Question

a) Mobile industry opted to use PSK (phase shift keying) over FSK (frequency shift keying). Explain the benefits of PSK over FSK.

b) Mobile phones are using quadruple PSK or higher order(s) of PSK. Discuss their throughput capability with a case example of the latest model.

c) Explain the differences between diffraction and refraction in electromagnetic wave propagations.

Answer 1

Phase Shift Keying PSK

PSK is the digital modulation technique in which the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time. PSK technique is widely used for wireless LANs, bio-metric, contactless operations, along with RFID and Bluetooth communications.

PSK is of two types, depending upon the phases the signal gets shifted. They are − Binary Phase Shift Keying BPSK

BPSK

This is also called as 2-phase PSK or Phase Reversal Keying. In this technique, the sine wave carrier takes two phase reversals such as 0° and 180°.

BPSK is basically a Double Side Band Suppressed Carrier DSBSCDSBSC modulation scheme, for message being the digital information.

Quadrature Phase Shift Keying QPSK

QPSK

This is the phase shift keying technique, in which the sine wave carrier takes four phase reversals such as 0°, 90°, 180°, and 270°.

Frequency Shift Keying FSK

FSK is the digital modulation technique in which the frequency of the carrier signal varies according to the digital signal changes. FSK is a scheme of frequency modulation.

The output of a FSK modulated wave is high in frequency for a binary High input and is low in frequency for a binary Low input. The binary 1s and 0s are called Mark and Space frequencies.

The following image is the diagrammatic representation of FSK modulated waveform along with its input.

To find the process of obtaining this FSK modulated wave, let us know about the working of a FSK modulator.

FSK Modulator

The FSK modulator block diagram comprises of two oscillators with a clock and the input binary sequence. Following is its block diagram.

The two oscillators, producing a higher and a lower frequency signals, are connected to a switch along with an internal clock. To avoid the abrupt phase discontinuities of the output waveform during the transmission of the message, a clock is applied to both the oscillators, internally. The binary input sequence is applied to the transmitter so as to choose the frequencies according to the binary input.

FSK Demodulator

There are different methods for demodulating a FSK wave. The main methods of FSK detection are asynchronous detector and synchronous detector. The synchronous detector is a coherent one, while asynchronous detector is a non-coherent one.

Asynchronous FSK Detector

The block diagram of Asynchronous FSK detector consists of two band pass filters, two envelope detectors, and a decision circuit. Following is the diagrammatic representation.

The FSK signal is passed through the two Band Pass Filters BPFsBPFs, tuned to Space and Mark frequencies. The output from these two BPFs look like ASK signal, which is given to the envelope detector. The signal in each envelope detector is modulated asynchronously.

The decision circuit chooses which output is more likely and selects it from any one of the envelope detectors. It also re-shapes the waveform to a rectangular one.

Synchronous FSK Detector

The block diagram of Synchronous FSK detector consists of two mixers with local oscillator circuits, two band pass filters and a decision circuit. Following is the diagrammatic representation.

The FSK signal input is given to the two mixers with local oscillator circuits. These two are connected to two band pass filters. These combinations act as demodulators and the decision circuit chooses which output is more likely and selects it from any one of the detectors. The two signals have a minimum frequency separation.

For both of the demodulators, the bandwidth of each of them depends on their bit rate. This synchronous demodulator is a bit complex than asynchronous type demodulators.

If this kind of techniques are further extended, PSK can be done by eight or sixteen values also, depending upon the requirement.

Benefits or advantages of PSK

Following are the benefits or advantages of PSK:
➨It carries data over RF signal more efficiently compare to other modulation types. Hence it is more power efficient modulation technique compare to ASK and FSK.
➨It is less susceptible to errors compare to ASK modulation and occupies same bandwidth as ASK.
➨Higher data rate of transmission can be achieved using high level of PSK modulations such as QPSK (represents 2 bits per constellation), 16-QAM (represents 4 bits per constellation) etc.

(B)Mobile phones are using quadruple PSK or higher order(s) of PSK.

A brief review of the principles and properties of F-QPSK (quadrature phase shift keying) modulation is given. Its spectral efficiency is compared with that of Gaussian minimum-shift keying (GMSK) using adjacent channel interference (ACI) as a parameter. It is established that in a noncellular environment, hardlimited F-QPSK has a spectral efficiency of 1.42 b/s/Hz, which is up to 51% more spectrally efficient than GMSK BT=0.5 for ACI=20 dB. F-QPSK's BER performance in AWGN and Rayleigh fading channels is shown to be superior to that of GMSK. The spectral efficiency of the modulations in cellular/microcellular environments where frequency is reused in geographically separate cells to achieve higher capacity is compared. It is shown that the application of F-QPSK in such environment would leads to a 95% increase of system capacity compared to GMSK. It is concluded that the power and spectrally efficient F-QPSK make it an excellent candidate for future high-capacity personal communication system (PCS) networks.

(C) Differences between diffraction and refraction in electromagnetic wave propagations.

2 Refraction: The bending of light ray due to a change in medium. I.E from either Rarer to Denser or Denser to Rarer.

3. Diffraction: The spreading of light when it passes thorough an edge on an object. Eg: when a boy throws a big stone in a pond, we see the waves which are Diffracted.