Digital Modulation

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In electronics and communications, modulation is the process of changing of some characteristics i.e. amplitude, frequency, phase of the carrier wave in accordance to the instantaneous amplitude of modulating signal.

There are two types of modulation -

1. Analog Modulation

1.1. Amplitude Modulation (AM)

1.2. Frequency Modulation (FM)

1.3. Phase Modulation (AM)

2. Digital Modulation

2.1. Amplitude Shift Key (ASK)

2.2. Frequency Shift Key (FSK)

2.3. Phase Shift Key (PSK)

Digital Modulation

In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion. Digital modulation in somewhat similar to the analog modulation except baseband signal is of discrete amplitude level. There are three types of digital modulation -

1. Amplitude Shift Keying (ASK)

2. Frequency Shift Keying (FSK)

3. Phase Shift Keying (PSK)

PSK or Phase shift key:

The phase of the carrier is shifted for this modulation. If the baseband signal m(t) =1 carrier in phase is transmitted. If m(t)=0 carrier with out of phase is transmitted i.e. cos(wct+П). If the phase shift is done in 4 different quadrants then 2bit of information can be sent at a time. This scheme is a special case of PSK modulation known as QPSK or Quadrature Phase Shift Key.

Application:

WiFi has different versions based on different IEEE standards. Different versions have different speeds and also uses different modulation Techniques. Out of them, IEEE 802.11b uses QPSK modulation and 802.11a and g use 16 or 64 QAM modulation for transmission. Difference between this two is the speed of data transfer. Obviously, 64-QAM is faster than QPSK.

Both QPSK and 8PSK are widely used in satellite broadcasting. QPSK is still widely used in the streaming of SD satellite channels and some HD channels. High definition programming is delivered almost exclusively in 8PSK due to the higher bitrates of HD video and the high cost of satellite bandwidth.

An enhanced version of GSM is known as EDGE ( Enhanced Data rates for GSM Evolution )uses 8-PSK modulation technique for faster data transfer as compared to GSM. GSM uses only 1 bit per symbol for data transfer while EDGE uses 3 bits per symbol for data transfer. It is commonly called as 2.75G network