Digital modulation techniqes (Phase-shift keying (PSK))

Digital modulation techniqes (Phase-shift keying (PSK))

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  DIGITAL MODULATION TECHNIQES Phase-Shiftkeying NOVEMBER 23, 2020 Supervised by: - Dr. Samy S. Soliman Prepared by: - Mohamed Sewailam
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  Modulation Modulation is somesort of operations to convert (Map) the baseband signal (Information) to be transmitted over transmission Media channel (Radio channel). There are three major types of digital modulation techniques used for transmission of digitally represented data:  Amplitude-shift keying (ASK)  Frequency-shift keying (FSK)  Phase-shift keying (PSK) The key difference between modulation techniques is how to represent the digital signal (Baseband) in proper way on carrier signal amplitude, frequency or Phase to be transmitted over define communication channel as show in below graph. Baseband signal can be represented by carrier Amplitude, frequency or Phase or combination of them to trade of between many elements or attributes (the highest bite rate, the least bite error rate, the signal immunity to noise (SNR), transmitter/Receiver complexity, ….). Here will talk about the Phase-Shift Keying technique and specially compare about three types of PSK which are QPSK, OQPSK and π/4-QPSK.
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  Phase-shift keying (PSK) PhaseShift Keying (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. As the phase of the output signal gets shifted depending upon the input. These are mainly of two types, namely BPSK and QPSK, according to the number of phase shifts it define the type of PSK as the Binary PSK is only two change in phase and for QPSK there are four changes in Phase. As show in below graph (Constellation Diagrams). Quadrature phase-shift keying (QPSK) This is PSK technique, in which the sine or cosine wave carrier takes four phases reversals such as 0°, 90°, 180°, and 270° Or 45°, 135°, 225°, and 315°…. Each Phase of the four ones (Symbol) represents two Bits. The following figure represents the QPSK waveform for two bits input, which shows the modulated result for different instances of binary inputs.
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  QPSK signal envelopesare the most severely affected by filtering. At the 180° phase transitions the signal envelope goes through zero amplitude as show in Constellation Diagrams). In QPSK these transitions occur when both the in-phase and quadrature (I and Q) channels change phase simultaneously. Such variation requires linear Power Amplifier to avoid spectral regrowth (side lobes). In order to eliminate the possibility of a 180° phase transition in the QPSK case, must ensure that the phase can only change a maximum of 90 or less than 180° at any given transition. When this is done the maximum envelope variation of the filtered signal will be 3dB.So OQPSK and pi/4 QPSK are variants of the basic QPSK modulation schemes. Offset quadrature phase-shift keying (OQPSK) Offset quadrature phase-shift keying (OQPSK) is a variant of phase-shift keying modulation using four different values of the phase to transmit. It is sometimes called staggered quadrature phase-shift keying (SQPSK). In OQPSK, after splitting the bit stream into odd and even (I and Q bits), one-bit stream is made offset by half a symbol period (one-bit period) with respect to the other. This ensures that both bits don’t change state at the same time, thus limiting the maximum phase change to 90 degrees and preventing any spurious high frequency components.
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  As shown abovein the constellation diagram OQPSK prevents phase transition from the origin by shifting one stream by a bit period and allowing only one bit to change between the transitions.
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  π/4 quadrature phase-shiftkeying (π/4 QPSK) In π/4 QPSK we achieve the same objective (avoid passing through origin) by rotating every other symbol by 45°. After splitting the bit stream into odd and even (I and Q bits), one-bit stream is made offset by 1/4 a symbol period (half of bit period) with respect to the other. Note in the transitions from any one symbol to the next, as shown in the diagram below, that it is not possible to go through the origin. Dual constellation diagram for π/4-QPSK. This shows the two separate constellations with identical Gray coding but rotated by 45° with respect to each other.
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  The modulated signalis shown above for a short segment of a random binary data-stream. The construction is the same as above for ordinary QPSK. Successive symbols are taken from the two constellations shown in the diagram. Thus, the first symbol (1 1) is taken from the "blue" constellation and the second symbol (0 0) is taken from the "green" constellation. Note that magnitudes of the two component waves change as they switch between constellations, but the total signal's magnitude remains constant (constant envelope). The phase-shifts are between those of the two previous timing-diagrams. The table below summarizes QPSK, OQPSK and pi/4 QPSK modulation types. QPSK OQPSK π/4 QPSK phase changes +/- 90 and +/-180 degrees phase changes +/- 90 Maximum phase changes +/-45 and +/-135 There is no offset between I&Q Bits There is Tb Offset between I&Q Bits There is Tb/2 Offset between I&Q Bits Requirements of linear amplifier as nonlinear amplifier cause spectral regrowth because of abrupt +/-180 degree transitions of the both bits change the phase at the same time. less demands of linear amplifiers, efficient nonlinear amplifier can be employed and they do not cause much spectral regrowth, as one of the bits changes the phase at a time and occurs twice during the symbol period with half the intensity of QPSK Phase transitions avoid zero crossing. This will remove design constraints on the amplifier, nonlinear amplifier can be employed
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  Index ASK Amplitude ShiftKeying FSK Frequency Shift Keying PSK Phase Shift Keying QPSK Quadrature Phase Shift Keying OQPSK Offset Quadrature Phase Shift Keying References https://en.wikipedia.org/wiki/Phase-shift_keying https://www.rfwireless-world.com/Terminology/QPSK-vs-OQPSK-vs-pi-4QPSK.html https://dsp.stackexchange.com/questions/41130/envelope-behavior-difference-between- qpsk-oqpsk-and-pi-4-qpsk https://www.slideshare.net/mansournch/oqpsk https://www.slideshare.net/naveenjakhar12/phase-shift-keying-4-quadrature-phase-shift- keying Thank you