10.3 MODULATION TECHNIQUES

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10.3 MODULATION TECHNIQUES

For designing an analog communication system, analog modulation techniques such as AM and FM are used. If you are designing a digital communication system, the choices are digital modulation techniques such as ASK, FSK, and PSK. Many variations of these basic schemes are available that give slightly different performance characteristics. Choice of a modulation scheme needs to take into consideration the performance requirements as well as the availability of hardware for implementing the modulator and demodulator.

In a communication system, an important design parameter is Bit Error Rate (BER). To achieve a good BER (to reduce the bit errors as much as possible), the Signal-to-Noise Ratio (SNR) should be high. SNR and E_b/N_o are related by the formula

where N_o is the noise power density in watts/Hertz. The total noise in a signal with bandwidth of B is given by N = N_o × B. Hence,

The BER can be reduced by increasing the E_b/N_o value, which can be achieved either by increasing the bandwidth or by decreasing the data rate.

The curves shown in Figure 10.1 give the performance of the modulation schemes. These curves are known as the waterfall curves. In designing a communication system, based on the required BER, the E_b/N_o value is obtained for a given modulation scheme. PSK and QPSK perform better compared to ASK and FSK because for a given BER, the value of E_b/N_o is less, and hence with less energy of the signal, we can achieve good performance. Another criterion to be considered is the ease of implementing the modulator/ demodulator. It is much easier to implement ASK and FSK modulators and demodulators as compared to PSK and its variations.

Figure 10.1: Performance curves for Digital Modulation Systems.

For digital communication systems, the BER is an important design parameter. BER can be reduced by increasing the value of E_b/N_o (where E_b is the energy per bit and N_o is the noise power density).

Note 

Choosing a specific modulation technique depends on two important factors—BER performance and complexity of the modulator/demodulator circuitry.

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