Phase modulation basics
Before looking at
phase modulation it is first necessary to look at phase itself. A radio
frequency signal consists of an oscillating carrier in the form of a
sine wave is the basis of the signal. The instantaneous amplitude
follows this curve moving positive and then negative, returning to the
start point after one complete cycle - it follows the curve of the sine
wave. This can also be represented by the movement of a point around a
circle, the phase at any given point being the angle between the start
point and the point on the waveform as shown.
Phase modulation works by modulating the phase of the
signal, i.e. changing the rate at which the point moves around the
circle. This changes the phase of the signal from what it would have
been if no modulation was applied. In other words the speed of rotation
around the circle is modulated about the mean value. To achieve this it
is necessary to change the frequency of the signal for a short time.
In other words when phase modulation is applied to a signal there are
frequency changes and vice versa. Phase and frequency are inseparably
linked as phase is the integral of frequency. Frequency modulation can
be changed to phase modulation by simply adding a CR network to the
modulating signal that integrates the modulating signal. As such the
information regarding sidebands, bandwidth and the like also hold true
for phase modulation as they do for frequency modulation, bearing in
mind their relationship.
Forms of phase modulation
Although phase
modulation is used for some analogue transmissions, it is far more
widely used as a digital form of modulation where it switches between
different phases. This is known as phase shift keying, PSK, and there
are many flavours of this. It is even possible to combine phase shift
keying and amplitude keying in a form of modulation known as quadrature
amplitude modulation, QAM.
The list below
gives some of the forms of phase shift keying that are used:
- PM
- Phase Modulation
- PSK - Phase Shift Keying
- BPSK - Binary Phase Shift Keying
- QPSK - Quadrature Phase Shift Keying
- 8 PSK - 8 Point Phase Shift Keying
- 16 PSK - 16 Point Phase Shift Keying
- QAM - Quadrature Amplitude Modulation
- 16 QAM - 16 Point Quadrature Amplitude Modulation
- 64 QAM - 64 Point Quadrature Amplitude Modulation
- MSK - Minimum Shift Keying
- GMSK - Gaussian filtered Minimum Shift Keying
These are just some of the major forms of
phase modulation that are widely used in radio communications
applications today. With today's highly software adaptable radio
communications systems, it is possible to change between the different
types of modulation to best meet the prevailing conditions.
Overview
Phase modulation, PM, is widely
used in today's radio communications scene, with phase shift keying
being widely used for digital modulation and data transmission. It is
used in all forms of radio communications from cellular technology to
Wi-Fi, WiMAX, radio broadcasting of digital audio and TV, and many more
forms of transmission.
communications. PSK, phase shift keying
enables data to be carried on a radio communications signal in a more
efficient manner than Frequency Shift Keying, FSK, and some other forms
of modulation.
With more forms of
communications transferring from analogue formats to digital formats,
data communications is growing in importance, and along with it the
various forms of modulation that can be used to carry data.
There are several flavours of phase shift keying, PSK
that are available for use. Each form has its own advantages and
disadvantages, and a choice of the optimum format has to be made for
each radio communications system that is designed. To make the right
choice it is necessary to have a knowledge and understanding of the way
in which PSK works.
Phase Shift Keying, PSK, basics
Like any
form of shift keying, there are defined states or points that are used
for signalling the data bits. The basic form of binary phase shift
keying is known as Binary Phase Shift Keying (BPSK) or it is
occasionally called Phase Reversal Keying (PRK). A digital signal
alternating between +1 and -1 (or 1 and 0) will create phase reversals,
i.e. 180 degree phase shifts as the data shifts state.
Binary phase shift keying, BPSK
The problem with phase shift keying is that the
receiver cannot know the exact phase of the transmitted signal to
determine whether it is in a mark or space condition. This would not be
possible even if the transmitter and receiver clocks were accurately
linked because the path length would determine the exact phase of the
received signal. To overcome this problem PSK systems use a differential
method for encoding the data onto the carrier. This is accomplished,
for example, by making a change in phase equal to a one, and no phase
change equal to a zero. Further improvements can be made upon this
basic system and a number of other types of phase shift keying have been
developed. One simple improvement can be made by making a change in
phase by 90 degrees in one direction for a one, and 90 degrees the other
way for a zero. This retains the 180 degree phase reversal between one
and zero states, but gives a distinct change for a zero. In a basic
system not using this process it may be possible to loose
synchronisation if a long series of zeros are sent. This is because the
phase will not change state for this occurrence.
There are many variations on the basic idea of phase
shift keying. Each one has its own advantages and disadvantages enabling
system designers to choose the one most applicable for any given
circumstances. Other common forms include QPSK (Quadrature phase shift
keying) where four phase states are used, each at 90 degrees to the
other, 8-PSK where there are eight states and so forth.
PSK constellation diagrams
It is often
convenient to represent a phase shift keyed signal, and sometimes other
types of signal using a phasor or constellation diagram. Using this
scheme, the phase of the signal is represented by the angle around the
circle, and the amplitude by the distance from the origin or centre of
the circle. In this way the can be signal resolved into quadrature
components representing the sine or I for In-phase component and the
cosine for the quadrature component. Most phase shift keyed systems
use a constant amplitude and therefore points appear on one circle with
a constant amplitude and the changes in state being represented by
movement around the circle. For binary shift keying using phase
reversals the two points appear at opposite points on the circle. Other
forms of phase shift keying may use different points on the circle and
there will be more points on the circle.
Constellation diagram for BPSK
When plotted using test equipment errors may be
seen from the ideal positions on the phase diagram. These errors may
appear as the result of inaccuracies in the modulator and transmission
and reception equipment, or as noise that enters the system. It can be
imagined that if the position of the real measurement when compared to
the ideal position becomes too large, then data errors will appear as
the receiving demodulator is unable to correctly detect the intended
position of the point around the circle.
Constellation diagram for QPSK
Using a constellation view of the signal enables
quick fault finding in a system. If the problem is related to phase, the
constellation will spread around the circle. If the problem is related
to magnitude, the constellation will spread off the circle, either
towards or away from the origin. These graphical techniques assist in
isolating problems much faster than when using other techniques.
QPSK is used for the forward link form the base
station to the mobile in the IS-95 cellular system and uses the absolute
phase position to represent the symbols. There are four phase decision
points, and when transitioning from one state to another, it is
possible to pass through the circle's origin, indicating minimum
magnitude.
On the reverse link from mobile to
base station, O-QPSK is used to prevent transitions through the
origin. Consider the components that make up any particular vector on
the constellation diagram as X and Y components. Normally, both of
these components would transition simultaneously, causing the vector to
move through the origin. In O-QPSK, one component is delayed, so the
vector will move down first, and then over, thus avoiding moving
through the origin, and simplifying the radio's design. A constellation
diagram will show the accuracy of the modulation.
Forms of phase shift keying
Although phase
modulation is used for some analogue transmissions, it is far more
widely used as a digital form of modulation where it switches between
different phases. This is known as phase shift keying, PSK, and there
are many flavours of this. It is even possible to combine phase shift
keying and amplitude keying in a form of modulation known as quadrature
amplitude modulation, QAM.
The list below
gives some of the more commonly used forms of phase shift keying, PSK,
and related forms of modulation that are used:
- PSK - Phase
Shift Keying
- BPSK - Binary Phase Shift Keying
- QPSK - Quadrature Phase Shift Keying
- O-QPSK - Offset Quadrature Phase Shift Keying
- 8 PSK - 8 Point Phase Shift Keying
- 16 PSK - 16 Point Phase Shift Keying
- QAM - Quadrature Amplitude Modulation
- 16 QAM - 16 Point Quadrature Amplitude Modulation
- 64 QAM - 64 Point Quadrature Amplitude Modulation
- MSK - Minimum Shift Keying
- GMSK - Gaussian filtered Minimum Shift Keying
These are just some of the major forms of
phase shift keying, PSK, that are widely used in radio communications
applications today. Each form of phase shift keying has its own
advantages and disadvantages. In general the higher order forms of
modulation allow higher data rates to be carried within a given
bandwidth. However the downside is that the higher data rates require a
better signal to noise ratio before the error rates start to rise and
this counteracts any improvements in data rate performance. In view of
this balance many radio communications systems are able to dynamically
choose the form of modulation depending upon the prevailing conditions
and requirements.
Overview
Phase shift keying, PSK, is a
particularly important form of modulation these days. With most of the
traffic on the newer radio communications systems and radio
communications links being carried as data and using forms of phase
shift keying, PSK, it is of particular importance.
Reason for Minimum Shift Keying, MSK
It
is found that binary data consisting of sharp transitions between
"one" and "zero" states and vice versa potentially creates signals that
have sidebands extending out a long way from the carrier, and this
creates problems for many radio communications systems, as any
sidebands outside the allowed bandwidth cause interference to adjacent
channels and any radio communications links that may be using them.
Minimum Shift Keying, MSK basics
The
problem can be overcome in part by filtering the signal, but is found
that the transitions in the data become progressively less sharp as the
level of filtering is increased and the bandwidth reduced. To overcome
this problem GMSK is often used and this is based on Minimum Shift
Keying, MSK modulation. The advantage of which is what is known as a
continuous phase scheme. Here there are no phase discontinuities
because the frequency changes occur at the carrier zero crossing
points.
When looking at a plot of a signal
using MSK modulation, it can be seen that the modulating data signal
changes the frequency of the signal and there are no phase
discontinuities. This arises as a result of the unique factor of MSK
that the frequency difference between the logical one and logical zero
states is always equal to half the data rate. This can be expressed in
terms of the modulation index, and it is always equal to 0.5.
Signal using MSK modulation
GMSK basics
GMSK modulation is based on
MSK, which is itself a form of phase shift keying. One of the problems
with standard forms of PSK is that sidebands extend out from the
carrier. To overcome this, MSK and its derivative GMSK can be used.
MSK and also GMSK modulation are what is known as a
continuous phase scheme. Here there are no phase discontinuities because
the frequency changes occur at the carrier zero crossing points. This
arises as a result of the unique factor of MSK that the frequency
difference between the logical one and logical zero states is always
equal to half the data rate. This can be expressed in terms of the
modulation index, and it is always equal to 0.5.
Signal using MSK modulation
A plot of the spectrum of an MSK signal shows
sidebands extending well beyond a bandwidth equal to the data rate. This
can be reduced by passing the modulating signal through a low pass
filter prior to applying it to the carrier. The requirements for the
filter are that it should have a sharp cut-off, narrow bandwidth and its
impulse response should show no overshoot. The ideal filter is known
as a Gaussian filter which has a Gaussian shaped response to an impulse
and no ringing. In this way the basic MSK signal is converted to GMSK
modulation.
Spectral density of MSK and GMSK signals
Generating GMSK modulation
There are two
main ways in which GMSK modulation can be generated. The most obvious
way is to filter the modulating signal using a Gaussian filter and then
apply this to a frequency modulator where the modulation index is set
to 0.5. This method is very simple and straightforward but it has the
drawback that the modulation index must exactly equal 0.5. In practice
this analogue method is not suitable because component tolerances drift
and cannot be set exactly.
Generating GMSK using a Gaussian filter and VCO
A second method is more widely used. Here what is
known as a quadrature modulator is used. The term quadrature means that
the phase of a signal is in quadrature or 90 degrees to another one.
The quadrature modulator uses one signal that is said to be in-phase
and another that is in quadrature to this. In view of the in-phase and
quadrature elements this type of modulator is often said to be an I-Q
modulator. Using this type of modulator the modulation index can be
maintained at exactly 0.5 without the need for any settings or
adjustments. This makes it much easier to use, and capable of providing
the required level of performance without the need for adjustments. For
demodulation the technique can be used in reverse.
Block diagram of I-Q modulator used to create GMSK
Advantages of GMSK modulation
there are
several advantages to the use of GMSK modulation for a radio
communications system. One is obviously the improved spectral
efficiency when compared to other phase shift keyed modes.
A further advantage of GMSK is that it can be amplified
by a non-linear amplifier and remain undistorted This is because there
are no elements of the signal that are carried as amplitude
variations. This advantage is of particular importance when using small
portable transmitters, such as those required by cellular technology.
Non-linear amplifiers are more efficient in terms of the DC power input
from the power rails that they convert into a radio frequency signal.
This means that the power consumption for a given output is much less,
and this results in lower levels of battery consumption; a very
important factor for cell phones.
A further
advantage of GMSK modulation again arises from the fact that none of
the information is carried as amplitude variations. This means that is
immune to amplitude variations and therefore more resilient to noise,
than some other forms of modulation, because most noise is mainly
amplitude based.
GMSK highlights
GMSK modulation is a highly
successful form of modulation, being used in GSM cellular technology,
and as a result, its use is particularly widespread. It is also used in
other radio communications applications because of its advantages in
terms of spectral efficiency, resilience to noise and its ability to
allow the use of efficient transmitter final amplifiers. Even though
other radio communications systems utilise other forms of modulation,
GMSk is an ideal choice for many applications.
Analogue and digital QAM
Quadrature amplitude modulation,
QAM may exist in what may be termed either analogue or digital formats.
The analogue versions of QAM are typically used to allow multiple
analogue signals to be carried on a single carrier. For example it is
used in PAL and NTSC television systems, where the different channels
provided by QAM enable it to carry the components of chroma or colour
information. In radio applications a system known as C-QUAM is used for
AM stereo radio. Here the different channels enable the two channels
required for stereo to be carried on the single carrier.
Digital formats of QAM are often referred to as "Quantised QAM" and
they are being increasingly used for data communications often within
radio communications systems. Radio communications systems ranging from
cellular technology through wireless systems including WiMAX, and Wi-Fi
802.11 use a variety of forms of QAM, and the use of QAM will only
increase within the field of radio communications.
Digital / Quantised QAM basics
Quadrature amplitude modulation,
QAM, when used for digital transmission for radio communications
applications is able to carry higher data rates than ordinary amplitude
modulated schemes and phase modulated schemes. As with phase shift
keying, etc, the number of points at which the signal can rest, i.e.
the number of points on the constellation is indicated in the
modulation format description, e.g. 16QAM uses a 16 point
constellation.
When using QAM, the constellation points are normally arranged in a
square grid with equal vertical and horizontal spacing and as a result
the most common forms of QAM use a constellation with the number of
points equal to a power of 2 i.e. 2, 4, 8, 16 . . . .
By using higher order modulation formats, i.e. more points on the
constellation, it is possible to transmit more bits per symbol. However
the points are closer together and they are therefore more susceptible
to noise and data errors.
To provide an example of how QAM operates, the table below provides the
bit sequences, and the associated amplitude and phase states. From
this it can be seen that a continuous bit stream may be grouped into
threes and represented as a sequence of eight permissible states.
Bit sequence | Amplitude | Phase
(degrees) |
000 | 1/2 | 0 (0°) |
000 | 1 | 0 (0°) |
010 | 1/2 | π/2 (90°)
|
011 | 1 | πi/2 (90°) |
100 | 1/2 | π (180°)
|
101 | 1 | π (180°) |
110 | 1/2 | 3πi/2
(270°) |
111 | 1 | 3π/2 (270°) |
Bit sequences, amplitudes and phases for 8-QAM
Phase modulation can be considered as a special form of QAM where the
amplitude remains constant and only the phase is changed. By doing this
the number of possible combinations is halved.
QAM advantages and disadvantages
Although QAM appears to
increase the efficiency of transmission for radio communications
systems by utilising both amplitude and phase variations, it has a
number of drawbacks. The first is that it is more susceptible to noise
because the states are closer together so that a lower level of noise
is needed to move the signal to a different decision point. Receivers
for use with phase or frequency modulation are both able to use
limiting amplifiers that are able to remove any amplitude noise and
thereby improve the noise reliance. This is not the case with QAM.
The second limitation is also associated with the amplitude component
of the signal. When a phase or frequency modulated signal is amplified
in a radio transmitter, there is no need to use linear amplifiers,
whereas when using QAM that contains an amplitude component, linearity
must be maintained. Unfortunately linear amplifiers are less efficient
and consume more power, and this makes them less attractive for mobile
applications.
QAM comparison with other modes
As there are advantages and
disadvantages of using QAM it is necessary to compare QAM with other
modes before making a decision about the optimum mode. Some radio
communications systems dynamically change the modulation scheme
dependent upon the link conditions and requirements - signal level,
noise, data rate required, etc.
The table below compares various forms of modulation:
Modulation | Bits per symbol |
Error margin | Complexity |
OOK | 1 | 1/2 | 0.5
| Low |
BPSK | 1 | 1 |
1 | Medium |
QPSK | 1 | 1 / √2 | 0.71
| Medium |
16 QAM | 4 | √2 / 6
| 0.23 | High |
64QAM | 6 | &radic / 14 |
0.1 | High |
Summary of types of modulation with data capacities
QAM summary
When choosing the form of modulation to use for a
radio communications system, it is necessary to take account of all its
attributes to assess whether it is suitable. QAM, Quadrature Amplitude
Modulation is being used increasingly because of the increased data
rate it offers compared to some of the simpler amplitude or phase only
formats. Also adaptive systems are being used to use QAM when
conditions are suitable, enabling high data rates to be transmitted
over the radio communications link.
QAM, Quadrature amplitude modulation is widely used in many digital
data radio communications and data communications applications. A
variety of forms of QAM are available and some of the more common forms
include 16 QAM, 32 QAM, 64 QAM, 128 QAM, and 256 QAM. Here the figures
refer to the number of points on the constellation, i.e. the number of
distinct states that can exist.
The various flavours of QAM may be used when data-rates beyond those
offered by 8-PSK are required by a radio communications system. This is
because QAM achieves a greater distance between adjacent points in the
I-Q plane by distributing the points more evenly. And in this way the
points on the constellation are more distinct and data errors are
reduced. While it is possible to transmit more bits per symbol, if the
energy of the constellation is to remain the same, the points on the
constellation must be closer together and the transmission becomes more
susceptible to noise. This results in a higher bit error rate than for
the lower order QAM variants. In this way there is a balance between
obtaining the higher data rates and maintaining an acceptable bit error
rate for any radio communications system.
QAM applications
QAM is in many radio communications and data
delivery applications. However some specific variants of QAM are used
in some specific applications and standards.
For domestic broadcast applications for example, 64 QAM and 256 QAM are
often used in digital cable television and cable modem applications.
In the UK, 16 QAM and 64 QAM are currently used for digital terrestrial
television using DVB - Digital Video Broadcasting. In the US, 64 QAM
and 256 QAM are the mandated modulation schemes for digital cable as
standardised by the SCTE in the standard ANSI/SCTE 07 2000.
In addition to this, variants of QAM are also used for many wireless and
cellular technology applications.
Constellation diagrams for QAM
The constellation diagrams show
the different positions for the states within different forms of QAM,
quadrature amplitude modulation. As the order of the modulation
increases, so does the number of points on the QAM constellation
diagram.
The diagrams below show constellation diagrams for a variety of formats
of modulation:
QAM bits per symbol
The advantage of using QAM is that it is a
higher order form of modulation and as a result it is able to carry
more bits of information per symbol. By selecting a higher order format
of QAM, the data rate of a link can be increased.
The table below gives a summary of the bit rates of different forms of
QAM and PSK.
Modulation | Bits per symbol | Symbol
Rate |
BPSK | 1 | 1 x bit
rate |
QPSK | 2 | 1/2 bit rate |
8PSK | 3 | 1/3 bit
rate |
16QAM | 4 | 1/4 bit rate |
32QAM | 5 | 1/5 bit
rate |
64QAM | 6 | 1/6 bit rate |
QAM noise margin
While higher order modulation rates are able to
offer much faster data rates and higher levels of spectral efficiency
for the radio communications system, this comes at a price. The higher
order modulation schemes are considerably less resilient to noise and
interference.
As a result of this, many radio communications systems now use dynamic
adaptive modulation techniques. They sense the channel conditions and
adapt the modulation scheme to obtain the highest data rate for the
given conditions. As signal to noise ratios decrease errors will
increase along with re-sends of the data, thereby slowing throughput.
By reverting to a lower order modulation scheme the link can be made
more reliable with fewer data errors and re-sends.
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