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DSSS in SmartRF for CC1200

Daniel M
Prodigy 80 points
Other Parts Discussed in Thread: CC1200, CC1125, CC1120

I tried to use DSSS with FEC in SmartRF for the CC1200.

It didn't work.


Is it a Bug or am I doing something wrong?

Thanks in advance

  • 0
    TI__Guru**** 317180 points

    Why do you want to use FEC in combination with DSSS since both use code gain? A quick look could indicate that it is not possible to run FEC and DSSS in Rx.

  • 0 in reply to TER
    Prodigy 80 points

    Hi TER,

    I would like to use DSSS+FEC in order to widen the BW and release the requirements from TCXO accuracy. I don't see conceptual/theoretical problem using DSSS with FEC. Their benefits should sum up without interference.

    So if I understood correctly, CC1200 does not support PN DSSS and FEC at the same time? I could not understand it from the datasheet or user's guide.

    Also, can you tell what is the coding gain of PN DSSS and 16bit Repeat DSSS modes? I have tested PN DSSS at 1.2kbps and I don't see 6dB coding gain as I expected (spreading x4). It may be because the despreading at receiver side is performed on hard- and not soft- bits?

  • 0 in reply to Daniel M
    Prodigy 80 points

    Hi,

    I'll be happy to get some answers. I think it is important information to understand the trade-offs while choosing CC1200 configurations.

    Thank you in advance.

  • 0 in reply to Daniel M
    TI__Guru**** 317180 points

    Sorry about the delay, this disappeared a bit due to vacation.

    CC1200 does not support FEC + DSSS at the same time. The short explanation is implementation limitations (time/ different focus)

    The receiver is non-coherent and hence the theoretic gain is not possible to reach. 

  • 0 in reply to Daniel M
    TI__Intellectual 960 points

    Daniel M, 

    I have measured the coding gain to be in the range of 3-4dB. The reason for not getting 6dB (10log4) (that would the the theoretical limit) is some implementation loss in the demodulator (speed, meaning few preamble bits needed has been prioritized). I agree that it would be nice if both FEC and DSSS, but unfortunate this is not possible. Note that the FEC that is implemented is defined by IEEE 802.15.4g.

    Another way of reducing the frequency accuracy for the 40MHz reference is to use the "feedback to the PLL"  (FB2PLL) features. In the SmarRF studio software, you can easily enable and get more info about this feature - just tick  the "Advanced RF parameter" box (up, middle). 

    If you want to go even lower in terms of RX bandwidth, then CC1120 and CC1125 is a better option. CC1125 can go down to 3kHz RX bandwidth