# 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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### How can IQ samples be obtained by shifting a signal down to DC?

In a direct sampling architecture where the first Nyquist zone is sampled, how can IQ samples be obtained by downconverting a recorded QPSK-modulated signal to DC? What is the relationship between ...
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### Simulation of Doppler shift in C/A code

I am trying to simulate C/A code under the effect of a doppler shift. Let's say the doppler shift is 5000Hz. I am assuming an ideal scenario where I can down convert the GPS to its baseband of 1....
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### Time-domain channel estimation using identical guard interval vectors

Assuming we have the received signal $y = h * x$ where $*$ is circular convolution, $x$ is the transmitted signal, $y$ is the received signal and $h$ is frequency selective channel. The signal $x$ is ...
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### BER of coherently detected DBPSK?

I understand that differential PSK is generally used when coherent detection is not possible, and so is usually detected non-coherently. However, for sake of theoretical analysis/understanding, what ...
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### BPSK decoding with a non-ideal transmission medium

I'm simulating tranmission via power line communications (PLC) using BPSK. Due to the coupling circuit and slightly inductive nature of the line, the BPSK signal received is far from ideal. In the ...
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### Related to SNR in wireless communication

I am working on a research paper related to wireless communication, wherein I am facing some doubt while writing expression of SNR (which is a ratio of Signal variance in numerator to Noise variance ...
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### Regarding SINR (Signal to Interference Plus Noise Ratio) in wireless communication

I am working on a research paper related to wireless communication, wherein I am facing some doubt while writing expression of SINR (which is a ratio of Signal variance in numerator to Interference ...
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### instantaneous frequency for a complex signal

Let $s(t)$ be a complex signal. I would like to know how to find a signal from its instantaneous frequency. I programmed the following function which calculates the instantaneous frequency. ...
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### Why is my Hard Decision Decoding of Convolutional codes not performing well?

I am encoding and decoding a randomly generated bitstream of data using a [5 7] convolutional code. For the decoding part I am using a Viterbi Decoder and trying both HDD and SDD. However, my results ...
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### Spreading sequence for dsss or cdma

In spread spectrum systems like DSSS or CDMA, we can take a pseudo-random sequence composed of 1 and -1. But can we use a complex number sequence between -1 and 1? Why is this type of sequence never ...
1 vote
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### A question of channel capacity in precoding case

I’ve a general question about precoding. The channel capacity after precoding is $\log_2(\det(I+F’H’HF))$ where $\det$ is the determinant, $I$ the identity matrix, $F$ the precoding matrix, $H$ the ...
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### pulse shaping filter and passband

Most of the time, the baseband transmission chain is: modulation->oversampling->shaping filter (SRRC)->bandpass conversion on a carrier frequency $f_0$. But can we change the previous order ...
1 vote
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### Does Filtering changes the noise characteristics in a signal?

So, I tried out what i am learning in digital communication using GNU radio. The plan is to modulate two baseband signals(sine and cosine) 1kHz with no modulation protocol(don't know any(yet)) to a ...
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### Issue with small signal ML TED with low RRC excess bandwidth

I have a simple BPSK system using RRC TX & RX filters with 10% excess bandwidth with a receiver that uses a symbol synchronizer of the common type consisting of a polyphase filter bank (performing ...
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### Unable to decode LDPC using Belief Propagation algorithm

I'm writing a MATLAB code to decode LDPC codes using the BP algorithm. I'm trying this with a (7,4) code. I tried the message-passing algorithm between variable and check nodes. L_r is the check to ...
1 vote
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### Energy of Transmit Pulses in Digital Communication

Consider the problem of transmitting just two SRRC pulses through an noiseless Channel. The two transmit bits are {+/-1}. In plots below, the individual transmit pulses are shown in black, while the ...
1 vote
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### Is there something wrong with MATLAB's pwelch function?

The normalized factor for windowing, is it $(\sum w[n])^2$ or $\sum w^2[n]$? I try to reproduce the pwelch function, if I set the normalized factor to $(\sum w[n])^2$, I get the same result as MATLAB ...
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### Free space path loss and ofdm

I want to apply the Friis equation of free space path loss on OFDM signal. Do I multiply signal by $\frac{c}{4\pi f_cd}$ before IFFT or after?
1 vote
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### Understanding the (complex) GNU Radio implementation of Mueller & Muller

I have some questions about the M&M implementation of GNU Radio that uses complex samples. I started reading the implementation for real values: ...
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### Mapping PAM-17 levels to PAM-5 levels

A $17$-level $\left\{ -4,-3.5,-3,\ldots,0, 0.5, 1,\ldots, 3.5, 4 \right\}$ digital signal coming at $125 \, \textrm{MHz}$ is passed through an FIR filter ($10$ taps) operating at $1250 \, \textrm{MHz}$...
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### real time streaming image encoding options

I'm interested in streaming video real time. There is an issue of deciding how to properly encoder the images and the format they should be. Since the data will be streamed over UDP, it needs to be ...
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### outage capacity of K user fading channel

Suppose you have $K$ users, each seeing a point to point fading channel with $n$ receive diversity. So the rate of the kth user is at most $\log(1+ SNR|h_k|^2)$. Now suppose, each user transmits at ...
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### Received signal from channel model

I'm struggling with this because I don't fully understand it. My goal is to estimate a received signal that is transmitted through a channel that follow a stochastic process. My doubt is, if the PDF ...
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### What exactly is transmitted in the channel in lossy compression?

Consider the problem of lossily compressing a memoryless binary source $X \sim \operatorname{Bernoulli}\left(\frac{1}{2}\right)$ at target Hamming distortion $D < \frac{1}{2}$. According to the ...
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### decision feedback equalizer building blocks (from Adaptive Filters: Theory and Applications” a book by Behrouz Farhang-Boroujeny)

Here you can see building blocks of DFE from "Adaptive Filters: Theory and Applications” a book by Behrouz Farhang-Boroujeny in chapter 17. Figure 17.9 shows the overall building blocks of DFE ...
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### How to detect when signals overlap in OFDM?

If you are in an environment with multiple transmitters of similar signal strengths, before band selection has taken place, how do receivers parse out overlapping OFDM signals from multiple ...
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### Distortion-Rate function for standard Gaussian signals quantized with a uniform alphabet

Let's say $X \sim N(0, 1)$ is a standard Gaussian input signal and $Y \sim U\{y_1, y_2, \cdots, y_{16}\}$ follows a discrete uniform distribution (in fact, $\{y_1, y_2, \cdots, y_{16}\}$ is a ...
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### Understanding frequency band specification in wireless networks

I am trying to clear my understanding of frequency bands. When a particular wireless network standard like WiFi 6 claims operation in the $2.4$ GHz or $5$ GHz band, what do these values refer to? I am ...
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### How to derive the error probability for pulse position modulation in digital communication?

I am trying to build a simulation on MATLAB for PPM transmission through AWGN channel. I have a function wrapping the transmitter, channel and receiver that takes a SNR value as input. My simulation ...
1 vote
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On page 301 of https://stanford.edu/~dntse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter7.pdf, equation (7.33) says that the magnitude of the dot product of two unit complex vectors is ...
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I have a multiuser uplink signal $$Y=h_1x_1+h_2x_2$$ where, $h_1$ and $h_2$ are channels for user 1 and user2 respectively. At the receiver how can I equalize the signal $Y$, assuming known $h_1$and \$...