# 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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### Power allocation for frequency selective channel [waterfilling ]

I am studying channel capacity with channel state information at a transmitter, where the water-filling algorithm is discussed. The algorithm should allocate more power to some subcarriers with larger ...
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### How $\Delta f=\frac{1}{2T}$ satifies the orthogonality condition?

Let $$s_{ml}(t)=\sqrt{\frac{2E}{T}}\exp(j2\pi\Delta fmt)$$ where $T$ is the time-period of signal $\Delta f$ is the frequency spacing The text says that two signals $s_{ml}(t)$ and $s_{nl}(t)$ are ...
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### Combiner vs mixer vs coupler?

All these three terms are often used in communication texts and apparently they are some how similar But what is exact difference between each of them?
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### Preamble length in the communication system

how many bits my preamble should be typically?? course carrier bits+ carrier recovery bits + timing recovery bits+ phase ambiguity ?? i have see typically carrier recovery bits + timing recovery ...
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### symbol synchronization

PLL is typically used in carrier synchronization and its diagram is well known, consisting of a multiplier, a low pass filter, and a VCO. In "Phase-locked Loop" at wiki (https://en.wikipedia.org/wiki/...
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### how to determine a length of a channel? ( given path, delay)

Given a channel matrices for each subcarrier of a frequency selective channel (OFDM): i.i.d.. channel =(randn(ntx,nrx, N)+1i*randn(ntx,nrx, N))/sqrt(2); simulated ...
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### how to use robot framework to launch unit tests on matlab?

I Know the syntax of rfw and and I want to make some unit test, for some kernels and functions in matlab and I need some help to begin with
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### What channel model to choose while simulating a system in passband with frequencies over GHz range?

I was wondering as to what kind of channel model do we use to test for BER vs SNR in a passband RF system. The system that I designed is basically a wideband frequency hopped OFDM system where I use ...
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### bandwidth vs. bandwidth range

As known, Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. OK, what means when saying that bandwidth range is 15 ~ 20 KHz ? Does that mean that ...
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### What is role of BW and frequency in internet speeds?

First the bandwidth (BW): It is the amount of data that can be transferred simultaneously.just like more no. of highway lanes so that many cars can travel side by side simultaneously. Frequency It ...
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### Calculation of channel autocorrelation for LMMSE

I am currently trying to implement an LMMSE estimator/equalizer for a python based ofdm receiver. The actual reception is done with a SDR and iq-samples are stored in a file for the "ofdm-receiver" ...
33 views

### what happens if you take the absolute value of a complex modulated signal? [closed]

I was going through some methods of eliminating carrier frequency offset (CFO). and one way is to take the absolute value of a received signal (somewhat like looking at the power of the signal) and ...
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### Negative frequency in the Fourier Transform [duplicate]

Why use this instead of a positive frequency? I asked on the math forum but it just wasn't helpful. I understand the idea of a negative frequency is important in general since many real signals like ...
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### Channel encoding after digital modulation

Generally, when using channel encoding followed by interleaving, that process should be performed before the modulator and the system structure should be like that : For me, I only use the ...
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### A Practical Method for Correcting Phase Errors in a Transceiver System Comprising of OFDM + DUC and DDC

I am back with a question related to my OFDM Transceiver System that I am designing where I use DUC and DDC. I could get rid of the high BER problem. I realized that the problem was due to the phase ...
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### How to compute (in non adaptative way based on MMSE FIR) the feedback and feedword filter for DFE equalization?

System Model Consider the single-input single-output (SISO) communication system in the figure below: where the information bit stream is encoded and mapped such that the output of the encoder is a ...
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So I am solving receiver figure of merit. I have gain in dB and noise temperature in K. I know [G/T] = [G] - [T] dB/K Here should I need to take T in dB or just as K?
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### what is the best technique to detect a given signal : energy detection, autocorrelation, and matched filter based sensing, CRC?

In order to detect the presence of a signal impaired by noise, what is the best technique (best metrics) one can use ? energy detection, autocorrelation, matched filter based sensing, CRC data ...
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### How to Compensate for Frequency Offset in single carrier transmission Using Coarse and Fine Compensation?

How to compensate for the frequency offset of a PAM, PSK, or QAM signal when we are using single carrier transmission, I am looking for algorithms, explanations to be able to carry out the ...
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### How to implement Signal transmitted by RF source

I am implementing a research paper, in which i have to implement signal transmitted by RF source in matlab. The equation given in this paper is : $s(n)e^{j \cdot 2 \cdot \pi \cdot f \cdot n}$. Any ...
23 views

### How to plot BER Vs SNR of a received signal

I am using USRP to transmit and received MIMO frames similar to Wi-Fi standard that I created on my own. After receiving the signal, it is processed as described in the standard. While I plot the ...
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### Why should the number of subcarriers of OFDM system be a power of 2?

I notice that almost all papers use the number of subcarriers of 64, 128,256.. so on, which is a number of $2^k$ and $k$ is an integer. Why is the number of subcarriers always like that? Can we use ...
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### Symbol Synchronization for GFSK/FSK signals in GNU Radio

I am trying to demodulate GFSK signal, and extract the packets using GNURadio. TX information: It is basically BLE transmission but customized. BW: 2MHz, data rate: 1MSymbols/sec, 1 bit per symbol. ...
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### How to implement Minimum mean-squared error (MMSE) equalization?

I have the channel estimation and noise power estimation and I need a matlab implementation of time domain siso channel Equalization. Theory: Consider the FIR filter of length $N+1:$  \hat{s}_{k}...
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### On the channel estimation based on vector

I have a matrix $X$ of size .i.e $(8,8)$ with $M$ columns and $N$ rows. Suppose I took it's $iFFT$ column wise resulting matrix $x$ of size $(8,8)$ too, $x = F^HX_{(:,m)},$ ...... (1), where ...
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### IFFT and sample count issue in OFDM

I wrote a python program to generate OFDM packets (i.e. the IQ samples) with a custom payload to, for example, be transmitted via an SDR. After mapping the chunk of bits corresponding to one ofdm ...
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### For Schwarz inequality for 2 signals $s_1(t)$ and $s_2(t)$, equality holds if $s_1(t) = c\cdot s_2(t)$ ($c$ is constant). Does $c$ have to be real?

I tried for 2 signals $s_1(t) = u(t)$ and $s_2(t) = (1+i) \cdot u(t)$. $i$ is the complex unit ($i = (-1) ^{0.5}$). I got the left hand side of the inequality as $(1-i)$ and the right hand side as \$(...
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### How do we know the receiver delay in an OFDM transceiver system?

I was simulating a simple OFDM transceiver system without a channel (just the transmitter and the receiver directly connected). I saw that if I use the "Find Delay" block in Simulink between the ...
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### Is DFE equalizer always blind or can we use reference signal?

I have made a blind DFE Equalization, I mean decision directed . But I am not sure how to make an implementation based on reference signal known by receiver and emitter. In the back forward filter ...
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### On the performance of DCT-based OFDM

I am trying to understand the OFDM based on DCT instead of DFT in order to compare between their performance. In case of AWGN, the performance is logic, but when I perform channel equalization in DCT-...
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### How to correct for delays in a PAM-X signal with orthogonal CDMA?

I have a receiver which receives data bits at a rate of 5kbit/s. The number of data bits per burst is low (say, 16 bits). These databits are obtained after despreading the incoming signal using an ...
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### Calculating dBFS for I/Q

I'm trying to figure out how to calculate the dBFS for a complex input. I have an AD9361 part which has a 16-bit A/D and I believe outputs I and Q samples. Now I know you can calculate the dBFS for a ...
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### Demodulation of 4-QAM

Any help to get me started is appreciated. I have an array of values for the 4-QAM where attach is a copy of part of the values of the array. I want to know how do I get the bits 0 and 1s from those ...
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### Combining CDMA with differential Manchester-like encoding?

I wonder to which extend it is possible to combine a self-clocking code (like Differential Manchester encoding) with synchronous CDMA, and how it would be done? Suppose I want to receive some bits ...
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### Interleaving or randomisation?

I am trying to implement a digital communication system as a learning exercise. For error correction, I am using two concatenated codes (RS and convolutional). However, I read that, in order to reduce ...
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### Technical difference between bent pipe and regenerative transponders

I am preparing a proposal for satellite transponders and need to choose transponder architecture. Based on primary research I realized that the regenerative transponders have a better characteristic. ...
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I have implemented QPSK but I have to make changes to OQPSK. On the transmit path I am clear, as I have to delay Q channel by 1/2 symbol. My receiver costas carrier recovery will not be the same. I ...
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### CDMA scheme that preserves constant envelope (NRZ/ASK shape)

(This is a followup question to Is this a "valid" CDMA modulation?) Is it possible to find a CDMA scheme in which the receiver can only have a simple slicer? In other words, for the ...