# 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
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### Frequency domain vs time domain equalization

It is said that a frequency domain equalizer is equivalent to an infinite length time domain equalizer, why is that? And if this is the case, why hasn't frequency domain equalization been used ...
2answers
49 views

### Minimum Number of Errors required for BER Curves in Matlab Simulation

what should be minimum Number of Errors required for getting BER Curves in Matlab Simulation for different number of transmitted bits ?
0answers
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0answers
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### Which channel is the suitable channel for the short distance?AWGN?rayleigh or rician channel?

What is the unsuitable channel for the short distance?The short distance i mentioned is 1m or 2m.Unsuitable channel I mean is that if i assume this channel in my system,it will be very strange. In my ...
0answers
29 views

### Why will the FDMA usually be used in transmitting analog signal,not in digital signal?

I saw this in the WiKi:https://en.wikipedia.org/wiki/Frequency-division_multiple_access FDMA can be used with both analog and digital signal but it generally used with analog signal Why?Don't we ...
0answers
8 views

### The dis-advantage that sensor use FDD (Frequency-Division Duplexing) to transmit the digital signal

We must need lots of sensors in IoT,sometimes these sensors have to transmit their information to other sensors at the same time.I want to say some disadvantages that sensor use FDD (Frequency-...
0answers
14 views

### Some confusion about the definition of NOMA,multiple access in Non-orthogonal multiple access means different information or different receiver?

The basic NOMA system model is as below,transmitter $T$ wants to sent the $x_1$ to $A$ receiver and $x_2$ to $B$ receiver.However,the channel between $T$ and $B$ is very bad,so T has to send the $x_1$ ...
1answer
52 views

### DSSS direct sequence spread spectrum - encoding each raw data bit (question)

For implementing DSSS, it is understood that each raw data bit needs to be encoded as a new recognizeable/identifiable bit pattern. In many literature involving DSSS, there is mention of using PN ...
1answer
26 views

### The meaning of correlating “Time Aligned” pseudo-random or walsh code sequences

Correlation is from a mathematical point of view a shift in time domain +multiplication + summation operation I thought that if 2 codes are "time aligned" then correlation reduces to a dot product ...
2answers
70 views

### Correlating a sequence or a code with itself at QPSK symbol rate

I am sending a QPSK signal that represents a certain sequence or code and that needs to be correlated with itself on the receiver (there are other sequences/codes in the system, and they are all ...
0answers
13 views

### Does anyone use the NOMA when the receivers are both in short distance?

The picture below is the basic NOMA (Non-Orthogonal Multiple Access) system model The basic setting of NOMA is that $d_1$ is longer than $d_2$ and $d_3$,so if $UE1$ want to receive the meassage from ...
0answers
22 views

### Detection Threshold Matched Filter

Let's say my digital communication system operates over a channel that is not AWGN but it has discrete noise levels added during each bit period. Transmitter sends +1V for "1" and -1V for a "0" and a ...
1answer
91 views

### Correlated phase noise and delay

2 identical signals generated by the same PLL/VCO should have the same phase noise profile and their respective phase noises should be correlated. Suppose one of them is delayed and I start sampling ...
1answer
26 views

### What factors affect the propagation delay at wireless communication?

Generally, the signal propagation delay is d/c. d is a distance between a transmitting antenna and a receiving antenna. c is light speed. However in NS-3, there is Random Propagation Delay Model ...
1answer
83 views

### Symbol Timing recovery for modulation producing ISI

I am interested in understanding why the common timing recovery algorithms function for modulation schemes which produce ISI. For example, suppose you are receiving at the output of a matched filter ...
1answer
198 views

### Why does signaling overhead for time synchronization scale up with the number of transmitter nodes in a multiple access system?

I was recently reading about random multiple access methods without time synchronisation (of the pure Aloha type). In this paper it is stated that, The need for transmitter synchronization is a ...
0answers
11 views

### Convolutional coding that payload length does not change

This is the question about Sigfox radio specification. The stack: Specifically, in Section 3.10 of the specification, we have a convolutional coding (after CRC) with three possible polynominals. I ...
1answer
335 views

### Important differences between BCJR and SOVA algorithms?

BCJR algorithm needs noise variance for Gamma (branch metrices) calculation. So I am wondering does SOVA (soft output Viterbi algorithm) needs noise variance?
1answer
131 views

### Viterbi Decoder without traceback

How can I use Viterbi algorithm for a real time scenario where I cannot afford to have delay in the tracing back the best path to the initial state?
0answers
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### What is the threshold to prefer polyphase channelizer over FIR filtering?

A wideband signal need to be processed to extract several narrowband channels from it. There are two straightforward approaches to do so: For each narrowband channel shift frequency and filter it ...
1answer
29 views

### Translating Matlab scrambler function to C

I am trying to implement the following matlab function in c: Mathworks scrambler Function Documentation This is the code I've written: ...
1answer
195 views

### Fractional spaced equalizer + timing (clock) recovery

I am trying to figure out how timing recovery is used after equalization (or, being more correct, when the equalizer is in the timing recovery loop), having a T/2 fractional spaced equalizer and using ...
1answer
97 views

### Frequency synchronization in OFDM

Could you please help me understanding how the null sub-carriers can be used to estimate the carrier frequency offset and thus performing frequency Synchronization at the receiver?
0answers
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### Can self-interference signal also be harvested if we cancel this self-interference signal?

A device with two antennas,one for transmitting signal by using RF ,one for receiving,and it also have a power splitter inside to split some of power to do energy harvesting,some of power do ...
2answers
232 views

### Inter Symbol Interference (ISI) / Equalization in Digital Signal Processing

I am studying the OFDM system and before to start I have decided to study the main concepts of modulation. And I have spent yet a lot of time to understand the concept of an equalization to reduce ISI....
1answer
495 views

### Cyclic prefix length in baseband OFDM with ZF equalizer

My question is about baseband OFDM. I try to plot the BER according to the SNR. I noticed that changing cyclic prefix length does not affect the performances. I.e., following the code below, ...
1answer
146 views

### From IQ to pi/4 DQPSK

I need to decode $\frac\pi4$-DQPSK packets from an SDR, preferably in C#. I have the SDR receiver part done. Now I'm trying to learn how to take the IQ stream and get the packet info from it. I've ...
0answers
15 views

### Why does this self-interference cancellation cancel other signal,not the self-interference?Do i misunderstand something?

In this paper, https://ieeexplore.ieee.org/document/8704259 ,i think transmitter with M antennas transmit $x_f[t]$ to Cell-center user ,and transmit $x_n[t]$ to the Cell-edge user,however,the distance ...
1answer
25 views

### Why when the SINR of transmitting signal is very large ,the device cannot decode the received signal at the same time? [closed]

$A$ transmit $x$ and receive the $y$ at the same time.If the SINR for transmitting $x$ is very large,$A$ cannot decode the $y$ signal.Why? In my opinion,If the SINR for transmitting $x$ is very ...
1answer
41 views

### How can i know the full duplex does work from the Shannon's theorem?

There is a full duplex system below,two user are $A$ and $B$,according the definition of full duplex ,$A$ and $B$ have to transmit and receive the signal at the same time.that is, transmitted signal ...
1answer
58 views

### Digital FM Demodulation

I have created a Common Lisp Library to receive samples of an FM signal from BladeRF1 SDR. I want to write a subroutine to implement FM demodulation. The input should be in the format of IQ data. ...
1answer
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### 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 ...
1answer
28 views

### Is there any advantage on using a low span value in a RRC filter?

I know that in a RRC filter a high value of the span gives a better response, in the sense that the RRC filter response is more near to the ideal RRC filter response. However, would there be any ...
1answer
92 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 ...
1answer
41 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 ...
1answer
53 views

### 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?
0answers
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### How to remove sum and difference terms while computing frequency from fft?

I am currently operating in vhf band AND trying to detect frequencies using Fast Fourier transform thresholding method. While detection of multiple frequencies , i received spurs(May not appropriate ...
1answer
41 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 ...
1answer
20 views

Let's suppose that I send the following sequence of bits: $x = [1, 0]$ I use a 7 length Barker code ($b = [−1,−1,−1,1,1,−1,1]$) in a BPSK modulation scheme, resulting in the following sequence: $... 1answer 28 views ### Definition of MIMO (Multi-input Multi-output ),does this output mean antenna or transmitter? I want to ask the definition of MIMO(Multi-input Multi-output ) If there are one transmitter and one receiver,each of them has$N$antennas,$N >1$. So i can say it is a MIMO system. But if now i ... 2answers 613 views ### Why does FEC seem to make BER worse at low Eb/N0? I've been plotting BER curves and am wondering why FEC seems to make a BER curve worse at low Eb/N0 than the un-coded theoretical BER curve? My example plot below is for Differential BPSK using a ... 1answer 41 views ### What is matched filter in QPSK signal? Consider we have received a QPSK signal. We than make I (In phase) and Q (quadrature component) which I understand is nothing but multiplying the received signal by the cos(2*pi*f) and -sin(2*pi*f) ... 2answers 88 views ### About understanding a Matched Filter I have some basic doubt in understanding a matched filter (MF). Please note that I have already seen very informative post in this context Understanding the Matched Filter In what follows, I list ... 0answers 29 views ### law for a random variable Let:$Y=|X_1+X_2|^2$with$X_1 \sim N(\mu_1,\sigma_1^2)$and$X_2 \sim N(\mu_2,\sigma_2^2)$.$X_1,X_2$are independent complex random variables. Consequently,$Y=Re(X_1+X_2)^2+Im(X_1+X_2)^2$My ... 0answers 21 views ### Channel information from power spectra Linked below is a previous post regarding a multipath channel. Equalisation of FFT spectra I am reusing the images here: The first image is that of the power spectra when there is no multipath and ... 1answer 40 views ### Is there any relation between QAM and streaming? I am trying to determine whether there is any relation between QAM and streaming (e.g., video streaming). I know that QAM can be used for broadcast TV. I'm not sure if QAM is used for WiFi bit ... 2answers 83 views ### MU-MIMO Sensitivity to Line of Sight (LOS) Why do researchers talk about MU-MIMO being "less sensitive to line of sight" / "LOS Okay"? For example in this video, time=[00:40,00:42] Similarly, "User separation based on spatial channel ... 0answers 14 views ### Do we need to distinguish which rate is it when the system is point to point?or we just say “rate”,not downlink rate or uplink rate If i have four transmitter,(t1,t2,t3,and t4), and one receiver(R),and each of them just have one antenna. When t1 is transmit signal to R,the other transmitters (t2,t3 and t4)harvest the RF ,when t2 ... 1answer 23 views ### What is the definition of half duplex?Is this system,$A$give$x$to$B$,then$B$give$x$to$C\$,called half duplex?

I saw the definition of half duplex in wiki,and it said half-duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a ...