Quadrature Phase Shift Keying (QPSK) / BPSK and QPSK / QPSK Waveform (Digita Modulation Techniques)
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- Опубликовано: 12 ноя 2024
- This video lecture discusses complete basics of Quadrature Phase Shift keying (QPSK). You will understand here, what is QPSK, why it is called as QPSK, how the QPSK is different from other digital modulation techniques like ASK,FSK, PSK and BPSK.
Here you will get an idea about, why QPSK is better than BPSK. This video also explains the waveform of QPSK. You will get here QPSK waveform diagram, QPSK waveform generation and examples of QPSK waveform.
This video explains, the comparison of bandwidth requirements in BPSK and QPSK. Quadrature phase shift keying allows 2 bits to be transmitted per symbol.
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What is Quadrature Phase Shift Keying (QPSK) Modulation?
In the last videos we have discussed ASK, FSK and BPSK i.e. Amplitude Shift Keying, Frequency Shift Keying and Binary Phase Shift Keying. In this video, we are going to discuss Quadrature Phase Shift Keying (QPSK). It is also a kind of phase shift keying but it is different from the binary phase shift keying.
So here we will understand what we mean by QPSK and why it is called as Quadrature Phase Shift Keying? what is 'Quadrature' here? and what is 'binary' in binary phase shift keying. We will also see the waveform of QPSK in this post and how it is formed. Here you will also get an idea about the benefits of QPSK over other digital modulation techniques.
How QPSK is different from ASK, FSK and BPSK
Actually we have seen in other posts on ASK, FSK, and BPSK that in these digital modulation techniques, the carrier wave assumes one of two possible states (symbols). Actually each discrete state of the carrier is called as symbol.
It means that if we talk about ASK, then we have two states (symbols) only of carrier wave's amplitude either 'No transmission of the carrier' or 'Transmission of the carrier'. In ASK, no carrier wave is transmitted, when we want to transmit binary '0' and when we want to transmit binary 1 then a continuous carrier wave is transmitted.
In the same way, in FSK, we have two levels of frequency- 'Low frequency' and 'High Frequency'. Low frequency corresponding to the transmission of binary '0' while high frequency corresponds to the transmission of binary '1'.
The same thing is with BPSK, here binary '0' and binary '1' corresponds to phase shift of '0 degree' or '180 degree', so there are two phases in binary phase shift keying, that's why it is known as binary phase shift scheme.
So you can see that in ASK, FSK and PSK, 1 bit per symbol is transmitted. But in case of QPSK, two bits per symbol are transmitted, i.e. in each symbol we have 2 bits.
Note that, with 2 bits we can have 4 possible combinations, therefore 4 phases. These 4 combinations with 2 bits (0 and 1) can be-
00, 01, 10 and 11
The 'Quadrature' word in Quadrature phase shift keying means that we have four phases here, Unlike the BPSK, where we have only two phases (0 degree and 180 degree)
Therefore in QPSK the total 360 degree phase is divided into four phases.
360/4 =90 degrees.
So here we have separation of 90° in phases. So in QPSK the 4 phases that are used are-
45°
45°+ 90° = 135°
135° + 90° = 225°
225° + 90° = 315°
So these all phases are at a separation of 90 degrees. Using the separation in phases helps in distinguishing easily at the receiving end and it can easily be demodulated without any mixing. That's why it is called as quadrature phase shift keying unlike to binary phase shift keying, where we use only 0 degree and 180 degrees. If they all were in the same phase then it would have been difficult to distinguish these messages.
QPSK Waveform Generation
Now look at the image given below-
This image shows two waveforms-
The first waveform is a complete cycle of a sinusoidal wave, showing angles (in degrees) on the x-axis,
While the second waveform is the QPSK waveform. This waveform shows only the phases (angles) that are used in quadrature phase shift keying.
We know that quadrature phase shift keying has the following phases-
45 degrees, 135 degrees, 225 degrees and 315 degrees. So this QPSK waveform shows only these four phases.
Note- Watch carefully both the waveforms and observe these four phases (angles) used in QPSK; in the first waveform of sinusoidal wave.
By observing the first waveform, you will clearly understand how we have phase shifted in QPSK.
Benefits of Quadrature Phase Shift Keying (QPSK)
Transmission of 2 bits in a symbol reduces the signaling rate. This reduces the frequency of the carrier wave needed for transmission and it results in reduced bandwidth.
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FINALLY!! I understand QPSK. Thank you.
Excellent explanation, you've gained a subscriber.
In the beginning I was worried about "demodulating" the accent, but as the video goes on it gets easier and more importantly your explanation is excellent and easy to understand. Thank you!
Thank You. You made this very clear for me. I had a lot of trouble visualizing this.
Thanks for the appreciation.
Fantastic Explanation
how much is the phase shift?? in 45 deg, is the o/p is 45 deg phase shifted, in 135 deg, is the o/p is 135 deg phase shifted????
Just awesome
Thank you so much dear
Thanks for the explanation
One doubt sir.. For a bit stream of 001011.. How u will draw the waveforms of qpsk..oqpsk, pi/4 qpsk
Sir,thanks for te video.Plz explain Gaussian frequency shift keying( GFSK)
You are welcome. Ok, I will try to make a video on it
Dashendra d
This Dude has got her nail polished
See what I did there...
any video on oqpsk
why sometime I see 0,180,90,270
great, I liked it, but I'm waiting also to explain those videos in MATLAB. that will make it clearer
Thank you so much...sure I will try to make videos using MATLAB. Keep watching and stay in touch.
Do you have any guide for doing QPSK in MATLAB? thank you so much
Please explain why you choose 45 degrees for the first phase
because, the si(t)= (√2eb/tb) x cos(2i-1) (π/4) where you substitute the value of i as 1,2,3 and 4
great video.. but i have a question why did the phase shift start with 45 degrees why not 35 then 35+90=125, 125+90=215, 215+90=305 ?
because, the si(t)= (√2eb/tb) x cos(2i-1) (π/4) where you substitute the value of i as 1,2,3 and 4
@@abhiramjoshi3517 thanks
Why you use 45 degree phase at first.
To stop wave collisions or cancellation. If you started at zero, then 90 degrees, then 180 degrees and so on, there would be collisions for the two waveforms that were exactly opposite each other. And don't thank me, thank the video for helping me sort this out in my head!
helpful
Nice
Thanks
Thanks
You are welcome... Stay connected and keep watching dear
Initially you shifted the phase by 45 degree and there after by 90 degree for four different symbols. Please make it clear.
it is the format you use to transmit a signal in QPSK you separate them by 90 degrees so the first representation starts at 45 and you increase it by 90 to get the next wave, 135. then 135+90 = 225 for the next one and 225+90 = 315 for the last point. to form a 360 circle separated at 90 degrees
DEAR SIR QPSK TECHNIQUE IS NOT SO CLEAR. COULD YOU PLEASE MAKE ME UNDERSTAND?
You should watch the video again or other videos available on RUclips.
Why 45 degree for the frst time
I too have the same question ..
Wave must never begin or end with value 0
because, the si(t)= (√2eb/tb) x cos(2i-1) (π/4) where you substitute the value of i as 1,2,3 and 4
Jai shree ram 🙏🙏
poorly explained.
Sorry for your bad experience but I will try to improve. You may watch my other videos if you like by visiting my channel...Engineering Made Easy
I am confuse
Why...where
Explain in normal english please
phase sift is wrong
how come is it wrong ??? can you please tell...