Modulation Techniques in Digital
Communications
Chapter 4
c.kabiri@ur.ac.rw
Outline
Digital modulation
Pulse shaping techniques
Linear modulation techniques
Constant envelope modulation
Modulation performance in fading and
multipath channels
Digital Modulation
Modern radio communication systems use digital modulation
techniques
Digital modulation offers many advantages over analog
modulation
Greater noise immunity
Robustness to channel impairments
Easier multiplexing of various forms of information
Greater security
Support complex signal conditioning and processing techniques
such as source coding, encryption, equalization, and error control
coding
Desired digital modulation should provide
Low bit error rates at low received SNR
Perform well in multipath and fading conditions
Occupies a minimum of bandwidth
Easy and cost-effective to implement
Pulse Shaping Techniques
Transmission of a rectangular pulse through a bandlimited
channel will result in intersymbol interference
Increase of channel bandwidth is not a solution to this problem
since radio communication systems operate within an allocated
and limited bandwidth
Out-of-band radiation in the adjacent channel in mobile radio
system should be 40dB to 80dB below that of the desired
passband
Techniques are required that
Reduce modulation bandwidth
Suppress out-of-band radiation
Reduce intersymbol interference
Use pulse shaping techniques
Time Waveforms of Binary Line
Codes
Power Spectral Density
Nyquist Criterion for ISI Cancellation
(I)
Nyquist Criterion for ISI Cancellation
(II)
Impulse that satisfies Nyquist condition
for ISI cancellation
Nyquist ideal pulse shape for zero ISI
Nyquist Criterion for ISI Cancellation
(III)
Raised Cosine Rolloff Filter (I)
Transfer function
Magnitude transfer function of a raised cosine filter
Raised Cosine Rolloff Filter (II)
Impulse response
Pulse shape having a raised cosine spectrum
Gaussian Pulse-shaping Filter (I)
Non-Nyquist techniques for pulse shaping which is particularly
efficient when used in conjunction with Minimum Shift Keying
(MSK) modulation
Transfer function of a Gaussian lowpass filter
where relates to the 3 dB bandwidth B of the baseband shaping filter
Impulse response of a Gaussian pulse-shaping filter
Gaussian Pulse-shaping Filter (II)
Impulse response
Linear Modulation Techniques
Binary Phase Shift Keying (I)
BPSK constellation diagram
Binary phase shift keying (BPSK) signal
Binary Phase Shift Keying (II)
Power spectral density (PSD) of a BPSK signal at RF
PSD of BPSK
Binary Phase Shift Keying (III)
BPSK receiver with carrier recovery circuits
Probability of bit error in AWGN
Q-function
Tabulation of the Q-function (complementary error function)
Differential Phase Shift Keying (I)
Differential Phase Shift Keying
(II)
Block diagram of a DPSK transmitter
Block diagram of DPSK receiver
Quadrature Phase Shift Keying (I)
QPSK constellation diagram
QPSK signal
Quadrature Phase Shift Keying
(II)
Block diagram of a QPSK transmitter
Quadrature Phase Shift Keying
(III)
PSD of a QPSK signal at RF
Quadrature Phase Shift Keying
(IV)
Block diagram of a QPSK receiver
Probability of bit error in AWGN
Offset QPSK
Offset QPSK (OQPSK) or staggered QPSK support more efficient
amplification
In OQPSK signalling, the even and odd bit stream mI (t) and
mQ(t) are offset in their relative alignment by one bit period
Example
Maximum phase shift of the transmitted signal at any given time
is limited to ±90.
Block diagram of a baseband differential detector
Constant Envelope Modulation (I) [Rappaport 311]
Many practical mobile radio communication systems use
nonlinear modulation methods, where the amplitude of the
carrier is constant, regardless of the variation in the modulating
signal.
The constant envelope family of modulations has the advantage
of satisfying a number of conditions, some of which are:
Power efficient Class C amplifiers can be used without introducing
degradation in the spectrum occupancy of the transmitted signal.
Low out-of-band radiation of the order of −60 dB to −70 dB can be achieved.
Limiter-discriminator detection can be used, which simplifies receiver design
and provides high immunity against random FM noise and signal fluctuations
due to Rayleigh fading.
While constant envelope modulations have many advantages,
they occupy a larger bandwidth than linear modulation schemes.
Constant Envelope Modulation (I) [Rappaport 311]
Binary frequency shift keying (BFSK)
The frequency of a constant amplitude carrier signal is switched
between two values according to the two possible message states.
Minimum shift keying (MSK)
MSK is a special type of continuous phase-frequency shift keying
(CPFSK) wherein the peak frequency deviation is equal to half the
bit rate.
Gaussian minimum shift keying (GMSK)
GMSK is a simple binary modulation scheme which may be viewed
as a derivative of MSK. In GMSK, the sidelobe levels of the spectrum
are further reduced by passing the modulating NRZ data waveform
through a pre-modulation Gaussian pulse-shaping filter.
Power Spectral Density of MSK Signals as
Compared to QPSK and OQPSK Signals
PSDs
Power Spectral Density of a
GMSK Signal
PSD
Modulation Performance in Fading and
Multipath Channels
To evaluate the probability of error of any digital modulation
scheme in a slow, flat-fading channel, one must average the
probability of error of the particular modulation in AWGN
channels over the possible ranges of signal strength due to fading
where
Slow Flat Rayleigh Fading Channel
Rayleigh distribution
where
Probability of error in slow flat Rayleigh fading channel
Probability of Error of a Particular Modulation Scheme in
Rayleigh Fading [Source: Rappaport]
Bit Error Rate Performance of Binary Modulation
Schemes in Rayleigh, Flat Fading Channel
BER for Pi/4 DQPSK in Flat Rayleigh
Fading Channel
BER for Pi/4 DQPSK in a 2-ray
Rayleigh Fading Channel
Exercises
Problem 1
Problem 2
Using the 4 QPSK signal of Problem 1, demonstrate how the received signal is
detected properly using a baseband differential detector. Assume the transmitter and
receiver are perfectly phase locked, and 0 0o.
Q&A
c.kabiri@ur.ac.rw
