# 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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2answers
85 views

### How to get sensor covariance matrix from CSI values?

I try to use musicdoa() function in Matlab to estimate the direction of arrival of the paths of signals received by the receiver in a wireless link. The function ...
1answer
56 views

### Inter-Symbol Interference (ISI) channels

Is there any open and free data-set for digital ISI channels? I want to have a variety of ISI channels' taps, such as $h[k]$ here. Especially looking for BPSK channels.
3answers
110 views

### Quality of experience engineer

I have noticed certain overlap between digital signal processing and quality assurance for jobs that require expertise in evaluating the quality of the transmitted signals on telecommunication systems ...
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 ...
1answer
76 views

### Phase synchronization between 2 PLLs

If 2 PLLs as the one shown are driven from the same reference/crystal and the ratio between divider N and R is an integer (note that divider R is outside the loop), does one need a sync pulse for ...
1answer
44 views

### how to set Equalizer's coefficient using generalized eigenvector.

In that paper https://jwcn-eurasipjournals.springeropen.com/articles/10.1186/1687-1499-2012-72 .. Equation 3, $J(w) = w^HBw/w^HCw$ .. $B$ and $C$ are matrices, $w$ the filter coefficients vector ...
2answers
103 views

### Averaging out noise and SNR improvement

Suppose I have a sequence of identical symbols carrying the same information at a sample rate equal to the symbol rate, why do we always talk about averaging them to average out noise, why not just ...
1answer
100 views

### Calculating the energy of trapezoid wave

Here is the trapezoid wave,although i know we can use $\int^{4}_0 |x(t)|^2dt$ to calculate its power,but i want to divide this trapezoid to right triangle$(t=0$~$2)$,and a square ($(t=2$~$4)$). I ...
1answer
374 views

### Noisy and Noiseless Channels

I am trying to understated the the achievable bit rate in noiseless and noisy channels. Particularly, I would like to solve the following problem: Let a real symbol $0\leq x_i < 1$ be ...
1answer
59 views

### External frequency divider in a PLL

Does a frequency divider external to the loop BW of a PLL cause any phase ambiguity ? and why? and is the result of dividing by an odd number different than dividing by an even number?
3answers
184 views

### “Complex Matched Filter”

What does a complex matched filter mean? . A complex bandpass filter can be formed by multiplying a low pass filter by exp(jwt) getting an analytic filter (one sided in the frequency domain) which is ...
1answer
38 views

### How does a telephone change your voice? [closed]

Is it due to vocal length normalization during MFCC? Please be kind enough to explain.
1answer
431 views

### Symbol timing synchronization using a high sampling rate

Symbol timing synchronization usually relies on a digital PLL procedure, an iterative process that relies on detecting a timing error signal that is used to control an interpolation filter that in its ...
1answer
59 views

### What does it mean that the normalized power of a received equals one?

I have read in that lecture, page 2, here that, the symbols are independent, and normalized to have power 1, which means $E(x x^H) = I$ .. what does that mean? could you please explain that for me?
0answers
162 views

### Dynamic loop bandwidth for a Costas loop

I'm using a Costas loop for carrier recovery. The BER vs SNR (from GNU Radio Simulation) curve for different loop bandwidths (0.03,0.05 and 0.07) is shown in the graph below. It can be seen that the ...
2answers
223 views

### Carrier Frequency Offset, Rotating Constellation and Correlator

I have been further reading up on carrier recovery methods and something is not clear to me: It's written that a carrier frequency offset results in a rotating constellation diagram but if you are ...
1answer
18 views

### What does SDMA's space actually mean?

What does SDMA's space mean? I mean,T for TDMA is time,and unity of time is second. F for FDMA is frequency,and unity of frequency is Hz.Now i know the S for SDMA is space or spatial,but i don't ...
1answer
65 views

### Phase versus time graph of QPSK

In wikipedia, there was an example on QPSK modulation where the main signal was 11000110 and I contained 1001 and Q contained <...
1answer
2k views

### What is the difference between Antenna port and Physical Antenna in LTE?

I have just started working on the 5G NR physical layer and I have difficulty understanding the concept of Antenna Port at the downlink. In release 10 there supposedly 15-22 antenna ports for CSIRS (...
1answer
58 views

### Frame acquisition vs symbol timing synchronization

For a known length of frames and using a known sequence in a preamble that is also of known length can auto correlation at the receiver be enough to acquire timing or is timing synchronization at ...
1answer
964 views

### Generate Complex White Gaussian Noise in MATLAB

n_3=sqrt(0.1)*randn(1,K); n_4=sqrt(0.1)*randn(1,K); beta_NLoS=(n_3+1i*n_4); % CN(0,0.1) I want to create a $CN(0,0.1)$,does my code have any problem?
2answers
174 views

### Timing error detector used for symbol timing synchronization

The Gardner timing error detector is independent of carrier phase and frequency error, and is also quite immune to self noise (to my understanding, I may wrong about that). Can one assume that it is a ...
2answers
73 views

### BPSK signal that has not been pulse shaped by any filter

Can one consider that a BPSK signal that has not gone through any pulse shaping filter to be equivalent to a signal that has gone through a rectangular filter of which the pass BW is equal to the ...
2answers
354 views

### Intersymbol interference due to limited channel bandwidth

What is the difference between limited-bandwidth induced intersymbol interference (ISI) and ISI due to multipath propagation. I noticed that the delay spread in indoor environment is in the range of ...
1answer
1k views

### Maximum Likelihood Detection of Signal Vectors in Gaussian Noise

Consider a binary-input additive white Gaussian noise channel. Let $\mathbf{x}_0 = (\sqrt{Es},\sqrt{E_s},⋯,\sqrt{E_s})$ and $\mathbf{x}_1 = (-\sqrt{E_s},-\sqrt{E_s},⋯,-\sqrt{E_s})$, be two codewords ...
2answers
113 views

### Signal bandwidth of an 802.15.4 signal at 2.4 GHz

This signal uses Offset QPSK with a half sine pulse shaping filter for the purpose of getting a constant signal envelop. Does anyone know the exact bandwidth of this signal after being pulse shaped, ...
4answers
72 views

### Bandlimiting and OFDM spectrum - what is considered “bandlimited”?

I have been reading about OFDM modulation, it's not clear to me how you can have band-limited OFDM but given its prevalence in wireless communication there must be some designation of approximately ...
2answers
1k views

### What is a constant envelope modulation?

The Book "Digital Modulation Techniques" by Fuqin Xiong groups digital modulation schemes into two classes: "constant envelope" and "nonconstant envelope." However, I was surprised by the inclusion ...
1answer
194 views

### Recursive Least Square Adaptive Linear Equalizer

For the adaptive filter to work properly, a desired signal d(n) needs to be provided. The output from the equalizer y(n) is subtracted from d(n) to produce an error signal, which is used to adjust the ...
2answers
232 views

### Software PLL tracking of carrier frequency in bandlimited transmission

My understanding is that a PLL(either in hardware or software) can recover the carrier frequency(or be used to correct the frequency offset) of a system. My question is how does this work when the ...
1answer
31 views

### Is there just a carrier in each frequency band in FDMA?

The figure is the FDMA,and i know that FDMA is that dividing the bandwidth into many blocks(frequency band),i want to ask Is there just a carrier in each frequency band in FDMA?i mean one carrier for ...
2answers
233 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....
0answers
184 views

### Blind equalization with constant modulus algorithm in GNU Radio

I'm currently simulating the effect of multitap channel on BPSK as well as its remedy i.e. equalization. The modem is expected to work with satellite TT&C transponder. The communication channel ...
1answer
79 views

### IQ generation separation

To generate I & Q samples from data samples S I = S(1:2:length(S)-1) * cos(2*pi*f*t) ; Q = S(2:2:length(S) ) * sin(2*pi*f*t) ; I & Q to integer ...
2answers
685 views

### Preamble Detection and Frequency Offset Estimation in radio packets

I have generated a radio packet of data with a preamble header of 0x55555555 and crc32 for the data only. The entire packet was separated in even and odd streams for I and Q. The I and Q were ...
1answer
59 views

### Doppler compensation method for cdma

Do you know of effective and simple methods to compensate for Doppler due to movement for the CDMA protocol?
2answers
70 views

### The relation of sub-carrier and FDMA

In Mengyu Liu, Yuan Liu, "Charge-then-Forward: Wireless Powered Communication for Multiuser Relay Networks", it is said that while information signals are conveyed by using FDMA over $N$ sub-...
0answers
107 views

### Why is channel estimation done after synchronization and why does it work?

I am interested in Software Defined Radio communications where almost all processing is pushed to the digital domain. I'm wondering why is channel estimation done after frequency and phase ...
5answers
112 views

### How can telecommunication system know which part is signal and which is noise? [closed]

When signal is received how can receiver know which part of received signal is signal and which part is noise??
0answers
27 views

### Question about direct spread spectrum in 802.15.4 at 2.4 GHz

I read the 802.15.4 standard and something is really confusing me: How is spreading achieved in 802.15.4? is it by multiplying, encoding or modulating each data symbol (4 bits) with a certain code ...
1answer
402 views

### What is the noise whitening process?

I came across this process when working on OFDM in wireless communication. It's mentioned if we have two copies of a signal resulted from SIMO system (Single Rx Multiple Tx), we can process the noise ...
1answer
202 views

### Digital Downconversion of Signal

DSP-Newbie here... Why are we using the ratio of the center frequency of the signal to the sampling frequency of the signal spectrum when we downconvert the signal? I have an Spectrum with Carriers ...
1answer
125 views

### PN code generation

Can we generate PN codes (maximal length sequence) using this technique in Matlab where L is the PN length? pn=2*(rand(1,L)>0.5)-1 If yes, why?
1answer
67 views

### Phase tracking in communication receivers [duplicate]

Why does Costa's loop PLL bandwidth need to be narrow? Text books mention that and yet give no explanation
1answer
59 views

1answer
102 views

### methods of crest factor minimization

i am studying the methodes CF minimization. and have a doubt. in each methods there are steps with DFT and IDFT or like this FFT and IFFT. Everyone provides the fact, that they use DFT and IDFT for ...