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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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Convolutional coding that payload length does not change

This is the question about Sigfox radio specification. The stack: Specifically, in Section 3.10 of the specification, we have a convolutional coding (after CRC) with three possible polynominals. I ...
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What is the threshold to prefer polyphase channelizer over FIR filtering?

A wideband signal need to be processed to extract several narrowband channels from it. There are two straightforward approaches to do so: For each narrowband channel shift frequency and filter it ...
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Translating Matlab scrambler function to C

I am trying to implement the following matlab function in c: Mathworks scrambler Function Documentation This is the code I've written: ...
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Why does this self-interference cancellation cancel other signal,not the self-interference?Do i misunderstand something?

In this paper, https://ieeexplore.ieee.org/document/8704259 ,i think transmitter with M antennas transmit $x_f[t]$ to Cell-center user ,and transmit $x_n[t]$ to the Cell-edge user,however,the distance ...
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Why when the SINR of transmitting signal is very large ,the device cannot decode the received signal at the same time? [closed]

$A$ transmit $x$ and receive the $y$ at the same time.If the SINR for transmitting $x$ is very large,$A$ cannot decode the $y$ signal.Why? In my opinion,If the SINR for transmitting $x$ is very ...
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1answer
43 views

How can i know the full duplex does work from the Shannon's theorem?

There is a full duplex system below,two user are $A$ and $B$,according the definition of full duplex ,$A$ and $B$ have to transmit and receive the signal at the same time.that is, transmitted signal ...
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1answer
87 views

Digital FM Demodulation

I have created a Common Lisp Library to receive samples of an FM signal from BladeRF1 SDR. I want to write a subroutine to implement FM demodulation. The input should be in the format of IQ data. ...
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1answer
29 views

Is there any advantage on using a low span value in a RRC filter?

I know that in a RRC filter a high value of the span gives a better response, in the sense that the RRC filter response is more near to the ideal RRC filter response. However, would there be any ...
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37 views

How to remove sum and difference terms while computing frequency from fft?

I am currently operating in vhf band AND trying to detect frequencies using Fast Fourier transform thresholding method. While detection of multiple frequencies , i received spurs(May not appropriate ...
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1answer
27 views

How to decode a Barker code

Let's suppose that I send the following sequence of bits: $x = [1, 0]$ I use a 7 length Barker code ($b = [−1,−1,−1,1,1,−1,1]$) in a BPSK modulation scheme, resulting in the following sequence: $...
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1answer
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Definition of MIMO (Multi-input Multi-output ),does this output mean antenna or transmitter?

I want to ask the definition of MIMO(Multi-input Multi-output ) If there are one transmitter and one receiver,each of them has $N$ antennas,$N >1$. So i can say it is a MIMO system. But if now i ...
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1answer
62 views

What is matched filter in QPSK signal?

Consider we have received a QPSK signal. We than make I (In phase) and Q (quadrature component) which I understand is nothing but multiplying the received signal by the cos(2*pi*f) and -sin(2*pi*f) ...
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29 views

law for a random variable

Let: $Y=|X_1+X_2|^2$ with $X_1 \sim N(\mu_1,\sigma_1^2)$ and $X_2 \sim N(\mu_2,\sigma_2^2)$. $X_1,X_2$ are independent complex random variables. Consequently, $Y=Re(X_1+X_2)^2+Im(X_1+X_2)^2$ My ...
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66 views

Implementing Costas loop for 4-PAM

EDIT 2: OK, regarding the code I’ve pasted below, and seeing the FFT plot, I suspect the LPFilters aren’t working since I’m getting $0.5*m(t)*[cos(2 \pi f_c t ( \theta_e)+cos(\theta_e)]$ instead of $0....
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22 views

Channel information from power spectra

Linked below is a previous post regarding a multipath channel. Equalisation of FFT spectra I am reusing the images here: The first image is that of the power spectra when there is no multipath and ...
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1answer
43 views

Is there any relation between QAM and streaming?

I am trying to determine whether there is any relation between QAM and streaming (e.g., video streaming). I know that QAM can be used for broadcast TV. I'm not sure if QAM is used for WiFi bit ...
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Do we need to distinguish which rate is it when the system is point to point?or we just say “rate”,not downlink rate or uplink rate

If i have four transmitter,(t1,t2,t3,and t4), and one receiver(R),and each of them just have one antenna. When t1 is transmit signal to R,the other transmitters (t2,t3 and t4)harvest the RF ,when t2 ...
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1answer
24 views

What is the definition of half duplex?Is this system,$A$ give $x$ to $B$,then $B$ give $x$ to $C$,called half duplex?

I saw the definition of half duplex in wiki,and it said half-duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a ...
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1answer
25 views

Find the ber for the cdma protocol

Do you know a reference (book, article) which gives (with a proof) the theoretical BER in the CDMA protocol for a differential modulation?
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2answers
37 views

Confused about the definition of point to point ,this point means single antenna or single device?

What is the definition of point to point ? does the "point" mean single device or single antenna? I mean,can one transmitter with multiple antennas and one receiver with multiple antennas be called ...
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Where will system,information and decoder are separated, be applied?why don't we consider maybe the splitter is too small to destroy the information?

In the research of energy harvest and SWIPT,some paper will design that information decoder and energy harvester will be in the same device,then use a power splitter to split the received power for ...
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1answer
47 views

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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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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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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1answer
39 views

Can we band pass filter QPSK signal?

Consider we have QPSK signal with carrier frequency 10kHz. Generally, we demodulate or use matched filter on the the QPSK signal and recover the transmitted bit or the symbols. If the QPSk signal is ...
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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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36 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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1answer
66 views

What is the minimum sampling rate for QPSK or what is the bandwidth of QPSK signal?

Consider we have created a QPSK signal with a carrier frequency 100kHz. Before applying matched filter or demodulation we need to take samples from the received signal. So what is the minimum ...
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1answer
24 views

What's the effect of spread spectrum on frequency- selective channel

In brief, we consider the channel as frequency-selective channel if the frequency of the signal is larger than then frequency of channel. So, does using the spreading spectrum change the frequency-...
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38 views

Is the zero part of a RRC filter generated signal sent?

When you use a RRC filter you add extra symbols at the end of your generated data because of the delay of the filter. When the data passes through the filter it gets up-sampled and the delay is ...
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22 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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2answers
107 views

About understanding a Matched Filter

I have some basic doubt in understanding a matched filter (MF). Please note that I have already seen very informative post in this context Understanding the Matched Filter In what follows, I list ...
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1answer
34 views

spectrum for a baseband communication signal

I have a question for the spectrum: In baseband before passing through channel, should the spectrum of a communication signal be symmetrical?
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46 views

Rate vs throughput vs capacity

What is the difference between Rate , throughput and capacity ? Their unit are all bps/s,but it seems that there are something different between them,can anyone tell me? My research is optimizing ...
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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
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Why does it seem most of people will optimize the downlink rate,not the uplink rate?

I search for some papers about energy harvest,SWIPT and optimization.And i found that there is just one paper to optimize the uplink rate,the others are for optimizing the harvested power,power budget ...
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1answer
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duration for a signal after filtering [duplicate]

I have a signal $s(t)$ of duration $T$. This signal doesn't have a limited bandwidth. To solve this problem, I filter it with a pulse shaping filter (raised cosine filter). My question: Does ...
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2answers
47 views

What will it happen when the outage occur?

I saw this explanation about outage probability,it said: it is the probability that an outage will occur within a specified time period. So what will it happen when the outage occur? stop ...
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1answer
34 views

Is it possible to spread signal using sum of more than one orthogonal code

I was wondering if we can spread the signal using more than one spreading code. Assume we have the Walsh code used to spread the transmitted signal, the Walsh code is, for example, gotten based on ...
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3answers
51 views

Why only odd harmonics after non-linear amplification?

Why do I get only odd harmonics appearing? This not only happens at saturation of the transfer curve, but also in the linear portion of the curve. I make a input signal that has too high power that ...
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1answer
139 views

Calculating Data Rate using Bandwidth, Transmission Power, Noise Power Spectrum Density and Channel Gain

I'm a Software Engineering student researching on Load Balancing in IoT and I need to calculate data rate. However, I don't know how to use these values to attain data rate despite searching on the ...
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1answer
33 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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1answer
29 views

Log Distance-Normal Shadow Model and Friis

Since I'm founding several but different results about this model, I would ask you if all it is OK with this formulas: 1) Friis Model --> $Pr_0 = Pt + Gt + Gr + 20\log_{10}\frac{\lambda}{4\pi d}$ 2) ...
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Dimension of Matrix in MIMO with multi-path

In a MIMO system, the received signal is supposed to be $y = Hx + n$, where $H$ is the channel of dimension $N_t \times N_r$, $x$ is the transmitted signal with dimension of $N_t\times 1$ , and $n$ is ...
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1answer
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When Two Sampled Sinusoidal Are Orthogonal?

If two analog sinusoidal are orthogonal with duration T, their minimum frequency difference should be 1/2T in case of no phase offset between them. And if there is phase offset, the difference is 1/...
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67 views

Frequency Domain Correction of Frequency Offset or Doppler Frequency in LTE

We are working on LTE Uplink CFO Correction for 3 MHz band. The time domain CFO correction is easy, which depends on the processing of 3840 samples step by step multiply by $\exp(-j 2\pi f_\text{...
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82 views

Does gray coding bring improvement to SER of QPSK, compared with natural coding?

I understand gray coding contributes to BER of QPSK, yet really don't understand why it brings improvement to SER. According to the relationship of SER and BER, it should be improved. Actually, when ...
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1answer
126 views

QPSK Phase Recovery : Maximum-likelihood Vs Costa's Loop

I have implemented a QPSK Costa's loop in GNU Radio and run some BER benchmarking. All is going well. Out of curiosity, I'm trying to implement the Maximum Likelihood phase recovery shown below. For ...
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25 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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1answer
224 views

Offset QPSK detection in GNU Radio - Sample delay

An OQPSK detector is being tested in GNU Radio. The architecture was obtained from Michael Rice's Digital Communications - A discrete-time approach. The flowgraph is shown below. The modulator ...