Chapter 4 Part 2(a) Digital Modulation Techniquesportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · Frequency Shift Keying (FSK) Quadrature Amplitude Modulation ... Microsoft

Chapter 4 Chapter 4

Part 2(a)Part 2(a)

Digital Modulation TechniquesDigital Modulation Techniques

OverviewOverview

�� Digital Modulation techniques Digital Modulation techniques

�� Bandpass data transmissionBandpass data transmission

�� Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)

�� Phase Shift Keying (PSK)Phase Shift Keying (PSK)

�� Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)

�� QuadratureQuadrature Amplitude Modulation (QAMAmplitude Modulation (QAM))


�� Digital modulationDigital modulation

� The process by which digital symbols are transformed into waveforms that are compatible with the characteristic of the channel.

�� BandpassBandpass modulationmodulation

� Process whereby the amplitude, frequency, or phase of an RF carrier, or a combination of them, is varied in accordance with the information to be transmitted.

Digital Digital BandpassBandpass ModulationModulation

�� A carrier signal has three parameters which can be used for A carrier signal has three parameters which can be used for

impressing:impressing:

)](cos[)()(

)()(

)(cos)()(

0

0

tttAts

ttt

ttAts

φω

φωθ

θ

+=∴

+=

=

Amplitude Frequency Phase


• If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK)

• If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)


� If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK)

� If both the amplitude and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)

Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)

� ASK demonstrates poor performance, as it is heavily affected by noise and interference.

� Used in radio telegraphy in the early 1900s

Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)

� M was chosen to be equal to 2, so it is corresponding to two waveform types.

� Also know as Binary ASK signaling

(also called on-off keying)

Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)

� Bandwidth occupancy of FSK is dependant on the spacing of the two symbols. A frequency spacing of 0.5 times the symbol period is typically used.

� FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states.

Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)

� M was chosen to be equal to 3, corresponding to the 3 waveform types (3-ary).

� Emphasize the mutually perpendicular axes.

� The signal set is characterized by Cartesian coordinates, such that each of the mutually perpendicular axes represents a sinusoid with a different frequency.

� Such mutually perpendicular vectors are called orthogonal signals.

Phase Shift Keying (PSK)Phase Shift Keying (PSK)

� Phase Shift Keying (PSK) demonstrates better performance than ASK and FSK.

� PSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states.

� Filtering can be employed to avoid spectral spreading.

� Widely used in both military and commercial communications

system. .


� M was chosen as to be as 2, and it is called binary PSK (BPSK)

� The modulating signal shifts the phase of the wave si(t) to one of two states, either zero or π (180º).

� For the BPSK example, the vector picture illustrates the two 180º opposing vectors.

� Signal sets that can be depicted with such opposing vectors are

called antipodal signal sets.


� Constellation of two-level PSK



� 4-PSK has more efficient usage of bandwidth than 2-PSK, because each signal unit has two bits. For the same bandwidth, the data bit rate doubles.


� Excellent performance of 2-PSK encourages us to go with 4-PSK, also called quadrature PSK (Q-PSK)


� The idea can be extended to 8-PSK, 16-PSK, 32-PSK,….

� The limitation is the ability of equipment to distinguish small differences in signal’s phase.

8 PSK

BPSK ModulatorBPSK Modulator

� Binary PSK (BPSK) modulation can be accomplished by simply multiplying the original signal d(t) (which is a binary random sequence) by the carrier signal, which is an analog sinusoidal oscillation. After multiplication a bandpass filter is required

QPSK ModulatorQPSK Modulator

QuadratureQuadrature Amplitude Modulation Amplitude Modulation

(QAM)(QAM)

� Combination of ASK and PSK which helps making a contrast between signal units. The number of amplitude shifts should be lower than the number of phase shifts due to noise susceptibility of ASK.

QuadratureQuadrature Amplitude Modulation Amplitude Modulation

(QAM)(QAM)

SummarySummary

Chapter 4 Chapter 4

Part 2(b)Part 2(b)


OverviewOverview

� Demodulation/Detection

� Coherent Detection

� Non-coherent detection

Demodulation/ DetectionDemodulation/ Detection

�� A binary A binary bandpassbandpass system will transmit one of two waveforms, denoted system will transmit one of two waveforms, denoted

s1(t) and s2(t). s1(t) and s2(t). Si(tSi(t) is used as a generic designation for a transmitted ) is used as a generic designation for a transmitted

waveform. waveform.

�� The received signal The received signal r(tr(t) degraded by noise ) degraded by noise n(tn(t) and is rewritten as ) and is rewritten as

�� n(tn(t) assumed to be a zero mean AWGN process. (degradation caused by) assumed to be a zero mean AWGN process. (degradation caused by

unavoidable thermal noise)unavoidable thermal noise)


Demodulation- Recovery of a waveform (to an undistorted pulse)Detection- decision making process of selecting the digital meaning of that

waveform.


�� Step 1:Step 1:

�� Frequency downFrequency down--conversion conversion

�� Frequency translation for signals operating at some radio Frequency translation for signals operating at some radio

frequency (RF)frequency (RF)

�� Receiving Filter (matched filter)Receiving Filter (matched filter)

�� To recover a pulse with the best possible signal to noise To recover a pulse with the best possible signal to noise

ratio (SNR), free of any ISIratio (SNR), free of any ISI

�� Equalizing FilterEqualizing Filter

�� Compensate for the distortion caused by both the Compensate for the distortion caused by both the

transmitter and the channel. transmitter and the channel.


� Predetection Point

� Sample / test statistic z(T) that has a voltage value directly proportional to the energy of the received symbol and that of the noise.

� Step 2:

� Threshold Comparison

� Decision is made regarding the digital meaning of that sample.


� Coherent Detection

� When the receiver exploits knowledge of the carrier’s phase to detect the signals.

� Non-coherent Detection

� When the receiver does not utilize such phase reference

information to detect the signals.

Coherent Detection of PSKCoherent Detection of PSK

�� Consider the following binary PSK (BPSK)Consider the following binary PSK (BPSK)

�� ϕϕ=arbitrary constant=arbitrary constant

�� E= signal energy per symbolE= signal energy per symbol

�� T = symbol duration.T = symbol duration.


� Convert the antipodal case into single basis function, ψ1(t) as

� Thus, express the transmitted signal, si(t) in terms of ψ1(t) and the coefficients ai1(t) as follows:


� For detection, the correlation receiver can be drawn as two product integrators, one of which is matched to s1(t), and the other is matched to s2(t)


� Assume that s1(t) was transmitted. Then the expected values of the product integrators as in Figure 5.1 with reference signals ψ1(t) , are found as

� E{.} denotes the expected value.

Coherent Detection of FSKCoherent Detection of FSK

� A typical set of FSK signal waveforms as below:

� E is the energy content of si(t) over each symbol duration T

� Assuming that the basis functions ψ1(t) , ψ2(t), .. ψN(t) form an orthonormal set, the most useful form for {ψj(t)} is


�� The linear combination of N orthogonal waveforms The linear combination of N orthogonal waveforms

�� These relationships are expressed in more compact notation asThese relationships are expressed in more compact notation as

�� where where

ψ1(t),ψ2(t)….

…ψN(t)


�� The amplitude normalizes the expected output andThe amplitude normalizes the expected output and can can

write as: write as:

�� Therefore, Therefore,

�� The The iithth prototype signal vector is located on the prototype signal vector is located on the iithth coordinate coordinate

axis at a displacement from the origin of the signal axis at a displacement from the origin of the signal space.space.

NonNon--coherent Detection of Differential coherent Detection of Differential

PSKPSK

� The term DPSK, refers to a detection scheme often classified as non coherent because it does not require a reference in phase with the received carrier.

� No attempt is made to determine the actual value of the phase from the incoming signal.

� Therefore, if the transmitted waveform is


PSKPSK

�� Inversely we can recover Inversely we can recover akak from from dkdk usingusing

�� akak = = dkdk ⊕⊕ dkdk−−11

�� If If dkdk and dkand dk−−1 are the same, then they represent a 1 of 1 are the same, then they represent a 1 of akak. .

If If dkdk and dkand dk−−1 are different, they represent a 0 of 1 are different, they represent a 0 of akak..

�� The sequence {The sequence {dkdk} is modulated onto a carrier with phase } is modulated onto a carrier with phase

0 or 0 or ππ..

ContCont’’dd……


PSKPSK

� The received signal can be characterized by

� α is arbitrary constant and is typically assumed to be a random variable uniformly distributed between zero and 2π

� n(t) is an AWGN process ( noise).

� If we assume that α varies slowly relative to two period times (2T), the phase difference between two successive waveforms,

θj(T1) and θk(T2) is independent of α; that is,

NonNon--coherent Detection of coherent Detection of

Differential PSKDifferential PSK

�� To send the To send the iithth message (i= 1,2,message (i= 1,2,……M), the present signal M), the present signal

waveform must have its phase advanced by waveform must have its phase advanced by ϕϕi= 2i= 2ππi/M i/M

radians over the previous waveform. radians over the previous waveform.

�� The detector measures the angle between the currently The detector measures the angle between the currently

received signal vector and the previously received signal received signal vector and the previously received signal

vectorvector

�� Figure: signal space for DPSKFigure: signal space for DPSK

NonNon--coherent Detection of FSKcoherent Detection of FSK

� The detection hardware must be configured as an energy detector,without exploiting phase measurement

� In-phase (I)

� Quadrature (Q)

NonNon--coherent Detection of FSKcoherent Detection of FSK

� The incoming signal will partially correlate with the cos (ω1t) reference and partially correlate with the sin (ω1t) reference.

� The non-coherent receiver require I and Q branch for each candidate signal in the signaling set.

� There are product integrators and squaring operation to prevent the appearance of any negative values.

� Each signal will be added from I and Q channel respectively.

� The final stage which the test statistic z(T) will be chosen based on which pair of energy detectors yield maximum output.

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Chapter 4 Part 2(a) Digital Modulation Techniquesportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · Frequency Shift Keying (FSK) Quadrature Amplitude Modulation ... Microsoft