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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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 sampls from the received signal. So what is the minimum sampling ...
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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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1answer
27 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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19 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
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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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27 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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29 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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32 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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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
23 views
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
46 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
29 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
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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
98 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
25 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
18 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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2answers
35 views
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
83 views
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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2answers
34 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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2answers
39 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
68 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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19 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
64 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 ...
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24 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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14 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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2answers
55 views
Why is bit error rate = symbol error rate / number of bits per symbol in QPSK
I saw a code computing the error probability of QPSK, and I found that in this code the bit error rate is computed as symbol error rate divided by $2$.
I think that $2$ is because the QPSK symbol ...
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1answer
17 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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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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69 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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2answers
67 views
What is the difference between frame error rate (FER) and symbol error rate (SER)?
What is the difference between frame error rate (FER) and symbol error rate (SER)?
I know the difference between frame error rate (FER) and bit error rate (BER) from the website below
https://www....
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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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1answer
79 views
FFT-based coarse carrier recovery for QPSK
An FFT-based coarse frequency acquisition block is being tested in GNU Radio. The algorithm used can be found on the GOES satellite user manual and is summarized in the figure below.
The algorithm ...
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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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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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1answer
48 views
Is it possible to use MRC with DS-CDMA-MIMO [closed]
I'm wondering how to use the Maximum Ratio Combining with MIMO-DS-CDMA, I have used the MIMO with DS-CDMA using Walsh code, then at receiver I couldn't recover the transmit signal using the MRC. here ...
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1answer
35 views
Continuous Phase Modulation
The wikipedia article on continuous-phase modulation states the following:
For instance, with QPSK the carrier instantaneously jumps from a sine to a cosine (i.e. a 90 degree phase shift) whenever ...
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1answer
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ADC and Matched Filtering
A basic theorem in communications is the matched filter maximizes the SNR at sampling. I'm a little confused on how this relates to discrete time systems and sampling rate.
Normally if you sample at ...
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1answer
29 views
Are the error probability of orthogonal signal and on/off signal the same?
I have to write the error probability of orthogonal signal and on/off signal,and now i have finished the error probability of on/off signal,but i don't know how to write the code about error ...
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1answer
61 views
Why do we need to write the noise as randn(2,1)./sqrt(2*SNR) instead of randn(1,1)/sqrt(2)?
I saw this code, about the error probability of on/off signal ,on the internet
...
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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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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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30 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
66 views
Maximum likelihood estimation complexity computation
I have a basic question about maximum likelihood (ML) estimator and its implementation.
I am trying to simulate a communication system, while using ML at the receiver side to find the transmit ...
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1answer
20 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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32 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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1answer
54 views
OQPSK vs QPSK envelope query
We use OQPSK because of envelope of signal but my question is
"In Phase Shift Keying(or QPSK) the amplitude of the transmitted signal is constant then how the envelope will change in OQPSK"??
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1answer
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Identifying the correct source of packets
I have a basic question regarding digital communication , especially pertaining to wireless internet .
A packet is basically a stream of bits . And a bit is detected as zero or 1 depending on ...
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1answer
32 views
Possibility of using Power allocation with Maximum likelihood estimation
I have a basic question about maximum likelihood (ML) estimator and its implementation with power allocation.
Suppose we are using $MIMO$ system with $4$x$4$ transmit and receive antennas, so the ...
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1answer
22 views
What is Assymptotic Equipartition Property?
I am currently studying about Polar Codes in 5G standard and while reading my paper I found something called AEP which is required for channel coding. I surfed the web but didn't found a satisfying ...
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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 ...
