Tuesday, 6 March 2012

Digital Modulation techniques

What Is The Modulation?

     Modulation is the process of encoding information from a message source in a manner suitable for transmission. 
It is generally involves translating a baseband message signal (called the source) to a bandpass signal at frequencies that are very high when compared to the base band frequency. 

The bandpass signal is called the modulated signal, and the baseband message signal is called the modulating signal. 

Modulation may be done by varying the amplitude, phase or frequency of a high frequency carrier in accordance with the amplitude of the message signal. 

Demodulation is the process of extracting the baseband message from the carrier so that it may be processed by the intended receiver.

Why we modulate signals?

  • In order to ease propagation process and use an antenna of a suitable length. Since the effective radiation of EM waves requires antenna dimensions comparable with the wavelength:
        eg.  -Antenna for 3 kHz would be ~100 km long. 
               -Antenna for 3 GHz carrier is 10 cm long.
  •  Sharing the access to the telecommunication channel resources:
       This is done by using FDM (Frequency division multiplexing) technique.
  •  In order to transmit larger power for wide area:
         If we amplify the data power using power amplifiers, it will be distorted, so we perform modulation and amplify the carrier power.
  •  In order to reduce noise effects in case of non-white Gaussian noise.
 Why Digital? (Analog versus Digital):

Modern mobile communication systems use digital modulation techniques.

Advancements in very large-scale integration (VLSI) and digital signal processing (DSP) technology have made digital modulation more cost effective than analog transmission systems.

Digital modulation offers many advantages over analog modulation.
Some advantages include greater noise immunity and robustness to channel impairments, easier multiplexing of various forms of information (e.g., voice, data, and video), and greater security.

Furthermore, digital transmissions accommodate digital error-control codes which detect and/or correct transmission errors, and support complex signal conditioning and processing techniques such as source coding, encryption, and equalization to improve the performance of the overall communication link.

New multipurpose programmable digital signal processors have made it possible to implement digital modulators and demodulators completely in software.

Instead of having a particular modem design permanently frozen as hardware, embedded software implementations now allow alterations and improvements without having to redesign or replace the modem. Table (1.1) shows a comparison between analog and digital modulation schemes to conclude the assessment of both modulation schemes usage in Wireless communication systems.

Table (1.1) comparisons between analog and digital modulation schemes

Factors that influence the choice of digital modulation:

A desirable modulation scheme should provide: Low bit error rates at low received signal to noise ratio.
  •  Performs well in multi-path and fading conditions, and in interference environment.
  •  Occupies a minimum bandwidth.
  •  Easy and cost-effective to implement.
  •  Cost and complexity of the receiver subscribers must be minimized.
  •  Modulation which is simple to detect is most attractive.
Note That: There is no modulation scheme that satisfies all these requirements, so trade-offs are made when selecting a modulation scheme.

The performance of a modulation scheme:

The performance of the modulation scheme is measured by
  •  Power efficiency (ηP).
  •  Bandwidth efficiency(ηB).
  •  Power spectral density.
  •  System complexity.
 Power efficiency ηP:

The power efficiency is defined as the required Eb/No (Ratio of the signal energy per bit to noise power spectral density) at the input of the receiver for a certain bit error probability Pb over an AWGN channel
Power efficiency describes the ability of a modulation technique to preserve the bit error probability of digital message at low power levels.

In digital modulation systems, in order to increase the noise immunity, it is necessary to increase the signal power, so there is a trade-off between the signal power and the bit error probability.

The power efficiency is a measure of how favorably this trade-off is made.

Bandwidth efficiency (Spectral efficiency) ηB:

Bandwidth efficiency describes the ability of a modulation scheme to accommodate data within a limited bandwidth. As the data rate increases, pulse width of the digital symbols decreases and hence the bandwidth increases.

 The system capacity of a digital mobile communication system is directly related to the bandwidth efficiency for a modulation scheme.
So a modulation scheme with greater value of ηB will transmit more data in a given spectrum allocation.
Note that the maximum possible bandwidth efficiency is limited by the noise in the channel according to Shannon's Theorem as:
Where C is the channel capacity in bps , and S/N is the signal to noise ratio .

Bandwidth efficiency, Power efficiency Trade-off:

Adding error control coding to message increases the required bandwidth, then ηBdecreases, but the required received power for a particular bit error rate decreases and hence ηP increases.

On the other hand using high levels M'ary modulation schemes (except in M‘ary FSK modulation which isn‘t bandwidth limited modulation scheme), decreases the bandwidth occupancy, ηB increases, but the required received power for a particular bit error rate increases and hence ηP decreases.
System Complexity:

System complexity refers to the amount of circuits involved and the technical difficulty of the system.

Associated with the system complexity is the cost of manufacturing, which is of course a major concern in choosing a modulation technique.

Usually the demodulator is more complex than the modulator. Coherent demodulator is much more complex than no coherent demodulator since carrier recovery is required.

For some demodulation methods, sophisticated algorithms like the Viterbi algorithm are required. Also note that, for all personal communication systems which serve a large user community, the cost and complexity of the subscriber receiver must be minimized, and a modulation which is simple to detection is most attractive All these are basis for complexity comparison.

Since power efficiency, bandwidth efficiency, and system complexity are the main criteria of choosing a modulation technique, we will always pay attention to them in the analysis of modulation techniques.

Other considerations:

While power and bandwidth efficiency considerations are very important, other factors also affect the choice of a digital modulation scheme.

For example The performance of the modulation scheme under various types of channel impairments such as Rayleigh and Rician fading and multipath time dispersion, given a particular demodulator implementation, is another key factor in selecting a modulation.

 In cellular systems where interference is a major issue, the performance of a modulation scheme in an interference environment is extremely important.

Sensitivity to detection of timing jitter, caused by time-varying channels, is also an important consideration in choosing a particular modulation scheme.

In general, the modulation, interference, and implementation of the time varying effects of the channel as well as the performance of the specific demodulator are analyzed as a complete system using simulation to determine relative performance and ultimate selection.

Digital modulation techniques may be classified into coherent and non-coherent techniques depending on whether the receiver is equipped with a phase-recovery circuit or not.

The phase recovery circuit ensures that the oscillator supplying the locally generated carrier wave in the receiver is synchronized (in both frequency and phase) to the transmitter oscillator.

Fig.(1.1) Digital modulation according to demodulation type

The modulation schemes listed in the fig. (1.2) and the tree are classified into two large categories: constant envelope and non-constant envelope.

 Under constant envelope class, there are three subclasses: FSK and PSK. Under non constant envelope class, there are three subclasses: ASK and QAM.

Fig.(1.2) Digital modulation hierarchy
Types of modulation schemes in different advanced digital communication systems:

Table (1.2) shows examples of the used modulation schemes in different wireless modern communication systems
Table (1.2) Modulation schemes used in advanced communication systems


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