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Non-Coherent Amplitude Shift Keying Chapter 2
CHAPTER 2
2.1 Theory of Digital Modulation
A digital communication system is one which sends number of signals from
one place to another in order to convey information.
Information can be presented as groups of (usually binary) digits. Such group
is called a digital Word. It is usually convenient to send the digit serially (one after the
other) and to put them together again as words at the receiving end.
Analog information, such as the voltage signal from a telephone, can be
converted to digital from, sent over a digital communication channel, and recovered to
analog form at the receiver.
In order to transmit a signal it is often modulated, and with digital signal the
modulation is referred to as “keying”. The amplitude shift keying (ASK) is considered
as the simplest way of shifting the frequency spectrum of a signal from base band to
some other band of frequencies.
The use of a higher frequency range reduces antenna size. In the modulation
process, the baseband signals constitute the modulating signal and the high-frequency
carrier signal is a sinusiodal waveform. There are three basic ways of modulating a
sine wave carrier. For binary digital modulation, they are called [ binary amplitude-
shift keying (BASK), binary frequency-shift keying (BFSK) and binary phaseshift
keying (BPSK)].
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Non-Coherent Amplitude Shift Keying Chapter 2
Modulation also leads to the possibility of frequency multiplexing. In a
frequency-multiplexed system, individual signals are transmitted over adjacent,
nonoverlapping frequency bands. They are therefore transmitted in parallel and
simultaneously in time. If we operate at higher carrier frequencies, more bandwidth is
available for frequency-multiplexing more signals.
Transmission of data across a noisy communications channel requires some
manner of separating the valid data from the background noise. The most common
way to accomplish this is to modulate the data at the transmission side and to
demodulate the data on the reception side, the end result being that the data coming
from the receiver are the same as the data being presented to the transmitter. The
efficiency of the modulation/demodulation process determines the accuracy of the
data coming from the receiver. Therefore, careful consideration must be given to the
selection of an appropriate modulation-demodulation scheme.
Figure 2-1…Different digital modulation techniques
2.2 Amplitude Shift Keying [ASK]
Amplitude shift keying -ASK- as shown in figure 19 in the context of digital
communications is a modulation process which imparts to a sinusoid two or more
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Non-Coherent Amplitude Shift Keying Chapter 2
discrete amplitude levels (Also called on-off keying - OOK). These are related to the
number of levels adopted by the digital message.
The digital data to be transmitted is the binary number . Two amplitudes are
used to directly represent the data, either 0 or 1. In this case, the modulation is called
binary amplitude shift keying or BASK
Figure 2-2…Binary Amplitude-Shift Keying (BASK)
A binary amplitude-shift keying (BASK) signal can be defined by
Equation 2-1…Characteristic of ASK
where A is a constant , fc is the carrier frequency, and Tb is the bit duration.
Figure 2-3 shows the BASK signal sequence generated by the binary
sequence 0 1 0 1 0 0 1. The amplitude of a carrier is switched or keyed by the binary
signal m(t). This is sometimes called on-off keying (OOK).
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Non-Coherent Amplitude Shift Keying Chapter 2
Figure 2-3…(a) Binary modulating signal (b) ASK signal.
There are sharp discontinuities shown at the transition points. These result in
the signal having an unnecessarily wide bandwidth. Bandlimiting is generally
introduced before transmission, in which case these discontinuities would be 'rounded
off'. The band-limiting may be applied to the digital message, or the modulated signal
itself.
It is a special case of amplitude modulation (AM). Amplitude modulation has
the property of translating the spectrum of the modulation to the carrier frequency.
The bandwidth of the signal remains unchanged.
The fact that AM simply shifts the signal spectrum is often used to convert the
carrier frequency to a more suitable value without altering the modulation. This
process is known variously as mixing, up-conversion or down-conversion. Some form
of conversion will always be present when the channel carrier occupies a frequency
range outside the modulation frequency range.
One of the disadvantages of ASK, compared with FSK and PSK, for example,
is that it has not got a constant envelope. This makes its processing (eg, power
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amplification) more difficult, since linearity becomes an important factor. However, it
does make for ease of demodulation with an envelope detector.
A block diagram of a basic non coherent ASK generator is shown in
Figure 2-4.
Figure 2-4…Transmitter Of ASK
While in figure 2-4 we can see a coherent ASK generator, the main difference
between the coherent systems and non-coherent systems that the coherent systems
needs carrier synchronization while this is not required in non coherent systems.
Figure 2-5…Coherent ASK transmitter
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2.2.1 Bandwidth modification
As already indicated, the sharp discontinuities in the ASK waveform of
Figure2-3 imply a wide bandwidth. A significant reduction can be accepted before
errors at the receiver increase unacceptably. This can be brought about by
bandlimiting (pulse shaping) the message before modulation, or bandlimiting the ASK
signal itself after generation, Both these options are illustrated in Figure22, which
shows one of the generators we will be modeling.
2.2.2 Using dual analog switch
The simplest method for binary ASK is to use a switch to gate the carrier on
and off, driven by the data signal as shown in figure 2-4 earlier while we can use any
mean of multiplication to achieve the goal which is modulating the carrier with the
data signal.
2.2.3 Symmetry in ASK
Spectrum of an ASK signal can be determined from its baseband data stream if
the ASK modulation process if seen as a multiplication or mixing of the baseband
symbol stream with the carrier wave.
Consider a single frequency cos(wmt) from within the baseband spectrum and
perform the mathematical multiplication with the carrier cos(wct) ...
2.2.4 Demodulation methods
For demodulation and detection purpose two techniques can be used.
Non-coherent (Threshold) detection to detect presence or absence of a carrier
signal.
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Coherent demodulation; multiplying the modulated signal by the carrier
signal to recover the baseband data signal. This detection is better for a noisy
environment.
2.2.4.1 Non-Coherent Detection
In the coherent detector, the exact reproduction of the carrier is needed (i.e.
requires carrier or phase synchronization).
Figure 2-6…Non coherent Detection
The diagram in figure 2-6 is a non-coherent detector which does not require carrier or
phase synchronization.
Both asynchronous and synchronous demodulation methods are used for the
demodulation of ASK signals. It is apparent that the ASK signal has a well defined
envelope. Thus it is amenable to demodulation by an envelope detector. A
synchronous demodulator would also be appropriate. We note that: Envelope
detection circuitry is simple. Synchronous demodulation requires a phase-Iocked local
carrier and therefore carrier acquisition circuitry.
With band limiting of the transmitted ASK neither of these demodulation
methods would recover the original binary sequence; instead, their outputs would be a
band limited version. Thus further processing - by some sort of decision-making
circuitry for example - would be necessary.
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Thus demodulation is a two-stage process:
Recovery of the bandlimited bit stream
Regeneration of the binary bit stream
Envelope demodulation
Having a very definite envelope, an envelope detector can be used as the first
step in recovering the original sequence. Further processing can be employed to
regenerate the true binary waveform this is can be.
Figure 2-7…Envelope detector operation
2.2.4.2 Coherent ASK Detection
A coherent detector figure 2-7 operates by mixing the incoming data signal
with a locally generated carrier reference and selecting the difference component from
the mixer output.
Figure 2-8…Coherent detection of ASK
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If the modulated data signal is a(t).cos(wct) and the reference carrier
cos(wct + q) where q is the phase error between the source and reference carriers, the
mixer output becomes:
a(t).cos(wct).cos(wct + q) = 0.5a(t).cos(q) + 0.5a(t).cos (2wct + q)
If q = 0 (reference carrier phase coherent) output is proportional to a(t)
Then coherent detection has better noise rejection
2.2.5 Coherent ASK Vs non-coherent ASK
In non-coherent detection, V is amplitude of the signal
Coherent ASK is more resistive to noise than non-coherent ASK, coherent
ASK is more complicated than non-coherent ASK due to the need of a local carrier in
coherent ASK receiver that is synchronized with the transmitted carrier (i.e. having
the same frequency and phase as the transmitted carrier).
2.3 Frequency Shift Keying [ FSK ]
Frequency-shift keying (FSK) is a method of transmitting digital signals. The
two binary states, logic 0 (low) and 1 (high), are each represented by an analog
waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is
represented by a wave at a different frequency. A modem converts the binary data
from a computer to FSK for transmission over telephone lines, cables, optical fiber, or
wireless media. The modem also converts incoming FSK signals to digital low and
high states, which the computer can "understand."
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Figure 2-9…Frequency shift keying
FSK is simplified a form of Frequency Modulation (FM). For good noise
performance and high bandwidth operation, FSK is the modulation technique of
choice. In true FM, an analog signal is represented with a linear frequency deviation
from center. FSK is a binary form of frequency modulation which uses hard shifts
between deviant frequencies to represent the data originally impressed on the carrier.
The magnitude of frequency shift is directly related to the magnitude of the
modulation source voltage.The modulation source is allowed two states: “on” and
“off”. When the modulation source is “off ” , the carrier frequency is shifted down
from the center frequency. When the modulation source is “on”, the carrier frequency
is shifted up from the center frequency. The amount that the carrier frequency is
shifted is referred to as the frequency deviation.
2.3.1 Modulation Process
In FSK, the instantaneous frequency of the carrier is switched between 2 or
more levels according to the baseband digital data, for example at logic 0 frequency a
wave of Acos wct is sent while when logic 1 data come, the system will produce a
wave of Acos n wct (where n = integer)
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Figure 2-10…Generated waveform of FSK
Also in FSK systems we can use two different methods of modulation, either
we use non-Coherent scheme or a coherent scheme.
A non-coherent FSK transmitter is shown in figure 2-11 where the simply data
source is connected directly to a voltage controlled oscillator that can give two
different frequencies, one sent at logic 1 and the other is sent at logic 0 inorder to
generate and FSK signal.
Figure 2-11…Non-coherent FSK transmitter
Unlike ASK, a carrier is always present with FSK modulation. This affords the
designer several benefits. First, the carrier will load the receiver at all times providing
greatly increased noise immunity. Secondly, the strength (or amplitude) of the carrier
can be used to determine the quality of the incoming signal.
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Non-Coherent Amplitude Shift Keying Chapter 2
As illustrated in figure 2-12 the coherent FSK system is more complex than
non-coherent one, but as said earlier the coherent systems have more immunity to
nose but they are more expensive, the same idea of non-coherent FSK is applied here.
Figure 2-12…Coherent FSK transmitter
2.3.2 Demodulation process
Demodulation of FSK depends on the modulation used either coherent or non-
coherent and below we can see the both receiver diagrams.
2.3.2.1 Non-Coherent FSK receiver
Figure 2-13 shown a non-coherent FSK receiver where the received signal
enters the system through two band pass filters so that we can separate each frequency
of the FSK signal either fc+fd or fc-fd after getting each frequency separated we pass
the signal top an envelope detector that will reconstruct the level of the data (the peak
voltage of the signal) the resultant is passed to a summer that will give us the final
value of the voltage the value will be compared with a reference voltage so that we
can regenerate the original data.
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Figure 2-13…Non-coherent FSK receiver
2.3.2.2 Coherent FSK receiver
Coherent FSK method uses a local carrier generator so that we can reconstruct
the data sent, this can be achieved by multiplying the entering signal by a carrier
signal as shown in figure 2-14.
Figure 2-14…Coherent FSK receiver
A drawback of the continuous carrier is that the transmitter is always drawing
power and generating an output. Therefore, the transmitter will ultimately require a
higher supply current than ASK-based systems. In addition, the output power cannot
be legally increased in countries
FSK is a Non Return to Zero modulation method. This means that the non-
modulated condition is between the “off” and “on” condition. In other words, the
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carrier should never be at the center frequency when modulation is present. The
benefit here is noise immunity. Hysteresis can be applied to the detector, eliminating
the effect of spurious frequency modulation generated from sources other than the
data stream.
Since FSK relies on frequency change, and not amplitude change, to indicate
data states, an FSK receiver is inherently immune to amplitude noise. This is of great
importance in bands which are extremely crowded and have a high potential for near-
band interference. This increased noise immunity suggests a potential for higher data
rates.
FSK modulation for the transmission of data has many features and limitations
to consider.
2.3.3 Advantages of FSK are:
Higher data rates
Continuous carrier presence
High noise immunity
2.3.4 Limitations of FSK are:
Higher cost
High power consumption
Larger size of equipments
FSK modulation should be used for applications where data rate, noise
immunity, and using one channel are of primary concern.
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2.4 Phase Shift Keying [PSK]
Phase-shift keying (PSK) is a method of transmitting and receiving digital
signals in which the phase of a transmitted signal is varied to convey information.
There are several schemes that can be used to accomplish PSK. The simplest
method uses only two signal phases: 0 degrees and 180 degrees. The digital signal is
broken up timewise into individual bits (binary digits). The state of each bit is
determined according to the state of the preceding bit. If the phase of the wave does
not change, then the signal state stays the same (low or high). If the phase of the wave
changes by 180 degrees -- that is, if the phase reverses -- then the signal state changes
(from low to high, or from high to low). Because there are two possible wave phases,
this form of PSK is sometimes called bi-phase modulation.
Figure 2-15…Phase shift keying
More complex forms of PSK employ four or eight wave phases. This allows
binary data to be transmitted at a faster rate per phase change than is possible with
biphase modulation. In four-phase modulation, the possible phase angles are 0, +90 ,
-90, and 180 degrees; each phase shift can represent two signal elements. In eight-
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phase modulation, the possible phase angles are 0, +45, -45, +90, -90, +135, -135, and
180 degrees; each phase shift can represent four signal elements.
Equation 2-2…Characteristic of PSK
Carrier frequency and amplitude remains same only the phase of the carrier is
shifted by 180 degree.For demodulation only Coherent technique can be used. In this
case the phase of received signal must be compared with a reference signal which is
the synchronised local carrier signal
Normally, PSK signals are transmitted by generating an audio signal which is
sent using SSB modulation. It is also possible to directly phase-key a radio frequency
carrier.
The technology used for receiving PSK signals is extremely complex with
compensation required for spreading of the signal energy between symbols (inter-
symbol interference) and for removing the effects of multipath in the HF environment
This is very much flavour of the moment in the amateur radio community at
present. This an experimental mode used by radio amateurs as a teleprinter signal
using extremely narrow bandwidths. This has the advantage that the mode is resistant
to noise and requires comparatively low power levels for successful communications.
Phase shift keying is a technique which shifts the period of a wave as we said
earlier ,this wave has a period of p. Also the start of the wave's period is at 0 as in
figure 2-16.
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Figure 2-16…Wave of period p and starts at 0
In figure 2-17 the same wave as the first, but its phase has been shifted. Notice
that the period starts at the wave's highest point (1).
Figure 2-17… Wave of period p shifted
It just so happens that we have shifted this wave by one quarter of the wave's
full period. We can shift it another quarter, if we wanted to, so the original wave
would be shifted by half it's period. And we could do it one more time, so that it
would be shifted three quarters of it's original period.
This means we have 4 separate waves. So why not let each wave stand for
some binary value? Since there are 4, we can let each wave signify 2 bits
(00,01,10,11) this can be summarized in table 2-1.
Bit value Amount of shift
00 None
01 ¼
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10 ½
11 ¾
Table 2-1…Phase shift of each bit value
2.4.1 Modulation Process:
In PSK, the phase of the carrier signal is switched between 2 or more values in
response to the baseband digital data the waveform is shown in figure 2-18. There are
two methods of generating PSK signals one of them is the coherent PSK and the other
is the differentially coherent PSK, the differentially coherent PSK is simpler than the
coherent PSK.
Figure 2-18… PSK signal waveform
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2.4.1.1 Differentially Coherent PSK Transmitter
Figure 2-19… Differentially Coherent PSK Transmitter
2.4.1.2 Coherent PSK transmitter
Figure 2-20…Coherent PSK transmitter
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2.4.2 Demodulation Process
2.4.2.1 Differentially coherent PSK Receiver
Figure 2-21… Differentially coherent PSK Receiver
2.4.2.2 Coherent PSK receiver
Figure 2-22…Coherent PSK Receiver
2.4.3 Summery of Digital modulation techniques
Modulation Type Data Transmitted signal
Amplitude Shift Keying0 0
1
Frequency Shift Keying0
1
Phase Shift Keying0
1
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2.4.4 Compression between digital modulation methods
Modulation Type Resistivitiy to noise Technical complexity
Amplitude Shift KeyingLess Resistivitiy to noise than
other systemsSimple hardware
Frequency Shift KeyingMore resistive to noise than ASK
and less than PSK
Complex hardware with
respect to ASK and simple
with respect to PSK
Phase Shift KeyingThe most resistive to noise with
respect to the other systems
Complex hardware with
respect to the other systems
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