OpenBTS - Prof. Dr.-Ing. Andreas Steil

Introduction

The OpenBTS project is an effort to provide an open-source Unix application that offers a completely regular GSM air interface (Um) to standard GSM handsets and connects calls using the Asterisk VoIP PBX. This 'flat' system architecture completely differs from the conventional GSM hierarchical architecture. Therefore, OpenBTS should better be refered to as 'GSM access point'. Within the scope of our research activities, we operate an OpenBTS access point in the 1800 MHz band.

This picture below gives an overview about the components used. The main hardware device is Ettus' Universal Software Radio Peripheral (USRP), connected via USB to a standard PC running the OpenBTS application and Asterisk. The RF RX part of the GSM access point consists of a simple ground plane antenna (triple leg) and a duplexer. The RF TX path is first  boosted by about 25dB and then fed through the duplexer. The TX power is about 0 dB ERP.

Hardware

USRP

The USRP is equipped with two 1800MHz daughterboards.

Amplifier

We are using a 25dB power amplifier, thanks to Communications Systems Service GmbH!

Duplexer

Kathrein Type No. 792 542 (1710-1785/1805-1880). Thanks to Kathrein!

Antenna

The triple leg antenna (obvioulsy non professional :-), a nicer one is under construction).

Software

OpenBTS

Implementation of the Um Interface.

Asterisk

A standard installation of Asterisk is required. OpenBTS uses Asterisk not only for handling VoIP calls, also for user authentication. Every mobile user has to be registered in the sip.conf with its particular IMSI.

Test Setup

Our test equipment consists of R & S FSP Spectrum Analyzer, R&S Universal Radio Communication Tester CMU 300, an Agilent E4433B ESG-D Series signal generator, several cell phones (Siemens A60, Nokia 3310, Sony Ericsson K770i) and two Cisco 7940 VoIP Phones. Thanks to R&S for lending the CMU 300!

= The internal quartz oscillator is by far not stable enough ;-(, so we are using an external clock.

 It works!

The old Siemens on the left indicates "CC 262 NC 09", which is our country and network code. Still displayed is the mobile providers branding "o2 - de".

Source : http://www.fh-kl.de/~andreas.steil/Projekte/OpenBTS/index.html