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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45 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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2answers
70 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 & ...
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57 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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131 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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46 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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117 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
192 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
38 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
63 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
100 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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13 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?
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1answer
61 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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25 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. ...
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1answer
55 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
38 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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35 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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86 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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52 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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2answers
104 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
129 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
86 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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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? ...
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2answers
38 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
143 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
64 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
24 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
123 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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481 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
104 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
20 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
43 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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1answer
319 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
74 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
59 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
92 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
93 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
38 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
88 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
33 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
65 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
159 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
53 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
65 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
148 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
40 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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42 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
67 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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1answer
28 views

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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49 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
36 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?