Electrical Engineering Stack Exchange
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Questions tagged [ofdm]

OFDM stands for Orthogonal Frequency Division Multiplexing. It is a communications transmission technique that breaks the carrier into multiple sub-carriers (hence, the multiplexing in frequency).

OFDM stands for Orthogonal Frequency Division Multiplexing. It is a communications transmission technique that breaks the carrier into multiple sub-carriers (hence, the multiplexing in frequency). Furthermore, the sub-carriers are orthogonal. OFDM accomplishes this through the application of an IDFT or IFFT in the transmitter. On the receiving side, an FFT or DFT is used.

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Guard bands in OFDM

Consider the use of OFDM in 802.11a (5GHz band) as shown in this picture taken from NI's White Paper: For a 20MHz channel with a frequency spacing of 0.3125MHz between the subcarriers, there are a total of 64 possible sub-channels. As per the…
V-Red
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MU-MIMO - do all antennas stream in different frequencies?

I have few questions on how MU-MIMO in WiFi works. Do all antennas stream in different frequencies or stream in same channel but different sub carriers(OFDMA?) Do MIMO antennas always work in OFDM technology?
Franc
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Minimum data rate for OFDM?

I'm relatively new to electronics, so sorry for my awkward understanding of topics. I'm just trying to wrap my head around some basic concepts here. From my understanding, in OFDM, the information that we want to send (with a high data rate) gets…
Noel93
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How are the neat sinc function shaped carriers produced in OFDM?

I am learning about OFDM and QAM. Every explanation I have read illustrates how the separate carriers are sinc functions that are arranged to overlap in a nice orthogonal way so as to cancel out interference from neighbouring carriers. The part I…
John Spence
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Maximum transmit power allowed for 802.11g

I'm learning about the 802.11 regs. In a book I'm using, "CCNA Wireless 200-355 Official Cert Guide (Certification Guide) 1st Edition by, David Hucaby (Author). Page 53", it has the following clause about 802.11g. Be aware that 802.11g has the…
Frank
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Wireless channel interpretation

Is there a way to estimate how fast a channel changes with respect to time in practical applications? While evaluating doppler effect for a user we use the frequency at which the channel changes. How is this frequency different from the carrier…
Mayank Mehra
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Symbol-level frequency-selective channel model representation (in OFDM)

The general symbol-level representation for a frequency-selective channel is: $$ y[k] = \sum_{l=0}^{L-1} s[k-l]h_{l} + z[k] $$ where \$s[k-l]\$ is transmitted symbols, \$z[k]\$ is noise, \$L\$ is the channel length. Here \$h_l\$ only have values…
dtouro
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Alternative to "Header/Payload Demux" block in GNU Radio for OFDM frames with a fixed length

I'm fairly new to GNU Radio and was looking for a way to communicate using OFDM without using packet headers. I am trying to implement a simple version of OFDM with fixed length frames. I have currently implemented a simple loop-back example without…
MattyK
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Can I reconstruct the channel impulse response from OFDM least square channel estimation?

I'm working on a successive interference cancellation (SIC) project for an OFDM system. I used pilots and least square channel estimation to estimate the channel frequency response (CFR.) It works just fine for recovering the QPSK data in the…
Han Miao
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What is the relation between symbol duration and subcarrier frequency spacing in OFDM?

What is the exact relation between symbol duration and frequency spacing? You may know in WiFi6, about 11ax band, there are more sub-carriers compared to WiFi5 because of using shorter frequency spacing, but also uses a longer symbol duration. I…
ssabassaba
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Why the does channel capacity not increase as the number of subcarriers increases?

We usually consider multi-carrier (interchangeably multi-channel) communications. By using the multi-carrier system, instead of one-broadband system, we can obtain many advantages, for example, able to send multiple data streams simultaneously,…
Danny_Kim
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OFDM IDFT implementation

i have some doubts with the implementation of the IDFT in OFDM systems. The question concerns the expression of the IDFT of the OFDM signal. During the symbols period \$T_s\$ we have the following base-band OFDM signal: $$x(t)=\sum_{m=0}^{N-1}X_m…
Marco Toni
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How to calculate subcarriers bandwidth and cyclic prefix length in an OFDM using the IFFT

I'm trying to understand how OFDM works thanks also to the Simulink OFDM block. Now, I have understood that the OFDM modulation "convert" a faster bit rate using a certain bandwidth into a number of slower bit rate streams using a series of…
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Why the frequency spectrum of my OFDM output signal has a series of spike?

I'm trying to understand OFDM modulation using Matlab Simulink and I've built the following system I sent to the input of the OFDM block a frame of 15 samples that comes from the 16-QAM modulator. The OFDM modulator also receives a frame of 15…
NicoCaldo
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How to calculate bit rate (physical layer) in ISDB-T (OFDM system)?

I was looking at OFDM (ISDB -T) specification. I couldn't figure out how they get the bit rate of 16.851 Mbps by having the following information? symbol length: 1.008 ms carrier interval: 0.992 kHz GI ratio: 1/8 G.I. length: 0.126 ms Bandwidth:…
TheFirefly
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