How can I improve RF timing performance?
I am using a RF communication system. Currently I'm getting a round trip time around 30mS and I want to reduce it having distance between the two nodes fixed.
I have following options:
1. Using BPSK @40kbps data rate
2. Using O-QPSK @ 250, 100 or 1Mbps data rate.
3. Using Directional antenna instead of omni directional antenna.
4. Increasing the power
Let me know which factor will have the impact on time.
Thanks,
Janakfun
It looks like you're going with 802.15.4 sub 1 GHz. All options you mention has probably nothing to do with your problem.
You should take a look to implemented protocol in your system.
Mazz
Yes Mazz,. You guessed it right...
I'm using IEEE 802.15.4 1 GHz radio..
But my question is
at least modulation scheme or data rate should get me timing improvement.
And if not then please let me know the reason..
Thanks
Why do you expect that modulation or data rate will affect it?
Data rate could do, but it seems strange that your payload is so big to take 30ms. If using 40kbps, it has to be larger than 150 bytes.
If this is your case, so higher data rate will reduce it for sure.
Mazz
the modulator and de-modulator in your case can improve the delay. So use faster modulator and de-modulator.
Mazz,
I have tried with the payload size of 42 bytes.
With BPSK@40 kbps it gives around 35 to 40 mS
and with QPSK @250kbps it gives around 22 to 25mS
Please suggest anything further to improve the time.
tony_lth: I dont have any option to use faster modulator and demodulator
I am very confused. How you want to decrease the communication timing between two nodes, increasing power or antenna gain?
Well those were the option... but I think power and gain would not help.
Any other thing can I do or any other option?
Modulation scheme?
datarate?
Maybe power and gain can improve delay. In some telecomm system, when de-modulation, if SNR is bad, it maybe need more time to de-modulator. It relates with the software in your system. Even in some hardware demodulator, say Veterbi decoder, the algorithm maybe relate to SNR. Bad SNR maybe need nore time. SO power and gain maybe help.
The rf portion of the radio is controlled by a microcontroller somewhere, either inside of the rf chip or on a 2nd chip. That microcontroller is the thing that decides when a signal gets transmitted (i.e. how long after receiving a signal it waits before responding). I would guess that 95% of your round trip time delay is in the digital lines of code in that controller, rather than having anything to do with the RF modulation rate.
I guess you could go with the highest data rate, in an attempt to get the header, data, and any crc done and over with as soon as possible. But I suspect you will not see a significant improvement until you rewrite the controller's code.
Janakfun
read carefully biff post; he explained what could be your problem and I believe he got the point.
Regarding time needed to send a single packet through two radios, you have to calculate it in this way (for example in BPSK@40 kbps).
In BPSK you send a symbol/bit=1 (1 bit=1symbol), so 42bytes=336 bits and packet duration is 8.4 ms. The radio will send additional bits (preamble, sync...) as it is explained in 802.15.4, so real packet duration is higher (I don't know the details, you should know them).
Modern radios has to wait that the full packet is received correctly to send it to the microcontroller, so at least 8.4 ms in the example. Than the micro has to do his own processing.
So overall time is the sum of all these operations. It has nothing to do with RF power, supposed your SNR is OK (that means, put the radios close each other to do these tests or monitor RSSI to see if received signal is higher enough from sensitivity level).
What you can do at radio level is use the highest available data rate. In this case you'll pay it with smaller max communication distance, this depends on your application.
Take a look at 802.15.4 standard. As it is not intended to high data rate application, maybe they have added something there that add delay, as I told you I don't know it deeply.
I hope it can help.
Mazz