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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Understanding FFT size in OFDM modulation

I know that the FFT size is the the number of subcarriers OFDM system have. But I also know that the FFT has to be $2^N$ big (in order to process N samples from time domain to frequency domain). ...
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bandwidth for a chirp

Let $s(t)$ an up-chirp with bandwidth $B$ and $y(t)$ an down-chirp with the same bandwidth. For a multi user chirp spread spectrum (2 users), I use the up-chirp and down-chirp as spreading signal. The ...
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OFDM system with cyclic prefix

I have: Bit rate $R_b = 25.6 [Mbit/s]$ Sampling frequency at the input of the receiver: $f_p = 8 [MHz]$ Protective spacing between successive OFDM symbols: $τ = 16 [µs]$ The combined length of ...
46 views

orthogonal family and pulse shaping filter

I am a beginner in digital communications and I have a question on the pulse shaping filter. For example, I have an orthogonal family $\{s_1,...,s_N \}$. Can we preserve the orthogonality with a ...
62 views

QAM constellation coder - transmitting a file

The file with size 6 MB is transmitted in 100 seconds using a constellation coder QAM with a constellation of 6 bit/symbol ${ M = 10^6, B = 8}$ Tasks: 1. What is the signal bandwidth if the band ...
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upsampling in the frequency domain

I have the Fourier transform $S(\nu)$. If I oversample by a factor $R$ in the frequency domain. Does the duration of the signal $s(t)=IFFT(S(\nu))$ change?
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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: <...
42 views

time for a signal after a sum

I have a signal $s(t)$ of duration $T$ with limited bandwidth. In matlab, I use a sampling frequency $fs$ and I have $L=T*fs$. After, I have $r=ones(size(s))+s$. To have a limited bandwidth, I filter ...
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What is the meaning of inverse signal bandwidth?

I have been studying the two ray model in communication systems. The text I'm studying has two signals, $u(t)$, which is Line of Sight and $u(t-t')$, which is the reflected signal. It quotes, ...
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Digital receivers for the reception of non pulse shaped signals [closed]

We are used to deal with pulse shaped signals, for example signals that go through root raised cosine filters at the transmit, and then matched with root raised cosine filters at the receive In ...
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Beamforming question

I'm working on a beamforming network. Every node in the network is half-duplex and has a linear antenna array that is capable of digital beamforming. A receiving node can be capable of forming any ...
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Multiplication and Convolution in Wireless Communications

In Wireless Communication, sometimes the received signal is expressed (in time-domain) as follows: $$r=hx+n$$ where $h$ is called a fading coefficient and sometimes it is expressed as: $$r=h*x+n$$ ...
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Energy in the mcss

In an MCSS (Multiple Chirp Spread Spectrum) communication, for example with a gaussian channel. Why should spreading chirps have the same energy?
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spectrum for a communication signal [closed]

I have a communication signal. Its bandwidth is limited. But does the $X(f)$ value is important? For example, I have $X(f)=6.10^3$ but for an other signal I have $X(f)=10^6$ (the channel is the same ...
24 views

Noise power for decorrelating detector

I have the following setup: $\mathbf{y}=\mathbf{R}\mathbf{s}+\mathbf{w}$, where the details of each entity can be found at http://bard.ece.cornell.edu/downloads/sim_tools/spanc_me/report/node12.html. ...
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differential phase tracking loop for the receiver

For a DBPSK modulation, how can we track the differential phase? I thought about using a phase tracking loop but I did not find articles for this.
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Modulating ofdm in audio spectrum

Trying to modulate audio with OFDM data. My fs = 44.1khz, i'm using 64 sub-carriers, but i'm only using 5 sub-carriers for data, so my bandwidth is about 3.445khz. I'm not sure how to properly write ...
136 views

Nyquist criterion for zero ISI in the frequency domain

Can someone explain in simple words /graphs the necessary and sufficient Nyquist criterion for zero ISI in frequency domain, namely: The constant folded Fourier Transform.