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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Matched filter for recieved RF digital packet by LPF

Im transmitting by cc1350 launchpad microprocessor digital RF Packets, transmitted modulation FSK. bit rate of transmitting is 50Kbouad, frequency of transmitting is 868MHZ . Im sampling the signal(...
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Help in designing filter for DUC (Digital Up-Converter) and DDC (Digital Down-Converter) using the built-in DUC and DDC blocks in MATLAB Simulink

Can anyone please help me in simulating a simple BPSK transceiver system that correctly receives a signal through a DUC and DDC? I have attached the screenshot of my Simulink system along with ...
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Residual carrier in PCM/PM/NRZ

As mentioned in the (Residual carrier tracking with PLL), residual carrier is created after PM modulation. I tried to do yhat in matlab softawre as: NRZdata: -1 1 -1 1 1 -1 -1 ,.... theta = 0.5 PM ...
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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 ...
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Doopler spread and delay spread in an equation of channel freq response

Assumed the transmission system with channel impulse response is determined by the matrices $H(\tau_l) \in C^{N_R \times N_T}$ ( $N_R$ and $N_T$ are numbers of received and transmit antennas) By ...
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BPSK-PM Modulation

As you know, in the Binary shift keying modulation(BPSK) with the change of phase, we can modulate signal, but recently in many resource(Residual carrier tracking with PLL) I see that after BPSK ...
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88 views

Timing recovery lock in receiver

I have done my receiver design with QPSK modulation. I am trying to communication between two systems at L band. My data rate is 1 Mbps. As my data rate is high I have allocated 64 bit for timing ...
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transmitting the real/imaginary parts of pass-band signal

As I know, the transmitted pass-band signal whose carrier frequency is $f_c$ can be written as follow: $s_1(t) = \text{Re}(a(t)e^{j2πf_ct})$ , where $a(t)$ is the complex modulated base-band signal. ...
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25 views

BW of a modulated signal

I come across different types of bandwidth. In digital modulation should it be null to null always? when do we use 3 db bandwidth?
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28 views

autocorrelation peak

Let $s(t)$ a signal of duration $T_s$ on a bandwidth $B$. The matched filtering of this signal gives: $R=max|s(t) \ast s^{\ast}(-t)|$ and here theoretically I saw that $R=BT_s$. But does anyone know ...
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201 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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How many bps would be required to reproduce the words in speech recognizably?

In Robert Gallager's lecture notes for 6.450 Principles of Digital Communications I, Exercise 2.1 asks an interesting conceptual question: Voice waveforms could be converted to binary data by ...
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continuous chain of ones or zeros for transmit

If my data has long chain of 1's or 0's. Can the data be modulated and transmitted as it is. Will this have any effect of the receiver adc as this may increase dc value of the signal or any other ...
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Relation between ICI and EVM in OFDM System

in an OFDM system with ICI, how can I calculate the ICI power using the error vector magnitude (EVM) constellation? thanks
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differential modulation and reception

I use a differential modulation with a SIMO system. At the reception, I have $2$ antennas.To find my symbol, I use the relation $r^{\ast}_i(n-1)r_i(n)$ to have the phase of the symbol with $r_i(n)=\...
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62 views

noise floor of attenuator

NF of attenuator is same as the attenuation value as the noise floor remains same but signal level gets attenuated? But NF of amplifier depends on the device as it increases both signal and noise?
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Bandwidth of a modulated signal in analog domain

If i am using QPSK modulation with data rate 4 Mbps with rolloff factor $0.5$, then the bandwidth will be symbol $\rm rate\times (1 + rolloff\ factor)$. So here it will be $\rm 2\ MHz\times 1.5 = 3\ ...
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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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223 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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62 views

Differential Encoding for M-PSK

Confused about a few topics w.r.t. differential encoding/decoding. First, consider the below BER curve taken from wikipedia ( https://en.wikipedia.org/wiki/Phase-shift_keying ) comparing BPSK/QPSK and ...
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27 views

What is the advantage of multiple antenna in receiver in energy harvesting ?harvested more power?

What is the advantage of multiple antenna in receiver in energy harvesting ? Assume there are $3$ transmitted antennas in the A device,and $1$ received antenna in the B device,now assume A device use ...
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27 views

Capacity and BER as performance of transmission system

We study the performance of the transmission system by channel capacity or BER/SER. If the system performance is studied as BER/SER, does it make sense to study a channel capacity?
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Resource block size in freq-selective channel

I asked about the basic of a frequency selective channel [here][1]. It was discussed Resource block (adjacent subcarriers and time indices together into "chunks") what parameters affect the ...
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“How is the noise floor of RF section is mapped to ADC noise floor for a 10MHz BW?”

My interpretation is if a gain of 40 is used before AGC -40 dBm become 0 dBm and -121dBm (noise level) becomes -81dBm SNR remain same (assuming NF =0 for theretical purposes). Noise floor of ADC is (6....
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21 views

Interference Distribution Assumption in NOMA IoT Scenario

I am assuming a NOMA IoT Uplink scenario, which is interference marred. I have to ascertain the SINR at the Access Point (AP) for the NOMA users. A sub-channel is allocated to various users and ...
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Do we have to insert the DC null in OFDM when using RF direct sampling ADC architecture?

Generally, DC Nulls are added in OFDM because zero-IF receivers don't pass the DC component. But what if I use an RF direct sampling architecture? Will there be DC offset? If not, then is it okay not ...
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123 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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Legacy LongRange Mode information (mode 1 in the my attached link)

Mode 1- Legacy Long Range Packet Format and Im attaching a photo of ti reference: link for more information about this mode category : TI SWRA642 I want to understand the packet format, what's my ...
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470 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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52 views

Low pass equivalent of bandpass white noise

Let $$n(t) = Re\{z(t)e^{j2\pi f_ct}\},\ $$ where $z(t)$ is the low pass equivalent of white noise $n(t)$. We know the autocorrelation of $n(t)$ is $$R_n(\tau) = \frac{N_0}{2} \delta(\tau),\ $$ $\delta(...
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35 views

A communication system

I am solving the following problem as an exercise. I have a signal $$z(t) = x(t)\cos(wt) + y(t)\cos(ut)$$ and we assume $x,y$ to be band-limited continuous time signals. In the frequency domain we ...
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What are the advantages of barker sequence over marker sequnce?

I read that barker sequence is used for synchronization among other uses. Barker sequences are rare and they have special properties - this part I understood. I am missing some thing when I think ...
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How do we know the receiver delay in an OFDM transceiver system?

I was simulating a simple OFDM transceiver system without a channel (just the transmitter and the receiver directly connected). I saw that if I use the "Find Delay" block in Simulink between the ...
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Types of equalizers

I can't get my mind around the different types of equalizers and the difference between some of them. I've been looking at different books unsuccessfully. So far, I have Wikipedia's list and this ...
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40 views

system bit rate vs channel coding

We are usually asked about the full system bit rate. However, we use channel encoding in practical systems, for example conventional coding with rate $(1/3)$. My question is about the full system bit ...
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23 views

channel impulse and frequency response: H?

The received signal of MIMO system is given as: $$ y= H \cdot x +n, (*)$$ where $H$ is a channel matrix. $h(\tau_l)$ is a channel impulse response. It is determined as a matrix $\in \mathbb{C}^{Nr \...
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57 views

Digital AGC signal level

Analog AGC is mainly used to increase the signal level to match the ADC sensitivity? Is Analog AGC also used to attenuate the signal?. Digital AGC is the first block used after ADC. Is the main ...
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How to measure EVM?

I am trying to understand the concept of EVM and how it is measured at different points in transceiver. I have a basic understanding that the error difference is calculated between two vectors (i.e. ...
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79 views

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 ...
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37 views

Difference: Ergodic channel capacity and spectral efficiency

Can someone explain what is the difference between the capacity and ergodic capacity? Ergodic capacity and spectral efficiency?
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38 views

How to prove a train of sinc pulses in digital communicaton system are orthogonal to each other?

Consider a train of sinc pulses: $$\phi_n(t)= \frac{\sin(\omega_M(t-nT_s))}{\omega_M(t-nT_s)}\quad; n=0,\pm1,\pm2,\dots$$ $\quad$where,$\quad T_s=\frac{\pi}{\omega_M}$ Now ,in order to show sinc ...
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52 views

Variations in MATLAB plot each time the program is executed

Every time I run the program my PLOT in MATLAB gets changed to some extent (some times to larger extent too). I understood that the channel is random and this might happen due to random distributions ...
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52 views

Signal bandwidth and channel bandwidth

I have a channel bandwidth $B_c$ and a signal bandwidth $B_s$ with $B_s<<B_c$. What is the problem when the signal bandwidth is smaller than the channel bandwidth? For example, my channel ...
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74 views

RRC filter order

I have a question on rrc filter. Hope somebody can answer it. Tx path: My symbol rate is 1 mbps. I am using rrc 8x which becomes 8mpbs. For four symbols the order of the filter becomes 32. Next ...
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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 ...
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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: <...
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81 views

How to compand an OFDM signal?

Im trying to run a simulation of an OFDM system with convolutional coding and u-law compander on MATLAB. The problem is I dont know how to compand the OFDM signal. Do I need to do something to the ...
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72 views

farrow resampler in timing recovery

My receiver contains: AGC,timing recovery,carrier recovey blocks. I am using gardner timing recovery with loop filter and farrow parabolic filter. The input to the farrow filter is the fractional ...
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51 views

Low data rate with high frequency offset

I am trying to implement a basic digital modulation scheme (BPSK) with a low data rate of 5 kbps. The requirement is to support a frequency offset of +/- 10 KHz. I would like to know what are the ...
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27 views

What is the unit-power information signal?

I am learning signal processing for my subject and have some problems with the unit-power information signal. I refer to this paper: https://arxiv.org/pdf/1906.03949.pdf In this paper, they formulate ...

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