How to improve 433MHz Bit Error Rate in environment with multipath propagation

Discussion in 'Wireless & RF Design' started by rf_noob, Jul 25, 2018.

  1. rf_noob

    Thread Starter New Member

    Jul 25, 2018
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    Hey RF experts, I hope I am not re-posting a similar question but I couldn't find any answers when I did a search. I am primarily a firmware programmer and have limited exposure to RF applications, and am running into RF performance issues with a garage door opener type of application. The product uses the ~433MHz band and we are using SOCs that do the modulation and demodulation for us. I am currently using FSK with Manchester decoding at 8K symbols/sec.

    I found that this works very well when there is Line Of Sight between the transmitter and receiver but whenever there is Non Line of Sight and in an enclosed environment like in a garage with reflecting walls and ceiling, the stream of data gets clobbered beyond repair by conventional forward error correction codes such as Hamming. I did not observed small bursts of corrupted bits but rather the whole packet is badly corrupted so that correctly preserved bits were the exception rather than the norm. I am guessing severe intersymbol interference?

    I am not able to disclose the modem chips used due to NDAs but I am wondering if there are any settings I can play with to improve the bit error rate. I tried to see if I am able to configure the sampling window of the receiver so that it uses the cleanest part of the symbols and avoid the parts that were spilled over from the previous symbol, but I can't seem to find any setting that allows me to. To begin with, I don't fully understand intersymbol interference. All the literature I found describes ISI of binary pulses. I don't really understand the way two adjacent RF symbols blend into each other.

    These settings are available according to the data sheets:

    Transmitter side:
    - Gaussian shaping
    - pre-emphasis

    Receiver side:
    - channel bandwidth

    Any way of improving the bit error rate? Thanks folks.
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Although probably not a viable option unless you are in a position to do low-level development work, concurrent-code spread spectrum has very high intrinsic immunity to multipath interference.

    We can discuss it further if you think you might be in a position to do the development work.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    Or just slowing your data rate way down might do the trick.
     
  4. rf_noob

    Thread Starter New Member

    Jul 25, 2018
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    Hey WBahn, thanks for the idea. However the hardware doesn't do frequency switching very well. Unless I write to the EEPROM and do a reset every time. We could upgrade to better hardware, but that's a different story.
     
  5. rf_noob

    Thread Starter New Member

    Jul 25, 2018
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    Really not an option! :)
     
  6. rf_noob

    Thread Starter New Member

    Jul 25, 2018
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    Would antenna diversity work in this case? Something cost effective.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    6,946
    Concurrent code spread spectrum doesn't require frequency switching. There are a few possible ways to implement it. The simplest is using OOK (on-off keying). Can your hardware do that?

    What is your required data rate (bits per second)?

    What is the maximum data latency that is tolerable?
     
  8. rf_noob

    Thread Starter New Member

    Jul 25, 2018
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    Hi WBahn, I am still waiting on the other team to provide me with the requirements, but I'd say 200ms latency and 8kbit/s data rate at least. The hardware does OOK and FSK, but not PSK.
     
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