To Question or Not to Question? That Is the Question !

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René Descartes was a French philosopher and his best known philosophical statement is “I think, therefore I am”. Statement itself can be argued. However, whole idea is that — if you can question about your own existence, it is enough proof of your existence. If you didn’t exist, you wouldn’t be able to question it.

Questions are important. It may sound trivial but asking right scientific questions is not an easy task. objectives of asking questions may differ, one asks questions to discover something new, one asks questions to understand already well established facts. Questions and subsequently answers give clarity of thought and you get clarity about something you did not understand.

However, most obvious question really is: Can I understand something new in science or technology without asking a question. Almost always answer to this question is ‘NO’. And, then another question is: What really is a good question specially in scientific context. Some would say that a good scientific question is something that can be answered by direct observations, experiments and results of those experiments. These questions are different from questions that are based on people’s opinion or their belief.

In this blog post (and upcoming blog posts) I want to list some of the question that can be asked to understand 5G-NR physical layer.

Question-1: In LTE, Cell Specific Reference Signal (CRS) is an Always-On signal and transmitted in 4 or 6 OFDM non-continuous symbols of each sub-frame. LTE and 5G-NR can share the same DL frequency. So, what are design choices made in design of 5G-NR DL signal so that collision with LTE-CRS can be avoided.

Question-2: In 5G-NR, PSS/SSS/PBCH signals are structured in time and frequency in a very specific way and this structure is known as SS-PBCH block (or SSB in short).

  • What is the reason that this structure uses 240 sub-carries in frequency domain and is 4 OFDM symbol long.
  • Why SSS is sandwiched between two PBCHs ?
  • Were there other alternatives arrangements discussed, what were the disadvantages of other alternatives ?

Question-3: In LTE, Synchronization signals are always located at the center of the RF carrier. There is no decoupling between channel raster and synchronization raster. When UE detects PSS/SSS, it also knows center of the RF carrier.

  • What is the reason that 5G-NR decouples channel raster and synchronization raster ?
  • How does UE know exact SS block frequency-domain location on the carrier ?

Question-4: Multiple PUCCH formats are specified in NR to carry various UCI payload sizes on different formats.

  • Why sequence based design was adopted for PF0 in 5G-NR ?
  • Why DMRS based design with evenly distributed pattern and 1/3 overhead was adopted for PF2 in 5G-NR ?
  • Why specific DMRS based design was adopted for PF1 and PF3 in 5G-NR ?

Question-5: Why specific design choices are made for synchronization signal design?

  • Why does 5G-NR use m-sequence for PSS instead of ZC-sequence based as in 4G-LTE ? 
  • Why does 5G-NR use gold-sequence for SSS instead of interleaved concatenation of two m-sequences with different cyclic shifts as in 4G-LTE ? 

.. To be Continued

~~Peace

Dheeraj