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
82 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 ...
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1answer
79 views

“Revolving” QPSK Constellation

I am trying to code a QPSK transmitter in VHDL but first I want to create one out of Simulink blocks and translate it into VHDL code. I am using an example project for the PlutoSDR as a guide. This ...
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1answer
60 views

OQPSK vs QPSK envelope query

We use OQPSK because of envelope of signal but my question is "In Phase Shift Keying(or QPSK) the amplitude of the transmitted signal is constant then how the envelope will change in OQPSK"??
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1answer
76 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 ...
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1answer
193 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 ...
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3answers
579 views

Compress a signal by storing signal diff instead of actual samples - is there such a thing?

I am working with EMG signals sampled at 2kHz and 16 bits, and noticed that they "look smooth", that is, the signals are differentiable, and if I apply a "diff" function (...
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3answers
761 views

Processing OBPSK as OQPSK

I'm trying to create and process an OBPSK signal, which, as I understand it, transmits even bits on one axis and the odd bits on the opposite axis, resulting in a constellation with 4 unique points. ...
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1answer
26 views

Identifying the correct source of packets

I have a basic question regarding digital communication , especially pertaining to wireless internet . A packet is basically a stream of bits . And a bit is detected as zero or 1 depending on ...
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1answer
32 views

Possibility of using Power allocation with Maximum likelihood estimation

I have a basic question about maximum likelihood (ML) estimator and its implementation with power allocation. Suppose we are using $MIMO$ system with $4$x$4$ transmit and receive antennas, so the ...
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1answer
333 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?
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1answer
125 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?
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1answer
190 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 ...
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1answer
22 views

What is Assymptotic Equipartition Property?

I am currently studying about Polar Codes in 5G standard and while reading my paper I found something called AEP which is required for channel coding. I surfed the web but didn't found a satisfying ...
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95 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?
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90 views

Why does power spectral density of OFDM signal changes with number of zero subcarriers

I am trying to plot the power spectral density (PSD) of an OFDM signal with $N_{dsc}$ subcarriers and $N_{zeros} = N_{fft}-N_{dsc}$ zero subcarriers. I am using the matlab function pwelch() to plot ...
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2answers
205 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....
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1answer
470 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, ...
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26 views

phase shift for a spreading signal

For a chirp spread spectrum with $2$ users, I can use an up-chirp and down-chirp for the spreading signals. This is my questions: 1) These $2$ signals have a phase shift. If my $2$ users are at ...
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1answer
123 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 ...
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1answer
47 views

BER vs Eb/N0 (db) Curve

So, I was trying to plot BER (Bit Error Rate) vs. Eb/No (in dB) curve using Python. Here is what I get, Eb/N0 is in dB on the x-axis But I know that this is not the correct graph. Here is my code. ...
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1answer
29 views

confused in practical example of shift invariance?

I am reading book of w smith ,"The scientist and engineer guide to digital signal processing" 2 edition I am trying to understand topic of shift invariance from theory given on pg 92 I have attached ...
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0answers
13 views

guard interval calculation for multi-cell massive MIMO

I am interested in a multi-call massive-MIMO system modeled as in the works of T. Marzetta, for example: "Inter-Cell Interference in Noncooperative TDD Large Scale Antenna Systems". In these works, ...
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1answer
21 views

Beamforming back lobes question

When performing a conventional digital beamforming I see in some textbooks they show beam patterns as where the arrival angle ranges between -90 to 90 degrees. But then other places show the full ...
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2answers
396 views

How to compute the channel gain from the path loss model of a wireless channel

I did my research but I did not get a satisfying answer. We have a transmitter and a receiver over a wireless communication channel. The channel's path loss is modeled as $ 128 + 37 \log_{10} (d)$, ...
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1answer
38 views

Why can the :$\mathbf W^*_{MMSE} $ be written as $E[\mathbf s \mathbf y^*]E[\mathbf y \mathbf y^*]^{-1}$

I read this paper: https://arxiv.org/pdf/1305.2460.pdf And there is a formula about MMSE:$\mathbf W^*_{MMSE}= E[\mathbf s \mathbf y^*]E[\mathbf y \mathbf y^*]^{-1}$ $\mathbf W _{BB}$:Baseband ...
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1answer
60 views

Carrier frequency of chirp signal

I am computing the link budget for a communication-radar (CoRadar) system. The formula for maximum distance, in communication, is: $d_{max} = \sqrt{\frac{P_t}{\tau}\frac{G_t G_{r, comm} \lambda^2}{(4\...
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1answer
81 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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11 views

Why can the C5 formula also means that $\sum\limits_{k=1}^{K}tr(\mathbf F_{K4E})>P_T$?

In this paper:https://arxiv.org/pdf/1805.08898.pdf I can understand why C4 means $P_E > P^*$,I just need to times $1-\rho_k$ in both two side,the C4 will become $(1-\rho_k)[\sum\limits_{k=1}^{K}\...
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1answer
34 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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18 views

Can the j be equal to k in the first formula ?In MISO system

In this system model,the received signal available for RF EH at each RX_k is given by However,its SINR formula is given by We can find that author said the j can't be equal to k,but their range are ...
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2answers
111 views

Why Convolution between Signal and a Channel Is Not Done Directly in SIMO Channel

In OFDM system, I need to transmit a signal $x$ in SIMO channel $H$ with one $Tx$ antenna and 4 $Rx$ receiver's antennas. The initial equation for that convolution is $r=Hx+n$ where $n$ is the noise. $...
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578 views

What Are Less Computationally Demanding Alternatives to the Viterbi Decoder?

What are less computationally demanding alternatives to the Viterbi Decoder? Ideally what I would like is a list of the most commonly used approximate methods, along with brief pros and cons.
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65 views

What are the limitations of using Magnetic induction rather than acoustic communication in underwater communication

In underwater environment, Why acoustic underwater communications are still famous more than magnetic induction communication however in magnetic induction underwater communication, higher data rate ...
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12 views

What is the unit of a complex AWGN variance? mW or W?

In this paper https://arxiv.org/pdf/1805.08898.pdf ,there is a variance be written as $\sigma^2_{h,k}=\theta d_k^{- \alpha}$, $\sigma^2_{h,k}$ is the variance of th channel,$d_k$ is the distance ...
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28 views

5G NR base matrix

The 5G NR specs for channel code are found here: https://www.etsi.org/deliver/etsi_ts/138200_138299/138212/15.02.00_60/ts_138212v150200p.pdf Starting on page 21 there is base matrix for the LDPC code ...
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1answer
33 views

Spatial diversity: symbol error probability, upper bound

I am trying to understand the calculation upper bound that is given in book. Edit 1: I added 3,44 Can someone explain to me how to come from (5.5) to (5.7) ?
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79 views

Working principal of MIMO(2 by 2)-OFDM (802.11n)

I have gained basic knowledge on MIMO-OFDM from online sources and in process of building 802.11n standard in GnuRadio+USRP B210. My goal is to analyze the channel state information and play with it ...
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1answer
39 views

Using Spreading spectral with Maximal likelihood

As asked here, HERE if we have the signal x = [0.7 + 0.7i; 0.7 - 0.7i; -0.7 + 0.7i; -0.7 -0.7i]; Which was spread over code ...
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1answer
41 views

Channel model: LOS Component and Rician K-factor

I am asking about Rician K- Factor in this model: I know that if $K=0$ then it is a pure Rayleigh fading, if $K= \infty$, then non fading Channel. What is it, if $0<K<\infty$ ? If i represet ...
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29 views

Training Fractionally Spaced Channel Estimator and Equalizer

If you are attempting least squares channel estimation with a fractionally spaced channel estimator, do you want the training sequence to also be fractionally spaced or symbol spaced? It looks like ...
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1answer
41 views

Channel estimation

In single-carrier systems, at the receiver, when we need to do channel estimation and equalization? Do we need to do before timing synchronization and CFO correction or before it? And, please let me ...
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43 views

Determining signal using Optimal receiver(Matched filter)

I'm given a signal that has been passed through a noisy channel using binary polar signaling with a pulse value of p(t) = 1. The receiver is coherent and the length of each pulse (in samples) is 10 ...
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1answer
12 views

operations valid for predicting new codewords for a Linear block code having linear encoder mapping

I want to know which operations are valid for predicting/finding a new codeword from a set of known codewords for a linear block code having linear encoder mapping, for eg.: Consider a linear ...
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1answer
77 views

Using MIMO with DS-CDMA and spread spectrum

I have a question about using MIMO with DS-CDMA. suppose we are using 4 TX antennas, and 4 RX antennas, then the Reyleigh ...
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3answers
1k views

Cyclic prefix vs. guard interval. Why use one over the other?

I'm currently studying some OFDM techniques, and have found that in order to avoid Inter-Symbol Interference (ISI), there needs to be some space in between symbols. Lets assume this length to be $\mu$....
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2answers
119 views

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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392 views

Modulation GMSK with I/Q

I have a doubt about something. GMSK Modulated signal is equal to I+Q (with I = cos and Q = sin) or I+i.*Q ? Thank you. Best regards.
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30 views

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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1answer
29 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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33 views

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 ...