OpenBTS RF Development Plans

These are our ideas for future RF development. If you are working on the RF task, feel free to edit and comment in this document.

Bear in mind that this device may not need FCC certification in its intended deployments.

Transmit Chain

USRP -> tx filter -> PA -> duplexer -> cables and antenna

  • The tx fitler provides suppression for out-of-band noise and is matched a standard GSM downlink band. The current candidate design is a stripline interdigital filter or low-cost cavity resonator (see below).
  • The PA is TBD, but will likely be similar to the Downeast PA3340 ([http://www.downeastmicrowave.com/PDF/3340pa.PDF]). Just get a good UHF module and let it do its job.
  • The duplexer proposal is a 2-cavity 1/4-wave design built from paint cans, coffee cans or large-diameter copper pipe. We're not joking here. We don't want a $4k hunk of precision machine work in the middle of a "low-cost" system.

Receive Chain

antenna and cables -> duplexer -> rx filter -> preamps -> USRP

  • The preamps need to provide at least 20 dB of gain to overcome noise inside the USRP. They need to be cheap, too. And they need enough headroom to overcome head-far problems. Even though GSM support active uplink power control, we do not want to be entirely reliant on it. The good news is that with the antenna up on a tower, there's a limit to how close handsets can be.
  • The rx filter is similar to the tx filter above, but passes the uplink band. In this side of the radio, though, insertion loss and a efficiency are more important.

USRP Changes

We are planning to build a new radio board based on the USRP, but specifically tailored for GSM applications.
Right now, OpenBTS uses a USRP with two RFX daughter cards. For actual deployment, it would be better to have a single board that combines all of these features:
  • a USRP-compatible control section, modeled on the USRP motherboard
  • two RFX-style receive sections with better RF isolation
  • one RFX-stype receive section
  • an OCXO running at a multiple of 13 MHz
  • on-board GSM-specific filters, either SAW or ceramic
    This board would still be software-compatible with the USRP, allowing developers to continue to use USRP-based kits for desktop testing.

Ticket #334 tracks the status of this work.

Other Details

Antenna

For omni cells, we can probably get a lot of service from marine cellular band antennas, built to be installed on boats and yachts. They are tough, efficient and only about $100 each. For sectorized installations, there are a number of panel style antennas on the market that should be suitable, including inexpensive designs built for the US 900 MHz ISM band.

Connectors

We only want two connector types in our RF system: SMA for low power and N for high power.






注：OpenBTS RF Development Plans（原文出处，翻译整理仅供参考!）