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Few questions on wireless and LTE

Started by Sharan123 3 weeks ago7 replieslatest reply 3 weeks ago86 views

Hello,

I have a few questions related to wireless and I will use LTE standard to put these questions.

1) if we look at the sub-carriers frequency plot, we see sub-carriers having peaks with deltaF carrier sub-spacing but the frequencies essentially spread out well into other sub-carriers although at a much lower amplitude. So, does this not impact in terms of interference?

2) On the modulation for transmission, the IQ data is mixed with carrier data, added and transmitted.

Is there a term for this modulation type?

Thanks a lot ...

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Reply by SlartibartfastSeptember 28, 2019

1) Inter Carrier Interference (ICI) is minimized by proper synchronization in both time and frequency.   Multipath and Doppler can still spread ICI energy around, so the short answer is, yes, there is impairment due to ICI.  This is why synchronization and system issues are important, to keep it low enough that the system still has acceptable performance.

2) The modulation type is Orthogonal Frequency Division Multiplexing (OFDM), and the mixing you are describing sounds like "direct conversion".

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Reply by Sharan123September 28, 2019
2) The modulation type is Orthogonal Frequency Division Multiplexing (OFDM), and the mixing you are describing sounds like "direct conversion"


As I understand, first constellation points are generated, OFDM is applied and then the IQ data is used for RF modulation before transmission.

The question is about the final modulation with IQ data. Are you sure this is OFDM?


BTW, added one more question below

3) when we refer to say 20 MHz bandwidth, does it refer to the rate at which the baseband data is used for final RF modulation?

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Reply by rrlagicSeptember 28, 2019

LTE runs at 30.72 MHz if that was 20 MHz bandwidth . Remaining parts are not used, so there are guard bands at sides.

Subcarriers are orthogonal, i.e. make no influence on each other ideally, but practically depends on sync.

IFFT is efficient way to produce that kind of signal.

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Reply by SlartibartfastSeptember 28, 2019

RF mixing or "modulating" doesn't change the signal, it just translates it in frequency.   It is still OFDM.

20MHz bandwidth means the signal occupies 20MHz of bandwidth.

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Reply by Sharan123September 29, 2019
20MHz bandwidth means the signal occupies 20MHz of bandwidth

How can I ensure that the signal occupies 20 MHz bandwidth.

I am sure it does not happen automatically. 

Is it through proper sampling rate?

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Reply by kazSeptember 29, 2019

The eNB (at base station) generates 20MHz (Downlink) by inserting 1200 subcarriers(tones) into iFFT of 2k then adds cyclic prefix and samples this ifft output on 30.72Msps.

This gives a bandwidth of 1200/2048*30.72 = 18Mhz. Leaving 1MHz guard on either side.


Further upsampling and frequency translation before DAC or at RF is then applied.

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Reply by Sharan123September 29, 2019

thanks a lot