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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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
24 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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50 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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10 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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please help with this QUESTION [on hold]
Explain how the data rate over a channel can be increased, without increasing the bandwidth? What is the disadvantage of this approach?
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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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28 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
60 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
18 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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1answer
18 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
35 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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Accepted delay in digital receiver?
At the receiver end, we will have a delay because of DSP such as delay of the digital filter, equalizer, decoder...,
My question is what is a reliable delay so that we can have synchronization with ...
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1answer
21 views
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
31 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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0answers
37 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
22 views
BER vs Eb/N0 (db) Curve
So, I was trying to plot BER (Bit Error Rate) vs. Eb/No (in dB) curve using Python. Here is what I get,
Eb/N0 is in dB on the x-axis
But I know that this is not the correct graph.
Here is my code.
...
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1answer
25 views
confused in practical example of shift invariance?
I am reading book of w smith ,"The scientist and engineer guide to digital signal processing" 2 edition
I am trying to understand topic of shift invariance from theory given on pg 92
I have attached ...
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3answers
537 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 (...
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1answer
44 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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0answers
8 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, ...
2
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1answer
19 views
Beamforming back lobes question
When performing a conventional digital beamforming I see in some textbooks they show beam patterns as
where the arrival angle ranges between -90 to 90 degrees. But then other places show the full ...
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0answers
52 views
GNU Radio | Transceiver loopback adapted | Transmission stops unexpectedly
(This is a follow up question to a previous question: link)
I have implemented a flowgraph of a transceiver loopback inspired by the examples found at ...
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1answer
36 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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1answer
37 views
Why can the :$\mathbf W^*_{MMSE} $ be written as $E[\mathbf s \mathbf y^*]E[\mathbf y \mathbf y^*]^{-1}$
I read this paper: https://arxiv.org/pdf/1305.2460.pdf
And there is a formula about MMSE:$\mathbf W^*_{MMSE}= E[\mathbf s \mathbf y^*]E[\mathbf y \mathbf y^*]^{-1}$
$\mathbf W _{BB}$:Baseband ...
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1answer
39 views
Carrier frequency of chirp signal
I am computing the link budget for a communication-radar (CoRadar) system. The formula for maximum distance, in communication, is:
$d_{max} = \sqrt{\frac{P_t}{\tau}\frac{G_t G_{r, comm} \lambda^2}{(4\...
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25 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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0answers
10 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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0answers
17 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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1answer
52 views
What are the limitations of using Magnetic induction rather than acoustic communication in underwater communication
In underwater environment, Why acoustic underwater communications are still famous more than magnetic induction communication however in magnetic induction underwater communication, higher data rate ...
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1answer
44 views
DSSS direct sequence spread spectrum - encoding each raw data bit (question)
For implementing DSSS, it is understood that each raw data bit needs to be encoded as a new recognizeable/identifiable bit pattern.
In many literature involving DSSS, there is mention of using PN ...
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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
72 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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0answers
10 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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1answer
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Spatial diversity: symbol error probability, upper bound
I am trying to understand the calculation upper bound that is given in book.
Edit 1: I added 3,44
Can someone explain to me how to come from (5.5) to (5.7) ?
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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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1answer
39 views
Using Spreading spectral with Maximal likelihood
As asked here, HERE if we have the signal
x =
[0.7 + 0.7i;
0.7 - 0.7i;
-0.7 + 0.7i;
-0.7 -0.7i];
Which was spread over code ...
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1answer
25 views
Channel model: LOS Component and Rician K-factor
I am asking about Rician K- Factor in this model:
I know that if $K=0$ then it is a pure Rayleigh fading, if $K= \infty$, then non fading Channel.
What is it, if $0<K<\infty$ ?
If i represet ...
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0answers
22 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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1answer
31 views
Channel estimation
In single-carrier systems, at the receiver, when we need to do channel estimation and equalization? Do we need to do before timing synchronization and CFO correction or before it? And, please let me ...
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0answers
36 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
30 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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1answer
11 views
operations valid for predicting new codewords for a Linear block code having linear encoder mapping
I want to know which operations are valid for predicting/finding a new codeword from a set of known codewords for a linear block code having linear encoder mapping,
for eg.:
Consider a linear ...
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1answer
59 views
Using MIMO with DS-CDMA and spread spectrum
I have a question about using MIMO with DS-CDMA. suppose we are using 4 TX antennas, and 4 RX antennas, then the Reyleigh ...
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2answers
51 views
Understanding FFT size in OFDM modulation
I know that the FFT size is the the number of subcarriers OFDM system have. But I also know that the FFT has to be $ 2^N $ big (in order to process N samples from time domain to frequency domain).
...
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26 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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0answers
26 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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0answers
36 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 ...
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1answer
53 views
QAM constellation coder - transmitting a file
The file with size 6 MB is transmitted in 100 seconds using a constellation coder QAM with a constellation of 6 bit/symbol
$ { M = 10^6, B = 8} $
Tasks:
1. What is the signal bandwidth if the band ...
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1answer
29 views
upsampling in the frequency domain
I have the Fourier transform $S(\nu)$. If I oversample by a factor $R$ in the frequency domain. Does the duration of the signal $s(t)=IFFT(S(\nu))$ change?
