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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34
votes
6answers
62k views

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 ...
24
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2answers
8k views

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 ...
16
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1answer
6k views

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: ...
13
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1answer
11k views

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 ...
12
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1answer
3k 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 ...
11
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4answers
17k views

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 ...
10
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3answers
1k 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 ...
10
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1answer
729 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 ...
10
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2answers
6k 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 ...
9
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1answer
8k views

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. ...
8
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2answers
3k views

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 ...
8
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2answers
335 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....
8
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1answer
250 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 ...
8
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1answer
520 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\...
7
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3answers
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 ...
7
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2answers
729 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 ...
7
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1answer
2k 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 ...
7
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2answers
735 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.
7
votes
1answer
224 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 ...
7
votes
1answer
268 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(...
6
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1answer
9k 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 $ ...
6
votes
3answers
365 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. ...
6
votes
2answers
111 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 ...
6
votes
2answers
724 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 ...
6
votes
1answer
332 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) ...
6
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1answer
1k 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 ...
6
votes
1answer
7k 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 ...
5
votes
3answers
638 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 (...
5
votes
2answers
537 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 ...
5
votes
1answer
5k 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 ...
5
votes
1answer
9k 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-...
5
votes
2answers
657 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 ...
5
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1answer
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 ...
5
votes
1answer
191 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 ...
5
votes
2answers
660 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 ...
5
votes
1answer
472 views

What is the cost of Hamming coding?

We know that (7,4) hamming code has 4 message bits and 7 code bits and can detect any single bit error. The (12,4) repetition code has 4 message bits and 12 code bits and can detect any single bit ...
5
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2answers
2k views

Golay complementary sequences for channel estimation - MATLAB

There are set of complementary sequences known as Golay sequences that are used for channel estimation because of the nice property they have which is that the sum of the autocorrelation function of ...
5
votes
1answer
12k views

QPSK Modulation/Demodulation in C++

I'm trying to implement a QPSK Mod/DeMod system in C++. So far i have a bit generator that generates N bits and stores it into an array: ...
4
votes
3answers
480 views

Lower digitisation rate for long distance communcation

Why is it that usually lower digitisation rates are used for long distance communications? e.g. sound is highly distorted in undersea communications, is this because of the lower digitisation rate ...
4
votes
4answers
6k views

Why does OFDM use cyclic-prefix while QPSK uses root-raised cosine filters?

My understanding is that BPSK/QPSK/... are often implemented with root-raised cosine (RRC) filters to reduce ISI and bandwidth. OFDM can be understood to be BPSK/QPSK/... implemented on multiple ...
4
votes
4answers
123 views

Why is phase range between $-\pi$ and $+\pi$ (instead of $0$ and $2\pi$)?

I wonder why the phase range in DSP applications is between $-\pi$ and $+\pi$. For example, why is that not between $0$ and $2\pi$? This question popped when I was reading about synchronization in ...
4
votes
2answers
291 views

Geometric representation of a signal using basis functions

I'm trying to solve the following problem where I need to express $x(t)$ in terms of the given $φ_i(t)$ functions. (It is proven that the $φ_i(t)$ functions are orthonormal).: Here's what I tried: In ...
4
votes
1answer
3k views

Why are Gold codes and Kasami codes used instead of pure m-sequences?

I'm trying to understand why Gold codes and Kasami codes are used instead of pure m-sequences, in direct-sequence spread-spectrum (DSSS) communication systems, to prevent interference between multiple ...
4
votes
3answers
8k views

What is the need for prediction filter in PCM and DPCM?

The DPCM works by the difference between actual samples and predicted samples. If we already have the actual samples, we can quantize it and encode it later. But why we use prediction filter? And ...
4
votes
3answers
8k 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 ...
4
votes
3answers
892 views

Optimal DC-free line encoding

I have a block of n bits that I'd like to encode into m bits that have an equal number of zeros and ones. I looked at protocols like 8b/10b but it wastes 25% of the data to achieve balance. As an ...
4
votes
3answers
519 views

Calculating penalty to SNR for non-matched filter

If a matched filter is optimal for detecting a known pulse in an AWGN channel, any other filter must have a degraded SNR. Given a discrete pulse shape and an arbitrary non-matched filter, is it ...
4
votes
2answers
1k views

Why does FEC seem to make BER worse at low Eb/N0?

I've been plotting BER curves and am wondering why FEC seems to make a BER curve worse at low Eb/N0 than the un-coded theoretical BER curve? My example plot below is for Differential BPSK using a ...
4
votes
3answers
4k views

Why is an interleaver often placed between an outer block code and an inner convolution code?

It seems that when doing FEC with a concatenated-code, an interleaver is often placed between the outer block code and the inner convolution code. The explanations I've seen say that this is because ...
4
votes
3answers
160 views

What causes this type of distortion in a constellation I/Q plot?

I'm transmitting QPSK through a digital pulse shaping filter (RRC, 35% excess BW, 2 samples per symbol, 64 taps) at a rate of 56Msym/sec. Receiving it on standard lab equipment (Vector signal ...

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