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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cyclic prefix in ofdm

In ofdm systems after applying IDFT we take the last N/x samples and place them before these N samples. This is called cyclic prefix(CP).So why to use a cyclic prefix? Why is the CP known to combat ...
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330 views

comparison between frequency offset estimators

I have been working frequency offset estimation in OFDM. The objective was to compare different frequency offset estimation techniques. By using MATLAB, I have simulated three different estimation ...
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0answers
129 views

TDE, OFDE and FDE equalizer simplest exmple in matlab

I want to implement the equalizer in matlab. I am just learning this. I want a good kick start code that can help me to learn it further. I searched a lot on Google but do not find a good reference ...
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1answer
43 views

Capacity computation in Costa sheme

I can not understand the following sentences, could you please anyone guide me ? "Comparing with the embedding in audio signals, the usable bandwidth is greatly reduced, from 22.05 kHz to 4 kHz. ...
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1answer
820 views

SNR relation with modulation index and noise power

What is the relation between the AM modulation index and the SNR? (I plotted this and it came as the shape of the exponential graph) Also what is the relation between the channel noise power and the ...
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2answers
1k views

Using Pilot tones to estimate carrier frequency offset in ofdm

I have been studying carrier frequency offset estimation techniques in OFDM systems. I have understood several of them. when using Pilot tones, as stated in paper (1), the process involves two methods ...
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1answer
455 views

Is zero forcing Optimization technique or Equalization technique?

I got some problem in the equalization techniques. I am new to communication simulations but I have done some data analysis labs in my college. I read some where on Internet that in a research paper ...
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95 views

Best strategy for sending digital signal over ultrasound

I'm interested in sending digital data over ultrasound. My initial thought is that it should be first converted to BMC: And then phase shift encoded (PSK): I initially thought to use a Goertzel ...
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1answer
900 views

QAM for analog and digital waves [closed]

I tried the following code for modulation of two analog waves. Can anyone point to how to modulate and demodulate for an analog and a digital wave? I tried by replacing the second signal (x2) with a ...
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792 views

How to display the output as the input with a fractional delay? [closed]

I am trying to introduce a fractional delay in my input vector x[n] so the output is something like y[n]=x[n-M/N]. I am not supposed to use the MATLAB fracdelay function.So, I used sampling and delay. ...
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1answer
59 views

Information to be transmitted being a stochastic process?

I am reading Digital Communication, 3rd edition by Barry & Lee, Book's Webpage: enter link description here This is an excerpt taken from this book (mentioned above) from Chapter 3, Page # 57, ...
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1answer
420 views

Carrier frequency offset (CFO) estimation based on Symbols

In (1), it's said that: If two identical training symbols are transmitted consecutively, the corresponding signals with CFO of $\epsilon$ are related with each other as follows: $y_2[n]=y_1[n]\exp(...
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51 views

How does µlaw encoding work with 16-bit signals?

I am new to DSP, so I wan't to know a couple of things. How does µlaw encoding function on a log scale such that you can input 16-bit signals and get the 8-bit µlaw signal output, after compression, ...
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2answers
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Doppler effect on Baud Rate

I'm having a hard time wrapping my head around doppler and how it affects a digital communications signal's baud rate. The examples that I've seen explain that the center frequency of a signal is ...
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2answers
95 views

On computing the number of independent values from a signal's bandwidth?

Can anyone help me understand where the value of $2$ comes from in the following statement? ...
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1answer
10k views

QPSK Modulation/Demodulation in C++

I'm trying to implement a QPSK Mod/DeMod system in C++. So far i have a bit generator that generates N bits and stores it into an array: ...
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1answer
123 views

State-of-art of telecommunications? [closed]

There seems to be a number of different ways for telecommunication. What is the most recent (and maybe standard) technique used today? It seems the simple AM/FM modulations are definitely somewhere in ...
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1answer
1k views

How to compute the free distance in TCM (Trellis-Coded Modulation)?

I would like to be able to compute the so-called "free distance" for a TCM with a rate 2/3 convolutional code (two inputs, three outputs) inside that I plan to use for 8PSK. My previous assumption was ...
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1answer
292 views

How is GPS used to synchronize oscillators in a distributed system?

I heard that it is important to have all oscillator clocks at different places be synchronized for example in a geographically distributed system such a telecommunication network. Of course to some ...
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1answer
470 views

WIDELY UNKNOWN?? Wireless Channel Transfer Function or freq response - frequency vs attenuation

We know that the signal attenuates out with distance and according to the channel transfer function or frequency response, the signal frequency components attenuate to different values based on ...
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196 views

Line coding for a modulation order that is not power of 2

In case a symbol encodes a non-integer number of bits, how is done the bit-symbol pairing? For example, we have a 3-levels FSK with bit rate of 1Mbps and a sequence 01011001. What is the symbol ...
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2answers
787 views

16 psk demodulation

I have to implement a complete receiver in digital communications. The transmitter gives me an output vector which has real and imaginary values. It uses 16psk for modulation. How would I ...
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0answers
196 views

PPM Variant - Analytic symbol error probability

I'm trying to calculate symbol error probability of variant of pulse position modulation considering only AWGN channel. The decoder is maximum likelihood so the implementation of the detector is by ...
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1answer
785 views

Using Doppler velocimetry with digital signals such as Bluetooth

I have found several sources describing how to use Doppler velocimetry/anemometry to measure the Doppler shift with analog signals, but i cannot find much about using this method for digital signals ...
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2answers
228 views

Entropy Of A Symbol

Does the entropy of a symbol represent the most optimal average number of bits that can be used to represent a symbol? For example take the example of tossing a coin: $$ H_{source} = -p_{head} ...
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1answer
150 views

Digital Communication System [closed]

How can we determine whether the receive filter is optimal or not in a digital receiver? For a decision region, how can we choose a noise value zk?
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2answers
360 views

Filter design for frequencies in range 0-60Mhz

First of all excuse me for asking a seemingly vague question. But i am going nowhere in the problem i am trying to solve. I am working on a powerline communication system where i need to upconvert ...
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1answer
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Bit scrambling as last state of transmitter - what advantages are there?

I believe that I read, for digital modulation with multiple levels, that the AGC makes use of the fact that the symbol sequence, before noise etc., can be modeled as a uniform distribution. $$\text{...
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1answer
4k views

Power Spectral Density and Fast Fourier Transform basic question

I am new to signal processing and I am trying to get the basic concepts right. I have a signal and I performed the FFT of the signal in MATLAB. I also did the ...
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1answer
7k views

What does the one-tap equalizer used with OFDM accomplish?

I've read that OFDM normally uses a one-tap equalizer. It seems that a one-tap equalizer could only scale and delay a signal. Why is this useful? I thought multipath was handled through the cyclic-...
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1answer
657 views

Barker Sequence for QPSK

I'm trying to implement a QPSK burst modulation and demodulation through a channel and I'm now considering the synchronization part. I've read that Barker sequence were excellent option for this as ...
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1answer
189 views

Rician factor effect

The Rician factor is the ratio between the mean power of the LOS path to the mean power of all other (non-LOS) paths. Increasing/decreasing this factor means increasing/decreasing the mean power of ...
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2answers
8k views

Is there a closed form for the average energy of a QAM constellation?

I'm trying to come up with a formula for the average energy of the signals in a M-QAM constellation. If $b$ is the number of bits represented by each point in the constellation, such that $M=2^b$. For ...
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1answer
888 views

Bandwidth of a communication system

As a rule of thumb, the bandwidth of a communication system with a complex signal must be equal to the bit-rate or symbol-rate? Besides, after reading this post, it is not clear for me how a band-...
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1answer
682 views

Manchester decoding

Is there any method of decoding a Manchester encoded signal (IEEE 802.3) that is independent of the bit period? Suppose the data I want to send is $\rm 10110$. The Manchester code of this data will ...
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1answer
146 views

Laser diode bitrate datasheet

I'm a student studying communications engineering. I'm trying to understand why this particular laser diode is made for 155 Mbps and 622 Mbps. I'm thinking that the bitrate is entirely dependent ...
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2answers
1k views

Bandpass filter BPSK

I have a question concerning BPSK and the bandpass filter that seems to be sometimes placed before the receiver circuit (the receiver is a mixer and lowpass filter). Basically, from what I've noticed,...
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6answers
52k views

Understanding the Matched Filter

I have a question about matched filtering. Does the matched filter maximise the SNR at the moment of decision only? As far as I understand, if you put, say, NRZ through a matched filter, the SNR ...
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1answer
197 views

What are the options for turning a bit stream into a byte stream?

I am creating a system (modulator and demodulator) for broadcasting binary data through an analog signal. I'm using BPSK with great success – at least, I'm generating great graphs. Green is input ...
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1answer
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OFDM training symbol format

For OFDM synchronisation, we are using two training symbols according to the Schmidl and Cox method of frequency synchronization and timing synchronization. The paper says: The first OFDM training ...
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1answer
617 views

Appropriate sampling frequency for bpsk modulated signal

I'm modulating a message with particular baudrate, say 2MHz using BPSK , Carrier frequency(Fc) is 8MHz, What should be appropriate Sampling Frequency(Fs) following Nyquist criteria, Should it be ...
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2answers
5k views

Need for Oversampling in OFDM and symbol time determination

I am trying to implement OFDM on gnuradio. I have two questions. The sampling frequency which is slightly higher than available bandwidth satisfies Nyqvist criterion for complex baseband signals. So ...
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3answers
567 views

OFDM coarse freq estimation

I am trying to read OFDM synchronization paper by Schmidl and Cox. The math is slightly overwhelming. What is the formula that they use in implementation that distinguishes between coarse estimation ...
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1answer
5k views

Determining sampling rate in OFDM demodulation

I have seen that in OFDM, the sampling rate is much lesser than twice the bandwidth of signal. why is it so? How is sampling rate calculated in OFDM? Does it depend on the modulation type used inside ...
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1answer
331 views

Find the time lag for a wave

suppose that we have a wave which has some peaks in some indexes like 2,12,25,... and otherwise zero. I want to extract the time lag or lags between peaks of a signal. could anyone help me ?
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2answers
457 views

Fixed point channel model for OFDM system

I'm testing an OFDM system implemented in fixed point. The data format is Q11. My system work's fine but I need to test it under some channel for evaluation of the design before field testing. The ...
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1answer
459 views

Remove spurious peaks of Hilbert transform

I want to eliminate spurious peaks of Hilbert transform for finding glottal closure in the linear prediction (LP) residual. I have the following 4 steps: Down-sample. Hilbert Transform. Identify ...
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1answer
4k views

Apply AWGN noise to QPSK-OFDM symbol

I'm working on an OFDM-QPSK based system and I'm having some trouble to match the BER of my system with the theoretical curve. My OFDM implementation fills all the subcarriers and have t_symbol s ...
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2answers
8k views

How to normalize the power of a QAM signal?

If I have, for example a 16-QAM signal, and the constellation points are (3,3),(3,1),(1,3),(1,1),...(-3,-3). Now I have a simulated in coming signal with arbitrary ...
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3answers
6k views

How to find the group delay of IIR filters in Python/SciPy

I sent a square wave signal (red) through a IIR filter (butterworth in this case), to make it more "realistic". Then there's a slight group delay in my output signal (blue). Is there a way to find out ...