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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23 views

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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41 views

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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41 views

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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60 views

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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33 views

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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34 views

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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1answer
19 views

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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23 views

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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42 views

Instantaneous SNR

A relay system with 3 nodes: S (power node), R (relay node), and D (destination node). S transmit the signal x to R, received signal at R: $$y_R=\sqrt{P_sd_1^{-\beta}} h_{SR}\,x + n_r$$ with: $...
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41 views

What's the computational complexity of MMSE equalizer in OFDM system

As I know that in case of one tap OFDM equalizer, (ZF equalizer), we should have $N$ division complexity since we divide every subcarriers by its correspondent channel tap in frequency domain. But I ...
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37 views

What is difference between Phase shift keying (PSK) and Quadrature Amplitude Modulation (QAM) mathematical equation

I went through a literature and found the following expression for PSK signal. On this basis, what will be the equation for **QAM signal **. Also from where i would be able to read about this equation ...
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How to reduce PAPR with constellations choice?

How to tune constellations mapping M ary-psk or x-qam to reduce PAPR ?
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2answers
53 views

How does subcarrier correlation affect the BER of OFDM?

I am studying the OFDM system and would like to examine it under different subcarrier correlation scenarios. I have calculated BER for the uncorrelated scenario and now I have rewritten it for the ...
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1answer
37 views

BFSK modulation coding on MATLAB

Like for BPSK we generate random bits and then data= round(rand(1,num_bit)); %Generate binary data bpskModulated = 2*data-1 % mapping to -1 and +1 Can we do same for BFSK like ...
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84 views

noise floor estimation

I want to estimate noise floor in the spectrum. I tested many methods using journals, but I couldn't find a stable method. Suggest that we have a spectrum consist of many signal and we want to ...
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34 views

Non coherent detection BFSK demodulation?

I am simulating BER performance of BFSK under AWGN and Rayleigh fading, BFSK has two symbols one entirely on the real plane while other on the complex plane for representing let's say zero and one, ...
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1answer
50 views

Carrier frequency offset estimation for QPSK and asymmetrical spectrum

I am working on estimation of carrier frequency offset for QPSK data from a satellite. From my understanding, there are two main approaches: either remove data from the carrier (by taking the fourth ...
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1answer
23 views

channel impulse and frequency response: H?

The received signal of MIMO system is given as: $$ y= H \cdot x +n, (*)$$ where $H$ is a channel matrix. $h(\tau_l)$ is a channel impulse response. It is determined as a matrix $\in \mathbb{C}^{Nr \...
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10 views

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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26 views

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" ...
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1answer
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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4answers
87 views

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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39 views

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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2answers
68 views

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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1answer
15 views

Receiver figure of merit

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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2answers
58 views

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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1answer
32 views

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 ...
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1answer
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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3answers
96 views

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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2answers
167 views

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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39 views

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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38 views

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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1answer
32 views

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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3answers
98 views

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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19 views

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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1answer
33 views

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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39 views

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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1answer
42 views

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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1answer
52 views

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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2answers
43 views

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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1answer
20 views

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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2answers
29 views

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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41 views

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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32 views

OQPSK receiver processing blocks

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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1answer
28 views

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 ...
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1answer
37 views

Is this a “valid” CDMA modulation?

I am banging my head against (specific aspects of) CDMA for a few days. At the basic level I am confused about the interrelation between implementations that are modeled as mixing and fully digital ...
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Any DVB-S2 sample streams publicly available on-line?

I'm working on a DVB-S2 receiver and I'd like to be able to test a part of the DSP chain with real-world signals, but getting good sample streams from a digitizer is not terribly easy. Can anyone ...

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