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#1 2017-Oct-11 20:44:35

madhu
Member
Registered: 2017-Oct-11
Posts: 2

Reg: Clarification on change of no. of subcarriers in OFDM

In WARP MIMO-OFDM example, the following code has been used. If I want to use any other subcarriers like 128, 256, etc, How to write (split) SC_IND_PILOTS and SC_IND_DATA manually?

        SC_IND_PILOTS           = [8 22 44 58];                           % Pilot subcarrier indices
        SC_IND_DATA             = [2:7 9:21 23:27 39:43 45:57 59:64];     % Data subcarrier indices
    N_SC                    = 64;                                     % Number of subcarriers
    CP_LEN                  = 16;                                     % Cyclic prefix length

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#2 2017-Oct-12 14:51:42

chunter
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From: Mango Communications
Registered: 2006-Aug-24
Posts: 1212

Re: Reg: Clarification on change of no. of subcarriers in OFDM

Just to clarify, this question is regarding the the WARPLab example? That example is pretty well parameterized, but I'm not entirely sure that you can change the number of sub carriers simply by adjusting N_SC and the pilot/data indices. You'll have to try it and see.

Those pilot and data locations for the normal 64 subcarrier case were inspired by the 802.11 standard, but the implementation isn't a compliant physical layer. So, you really aren't bound to anything when extending to higher number of subcarriers. Of the 64 subcarriers, 48 are carrying data. The remaining subcarriers are used for pilots (4 pilots) and some are simply unused because they are near the radio's filters (e.g. the DC bin at index 1 and the high frequency bins from indices 28 to 38). It's your choice how to divvy up your 128 or 256 subcarriers.

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#3 2017-Oct-12 20:25:25

madhu
Member
Registered: 2017-Oct-11
Posts: 2

Re: Reg: Clarification on change of no. of subcarriers in OFDM

These codes are given in example WARP lab code for MIMO-OFDM. Is it possible to split the range of data subcarriers and pilots on my own? Because, the difference between the numbers are different. Could you please answer for the above? Based on the answer, I need to proceed further. Thanks.

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#4 2017-Oct-12 22:24:22

murphpo
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From: Mango Communications
Registered: 2006-Jul-03
Posts: 5159

Re: Reg: Clarification on change of no. of subcarriers in OFDM

The WARPLab OFDM examples use the 64-subcarrier scheme defined in the 802.11a/g spec. The rest of the example OFDM implementation is not compliant to the 802.11 spec.

It would be straightforward to use 64 subcarriers but change the indexes of pilot and data subcarriers; this should only require changing the SC_IND_ variables. If you want to use more subcarriers you will need to study the code in detail and change all the places that assume 64 subcarriers. This includes the SC_IND_ variables, plus the definitions of the training symbols in the preamble, the computation of channel estimates, CFO and SFO estimation, etc. It's definitely feasible to make these changes but, as we've never done this, we cannot instruct you on how to do it.

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