity, produce six ‘false signals’ (3-rd order intermodulation products) at the following frequencies: – Fia = 2*F1 - F2 – Fib = 2*F2 - F1– Fic = 2*F1 + F2– Fid = 2*F2 + F1– Fie = 3F1– Fif = 3F2
Even if F1 and F2 do not interfere one with another, intermodulation products can interfere with one or another. Frequencies Fia, Fib, can be close to F1 or F2. More ‘real’ signals, more the number of ‘false signals’
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F1 F2
2F1-F22F2-F1
2F22F1
2F1+F2 2F2+F1
3F1 3F2
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• All above parameters are defined under assumptions of small-signal and systems with third-order nonlinearity
• IP1dB and IP1dB_b can be directly measured• IIP3 and IIP3_h cannot be directly measured: they can
only be extrapolated from small signal measurements as they are defined under small signal assumptions.
• The numerical values of the parameters are inter-related:• IP1dB = IIP3 – 9.6 dB• IP1dB_b = IIP3 – 12.6 dB• IIP3_h = IIP3 + 5 dB
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Summary
• Radiocommunication channel• Modulation • Spreading spectrum • Nonlinearities & intermodulation
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References
• Campbell AT. Untangling the Wireless Web – Radio Channel Issues, Lecture Notes E6951, comet.columbia.edu/~campbell
• Proakis J. Digital Communications, McGraw & Hill Int.
• Rappaport TS. Wireless Communications, Prentice Hall PTR
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Any question?
Thank you for your attention
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• Copyright © 2005 Ryszard Struzak. This work is licensed under the Creative Commons Attribution License http://creativecommons.org/licenbses/by/1.0
• These materials may be used freely for individual study, research, and education in not-for-profit applications. Any other use (and/or displaying the material in the WWW) requires the written author’s permission
• If you cite these materials, please credit the author. If you have comments or suggestions, please send these directly to the author at r.struzak@ieee.org.
Copyright note
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