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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 PSD: Why Does Power at High Frequencies Affect Low Frequencies?

I'm trying to wrap my head around power spectral density on a conceptual level, but I am having some difficulty. Suppose I have a communication system where I am receiving and sampling white Gaussian ...
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Can you use the theory of Signal Space to study Band Base digital systems?

Introduction: There is one way to study different modulations in band pass digital systems and is working with "signal space" and the Constellation Diagram. My questions about this are: 1) Is it ...
545 views

Cyclic prefix length in baseband OFDM with ZF equalizer

My question is about baseband OFDM. I try to plot the BER according to the SNR. I noticed that changing cyclic prefix length does not affect the performances. I.e., following the code below, ...
564 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 ...
182 views

Rake Receiver vs OFDM and multi-path channel

My Question is, Can we use the Rake Receiver with OFDM modulation? As you know, Rake receiver is usually used with DSSS modulation, so can we use it with OFDM? in other words, Does the Rake Receiver ...
239 views

FFT based symbol synchronization for digital demodulation

Where can I read more about "slow search" methods for symbol synchronization based on FFT for extracting a clock signal from the modulated signal? I have read here (8.7 Symbol synchronization) One ...
103 views

Explanation of fractional sampling in OFDM [closed]

What is the concept fractional sampling? Is there a relationship between the fractional sampling and interpolation? And how does the fractional sampling help in convert the SIMO (Signal input multi-...
428 views

What techniques are available for correcting constellation rotation due to phase ambiguity?

In order for a receiver to recover a baseband signal, it needs a carrier signal equivalent to that used by the transmitter for upconverting the original baseband signal. Unless pilot tones are used, a ...
897 views

Choosing samples per symbol for modem pulse

One of the parameters in my DSP library for pulse design is "samples per symbol", and I would like to get some advice about choosing this parameter when designing a modem. The smaller the pulse width,...
385 views

what is the frequency selective channel and doubly-selective channel

Could anyone please help me to explain the frequency-selective channel and doubly-selective channel and the difference between them in brief and easy way. I have tried to check on internet, I couldn'...
121 views

Demodulation in Rayleigh Flat Fading Channels

A core part of demodulation in Flat-Fading channels is (assuming) the channel as a(random) constant complex number multiplied with your signal. This is done in order to de-rotate the symbol for ...
84 views

DSP using audio IO from PC using C++

This is my first question on dsp. I am looking for something very simple here and I am sure there are libraries that will do pitch recognition and FFT but my requirement is very simple. I have a PC ...
63 views

Why do we use conjugate transpose IFFT in OFDM

I have a question regarding the OFDM and using the conjugate transpose of vector (matrix). As you know, if we have a block of data,let's call it $X_n$, consisting of $N$ data symbols, we should ...
315 views

difference between pi/4 qpsk and differential qpsk

What is the difference between a $\pi/4$-QPSK modulation and a QPSK differential modulation (DQPSK)? Are these modulations identical?
76 views

Channel Shortening for underwater acoustic channel communication

I'm new in this forum, hope that we can learn from each other well. I need to ask about the channel shortening for underwater acoustic channel. what are the common algorithms in that field? is ...
189 views

How can I make a mmWave frequency selective channel in matlab?

$$H_d=\sum_{\mathcal{\ell}}^{N_p}\alpha_\ell p(dT_s-\tau_\ell)\mathbb{a}_R(\phi_\ell)\mathbb{a}^*_T(\theta_\ell)$$ I'm trying for simulation of a frequency selective channel in matlab. my problem is ...
98 views

Viterbi Algorithm - Selecting survivor paths which both contain the same Hamming distance

To my (limited) knowledge, when determining which survivor path to choose to produce an error-corrected output of a binary Viterbi decoder, the path with the smallest Hamming distance is the most ...
192 views

Confusion in Peak to Average Power Ratio (PAPR)

(1) What does it mean by 'a system with low peak to average power ratio requirement'? Doesn't every system idealises it to be lower? Or there are some exceptions to it? (2) Higher PAPR can be a ...
133 views

problem with the matched filter

Let $y(t)=s(t)+n(t)$ For the matched filter h(t) on $[0,T]$ we have, $h(t)=s(T-t)$. And the output is: $z(t)=\int_0^T y(u)h(t-u)du=\int_0^T y(u)s(t-(T-u))du=\int_0^T y(u)s(u-(T-t))du$ But sometimes ...
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What is the acceptable range for bandwidth utilization factor in OFDM systems?

The bandwidth utilization factor is defined by (alpha=bite rate/bandwidth), for OFDM system, what is the acceptable range for alpha?
64 views

Beamforming with NDP frames

During beamforming, the Tx first sends NDP Announcement (which is a broadcast frame) thereby reaching all Rx in its area. And then it follows by sending NDP. Is the 2nd frame (NDP) is also a ...
73 views

Determining the features of the DSP

I need to sample a signal of 40MHz with a sampling frequency of 120MHz at least with a 8/12 bits ADC and send them through the USB serial port of a PC. That means the serial communication bandwith ...
64 views

Transmission frquency in MIMO-OFDM system

What I understand in MIMO system, the user data is broken down into several chunks and transmitted over the antennas (say 3 Tx antennas) by using same frequency Fc. In order to use the interference as ...
843 views

How channel state information is calculated from Sounding Packet

What I understand is for Explicit Transmit Beamforming scenario, the Transmitter sends an NDP packet(aka HT Sounding) which consist of OFDM training symbols in packet preamble (HT-LTF) which are ...
167 views

How to make the impulse response sparse? How does one know that the channel is sparse?

I am new to sparse channel estimation algorithms and reading research articles. One such paper is blind sparse channel estimation using a modification of the BOMP technique titled, "Blind Acoustic ...