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

Capacity and BER as performance of transmission system

We study the performance of the transmission system by channel capacity or BER/SER. If the system performance is studied as BER/SER, does it make sense to study a channel capacity?
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Resource block size in freq-selective channel

I asked about the basic of a frequency selective channel [here][1]. It was discussed Resource block (adjacent subcarriers and time indices together into "chunks") what parameters affect the ...
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28 views

noise floor of attenuator

NF of attenuator is same as the attenuation value as the noise floor remains same but signal level gets attenuated? But NF of amplifier depends on the device as it increases both signal and noise?
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“How is the noise floor of RF section is mapped to ADC noise floor for a 10MHz BW?”

My interpretation is if a gain of 40 is used before AGC -40 dBm become 0 dBm and -121dBm (noise level) becomes -81dBm SNR remain same (assuming NF =0 for theretical purposes). Noise floor of ADC is (6....
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Differential Encoding for M-PSK

Confused about a few topics w.r.t. differential encoding/decoding. First, consider the below BER curve taken from wikipedia ( https://en.wikipedia.org/wiki/Phase-shift_keying ) comparing BPSK/QPSK and ...
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Do we have to insert the DC null in OFDM when using RF direct sampling ADC architecture?

Generally, DC Nulls are added in OFDM because zero-IF receivers don't pass the DC component. But what if I use an RF direct sampling architecture? Will there be DC offset? If not, then is it okay not ...
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Legacy LongRange Mode information (mode 1 in the my attached link)

Mode 1- Legacy Long Range Packet Format and Im attaching a photo of ti reference: link for more information about this mode category : TI SWRA642 I want to understand the packet format, what's my ...
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1answer
40 views

system bit rate vs channel coding

We are usually asked about the full system bit rate. However, we use channel encoding in practical systems, for example conventional coding with rate $(1/3)$. My question is about the full system bit ...
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25 views

How to measure EVM?

I am trying to understand the concept of EVM and how it is measured at different points in transceiver. I have a basic understanding that the error difference is calculated between two vectors (i.e. ...
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1answer
56 views

Digital AGC signal level

Analog AGC is mainly used to increase the signal level to match the ADC sensitivity? Is Analog AGC also used to attenuate the signal?. Digital AGC is the first block used after ADC. Is the main ...
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36 views

Difference: Ergodic channel capacity and spectral efficiency

Can someone explain what is the difference between the capacity and ergodic capacity? Ergodic capacity and spectral efficiency?
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How to prove a train of sinc pulses in digital communicaton system are orthogonal to each other?

Consider a train of sinc pulses: $$\phi_n(t)= \frac{\sin(\omega_M(t-nT_s))}{\omega_M(t-nT_s)}\quad; n=0,\pm1,\pm2,\dots$$ $\quad$where,$\quad T_s=\frac{\pi}{\omega_M}$ Now ,in order to show sinc ...
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51 views

Low data rate with high frequency offset

I am trying to implement a basic digital modulation scheme (BPSK) with a low data rate of 5 kbps. The requirement is to support a frequency offset of +/- 10 KHz. I would like to know what are the ...
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What is the unit-power information signal?

I am learning signal processing for my subject and have some problems with the unit-power information signal. I refer to this paper: https://arxiv.org/pdf/1906.03949.pdf In this paper, they formulate ...
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52 views

Signal bandwidth and channel bandwidth

I have a channel bandwidth $B_c$ and a signal bandwidth $B_s$ with $B_s<<B_c$. What is the problem when the signal bandwidth is smaller than the channel bandwidth? For example, my channel ...
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24 views

Estimation algorithms for Multi-User MIMO

I am interested in a question how to estimate a channel/symbol in multi-user system. In the case of a single-user, it can used ZF/ pilot estimation/ MS/ LS algorithms. Do we have a difference in using ...
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The similitude of interference cancellation and the use of earplugs

I am trying to model a network for performance evaluation on a wireless system with interference cancellation. Then I stumbled upon a question that I needed to answer before I can proceed. Can ...
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25 views

How can i identify number of return of the ping in the sonar?

I am writing a fake function which is actually doing active pinging (basically SONAR). I am little confuse here about how to identify the number of sonar return i got ? Let's say i am continuously ...
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1answer
38 views

How to check my CRC of my RF packet?

The "problem" is that I want to validate if my received RF packet is valid or not valid, in other words I want to validate if my RF packet corrupted or not corrupted . the transmitter that ...
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1answer
25 views

Implementation of modulation/demodulation models

I am trying to implement mod demod for data rate of few kbps . I would like to know is it better/ easier to implement in DSP/FPGA. I am aware of both C and VHDL programming. I think the code flow and ...
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2answers
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Conventional channel coding/decoding

I am trying to understand the algorithms of channel coding. Regarding the conventional channel which is, for me, the easiest one to implement, I have a question regarding the output of its decoder. ...
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8 views

What is the region in the middle of expected input range that is considered indeterminate called?

In using something like voltage levels to encode a binary signal, there's a region between the two states of the line that is considered unclear and belongs to neither state. Example: From 0v to 2v ...
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1answer
25 views

tx and rx model supporting all data rates

i am trying to build complete tx and rx path which can support varies data rates. for example if my data rate have to support from 10 kbps to 1000 kbps. Can i keep the baseband processing fixed for ...
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1answer
47 views

Digitalization of a signal

Suppose I have an analog signal I want to transmit. After sampling and quantization, I can map the samples to bits. So now I have a bit stream that represents the original analog signal. My questions ...
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47 views

LDPC for channel coding

I am working on OFDM over harsh channel, so the channel coding is an essential thing for achieving a reliable communication. According to my reading, LDPC is almost the best channel coding we can use ...
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2answers
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Multi-input Single-output communication system

In case of communication in MISO system, if we have a system of 4 x 1, which means 4 transmitter with 1 receiver. Will the received signal be a vector of dimension 4 x 1 ? Or it will be 1 x 1 since ...
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1answer
42 views

LPF Filtering without changing DC value of signal

I have sampled my transmitted signal, it's looks like sinusoidal , frequency sampling is =2048Khz, bitrate of transmittion=100Kbaud , and my problem is that I have designed in matlab my LPF filter ...
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52 views

Low pass equivalent of bandpass white noise

Let $$n(t) = Re\{z(t)e^{j2\pi f_ct}\},\ $$ where $z(t)$ is the low pass equivalent of white noise $n(t)$. We know the autocorrelation of $n(t)$ is $$R_n(\tau) = \frac{N_0}{2} \delta(\tau),\ $$ $\delta(...
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1answer
35 views

A communication system

I am solving the following problem as an exercise. I have a signal $$z(t) = x(t)\cos(wt) + y(t)\cos(ut)$$ and we assume $x,y$ to be band-limited continuous time signals. In the frequency domain we ...
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1answer
42 views

Sampling of bandpass signal with bandpass filter

Trying to solve the following problem about bandpass signal sampling. I have a signal whose Fourier transform is such that $X(w)= 0$ if $w>w_h$ or $w<w_l$. The reconstruction is done using a ...
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1answer
24 views

the best pre emphasis for MMSE equalizer

The system consists of multiple low-pass filter and noise, $h_1$, $h_2$, $h_3$, ..., $n_1$, $n_2$, $n_3$, ... At the transmitter, the power of signal is fixed; and at the receiver, MMSE equalizer is ...
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22 views

Matched filter for recieved RF digital packet by LPF

Im transmitting by cc1350 launchpad microprocessor digital RF Packets, transmitted modulation FSK. bit rate of transmitting is 50Kbouad, frequency of transmitting is 868MHZ . Im sampling the signal(...
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33 views

ZP-OFDM vs. CP-OFDM channel equalization

Now, I am working on OFDM equalization, according to what I understood, Adding CP into the OFDM block change the convolution from linear to circular. That circular convolution enables using point-wise ...
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24 views

Does the channel impulse response change over time?

Let's suppose we have an emitter and a receiver in fixed positions the emitter sends a signal $s_1(t)$ and after a period $T$ it sends $s_2(t)$, the receiver receives $r_1(t)$ and $r_2(t)$. My ...
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2answers
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filtering my received Signal by LPF

I've received my transmitted signal that I sampled it on 20480000 frequency , transmitted frequency 868MHZ , bitrate 100KHZ , I see the "bits" that Im transmitting in my plot in matlab it ...
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1answer
48 views

why don't we use Broadband technology to transmit digital signals ,and use Baseband technology to transmit analog signals? [closed]

I learn the Broadband technology and baseband technology in this website: https://www.computernetworkingnotes.com/networking-tutorials/differences-between-baseband-and-broadband-explained.html ...
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autocorrelation peak

Let $s(t)$ a signal of duration $T_s$ on a bandwidth $B$. The matched filtering of this signal gives: $R=max|s(t) \ast s^{\ast}(-t)|$ and here theoretically I saw that $R=BT_s$. But does anyone know ...
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1answer
27 views

continuous chain of ones or zeros for transmit

If my data has long chain of 1's or 0's. Can the data be modulated and transmitted as it is. Will this have any effect of the receiver adc as this may increase dc value of the signal or any other ...
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1answer
64 views

Use of I/Q representation for unmodulated Rx Signal

it may be a stupid question but while learning about DSP and Modulation I was wondering, if the I/Q representation of an RF Signal is of any use, when the received signal does not contain any ...
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Converting Received RF packets to binary file that RF packets sampled by dongle rtl-sdr

Im using #SDR software that I transmit packets from cc1350 launchpad to the air and I want to validate that if those packets are valid or not by checking its CRC. my problem is actually the step to ...
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1answer
41 views

Bandwidth of a modulated signal in analog domain

If i am using QPSK modulation with data rate 4 Mbps with rolloff factor $0.5$, then the bandwidth will be symbol $\rm rate\times (1 + rolloff\ factor)$. So here it will be $\rm 2\ MHz\times 1.5 = 3\ ...
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1answer
82 views

Find continuous signal given a condition on its samples

Let $x(t)$ be band-limited with $B = \omega_m$. Sampling gives us $$x(nT_s) = \begin{cases} 1, & n = 0 \\ 0, & n \not = 0 \end{cases}$$ And $\omega_s = 2\omega_m = \frac{2\pi}{T_s}$. Find ...
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1answer
64 views

Capacity and channel estimation algorithms: explanation of results

I study a spectral efficiency of a system with a precoding scheme with different channel schemes. Algorithm A for channel estimation gives a smaller estimation error instead of Algorithm B. I have ...
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29 views

For an LTI system, the zeos and poles are defined in the z plane. How to add additional poles and zeroes so LTI system is real valued?

The question already has a zero and a pole in the complex plane. It is asking to add additional zero and pole so LTI becomes zero valued. zeros=-1/2 + 1/2 i , poles = -1/3 -1/3 i . What I understand ...
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What causes this type of distortion in a constellation I/Q plot?

I'm transmitting QPSK through a digital pulse shaping filter (RRC, 35% excess BW, 2 samples per symbol, 64 taps) at a rate of 56Msym/sec. Receiving it on standard lab equipment (Vector signal ...
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45 views

Spectral Inversion in Zero IF Receivers

Is spectral inversion possible in zero IF receivers? If yes, then can somebody explain with an example how that happens and what can be done to correct it. The following problem can be taken as ...
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1answer
28 views

Filters used in FBMC

On page 137, chapter 5 in the book titled 5G Physical Layer: Principles, Models and Technology, the author says: An important attribute of the FBMC waveforms is that they typically do not employ any ...
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36 views

Why is it that the input to an IFFT is first reordered in an OFDM Modulator

I am back with my questions! I recently came across a paper titled "Low Latency IFFT Design for OFDM Systems Supporting Full-Duplex FDD" where they claim a new method of IFFT input mapping ...
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27 views

burst qpsk timing recovery

I am working on the burst qpsk transmit receive with data rate varying from 100 kbps to 1 mbps. My modem works fine for continuous tx & rx with costas loop for carrier recovery and Gardner for ...
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3answers
74 views

Meaning of sampling frequency?

Say there is a communication system, which can manage sampling frequency up to 4 MHz (given by software). Can the system be able to process signals with maximum frequency more than 2 MHz? Because ...

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