NPR: Hamnet over 70cm

Some days ago, Guillaume F4HDK emailed me to introduce me his latest project, NPR (New Packet Radio). This is an open-source modem designed to carry IP traffic over the 70cm Amateur radio band, with data rates of up to 500kbps. The goal of this modem is to be used for the Hamnet Amateur radio IP network, to give access to end users where coverage on the 2.3GHz and 5GHz bands is poor due to the terrain.

Guillaume knew that I had worked on IP over 70cm with my CC1101 and Beaglebone black project, so he wanted to know what I though about NPR. After reading all the available documentation, I found NPR very interesting. Indeed, Guillaume has come up with clever ways of solving some of the difficulties I foresaw when planning out my experiments with the CC1101.

The most important aspect about NPR is that it is already a finished product that people can build as a kit and start using. My experiments with the CC1101 were a mixture of proof of concept and play around, and never progressed from that stage due to lack of interest in my local Amateur community. However, Guillaume has put a lot of time, thought and effort in developing NPR. Of course the project can evolve further, but it is usable in its present stage. In what follows, I do a detailed analysis of the technical aspects of NPR.

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Using the CC1101 and Beaglebone black for IP traffic on 70cm

Lately, I have been experimenting with using a CC1101 chip together with a Beaglebone black single board ARM computer to transmit IP traffic over the 70cm Amateur band. There has been a similar project from OEVSV, but I’ve never seen this project reach a final form. Edouard F4EXB has some code that uses the Raspberry Pi instead. Presumably, this will suffer from problems when using the higher data rates supported by the CC1101, as his software is not real-time.

The goal of my project is to build an affordable 70cm IP transceiver with a power of a few Watts. This can be used in the Hamnet Amateur Radio IP network. The modulation should not use more than a couple of hundreds of kHz’s of spectrum, as it doesn’t seem very sensible to take up much more spectrum in the 70cm band. Although the usual maximum bandwidth in the 70cm band is 20kHz, the IARU R1 bandplan allows for wideband experiments around 434.000MHz. A data rate of 128kbps with MSK modulation seems about right, as it uses roughly 200kHz of spectrum. Further on-the-air tests will perhaps change these parameters a bit.

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