Ettus Research USRP

All versions of the USRP from  Ettus Research are fully supported in the public version of OpenBTS.

The type of device is automatically-detected* and device specific parameters (sample rate settings, clocking frequency, etc.) are configured by OpenBTS internally. User controlled settings such as operating frequency (ARFCN), RF gain levels, etc. must still be configured separately by if non-default settings are desired.

See the build guide for setup and installation instructions.

* USRP1 requires special handling (see below)

B200 / B210

The recently released B200 / B210 devices with the new Analog Devices AD9361 RFIC are supported by OpenBTS.

Use of an external 10 MHz reference is recommended, but not mandatory, in order to run OpenBTS. When no external reference is available, the TCXO on the USRP B200 series is rated at +/- 2.0 ppm, which may cause instabilities with handset camping and registration. Please consider this factor during testing.

B210 Dual Channel

The second channel of the B210 is not supported by OpenBTS. For GSM operation ofthe B210 in a dual channel configuration, users may consider the  Osmocom GSM stack using  OsmoTRX, which supports the use of 2 ARFCN signals in a dual channel configuration.

B100 / B110

The current release of OpenBTS supports the B100 series with a 64 MHz specific polyphase resampler and updated GSM receiver design. These changes resolve various issues previously seen with B100 and earlier versions of OpenBTS, which were due to sample rate issues at 52 MHz and level control.

The use of GPS or the 10 MHz external reference is generally not necessary for desktop testing and other casual use cases with the B-Series. Commercial and operator class applications that require continuous operation and the highest level of reliability should rely on a separate precision reference source.

Along with the B200, the B100 provides the lowest cost OpenBTS device from Ettus Research. The primary difference between the two devices is the RFIC based RF front end on the B200 versus discrete RF daughterboards on the B100.

E100 / E110

Limited OpenBTS support is provided for OpenBTS on the embedded platforms E100 and E110. Performance may vary as the resource constrained environment does not allow for operation of a fully configured 8 time slot ARFCN. For additional embedded functionality, users may consider the  Osmocom GSM stack using  OsmoTRX, which includes options for ARM optimized SIMD instructions.

N210 / N200

The N-Series of USRP devices is the highest performing line from Ettus, which is capable of providing the highest dynamic range and lowest phase error of current Ettus devices.

With calibration, these devices can pass GSM transmit signal tests such as GMSK phase error and spectrum mask with significant margins. Modulation and measurement requirements are specified in the following standards.

  • 3GPP TS 05.05 "Radio Transmission and Reception"
  • 3GPP TS 05.04 "Modulation"

The use of GPS or the 10 MHz external reference is generally not necessary for desktop testing and other casual use cases with the N-Series. Commercial and operator class applications that require continuous operation and the highest level of reliability should rely on a separate precision reference source.

http://tsou.cc/gsm/osmo-trx-phase75.gif

http://tsou.cc/gsm/osmo-trx-spectrum75.gif

X300 / X310

The USRP X300 series is not supported at this time.

USRP2

The USRP2 is functionally equivalent to the N-Series, but does not include an on-board reference or support the optional internal GPSDO module.

An external 10 MHz reference signal is required in all cases for USRP2 operation with OpenBTS.

USRP1

Required Hardware Modification

Use of the USRP1 with OpenBTS requires hardware modification. Because of the specification of the on-board frequency reference, the device must use an external reference for greater frequency accuracy. Unlike other USRP devices, the USRP1 does not accept a 10 MHz reference signal and, instead, must be modified to use an external 52 MHz clock.

The USRP1 is not supported by OpenBTS without the hardware modification.

Both RF chains can be used to provide separate uplink and downlink signals in a single ARFCN channel configuration.

OpenBTS does not support the use of two daughterboards in dual-channel (2 uplink and 2 downlink signals) operation.

Supported Daughterboards

OpenBTS with USRP1 supports a limited number of Ettus daughterboards. Note that certain recent daughterboards are not supported.

  • Supported: RFX900 and RFX1800, WBX boards prior to version 3
  • Not Supported: SBX and CBX

This support list applies only to USRP1 with OpenBTS. Other USRP devices - and USRP1 with the UHD driver for non-OpenBTS applications - do not have these limitations.

Driver

As an older device, the USRP1 does support FPGA based timestamps in the Ettus UHD driver. Instead, the libusrp driver found in old versions of GNU Radio must be used with OpenBTS, so software installation is somewhat more complicated than the other USRP devices.

Frequency Reference

GSM base station operation requires a stable reference oscillator for frequency stability and reliable operation with mobile devices, which expect the base station to serve as a timing reference. The GSM stability requires are specified in the following standards.

  • 3GPP TS 05.05 "Radio Transmission and Reception"
  • 3GPP TS 05.10 "Radio subsystem synchronization"

The general requirement is stated in TS 05.10,

The BTS shall use a single frequency source of absolute accuracy better than 0.05 ppm for both RF frequency generation and clocking the timebase. The same source shall be used for all carriers of the BTS.

In commercial infrastructure applications, this level of accuracy is usually provided by some combination of one or more GPS, OCXO (oven-controlled crystal oscillator), or rubidium time standard components.

For less mission-critical use cases, such as the type of testing often performed with OpenBTS, the high level of specified frequency accuracy can be slightly relaxed.

The following recommendations are for desktop testing and non-critical applications with Ettus USRP's. For operator or carrier-grade use cases, GPSDO or comparable external referencing remains recommended for all devices.

Ettus ResearchNotes
USRP1Requires clocking modification and 52 MHz reference
USRP210 MHz reference required
B100
B110
B20010 MHz reference recommended
B21010 MHz reference recommended
N200
N210
E100Embedded (limited support)
E110Embedded (limited support)

Reference Options

  • GPS locked reference from Ettus Research
  • External reference signal

USRP GPS-Disciplined Oscillator

Highly accurate  GPS disciplined oscillator units are available from Ettus Research.

External Reference

A 10 MHz reference out signal, commonly found on the back of many types of electronic test equipment, can be used as a reference source for USRP devices. Please note the reference specifications vary by device and manufacturer.

http://sdrconcepts.com/images/external_ref_small.jpg