[Paparazzi-devel] 868MHz duty cycle (was Xbee 868 uplink)

[Steve Joyce]

I put this question about transmit duty cycle limiting to Aerocomm
(Lairdtech) tech support some time ago.  The answer you may find surprising:
He claims the duty cycle is already limited in the modules.  strange because
I seem to recall the manual states that compliance is the responsibility of
the OEM. 

 

About the Xbee uplink problem- I haven't tried the Xbee module, but I know that I saw the same thing with Aerocomm 868MHz modules until I switched to operate in full duplex mode. i.e. half the (already small) bandwidth is reserved for uplink.

 

Cheers,
Steve

-----Original Message-----
From: Steve Joyce [mailto:address@hidden
Sent: Tuesday, April 01, 2008 12:35 PM
To: wireless.support
Subject: question about transmit duty cycle AC4868-250

greetings,

I am working with AC4868-250 modules in a datlink for unmanned aircraft.
I understand that to comply with regulations, we are supposed to limit
the on-air transmit time to 10% (over an hour), but I am not sure how to
achieve this in practice.

Actually, looking through the manuals I am unclear about how the data
rate relates to the transmit duty cycle. It seems the maximum RF data
rate is 28.8k, but you can run the interface at 57.6k. How is this
possible? Am I missing something?

If we run it full speed (at 28.8 RF baud rate), does this mean the
module is transmitting 100% of the time and we could only run it for 6
minutes of an hour? conversely, if we want to run it continuously for
an hour, are we limited to 2800baud?

Would greatly appreciate some clarification on the topic. especially
how to measure and limit the transmit cycle.

Regards,
Steve Joyce

-----------------------------
From address@hidden Fri Apr 4 15:51:47 2008

Steve,

The Duty Cycle in the radio is fixed in the firmware, there is
nothing you need to do to ensure compliancy. It is transparent to the
end user.

The interface baud rate is adjustable to allow the radio to be used in a
wide variety of applications, but does not necessarily indicate
throughput. You are correct that if you attempt to run at full 57.6k
on the interface, the radio will not be able to keep up and you will
lose data. Actually due to retransmissions and RF overhead, the
actual throughput of the radio will be less than even the 28.8k.

No, the TX time is controlled internally and is transparent to the end
user. The 28.8k RF Baud takes into account the duty cycle.

Thanks,
Chris

----- Original Message -----

From: Rui Costa

To: address@hidden

Sent: Monday, May 25, 2009 12:16 PM

Subject: Re: [Paparazzi-devel] Xbee 868 uplink

I have the Aerocomm AC4868 and when I read your e-mails talking about  the duty cycle. I'm sad.

 

 

"EUROPEAN REGULATIONS
ETSI requirements as specified in the ETSI EN 300 220-1 define the requirements for the 868-870MHz bands. For a
conducted output of 180mW and radiated outputs up to 500mW, the maximum allowed duty cycle is <10%. This
duty cycle is measured as the amount of TX time on, monitored over one hour and relative to a one hour period. Thus
for the AC4868-250, the maximum “on” time in an hour cannot exceed six minutes."

 

There is any solution better out there. With this duty cicle my project will not work. I'm designing an UAV for civilian use. I need a data-link solution for Europe. Any ideia? Or I have to gave up of all?