Digital Modulation Techniques
In Baseband pulse transmission, the input data in baseband
pulse transmission is represented by a discrete PAM signal (Line codes). At low
frequencies, the baseband signals have high power. As a result, they can
be transmitted through a pair of wires or coaxial cables.
Due to the impracticality of using huge antennas,
baseband signals cannot be sent across radio connections or satellites. As
a result, the communication signal's spectrum has to be moved to higher frequencies.
This is performed by modulating a high-frequency sinusoidal carrier with the
baseband digital signal. The modulated signals are transmitted through a
bandpass channel, such as a microwave radio link, a satellite link, or an
optical fiber link. This process is termed Digital carrier modulation, or
(digital passband communication).
DIGITAL MODULATION:
The mapping of a sequence of input binary digits into a
series of corresponding high-frequency signal waveforms is known as digital
modulation. These modulated waveforms can change in amplitude, frequency,
phase, or a combination of these signal properties (Amplitude and phase or
frequency and phase).
Digital Modulation Techniques
There are mainly two types of digital modulation techniques.
They are :
1. Coherent digital modulation techniques
2. Non-Coherent digital modulation techniques
1. Coherent
Digital Modulation Techniques
Coherent detection is used in coherent digital modulation
techniques. The local carrier generated at the receiver is phase synchronized
with the carrier at the transmitter in coherent detection. As a result,
detection is achieved by comparing the received noisy signal to the locally
generated carrier. Synchronous detection is a coherent detection. Coherent
detection techniques are more complex, but they can provide better
performance than non-coherent detection.
2.
Non-Coherent Digital Modulation Techniques
Non-Coherent detection is used in these techniques. The
receiver carrier does not need to be phase synchronized with the transmitter
carrier for the detection process. As compared to coherent
detection, Non-Coherent detection techniques are easy to implement. However,
compared to Coherent detection, the probability of error is high in
non-coherent detection.
Listing of
various types of digital modulation methods:
Based on the
mapping techniques, we can broadly classify the digital modulation methods.
I. Binary
Scheme / M-ary Scheme:
During each
signaling interval of duration Tb, we send one of the two possible signals in a
binary scheme. The examples for the binary scheme are:
1. Amplitude
Shift Keying (ASK), 2. Frequency Shift Keying (FSK) and 3. Phase Shift Keying
(PSK)
M-ary
Scheme:
During each
signaling period 'Tb' in the M-ary system, we can send any of the M possible
signals. The examples are::
1. M-ary ASK
2. M-ary FSK
3. M-ary PSK
4. Minimum
shift keying (MSK) is a type of phase frequency shift keying in which the
minimum shift is used (CPFSK).
5. M-ary PSK
with M=4 is represented by quadriphase shift keying (QPSK). Quadrature carrier
multiplexing systems are of two types: MSK and QPSK.
6. Amplitude
Modulation in M-ary Quadrature (M-ary QAM)
M-ary
Amplitude-Phase Keying is the result of combining discrete changes in the
amplitude and phase of a carrier (APK). M-ary QAM is a special form of this
hybrid modulation.
II. Based on the performance of the modulation scheme and
properties of a modulated signal.
1. Power efficient scheme / Bandwidth efficient scheme
2. Continuous phase (CP) modulation / In phase Quadrature
phase (IQ) modulation
3. Constant envelope modulation / Non-Constant envelope
modulation
4. Linear modulation / Non-linear modulation
5. Modulation scheme with memory/modulation scheme without
memory.
Design Goals of Digital Communication System
A digital communication system designer has a variety of
modulation/detection techniques. Each design has its own set of trade-offs. The
use of available primary communication resources, transmission power, and
channel bandwidth determine which modulation/detection system is used. The
choice depends on achieving as many of the design goals as possible.
1. Maximum data rate
2. Minimum possibility of symbol error
3. Minimum transmitted power
4. Minimum channel bandwidth
5. Maximum resistance to interfering signals
6. Minimum circuit complexity.
Gram-Schmidt Orthogonalization Procedure
The process of converting an incoming message mi into a modulated wave Si(t) may be broken down into discrete-time and continuous-time procedures. The Gram-Schmidt orthogonalization process allows any collection of M energy signals, S(t), to be represented as linear combinations of N orthonormal basis functions. As a result, the provided collection of real-valued energy signals S1(t), S2(t),... Sm(t), each with a period of T seconds, may be represented in the form
The real-valued basis functions ฯ1(t), ฯ2(t), …… ฯN(t) are orthonormal. Hence we have
In the first condition, each
basis function must be normalized to have Unit energy. The second
condition specifies that throughout the interval 0≤๐ก≤๐, the basis functions ฯ1(t), ฯ2(t),
…… ฯN(t) are orthogonal to each other.
In equation (1), the coefficients of the expansion can be defined as:
Modulator Design:
Assume that the set of coefficients {Sij}, j = 1, 2, … N is operational. Then, as shown in Figure, we can apply the equation to create the signal Si(t), where I = 1, 2,... M, from equation (1)
Figure -
Scheme for generating the signal Si(t)
It is composed of a bank of N multipliers, each with its basic
function, followed by a 'summer'. This method functions similarly to a
modulator in a transmitter.
Detector Design:
Figure - Scheme
for generating the set of coefficients {Sij}
Assume that the collection of signals {๐(๐ก)},๐=1,2,… ๐, is
operating as input. To calculate the set of coefficients {๐๐๐}, j = 1,
2, ….N as per equation, we may utilize the scheme shown in figure (3). A
bank of N product integrators or correlators with a common input makes up this
method. Every multiplier has its basis function. This method is comparable to
the receiver's detector.