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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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Why is the received signal=information+RF signal+noise,not just information+noise?

In this paper: https://www.researchgate.net/publication/325008791_Beamforming_Design_for_Max-Min_Fair_SWIPT_in_Green_Cloud-RAN_with_Wireless_Fronthaul Here is its system model In this model,it said ...
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26 views

What is SIC? And why should we decode some signal first when we know that we will decode it again in the future?

I read the paper :http://jultika.oulu.fi/files/nbnfi-fe2019040110636.pdf The author said : First ,$x_s=x_1+x_2$,and the transmitter transmit the $x_s$ to $U_1$,and the received signal in the $U_1$ ...
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15 views

What is the advantage of this system (DA port in the DAS is usually equipped with its own power amplifier at the analog front-end)

I saw a paper,https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8283591 ,and it said Each DA (distributed antenna) port in the DAS (distributed antenna system) is usually equipped with ...
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22 views

preamble for doppler shift estimation

To estimate the doppler shift in a multiuser communication, I use preambles (PN codes). When I send the preambles with the datas (BPSK symbol), the estimation for the doppler shift are not correct. ...
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37 views

How to calculate Eb/N0 for a digital BPSK modulated multicarrier waveform

Suppose that I have a BPSK modulated multicarrier carrier waveform $L$ samples long at a sampling rate of $f_{s}$: $$ x_{tx}[l] = \sum\limits_{n=0}^{N-1} A \cdot \cos\left(2 \pi \frac{f_{n}}{f_{s}} ...
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23 views

Discrete Frequency Equation and Relation

I think I am looking at conflicting equations from a few sources, or maybe I just dont understand it. $ \Omega $ is Discrete frequency $ N $ is Discrete Period in number of samples $ \omega $ is ...
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33 views

How to measure a signal's initial phase given a buffer of received samples of the signal?

For CPM and FSK, how to measure a received signal's INITIAL phase based on its I and Q components given a buffer of received samples of the signal?
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1answer
35 views

frequency/phase offset estimation in satellite DOA beamforming applications

In satellite QPSK demodulation, we first estimate and remove the phase offset introduced by the channel. Then, we demodulate the baseband information bits (I & Q channels). In satellite DOA ...
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26 views

IT++ and Alternatives DSP Libraries

I want to implement some signal processing blocks in C++ so I did a bit of search on the available libraries for signal processing in C++ and I found that IT++ provides comprehensive set of functions, ...
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36 views

Binary vs 4-PAM PSD

I'm struggling to understand how you would form an expression for the PSD of a 4-PAM signal which is using rectangular pulse shaping. I understand for binary the likelihood of +/1 is equal so the ...
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19 views

data rate cdma chirp

I combined a chirp with a CDMA and I would like to know how to find the data rate. To begin, I spread the datas with the chirp and after I spread the previous signal with the PN code. My bandwidth is ...
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1answer
27 views

Cellular communication uplink

I can see, from my apartment, a cell tower. From what I have learned, in a time division duplex system the uplink and downlink communication are kept separate in time. Uplink from some time, then ...
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16 views

Average number of time slots until successful transmission in adaptive tree walking protocol

i was searching everywhere and couldn't find in any paper or book the average number of time slots until a successful transmission. for simplicity sake, in a simple network as given in the picture, ...
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146 views

How is DJI OcuSync wireless link physical layer implemented?

I'm looking for documentation on the wireless command link of the DJI OcuSync. I found this website (in Chinese). It seems like an OFDM link that uses Alamouti codes? I'm specifically interested in ...
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15 views

The theoretical formula of error probability and the data rate in MIMO,and the relation between $G^T$ and $G^R$ and $M$

I want to use Matlab to plot these two figures,fig(a) is the error probability,and as we can see,the higher M is,the lower error probability it has in the same SNR . The fig(b) is data rate,the higher ...
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17 views

waveform and multiuser communication

Sorry for my very simple question but I am a beginner in digital communication. Why in a multiuser communication, we have to have the same waveforms for each user? Is this an important rule for ...
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25 views

bandwidth and multiuser communication

I assume $3$ users. In the CDMA, the signals after the spreading for the $3$ users have the same bandwidth $B$. Is it possible to have $3$ spreading signals of the same band $B'$ such that $3B'=B$? ...
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22 views

Parity Check matrix from degree and check nodes polynoms, LDPC codes

I'm trying to simulate LDPC codes which were optimized for the BEC channel. I generated the degree and check nodes polynomials according to chapter 3.15 in "Modern Coding Theory" by Richardson and ...
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36 views

Symmetric Capacity of a Binary Discrete Memoryless Channel

While reading this paper https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5075875, I found that the the symmetric capacity of BDMC $W$ is given by $$I(W) = \sum_{y\in Y}\sum_{x\in X}\...
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39 views

PLL carrier recovery: intermediate versus baseband

When looking at carrier recovery, a PLL is often applied to correct the frequency offset between the local oscillator and the transmitter after down conversion. I'm wondering what happens to the PLL ...
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1answer
28 views

Synchronization Accuracy Required For Communications

One of the biggest problems in phase modulated communication is discovering when your signal has arrived. High accuracy in this area is important, as even a quarter wavelength error can cause the ...
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35 views

data rate and PN code

I have a question about the spreading sequence for CDMA. Normally, we use a PN code but imagine that we use a cosine wave combined with a PN code where the PN code has for length $T * fs$. With $T$ is ...
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156 views

Why does power spectral density of OFDM signal changes with number of zero subcarriers

I am trying to plot the power spectral density (PSD) of an OFDM signal with $N_{dsc}$ subcarriers and $N_{zeros} = N_{fft}-N_{dsc}$ zero subcarriers. I am using the matlab function pwelch() to plot ...
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1answer
122 views

Time delay estimation between two complex signals(IQ)

I need to time align two complex signals. The two signals obtained were at different RF frequency before digitisation. So,I have two complex signals(IQ) and I need to do delay estimation and apply ...
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16 views

guard interval calculation for multi-cell massive MIMO

I am interested in a multi-call massive-MIMO system modeled as in the works of T. Marzetta, for example: "Inter-Cell Interference in Noncooperative TDD Large Scale Antenna Systems". In these works, ...
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1answer
56 views

Time frequency analysis for a spreading signal

I am a beginner in digital communications: I am studying the spread spectrum communication and I have a question on the spreading signals. For example I have 2 spreading signals and I do a time ...
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26 views

phase shift for a spreading signal

For a chirp spread spectrum with $2$ users, I can use an up-chirp and down-chirp for the spreading signals. This is my questions: 1) These $2$ signals have a phase shift. If my $2$ users are at ...
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11 views

Why can the C5 formula also means that $\sum\limits_{k=1}^{K}tr(\mathbf F_{K4E})>P_T$?

In this paper:https://arxiv.org/pdf/1805.08898.pdf I can understand why C4 means $P_E > P^*$,I just need to times $1-\rho_k$ in both two side,the C4 will become $(1-\rho_k)[\sum\limits_{k=1}^{K}\...
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20 views

Can the j be equal to k in the first formula ?In MISO system

In this system model,the received signal available for RF EH at each RX_k is given by However,its SINR formula is given by We can find that author said the j can't be equal to k,but their range are ...
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12 views

What is the unit of a complex AWGN variance? mW or W?

In this paper https://arxiv.org/pdf/1805.08898.pdf ,there is a variance be written as $\sigma^2_{h,k}=\theta d_k^{- \alpha}$, $\sigma^2_{h,k}$ is the variance of th channel,$d_k$ is the distance ...
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1answer
186 views

From IQ to pi/4 DQPSK

I need to decode $\frac\pi4$-DQPSK packets from an SDR, preferably in C#. I have the SDR receiver part done. Now I'm trying to learn how to take the IQ stream and get the packet info from it. I've ...
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54 views

5G NR base matrix

The 5G NR specs for channel code are found here: https://www.etsi.org/deliver/etsi_ts/138200_138299/138212/15.02.00_60/ts_138212v150200p.pdf Starting on page 21 there is base matrix for the LDPC code ...
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118 views

Working principal of MIMO(2 by 2)-OFDM (802.11n)

I have gained basic knowledge on MIMO-OFDM from online sources and in process of building 802.11n standard in GnuRadio+USRP B210. My goal is to analyze the channel state information and play with it ...
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31 views

Training Fractionally Spaced Channel Estimator and Equalizer

If you are attempting least squares channel estimation with a fractionally spaced channel estimator, do you want the training sequence to also be fractionally spaced or symbol spaced? It looks like ...
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56 views

Determining signal using Optimal receiver(Matched filter)

I'm given a signal that has been passed through a noisy channel using binary polar signaling with a pulse value of p(t) = 1. The receiver is coherent and the length of each pulse (in samples) is 10 ...
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1answer
42 views

chirp spread spectrum with 2 users

I am a beginner in digatal communications. I have 2 users and I want use a chirp spread spectrum. Can we take an up-chirp and a down-chirp as spreading signals for the 2 users? and if yes, why? The ...
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31 views

bandwidth for a chirp

Let $s(t)$ an up-chirp with bandwidth $B$ and $y(t)$ an down-chirp with the same bandwidth. For a multi user chirp spread spectrum (2 users), I use the up-chirp and down-chirp as spreading signal. The ...
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55 views

OFDM system with cyclic prefix

I have: Bit rate $ R_b = 25.6 [Mbit/s]$ Sampling frequency at the input of the receiver: $ f_p = 8 [MHz]$ Protective spacing between successive OFDM symbols: $ τ = 16 [µs] $ The combined length of ...
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119 views

Preamble detection using cross-correlation

I am generating a preamble with a sequence of [1 -1 1 -1 1 -1....] and spread with a barker-11 sequence. Now, the transmitted signal becomes [B(Barker), -B(Inverse of Barker), B, -B,....]. In channel, ...
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1answer
185 views

What is the meaning of inverse signal bandwidth?

I have been studying the two ray model in communication systems. The text I'm studying has two signals, $u(t)$, which is Line of Sight and $u(t-t')$, which is the reflected signal. It quotes, ...
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13 views

Energy in the mcss

In an MCSS (Multiple Chirp Spread Spectrum) communication, for example with a gaussian channel. Why should spreading chirps have the same energy?
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24 views

Noise power for decorrelating detector

I have the following setup: $\mathbf{y}=\mathbf{R}\mathbf{s}+\mathbf{w}$, where the details of each entity can be found at http://bard.ece.cornell.edu/downloads/sim_tools/spanc_me/report/node12.html. ...
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33 views

Phase estimation in Beamforming

If I have a modulated signal $A\cos(2\pi f c t + \phi)$, I can demodulate it using IQ demodulator and estimate the phase using I and Q channel information. When it comes to MIMO case (specifically ...
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83 views

Phase demodulation vs frequency demodulation using a PLL

When demodulating a phase modulated signal using a PLL it is recommended to use a narrow loop BW, so that the VCO does not really follow the input phase modulated signal but outputs a reference ...
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51 views

Multiple beam beamforming

For digital beamforming, we can form as many beams as we want. No matter where I read about this subject I cannot find any discussion of the interference that one would expect between beams (not ...
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34 views

Fourier transform for a signal

If we want to sample a signal $s$ for digital transmission, the modulus of its Fourier transform, $|S|$, must be non-zero only in a frequency band $B$. But are the magnitudes of $|S|$ are important? ...
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1answer
64 views

Error detection and correction algorithms for Wireless Body Area Networks

I am working on transmitting ECG signals over wireless body area network based on IEEE 802.15.4 standard. To reduce the BER and increase the signal reconstruction quality, I think that I need to ...
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1answer
90 views

Modulation and Noise

I am fairly new to studying communication systems. When it comes to different digital modulation techniques, does the effect of an additive complex noise (a) introduced by the system, and (b) by the ...
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1answer
147 views

Nyquist criterion for zero ISI in the frequency domain

Can someone explain in simple words /graphs the necessary and sufficient Nyquist criterion for zero ISI in frequency domain, namely: The constant folded Fourier Transform.
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130 views

Frequency domain vs time domain equalization

It is said that a frequency domain equalizer is equivalent to an infinite length time domain equalizer, why is that? And if this is the case, why hasn't frequency domain equalization been used ...