Questions tagged [demodulation]

Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.

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309 views

Denoising effect in GnuRadio OFDM Serializer block

Why does the OFDM serializer has such a strong denoising effect in the flow graph below ? Is that normal ? The upper constellation is AFTER the OFDM serializer block and the lower is BEFORE the OFDM ...
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2answers
5k views

Frequency Shifting in MATLAB

I am currently attempting to demodulate an audio file using MATLAB. After taking the fft of the signal, I receive this: After some more coding, I found the following values: Fs = 44100; Window ...
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2answers
109 views

Must OFDM signal be time-division in a multiple-transmitter and SFN system when the signal is free of spread-spectrum?

The OFDM baseband signal can be demodulated since each subcarrier is orthogonal to another. But In SFN system, i.e., Single Frequency Network, all OFDM signals from several transmitters are in the ...
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1answer
923 views

IQ sampling vs 2X f0 sampling for SDR

Are there any modulation schemes (or other situations) where sampling at or very near twice the carrier frequency would be better than IQ or quadrature sampling using a local oscillator at or near the ...
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1answer
1k views

ASK and Manchester Encoding

Firstly, the context: I've been given a signal in a wav file, the sampling rate and carrier frequency along with the fact it is Manchester encoded and has been ASK Modulated. Five periods of a ...
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2answers
1k views

Possible to break out complex WiFi signal?

Is it possible to break out the i and q of a complex WiFi signal? If I have a laptop with a standard WiFi card in it, could I break out the i and q in analog, or upon sampling in an FPGA via digital ...
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1answer
726 views

DC to phase relationship with Low pass filtered sine and square wave multiplication and it's pitfalls. IQ Demodulation

From what I have read about signal (de)modulation I understand that multiplying two sine waves, both with the same frequency, gives a two frequency component sine wave. One at DC and one at $2\omega$. ...
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2answers
2k views

How to compute bit level soft decisions from an M-ary symbol?

When working with a decoder such as LDPC that can take in soft-decision inputs (or channel posterior probabilities as I've seen them called as well), how are these soft-decision values calculated for ...
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1answer
322 views

OFDM Phase compensation

I need to compensate a OFDM-16_qam constelation rotation but I could not find out how to implement it with MATLAB I have a Matrix [48 x 25] where each colum is a OFDMsymbol without CP , pad and pilots,...
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0answers
122 views

How does a Costas Loop lock to one of multiple BPSK signals

I've been looking into how GPS receivers work in terms of signal processing. When a GPS receiver tracks the received signal, it often uses a Costas loop to synchronize the carrier. How that works ...
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207 views

How to calculate the accurate BER expression of circular 8-QAM constellation modulation?

It is easy to understand the BER derivation of square QAM, but the circular 8-QAM? I'm quite confused. The decision region is not in a rectangular shape anymore, meaning regular way does not work here....
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90 views

Does anyone recognize this modulation?

I'm trying to extract the code of my garage remote, but i have some difficulty to find witch modulation has been used. At first I thought it was a simple digital modulation, but there is some strange ...
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430 views

Demodulation distortion in single-sideband and double-sideband

I am trying to compare the demodulation for single sideband (SSB) and dual sideband modulation formats (DSB) when the reconstructed carrier is not perfect (i.e. shifted in both frequency and phase). I ...
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90 views

Filtering and Differentiating phase-modulated signals

For a project work, I need to demodulate data from a Laser Doppler Vibrometer. The distance information is phase-modulated, therefore the velocity information is frequency-modulated: $$x \propto \...
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252 views

Amplitude/phase recovery on noisy signal

I have a noisy signal that has an ac component of fixed frequency but variable amplitude and phase. I'd like to recover the ac component. The signal (blue trace) is mostly smooth, but has a few ...
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1answer
1k views

I/Q Sampling and Demodulation

A bit stream is mapped to symbols during baseband modulation, each symbol (say for QPSK) represents two bits. Each symbol has a corresponding I & Q value, which in QPSK, are used as the amplitude ...
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1k views

How to implement GMSK demodulation

I am currently trying to implement a GMSK demodulator, using Java. I am actualy trying to use the model of this block diagram. However, I really do not know what goes on within the mode selector and ...
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308 views

AFSK clock recovery in noise

What algorithms are available for optimizing estimated bit clock recovery of an audio FSK modulated signal over an extremely low S/N ratio channel? (e.g. barely audible due to high ambient noise and ...
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2answers
2k views

Need help understanding 16QAM constellation diagram

Is the identification of each point on the constellation diagram arbitrary as long as the sender and receiver know the mapping? I have seen some diagrams which order in an outward spiral, while ...
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2answers
90 views

Given a real signal, increase modulation frequency

Lets say I receive the signal $$x(t)\cos(\omega_c t)$$ and I want to change the the carrier frequency $x(t)\cos(1.001\omega_c t)$ How can this be done? If I modulated it with a complex exponential, ...
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3answers
234 views

Bpsk demodulator implementation

For a project I'm trying to implement a BPSK demodulator on some random wave files, I'm getting a few difficulties: How do I know my bit rate, i.e. how many bits each 'real data bit' I'm having. How ...
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1answer
832 views

Beginner BPSK demodulation question

I'm writing a simple application in Python that sends and receives data over binary PSK using audio signal. I used cos() for 0 and -cos for 1. On the receiver side I figured I can demodulate by ...
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2answers
171 views

Decoupling two signals where one is a delayed version of the other

I have a signal $y(t)$ that consists of the sum of two signals where one is a time delayed version of the other. $ y(t) = x(t) + x(t-t_{d}) $ $x(t)$ is known but the time delay $t_d$ is not. I ...
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2answers
3k views

I/Q Sample demodulation to input data

I am trying to convert some raw unscaled I/Q sample data back to the original input data. I've been reading a lot of different posts and articles in I/Q demodulation and the focus is almost solely on ...
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3answers
756 views

OFDM sensitivity to sampling time offset (with half of time interval between samples)

In simple QAM modulation symbol synchronization is crucial: the samples need to be taken (or interpolated and resampled to ) exactly corresponding to the time when the symbol was emitted. This is ...
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1answer
4k views

How to correct the phase offset for QPSK I-Q data

I have an I-Q data of QPSK modulation, the data is is smeared and I have to correct the phase offset that causes the smearing problem, how can I apply the Phased-locked loop or the Costas loop on this ...
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2answers
802 views

Why is a precoder necessary for DQPSK and what does it accomplish?

I've implemented a soft-decoder for DQPSK using the wonderful answers I received here: How to soft decode DQPSK? To get the soft-decoder working properly I needed to precode the data I was sending ...
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1answer
116 views

How can an SDR recover a high-frequency signal?

How can software-defined radios operate at high frequency? The Nyquist rate dictates that you need to sample at twice the frequency to fully recover the signal. If my signal of interest is modulated ...
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1answer
516 views

Prepare IQ Data for Goertzel Algorithm

I have a quadrature (IQ - 0..255) data byte stream that I receive from an rtl-sdr dongle as IQIQIQIQ..... That means that negative values are < 128 and positive values >= 128 I am attempting to ...
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1answer
874 views

How to determine the symbol rate of a 2-FSK signal?

Let's say that I have a 2-FSK signal recorded so that the SNR is around 20 dB and the noise seems to be more or less white. The distance between tones is greater than the minimum distance needed for ...
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1answer
1k views

How does carrier frequency offset effect the constellation diagram in OFDM?

In the case of OFDM modulation, if the carrier frequency is slightly off then how does that effect the constellation diagram ? Is there some general rule how it will change (such as rotate) ? Or ...
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3answers
436 views

Can OFDM modulate analog signal?

I suppose that OFDM can't modulate analog signal. The basic mathematical description for OFDM is $$ v(t)=\sum_{k=0}^{N-1}X_{k}\exp(j2\pi k t /T),\quad 0<t<T$$ In this equation, $X_k$ is a ...
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1answer
759 views

Demodulation with MATLAB

I am trying to listen VLF radio signals. I have a recorded wave file (download) and here is the frequency spectrum: I have produced this spectrum using this matlab code: ...
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1answer
169 views

What does the phase discriminator portion of the Costas Receiver do mathematically?

What does the phase discriminator portion of the Costas Receiver do mathematically? The output of the I-channel is $ \frac{1}{2}A_C \cos \phi \, m(t) $. Which means for small deviation of phase $ \...
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1answer
184 views

RF demodulation for dummies

Given a signal that's been FM encoded on a carrier wave, what are the individual steps I need to take to demodulate it, and recover the original signal? I don't have a background in signal processing,...
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2answers
104 views

Contradiction between complex baseband and real-valued baseband

Suppose a direct-conversion transmitter+receiver (ideal transmitter) along with its filters. In complex baseband, the filters in the signal chain can be modeled as a complex-valued FIR filter ($y$, $c$...
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1answer
4k views

Why isn't this AM Demod GNU Radio flow graph working with different sampling rates?

I am trying to demodulate an AM signal in GNU Radio, and I am having problems with the sampling rates. I will start off with an example where the flow graph was working. The sampling rate is 256kHz, ...
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1answer
79 views

resonance demodulation for bearing faults [closed]

Can someone explain me how are the different frequency bands useful for detecting bearing faults using resonance demodulation?
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1answer
2k views

Practical Gardner timing error implementation

I'm trying to demodulate a QPSK communication. For the moment I do: Signal detection by computing the ratio between the power of the carrier wave compared to the signal power. Initial clock ...
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0answers
31 views

RF direct conversion down to DC and signal recovery from IQ

I've previously worked with super-heterodyne receivers where the IF spectral content has been above DC. Reading the below tutorial; direct down conversion receivers are used to mix a signal of ...
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0answers
64 views

What's wrong with my M-ary ASK modulation demodulation MATLAB code?

My MATLAB code linked below implements ASK modulation of an M-ary signal, impresses it on a carrier, adds AWG noise of various SNRs. Then, for each SNR, demodulates the signal and calculates the ...
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1answer
14 views

Term for signal waveform correction or demodulation

Let say a periodic signal waveform $v(t)$ can be modeled as: $v(t) = f(\phi) A \sin(2\pi ft + \psi)$ where $A$ is a constant amplitude, $f$ is signal frequency, $t$ is time and $\psi$ is a constant ...
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0answers
34 views

Help in identifying low speed image transmission protocol over RF

In the seventies I worked for Muirhead on press fax machines that used to transmit pictures over radio and landline as I recall two FM systems were used a carrier 800 Hz modulated +- 150 Hz and 1900Hz ...
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128 views

What are the pros and cons of common modulation techniques? [closed]

What are the pros and cons of commonly used modulation techniques? Why is one preferred over another in a given situation? Why do so many exist? Are some better at resisting noise, travelling long ...
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48 views

Mathematics in selection of RC time constant for an envelope detector for AM

Let the capacitor be charged to voltage $ V(t_0) $ at $ t=t_0 $ and then start discharging. At time $ t \, (>t_0)$, the delayed capacitor voltage is $$ v(t)=V(t_0)\operatorname{exp}(- \frac{t-t_0}{...
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0answers
112 views

How to find and fix the delay of a modulation

I am creating a model in MATLAB to simulate power line communication, but I found a problem. I'm very new to the DSP area, I get my code below from a MATLAB example. ...
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2answers
3k views

How to apply low-pass filter to an I/Q signal to get baseband signal?

Can anyone help me with the demodulation technique of a modulated signal? I mixed it with I/Q signal. But I am very confused about low-pass filtering. I am trying to apply a low-pass filter, but I am ...
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0answers
328 views

FSK demodulator by integration if the samples per symbols is 2

Say I have a binary FSK with +f and -f frequencies and I have two samples per symbol. Let's consider the "1) multiply with <...
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0answers
3k views

Doing analysis using IQ demodulation in Python

I'm trying to doing an analysis using IQ demodulation. I'm doing this by taking a noisy signal which has a desired signal in it. The desired signal has a frequency that is almost stable, which I'm ...
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0answers
67 views

Recover signal from sum of AM and FM

I have samples of a signal that contains an AM signal added to an FM signal with the same carrier frequency $f_c$. The AM signal is a sinusoid with frequency $f_{AM}$ and the FM signal is a sinusoid ...