Questions tagged [ofdm]

OFDM stands for Orthogonal, Frequency-Division Multiplexing.

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Channel state information from non-HT packets: is that possible?

I read several papers about CSI data, but one thing is not clear. I would like to extract csi from legacy rate packets. I read some forums related to existing CSI tools and they only state that CSI ...
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Understanding the concept of semi-blind receivers

In the book "Signals and Images: Advances and Results in Speech, Estimation, Compression... ", an overview of using tensor analysis in signal processing is given. On page 243 it is explained that an ...
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Can you explain me in a easy way what the use of Cyclic Prefix in OFDM is? [duplicate]

I cannot understand why cyclic prefix is used. I know that it mitigates the effect of ICI but I can't see how. Can you explain please? Formulas and mathematics is well accepted. Thanks. I read this ...
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51 views

Explanation a flat fading channel in case MIMO OFDM

I am trying to understand flat fading and frequency selective channels. As I understood, in the case of flat fading channels MIMO channel elements, $h_{i,j}$, are random variables. Which assumption ...
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Derive the LFSR sequence from demodulated bitstream

I'm new here and hope to have good feedback from you. I'm trying to blindly demodulate an RF signal and managed to get a sequence of bits for a preamble. After some research, I'm in fact exactly in ...
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qam demapper why are soft-outputs used?

I'm starting to learn about OFDM QAM demapping, and a lot of documentation points to using "soft-output" algorithms that calculate Log-Likelihood Ratios (LLRs). Why is this method advantageous? My ...
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24 views

what should the the output of an FIR filter (root raised cosine) look like?

I am implementing a filter that is FIR on verilog HDL ,, I understand what happen in the filter well, I mean the delay, multiply and sum, the problem that I can't imagine how can I get the desired ...
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87 views

How is DJI OcuSync wireless link physical layer implemented?

I'm looking for documentation on the wireless command link of the DJI OcuSync. I found this website (in Chinese). It seems like an OFDM link that uses Alamouti codes? I'm specifically interested in ...
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How to relate power spectral density with the number of resource blocks in LTE?

I am running a simulation with 100 Resource blocks for LTE OFDM and I plotted a PSD for it. Now I am able to bring down the out of band emissions when I perform an algorithm on it. Based on the ...
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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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11 views

getting channel matrix from rayleighch matlab object

If I use "rayleighchan" of the communication toolbox, and I set the Doppler frequency, number of paths, path gain, etc . How can I extract the channel response to use it in MMSE channel estimation?
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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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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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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 ...
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Calculating the Channel state information for localization

I want to build a localization system using 4G LTE for a project. Intuitively, I can get the position measuring the time a signal leaves the Base Station to the User Equipment. Knowing that these ...
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55 views

OFDM modulator: Transmission time calculation

I have a problem with a certain task. The frequency of the signal sampling at the OFDM modulator output is 20 MHz. The length of the protection period is 2 microseconds and it is 20% of the ...
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70 views

In OFDM, what is the length of the cyclic suffix that is added before windowing?

I searched for length of cyclic suffix but all I can find information about is the cyclic prefix, I want to know also if there is short guard interval and long guard interval does it affect the length ...
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37 views

Why do we need two Long training sequences for Channel Estimation in 802.11a and 802.11p systems?

I am working on 802.11p and was wondering about the channel estimation part. At physical layer (PPDU), The preamble contains 10 Short Training Sequence and two Long training same sequences. Why do ...
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49 views

What is fft input size for ofdm?

From the Transmitter comes a block of size N+L (where N is the number of subcarriers and L is the cyclic prefix length). Then the block goes through a Channel which adds delay and AWGN, so this ...
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40 views

Quantized error in scatter plots for a hamming windowed LTE signal

I have a time domain signal based on LTE standards. Carrier spacing = 15KHz, Symbols per slot = 6, extended Cyclic Prefix = 512 samples FFT size = 2048, No. of sub carriers = 1200, channel BW = 20MHz, ...
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43 views

AWGN interpretation of bit error rate in time domain

I'll consider a 20 MHz bandwidth between 1.99-2.01 GHz. The carrier has been modulated by an OFDM signal (I/Q modulation) and is passed through a narrowband filter at the receiver. The noise that is ...
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64 views

Digital Communications block diagram question

I am new to digital communications and trying to understand the basic underlying concepts of digital comm systems. So far , studying the course , I have some questions which if anyone can answer would ...
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1answer
93 views

How to filter an OFDM signal? [closed]

Across the channel bandwidth there shoudd be 100 RBs (central 1200 sub carriers in a 2048 bin). Max power normalized to 0dB, the first OOB lobe should be -34dB. This 1st lobe on either side is at 5 ...
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2answers
45 views

What should be the cut off frequency of a Filter operating on an OFDM signal?

I have a time domain OFDM signal and to this I need to apply different linear filter however I am not sure on the cut off frequency. I am simulating for multiple symbols where each symbol has 1200 ...
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90 views

No. of Cyclic prefix for 5G NR signal with 100Mhz bandwidth different subcarrier spacing?

I want to generate a 100Mhz bandwidth 5G NR signal with different subcarrier spacing(15,30,60,120Khz) in matlab. What is the no. of CP to be added for 1st OFDM symbol and the other symbols and also ...
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72 views

Applying DFT to OFDM signals above Nyquist rate

Consider an OFDM signal which has $N=8$ subcarriers and is band-limited to $[-B/2,B/2]$, $\sum_k x_k e^{j2\pi kf_0t}$ where $f_0=\frac{B}{N}$. For this case the Nyquist rate is $2\times B/2=B$, i.e., ...
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30 views

How to window a time domain signal for 6 symbols with cyclic prefix length of 512 samples?

Based on LTE stds. I am running a simulation for 6 symbols (each having 1200 central subcarriers) each in a 2048 FFT/IFFT bin. To these symbols are appended 512 samples as cyclic prefixes. I have to ...
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26 views

Applying window function to reduce out of band emission for a symbol in LTE simulation?

I have to apply a window to an IFFT'ed signal (based on LTE standards, simulated for 1 symbol - 100 resource blocks - 1200 subcarriers). I have a signal which I have simulated till the IFFT. Now I ...
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91 views

Sampling Frequency in a OFDM technique:

I am a new learner of OFDM. One source I refer explained OFDM in this way: Consider 4 symbols $X_0$, $X_1$, $X_2$, $X_3$ $\in \mathbb C$. They are modulated using Multi-Carrier Modulation. The time ...
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48 views

Modulating ofdm in audio spectrum

Trying to modulate audio with OFDM data. My fs = 44.1khz, i'm using 64 sub-carriers, but i'm only using 5 sub-carriers for data, so my bandwidth is about 3.445khz. I'm not sure how to properly write ...
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51 views

Is CMA equalization applicable for OFDM

I have a question regarding the constant modulus algorithm (CMA), as you know it's one of the known algorithms used for blind equalization. The cost function of the CMA is build blindly, it means ...
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132 views

What is the index modulation in OFDM and Subcarrier Index modulation?

I'm new here, I'd like to ask about the OFDM-IM (OFDM - Index modulation), what is it and how it works? Is it the same as SIM-OFDM? I checked white papers, I see it's using the index of sub-...
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121 views

Ambiguity in the IFFT process in OFDM

I am still trying to iron out some ambiguities in my understanding of the IFFT component of OFDM modulation schemes. So here we have a QAM symbol $s_0$ being multiplied with the subcarrier for that ...
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1answer
70 views

Visualising OFDM subcarriers in the frequency domain

Im struggling to understand why in some images the frequency domain shows clear bars with amplitude and phase and with 0 frequency variation (it is clear that the frequency of the waves does not ...
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1answer
92 views

OFDM Simulation process

I'm trying to understand OFDM by making a simulation. Are these steps correct? generate M random complex QAM symbols. example: (1+j,1-j,-1-j,1-j....) Split my M samples into a N 2048 sized arrays ...
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138 views

What is the mixed numerology in OFDM

In order to meet the user requirements in the next generation radio access technology, the ongoing standardization within 3rd Generation Partnership Project (3GPP) offers flexibility in terms of ...
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55 views

OFDM in Time domain

This tutorial shows the idea of analog time domain OFDM. My question is if we can sample the analog signal and use time-domain OFDM in a similar fashion. Why do we need to apply IFFT/FFT to the signal ...
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122 views

OFDM RF signal waveform

From the Wikipedia article: "An inverse FFT is computed on each set of symbols, giving a set of complex time-domain samples. These samples are then quadrature-mixed to passband in the standard way. ...
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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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217 views

Use of Long Training Field (LTF) of 802.11 standard

In 802.11a/n the preamble part has L-LTF(Legacy-Long Training Field) and HT-LTF(High Throughput-Long Training Field). I want to clarify my understanding of these two fields: Are the only difference ...
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68 views

What's the difference between SISO and SIMO in using matched Filters

Suppose we are transmitting a signal $x$ in $OFDM$ with $N$ subcarriers, After performing Parallel/Serial conversion and then N-points FFT, we suppose to have signal $s = IFFT^Hx$, Then adding the ...
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171 views

OFDM Short Training Field clarification

I want to clarify my understanding of OFDM training fields. Please correct me if I am wrong as I am getting 2 different information from online resources. For 802.11a standard the preamble contains ...
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2answers
125 views

How to determine the original size of the FFT used in unknown OFDM symbol

Given an unknown OFDM symbol consisting of 9144 samples, interested to know the size of the FFT used in the TX/Number of bins. Down-sampling to 512,256,128,64,32 yields the following. Do not see how ...
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90 views

Is sparse signal in time domain is sparse in Frequency domain

In OFDM system, if we have a multi-path environment, we supposed to have a sparse received signal in time domain, then $FFT$ is performed to convert it into frequency domain. My question, does the ...
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Is the Channel shortening similar to Time domain Equalizer (TEQ)

I used to read about Time domain equalizer (TEQ) in many lectures, i.e here , But in many other papers, that's called Channel shortening i.e here . However, in the a two above links, the cost ...
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138 views

What is about the circular convolution in OFDM

In an OFDM system, serial-to-parallel conversion for data is done, then the DFT is performed and then adding the cyclic prefix (CP). My question is related to that step of adding a CP. As I know, ...
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1answer
43 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 ...
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139 views

How I can perform SIMO channel convolution in MATLAB

In OFDM system, I need to transmit a signal $x$ in SIMO channel $H$ of single Tx antenna and $4$ Rx antennas. The initial equation for that convolution is $r = Hx + n$ where $n$ is the noise. As ...
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99 views

ADC sampling rate in OFDM baseband processor

I have optical OFDM baseband TX and RX implemented in FPGA. The design is tested using one FPGA board in loopback connection from DAC to ADC via coaxial cable. It works fine, the BER is about 1.10^-8 ...
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147 views

OFDM: How to do the carrier recovery of the quadrature(I/Q) demodulation

For an OFDM system, after the OFDM modulation, the quadrature modulation is used to move the signal from the baseband to the pass-band. But when it comes to the receivers, I can't find any way to make ...