Theory behind BPSK and O-QPSK

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What is the best modulation to choose when targeting maximum range in sub-1G?
My (basic) knowledge tells me that I should go for BPSK as the receiver has a higher senisitvity. I can easily cope with the lower transmission rate.

Are there any other advantages with OQPSK? The only one I can think of is that it uses less RF bandwidth and might be less sensitive of being jammed by nearby transmitters.

Thanks

Last Edited: Fri. Oct 16, 2015 - 02:18 PM
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"BPSK, 1/2 the data rate = 3dB overall system S/N

O-QPSK peak to average ratio is less which might make things easier for the TX."

Resulted in a 1/2 hour discussion with my boss, as I didn't know what O-QPSK was.

Maybe, send me a PM.

Dave

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O-QPSK is used in IEEE 802.15.4 for 2.4GHz. Reduces cost of transmitter's amplifier as said above.

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It will be used for sub-1G. And the chip I am using does support either modulation.
So, it is really about which one to choose

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In theory BPSK should be better. In practice, depends on how the receiver is designed, but I'd assume it should be efficient. And it's not like it's very hard to change if tests will show that O-QPSK is better.

NOTE: I no longer actively read this forum. Please ask your question on www.eevblog.com/forum if you want my answer.

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If you refer to the AT86RF212 datasheet, you will find in section 10.7 the receiver sensitivity values. They are showing that for 20 byte frames you will gain for BPSK20 up to 9dB more receiver senstivity compared with OQPSK250.

The line of sight distance is a function of the so called
Link Budget, which is the difference of TX power (e.g. 4dB for RF212) and reveicer sensitivity.

http://en.wikipedia.org/wiki/Lin...

So the link budget for BPSK20 is 114dB and for OQPSK 105dB.

Most of the link budget can be lost by suboptimal RF design. Also line of sight is in most cases a theoretical assumption. So we are talking here about theoretical limits.

As a rule of thumb: go with BPSK if you want to increase the distance between the stations.

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Yes, that's exactly what I am referring to. I am actually using a ATZB-900 module which is based on the RF212, just with slightly modified firmware.
Of course, I have to do real-life field tests (once the rain period is over, we currently do have a quite cold and wet winter) to compare both modulation methods but as the theoretical receiver sensitivity numbers show, I would expect a significantly shorter range when using OQPSK.

On the other hand, when looking at the RF212 datasheet, OQPSK does use more signal bandwith.

The problem with a single performance test is that I am planning to create many systems and I can't fully predict where they are going to be used, therefore I can't tell what the environment will look like in terms of other RF systems.

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If it is outdoor equipment, you can add some dip switches to the device, so the end user can tune the device settings for his needs without the need of a PC for parametrization.

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isn't it significant that BPSK is 1 bit per symbol vs. 2 bits per symbol in QPSK/O-QPSK?
Thus QPSK has a higher rate, but requires a few dB more SNR.

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Baycity wrote:

The problem with a single performance test is that I am planning to create many systems and I can't fully predict where they are going to be used, therefore I can't tell what the environment will look like in terms of other RF systems.

It is definitely also be possible to tune the transceiver after installation in field as well :lol:

But this requires some sort of intelligence in the application code.

Just a thought... :roll: :idea: