OFDM stands for **o**rthogonal, **f**requency-**d**ivision **m**ultiplexing.

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Variation of channel attenuation with frequency

Consider a standard RF channel over which signals are being transmitted over a 10 MHz bandwidth at 1800 MHz (using OFDM). How does the channel's intrinsic attenuation vary over frequency? More ...
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Bandwidth of ofdm symbol in a visible light communication system

What is the difference of the bandwidth of the LED and the optical channel? If ofdm symbols are transmitted over this system, is the bandwidth of ofdm symbol equal to BW of the LED or of the channel.
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Generating two bursty signals that are unsynchronized with each other

I am testing adjacent (and non-adjacent) channel rejection for an OFDM signals. (Clause 18, IEEE 802.11-2012) In general the requirement is to generate test frames on what is referred to as the test ...
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38 views

Visible light communication and LED modulation

I work on a VLC system in which 16-PAM symbols with complex pilot symbols are as ifft input. I made the ifft output real and positive. Now how should i use these ofdm symbols to modulate LED? Their ...
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28 views

Spectrum of OFDM with raised cosine window(overlap and add)

I'm having trouble observing OFDM with a raised cosine (RC) window in Matlab. I know how to generate an OFDM signal with a raised cosine (RC) window, I just don't understand when the last W samples ...
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97 views

Why do the OFDM training (and payload) symbols have silent sub-carriers (aka virtual carriers)?

I am examining an OFDM signal. Training symbol: The first two OFDM symbols are training symbols, they are 2PSK modulated (within the subcarriers). In the first OFDM symbol the $(0+18N)^{\rm th}$ ...
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23 views

Frequency autocorrelation function - Communication channel

If we have two discrete stochastic spectra and each spectra has $N$ points. We want to calculate the autocorrelation of these spectra, what should be the number of points in the resulting ...
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56 views

OFDM sensitivity to sampling time offset (with half of time interval between samples)

In simple QAM modulation symbol synchronization is crucial: the samples need to be taken (or interpolated and resampled to ) exactly corresponding to the time when the symbol was emitted. This is ...
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39 views

Adaptive LMS(Least Mean Square) Equalization in OFDM

I have a problem with the adaptive LMS frequency domain equalization in OFDM. When I assume that the receiver knows the channel perfectly and apply zero forcing equalizer, there is no problem. Also ...
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71 views

OFDM symbol acquisition block in GNU Radio gives rotating constellation even if there is no carrier offset. How is this possible?

Why does the constellation rotate when using OFDM Symbol Acquisition block in this flow graph below ? The only explanation I could think of would be carrier frequency offset but in this flowgraph ...
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39 views

Denoising effect in GnuRadio OFDM Serializer block

Why does the OFDM serializer has such a strong denoising effect in the flow graph below ? Is that normal ? The upper constellation is AFTER the OFDM serializer block and the lower is BEFORE the OFDM ...
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38 views

Cyclic prefix vs. guard interval. Why use one over the other?

I'm currently studying some OFDM techniques, and have found that in order to avoid Inter-Symbol Interference (ISI), there needs to be some space in between symbols. Lets assume this length to be $\mu$....
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25 views

How does carrier frequency offset effect the constellation diagram in OFDM?

In the case of OFDM modulation, if the carrier frequency is slightly off then how does that effect the constellation diagram ? Is there some general rule how it will change (such as rotate) ? Or ...
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36 views

OFDM demodulation, how to set the sampling rate?

I have a signal for which I know the OFDM burst length (from the autocorrelation function of the signal) T_burst and the FFT size 1024 (by measuring the channel ...
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34 views

Transmit power of non-orthogonal FDM

It is known that the total power of an OFDM signal equals to the sum of the powers on each individual subcarriers. I am a bit puzzled by the total transmit power of a generalized FDM system if ...
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19 views

What is the difference between multipath channel and fading channel? [duplicate]

what is the formula of channel reponse of multipath fading? is it h = 1/sqrt(2)[randn(1,N) + irandn(1,N)]; % with N: length of signal or 1/sqrt(2)1/sqrt(Ntap)[r andn(1,Ntap) + i*randn(1,Ntap)];%...
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42 views

Average power of orthogonal and non-orthogonal transmission

It is known that the average transmit power of OFDM system equals the sum of the power on each subcarrier due to the orthogonality condition. This can be verified as follows: $$s(t) = \sum_{k = 0}^{N-...
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19 views

Is adding zeros part of the Hermitian Symmetry process?

I have been reading about DMT/OFDM and every source I find defines their system differently. I simply want to know if zero insertion is part of the Hermitian symmetry process that produces real OFDM ...
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13 views

Is it possible to estimate channel using the pilot symbols amplitude only, NOT using phase?

I want to estimate a rayleigh fading channel using pilot symbols. I want to use linear interpolation, Comb-type pilot arrangement and Least Square Estimator. Is it possible to use pilot symbols' ...
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22 views

OFDM Channel estimation using pilots

I'm simulating various scenarios of channel estimation using pilots in OFDM. The FFTSize and number and position of pilots may vary but the pilot scheme is identical, meaning that pilots exist in ...
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46 views

Multipath resistance of spread spectrum systems

I have been reading about various telecommunications technologies and one thing keeps being repeated many times without a clear explanation - that spread spectrum is not only resistant to multipath ...
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79 views

Effect of cyclic prefix and zero-padding in OFDM

What happens if zeros are added instead of cyclic prefix in OFDM system? How does it affect performance of system?
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Problem using MMSE estimation of channel frequency response

I need to model the minimum mean square error (MMSE) performance in estimating channel frequency response. I have channel's power delay profile (PDP) as a table with tap delays and powers. The ...
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40 views

Coherence bandwidth/delay spread in OFDM

Say we want to design an OFDM system and we have some parameters given to us, i.e. the coherence time $T_c$ and the max delay spread $T_m$. It's clear to me why we need to ensure $T_s(N+N_{cp}) \ll ...
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42 views

IFFT-based OFDM system

Please explain, mathematically, how can a bank of modulators be replaced with the IFFT in an OFDM system?
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Inconsistency in LTE UE-Specific Reference Signal for Special Subframe Configurations 9

Refer 3GPP 36.311 (Release 11 or higher) section 6.10.3.2 or Figure 6.10.3.2-3 shown below, Here UE-RS is defined for Special Subframe configuration# 9 as seen in the middle row of sub figures in ...
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21 views

How to determine if and where are pilot subcarriers in an OFDM signal?

I have an OFDM signal, recorded at 2.45 GHz , coming from a camera. How can I determine if there are pilot subcarriers in the signal ? I looked at the signal with high resolution FFT waterfall ...
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109 views

Is this waterfall characteristic of QAM or OFDM (or something other) modulation?

I recorded this signal at 2.45GHz, it is coming from a camera. Is this QAM or OFDM ? Based on those side-lobes I would guess OFDM but I am not sure. Also there seems to be an interesting preamble ...
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98 views

What benefits can we get from the use of GFDM?

Existing literatures say that the generalized frequency division multiplexing (GFDM) has lower out-of-band emission (OOBE) levels compared to orthogonal frequency division multiplexing (OFDM). However,...
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73 views

How to calculate energy of an OFDM signal?

What is the difference between power and energy of OFDM signal. How to calculate energy of an OFDM signal, if we consider OFDM symbol size is 64, and QPSK is used as modulation scheme?
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channel estimation in OFDM systems(MMSE Estimator)

I want to use the mmse estimator to estimate the channel in ofdm systems. I find the formula: h=Rgy*(Ryy)^-1*y[1],while Rgy is RggF'X'. My question is: in the ...
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106 views

OFDM channel estimation

As we know, if we want to get the time domain response of the channel $h[n]$, we can first get the frequency domain response of the channel $H[k]=Y[k]/X[k]$. And then we do IFFT. But now one OFDM ...
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125 views

How to simulate High Power Amplifier(HPA), like Solid State Power Amplifier (SSPA)

I cannot understand how to simulate Solid State Power Amplifier (SSPA). When I directly write the Rap's SSPA model in Matlab, it does not produce BER of OFDM like published papers by other researchers'...
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75 views

Crest Factor of OFDM signal

I have been given a OFDM signal of crest factor of 12dB. The crest factor is the ratio of peak power to the average power of the signal. Does anyone have any idea how crest factor of 12dB is derived ...
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Practicality of the assumption of i.i.d. for Rayleigh channels

I would like to understand how far the assumption of having i.i.d. is accurate/valid (from the practical point of view) when an OFDM system is working in Rayleigh channels. Is this means the channel ...
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175 views

Number of samples in one period of subcarrier after IFFT

I have a quick question about the number of samples in one period of a sub carrier in OFDM systems. The IFFT process generates N-parallel sub carriers in discrete time domain which are sent ...
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Conceptual questions on MC-CDMA - where to perform equalization?

I have some basic questions on MC-CDMA technology. Below are the steps involved and the Questions. Please correct me if there is any mistake in the steps. An intuitive answer will be very helpful. ...
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Guard Interval Correlation for a given Impulse Response in OFDM system

I've been given a channel response of $$ h(n) = \delta(n) + A \delta(n -D) $$ where A and D are arbitrary constants. What would be the output of Guard Interval Correlation (GIC) symbol ...
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66 views

Bandwidth of OFDM symbol

In 802.11a, channels of 20 MHz bandwidth are available for wireless transmission. With OFDM modulation, the protocol uses 64 pt IFFT for transmission. This means we take a stream of symbols apply ...
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38 views

A General Question regarding OFDM symbol

I have cross across an OFDM modulator that says that A frequency domain sequence is passed into a 64 point IFFT. A cyclis prefix is added (0.8 $\mu$sec) and creates as a result a 4 $\mu$sec time ...
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33 views

Which MIMO scheme is implemented in a modem?

Hope I phrased the title in a sense. I'm trying to get more knowledge on MIMO , so I read a few paper and i've understood that MIMO system can provide 3 types of improvements : array gain, diversity ...
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109 views

Can we do cyclic prefix with zeros in OFDM rather than with samples from end of the ofdm symbol?

considering an OFDM symbol of x(0)x(1)x(2)x(3)x(4)....x(n-2)x(n-1)x(n). To perform cyclic prefix we prefix some samples from end of this OFDM symbol to the beginning of the OFDM symbol, like x(n-3)...
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479 views

Purpose of interleaving in OFDM systems

Interleaving is often encountered in digital communication systems. I have seen interleaving between codewords in a single OFDM symbols as follows Assume the bits are denoted by $$\{c_o,c_1,c_2,\...
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157 views

Necessity of Cyclic Prefix in OFDM

I have a question on CP-OFDM. In recent years people are talking about filter bank approach where they are trying to filter out the out of band side lobes using filters. Besides they are presuming ...
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60 views

A sequence OFDM modulated with normalization factor

The following sequence is used to modulate 52 subcarriers. Twelve out of 52 are non zero. I was wondering $S_{[–26, 26]} = \sqrt{13/6} \{0, 0, 1+j, 0, 0, 0, –1–j, 0, 0, 0, 1+j, 0, 0, 0, –1–j, 0, 0, ...
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383 views

Pilot symbols in IEEE 802.11a

In IEEE 802.11a standards, there are many fields. Of which is the Data field which has 52 available sub-carriers used for transmission of coded bits. If the subcarrier locations are numbered from 32,-...
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Sparse Channel Estimation in OFDM System

Compressive sensing methods are used in channel estimation when the channel is thread as sparse where there are zero points in the signal representation. I am in the beginning of the sparse signal ...
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365 views

DAC implementation in OFDM

In an OFDM transmitter, after IFFT & CP block, a DAC block is used.Since the output of IFFT is complex, and the input to DAC should be an integer, how can we integrate the two blocks? I ...
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SFBC Alamouti Scheme LTE

A sentence I copied from book. That a 2$\times$1 Alamouti SFBC is referred to as a rate 1 code, two symbols are sent over two adjacent subcarriers. This question is regarding this SFBC encoder (...
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OFDM modulator in LTE

My understanding of what an OFDM modulator does, is that it takes a parallel sequence of QAM symbols of size M, pads it with zeroes corresponding to the zero subcarriers, then passes them through an ...