Questions tagged [ofdm]

OFDM stands for Orthogonal, Frequency-Division Multiplexing.

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58 views

IFFT and DAC relationship in OFDM Transmitter

Say, for LTE BW 20 MHz, I have total 2048 SubCarrier at IFFT Input leading to 2048 Output (Complex) at IFFT Output, and in 1 OFDM Symbol (useful symbol length) of time 66.67 usec, the sampling rate ...
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45 views

How is SNR Calculated in OFDM Reciever? [closed]

Can you please guide me on the information to calculate SNR for OFDM Reciever? Is OFDM Pilots used for it? And do we need to follow channel estimation like steps of interpolation to determine it? I'm ...
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54 views

OFDM Query : Why In Phase and Quadrature Phase RF Conversion is needed? [duplicate]

OFDM Basic Query- x[n] produced at output of IFFT is in discrete time, so we can use DAC and then RF Upconversion (multiply it with LO Frequency say f) and send it, why are we doing RF up conversion ...
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26 views

MIMO Noise Power Calculation Using SNR

i'm in the process of simulating a MIMO-OFDM transceiver, in which we have $N_T$ transmit and $N_R$ receive antennas ($N_R \geq N_T$), as shown in the image below for the 2x2 case What i'm having ...
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34 views

What's wrong with a filter for subband OFDM?

I am confusing about why it is necessary to pack the QAM symbols onto the frequency bins, such that the allocated range of frequency bins is located in the middle of the subband (as in lines 26-29). ...
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39 views

One subcarrier lost after using scipy resample in an OFDM system with multipath channel

I simulate with Python a simple OFDM communication chain with a multipath propagation channel and no noise. Here are the details: Generate 10k independant batches of 128 random 4-QAM symbols. The 128 ...
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30 views

Why we should pack data symbols in the band center before convolution with a lowpass filter?

I am investigating the spectrum shaping filter for OFDM modulation, and following an example on https://www.mathworks.com/help/comm/ug/f-ofdm-vs-ofdm-modulation.html In the part F-OFDM Transmit ...
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1answer
30 views

Filter for ISI Channel

I know that OFDM has many advantages in removing ISI. But I think there is one more way of removing ISI(equalizing or inverting technique). Syppose there is a channel $H(\omega)$ and we have our ...
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32 views

Parsing downlink cellular towers signals

I want to parse signals coming from 4G/5G towers to extract only the ID of the tower and the transmission time (on simplex communication: only downlink). Are these packets (or frames) sent on a ...
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1answer
26 views

First element of the complex input array for the IFFT with OFDM

I am simulating OFDM by randomly selecting members of a constellation. That is, randomly selecting the I & Q from a 64QAM constellation. This gives me 64 possible phase & amplitude values. I ...
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51 views

Square Root MMSE

Could anyone please explain how to perform the derivation of the square-root MMSE equation using an extended channel matrix H (as it is said in the "A LOW COMPLEXITY SQUARE ROOT MMSE MIMO DECODER&...
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29 views

Pilot Signal Overhead

I have a doubt regarding Pilot signals such as CSI-RS in NR. Using a large number of CSI-RS affects Datarate significantly? or it is negligible? Because I wonder it is meaningful that optimize the ...
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31 views

802.11a OFDM Long Preamble Cyclic Prefix

im currently looking at an implementation of the preamble for OFDM in the 802.11a standard, as shown in the pic below What I dont understand is the cyclic prefix (labelled GI2). Based on this picture ...
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108 views

What is the difference between a guard band and a cyclic prefix in OFDM?

im trying to understand the difference between a guard band, and the cyclic prefix, or at least the relationship between the two. Prior to now, I was under the interpretation that the cyclic prefix ...
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48 views

I am trying to demodulate an OFDM signal

I am trying to demodulate an OFDM signal which was captured using a spectrum analyzer. The transmitted signal contained a 16 QAM modulation format, the received signal and the transmitted signal are ...
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48 views

OFDM transmit and receive steps + RF

Would someone help confirm whether the steps I have here for OFDM communication chain make sense? At the tx bits - symbols - IFFT+CP - precoding - DAC - filter - PA - up conversion At the rx side ...
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53 views

How can I demodulate OFDM signal having low SNR?

So I am trying to demodulate an OFDM signal with SNR = 10 dB, the parameters of modulation are as follows, oversampling factor = 8, cyclic prefix length = 144, number of Sub carriers used = 3600, ...
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55 views

PAPR effect on LNA, ADC and PA

It is well known the OFDM systems suffer from PAPR given that the OFDM symbol is a combination of all of the subcarriers. Subcarrier voltages can add in-phase at some points within the symbol, ...
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32 views

Differences between cyclic prefix, postfix and zeropadding [duplicate]

I was looking into details of OFDM and how the CP is copied last few sample of OFDM and placed in the beginning to cause circular convolution and mitigate is. My understanding is this cyclic prefix ...
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78 views

Frequency Error/CFO effect on Constellation for OFDM

Very often in literature we see that effect of carrier frequency offset is seen as rotating of a constellation points. And explanation is generally that after down converting and sampling the time ...
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52 views

Frequency offset estimation accuracy

To improve frequency offset estimation, we could: Use the second order interpolation / equation in the frequency domain based on the coarse estimation fc, e.g. BPSK frequency offset estimation; Zero ...
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78 views

How to calculate the data rate of OFDM system

I have OFDM system with following parameters, Bandwidth $B = 5$ KHz, Number of sub-carriers $N = 64$, guard interval $N_g = 8$; Sampling frequency $F_s = 10000$ and Modulation order $M$ is QPSK. Is it ...
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48 views

How to calculate received signal power for a Multi-User MIMO OFDM system?

I have a Multi-User MIMO system where there are 2 users served by a base station. The base station consists of 64 transmit elements and the users are equipped with linear antenna array consisting of 3 ...
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50 views

Do the effects of OFDM pilot tones spacing appear in time or in frequency domain?

As known, the pilot tones are transmitted via subcarriers predefined and known in the transmitter and the receiver. The pilot spacing is repeated with a certain rate that depends on how fast the ...
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67 views

In practice, why is there any overlap in frequency-division multiplexing?

Okay, so I am aware of the math behind perfect reconstruction in filterbank and wavelet theory when there is half-band symmetry. But I don't understand, when we have the computational power to do ...
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45 views

OFDM IFFT/FFT processing

After the serial-to-parallel to break input bitstream into groupings we do the constellation mapping into values of phase/amplitude (or I, Q). For 64 sub-carriers this gives us 64 pairs of amplitude(n)...
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211 views

Setting the duration of subcarriers in OFDM system

If have an ofdm system with $N$ subcarriers, and sampling frequency $F_s$, that means that duration of each subcarrier is $T_s = \ {1/F_s}$ and the duration of whole symbol (which includes $N$ ...
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107 views

Understanding MMSE equalizer equation

I have read many online materials and journal paper where the equation for MMSE equalizer is given as: $G_{MMSE}$ = $H^H(HH^H+\nu^2I )$ where $H$ is the channel, $H^H$ is its hermitian. $\nu^2$ is ...
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80 views

Modulation order in real signal and complex signal modulation

In real signal modulation, e.g. ofdm based on DCT transform, the modulation order $m$ should be $\sqrt{M}$ where $M$ is the modulation order of the complex signal. for example, if we are using QPSK ...
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31 views

Alamouti scheme and MIMO OFDM model, theoretical or practical concept?

I am not an expert in signal processing but last week I attended a class of signal processing. We discussed MIMO, OFDM and Alamouti scheme. On this lecture it was represented a lot of math equations, ...
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38 views

3G CDMA vs. LTE OFDM

I have some question about wireless evolution. It's about 3G to 4G. Why LTE use OFDM not CDMA? What are the advantage and disadvantage of these two multiple access? Cause I'm not familiar with CDMA, ...
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60 views

OFDM signal in time and frequency domain and its PSD

My question is on OFDM signals for LTE. I been reading a tutorial on LTE where ofcourse the modulation is OFDM and it is explained that an OFDM signal is a rect function in time domain. The duration ...
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33 views

implmentation of FFT using lower N point

I have random data which was generated using randi funtion of 1024 bits.. I will modulate this using QPSK and will pass through 512 point IFFT.. While doing reverse process, I want to use 256 point ...
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36 views

DFT and baseband wave forms in OFDM

In most of the textbooks, the motivation for the usage of DFT in OFDM goes as follows. Let $X[0],\cdots,X[N-1]$ be complex symbols, and $$s(t)=\sum_{k=0}^{N-1}X[k]e^{j2\pi\frac{k}Tt}\qquad (0\leq t\...
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3answers
115 views

How do we impose Hermitian Symmetry in optical OFDM?

I’m currently trying to wrap my head around optical OFDM. I’m reading various tutorials and articles, but I’m hitting a stumbling block trying to understand Hermitian Symmetry and how it is applied. I’...
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In OFDM, does the parameter time offset is important if there is only one mobile in the cell?

I am learning about OFDM, and in my reading, I have noticed that (mentioned everywhere) synchronization is a very critical aspect of OFDM. Here, I am writing in the context of time synchronization (...
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78 views

Using large DFT for OFDM to avoid the guard interval

In OFDM, we usually insert the guard interval to avoid the inter-symbol interference at the expense of achieved data rate. that guard interval can be either zeros or CP guard interval, which it must ...
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2answers
292 views

OFDM use a pulse shaping filter or not?

I've heard in various literature that band limited channels use pulse shaping filters. By the way, In OFDM it use a cyclic prefix to overcome ISI, so I wonder if OFDM does not use pulse shaping filter....
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70 views

power spectrum of passband of ofdm is it right shape or not?

I tried to get power spectrum of ofdm passband signal. Below is the whole matlab code for that with BPSK+ofdm. and I got the power spectrum as below. I wonder that is that right form or not? ...
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36 views

Spectral efficiency of with OFDM vs without OFDM?

OFDM has advantage over frequency selective channel with addition of cyclic prefix after IFFT of MPSK or MQAM symbol. but I wonder that the spectral efficiency of with OFDM vs without OFDM.
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59 views

Simulating Noise Variance in OFDM with MATLAB

I got a task to simulate an OFDM system. It works quite, but my problem is the calculation of the noise variance (and further noise for AWGN). Firstly, I'm calculating my noise variance in linear ...
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33 views

Can the DFT can be performed without normalization?

If we have OFDM system with $N$ sub-carriers, the DFT matrix can be expressed as follows: $F = dfmtx(N)/sqrt(N)$ My question, is it possible and practical to use the matrix without Normalization, it ...
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22 views

timing issue in OFDM

suppose I have this OFDM signal the pink part is cyclic prefix. I should sample at point b. Because of lack of Timing I started to sample at point a, but I get wrong detection. How can I compensate ...
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81 views

Mathematical expression of Multicarrier frequency equalizer

I have a signal $X$ with length of $N$, multiplied with any unitary matrix, i.e the transpose of DCT matrix as: $x = D' X$ where $D'$ is the transpose of the of DCT matrix. Then let's add $CP$ guard ...
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56 views

Possible to Implement Baseband OFDM on Microcontroller?

I am wondering if one can implement OFDM on any available microcontroller or Development board in the present Market. Definitely not looking FPGA solution. Specifically, on any Microcontroller is it ...
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66 views

Why does carrier frequency offset (CFO) cause carrier phase error (CPE)?

I've already asked 2 questions on this but there is one discrepancy I cannot reconcile and is the crux of the problem: \begin{align} \newcommand{\prefix}{\frac1N\sum\limits_{n=0}^{N-1}} R_l &= \...
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116 views

Why does fine CFO cause rotation of the constellation?

Just thinking about it, why does fine CFO cause rotation only? I only see diagrams that show rotation of the constellation. We assume this is a complex magnitude frequency domain plot In order for ...
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1answer
55 views

The cost function of LS algorithm

I'm trying to understand the derivation of LS algorithm for channel estimation in OFDM. Could anyone explain why do we need complex conj transpose in the below equation?
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42 views

Sampling frequency error in OFDM [duplicate]

If the sampling rates at receiver and at transmitter are different, there will be ICI. However I don't find any practical technique to estimate and compensate this kind of error. Is it true that we ...
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66 views

What is the benefit of adding zeros subcarriers in frequency-domain OFDM signal?

I have noticed in some OFDM systems, some null subcarriers are added in frequency domain. What's the benefit of those zero subcarriers, and is that mandatory ? An example I have read about that adding ...

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