Comments on: The basics of 5G’s modulation, OFDM

By: SAIMANIKANTHA GOPALAM

SAIMANIKANTHA GOPALAM — Thu, 08 Sep 2022 19:59:51 +0000

“that bandlimited orthogonal signals can be combined with significant overlap while avoiding interchannel interference.”
Dear sir,
Here what is the meaning of avoiding interchannel interference (How)

By: Bob Witte

Bob Witte — Mon, 21 Feb 2022 05:10:45 +0000

In reply to Manjunatha K H.

In general, any modulation scheme can be used across a wide variety of frequencies. Modulation type is usually driven by the application, the available bandwidth, and regulatory requirements.

5G spectrum is divided into Frequency Range 1 and 2 (FR1 and FR2). FR1 is below 6 GHz and FR2 is ~24 to 52 GHz. So the 5G modulation methods may be used in those frequency ranges. Also, Wi-Fi 4, 5, and 6 use the 5 GHz band with OFDM and a variety of modulation types. See https://www.eeworldonline.com/ofdma-improves-spectrum-use-in-wi-fi-6/

The FCC frequency allocations between 5 GHz and 10 GHz include radiolocation (RADAR) and satellite communications. Satellite comms are usually wideband digital signals and may use OFDM. See https://blogs.keysight.com/blogs/tech/rfmw.entry.html/2020/08/24/modulation_schemesf-6MpZ.html

By: Manjunatha K H

Manjunatha K H — Tue, 15 Feb 2022 10:27:54 +0000

Which modulation scheme can be used at 5.1 GHz to 10 GHz applications

By: Bob Witte

Bob Witte — Mon, 03 Jan 2022 17:53:55 +0000

In reply to Kadhiem. The lobes shown in Figure 4 are the result of the iFFT and occur before the DAC. Mathematically, (sin x)/x or sinc functions create these lobes. The figures in Ref 4 may be helpful. Of course, the DAC may also introduce additional frequency components.

By: Martin Rowe

Martin Rowe — Mon, 03 Jan 2022 15:20:51 +0000

In reply to Robert Ladley.

Robert,

Thanks for your comment. We just posted an article on the altimeter issue. See https://www.5gtechnologyworld.com/5g-altimeter-interference-aviation-versus-telecoms/

By: Robert Ladley

Robert Ladley — Wed, 22 Dec 2021 17:04:38 +0000

Bob.
great article particularly to me as an aerospace avionics engineer with a continuing interest in the airborne FMCW Radio Altimeter interference potential . I did not appreciate the fact that 5G uses OFDM both from the base station and the user device unlike the one way 4G.
Whilst the aerospace community likes devices to be “OFF” or aeroplane mode in flight this is commonly not the case and it is almost impossible to “police” I would like to see some extensive testing to ascertain the interference potential prior to full implementation of 5G. Current discussion seems concentrated on the ground stations with little consideration of the PED activity.

By: Kadhiem

Kadhiem — Fri, 03 Dec 2021 03:03:21 +0000

Thanks Bob for the clear explanations.
From Figure 3 to figure 4:
I assume an iFFT generates exact tones with single nonzero sample and that when we talk about lobes we mean the frequency domain in real world (after DAC).
The lobes of figure 4 may also occur before DAC if we consider phase discontinuities of a stream of symbols.
Am I right?

By: Bob Witte

Bob Witte — Tue, 12 Oct 2021 21:34:29 +0000

In reply to Lewis Bates.

Lewis,
I think you just wondering what causes the symbol time to be the reciprocal of the subcarrier spacing. The short answer is that it is a characteristic of the FFT.

Here’s the slightly longer answer:
The time duration of a symbol will be the time associated with N samples: N (1/fs) = N/fs.
where fs is the sample rate.
The spacing between subcarriers is also tied to fs and the number of samples N. Again, this is a characteristic of the FFT. The N subcarriers in the frequency domain are spread evenly across the fs frequency range. That gives us the subcarrier spacing of fs/N.
So the symbol time is N/fs, which is the reciprocal of fs/N.

By: Lewis Bates

Lewis Bates — Wed, 06 Oct 2021 05:44:35 +0000

Can you please explain the relationship between subcarrier spacing and symbol duration? I understand that symbol time is the reciprocal of the subcarrier spacing, and that larger sub-carrier spacing therefore yields a shorter symbol/slot duration (which enables, for example, both eMBB and URLLC services in 5G), but I don’t understand exactly what the basis of that relationship is. Thanks.

By: Bob Witte

Bob Witte — Wed, 24 Feb 2021 00:16:40 +0000

Ted,
Good question and I am not completely sure of the answer.
In general, OFDM is very robust in dealing with channel imperfections. For example, it handles multipath distortion well, which is common in mobile wireless systems. Based on the literature I’ve read, the concerns around going higher in frequency are mainly due to increased channel loss, not loss of orthogonality.

Bob