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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RRC filter order

I have a question on rrc filter. Hope somebody can answer it. Tx path: My symbol rate is 1 mbps. I am using rrc 8x which becomes 8mpbs. For four symbols the order of the filter becomes 32. Next ...
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79 views

Why is the received signal=information+RF signal+noise,not just information+noise?

In this paper: https://www.researchgate.net/publication/325008791_Beamforming_Design_for_Max-Min_Fair_SWIPT_in_Green_Cloud-RAN_with_Wireless_Fronthaul Here is its system model In this model,it said ...
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45 views

How can I spread a modulated signal over walsh code

I need to confirm if spreading signal by using walsh code in MATLAB is right or no, Suppose I'm using QAM modulation, so I need to spread the signal over walsh code, is the below step is right: <...
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81 views

How to compand an OFDM signal?

Im trying to run a simulation of an OFDM system with convolutional coding and u-law compander on MATLAB. The problem is I dont know how to compand the OFDM signal. Do I need to do something to the ...
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72 views

farrow resampler in timing recovery

My receiver contains: AGC,timing recovery,carrier recovey blocks. I am using gardner timing recovery with loop filter and farrow parabolic filter. The input to the farrow filter is the fractional ...
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27 views

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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12 views

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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2answers
78 views

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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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
39 views

How to check my CRC of my RF packet? [duplicate]

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
26 views

Trying to find an intuitive understanding of OQAM/FBMC

I am trying to understand, possibly in an intuitive way, why would OQAM/FBMC when used with a Phydyas filter lead to a signal that does not need any cyclic prefix just like in any OFDM signal also ...
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27 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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64 views

processing gain in the receiver

I am always confused about the processing gain in the receiver. assume my receiver had LNA-> mixer->filter->ADC. does the filter in the analog section has any role to play with the processing gain? ...
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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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138 views

explanation on the mimo technique

In MIMO systems we have: $y=Hx+n$ with $H$ is a matrix $N_r \times N_t$. But the previous equation is only true for non-frequency selective channels. What is the formula for frequency-selective ...
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55 views

Symbol synchronization with unknown symbol rate

I'm learning about the early late gate symbol synchronization and understand that it needs to be given the symbol rate ahead of time. How could a multi-symbol rate system use an early late gate? It is ...
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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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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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Study resource for Digital communication

I am in my 5th semester of computer science and we have a digital communication course. All the textbooks of digital comm. available in my library demand a prerequisite of Circuit analysis and Signals ...
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71 views

chirp spread spectrum with 2 users

I am a beginner in digatal communications. I have 2 users and I want use a chirp spread spectrum. Can we take an up-chirp and a down-chirp as spreading signals for the 2 users? and if yes, why? The ...
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1answer
45 views

phase response of an analog filter

I am trying to understand the relation between phase response of an analog filter and its influence on the quality of a signal with PSK modulation. For example I want to design a BPF for HF ...
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1answer
48 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
16 views

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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37 views

correlator for cdma

I use a CDMA protocol for a multiuser communication. Let $c_i(t)$ the signature for the $i$-th user and $d_{i,k}$ the $k$-th modulated symbol for the $i$-th user. At the reception, we have $$r(t)=\...
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28 views

Confusion about the impact of flat fading and frequency selective fading on OFDM

OFDM divides the signal in frequency domain into orthogonal and relatively narrow subcarriers where fading can be considered to be flat Flat fading is not associated with delay spread and is corrected ...
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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
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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1answer
27 views

Most reliable way to decode PAM2 signal with unknown signal levels?

I assume this is pretty much a standard problem. I have a PAM2 signal that comes from a complex envelope (i.e., it is not zero-centered) and is heavily imposed by noise. This shows 20 different ...
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49 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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67 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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1answer
55 views

AGC noise level

Typically AGC is used to increase the noise level. If my receiver sensitivity is -100 dBm with 60 gain After AGC it becomes -40 dBm. If the noise figure of AGC is less. So the noise also increased by ...
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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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119 views

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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70 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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1answer
84 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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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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76 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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31 views

My preamble bits not enough for locking the timing recovery for a gaussian channel

I have chosen the preamble to be 48 bits (16 bits for carrier recovery+16 bits for timing recovery+ 16 bits for AGC of 30dB + 16 bits for phase ambiguity) As channel is gaussian and there is no much ...
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2answers
42 views

Can two modulations be used in the same frame

Can two modulations be used in the same frame? For qam/qpsk/psk/ modulation scheme for the data bits can i have a fixed preamble with psk modulation? does this help in achieving faster carrier freq ...
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1answer
38 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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3answers
77 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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1answer
30 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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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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1answer
112 views

SNR Signal to noise ratio of Pseudo random bit sequence (PRBS)

How should I estimate the Signal to noise ratio of a 20 Gbps NRZ PRBS pattern from power spectral density--where should I take the noise floor if I can assume well the noise is AWGN? I know roughly ...
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1answer
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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102 views

Sample rates, Samples per Symbol, and Digital Pulse Shaping

Having some confusion about digital pulse shaping for complex baseband (passband) signals. The complex baseband linear modulation equation is $$s(t)=\sum_{m=-\infty}^{\infty}\text{Re}\{a_m\}h(t-mT)+j\...
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2answers
31 views

How are the constellation point coordinates determined in digital modulation

I need to understand where the concept of the distance comes from. What I mean is, how do we know what coordinates are to assign to points in the constellation. For example here for QPSK the points ...
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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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