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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1answer
50 views

Confussion on roll-off factor, sampling time and Raised Cosine pulses.

I am reviewing some bandlimited communications theory and something is bothering me - My book says increasing the roll-off factor makes the received sequence more tolerant to timing recovery errors ...
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1answer
141 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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1answer
58 views

Where in the signal chain is Channel Equalization applied?

I am reading about channel equalization and I am somewhat confused on where in the signal chain channel equalization is normally applied and if it's before or after a correlator detector or matched ...
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1answer
128 views

The benefits of a fractionally spaced equalizer

I often read that one of the benefits of FSE vs a symbol rate equalizer, besides avoiding any aliasing in the roll off region, is that it is less sensitive to the timing phase. What does that really ...
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1answer
92 views

OFDM Simulation process

I'm trying to understand OFDM by making a simulation. Are these steps correct? generate M random complex QAM symbols. example: (1+j,1-j,-1-j,1-j....) Split my M samples into a N 2048 sized arrays ...
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1answer
32 views

Narrow band Interference affecting timing acquisition in spread spectrum communication systems

It is often said that a narrow band "in band" interferer will collapse and its BW will spread when it is correlated with a spreading code at the receiver of say a CDMA system while the signal of ...
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1answer
47 views

bandwidth for a chirp spread spectrum

For a PSK communication with a symbole time $T$, the bandwidth of the system is $B=\frac{1} {T}$. But for a chirp spread spectrum with the same symbol time $T$ and the bandwidth of the chirp is $...
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1answer
232 views

White noise and entropy confusion about concepts

The spectral entropy (SE) of a signal is a measure of its spectral power distribution. The concept is based on the Shannon entropy, or information entropy, in information theory. The SE treats the ...
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1answer
41 views

Using IFFT for the purpose of “Multiple Orthogonal Frequency Shift Keying”

Is it possible to create a set of complex orthogonal sinusoids using FFT or IFFT In other words a kind of "Multiple Frequency Shift Keying" using multiple sinusoids that are created with a fourier ...
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1answer
42 views

What is an intuitive understanding of the difference between the minimum required bandwidth and actual bandwidth?

When using for example, polar signaling of full width rectangular pulses, the bandwidth is equal to the pulse rate $Rb$, which is also the bit rate. $B=R_b$ for full width rectangular pulses. But ...
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1answer
133 views

Doppler Shift of Phase Coded Pulse Compression Waveform

Suppose I want to transmit the signal: $$ x(t) = g\Big(\frac{t-t_{0}}{T}\Big) \cdot \cos(2 \pi f_{0} t + \theta_{0}) + g\Big(\frac{t-t_{1}}{T}\Big) \cdot \cos(2 \pi f_{1}t + \theta_{1}) $$ Where $g(...
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2answers
581 views

Preamble Detection and Frequency Offset Estimation in radio packets

I have generated a radio packet of data with a preamble header of 0x55555555 and crc32 for the data only. The entire packet was separated in even and odd streams for I and Q. The I and Q were ...
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6answers
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Understanding the Matched Filter

I have a question about matched filtering. Does the matched filter maximise the SNR at the moment of decision only? As far as I understand, if you put, say, NRZ through a matched filter, the SNR ...
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1answer
217 views

Use of Long Training Field (LTF) of 802.11 standard

In 802.11a/n the preamble part has L-LTF(Legacy-Long Training Field) and HT-LTF(High Throughput-Long Training Field). I want to clarify my understanding of these two fields: Are the only difference ...
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3answers
109 views

Quality of experience engineer

I have noticed certain overlap between digital signal processing and quality assurance for jobs that require expertise in evaluating the quality of the transmitted signals on telecommunication systems ...
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1answer
39 views

Generation of a stream of data bits 0s and 1s at a certain bit or data rate [closed]

This is a very basic question ,but basics important, : How is a stream of data bits (zeros and ones that express a certain information) and at a certain rate being generated? what is involved in terms ...
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1answer
171 views

OFDM Short Training Field clarification

I want to clarify my understanding of OFDM training fields. Please correct me if I am wrong as I am getting 2 different information from online resources. For 802.11a standard the preamble contains ...
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2answers
129 views

Why would we simulate baseband rather than passband

My question is, Why do we usually use baseband in simulation rather than passband? however, if we want to test our simulated idea in real environment, it must be sometimes in passband !! So, should ...
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1answer
73 views

Complex filters used in analog domain [closed]

Complex filters can be used in low IF architecture especially in DSP in order to phase out (reject) the image signal. How are complex filters implemented in the analog domain?
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1answer
51 views

Time for a preamble (underwater acoustic communication)

What is the maximum duration of a preamble for underwater acoustic communications? For example, can we have a preamble $>10 \textrm{ sec}$?
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1answer
123 views

Why must the Region Of Convergence (ROC) contain infinity and the system function be a right-sided sequence for it to be causal?

I need the simple logic on the condition a system becomes causal. We know that causal contains only past values. I can't relate this with the Region Of Convergence (ROC) concept.
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0answers
52 views

How general are adaptive-filtering techniques? [closed]

How often do problems arise that let you use adaptive filters? Unless I am understanding something incorrectly it seems the requirement that the input signal be stationary(or even WSS) is too strong ...
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1answer
90 views

Is sparse signal in time domain is sparse in Frequency domain

In OFDM system, if we have a multi-path environment, we supposed to have a sparse received signal in time domain, then $FFT$ is performed to convert it into frequency domain. My question, does the ...
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3answers
5k views

How to find the group delay of IIR filters in Python/SciPy

I sent a square wave signal (red) through a IIR filter (butterworth in this case), to make it more "realistic". Then there's a slight group delay in my output signal (blue). Is there a way to find out ...
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1answer
48 views

Inter-Symbol Interference (ISI) channels

Is there any open and free data-set for digital ISI channels? I want to have a variety of ISI channels' taps, such as $h[k]$ here. Especially looking for BPSK channels.
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2answers
340 views

Intersymbol interference due to limited channel bandwidth

What is the difference between limited-bandwidth induced intersymbol interference (ISI) and ISI due to multipath propagation. I noticed that the delay spread in indoor environment is in the range of ...
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1answer
70 views

Phase synchronization between 2 PLLs

If 2 PLLs as the one shown are driven from the same reference/crystal and the ratio between divider N and R is an integer (note that divider R is outside the loop), does one need a sync pulse for ...
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2answers
98 views

Averaging out noise and SNR improvement

Suppose I have a sequence of identical symbols carrying the same information at a sample rate equal to the symbol rate, why do we always talk about averaging them to average out noise, why not just ...
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1answer
361 views

Noisy and Noiseless Channels

I am trying to understated the the achievable bit rate in noiseless and noisy channels. Particularly, I would like to solve the following problem: Let a real symbol $0\leq x_i < 1$ be ...
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1answer
43 views

how to set Equalizer's coefficient using generalized eigenvector.

In that paper https://jwcn-eurasipjournals.springeropen.com/articles/10.1186/1687-1499-2012-72 .. Equation 3, $J(w) = w^HBw/w^HCw$ .. $B$ and $C$ are matrices, $w$ the filter coefficients vector ...
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3answers
173 views

“Complex Matched Filter”

What does a complex matched filter mean? . A complex bandpass filter can be formed by multiplying a low pass filter by exp(jwt) getting an analytic filter (one sided in the frequency domain) which is ...
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1answer
55 views

External frequency divider in a PLL

Does a frequency divider external to the loop BW of a PLL cause any phase ambiguity ? and why? and is the result of dividing by an odd number different than dividing by an even number?
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1answer
87 views

Calculating the energy of trapezoid wave

Here is the trapezoid wave,although i know we can use $\int^{4}_0 |x(t)|^2dt$ to calculate its power,but i want to divide this trapezoid to right triangle$(t=0$~$2)$,and a square ($(t=2$~$4)$). I ...
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5k views

What is difference between OFDM and GFDM?

I am trying to understand the difference(s) between OFDM (Orthogonal Frequency Division Multiplexing) and GFDM (Generalized Frequency Division Multiplexing) which are used as a multicarrier modulation ...
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1answer
393 views

Symbol timing synchronization using a high sampling rate

Symbol timing synchronization usually relies on a digital PLL procedure, an iterative process that relies on detecting a timing error signal that is used to control an interpolation filter that in its ...
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1answer
36 views

How does a telephone change your voice? [closed]

Is it due to vocal length normalization during MFCC? Please be kind enough to explain.
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1answer
1k views

How to Find the Frequency Response of a Communication Channel from Input and Output Symbols in MATLAB?

I want to find the channel frequency response of a digital communications system. I have the functions of the input symbol (a triangle) and the output symbol - a distorted triangle: $$\dfrac{1}{1 + \...
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1answer
55 views

What does it mean that the normalized power of a received equals one?

I have read in that lecture, page 2, here that, the symbols are independent, and normalized to have power 1, which means $E(x x^H) = I$ .. what does that mean? could you please explain that for me?
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0answers
138 views

Dynamic loop bandwidth for a Costas loop

I'm using a Costas loop for carrier recovery. The BER vs SNR (from GNU Radio Simulation) curve for different loop bandwidths (0.03,0.05 and 0.07) is shown in the graph below. It can be seen that the ...
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2answers
208 views

Carrier Frequency Offset, Rotating Constellation and Correlator

I have been further reading up on carrier recovery methods and something is not clear to me: It's written that a carrier frequency offset results in a rotating constellation diagram but if you are ...
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1answer
18 views

What does SDMA's space actually mean?

What does SDMA's space mean? I mean,T for TDMA is time,and unity of time is second. F for FDMA is frequency,and unity of frequency is Hz.Now i know the S for SDMA is space or spatial,but i don't ...
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1answer
64 views

Phase versus time graph of QPSK

In wikipedia, there was an example on QPSK modulation where the main signal was 11000110 and I contained 1001 and Q contained <...
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1answer
1k views

What is the difference between Antenna port and Physical Antenna in LTE?

I have just started working on the 5G NR physical layer and I have difficulty understanding the concept of Antenna Port at the downlink. In release 10 there supposedly 15-22 antenna ports for CSIRS (...
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5answers
112 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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2answers
300 views

Maximum Likelihood for Colored Noise

I have the following question about the maximum likelihood (ML) in presence of inter-symbol interference and colored noise. Assume the communication system is as follows. Information source, ...
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1answer
57 views

Frame acquisition vs symbol timing synchronization

For a known length of frames and using a known sequence in a preamble that is also of known length can auto correlation at the receiver be enough to acquire timing or is timing synchronization at ...
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1answer
873 views

Generate Complex White Gaussian Noise in MATLAB

n_3=sqrt(0.1)*randn(1,K); n_4=sqrt(0.1)*randn(1,K); beta_NLoS=(n_3+1i*n_4); % CN(0,0.1) I want to create a $CN(0,0.1)$,does my code have any problem?
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Timing error detector used for symbol timing synchronization

The Gardner timing error detector is independent of carrier phase and frequency error, and is also quite immune to self noise (to my understanding, I may wrong about that). Can one assume that it is a ...
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1answer
68 views

Modeling channel response as complex vs splitting I and Q

If I have a communications channel where a digital QAM signal is transmitted, I can either model the channel's impulse response as a complex $h(t)$, or as 2 separate impulse responses $p(t)$ and $q(t)$...
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2answers
550 views

Difference between diversity gain and coding gain

I am reading about space-time codes. They break down further into 2 sub categories called space-time trellis codes and space-time block codes. It says that trellis codes offer both diversity and ...