Pass band in digital up conversion of LTE signals

Started by LabPe43 3 years ago6 replieslatest reply 3 years ago247 views

Hi,

I am new to filters and DUC (digital up conversion). I was reading an article from xilinx regarding DUC.

It was using a passband of 9.015 MHz in its first stage of filters (Single rate filter) for a 20 MHz bandwidth with Fs = 30.72 MHz

In #LTE 20 MHz, the data carriers occupy 18 MHz so wonder why the pass band is chosen to be half of occupied bandwidth? 

Thanks,

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Reply by Tim WescottJune 10, 2016

Much of the story is on Figure 5 on page 5.  The filter (and signal) that they're referring to is at baseband.  At baseband, signal bandwidths are measured from 0 to a positive frequency -- but when you upconvert from baseband, you upconvert both the positive and negative image of the signal.  Depending on your point of view, either the original bandwidth term was off by a factor of two (but try changing it now -- Hah!) or going up from baseband is a special case.

Going back to Figure 5, the occupied bandwidth as shown is from -10MHz to +10MHz, but nearly any sane engineering specification will call it out as 10MHz.  When you up-convert, suddenly the occupied bandwidth doubles, going from carrier-10MHz to carrier+10MHz.

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Reply by oliviertJune 10, 2016

Hi LabPe43  (are you a cousin of C-3PO?),

The signal to filter is complex and uses the band [-9MHz; 9MHz].

The filter itself is real that's why we choose 9MHz of bandwidth, as a real filter will keep the overall band [-9;9] MHz.

This real filter is used on the real part and the imaginary part. At the end you will keep the complex signal within the band [-9;9]MHz.

Olivier

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Reply by Rick LyonsJune 10, 2016

LabPe43,

How can we explain the signal processing described in a Xilinx article if you didn't give us a web link so we can read the article?

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Reply by LabPe43June 10, 2016

The article can be found at http://read.pudn.com/downloads160/doc/718492/LTE%20DFE%20App%20Note.pdf

line 6, page 12.

However, I think this is something common to all wireless standards. The filter passband is chosen to be BWoccupied/2

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Reply by raphJune 12, 2016

Dear LabPe43

According to your description, xilinx advise to use a passband of 9.015 MHz in its first stage of filters (Single rate filter) for a 20 MHz bandwidth with Fs = 30.72 MHz.

I understand Xilinx refer to complex filter (I/Q implementation). Which means the spectrum view of bandwidth is -9.015 to +9.015MHz. The total bandwidth is 18MHz. The up converter IP is able to deal with complex input.

Page 15, there is a description of this complex implementation of the filter.

Best regards,
Raph


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Reply by kazJune 12, 2016

no the filter need not be complex.I and Q are filtered separately with a filter of passband = half of lte bandwidth in air. This was already explained by Tim. Baseband width is half upconverted bandwidth


Kaz