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I am developing a VOIP application. Most clients use a wired Ethernet connection, but some of the clients need to have a wireless connection. Currently the core board is using:

  • Cortex-A8 (TI AM3359)
  • 256MB RAM
  • 2GB Flash
  • WiFi 802.11N

But the range of WiFi is very terrible, compared to ~5Km walkie talkie, I know XBee might work but I have never used it in either production or development. My requirements for the digital wireless communication are:

  1. 1-5Km in Range (1Km with minor obstacles or 5Km line of sight)
  2. 100Kbps (12.5KB/s) lowest transfer rate condition.
  3. multiple clients (100 clients)
  4. relatively low price.
  5. preferably: support TCP/IP, support simple encryption (so no need to modify software stack much)

So should I use XBee? or any other alternatives? Thanks.

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Xbee modules are an option, as are other 800 to 900 Mhz RF modules, but subject to some constraints.

I suggest you make your choice by looking at Digi's comparison matrix table of the different Xbee options; there are at least three that suit your application, and they are point-to-multipoint capable.

The table states you can get outdoor range up to 40 km (for example, with the XBee-PRO 868), but note a couple of things:

  • this is outdoors range; if the same transmission were attempted in an indoors environment (or generally a situation where there are intererence sources within the Fresnel zone), your range will drop markedly.

  • this requires using a high-gain antenna, so that may be a constraint depending on what your device construction allows (in fact, I think most of the distances specified in that table are with a dipole antenna).

  • some of the model variants are only available selectively in Europe or North America, if this matters.

  • for some insight on performance metrics, it's worth reading this app note, where they demonstrate nearly 0% packet loss at distances up to 40 km range, but with specific conditions.

Note that the data rate for the long-range models might not meet your requirement of 100 Kbps, which is a tough ask although...

Alternately, GSM modules perform your bandwidth requirement easily in a different context, which brings up an alternate idea: You can also consider instead using GSM modules for your communication, although this obviously might turn pricey since you state there are on the order of a 100 users.

One final suggestion is to use the low- or medium-range Xbee models, but use extenders/repeaters/relay-units to cover the required distance (or in case of the Zigbee variant of Xbee, you can even use the units in the mesh as extenders themselves). Setting up a robust Zigbee network takes some development effort, but this is somewhat simplified with the firmware stack on the Xbee ZB units.

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