Questions tagged [digital-communications]

Digital Communication is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channel.

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Understanding the Matched Filter

I have a question about matched filtering. Does the matched filter maximise the SNR at the moment of decision only? As far as I understand, if you put, say, NRZ through a matched filter, the SNR ...
user968243's user avatar
24 votes
2 answers
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Extracting Binary Magnetic-Strip Card Data from raw WAV

I am faced with a tricky challenge: To extract binary data from an iPhone magnetic strip card reader. This is what the magnetisation on the card looks like: Source Here is the .WAV the iPhone ...
P i's user avatar
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17 votes
1 answer
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How to estimate and compensate for doppler shift in wireless signals?

I was wondering what good method(s) exist for the estimation (and subsequent compensation) of doppler shift for transmitted signals, be they acoustic or RF, in the context of comms. The question: ...
Spacey's user avatar
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13 votes
1 answer
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How to perform carrier phase recovery in software?

What are the options for recovering the phase of a BPSK signal in software? The only resources I can find online contain circuit diagrams – there seems to be no one willing to explain this to people ...
Keith's user avatar
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12 votes
4 answers
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How is symbol synchronization with OFDM done?

I'm trying to understand how symbol synchronization is done in OFDM using pilot tones, cyclic prefixes, or any other technique. I've read the following answers which provide some explanation, but I ...
Dan Sandberg's user avatar
12 votes
1 answer
4k views

Optimum matched filter without ISI

Given a filter used to shape the digital signal, $p(x)$, and given that we do not want the filter combination to cause any ISI, what "matched" filter, $q(x)$ will maximise the SNR? Matched filters ...
gyroidben's user avatar
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11 votes
6 answers
5k views

How can I work on DSP using C/C++?

I have been working on MATLAB for signal processing for a while. Many companies from DSP for a communication background has asked me whether I know C/C++ very well. I am confused as to why the ...
Sukshith Shetty's user avatar
11 votes
1 answer
773 views

What are thoughts on best modulation to use for underwater acoustic communications at low frequencies?

I wanted to ping the DSP hivemind for general thoughts on what would be the best modulation type to use for low frequency underwater communications. I have chosen this project since I can learn a lot ...
Spacey's user avatar
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11 votes
2 answers
7k views

What is difference between OFDM and GFDM?

I am trying to understand the difference(s) between OFDM (Orthogonal Frequency Division Multiplexing) and GFDM (Generalized Frequency Division Multiplexing) which are used as a multicarrier modulation ...
tuner's user avatar
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10 votes
3 answers
2k views

Capacity of AWGN channel

I am confused understanding basic concepts of communication over AWGN channels. I know the capacity of a discrete time AWGN channel is: $$C=\frac{1}{2}\log_2\left(1+\frac{S}{N}\right)$$ and it is ...
Mah's user avatar
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10 votes
1 answer
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Information Theory - units of channel capacity

In a first course in Information Theory, when the operational interpretation of channel capacity is introduced, it is said to be the highest data rate (in bits/channel-use) of reliable communication. ...
rk2's user avatar
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9 votes
2 answers
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Why are QAM Constellations regular and rectangular?

In most IQ graphs of QAM noise, it seems like the constellation ends up with more distortion the farther you get from the center: What causes this non-linear distortion, and why are QAM ...
Mitch's user avatar
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9 votes
3 answers
1k views

What is the definition of a "Symbol"

I’m looking for the simplest, precise definition of a Symbol e.g. The symbols found within a signal The signal has a N symbols per second I'm following this tutorial. The fellow defines the term ...
user13764383's user avatar
9 votes
3 answers
4k views

PSD of complex white gaussian noise

It may be a really simple question, but I'm not sure about this one: Given a complex white Gaussian noise process with iid real and imaginary parts and a double sided power spectral density of $N_0/2$...
Phobos's user avatar
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9 votes
1 answer
298 views

Practicality of the assumption of i.i.d. for Rayleigh channels

I would like to understand how far the assumption of having i.i.d. is accurate/valid (from the practical point of view) when an OFDM system is working in Rayleigh channels. Is this means the channel ...
Noor's user avatar
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8 votes
3 answers
2k views

Common methods of transmitting digital data through an extremely noisy audio signal?

I am developing software for transmitting data from a computer to a mobile device using audio. Specifically, from the computer's speaker to the device's microphone. (I am developing both the sender ...
Keith's user avatar
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8 votes
2 answers
948 views

QAM constellation slicer with unknown carrier phase

As shown in the image, I have a 16QAM constellation that is misaligned due to a phase rotation. In this case you can see that rotation amount is approximately $\theta = \pi/4$, but this won't be the ...
user827822's user avatar
8 votes
2 answers
382 views

What input to use for an AR model of a vowel sound?

I've recorded a 2-sec pronunciation of a vowel sound. The first 0.12 or so seconds of the signal are shown below. Now, I've constructed an auto-regressive (AR) 8th-order model to compress this signal....
Andreas's user avatar
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8 votes
1 answer
285 views

How to send / receive a short impulse in an extremely noisy signal with high timing accuracy?

I am pretty new to DSP. I need a computer to transmit data to a receiver through audio signals in a loud outdoor environment. (I am writing the software for both sender and receiver so I can design ...
Keith's user avatar
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8 votes
1 answer
638 views

Which digital modulation scheme would be suitable for transmitting digital data through the output of a PC soundcard

I need to output a data stream at $40\textrm{ kbit/s}$ or faster out of a computer system whose only accessible output peripheral is the audio interface. This interface has reasonable specs, $96\...
pichenettes's user avatar
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8 votes
1 answer
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Whitened Matched Filter

I am seeking for an advice on whitened matched filtering technique. I have looked into the literature and I do understand its the purpose and how to select the filter in order to achieve the desired ...
Milos Milosavljevic's user avatar
7 votes
1 answer
13k views

BPSK vs $\pi/2$ BPSK, what the benefits?

I have come across modulation techniques that have $\pi/2$ phase shifts from the regular constellations. For example $\pi/2 $ BPSK vs constellation the regular BPSK. Other examples include $\pi/2 $ ...
Tyrone's user avatar
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7 votes
1 answer
4k views

Location of Matched Filter

In a digital communication receiver, matched filter(SRRC filter) is used. In different literatures, the location of the matched filter is mentioned in two different places. One is just after the ADC ...
avi1987's user avatar
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7 votes
2 answers
768 views

Alternative to Orthogonal Matching Pursuit (OMP) Algorithm

In the Compressed Sensing context, assume there is a signal $ x \in {\mathbb{R}}^{n} $ which is $ k $ sparse. Namely its Pseudo $ {\ell}_{0} $ Norm is $ {\left\| x \right\|}_{0} = k $ (The signal has ...
Digi1's user avatar
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7 votes
2 answers
946 views

What Are Less Computationally Demanding Alternatives to the Viterbi Decoder?

What are less computationally demanding alternatives to the Viterbi Decoder? Ideally what I would like is a list of the most commonly used approximate methods, along with brief pros and cons.
gyroidben's user avatar
  • 293
7 votes
3 answers
911 views

Maximum Likelihood for Colored Noise

I have the following question about the maximum likelihood (ML) in presence of inter-symbol interference and colored noise. Assume the communication system is as follows. Information source, ...
Noor's user avatar
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7 votes
1 answer
9k views

Simulating BPSK Costas Loop in Matlab

I am trying to implement BPSK demodulator in FPGA. First, I am modeling the system in MATLAB. Now, I am trying to design Costas Loop for carrier synchronization. This is the block diagram I'm trying ...
Siraj Muhammad's user avatar
7 votes
1 answer
318 views

Why would I want to define a modulation index for each tone (DSB-FC)?

So the exercise is basically a signal $f(t)$ that is going to modulate the carrier $A\cos(\omega_ct)$ using a modulation index of $m=1$. I have to find $A$ and the power of the modulated signal: $$ f(...
user3680's user avatar
6 votes
2 answers
6k views

What is a constant envelope modulation?

The Book "Digital Modulation Techniques" by Fuqin Xiong groups digital modulation schemes into two classes: "constant envelope" and "nonconstant envelope." However, I was surprised by the inclusion ...
nispio's user avatar
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6 votes
1 answer
3k views

ADC quantization +thermal noise

ADC noise can affect the overall noise performance of a receiver. The noise is partially quantization noise and partially thermal noise and as a general rule it extends up to the Nyquist frequency = ...
Hatem Tawfik's user avatar
6 votes
3 answers
611 views

Why are we always interested in mean-squared distortion?

When dealing with quantizers, and in many other communications problems, the interest is usually on the mean-squared distortion or mean-squared error, rather than mean absolute error or anything else. ...
gtak's user avatar
  • 83
6 votes
2 answers
6k views

How to Simulate AWGN (Additive White Gaussian Noise) in Communication Systems for Specific Bandwidth

I am trying to generate a AWGN waveform to add it to the signal of my simulated communication system. The operating bandwidth of the communication system is about ...
user113478's user avatar
6 votes
1 answer
235 views

Does any error correction code still work in such situation?

I'm looking for a kind of error correction code or solution that can correct my codeword in this case: My message holds $k$ bits, and $2k$ bits codeword (rate is $1/2$) is produced by the generator ...
WangYudong's user avatar
6 votes
2 answers
1k views

Intersymbol interference due to limited channel bandwidth

What is the difference between limited-bandwidth induced intersymbol interference (ISI) and ISI due to multipath propagation. I noticed that the delay spread in indoor environment is in the range of ...
user24907's user avatar
  • 313
6 votes
2 answers
298 views

orthogonal family and pulse shaping filter

I am a beginner in digital communications and I have a question on the pulse shaping filter. For example, I have an orthogonal family $ \{s_1,...,s_N \}$. Can we preserve the orthogonality with a ...
user40695's user avatar
6 votes
1 answer
374 views

How to derive $r(t) = c(t) \circledast \frac{1}{2} h_b(t, \tau)$?

Consider a linear time-variant channel. The transmitted signal is $x(t)$, the channel impulse response is $h(t, \tau)$, and the received signal is $y(t)$. Then $$ y(t) = \int_{-\infty}^\infty x(\tau) ...
Wei-Cheng Liu's user avatar
6 votes
1 answer
2k views

How to measure/estimate symbol rate of a signal with unknown ( but probably QAM ) modulation?

Is it possible to estimate the symbol rate of a signal whose modulation is not know exactly but it can only be guessed that it is probably some kind of QAM (probably 64 or 256). What is the best way ...
jhegedus's user avatar
  • 247
5 votes
3 answers
678 views

Compress a signal by storing signal diff instead of actual samples - is there such a thing?

I am working with EMG signals sampled at 2kHz and 16 bits, and noticed that they "look smooth", that is, the signals are differentiable, and if I apply a "diff" function (...
heltonbiker's user avatar
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5 votes
2 answers
585 views

Why not use zero-signal in digital transmission?

This is something that bothers me from time to time and has always been hand-waved away with some plausible but non-rigorous explanation. For clarity, consider the QPSK case where we have the ...
Benjamin Lindqvist's user avatar
5 votes
3 answers
11k views

How to find the group delay of IIR filters in Python/SciPy

I sent a square wave signal (red) through a IIR filter (butterworth in this case), to make it more "realistic". Then there's a slight group delay in my output signal (blue). Is there a way to find out ...
LWZ's user avatar
  • 181
5 votes
2 answers
326 views

Amplifier Placement in a Wireless Receiver

In the context of wireless receiver system design, what is the optimum choice of amplifier placement? Should the received signal be amplified first then filtered or should it be the other way round? ...
malik12's user avatar
  • 488
5 votes
2 answers
886 views

Why is CDMA not enough for wireless communication?

I have a question about CDMA, FDMA and TDMA. I don't understand why we suffer from limited capacity when we have CDMA. I can understand the limitation in FDMA and TDMA, as time and frequency are ...
Mio's user avatar
  • 59
5 votes
1 answer
8k views

Phase synchronization in BPSK

In BPSK for demodulation at the receiver end we do phase recovery between received wave and carrier wave. We can find the phase difference between the carrier and received waves using : PLL And ...
Sate's user avatar
  • 121
5 votes
2 answers
591 views

Quadrature modulation

I have a question: What is the difference between $ I \cos(t) - Q \sin(t) $ and $ I \cos(t) + Q \sin(t) $ except that they are phase shifted 90 degrees ? On the internet I can find both ...
Przemek Lewandowski's user avatar
5 votes
2 answers
11k views

What does the one-tap equalizer used with OFDM accomplish?

I've read that OFDM normally uses a one-tap equalizer. It seems that a one-tap equalizer could only scale and delay a signal. Why is this useful? I thought multipath was handled through the cyclic-...
Dan Sandberg's user avatar
5 votes
2 answers
2k views

Difference between these two digital down conversion methods

I can think of two possible methods of digital down conversion. One seems superior to the the other for most cases, but I'd like to get some DSP experts' comments on the practical differences between ...
Void Star's user avatar
  • 251
5 votes
1 answer
116 views

Does the Quality of the Pictures Taken by a USB 3.0 Camera Depend on Cable Length?

I've got a JAI GO-5000-USB camera and have tried several cable lengths, ranging from 40cm to 5m. So far, I haven't observed many differences in the quality of the pictures I've taken. Since I want ...
Mickey's user avatar
  • 153
5 votes
1 answer
2k views

OFDM training symbol format

For OFDM synchronisation, we are using two training symbols according to the Schmidl and Cox method of frequency synchronization and timing synchronization. The paper says: The first OFDM training ...
Karan Talasila's user avatar
5 votes
3 answers
4k views

Do BPSK vs QPSK have the same performance?

I'm trying to re-learn digital communication theory again and am creating a bit error probability simulation for various modulations. So far I've added BPSK, QPSK, and 8PSK and below is a plot that ...
jstraugh's user avatar
5 votes
2 answers
2k views

Timing error detector used for symbol timing synchronization

The Gardner timing error detector is independent of carrier phase and frequency error, and is also quite immune to self noise (to my understanding, I may wrong about that). Can one assume that it is a ...
Hatem Tawfik's user avatar

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