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10 votes
Accepted

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). ...
Maximilian Matthé's user avatar
8 votes

Practicality of the assumption of i.i.d. for Rayleigh channels

I am a little late but I post my answer anyway so that someone having the same question will find it interesting and discuss. The discrete baseband multipath channel can be modeled as a FIR, i.e. $$...
AlexTP's user avatar
  • 6,595
8 votes

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 ...
AlexTP's user avatar
  • 6,595
7 votes

OFDM intra-block multipath interference

By using cyclic prefix and its associated demodulation techniques, LTE OFDM (nearly) eliminates the inter-symbol inteference (ISI). The intra-block inteference, if I understand well your question, ...
AlexTP's user avatar
  • 6,595
7 votes

Effect of cyclic prefix and zero-padding in OFDM

The main idea of using Cyclic prefix in OFDM systems is because of 2 reasons, 1) To eliminate ISI from previous symbol. 2) To model linear convolution as circular convolution If an OFDM symbol ...
spectre's user avatar
  • 575
7 votes

What's the reasoning behind changing pilot tones in OFDM?

Advantage of pilot pseudo-random sequences in standards (with examples are taken from LTE) : Interference from other sources with different identities (UE ID, Cell ID, ...) are mitigated if detection ...
AlexTP's user avatar
  • 6,595
7 votes
Accepted

How the increasing of sampling frequency in OFDM didn't cause increase on the required channel bandwidth

You can say the null subcarriers have their own bandwidth if you define an alphabet including "zeros" and use null carriers to transmit these "zeros". As the comment of MBaz, If we start with the ...
AlexTP's user avatar
  • 6,595
6 votes

Why is LTE either FDD or TDD?

You are mixing two things modulation and multiple access. SC-FDMA has been chosen because it is battery-friendly modulation. SC-FDMA is used in both FDD and TDD modes. There are several reasons why ...
AlexTP's user avatar
  • 6,595
6 votes

Sending complex signal over a single wire

Note that signals sent over wires (and over the air, and over any medium) are always real. What quadrature means is that, on a passband channel (wired or otherwise), you can transmit two signals at ...
MBaz's user avatar
  • 15.3k
6 votes
Accepted

What is the difference between (Sample Timing Offset), (Carrier Frequency offset), and (Sampling Frequency Offset)?

I would have the easiest time explaining these briefly with the aid of an eye diagram as depicted in the graphic below. In this example we see the constellation pattern of a raised cosine QPSK ...
Dan Boschen's user avatar
  • 51.9k
6 votes

OFDM use a pulse shaping filter or not?

The purpose of pulse shaping filters is not to overcome ISI as is implied in the OP's question. The only reason for using a pulse shaping filter is spectral efficiency, and in the process ISI can be ...
Dan Boschen's user avatar
  • 51.9k
6 votes
Accepted

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-...
Dan Boschen's user avatar
  • 51.9k
5 votes

IFFT-based OFDM system

Say you want to transmit complex symbols $s_0,s_2,\ldots,s_{N-1}$ using frequency-division multiplexing. In the "bank of modulators" approach, you would assign a carrier frequency $f_k$ to each ...
MBaz's user avatar
  • 15.3k
5 votes

Subcarrier spacing in OFDM

There are many intepretation of OFDM to answer your question. One way I find intuitive is looking at the resolution of (Discrete) Fourier Transform. By capturing a signal during time $T$, and by ...
AlexTP's user avatar
  • 6,595
5 votes

What is the mixed numerology in OFDM

In general, it means several waveforms will coexist in 5G ecosystem. In the context of LTE-based 5G (up to now), several subcarrier spacings (hence CP) coexist. The first and foremost motivation for ...
AlexTP's user avatar
  • 6,595
5 votes
Accepted

Frequency domain samples to time domain without knowing the sampling rate?

Generally the relationship between the sampling frequency $f_s$ and the frequency spacing of each bin $\Delta f$ is given by: $$\Delta f= f_s/N$$ For example if you have 1000 bins and the sampling ...
Dan Boschen's user avatar
  • 51.9k
5 votes
Accepted

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, ...
jithin's user avatar
  • 2,263
4 votes

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, ...
Peter K.'s user avatar
  • 25.8k
4 votes
Accepted

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 ...
Jason R's user avatar
  • 24.6k
4 votes

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,...
Marcus Müller's user avatar
4 votes
Accepted

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 ...
Zeyad_Zeyad's user avatar
  • 1,056
4 votes
Accepted

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 ...
Marcus Müller's user avatar
4 votes
Accepted

Setting Parity Check Matrix for LDPC channel coding

As specified in documentation, using 'ParityCheckMatrix' you can configure the Parity Check Matrix (PCM) during the constructions of the encoder/decoder objects. ...
AlexTP's user avatar
  • 6,595
4 votes
Accepted

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 ...
MBaz's user avatar
  • 15.3k
4 votes
Accepted

Intercarrier Interference (ICI) with OFDM signals

As Marcus mentioned in the comments under the original question, this is complicated with lots to understand. Thankfully, the framework is already here with other posts to at least pursue Marcus' ...
Dan Boschen's user avatar
  • 51.9k
4 votes
Accepted

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 ...
Marcus Müller's user avatar
4 votes
Accepted

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, ...
Marcus Müller's user avatar
4 votes

What type of LO lekage Correction algorithm is used in 5GNR?

In direct conversion transceivers, the LO leakage is pretty much a given. Some systems, mainly OFDM systems like Wifi, deal with that by simply leaving the DC carriers unused. And that's it. No ...
Marcus Müller's user avatar
4 votes

What type of LO lekage Correction algorithm is used in 5GNR?

Just want to provide some details to complement Marcus's answer. (1) 5G NR does not avoid DC carrier, (2) it is possible to use it to modulate data (accepting degradation), and (3) 3GPP does not ...
AlexTP's user avatar
  • 6,595
3 votes

Beginner's guide to OFDM

In time domain, OFDM breaks one serial fast bit stream into many parallel slow bit streams. Then, these parallel slow bit streams are multiplied with orthogonal sinusoids, where orthogonality between ...
QMC's user avatar
  • 784

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