# Tag Info

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### Flat fading criterion of OFDM subcarrier spacing

Your mathematical derivation is correct, your $H[k]$ is the single-tap equalizer (i.e. one tap for each subcarrier, and the subcarriers do not mix with each other. That's the orthogonal in OFDM). ...

### Coherence bandwidth/delay spread in OFDM

Indeed, the constraint $T_s(N+N_{cp}) \gg T_m$ should be derived as $T_s(N+N_{cp}) \gg T_s N_{cp} \geq T_m$. The first inequality to make sure that the CP overhead is as small as possible. The second ...

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### OFDM time vs. frequency domain channel estimation/equalization

Traditionally, OFDM became popular in WiFi and LTE because the channel model consisted of multi-path. That is, the radio signal transmitted in 1-6GHz frequencies bounced from various obstacles (walls, ...
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### how to design the $\text{sinc}(\cdot)$ filter for OFDM oversampled system

Resampling the transmit OFDM waveform in the frequency domain by zero-padding the FFT prior to taking the IFFT has the advantage of simplicity and maintaining sub-carrier orthogonality (no inter-...

### Number of samples in one period of subcarrier after IFFT

You need as many samples as you have subcarriers. If you have 64 carriers, then you need 64 samples.

### Multipath resistance of spread spectrum systems

The key idea in spread-spectrum communications is to use much more bandwidth than the minimum required for the signal being sent. Multipath effects occur when the sent signal $s(t)$ and a delayed, ...
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### The necessity of null subcarrier in OFDM?

The only portion of the spectrum that is "occupied" or "transmitted" are the subcarriers that are actually used (i.e. 10 MHz instead of 15 MHz). The null subcarriers referenced in your question are ...

### Difference between Fine and Coarse Frequency Offset in OFDM

If in doubt, read the original Schmidl&Cox Paper Robust Frequency and Timing Synchronization for OFDM. It's actually a good read, and a good paper with respect to demonstration of state of the art,...
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### Why Convolution between Signal and a Channel Is Not Done Directly in SIMO Channel

I've said, in another comment, convolution using the function conv "i.e in MATLAB" and convolution using the Toeplitx matrix must give the same results. That's ok. Now, according to your code, the ...
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### What is the forward error correction in 802.11a?

From the official standard, IEEE Std 802.11a-1999, Part 11, section 17.3.5.5 Convolutional encoder, see the rate 1/2 code: The resulting two streams A and B are interleaved bit-alternatingly. The ...
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### Spectral efficiency of with OFDM vs without OFDM?

The spectral efficiency of OFDM is strictly worse than that of a pulse-shaped QAM signal with the same rate. OFDM requires a guard interval, on which no useful information is transmitted. It is ...
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### Can an OFDM waveform be spread (DSSS)?

You can do a lot of things. Some are sensible! Remember why you're using OFDM (frequency-domain equalization) and remember that the temporal structure of the OFDM symbol (as the IDFT of the data ...
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### Frequency-domain vs Time-domain OFDM oversampling

Your first method is just not using the subcarriers at the band edges (the ones that you zero-pad). It is hence equivalent to a sinc-interpolation in the time domain. Finally, a cyclic prefix (or ZP, ...
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### OFDM demodulation, how to set the sampling rate?

Your receiver will consist of, apart from synchronization blocks, an FFT. You want that FFT to generate one output bin for each of the subcarriers. Therefore, you should make use of the relationship ...
In your example, the sinusoids are still orthogonal to one another; the key is that you must integrate over the correct period when calculating $P$. All of the sinusoids have frequencies that are ...