# Questions tagged [ofdm]

OFDM stands for Orthogonal, Frequency-Division Multiplexing.

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13 views

### Free space path loss and ofdm

I want to apply the Friis equation of free space path loss on OFDM signal. Do I multiply signal by $\frac{c}{4\pi f_cd}$ before IFFT or after?
607 views

### Constellation Mapping In OFDM

If i am not mistaken, the two following hold : a. In OFDM Conceptualization, the constellation mapping at Tx (or de-mapping at Rx) is done in Frequency domain b. However the basic theory (Including ...
48 views

### Do you have to receive a whole OFDM signal to be able to demodulate?

An OFDM signal is made of subcarriers that are orthogonal to each other. This terminology makes me think that if one is only interested in information on a specific subcarrier, only that subcarrier ...
74 views

### OFDM: From DFT to time-continuous signal

I try to write down the equations that show how constellation symbols (PSK, QAM etc.) are modulated into a time-continuous signal. I read the articles referenced at the end. However, I am still stuck ...
93 views

### How are subcarrier symbols modulated into an analog OFDM signal?

I am currently researching about OFDM models (formulae) that show how the individual data symbols (eg., QAM symbols) are converted to OFDM symbols and how the OFDM symbols are turned into an analog ...
57 views

### How to detect when signals overlap in OFDM?

If you are in an environment with multiple transmitters of similar signal strengths, before band selection has taken place, how do receivers parse out overlapping OFDM signals from multiple ...
394 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 ...
166 views

### How to convert an OFDM symbol to a signal?

How can I convert an OFDM symbol generated from an IFFT block to an OFDM signal for the purpose of transmitting it?
1 vote
282 views

### How does DFT-s-OFDM precoding reduces PAPR?

I would like to understand how does DFT-s-OFDM reduces PAPR compared to CP-OFDM. I am not able to locate any mathematical explanation for this. If anybody can provide an explanation it will be great. ...
293 views

### How to estimate the average distance between peaks (maxima) of the magnitude of an OFDM signal?

Given an arbitrary configuration of carriers having bandwidths that belong to the 3GPP standard (5, 10, 15, 20, 25, 30, 35, 40, 50, ..., 100 MHz), is there a method/rule-of-thumb to estimate the ...
1 vote
39 views

### spectral density of Modulated ofdm signal

I have formed my OFDM symbol his length is 88 (Nsubcarrier=72,Ncp=16) with Fs=1080000 Hz we call this symbol as Signal_from_baseband I want to modulate the OFDM symbol with Fc carrier=2.1 GHz and plot ...
1 vote
2k views

### OFDM - why do we need IFFT?

This question is really related to - is there any need for IFFT in the first place? If we want to send a bunch of complex-numbers (vectors), then most OFDM discussions involve presenting those ...
10k views

### IFFT Concept in OFDM

can any one explain how actually IFFT generating sub carriers in OFDM.... when we are simulating OFDM transmitter in matlab, we are taking input data stream, encoding it and modulating the encoded ...
421 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 ...
1k 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 $N$ ...
33 views

### In wireless communication, is pulse shaping applied to each BPSK signal before OFDM is used, or not?

I have read a number of posts that seem to ask this question but have not been able to find a definitive answer. Here goes... I understand that BPSK with NO pulse shaping would produce a signal in ...
296 views

### OFDM Signal and IQ Paths

I understand that we have analog complex signal after it passes DAC after coming out of IFFT in OFDM Transmitter. After that we do orthogonal upconversion on it and send it. I've question now for OFDM ...
21 views

### Channel Estimate for OFDM shows distortion for first last order samples

The Channel Gain Estimation was obtained using Least Square method as follows: Hk= mean(Rx_LTS ./ Tx_LTS, 2); k= -26 to 26 Where Rx_LTS is the received long training sequence (52 samples after ...
19 views

### How to measure the performance of OFDM System using Error Vector Magnitude

I have used BER vs. Tx-Gain in dB curve as one tool to measure my ofdm system's performance. Besides, I would like to measure the performance using the EVM. How I can implement this EVM measurement in ...
28 views

### Channel Estimation of OFDM using Long training Sequences

Based on IEEE 802.11a Standards, I am using two Training Sequence-Preamble for Synchronization and Channel estimation. I have read in some papers that the channel estimation is performed by taking the ...
1 vote
61 views

### OFDM samples after fft show Amplitude Compression

I transmit OFDM signal with bandwidth 20 MHz via ADALM-Pluto SDR device. To study the effect of the wireless channel, I used the same device for transmission and reception. The maximum bandwidth of my ...
1 vote
93 views

### Complex samples in OFDM signal, after fft-block at the receiver, have rotating amplitude towards zero even after phase correction

First of all, please consider my lack of experience/background in real time transmission using SDR especially if my explanation was expressed technically well.enter image description here I have ...
1 vote
47 views

### How to calculate Coarse & Fine Carrier freqeuncy Offset using Short and Long training Preamble respectively! Equations not understood

I am transmitting an OFDM signal. According to 802.11a Standards, I am using Short and long training Preamble each has length of 160 samples for time and frequency synchronization. According to Essam ...
96 views

### Why does one of my OFDM subcarriers have a much higher gain than the rest in GNUradio?

I'm using a fairly standard setup after the ofdm flowgraph examples provided by gnuradio. When I look at the absolute value of the received complex symbols in the first sync word (and every other ...
50 views

### OFDM Problem with Simulink IFFT

I'm studying OFDM modulation and trying to implement the Tx system on Simulink. I did what is shown in the picture. I was expecting to see subcarriers but I got "nothing". What am I doing ...
18 views

### OFDM interpretation data

I'm studying OFDM modulation. I understand that a bit stream comes in and you get the modulated signals (for example QPSK). Let's say we have 4 subcarriers. At the entrance to the P/S I have [1+j,1-1j,...
36 views

### How to do MIMO precoding for OFDM?

I studied OFDM, MIMO, and MU-MIMO, but I'm not sure how it works when MIMO and OFDM are combined. I can't find good literature that talks about MIMO-OFDM. I want to understand how to use eigenmode ...
68 views

### Understanding FFT size in OFDM

The number of subcarriers = the size of the FFT? The size may be the same or different depending on the document.
26 views

### Is OFDM equalization performed every 14 symbols?

The number of symbols and slot in a subframe differs depending on the subcarrier spacing Is equalization processing performed for each slot? Or per subframe?
877 views

What are the advantages and disadvantages of using VOFDM over OFDM? I know that one advantage is frequency diversity, what are others? edit : By VOFDM I mean Xia's method as shown here.
1 vote
114 views

### 5G strandard pilot allocation, channel estimation method

I'm trying to create an OFDM simulation, but I don't know the 5G standard pilot allocation estimation method. Is this simulation good? One transmission unit has $64$ subcarriers and $14$ OFDM symbols. ...
1 vote
74 views

### Can the number of subcarriers/user be greater than 12 in 5G?

In 5G, I think 1 resource block (RB) carrier is 12 subcarriers. So one user uses 12 subcarriers. But in some research, 612 subcarriers are used for channel estimation research. Is it okay to use more ...
3k views

### what does the output of ifft in ofdm look like?

i have a problem in ofdm modulation .... there is a stream of bits that is converted from serial to parallel and is applied to an IFFT block. based on the definition of IFFT the output should be an ...
1 vote
43 views

### simulation models unique to each researcher? Or use a specific common model?

Are the simulation models used by researchers at each university unique to each university? Or use a specific common model? I started working on OFDM research at university, but I'm at a loss as to ...
89 views

### Need to write a simple code in matlab of training sequence or zadoff Chu sequence for my OFDM signal-synchronization

I have written a complete code, in matlab, for my ofdm signal for the transmitter and receiver. Now I want to transmit my ofdm signal via wireless channel using SDR. Now, I need to combine a simple ...
1 vote
77 views

### OFDM complex symbols have dynamic phase shift after the fft-block & before the BPSK demodulator at the receiver ,

I have transmitted and received an OFDM-signal using the same ADALM-Pluto-SDR's Tx/Rx Antenna. I have used BPSK baseband modulation with symbol freq=312.5 KHz, L (length of ifft)=64, fsampling=fsym*L=...
34 views

### IQ time domain data to Constellation regeneration for a OFDM 16 QAM signal at different power levels to obtain EVM vs Pout information

16 QAM OFDM signal at output of Power amplifier at fixed frequency and different input power levels are received as a IQ data (time domain) by Spectrum analyzer, I would like to generate 16 QAM ...
11k 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-...
1 vote
37 views

### Analyzing the OFDM out-of-band effect when using different guard interval

I am trying to understand the concept of out-of-band (OOB) which occur with OFDM, I have tried to read about it, but I couldn’t understand how I can illustrate that effect with MATLB. This is the ...
1 vote
56 views

### Correct OFDM oversampling method

I need to oversample my OFDM signal, however I have issues understanding which OFDM oversampling method is the recommended one. First method : Before taking the IFFT, the OFDM subcarriers are padded ...
1 vote
46 views

### OFDM design considerations for choosing the number of subarriers $N$

I was trying to think of all possible constraints or factors that needs to be taken into consideration while designing an OFDM-based wireless communication system (For example, LTE, 5G technologies ...
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### OFDM parameters vs channel parameters

I would like to understand the relation between OFDM parameters and channel parameters. I know the following: length of cyclic prefix > delay spread ofdm symbol time > coherence time of the ...
1k 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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### What is use of Quadrature Converter in OFDM?

I'm learning about OFDM Transmitter and see that output of IFFT Block produces complex digital signal and we need DAC to convert it to analog and then use RF Frequency mixing to send it out of ...
140 views

### Relation between sampling time, subcarrier spacing and number of subcarriers

For OFDM, will the following relation always hold: subcarrier_spacing * sampling_time = 1/N N: number of subcarriers which is the size of IFFT/FFT This will imply that sampling frequency is exactly ...
1 vote
106 views

### Perfect channel estimation for OFDM (via H=Y/X) in MATLAB

Despite using the same channel for convolution, each time I get a different estimated channel. Could you please tell me what I'm doing wrong? I want to demonstrate that I can estimate the same channel ...
1 vote
77 views

### Is it possible to consider circular convolution in case of appending the guard interval as a suffix instead of a prefix

In OFDM system, the cyclic prefix is added into the head of time-domain signal to enable the circular convolution with the channel, and then perform one-tap frequency-domain equalization at the ...
97 views

### Modeling an OFDM signal and comparing it with QPSK

I am trying to model an OFDM signal based on original QPSK stream and compare their baseband spectrums. I've used this article as a reference for plotting spectrum and this as a MATLAB OFDM reference ...