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LTE OFDMA Physical layer basic question
Posted on 03. May, 2013 by shethtejas104
in LteWorld Forum
I have only recently started understanding the PHY layer of ODFMA based LTEi. Visualizing what exactly happens at Layer 1 is a little more challenging than at protocol stack level. I have so far understood clearly the meaning of OFDMA frame structure (TDD for example sake), and details like the length of a frame/subframe/slot/OFDMA symbol in time domain and subcarrier spacing in frequency domain and ResourceBlock size etc. Understanding these details is no big deal.
The problem follows after that.
What exactly is "one OFDMA symbol" and how much data does it carry? I read somewhere and I quote "One symbol is a certain time span of a signal that carry one spot in the I/Q constellation". I understand I/Q constellation and I also understand one OFDMA symbol as per LTE specs. But One OFDMA symbol extends across 1200 subcarriers in a 20 MHz band. And as per 36.211, there are 3,07,200 samples in a frame, 30720 samples in a TTI or subframe, and 15360 samples in a slot (= 7 OFDMA symbols).
Now if I try to combine all the concepts, one OFDMA symbol (in time) has 1200 center frequencies or subcarriers' information over time span of (66.7) microseconds and this occupies on spot in I/Q constellation. This doesn't make sense. Something is wrong. Please help me clarify this. I hope I have been able to word my question properly. Any link on this matter will also help greatly.
As an example to explain my confusion, let us take PRACH Physical channel for example :
The spec says it occupies 6 Resource blocks, which is equal to 1.08 MHz, and it has 839 subcarriers (plus 25 guard subcarriers). This is in Frequency domain. In terms of time domain, its a preamble which can occupy different lengths depending on its format. If I take format zero, it occupies a total of 27,744 samples including CP. So here's the question : You have 27744 samples in time domain. This equates to roughly 903.125 microseconds. So you have 27744 samples transmitted over a period of 903.125 seconds, and during this time period, 839 subcarriers get transmitted. Does it mean that each of the subcarriers had 27744 samples' data?
