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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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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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1answer
51 views

spectrum for a communication signal [on hold]

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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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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1answer
45 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 $...
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1answer
22 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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18 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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41 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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38 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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38 views

What is the average energy in QAM and QPSK constellation

Average energy in QAM constellation is explained well in this link Is there a closed form for the average energy of a QAM constellation? We can also simplify it more, the average energy is $E_{...
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1answer
16 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
38 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 ...
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31 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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2answers
28 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 ...
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1answer
128 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 ...
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1answer
45 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
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 $s_i(t)$, ...
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1answer
25 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?
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2answers
52 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
39 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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Is there a naming convention of signals at different stages?

I am learning with the book "Introduction to Digital Communications" by Göran Lindell and he uses a wide variety of letters for signals at different stages, which confuses me a lot. Especially as ...
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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 ...
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1answer
25 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 ...
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0answers
22 views

BER performance over fading channel

Suppose the channel model is a frequency-nonselective Rayleigh fading multipath channel. The received signal is expressed as \begin{equation} r(t) = \sum_{l=1}^{L}\alpha_l s(t - \tau_l) + n(t) \end{...
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1answer
27 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, ...
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1answer
32 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 ...
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1answer
29 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 ...
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1answer
35 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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2answers
38 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 ...
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1answer
32 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
70 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.
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1answer
28 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
48 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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1answer
25 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 ...
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1answer
38 views

About combination between QAM and PAM

I have confused question regarding the possibility of combination between QAM modulation and PAM in order to improve the spectral efficiency. Is that possible? As you see in the below Figure, it's ...
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1answer
38 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 ...
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1answer
19 views

About Data spreading in CDMA

If we have data we need to transmit which is $i$, so we should multiply is using XOR function with spreading code $y$. How many bits (Maximum and Minimum) should $i$ and $y$ can have ?
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Why are componets of Complex baseband noise uncorrelated?

Many articles and papers on digital communications use the assumption of an AWGN environment. They write that after the signal is converted to baseband the resulting In-Phase and Quadrature components ...
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1answer
30 views

The notion of white and colored noise

The autocorrelation function of AWGN is an impulse response at t=0, and its fourier transform or its spectral power density has a flat infinite BW Now any receiver will have bandpass or low pass ...
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1answer
36 views

Is CMA equalization applicable for OFDM

I have a question regarding the constant modulus algorithm (CMA), as you know it's one of the known algorithms used for blind equalization. The cost function of the CMA is build blindly, it means ...
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0answers
53 views

Phase shift of a carrier with IQ data

A sine wave of amplitude 1 and phase $\theta$ has IQ data of $I=\cos\theta$ and $Q=\sin\theta$ It will be modulated by a sine wave to do a phase shift, modulation has amplitude 1 and phase $\phi$ ...
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1answer
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The meaning of correlating “Time Aligned” pseudo-random or walsh code sequences

Correlation is from a mathematical point of view a shift in time domain +multiplication + summation operation I thought that if 2 codes are "time aligned" then correlation reduces to a dot product ...
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2answers
34 views

Minimum Number of Errors required for BER Curves in Matlab Simulation

what should be minimum Number of Errors required for getting BER Curves in Matlab Simulation for different number of transmitted bits ?
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2answers
34 views

How to calculate $p_c$ (binomial probability)

In the given image I don't understand how to calculate $p_c$, can anyone please explain?
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2answers
40 views

How to calculate $E_b/N_0$ for a 8-PSK?

How to calculate $E_b/N_0$ for a 8PSK? My BER is given. But I don't understand how to calculate $E_b/N_0$ as it is inside a Q function.
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1answer
83 views

Average Energy of modified QAM Constellation

How to compute the average energy for the modified 16-QAM constellation below? I know that for regular/rectangular 16-QAM we can utilize the 4-PAM along each axes but I'm not sure how can we find $E_{...
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0answers
28 views

How symbol synchronization is achieved in 802.11n standard

According to the 802.11n standard, packet detection and synchronization is done with the preamble part of the frame. A packet is detected with the correlation method, that is comparing the short ...
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1answer
40 views

Symbol Timing recovery for modulation producing ISI

I am interested in understanding why the common timing recovery algorithms function for modulation schemes which produce ISI. For example, suppose you are receiving at the output of a matched filter ...
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1answer
31 views

“Revolving” QPSK Constellation

I am trying to code a QPSK transmitter in VHDL but first I want to create one out of Simulink blocks and translate it into VHDL code. I am using an example project for the PlutoSDR as a guide. This ...
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Symbol timing synchronization and orthogonality of Walsh codes coming in with different delays

Walsh codes are perfectly orthogonal as long as they are perfectly aligned in time. Suppose a preamble is used for symbol timing synchronization of a signal consisting of a certain Walsh code ...