New answers tagged digital-communications
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With these parameters, Can I define a length of the channel
There's now $n_{tx} \times n_{rx}$ (or, depending on how you consider the this, $n_{rx}$), with potentially different lengths, not just one channel!
The only thing that makes sense to define as "the channel length" is the maximum length among these many channels.
However:
The fact that you're ...
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However, when I add an AWGN channel, I get pretty low BER of 10^-5 in passband as compared to around 10^-4 BER in baseband under same AWGN channel of SNR 10 dB. Why is it happening?
You've got a bug in either passband channel or baseband channel model, or in how you transform your noise or your signal – end of story!
I thought that there would be more ...
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Range is the minimum to the maximum value of something. Since bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies, then the range of bandwidth of 15 to 20 KHz would then mean that the minimum difference (bandwidth) would be 15 KHz and the maximum bandwidth would be 20 KHz. If the saying was instead that the ...
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For a SISO channel, assuming you have estimated channel coefficients for each sub-carrier $k$, the MMSE equalizer is
$$
\hat{h}_k=\frac{h_k^*}{|h_k|^2 + \frac{\sigma_x^2}{N_0}}
$$
So you can see already there is a multiplication in the numerator ($h_k^*$), and then there is a division by a term in the denominator. So for all $N$ subcarrier, this itself will ...
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Peak to Average Power Ratio (PAPR) is defined as:
$$
\text{PAPR} = \frac{\text{Peak Power}}{\text{Average Power}}
$$
If you're working with a $M$-PSK constellation, then the peak power is $P_s$ and the average power is also $P_s$ assuming each symbol is equally likely. For $M$-PSK, the $\text{PAPR}=1$.
If you working a $M$-QAM constellation, then the peak ...
1
OFDM subcarriers are packed relatively tightly together. If you look at the original OFDM signal in the frequency domain, you may wonder why adjacent subcarriers are not interfering with each other. The answer is that subcarriers are orthogonal to each other. Even adjacent subcarriers, have 0 influence on each other, and are independent in that sense.
It ...
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The correlation you're speaking of is in the receiver, not between the subcarriers formed at the transmitter!
The "O" in OFDM says that one carrier should have 0 influence on the others. So, if you see a correlation between subcarriers in the receiver that hasn't been there in the data modulated onto the subcarriers, then you're violating the "O" in OFDM.
...
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BFSK is done by selecting between two frequencies for each symbol (Binary Frequency Shift Keying). What the OP is showing is samples from M-PSK (Phase Shift Keying); selecting between M phases for each symbol (which would result in two distinct dots on the IQ constellation). Here the two symbols selected are phases 0° and 90°, so these could be 2 of the 4 ...
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You can get help best if you can share what kind of signal do you have. For example, in scenarios where you have a multipath channel time domain response or 'channel impulse response', you can use mdeian of median of windows as your noise floor.
Because 'channel impulse response' will have multiple narrow peaks in time domain corresponding to each path, ...
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It is most convenient for implementation of the IFFT and FFT, that the number of subcarriers is a power of 2. However, it is not necessary.
Even in practical systems, it may not be a power of 2. In LTE, for example, there are 6 allowed bandwidths, ranging from 1.4 MHz to 20 MHz. For the 15 MHz deployment bandwidth, the number of subcarriers is 1536. See ...
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A likely source of the amplitude imbalance given it is on the actual signal is filtering distortion along the signal chain such that the gain is not flat across the bandwidth of the spectrum. This would not be uncommon with analog filtering and can be compensated for with equalization (if needed).
For an alternate approach to carrier tracking applicable to ...
1
No, $H_{rx,tx}$ is the channel matrix with $(r,t)^{th}$ element representing channels attenuation and phase rotation on transmit symbols $\vec{x}_{tx}$, from $t^{th}$ transmit antenna to $r^{th}$ receive antenna. The equation is neither in time-domain nor in frequency domain. It is on transmit symbols. For example, in OFDM systems, you get $y_{rx}$ (received ...
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Are my assumptions correct?
no, sorry.
(BW) It is the amount of data that can be transferred simultaneously.just like more no. of highway lanes so that many cars can travel side by side simultaneously.
We need to talk about different meanings of the word "Bandwidth".
In networks technology, that often means "number of bits per second that this system ...
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Taking absolute value will certainly eliminate the phase component but it depends on what kind of modulation you use. The cost of it will be huge for modulation schemes which depend on phase of signal (BPSK/QPSK/QAM etc).
For example for QPSK, suppose your transmitted baseband signal is $1+j1$, that is $\phi=\pi/4$. Due to CFO phase error of more than $\pm \...
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While reading the above few answers, it’s been stated like negative frequency has no physical significance, but it is completely wrong.
Fourier transform is used to pull out - that component of a signal (under test)which is having a particular repetition frequency( or rotation frequency in that sense).
If you agree to the fact that complex exponential ...
1
That is a very poorly written article IMO.
In simple rule of thumb terms:
For real valued tones, there isn't a qualitative distinction between negative and positive frequencies, so there is no point in using negative frequencies. Choosing not to use them by convention is quite different than they don't exist.
For complex valued tones, the negative and ...
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In the FT "negative frequencies" means nothing physically, however they are essential in the maths behind the FT.
The FT is nothing more than a change of base.
If we take an example with a door function p(t),
\begin{equation}
p(t)
\begin{cases}
1 & \text{ if } -\frac{1}{2}\leq t\leq\frac{1}{2} \\
0 & \text{ elsewhere }
\end{cases}
\end{equation}
...
1
I think you may be confused about the use of negative frequencies and the sign of the integration coefficient.
Let's take a look at the inverse Fourier Transform
$$ x(t) = \int_{-\infty}^{+\infty}F(\omega)\cdot e^{ 2 \pi i \omega} d \omega $$
That basically means that you can construct any time domain signal as the from a set of complex exponential. So ...
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The role of the modulator in your block diagram is to make an analog signal, while in classical communications, the data and the channel coder is discrete. So no, in the sense of your block diagram, that can't work.
Note that you stated trouble because your signal wasn't binary in your approach: Not all channel coders work on binary symbols. For example, ...
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Consider some actual use cases for further insight:
Unmodulated Carrier: The peak to average ratio is 3 dB given the relationship between the rms and peak level of a sine wave.
Unfiltered Single-Carrier BPSK or QPSK: This would be the modulation of rectangular shaped symbols, so would have a 3 dB peak to average ratio just as in the case of an unmodulated ...
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SC-FDMA is not actually a single carrier signal. SC-FDMA is sometimes also known as DFT-spread OFDM or DFT-precoded OFDM, which I think is a better name to characterize it. It is still a multi-carrier signal, but with the DFT applied before the OFDM transmission. The DFT spreading helps to reduce PAPR compared to a normal OFDM signal, but that doesn't really ...
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I don't think it would be reasonable to expect the phase to be exactly the same. The raised cosine filter for example has a linear phase response. Also don't forget that the auto-correlation for the cyclic prefix detection might find the frame starting at any of the 16 samples of the cyclic prefix, inducing an undetermined phase shift.
Do you have pilot sub-...
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The Symbol Synchronizer block is a PLL-based synchronizer that is trying to estimate the symbol clock period and symbol clock phase (aka timing offset) based on the samples coming in that represent the data symbols.
Being a PLL configured with static parameters, there is a fundamental trade off between acquisition speed and tracking stability of the symbol ...
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The relationship [G/T] = [G]-[T] dB/K is incorrect.
G/T when in units are already given in dB/K is simply the gain in dB divided by the temperature in K.
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Coarse and fine compensation are talking about first fixing the impairment using a rough estimate then again fix/fine-tune using a better estimate. One way to do this is by using the so called $M^{th}$ power estimator for PSK signals. The version I'm talking about will use the received symbols after timing synchronization is done. That is, the input ...
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Not necessary. In DRM (Digital Radio Mondaile) specification, for some channels number of careers are not the power of 2. Power of 2 makes the implementation of FFT and IFFT simpler.
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There is no best rule, it depends on what we know about the signal.
Energy Detector: The energy Detector makes a decision on presence or absence of a signal based on sum of squared samples. This rule comes from the fact that the signal to be detected is inherently assumed to be random following a wide sense stationary process with a known PDF, the ...
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To compensate for a frequency offset first it needs to be measured, let us take the example of PSK.
At the trasnmitter : Suppose the signal is upconverted at the transmitter to $F_c$, then the received signal is given by $$s(t)e^{j2\pi F_ct}$$ here $s(t)$ is the baseband PSK signal.
At the Receiver: Suppose the received signal has a frequency offset, ...
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I assume this would be as follows:
s*exp(j*2*pi*f*n)
Where:
n: sample index
s: baseband signal, magnitude and phase for each sample
f: carrier frequency in cycles/sample
Note how the carrier as a discrete signal is given in cycles per sample. This is the normalized frequency and is related to the desired frequency $F$ in Hz by dividing by the ...
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From the plot it seems the transmitted QAM points are $(0.5, 0.5),(-0.5,0.5),(-0.5,-0.5),(0.5,-0.5)$, and if you can model your comm system as just AWGN channel, then your signal amplitude will just be $\sqrt{0.25+0.25} = \sqrt{0.5}$ and hence signal power will be $0.5$. In doing so, I have assumed all 4 QAM symbols equiprobable.
You can get your noise ...
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Implementing an IFFT and FFT engine which is a power of $2$ is easier in hardware and hence if an OFDM system is talked about, it is talked about in $2^k$ length FFT. All practical Communication system based on OFDM use $2^k$ length FFT-IFFT engines.
However, for OFDM in principle, it is not required to have FFT-IFFT in power of $2$.
There are different ...
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First value is from cyclic prefix and last value is the last sub-carrier which you are messing up by windowing of 1 sample. As a result of this, the frequency orthogonality of the sub-carriers is lost. And this inter-carrier interference is causing your QAM sliced symbols to randomly change phase and amplitude around true value.
The way actually windowing ...
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(See the helpful comments below by @AndyWalls who wrote the timing recovery block for GNU Radio)
The loop bandwidth parameters do not sound right. Consider that once the loop has converged to a linear operation the settling time is proportional to the inverse of the bandwidth (for a first order loop the 10% to 90% settling is $0.35/BW$). Acquisition is ...
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I can answer for adaptive filter containing feedback path alone as I have implemented it recently (both Blind DFE and with Reference Symbols), but it should work for DFE containing feed-forward path as well. In Blind DFE while computing the error, $e[n] = x[n] - \hat{x}[n]$, where $x[n]$ is the equalizer output, $\hat{x}[n]$ is the corresponding Hard ...
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