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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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56 views

Doppler compensation method for cdma

Do you know of effective and simple methods to compensate for Doppler due to movement for the CDMA protocol?
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2answers
64 views

The relation of sub-carrier and FDMA

In Mengyu Liu, Yuan Liu, "Charge-then-Forward: Wireless Powered Communication for Multiuser Relay Networks", it is said that while information signals are conveyed by using FDMA over $N$ sub-...
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0answers
99 views

Why is channel estimation done after synchronization and why does it work?

I am interested in Software Defined Radio communications where almost all processing is pushed to the digital domain. I'm wondering why is channel estimation done after frequency and phase ...
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5answers
111 views

How can telecommunication system know which part is signal and which is noise? [closed]

When signal is received how can receiver know which part of received signal is signal and which part is noise??
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0answers
27 views

Question about direct spread spectrum in 802.15.4 at 2.4 GHz

I read the 802.15.4 standard and something is really confusing me: How is spreading achieved in 802.15.4? is it by multiplying, encoding or modulating each data symbol (4 bits) with a certain code ...
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1answer
332 views

What is the noise whitening process?

I came across this process when working on OFDM in wireless communication. It's mentioned if we have two copies of a signal resulted from SIMO system (Single Rx Multiple Tx), we can process the noise ...
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1answer
156 views

Digital Downconversion of Signal

DSP-Newbie here... Why are we using the ratio of the center frequency of the signal to the sampling frequency of the signal spectrum when we downconvert the signal? I have an Spectrum with Carriers ...
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1answer
118 views

PN code generation

Can we generate PN codes (maximal length sequence) using this technique in Matlab where L is the PN length? pn=2*(rand(1,L)>0.5)-1 If yes, why?
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1answer
61 views

Phase tracking in communication receivers [duplicate]

Why does Costa's loop PLL bandwidth need to be narrow? Text books mention that and yet give no explanation
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1answer
56 views

Why Isn't the ML Estimator (MLE) in MIMO Spatial Multiplexing Obtained by the Least Squares Solution?

In the simplest scenario of MIMO spatial multiplexing: $$\mathbf{y} = H\mathbf{s} + \mathbf{n}$$ where: $\mathbf{s}=[s_0,s_1,...s_{M-1}] \\\mathbf{y}=[y_0,y_1,...y_{N-1}]$ $\mathbf{n}=[n_0,n_1,......
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1answer
152 views

What's the relation between the sampling rate and FFT size in OFDM

In OFDM system, Is there relationship between Sampling rate and FFT size? For example, if we downsample or oversample the received signal, Should we decrease or increase the FFT size accordingly? ...
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0answers
269 views

DPCM Encoder and Decoder Simulation in MATLAB

I am trying to simulate DPCM in order to understand how it works. I am trying to implement the simplest version of DPCM which has a system scheme like in figure 1. Sorry for the quality of the ...
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1answer
130 views

Get I and Q components from a signal (multple frequency signal)?

Is there anyway to get the I and Q components from a signal in the time domain? The signal has multiple frequencies. So far I have been doing this in a SIMPLE situation: Fast fourier transform to ...
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0answers
234 views

GMSK/GFSK BER simulation in GNU Radio : How to specify signal energy

In GNU Radio, BER vs SNR can be measured by fixing the signal energy and varying the amplitude of the noise source block. For example, in BPSK, we have transmitted signal $s\left(t\right)$ with a ...
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0answers
46 views

Can we sample the signal with any period as long as the Nyquist rate is respected

I have a question regarding the sampling rate. Suppose that we transmitted a signal $m(t)$ then at receiver side, we received the continuous signal $x(t)$, which is time varying and sampled with a ...
0
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1answer
90 views

methods of crest factor minimization

i am studying the methodes CF minimization. and have a doubt. in each methods there are steps with DFT and IDFT or like this FFT and IFFT. Everyone provides the fact, that they use DFT and IDFT for ...
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1answer
55 views

Homodyne concept of receiver

I would like to understand a concept of homodyne architecture of I-Q demodulator and how the wave looks like at each step. It is a bandpass Signal: Step 1: multiplication with cos and sin wave. I ...
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1answer
55 views

Don't understand the relation about the wireless communication flow chart and TDMA illustration diagram

The first picture is the wireless communication flow chart,and the second is the TDMA illustration diagram,Which part of first picture is TDMA ? or if first picture is the standard process for TDMA,...
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2answers
627 views

Does a matched filter really improve SNR?

Does the matched filter actually improve SNR or does it maximize it (making sure that it is not less than what it should be) ? improving SNR would not make sense to me as suppose a receiver has a ...
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1answer
28 views

Relation between Wireless Power Transfer, Wireless Information Transfer and the definition of sum rate

I read this paper Charge-then-Forward: Wireless Powered Communication for Multiuser Relay Networks ,and it said it on the second page ,and I don't really understand why is the theory ,the author said,...
0
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1answer
18 views

The difference about WPCN and TDMA or FDMA

I am a little bit confused about the difference WPCN and TDMA or FDMA,i mean the level difference. WPCN is a wireless powered communication network,as the name implies,it can transmit the information ...
0
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1answer
266 views

number of subcarriers for an OFDM system

For an OFDM system, it is necessary to know the number of subcarriers. For example for the subcarrier $k^e$, we must have: $B_k <B_c$ with $B_c$ the channel coherence bandwith. And $B_k=\frac{1} {...
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2answers
35 views

Rate in CDMA setup

I am working on a CDMA system with only 4 chips/symbol. I want to use $R=\frac{1}{2}log_2(1+SNR)$ chips/channel use to calculate what $SNR$ I require to successfully receive a given transmission. In ...
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0answers
65 views

minimum usable 5G cellphone signal level?

I'm a programmer (not and EE) working on a project to simulate coverage and capacity for 5G cellphone signals at 26 GHz. But I've done enough path loss calculations for terrestrial and satcom radio ...
0
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1answer
59 views

IQ modulation principle

I am new in the signal processing. I hope I can get help. I want to ask a question regarding IQ modulation work . Why can’t I simply multiply a complex base-band signal with a cosine to modulate it?
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0answers
108 views

How can I oversample the received signal by double of sampling time and have two signals

I have a question, how can I make the sampling in Matlab for the received the signal by half of time duration of symbol (Ts), and have two copies of signal (that's the process of fractional sampling ...
2
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2answers
92 views

How to solve equations for the MMSE receiver?

Given the following equations for the achievable rate of the Minimum Mean-Squared Error Receiver [1]: $$\mu = \frac{1}{K-1} \sum_{i=1, i \neq k}^{K}{\frac{1}{Mpd_{i}\left(1 - \frac{K-1}{M}+ \frac{K-1}...
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2answers
158 views

matched filter for doppler

To remove the Doppler shift, we can use a bank of matched filter. But, to use this method, we need an interval $[-f_{max};f_{max}]$. How can we approximate this interval?
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2answers
139 views

Phase ambiguity in QAM modulation

I'm currently studying a decision directed carrier recovery scheme. Basically a PLL try to estimate the phase error in the current received symbol, the phase detector compensate the incoming symbol's ...
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2answers
168 views

Understanding PSD: Why Does Power at High Frequencies Affect Low Frequencies?

I'm trying to wrap my head around power spectral density on a conceptual level, but I am having some difficulty. Suppose I have a communication system where I am receiving and sampling white Gaussian ...
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0answers
18 views

Can you use the theory of Signal Space to study Band Base digital systems?

Introduction: There is one way to study different modulations in band pass digital systems and is working with "signal space" and the Constellation Diagram. My questions about this are: 1) Is it ...
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1answer
450 views

Cyclic prefix length in baseband OFDM with ZF equalizer

My question is about baseband OFDM. I try to plot the BER according to the SNR. I noticed that changing cyclic prefix length does not affect the performances. I.e., following the code below, ...
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2answers
517 views

QAM constellation slicer with unknown carrier phase

As shown in the image, I have a 16QAM constellation that is misaligned due to a phase rotation. In this case you can see that rotation amount is approximately $\theta = \pi/4$, but this won't be the ...
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1answer
155 views

Rake Receiver vs OFDM and multi-path channel

My Question is, Can we use the Rake Receiver with OFDM modulation? As you know, Rake receiver is usually used with DSSS modulation, so can we use it with OFDM? in other words, Does the Rake Receiver ...
3
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1answer
190 views

FFT based symbol synchronization for digital demodulation

Where can I read more about "slow search" methods for symbol synchronization based on FFT for extracting a clock signal from the modulated signal? I have read here (8.7 Symbol synchronization) One ...
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1answer
95 views

Explanation of fractional sampling in OFDM [closed]

What is the concept fractional sampling? Is there a relationship between the fractional sampling and interpolation? And how does the fractional sampling help in convert the SIMO (Signal input multi-...
3
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1answer
344 views

What techniques are available for correcting constellation rotation due to phase ambiguity?

In order for a receiver to recover a baseband signal, it needs a carrier signal equivalent to that used by the transmitter for upconverting the original baseband signal. Unless pilot tones are used, a ...
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2answers
601 views

Choosing samples per symbol for modem pulse

One of the parameters in my DSP library for pulse design is "samples per symbol", and I would like to get some advice about choosing this parameter when designing a modem. The smaller the pulse width,...
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2answers
296 views

what is the frequency selective channel and doubly-selective channel

Could anyone please help me to explain the frequency-selective channel and doubly-selective channel and the difference between them in brief and easy way. I have tried to check on internet, I couldn'...
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2answers
111 views

Demodulation in Rayleigh Flat Fading Channels

A core part of demodulation in Flat-Fading channels is (assuming) the channel as a(random) constant complex number multiplied with your signal. This is done in order to de-rotate the symbol for ...
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1answer
75 views

DSP using audio IO from PC using C++

This is my first question on dsp. I am looking for something very simple here and I am sure there are libraries that will do pitch recognition and FFT but my requirement is very simple. I have a PC ...
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0answers
56 views

Why do we use conjugate transpose IFFT in OFDM

I have a question regarding the OFDM and using the conjugate transpose of vector (matrix). As you know, if we have a block of data,let's call it $X_n$, consisting of $N$ data symbols, we should ...
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1answer
248 views

difference between pi/4 qpsk and differential qpsk

What is the difference between a $\pi/4$-QPSK modulation and a QPSK differential modulation (DQPSK)? Are these modulations identical?
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1answer
70 views

Channel Shortening for underwater acoustic channel communication

I'm new in this forum, hope that we can learn from each other well. I need to ask about the channel shortening for underwater acoustic channel. what are the common algorithms in that field? is ...
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1answer
160 views

How can I make a mmWave frequency selective channel in matlab?

$$H_d=\sum_{\mathcal{\ell}}^{N_p}\alpha_\ell p(dT_s-\tau_\ell)\mathbb{a}_R(\phi_\ell)\mathbb{a}^*_T(\theta_\ell) $$ I'm trying for simulation of a frequency selective channel in matlab. my problem is ...
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1answer
81 views

Viterbi Algorithm - Selecting survivor paths which both contain the same Hamming distance

To my (limited) knowledge, when determining which survivor path to choose to produce an error-corrected output of a binary Viterbi decoder, the path with the smallest Hamming distance is the most ...
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1answer
141 views

Confusion in Peak to Average Power Ratio (PAPR)

(1) What does it mean by 'a system with low peak to average power ratio requirement'? Doesn't every system idealises it to be lower? Or there are some exceptions to it? (2) Higher PAPR can be a ...
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1answer
107 views

problem with the matched filter

Let $y(t)=s(t)+n(t)$ For the matched filter h(t) on $[0,T]$ we have, $h(t)=s(T-t)$. And the output is: $z(t)=\int_0^T y(u)h(t-u)du=\int_0^T y(u)s(t-(T-u))du=\int_0^T y(u)s(u-(T-t))du$ But sometimes ...
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1answer
81 views

Dimension of channel matrix elements in MIMO systems

In a MIMO system we have: $y= \mathbf{H} \star \mathbf{x} +n$ with $x \in \mathbb{C}^{N_t \times 1}$ ($N_t$ the number of transmission antennas) and $\star$ the convolution product. For the matrix, $\...
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0answers
37 views

Required number of measrments for signal recovery in a compressed sensing MMV problem?

For multiple measurement vector (MMV) problem $Y=AX$ where $A$ is $m \times n$ sensing matrix and $X$ is $n \times L$ matrix haveing K non zero rows. What are the necessary conditions on the ...