# Tag Info

### Envelope behavior dIfference between QPSK, OQPSK and $\pi/4$-QPSK

Let's look at each in turn: Standard QPSK: With standard QPSK, each of the signal points are in quadrature (Note that the signal points in the constellation can be at any arbitrary phase really; ...
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### What is a constant envelope modulation?

You've pointed out a very important distinction between theory and practice. In theory, as suggested by your book and in Fat32's answer, modulation schemes where all information resides in the phase ...
• 79.2k
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### Gardner Timing Recovery for Repeated Symbols

Your Gardner Loop would properly "Flywheel" in the absence of symbol transitions. So it would slowly drift off of synchronization, but would not spiral out of control as you describe. This is ideal ...
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### Gardner timing recovery and constellation diagram

Your timing error detector should be run once for each input symbol. As you noted, your signal is far more oversampled than you need to be, so you won't end up using most of them for the purposes of ...
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### Recovering signal for psk

To add to the excellent information given by Cassman in his response, here is a block diagram of a carrier recover loop for QPSK and QAM modems using a decision directed approach. I have detailed the ...
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### Recommendation for good ressources on digital processing and D-QPSK

Wow, I'm honored by Matt L. doing what I'm often doing: Referring people to GNU Radio. The project actually has a list of recommended literature, but I don't know how well that'd fit you. It's ...
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### Block diagram of M-PSK modulation and demodulation

So this seems to be a general block diagram for these types of modulations. No, this is a block diagram of a IQ upconverter with some unspecified digital data modulator on the input. You need to ...
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### The difference between MSK as CPFSK and OQPSK

The answer to your question is yes, these two representations are equivalent. The first important thing to note is that in your formula, $d_k$ must be either $1$ or $-1$, because the frequency ...
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### Noise variance calculation for 16APSK Demodulator

This is to take into account the oversampling operation. Symbol time = sample time implies no oversampling. See AWGN model for more details about the conversion among EbN0, EsN0 and SNR. For ...
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### What is the theoretical bit error rate for QPSK Matlab simulation when constellation points are transmitted (no symbol duration)?

TL;DR The theoretical BER of an ideal QPSK system is $\displaystyle Q\left(\sqrt{\frac{2E_b}{N_0}}\right)$ where $E_b$ is the energy per bit and $\frac{N_0}{2}$ is the two-sided power spectral density ...
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### Sample rates, Samples per Symbol, and Digital Pulse Shaping

Yes the OP is correct in that you can implement pulse shaping in less than 2 samples per symbol for exactly the reasons that was outlined. However importantly we must also keep in mind having excess ...
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### When I double bit rate I do not see a difference in I_spectrum and Q_spectrum, but why?

Do you know how ADS is plotting the spectrum? Plotting the spectrum without doing some kind of normalization will give you a higher magnitude. Once you determine the size of the FFT, normalizing by ...
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### Adding AWGN noise with a correct noise power to the signal

The kind of AWGN channel you're simulating is sometimes called a "discrete time" channel. That means that you're going to simulate the bit error rate at the output of a decision device, the input to ...
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### Rectangular 8 QAM bit error rate calculation

Neglecting square terms of the Q function, your expression is correct, but it is the symbol error probability, not the bit error probability: $$P_s=2.5\,Q\left(\frac{d}{2\sigma}\right)\tag{1}$$ With ...
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### GNU Radio - PSK Mod block - unexpected constellation diagram

What you are seeing is the transitions from one constellation point to another. In order to reduce the signal bandwidth, the baseband signal is low-pass filtered. This causes the transitions to not ...
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### Why does DPSK constellation shrink when noise is added?

It looks like your circle got bigger. The constellation is fine.
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### How to correct the phase offset for QPSK I-Q data

Assuming we have timing recovery resolved (meaning our system knows the correct time locations for our symbol decisions), then we can use the decided symbols along with our pre-decision values in a ...
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### Processing OBPSK as OQPSK

OBPSK in the way that you describe it is also called $\pi/2$-BPSK meaning that the signal constellation is different from bit to bit, rotating by $\pi/2$ after it is used. There are two variations of ...
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### Recovering signal for psk

So not only do you need to do timing synchronization to recover the baud clock, you need to do some form of turn on detection. You could start your PSK synchronization at the first sample, but the ...
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### Constellation diagram for QPSK modulation

I think you may be missing some fundamental understanding of QPSK modulation, which was likely the point of the homework exercise so let me hopefully fill in some blanks, from which you can figure out ...
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### Carrier Tracking Loop: Tracking noise vs Phase Noise Trade-off

The question is specific to optimizing loop bandwidth for the decision directed carrier tracking loop with one sample per symbol. In other posts such as this one PLL for Phase Demodulation and Carrier ...
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### Phase Noise in Constellation Plot

The spread in the plot is the peak to peak phase, while the integrated phase noise is an rms quantity- The OP in his comment below this answer reported a measurement of 35° in the constellation and a ...
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### Use of pulse shaping in digital modulation

QPSK in its raw form would appear just as the OP has shown, where the information is encoded into four symbols given by the four phase states alone. The problem is this has a very wide transmit ...
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### Spectral properties of QPSK modulation

The spectrum is that of the base pulse used in the modulation, so in this case a rectangular pulse. A single rectangular pulse in time, as given by the Fourier Transform, is a Sinc in Frequency with ...
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### Why does OFDM use cyclic-prefix while QPSK uses root-raised cosine filters?

The reason cyclic prefix is used in OFDM systems is to avoid complex equalizers in receivers. Cyclic prefix converts linear convolution of fading channel(h) & ...
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### Why oversample data before using raised cosine filter?

In digital communications, you can transmit a sequence of numbers using a train of pulses that don't interfere with each other. A given raised cosine pulse $p(t)$ has zero crossings every $T$ seconds ...
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### Sphere decoding for non-binary modulation

This channel model is known as a multiplicative channel, and it differs from the AWGN channel in that the signal amplitude is scaled by a gain factor. If the gain factor is real (that is, if the ...
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### what is the difference between QPSK and 4QAM?

There are different types of QPSK: Offset (staggerd) QPSK, $\pi/4$-QPSK, etc. In its most known form, QPSK is identical to $4$-QAM or $4$-PSK. That is, the same constellation can be referred to by ...
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### Envelope behavior dIfference between QPSK, OQPSK and $\pi/4$-QPSK
The main purpose of OQPSK, $\pi/4$-QPSK and other variants is to prevent the signal frequently transition through zero. When a digitally modulated signal stream input to a power amplifier passes from ...