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Unformatted text preview: All About Modulation – Part I 1 Intuitive Guide to Principles of Communications
www.complextoreal.com All About Modulation
Basic Concepts, Signal Space, Constellations and Phase Shift Keying
modulations (PSK, QPSK, OQPSK, MPSK, π/4QPSK, MSK, and GMSK) Basic Concepts of modulation
Three kinds of modulations
Modulation is the process of facilitating the transfer of information over a
medium. Sound transmission in air has limited range for the amount of power
your lungs can generate. To extend the range your voice can reach, we need to
transmit it through a medium other than air, such as a phone line or radio. The
process of converting information (voice in this case) so that it can be
successfully sent through a medium (wire or radio waves) is called modulation.
We begin our discussion of digital modulation by starting with the three basic
types of digital modulation techniques. These are;
AmplitudeShift Keying (ASK)
FrequencyShift Keying (FSK)
PhaseShift Keying (PSK)
All of these techniques vary a parameter of a sinusoid to represent the information
which we wish to send. A sinusoid has three different parameters than can be
varied. These are its amplitude, phase and frequency. Modulation is a process of
mapping such that it takes your voice (as an example of a signal) converts it into
some aspect of a sine wave and then transmits the sine wave, leaving the actual
voice behind. The sine wave on the other side is remapped back to a near copy of
your sound.
The medium is the thing through which the sine wave travels. So wire is a
medium and so are air, water and space. The sine wave is called the carrier. The
information to be sent, which can be voice or data is called the information signal.
Once the carrier is mapped with the information to be sent, it is no longer a sine Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 2 wave and we call it the signal. The signal has the unfortunate luck of getting
corrupted by noise as it travels.
In ASK, the amplitude of the carrier is changed in response to information and all
else is kept fixed. Bit 1 is transmitted by a carrier of one particular amplitude. To
transmit 0, we change the amplitude keeping the frequency constant. OnOff
Keying (OOK) is a special form of ASK, where one of the amplitudes is zero as
shown below. Figure 1  Baseband information sequence – 0010110010 ASK(t ) = s(t ) sin(2πft ) Figure 2  Binary ASK (OOK) signal
In FSK, we change the frequency in response to information, one particular
frequency for a 1 and another frequency for a 0 as shown below for the same bit
sequence as above. In the example below, frequency f1 for bit 1 is higher than f2
used for the 0 bit. ⎧ sin( 2πf1t )
FSK ( t ) = ⎨
⎩sin( 2πf 2 t ) for bit 1
for bit 0 Copyright 2002 Charan Langton , revised Dec 2005 1 www.complextoreal.com All About Modulation – Part I 3 Figure 3  Binary FSK singnal
In PSK, we change the phase of the sinusoidal carrier to indicate information.
Phase in this context is the starting angle at which the sinusoid starts. To transmit
0, we shift the phase of the sinusoid by 180°. Phase shift represents the change in
the state of the information in this case.
⎧ sin(2π f t ) for bit 1
PSK (t ) = ⎨
⎩sin(2π f t + π ) for bit 0 2 Figure 4  Binary PSK Carrier (Note the 180° phase shifts at bit edges)
ASK techniques are most susceptible to the effects of nonlinear devices which
compress and distort signal amplitude. To avoid such distortion, the system must
be operated in the linear range, away from the point of maximum power where
most of the nonlinear behavior occurs. Despite this problem in high frequency
carrier systems, Amplitude Shift Keying is often used in wirebased radio
signaling, both with or without a carrier.
ASK is also combined with PSK to create hybrid systems such as Quadrature
Amplitude Modulation (QAM) where both the amplitude and the phase are
changed at the same time. What is digital, what is analog?
There are three parts to a communications system.
1. The information, also called the baseband Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 2. The medium 3. 4 The carrier Information can be defined in two forms, digital or analog. Analog signal is
considered continuous. Its signal amplitude can take on any number of values
between the signal maximum and minimum. Voice is analog and can take any
number of volume levels between its “dynamicrange” which is the range of
volumes your vocal cords can produce. Digital devices convert analog voice to a
digital signal by process of sampling and quantization. The analog signal is first
sampled and then quantized in levels and then each level is converted to a binary
number. For example, we may quantize your voice in 16 levels. Each of these
levels can be represented by four bits.
Perhaps you remember when your telephone system went to the “tone” dialing. It
went from being a pure analog system to a digital system based on sampling and
quantization. Other examples of analog information are music and voice
transmitted via FM and AM radio transmissions. Nearly everything else
nowadays is digital.
The medium is thing the signal travels through. It can be air, space or wires of all
sorts. Each of these mediums offers its own unique set of advantages and
distortions that determine what is used as a carrier. A short wire in a chip for
example may not need a carrier at all. A signal through space such as for satellite
transmission may need a very high frequency carrier that can overcome space loss
and other atmospheric losses.
If medium is the road taken, then carrier is the truck that carries the information
hence we call it Carrier. It is a sinusoid in our case. Depending on the medium, it
will have a frequency appropriate to the medium. It can be at light frequencies as
in optical fiber or a microwave frequency as for mobile communications. An
electromagnetic carrier can be of any frequency depending on the medium and the
communication needs. Most mediums dictate what type of carrier (its frequency,
amplitude) can propagate through it and the type of distortions it will suffer while
traveling through it.
Anything that is wireless is analog – always. Wired signals can be digital or
analog. Communications inside a computer are examples of pure digital
communications, digital data over digital medium. LAN communications are
digital data over analog medium. The AM and FM radios are examples of analog
data over analog medium.
In general when we talk about a digital system, we are usually talking about
digital information over an analog medium. However, there are exceptions. Pulse
Coded modulation (PCM) is a form of modulation where there is no carrier, so
that makes it a pure digital system. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 5 The “Shift Keying” the second two terms in the name of these modulations imply
that they are digital modulations, i.e. the information is also digital. Signal Spaces and basis functions
The study of signal spaces provides us with a geometric method of
conceptualizing the modulation process. In a physical space when we describe a
vector by its coordinates (x, y); the vector is being described by a linear
combination of two functions (1, 0) and (0, 1). Any vector can be written as a
linear combination of these two functions. These functions are called basis
functions and are orthogonal to each other.
Another example of such a family of functions are the unit width pulses separated
in time shown below. Each of these is independent of others and clearly we can
use these functions to create any random data sequence consisting of square
pulses. Each one of these single pulses is a basis function. However, this is not a
very efficient set of basis functions as it takes a large number of these functions to
create a random signal.
φ0(t)
φ1(t)
φ2(t)
φ3(t) Figure 5  Orthonormal basis set
Ideally we want as few basis functions as possible which when combined can
create a large number of independent signals, both digital and analog. In general,
basis functions should
• Have unit energy, such as the (1, 0) and the (0, 1) vectors and the above
unit pulses. • They should be orthogonal to every other function in the set, represented
mathematically by ∫ +∞ −∞ ⎧1 i = j
⎩0 i ≠ j φ i ( t )φ j ( t ) = ⎨ 3 An important example of terrific basis functions is the pair of sine and cosine
waves of unit amplitude. This special basis set is used as carriers in all real
communications systems. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 6 φ1(t) φ2(t) 1 1 0 t 1 1 0
1 2 t
1 2 Figure 6  Sine and cosine, two orthogonal functions are the basis set for all
modern communications The concept of I and Q Channels
Without worrying about what a signal is, let’s just define it as a vector. Below you
see two views of a signal space. One shows a signal in rectangular and the other
in its polar form. We can describe the signal it in polar form by its magnitude and
it phase (angle) or by its rectangular projections, such as s11 and s12. φ2(t)
φ2(t) s11, s21
s12
40.4° s11 φ1(t) φ1(t) (a) I and Q projections (b) Polar form Figure 7  Signal vector plotted on signal space
In Figure 7(a) the x and yaxis are called Inphase and Quadrature projections of
the signal. Quantity s11 is I projection and s12 is the Q projection of the signal.
Figure 7 (b) shows the same signal in polar form, with its length equal to its
amplitude and the angle is equal to its phase. These are two canonical ways of
representing signals.
The coefficients s11 represent the amplitude of I signal and s12 the amplitude of the
Q signal. These amplitudes when plotted on the x and y axis respectively, give the
signal vector. The angle the signal vector makes with the xaxis is the phase of
this signal. Magnitude of signal S = I 2 + Q 2 Copyright 2002 Charan Langton , revised Dec 2005 4 www.complextoreal.com All About Modulation – Part I Phase of the signal = tan −1 7 I
Q In itself, this is simple enough but gets confusing when related to modulation as
we shall see. Symbols, bits and bauds
A symbol is quite apart from a bit in concept although both can be represented by
sinusoidal or wave functions. Where bit is the unit of information, the symbol is a
unit of transmission energy. It is the representation of the bit that the medium
transmits to convey the information. Imagine bits as widgets, and symbols as
boxes in which the widgets travel on a truck. We can have one widget per box or
we can have more. Packing of widgets (bits) per box (symbols) is what
modulation is all about.
In communications, the analog signal shape, by preagreed convention, stands for
a certain number of bits and is called a symbol.
Digital world of bits and
information Analog world of signal
symbols Bit baseband amplitude Carrier amplitude Time
Process of modulation
converts bits to symbols. Each of these is one bit
of information, 0's and
1's. This is a carrier symbol
that represents one
information bit. Figure 8 – Digital information travels on analog carrier
A symbol is just a symbol. It can stand for any number of bits, not just one bit.
The bits that it stands for are not being transmitted, what is transmitted is the
symbol or actually the little signal packet shown above. The frequency of this
packet is usually quite high. The 1 Hz signal shown above is just an abstraction.
A baud is same as the symbol rate of a communication system. So if we send 200
bauds, then we are send 200 symbols per second. PSK modulations
BPSK
Let’s imagine a ship lost at sea with no communication system. It sees an airplanes flying
overhead and wants to communicate its plight to the airplane while it is overhead. The
Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 8 captain marks two spots on each side of the mast as shown below. Now he holds a bright
light and runs back and forth between the marked spots to signal a message. Spot to the
right means a 1 and spot to the left means a 0. We assume that all airplanes seeing this
know that what each light stands. Figure 9 – Two signaling spots, a simple modulation system
This is a one dimensional signal, because the captain uses only one dimension (running
from left to right) to indicate a symbol change.
The shining of the light is a symbol. There are two light positions, so those are two
symbols. Let’s give these two symbols names of s1 and s2. Simplest thing is to have the
symbol stand for just one bit. This method of transmitting information, i.e. the bits, is
essentially a Binary Phase Shift Keying (BPSK) modulation. We utilize just one sinusoid
as the basis function. We vary the phase of this signal to transmit information which is
identical in concept to the example of shining the light from the deck. Each symbol is
signaled by a change in position (really the phase) of the light as in this example. In
BPSK we define two little packets of the cosine wave, one with zero phase and second
one with a 180 degree different phase.
The BPSK signal is special. It lies totally in one axis, xaxis. It has no yaxis projection.
The vector flipflops on the xaxis depending on the value of the bit. Table 1 lists the two
symbols and the signals used to represent them. (The carrier signal shown is for f = 1
Hz.) The I and Q amplitudes are the x and y projections computed by setting fc = 0, and
2 Es
= 1 , then we get, I = 1 for the first symbol and 1 for the second symbol. Q
T amplitude is zero for both symbols because sin of both 0° and 180° is zero.
Symbol S1 S2 Bit 0 1 Expression Modulation Signal
at fc = 1 Hz I Q At fc = 0 At fc = 0 2 Es
cos( 2π f c t + 0)
T 1 0 2 Es
cos( 2π ft + π )
T 1 0 Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 9 Table 1 – Mapping rules for BPSK
Look at the modulation signals in the above table. The little signals shown are at fc = 1
Hz. They are there to give you a feel for what the transmitted signal looks like. Of course,
in a real system these would be at much higher frequency. This is what is transmitted in
response to the bits not the bits themselves. But what are those funny coefficients in front
of the expressions above?
Recall from Tutorial 1 that energy of a signal is equal to
Es = A2T
2 5 So instead of writing a amplitude term to make the expression general, we write the
equation in terms of energy, where A = 2 E s / T . When referring to carrier signals, we
typically talk in terms of signal or bit energy, so it makes sense to write the equations in
terms of energy, which is what this scaling factor is. Now we can scale the carrier signal
for the power with which it is transmitted. Creating a BPSK carrier
How would we send a bit sequence 0111 0101 0010 1011 using BPSK signaling
technique? To transmit this sequence, we need 16 symbols since each BPSK symbol
stands for one bit. These are
s1 s2 s2 s2 s1 s2 s1 s2 s1 s1 s2 s1 s2 s1 s2 s2
Now string together the appropriate symbol signal packets from Table 1 in the right
order. Figure below is the modulated carrier that would be transmitted for this sequence
if we use the mapping in Table I.
2 2 carbpsk ( t )
0 s( t ) 2 2
0
0 2 4 6 8
t 10 12 14 16
16 Figure 10  A BPSK signal for bit sequence 0111 0101 0010 1011
If you could catch the modulated carrier and look at it on a network analyzer, you would
see the above. However, the above picture is at a carrier frequency of 1 Hz, which is not
realistic. In real systems, the carrier frequency is very high and we would see a signal that
covers a lot of cycles between each transition. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 10 What is a transition? A transition is the time at which we switch from one symbol to the
next. What happens at the transition boundary is different for various modulations and is
quite an important thing. In the case of BPSK, at every bit transition the signal does a 180
degree phase shift.
We worry about what the signal does at transitions because of amplifier nonlinearities.
Amplifiers used in communications have a very hard time with sudden changes in signal
amplitudes and introduce distortions. Since this makes it harder to decode the symbol, we
try to control these transitions. QPSK
Now imagine a different ship. Its captain thinks up a different signaling arrangement.
Here the he has marked out four spots on the deck, to the East and West and North and
South. He assigns four different combinations to each of the spots as shown below. He
can send two bits, with each flash of the light. If he can do it in the same time period as
the first ship, then this person would be able to communicate twice as fast.
L ig ht to t he N ort h
o f mast mean s 0 1 Li gh t to th e Wast
of mast means 0 0 Lig ht to t he E ast
o f mast mean s 1 1 Lig ht t o t he South
o f mas t mean s 10 Figure 11 – A two dimensional signaling system
By creating four signaling spots, he has added another dimension. This gives two basis
functions, the EastWest and the NorthSouth movements. Now there are four different
symbol positions possible and we can assign 2 bits to each unique symbol.
The dimensionality of a modulation is defined by the number of basis functions used.
That makes QPSK a twodimensional signal. Not because it sends two bits per symbol,
but because it uses two independent signals (a sine and a cosine) to create the symbols.
All PSK modulations we will discuss here are twodimensional.
Now some light math –
QPSK signal is an extension of the BPSK signal. Both of these are a type of Mary
signals. We can write the process that describes the modulated signal in a polar form as
2π i ⎞
⎛
6
si (t ) = Ac ps(t ) cos ⎜ 2π fct +
M⎟
⎝
⎠ Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 11 Where ps(t) is the pulse shaping function. In digital phase modulation, the phase of the
sinusoid is modified in response to a received bit. The changing phase is shown in blue.
A sinusoid can go through a maximum of 2π phase change in one period. So the
maximum phase we can change at any one time is 180°. We can use M quantized levels
of 2π, to create a variety of PSK modulation. The variable i is a number from 1 to M. The
allowed phases are given by
Modulation angles θ i = 2πi
M 7 M stands for the order of the modulation. M = 2, makes this a BPSK, M = 4 is QPSK, M
= 8, 8PSK and so on. Following diagram shows three of these modulations and their
“constellations.” A rotation of the second resulting in the third figure does not change the
modulation, its power or performance. These modulation are called rotationally invariant. 01 1 Es 0 01 00 011 010
001 00 11 Es 111 000 Es
Es 110
11 10 10 100
101 Figure 12 – MPSK modulations, a. BPSK, b. QPSK, c. also QPSK, d. 8PSK
For baseband PSK signals, we use a square pulse. The pulse has an amplitude of A. The
energy in this pulse is equal to the power of the signal times the duration, T it lasts.
Power is equal to A2 with R = 1 ohm and T is the symbol time. A T E =1=
A= t A 2T
2 1
T Which gives this equation for the pulse; the pulse has this amplitude over a period T secs. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 2
T ps (t ) = 12 0≤t ≤T 8 Substitute this into equation 6, we get
s i (t ) = Ac 2
2πi ⎞
⎛
cos⎜ 2πf c t +
⎟
T
M⎠
⎝ 9 The carrier amplitude Ac, lets just set it at E s . Now we have the modulation equation of a general MPSK signal.
⎛
⎞
⎜
⎟
2 Es
⎜ 2π fct + 2π i ⎟
si (t ) =
cos
T
M⎟
⎜ Changing
⎜ with time Changing ⎟
Cons tan t
with inf o ⎠
⎝ i = 0,1, .... M 10 An arbitrary plot of this equation looks as shown below. At each tick which is the symbol
time, there is a phase shift.
2 2 car( t ) 0 −2 2 0 1 2 3 0 4 5 6 t 7 8
8 Figure 13 – An arbitrary modulated signal which shows phase shifts at each time
tick
Here we see several phase shifts, some 90° as at t = 1 and others 180° as we see at t = 6.
A modulated signal for BPSK has only 180° degree phase shifts, whereas a QPSK has
90° degrees and 180° degree phase shifts.
The modulation equation 10, is much more useful in understanding modulation. The first
part is the magnitude or the amplitude of the signal and is a constant. The rest is a
function of the phase angle just as we would like it to be since we are doing phase
modulation.
Now let’s expand this equation using the trigonometric identity
cos( A + B) = CosACosB − sin A sin B Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I si (t ) =
= 13 2 Es
2π i π ⎞
⎛
+⎟
cos ⎜ 2π fct +
T
M
4⎠
⎝ 2 Es
T ⎡
⎛ 2π i π ⎞
⎛ 2π i π ⎞ ⎤
⎢cos (2π fct ) cos ⎜ M + 4 ⎟ − sin ( 2π fct ) sin ⎜ M + 4 ⎟ ⎥
⎝
⎠
⎝
⎠⎦
⎣ 11 at fc = 0, we get four baseband signals we will use for signaling. We have initialized the
phases to start at 45°. This shift has no effect on the modulation.
Now come the I and the Q channels, remember the basis set. φ1 (t ) = cos ωc t
φ2 (t ) = sin ωc t
Any two signals created through the scaled versions of these basis signals are also
orthogonal. So let’s scale these and call them I and Q channels. I= 2 Es
cos (2π fct )
T 12 2 Es
Q=
sin (2π fct )
T
The above are clearly orthogonal because we just multiplied the basis functions with a
constant. Now multiply them with the angle part from Eq. 11 part also. For i = 0, 1, 2, 3
and M = 4, a QPSK constellation, the second part is also a constant. So the two equations
remain orthogonal. I= ⎛
2 Es
⎛π ⎞
⎛ 3π ⎞
⎛ 5π ⎞
⎛ 7π
cos ( 2π fct ) ⎜ cos ⎜ ⎟ or cos ⎜ ⎟ or cos ⎜
⎟ or cos ⎜ 4
T
⎝4⎠
⎝4⎠
⎝4⎠
⎝
⎝ Q= ⎛ ⎛π ⎞
2 Es
⎛ 3π
sin ( 2π fct ) ⎜ sin ⎜ ⎟ or sin ⎜
T
⎝4
⎝ ⎝4⎠ ⎞
⎛ 5π
⎟ or sin ⎜ 4
⎠
⎝ ⎞
⎛ 7π
⎟ or sin ⎜ 4
⎠
⎝ ⎞⎞
⎟⎟
⎠⎠ ⎞⎞
⎟⎟
⎠⎠ Actually true orthogonality requires that fc be a integer multiple of T/M but for large
carrier frequencies, this is not so important. Now we can express the modulation equation
as
s(t ) = 2 Es
2 Es
cos (θ (t ) ) cos (2π fct ) −
sin (θ (t ) ) sin (2π fct ) 13
T
T
1 2 This is called the quadrature form of the modulation equation. The two signals are
orthogonal. The colored part is the amplitude of the I and the Q channels. The values of
Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 14 Es . So a phase
modulated signal can now be seen as a combination of two quadrature signals, the
amplitude of which changes in response to the phase change. The modulating signal can
be seen as a vector with I and Q as its x and y components.
these is the same as the x and yaxis projection of the signal of energy Although we created the BPSK modulated signal by stringing together the appropriate
packets of signals, in real systems, we can not create a modulated carrier this way. What
we have at our disposal are oscillators that can produce continuous sines and cosines. We
can not just use a certain part of the signal as if it was sitting on a shelf for us to grab the
needed piece. We need a way to create a signal packet of a particular phase when needed
out of a freerunning sine or cosine. This is where Quadrature Modulation with I and Q
channels come into play. I and Q channels are not just concepts but also how modulators
are designed. However, the signal created by I and Q channels is not what is transmitted,
it is the sum or the difference (makes no difference as long the polar form is consistent)
of these two, and that is the real modulated signal.
How the bits are mapped to the possible phases can be done in many ways, especially
when you have a lot of symbols. Try this with 8PSK: The bit combinations are 001, 000,
100, 101, 111, 110, 010, 011. There are many different ways of doing this. The best way
to do this is to number them such that each adjacent phase means just one bit difference.
So that when a phase mistake is made and the most likely one is the nearest phase, then
only one bit is decoded incorrectly. This is called Gray coding and is always applied in
PSK. In QPSK we can do this perfectly. In other higher order (M > 4) PSK modulations,
perfect gray ordering is not always possible.
For QPSK, we have four symbols, each stands for two bits. Nominally we start the first at
45 and then change phase by 90 each time to get the next symbol. The I and Q values are
computed by setting fc = 0, and
2 Es
= 2 . For the first symbol which lies in the first quadrant, the I and Q values are
T both +1. Similarly for others as shown in this table.
Symbol Bits S(t) Phase,
(Deg.) S1 00 2 Es
cos( 2π f c t + π / 4)
T 45° S2 01 2 Es
cos(2π f c t + 3π / 4)
T 135° Copyright 2002 Charan Langton , revised Dec 2005 Mod. Signal
At fc = 1 I Q 1 1 1 1 www.complextoreal.com All About Modulation – Part I 15 S3 11 2 Es
cos( 2π f c t + 5π / 4)
T 225° S4 10 2 Es
cos( 2π f c t + 7π / 4)
T 315° 1 1 1 1 Table 2 – Mapping rules for QPSK Constellation of QPSK
A constellation is a plot of the symbols on the rectangular space. We create this for PSK
by first drawing a circle of radius = Es . Since we like the I and Q channels to have
amplitudes of 1, then the radius of the circle becomes 1.414. Now compute the
modulation angle, which is 360° divided my M. For PSK that is 90°. So that’s four points
each 90° apart on the circle. If the modulation is 16 OSK, the points or symbols would be
22.5° apart. Now compute the x and y projections for each symbol. These are the
amplitudes of the I and Q channels. Once you know that, you can create I and Q channels
and the real modulated signal which is a sum of I and Q channels.
Visually the constellation diagram which is what this picture is called, shows the phases
of the symbols and their relationship to each other. The xaxis projection for each symbol
is the I channel amplitude and yaxis projection is the Q channel amplitude. Each signal
is shown with a little packet of signal that goes with it. The constellation diagrams is
always done at baseband, i.e. fc = 0. So the signal is just a point. But I have added the
little packet of modulated signal at fc = 1, just so you can get an intuitive feel for what we
are doing. Depending of the fc, it is these little packets that are transmitted.
Q S= 1 1, + 1 +1, +1 S= 0 Phas e angle I S= 3 1, 1 +1, 1 S= 2 Figure 14 – Constellation points are the tips of the modulating signal
QPSK example
Signal s1 shows a string of symbols, numbers from 0 to 4 signifying a random symbol
that needs to be transmitted. We map these into I and Q by using the figure above. For Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 16 example, the second integer in signal s1 below is 2. Its I channel value is 1 and its Q
value is 1. Figure 15 – QPSK, s1 – arbitrary integer stream indicating a bit stream of 2 bits per
integer, s2 – the I channel mapping, s3. the Q channel mapping
Symbol 0 3 2 0 3 3 2 3 3 2 I Data 1 1 1 1 1 1 1 1 1 1 Q Data 1 1 1 1 1 1 1 1 1 1 Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 17 Figure 16 – QPSK modulation s4 a cosine wave of frequency 1 Hz, s5 – s4
multiplied by s2, s6 – a sine wave of frequency 1 Hz, s7 – s6 multiplied by s3, s8 –
add I and Q channels to get the real modulated signal
Signal s4 is the I carrier or the cosine wave of frequency 1. Signal s5, multiplies I channel
values from signal s2 with carrier signal of s4. Signal s6 is the Q carrier or the sine wave
of frequency 1. Signal s7 multiplies the carrier s6 with Q(t) of signal s3. Signal s7 and
signal s5 are Q and I channel respectively. We add I and Q to get the real modulated
signal in s8. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 18 The following diagrams show the two conventional ways of doing modulation.
Information
s ignal m(t) s (t ) = A(t ) cos(ωct + θ (t )) A(t )
Bas e band
s ignal
proce ssing Modulate d s ignal θ (t )
Phas e
Modulator cos(ωc t + θ (t )) Carrie r
s ignal, fc Method I – The polar form of modulation
I nphas e channe l
Information
s ignal m(t) + x (t )
Bas e band
s ignal
proce ssing Σ s (t ) = x(t ) cos(ωct ) − y(t ) sin(ωct )
− Modulate d s ignal y (t )
Quadrature
channe l cos(ωc t )
Carrie r
s ignal, fc sin(ωc t )
900 de g
phas e s hift Figure 17  Method II – The quadrature form of modulation using I and Q
channels Method 1 is most straight forward but requires multiplications and square roots. Method
II, the quadrature method is easier to implement in hardware so is the predominant
method used in digital modulation.
These are called the canonical forms, which just means that this is the conventional way
of doing this type of modulation.
You need to recognize that in quadrature modulation, channel I and Q are not transmitted.
Only the real signal is transmitted.
Modulation Index of a QPSK signal
The modulation signal can also be written like this. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 19 s(t ) = Ac cos ⎡ωct + Dp m(t )⎤
⎣
⎦ 14 where m(t) is the information signal, varying between +1 and 1 with a rectangular pulse
shape. Dp is called the phase sensitivity. It is equal to the peak phase deviation over one
symbol. Its units are radians per volt. Plotting above equation, we get the modulated
signal as a function of the phase sensitivity.
The phase sensitivity of a modulated signal is related to the traditional modulation index
as
2∆θ 2 Dp
=
h=
15 π π For a true, carrier suppressed PSK signal, the modulation index is equal to 1 since the
peak phase variation is 90 degrees. PSK signals are all carrier suppressed and all have a
modulation index of 100%.
1
0. 8
0. 6
0. 4
D=2 0. 2 D = 1. 57 0
0. 2 0 10 20 30 40 50 60 70 80 90 100 D=1
D = .6 0. 4
0. 6
0. 8
1 Figure 18 – The modulation index of a PSK signal is 100% Shaping the pulse to reduce bandwidth
The square pulses shown here are not practical to send. They are hard to create and
require a lot of bandwidth. So in their lieu we send shaped pulses that convey the same
information but use smaller bandwidths and have other good properties such as
intersymbol interference rejection. One of the most common pulse shaping is called “root
raised cosine”. This pulse shaping has a parameter called the rolloff which controls the
shape and the bandwidth of the signal.
Some common pulse shaping methods are
• Root Raised cosine (used with QPSK) • Halfsinusoid (used with MSK) • Gaussian (used with GMSK) Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I • 20 Quadrature partial response (used with QPR) In Figure 19 we see the time domain traces of a QPSK I and Q channel where the signal
has been shaped by root raised cosine pulses. The general bit shape can be seen easily. Figure 19 – Rootraised cosine shaped pulses We can draw a constellation diagram of this signal by sampling this signal every
20 samples, and then plotting the measured I values against Q values. We get the
following constellation diagram. Figure 20 – Constellation diagram of a rootraised cosine shaped signal Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 21 Note that the RRC constellation diagram has a scatter around the ideal points.
This is an inevitable consequence of any pulse shaping and relates to increased bit
error rate for the signal.
A Gaussian pulse shape replaces the square pulses with a Gaussian pulse. Here is
a time domain trace for a Gaussian pulse shaped signal. Figure 21 – The time domain signal shaped by Gaussian pulses. This is how we do it in hardware.
I Channel
Rb / 2 Set I
amplitude x RRC
shaping Information
bitstream
Serial to Parallel
Converter
Rs = Table
Lookup ∑ cos(2π f c Tb ) π Rb
2 2 Σ
QPSK
Carrier delay π cos(2π f c Tb + )
2 Rb / 2 Set Q
amplitude RRC
shaping x Q Channel Figure 22 – Hardware implementation of the MPSK modulator Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 22 Summary: The Serial to parallel converter takes the bit stream coming in at a bit
rate of Rb and splits it into two streams, each of half the bit rate. Depending on
the dual bit pattern coming in, I and Q amplitudes are set from a table lookup
function. Each of these is then individually modulated by a sine or a cosine wave
of carrier frequency ω after being shaped into a root raised cosine pulse. These are
added together to get the transmitted signal. Constant Envelope modulation
QPSK is part of a class of signals called constantenvelope signals. There is no
rigorous definition of a constant envelope signal. One definition is; when sampled
at the symbol rate, the sampled value of the amplitude is constant. Another is that
there are no discontinuous phase changes. Yet another is that the maximum and
minimum amplitude attained by the signal over one period is constant. The sine
wave is an ideal constant envelope signal.
Constant envelope signals suffer less distortion in high power amplifiers and are
preferred for wireless applications. The reason is that amplifiers work by
changing a signal's amplitude, either increasing or decreasing it. To increase a
signal’s power is to increase its amplitude. A nonlinear amplifier changes the
signal amplitude by differing amount depending on the instantaneous amplitude
of the signal. The more the amplitude of a signal varies, the more nonlinear
amplification occurs and this results in a distorted signal. QPSK is not technically
a constant envelope because of its discontinuous phase shifts but is considered
nearly so. A shapedpulse signal is also not constant envelope but is nearly so.
1.5 fsk( t )
s( t ) 0 1.5
0
0 2 4 6
t 8 10
10 Figure 23 – FSK is definitely a constant envelope modulation. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 23 ask ( t )
0 s( t ) 0 2 4 6 8 10 t Figure 24  ASK is definitely not a constant envelope modulation Offset QPSK
Offset QPSK is a minor but important variation on QPSK. In Offset QPSK, the Q
channel is shifted by half a symbol time so that I and Q channel signals do not
transition at the same time. The result of this simple change is that phase shifts at
any one time are limited and hence offset QPSK is more “constantenvelope” than
straight QPSK. In high power amplifiers and for certain satellite applications,
Offset QPSK offers better performance. Although in a linear channel its bit error
rate is the same as QPSK, in nonlinear applications, its BER is lower when
operating close to the saturation point of the transmitting amplifier. Offset QPSK
(OQPSK) is also called staggered QPSK (SQPSK).
I Channel
Rb / 2 Set I
amplitude x RRC
shaping Information
bitstream ∑Σ Serial to Parallel
Converter
Rs = cos(2π f c Tb ) π Rb
2 2 QPSK (or OQPSK)
Carrier
delay π Rb / 2 Set Q
amplitude
Q Channel cos(2π f c Tb + )
2 RRC
shaping Tb delay x Symbol time delay
for OQPSK only Figure 25 – QPSK modified to become OQPSK Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 24 Figure 26 – I and Q channel mappings of an Offset QPSK signal, the symbol
transitions do not occur at the same time. Unlike QPSK, I and Q channels of an OQPSK signal do not transition at the same
time. One consequence of this is that when we look at the constellation diagram
of the OQPSK, the symbol transitions occur only to neighbors. This means that
the transitions are never more than 90°. At any symbol change, for either I or Q
channel, only one axis can change at a time, either I or the Q but not both. (At any
transition, only I or the Q changes but not both.) In constellationspeak, if the
signal was in the right upper quadrant, the next signal can only go to either the
lower right quadrant or to upper left quadrant but not across. Note how this is
different from QPSK, where all transitions can occur. Figure 27 – OQPSK and QPSK constellation diagrams, (1) OQPSK, (2)
QPSK Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 25 (a) OQPSK – All phase shifts are 90°. (b) QPSK  Note the 180° phase shift.
Figure 28 – The phase jumps at the symbol transition for OQPSK are
smaller. (Note that the figures above are not of the same scale in time.) Figure 28 compares the OQPSK signal with a QPSK signal. Note that the OQPSK
signal never transitions more than 90°. QPSK on the other hand goes through
phase change of 180° for some transitions. The larger transitions are a source of
trouble for amplifiers and to be avoided if possible. In satellite transmission,
QPSK reigns supreme, it is easy to build and operate. Military often uses OQPSK
because of its need to use low power radios and minimum adjacent channel
interference issues.
How OQPSK differs from QPSK: The Q channel of OQPSK is delayed by a
half a symbol time, staggering the two quadrature channels. Minimum Shift Keying (MSK)
Although MSK is often classified as FM modulation, it is also related to offsetQPSK owing to the dual nature of FSK and PSK modulations. OQPSK is created
from QPSK by delaying Q channel by half a symbol from I channel. This delay
reduces the phase shifts the signal goes through at any one time and results in an
amplifierfriendly signal.
MSK can be derived from OQPSK by making one further change  OQPSK I and
Q channels use square rootraised cosine pulses. For MSK, change the pulse
shape to a halfcycle sinusoid. Figure 29 shows a MSK pulse signal and then
multiplication by the carrier. The red curve is the carrier signal, and the blue the
MSK pulse shape and the black the multiplication of the pulse shape and the
carrier giving the modulated carrier. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 26 (a) MSK pulse and carrier for a 1 bit (b) MSK pulse and carrier for a 0 bit Figure 29 – MSK pulse shaping is a halfsine wave shown in blue, positive for
a 1 and negative for a 0. The carrier signal expression for MSK is
⎛π ⎞
⎛π ⎞
⎛ π ⎞ ⎛π ⎞
c(t ) = a(t )sin ⎜
t ⎟ cos ⎜ t ⎟ + a(t )sin ⎜
t ⎟ sin ⎜ t ⎟
⎝ 2T ⎠
⎝T ⎠
⎝ 2T ⎠ ⎝ T ⎠ with the underlined portion, the halfsinusoid pulse shape. Figure 29 shows how
MSK pulses look compared to QPSK square pulses. Remember in QPSK, the
square pulse itself equates to a discrete phase. In MSK, the shape is continuous
changing, so there is no discrete jump in the modulated signal at the symbol edge
as there is QPSK. For this reason the modulated signal in Fig. 32 has no
discontinuities as compared to MPSK signals. Figure 30 – MSK pulse, each pulse is a half cycle sine wave. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 27 The dashed line is the QPSK I and Q channel symbols and the solid lines show
how these have been shaped by the half sine wave. The I and Q channels are
computed by
⎛ πt ⎞
MSKI (t ) = QPSKI (t ) sin ⎜
⎟
⎝ 2T ⎠
⎛ π (t + .5T ) ⎞
MSKQ (t ) = QPSKQ (t + .5T ) sin ⎜
⎟
2T
⎝
⎠ The I and Q channels are then multiplied by the carrier, cosine for the I channel
and sine for the Q channel. Note that the period of pulse shape is twice that of the
symbol rate.
⎛ πt
MSKcarrI (t ) = QPSKI (t ) sin ⎜
⎝ 2T ⎞
⎛ πt ⎞
⎟ cos ⎜ ⎟
⎠
⎝T ⎠ ⎛ πt
MSKcarrQ (t ) = QPSKQ (t ) sin ⎜
⎝ 2T ⎞
⎛ π (t + .5T ) ⎞
⎟ s in ⎜
⎟
T
⎠
⎝
⎠ Figure 31 – MSK I and Q modulated carriers. Adding I and Q components gives the MSK carrier of Figure 31. Compare this
carrier to a QPSK carrier. This one has much smoother phase shifts at the symbol
boundaries. This results in lower side lobes which is an advantageous property for
wireless signals since it results in less adjacent signal interference. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 28 Minimum Shift Keying (MSK) is also called continuous phase (CP) Frequency Shift
Keying (FSK). MSK is a class of continuous phase modulations. These are
particularly suited to media which uses nonlinear amplifiers. FSK is the digital version of analog Frequency Modulation (FM) and MSK is also
a form of FSK, where modulation index is equal to .5 which results in a minimum
frequency separation such that the modulation frequencies are still orthogonal.
(See FM tutorial) Figure 32 – MSK modulated carrier
I Channel
Rb / 2 x Information
bitstream x Serial to Parallel
Converter
Rs = Rb
2 Σ
⎡ ⎛1⎞⎤
sin ⎢ 2π ⎜
⎟t⎥
⎣ ⎝ 4Tb ⎠ ⎦ MSK
Carrier cos(2π f c Tb ) π Tb advance 2 delay π 1
Rb =
Tb x
Bit rate in bits/sec cos(2π f c Tb + )
2
Tb delay x
Q Channel Figure 33 – MSK modulator block diagram Figure 32 shows the modification made to the QPSK modulator to create the
MSK signal. Only the pulse shaping has been changed. The half cycle time shift
of the OQPSK stays.
How MSK differs from QPSK: MSK is generally considered a FSK modulation
but it is exactly the same as OQPSK except that it uses a halfsinusoid for pulse
shaping instead of rootraised cosine pulses. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 29 Gaussian MSK (GMSK)
We created MSK by applying a half sinusoid to the square pulse. By using a
Gaussian pulse shape, the result can be improved even further. The modulation
obtained this way is called GMSK.
GMSK is used in several mobile systems around the world. Global Speciale
Mobile (GSM), Digital European Cordless Telephone (DECT), Cellular Digital
Packet Data (CDPD), DCS1800 (Digital Communications System in the 1800
MHz band) in Europe, and GSMbased PCS1900 (Personal Communications
Services in the 1900 MHz band) in the U.S. uses GMSK.
Recall that the rootraised cosine pulse has a roll off factor, α. The rolloff factor
determines how sharply the pulse rolls off to zero energy. A Gaussian pulse
similarly has a BT factor that determines how sharply it rolls off. A BT of .3 is
used commonly.
MSK and GMSK, both being related to FM modulation, can both be created two
ways, 1. as a PSK signal and 2. as a FSK signal. Both are most commonly
implemented as a FSK technique.
The Gaussian pulse shape used instead of the halfsinusoid or the root raised
cosine is given by
g (t ) = 1⎛ ⎛
t − .5T
⎜ Q 2π Bb (
2T ⎝ ⎜
ln 2
⎝ t + .5T
⎞
⎛
⎟ − Q ⎜ 2π Bb (
ln 2
⎠
⎝ ⎞⎞
⎟⎟
⎠⎠ where
Q (t ) = ∫
t 1 − ( x2 / 2)
e
dx
2 Quantity Bb is signal bandwidth. BT factor is equal to this number times the
symbol time, T. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 30 Figure 34 – Modulated I and Q GMSK carriers Figure 35 – Add I and Q GMSK carriers to obtain the composite carrier The GMSK modulated carrier is even better at transitions than MSK and this is
the main reason it is used as a standard in some cellular systems.
How GMSK differs from MSK: GMSK is nearly always implemented as a FM
modulation. However conceptually it is same as MSK except instead of halfsinusoid as a pulse shape a Gaussian pulse shape is used instead. 8PSK
Imagine once more the man on the ship, he figures he’s got the space on the deck,
so why not add more signaling positions. He marks out a circle and doubles the
number of places where he will stand to flash up the signal. We can see the
problem right away, how is the airplane going to make out where he is standing.
But never mind, he goes ahead with his plan. Here is how his new signaling
positions look Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 31 010 Lev el 2 00 1 011 Lev el 1 00 0 111
11 0 10 0 d, di st ance
between l ight s 101 Figure 36 – 8PSK uses eight different unique signals He assigns bit values to each of the eight positions as shown. Note that each set of
bits is just one bit different from its neighbor. So if the airplane does make an
error in reading his position, most likely this will result in only one bit being
misinterpreted. The eight positions are created with x and y distances or by phases
of sines and cosines in communications.
Q
0, +1. 414
S= 2
S =1 1, +1 +1, + 1 S= 3 1.414, 0 Phas e angle S= 4 S =0 S=5 + 1.414, 0 I S= 7 1, 1 +1, 1
S=6
0, 1.414 Figure 37 – 8PSK constellation diagram and the I and Q channel amplitudes We have two basis functions again, a sine and a cosine and each configuration has
a different phase to indicate a specific bit pattern. We use four different phase
values, namely π / 8, 3π / 8, 5π / 8 and 7π / 8 . Each of these phase shifts is 45
degrees apart. Each of these is applied to the sine and the cosine to give us a total
of eight values.
(Assume
Symbol S1 Bits 000 2 Es
= 1.414)
T Expression s1 (t ) = 2 Es
cos(ωt )
T Copyright 2002 Charan Langton , revised Dec 2005 Phase 0° Signal, fc = 1 I Q 1.414 0 www.complextoreal.com All About Modulation – Part I 001 32 S3 S4 2 Es
cos(ωt + π / 4)
T 45° 1.0 1.0 s3 (t ) = 2 Es
cos(ωt + π / 2)
T 90° 0 1.414 s4 ( t ) = 011 s2 ( t ) = 2 Es
cos(ωt + 3π / 4)
T 135° 1.0 1.0 s5 (t ) = 2 Es
cos(ωt + 5π / 8)
T 180° 1.414 0.0 s6 (t ) = S2 2 Es
cos(ωt + 7π / 8)
T 225° 1.0 1.0 s7 (t ) = 2 Es
cos(ωt + 9π / 8)
T 270° 0 1.414 010 S5 S6 S7 110 111 101 Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I S8 100 s8 (t ) = 33 2 Es
cos(ωt + 11π / 8)
T 315° 1.0 Table 3 – 8PSK signals
8PSK stepbystep Step 1  The bits stream to send is: 100 111 111 111 111 001 …. Figure 38 – Bit stream to be modulated using 8PSK, 100 111 111 111 111 001 Let’s name these bit packets for convenience.
Symbol sequence: s8 s6 s6 s6 s6 s2 ….
Map each symbol to I and Q using the amplitudes in Table 4.
s8 s6 s6 s6 s6 s2 I 0.707 0.707 0.707 0.707 0.707 1.414 Q 1.414 1.414 1.414 1.414 1.414 0.707 Each of these threebit packets are mapped to levels of I and Q channel from the
table above.
After the complex multiplication with cosine and sine wave of the carrier
frequency we get the signals shown in Figure 39. When I and Q are added
together, you get the composite signal in Figure 40. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com 1.0 All About Modulation – Part I 34 Figure 39 – 8PSK mapping of I and Q for sequence in Figure 36. Note that
both I and Q can take on 4 different values. Figure 39 –I and Q modulated sequences Figure 40 –Adding I and Q to obtain the composite carrier Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 35 8PSK transmitted signal shows smaller phase transitions (on the average) then
QPSK which is a good thing but since the signals are also less distinctly different
from each other, makes 8PSK prone to higher bit errors. Why then would we
want to use 8PSK? Because, we can pack more bits per symbol, with each
symbol transmitted, we can convey three bits. The throughput of 8PSK is 50%
better than QPSK which can transmit just 2 bits per symbol as compared to 3 for
8PSK. 8PSK is the first of the bandwidthefficient modulations. π/4QPSK – a variation on both QPSK and 8PSK
This a variation of QPSK that mimics 8PSK. Like QPSK, π/4QPSK transmits
two bits per symbol. So only four carrier signals are needed but this is where the
twist comes in. In QPSK we have four signals that are used to send the four twobit symbols. In π/4QPSK we have eight signals, every alternate symbol is
transmitted using a π/4 shifted pattern of the QPSP constellation. Symbol A uses a
signal on Path A as shown below and the next symbol, B, even if it is exactly the
same bit pattern uses a signal on Path B. So we always get a phase shift even
when the adjacent symbols are exactly the same.
The constellation diagram looks similar to 8PSK. Note that a 8PSK constellation
can be broken into two QPSK constellations as show below. In π/4QPSK, one
symbol is transmitted on the A constellation and the next one is transmitted using
the B constellation. Even though on a network analyzer, the constellation looks
like 8PSK, this modulation is strictly a form of QPSK with same BER and
bandwidth. Although the symbols move around, they always convey just 2 bits
per symbol.
B2 (01)
A2 (01) Path A
A1 (00) B3 (11) B1 (00) A3 (11) A4 (10)
B4 (10) Path B Figure 41  π/4QPSK constellation mimics 8PSK but it is two QPSK
constellations that are phase shifted.
Stepbystep π/4QPSK We wish to transmit the following bit sequence. We divide the bit sequence into
2bit pieces just as we would do for QPSK Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 36 Bit sequence: 00 00 10 00 01 11 11 00 01 00
Transmit the first symbol using the A constellation shown in Figure 41 and the
next symbol uses the B constellation. For each 2bit, the I and Q values are the
signal coordinates as shown below. Symb
ol 1
2
3
4
5
6
7
8
9
10 Bits Symbol
ID I
coordina
te 00
00
10
00
01
11
11
00
01
00 A1
B1
A4
B1
A2
B2
A3
B1
A2
B1 .707
0
.707
0
.707
1
.707
0
.707
0 Q
coordinat
e .707
1
.707
1
.707
0
.707
1
.707
1 Table 5  π/4QPSK symbols mapping to I and Q The I and Q channels for a π/4QPSK signal are shown below in Figure 42. Note
that there are five possible levels (1, .707. 0 .707, 1) and I and the Q channel
show this variation in response to the symbols. Figure 42 – I and Q mapping of π/4QPSK symbols Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 37 Step 1 – Map bits to symbols
Bits
Symbols 00
A1 00 10 00 01
B1 A4 B1 A2 11
B2 11 00 01 00
A3 B1 A2 B3 Step 2  Multiply the I and Q with a carrier (in the example below, the carrier
frequency is 1 Hz.) and you get an 8PSK signal constellation.
B1 (00)
A2 (01) A1 (00) B2 (01) B4 (10) A3 (11) A4 (10)
B3 (11) Path B Figure 43 – of π/4QPSK symbols traverse over a 8PSK constellation The constellation diagram is a path that the symbols have traced in time as we can
see in the above diagram of just the symbols of this signal. The path stars with
symbol A1, then goes to B1 which is on path B. From here, the next symbol A2 is
back on Path A. Each transition, we see above goes back and forth between Path
A and B. Figure 44 – π/4QPSK modulated I and Q Channels Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 38 Figure 45 – π/4QPSK modulated carrier
What is the advantage of doing this? On the average, the phase transitions are
somewhat less than a straight QPSK and this does two things, one is that the side
lobes are smaller so less adjacent carrier interference. Secondly the response to
Class C amplifiers is better. This modulation is used in many mobile systems. There is also a modification to this modulation where a differential encoding is
added to the bits prior to modulation. (More about differential encoding in
Tutorial 2.) When differential coding is added, the modulation is referred to as
π/4DQPSK. 16PSK
We can keep on subdividing the signal space into smaller regions. Doing so one
more time for 8PSK so that each signal is now only 22.5° apart, gives us 16PSK. This will give 16 signals or symbols, so each symbol can convey 4 bits. Bit
rate is now four times that of BPSK for the same symbol rate. The following
figures show the 16PSK signal at various stages during modulation. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 39 (a) 16PSK symbol mapping to I and Q channels. (Now the signal has 8
levels.) (b) 16PSK modulated I and Q Channels (c) 16PSK modulated carrier
Figure 46 – 16PSK modulated signal We can see where this is going. We can keep on increasing bits per symbol this
way. However, 16PSK is rarely used. Despite the fact that 16PSK is bandwidth
efficient is that it has higher bit error rate than a common modulation from the
class of Quadrature Amplitude Modulation called 16QAM which has the
same bit efficiency. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 40 16 QAM The modulation equation for QAM is a variation of the one used for PSK. The
generalized PSK allows changing both the amplitude and the pghase. In PSK all points
lie on a circle so the I and Q values are related to eachother. PSK signals are constant
envelope because of this. All points have the same amplitude. If we allow the amplitude
to change from sysmbol to symbol, then we get a modulation called quadrature amplitude
modulation (QAM). It can be considered a linear combination of two DSBSC signal. So
it is a AM and a PM modulation at the same time. s(t ) = 2 Es
2 Es
cos (θ (t ) ) cos (2π fct ) −
sin (θ (t ) ) sin (2π fct )
T
T
I Q This equation can be used to create a hybrid type of modulation that varies both the
amplitude and the phase. Let’s say that M = 16, so that we have 16 symbols, each
representing a four bit word. We can lay these out in a circle but they would be too close
and the error rate is likely to be high. So how about this constellation?
Q ax is
1 101 1001 0001 0101 1100 1000 0000 0100 3
1
1110 1010 0010 1111 1011 0011 I ax is 0110 0111 1
3 Figure 47 – 16QAM constellation in the IQ plane It turns out that this constellation which has 16 points all spread in the xy plane, instead
of 16 points all on the circle, performs better in some situations. In QAM, the signal
points lie in rectangle instead of a circle.
In MQAM, and this one is for M = 16, we vary not just the phase of the symbol but also
the amplitude. In PSK, all symbols sat on a circle so they all had the same amplitude.
Here the points closer to the axes have lesser amplitudes and hence energy than some
others. We can compute the x and y axis values of each of these points and depending on
the total power we want, we can set the value of a. For typical constellation, set a = 1. If
we call the symbols integers then they range from 0 to 15. We show a sequence of
random integers up 15 in signal s1 below that we will use these to create a 16QAM
signal. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 41 Table 4 – 16 QAM Mapping
Symbol
Bits S1 s1 (t ) = 0000 Expression Phase 2 Es
cos( 2π f c t + π / 4)
0°
T I Q 1 1 (Fill in the rest of the table.)
The first integer in s1 is 0. Its x coordinate is 3 shown in s2. its y coordinate is also 3. The
second integer is 15. Its xcoordinate is 3 and its y coordinate is 1.
Having created the I and the Q channels this way (it is a table look up function, also
called baseband processing), we can now multiply these signals with the cosine and the
sine wave carriers. Then add (or subtract) the two and you have the modulated carrier
shown in s6. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I Copyright 2002 Charan Langton , revised Dec 2005 42 www.complextoreal.com All About Modulation – Part I 43 Figure 48 – Generation of a 16QAM signal 64 QAM
These figures belong to a 64 QAM signal. There are 64 symbols, which means the
incoming integers are from 0 to 63, each representing 5 bits. Using a table lookup, we
create the I and Q channels, and then the modulated signal from thereon as before. Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 44 Figure 49 – Generation of a 64QAM signal
Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 45 More Example signals 8PSK Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 46 Figure 50 – Generation of a 8PSK signal Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 47 Figure 51 – Generation of a 16PSK signal Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com All About Modulation – Part I 48 Copyright 1998, 2006 Charan Langton Thanks to Rolando Menendez for his help and corrections. I can be contacted at
[email protected]
Other tutorials at
www.complextoreal.com Copyright 2002 Charan Langton , revised Dec 2005 www.complextoreal.com ...
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This note was uploaded on 02/07/2011 for the course EEE EE567 taught by Professor Tutorials during the Spring '11 term at Birla Institute of Technology & Science, Pilani  Hyderabad.
 Spring '11
 Tutorials
 Phase Shift

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