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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2answers
21 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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0answers
15 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
17 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 ...
4
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0answers
23 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
48 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
22 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?
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1answer
26 views
How can I spread a modulated signal over walsh code
I need to confirm if spreading signal by using walsh code in MATLAB is right or no,
Suppose I'm using QAM modulation, so I need to spread the signal over walsh code, is the below step is right:
<...
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1answer
25 views
time for a signal after a sum
I have a signal $s(t)$ of duration $T$ with limited bandwidth. In matlab, I use a sampling frequency $fs$ and I have $L=T*fs$. After, I have $r=ones(size(s))+s$. To have a limited bandwidth, I filter ...
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0answers
24 views
How to implement hermitian symmetry before ifft in matlab? [on hold]
If I use hermitian symmetry after IFFT and clip negative part to make it only positive signal for VLC system. I am getting an only straight line in BER curve. Can anyone help me with this problem? I ...
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2answers
59 views
About using Walsh matrix as spreading sequence
I'm asking about some details about using the Walsh matrix as spreading sequence code. For example, suppose I'm using a $4\times 4$ Walsh matrix given by
$$H = \begin{pmatrix}
1 & 1 & 1 & ...
1
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0answers
28 views
Band Edge Filters
I am interested in using Band Edge Filters to do carrier frequency recovery but this is kind of secondary to my question which is:
How do you build an FIR filter which isn't symmetric about the ...
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0answers
23 views
Preamble detection using cross-correlation
I am generating a preamble with a sequence of [1 -1 1 -1 1 -1....] and spread with a barker-11 sequence. Now, the transmitted signal becomes [B(Barker), -B(Inverse of Barker), B, -B,....]. In channel, ...
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2answers
43 views
What is the duration of the spreading sequence
I have a question regarding the spreading sequence duration using Walsh matrix. I've read a paper related to that, they consider the signal $S = R(S) + I(S)$, where $R(S), I(S)$ are the real and ...
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2answers
46 views
How to find the instantanous SNR over Fading channel
I would like to prove that the instantnous SNR of flat fading channel
$$y=hx+n$$
is
$$\gamma=\frac{|h|^2}{N_0}P$$
where $E\{z\}$ is the expectation of $z$ and
$$
E\{x\}=P
$$
$$
E\{n\}=0
$$
$$
E\{...
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1answer
43 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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1answer
32 views
Digital receivers for the reception of non pulse shaped signals [closed]
We are used to deal with pulse shaped signals, for example signals that go through root raised cosine filters at the transmit, and then matched with root raised cosine filters at the receive
In ...
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1answer
34 views
Beamforming question
I'm working on a beamforming network. Every node in the network is half-duplex and has a linear antenna array that is capable of digital beamforming. A receiving node can be capable of forming any ...
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1answer
22 views
Multiplication and Convolution in Wireless Communications
In Wireless Communication, sometimes the received signal is expressed (in time-domain) as follows:
$$r=hx+n$$
where $h$ is called a fading coefficient
and sometimes it is expressed as:
$$r=h*x+n$$
...
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0answers
11 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?
-3
votes
1answer
53 views
spectrum for a communication signal [closed]
I have a communication signal.
Its bandwidth is limited. But does the $X(f)$ value is important? For example, I have $X(f)=6.10^3$ but for an other signal I have $X(f)=10^6$ (the channel is the same ...
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0answers
18 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. ...
2
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1answer
49 views
Is there a relationship between Average energy of transmission and power saving
As mentioned in many books, the average energy of transmitted symbol is given by:
$E_s = (2/3)(M - 1) $
where $M$ indicated the modulation order.
Suppose that we are using in the first scenario $...
0
votes
1answer
24 views
Modulation scheme for a noisy channel with a complicated transfer function
I want to build an audio modem with a speaker in a room and a smartphone. The channel is quite bad:
Far field, so there is potentially significant reverb
Typical residential background noise
phone ...
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0answers
20 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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0answers
47 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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0answers
40 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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votes
2answers
41 views
MU-MIMO Sensitivity to Line of Sight (LOS)
Why do researchers talk about MU-MIMO being "less sensitive to line of sight" / "LOS Okay"? For example in this video, time=[00:40,00:42]
Similarly, "User separation based on spatial channel ...
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1answer
21 views
AM-AM and AM-PM plots
I have complex baseband signal and its output when passed through power amplifier. How to plot the AM-AM and AM-PM plots?
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1answer
47 views
data rate in digital communications
I am a beginner in digital communications and I have question for the data rate with CDMA. Let, the length of the spreading sequence is $L_c$, The modulation is BPSK and the symbol time is $T_s$.
To ...
0
votes
0answers
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? ...
0
votes
2answers
30 views
Sampling vs repetition code
I'm working on a multiuser wireless system simulation and have run into something regarding sampling rates. From everything I've known for a matched filter receiver, the correlation step is performed ...
1
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1answer
129 views
Concatenated Deletion & Sticky Channel Capacity
Suppose you are sending information through a deletion channel with a deletion rate of .5 and then the output of this channel is sent through a sticky channel with a repeat rate of .5 also. No error ...
0
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1answer
52 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 ...
-1
votes
1answer
19 views
multi-user communication
For a multi-user communication, I do many signals $s_i(t)$ with $i \in \{1,...,N \}$, $N$ the user number. The bandwith for each $s_i(t)$ is the same.
But when I do the Fourier transform of $s_i(t)$, ...
0
votes
1answer
28 views
data rate with a FSK modulation
With a BPSK modulation the data rate is $D=\frac{1}{T_s}$ where $T_s$ is the symbol time. But for a BFSK modulation, how do we find the data rate?
2
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2answers
72 views
PSD of complex white gaussian noise
It may be a really simple question, but I'm not sure about this one:
Given a complex white Gaussian noise process with iid real and imaginary parts and a double sided power spectral density of $N_0/2$...
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2answers
45 views
What is the advantage of MISO technology in communications?
What is the advantage if the MISO (Multiple Input Single Output) technology in communications? I have been searching it on the internet, but there are just about their model and full name.
For ...
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1answer
16 views
Why do we need to minimize the TX power to require for meeting QoS?
Here is the part of the paper,
Based on these architectures, a lot of recent developments have lately appeared intending at enhancing the rate-energy performance of multiuser Multiple
Input ...
-1
votes
1answer
27 views
power of the preamble
In a modulated communication signal, the power of the signal to be sent or received must be equal to 1 to after calculate the BER.
But if I add a preamble at the beginning of the signal for example ...
1
vote
1answer
41 views
How to sketch a frequency spectrum for an AM signal?
So I have an AM signal, given by:
$$v(t)=50\cos(\pi 10^{6} t )+ 20\sin(\pi 10^3t)\cos(\pi 10^6 t)$$
I was asked to sketch the spectrum of the signal obtained, so I found the fourier transform of it, ...
0
votes
1answer
35 views
Converting a non-stationary random process into a WSS process by adding a random phase
Here is an example where this method has been implemented.
We were trying to calculate the spectrum of a transmitted signal(Random signal/weighted pulse)
The auto correlation function of the pulse ...
1
vote
1answer
31 views
upsampling for a signal
We have a signal $s(t)$.
If we do an upsampling, does the signal duration increase?
What is the point of upsampling if the signal time increases?
Can we do an upsampling if we don't use a shaping ...
0
votes
1answer
39 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 ...
-1
votes
2answers
45 views
Digital communication BER performance
I've searched for the theoretical equations of bit error rate over Rayleigh flat fading channel basically for binary phase shift key and 16-Quadrature modulation but i didn't find them, could any one ...
1
vote
1answer
33 views
Questions about reproducing results of a paper on Communication Systems and Wireless Communication
I'm trying to reproduce the results of a paper on Device-to-Device Communication, channel assignment and mode selection (G. Yu, L. Xu, D. Feng, R. Yin, G. Y. Li and Y. Jiang, "Joint Mode Selection and ...
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3answers
72 views
Representing data in combination of QAM and BPSK
I have a question regarding the data representation in different modulations. Assuming we are using MIMO system whith $4$ $T_x$ x $4$ $R_x$ which means four transmission antennas by four receiving ...
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1answer
20 views
differential phase tracking loop for the receiver
For a DBPSK modulation, how can we track the differential phase? I thought about using a phase tracking loop but I did not find articles for this.
1
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1answer
29 views
Modulating ofdm in audio spectrum
Trying to modulate audio with OFDM data.
My fs = 44.1khz, i'm using 64 sub-carriers, but i'm only using 5 sub-carriers for data, so my bandwidth is about 3.445khz. I'm not sure how to properly write ...
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1answer
62 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.
0
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1answer
29 views
phase tracking loop for dbpsk
I have a communication with a DBPSK modulation. The communication is disrupted by a Doppler shift (due to movement) and Doppler spread (due to channel). At the reception, after Doppler shift ...
