# 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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### NDA timing estimation for mutli-level QAM(RC pulse)

What non-data0aided techniques are used to estimate the timing for multi-level QAM? For single level QAM(so 4 QAM) you can use variance minimization or power maximization but I'm not sure these ...
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### Can we really use equal gain combining in the receiver in the real world?

Can we really use equal gain combining(EGC) in the receiver in the real world? Because we have to know the phase of elements in the channel matrix first before we do the EGC,however,the channel should ...
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### Lagrange Vs Sinc interpolation

I was wondering what is the practical difference between Lagrange Interpolation using Farrow Structure and Sinc Interpolation? Both require pre-computation of time offset coefficients using a lookup ...
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### Zero Forcing Equalizer in ISI channel

I'm studying on designing 9-tap zero forcing equalizer to remove ISI in the channel. I assumed that the channel response is ht = [0.41 0.815 0.41] and I tried to calculate bit error rate. When I ...
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### up chirp and down chirp

The expression for a HFM up-chirp is $$x_1(t)=e^{-j2\pi K\log(1-\frac{t}{t_0})}$$ with $\frac{-T}{2} \leq t \leq \frac{-T}{2}$ and $K=\frac{Tf_lf_h}{2(f_h-f_l)}$, $t_0=\frac{T(f_h+f_l)}{2(f_h-f_l)}$....
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### Understanding Asymptotic Equipartition Property

I have some problems in understanding the precise meaning of the Asymptotic Equipartition Property, related to a large number n of independent and identically distributed random variables (X1, X2, ...,...
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### Precoding algorithm for OFDM

I working on an implementation of a linear precoding algorithm for MIMO OFDM system and I have an issue with symbol estimation part. Let me briefly explain what I am doing: H- channel matrix ...
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### About the post “How to use “Filter” matlab function to apply channel to signal?”

I would like to add my question to the post How to use “Filter” matlab function to apply channel to signal?, but I don't have permission to do that. The user @Anna Koroleva discussed with @Enginner ...
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### How to find similar channels in freq-selective channel?

Early I have asked about precoding in frequency selective channel "Frequency-selective channel: Basic in theory" and got the following answer: In practice you might not want to do this as the ...
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### How are CDMA codes distributed?

I think this question is so trivial that it's nowhere explained (I would not find it) and the answer is probably trivial too. But I still wonder how codes in a typical CDMA system are distributed? ...
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### Definition of Perfect Code (and example of Hamming Code)

Let's consider the following sentences about coding a message before transmitting it in a communication system (here you find the complete pdf) First question: can you explain me these two ...
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### Precoder selection in LTE

I am reading papers about LTE and "resource block (RB)" was mentioned in each of them and also "precoder selection" What kind of parameter is used to select precoder for each of RB? What is ...
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### Create QPSK data In Matlab

How to form QPSK data in Matlab? I have written it as: Mod=4; s = randi([0 Mod-1], N*MT, 1 data=pskmod(s,Mod); Will be it enough to form QPSK data?
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### How to use rayleighchan(ts,fd,tau,pdb) to form a channel matrix?

I am starting a transmission system simulation. To create a channel matrix I have used rand function in Matlab. But Matlab has a ...
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### Understanding and converting Matlab's coarse frequency compensation function

I am trying to understand and convert to C Matlab's coarse frequency compensator the documentation for which can be found here: Documentation Matlab's design itself is based on this IEEE paper: ...
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### Should we use propagation losses channel when we are simulation the system which is for the sensor communication?

Should we use propagation losses channel when we are simulation the system which is for the sensor communication?The sensor communication i said means that sensor will exchange their information to ...
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### Why does almost all paper usually add the RF wave power directly?it seems like feed the RF-DC converter,then sum the DC power

In the energy harvest, we need to decoding and harvest the energy,but i am confused with something,i find that if the receive antenna are not just one, when we calculate the harvest power,we just sum ...
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### Small Scale Fading time varying or position varying

I would like to ask how small scale fading can affect a transmission with time. The particular system is an antenna transmitting an ASK modulated signal at 915MHz and with bandwidth between 200KHz to ...
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### Does any paper use energy harvest to in the baseband?

Most of RF energy harvest system just harvest $\rho$ of the RF signal for charging the battery,and the other part RF energy will be processed to baseband signal by RF Chain to do the information ...
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### Understanding Matlab's Automatic Gain Control function

I am trying to understand the automatic gain control block provided by the communications toolbox. The documentation is here: Documentation My questions are in regards to two of the configurable ...
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### How to use “Filter” matlab function to apply channel to signal?

I need to use filter Matlab function to describe the following : H: MR x MT ...
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### Is my concept of receive diversity right? EGC,MRC and SC

Is my concept of receive diversity right?There are three combining technology belong to receive diversity. 1.Equal gain combining(EGC) $\sum e^{-j\theta_k}(\sqrt{P}|h_k| e^{j\theta_k}x[n]+w[n])$ ...
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### Can we use beamforming to make sure that the signal will not make interference together in Full duplex?if yes,why?

To avoid signals will make interference to each other in full duplex,there are two methods to solve this problem. Time-division duplexing and Frequency-division duplexing,they just use different ...
34 views

### Length of CDMA code and message length vs. number of participants

What is the minimum code size for CDMA codes for synchronuous CDMA (not asynchronous CDMA!) with N number of participants? Furthermore, suppose N parties want to uniquely transmit a message of length ...
59 views

### Why do we need to increase sampling frequency at the transmitter?

I've thinking about this for some time now and I was wondering why do we need to increase smapling rate in the transmitter? I will explain a bit more. From the point of view of a software-defined ...
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### Why can we know the best precoding direction after we do the SVD to the channel?

Why can we know the best precoding direction after we do the SVD to the channel?for example $\mathbf y=\mathbf H\mathbf s+\mathbf n$,now we do the SVD to the $\mathbf H$ If the $\sigma _1$ is the ...
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### Does zero-forcing equalizer need known channel impulse response?

I'm studying some of the basic equalizing structres and I understand how Zero-forcing works, but it seems to me that a known channel impulse response is needed. Am I right? If so, what's the point? I ...
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### Types of equalizers

I can't get my mind around the different types of equalizers and the difference between some of them. I've been looking at different books unsuccessfully. So far, I have Wikipedia's list and this ...