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).
...
- 6,068
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 ...
- 5,760
7
votes
Accepted
Purpose of interleaving in OFDM systems
Interleaving is not specific to OFDM. It is an age old technique used for improving error correcting codes.
Basically, most error correction coding techniques are good at suppressing bit-wise errors ...
- 5,589
7
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.
$$...
- 5,760
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 ...
- 5,760
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 ...
- 5,760
6
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, ...
- 5,760
6
votes
What Is the Difference between RLS, LMS and Wiener Filter? When Is One Preferred Over Another?
All three are Estimators / Predictors.
All of them try to estimate the coefficients of Linear Filter which minimizes an MMSE Cost Function.
The Wiener filter assumes all data is given and sets the ...
- 40.7k
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 ...
- 5,760
5
votes
Accepted
why LTE doesn't use DC subcarrier in the downlink?
The DC subcarrier is normally omitted because in a zero-IF receiver it overlaps with DC offsets in the hardware which are very difficult to remove.
- 570
5
votes
Accepted
Really basic question about OFDM
Either of your suggested approaches would work (adapting modulation/coding scheme on a per-subcarrier basis or changing all subcarriers at once). There's no theoretical constraint that prevents you ...
- 23.7k
5
votes
Hermitian symmetry in OFDM systems
The Hermitian symmetry is used to obtain a real-valued time-domain signal. It is a special case of OFDM called discrete multitone (DMT). It exploits a property of the discrete Fourier transform (DFT), ...
- 4,071
5
votes
Accepted
Alamouti code after or before IFFT block in OFDM
The basic concept of OFDM is to divide a high-bitrate datastream into $N$ low-bitrate datastreams and to multiplex these low-bitrate datastreams in frequency. That is, every datastream is assigned to ...
- 4,071
5
votes
What is the advantages of BPSK+OFDM over usual BPSK (with no OFDM Modulation)?
In a pure AWGN channel there is no benefit from OFDM because the noise will affect the received signal in the same way regardless of whether sent as a higher bandwidth serial stream or lower rate ...
- 51
5
votes
Accepted
Can we do cyclic prefix with zeros in OFDM rather than with samples from end of the ofdm symbol?
There are two major approaches for cyclic extension in OFDM systems - CP (cyclic prefix) and ZP (zero padding, also called Trailing Zeros, TZ). Generally they show the same performance, I mean in AWGN ...
- 1,023
5
votes
Number of samples in one period of subcarrier after IFFT
If you'd like to think analog, an OFDM signal can be written as a sum of weighted complex sinusoidals:
$$
x(t)=\sum_{k=0}^{N-1}X_k \exp{\left( \mathrm j 2\pi\frac{kf_\mathrm{s}}{N}t \tag{1}\right)},
$$...
- 4,071
5
votes
IFFT-based OFDM system
Say you want to transmit complex symbols $s_0,s_2,\ldots,s_{N-1}$ using frequency-division multiplexing. In the "bank of modulators" approach, you would assign a carrier frequency $f_k$ to each ...
- 13.8k
5
votes
Sending complex signal over a single wire
Note that signals sent over wires (and over the air, and over any medium) are always real. What quadrature means is that, on a passband channel (wired or otherwise), you can transmit two signals at ...
- 13.8k
5
votes
Subcarrier spacing in OFDM
There are many intepretation of OFDM to answer your question. One way I find intuitive is looking at the resolution of (Discrete) Fourier Transform.
By capturing a signal during time $T$, and by ...
- 5,760
5
votes
What is the mixed numerology in OFDM
In general, it means several waveforms will coexist in 5G ecosystem.
In the context of LTE-based 5G (up to now), several subcarrier spacings (hence CP) coexist. The first and foremost motivation for ...
- 5,760
5
votes
Accepted
What is the difference between (Sample Timing Offset), (Carrier Frequency offset), and (Sampling Frequency Offset)?
I would have the easiest time explaining these briefly with the aid of an eye diagram as depicted in the graphic below.
In this example we see the constellation pattern of a raised cosine QPSK ...
- 37.1k
5
votes
Accepted
Frequency domain samples to time domain without knowing the sampling rate?
Generally the relationship between the sampling frequency $f_s$ and the frequency spacing of each bin $\Delta f$ is given by:
$$\Delta f= f_s/N$$
For example if you have 1000 bins and the sampling ...
- 37.1k
4
votes
Accepted
What type of correlation is this equation?
I understand your confusion because the equation is barely understandable from the information given in the book. It becomes more clear from the original paper by Classen and Meyr [1] from which it ...
- 4,071
4
votes
OFDM modulator in LTE
Definitely yes. But after IFFT operation data block is CP extended before transmission.
Each sample of OFDM symbol from the output of IFFT is weighted sum of all QAM/PSK modulated subcarriers from the ...
- 1,023
4
votes
Accepted
OFDM modulator in LTE
Yes. Cyclic Prefix and windows(prefix and postfix for pulse shaping) are added to the OFDM symbol, but windows are only for overlap-and-add for pulse shaping, so the effective length is M+CP_length ...
- 496
4
votes
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.
- 11.8k
4
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 ...
- 505
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, ...
- 22.5k
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 ...
- 1,046
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 ...
- 26.3k
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