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Performing measurements on the common public radio interface

As in many other areas, digital interfaces are also being used increasingly in wireless communications. Instead of using analog I/Q signals, the baseband and RF modules of base stations communicate by means of digital baseband signals.


Wireless communications network operators, in particular, are demanding the possibility of combining base station modules from different manufacturers to avoid being completely dependent on one supplier. The interfaces must therefore be standardized.

The common public radio interface (CPRI) standard is an attempt at creating a standardized platform for digital communications between baseband and RF. It defines the interface between the baseband unit of a base station (the REC, radio equipment control) and the RF unit (the RE, radio equipment).

The trend toward digital interfaces also creates new requirements regarding measurement methods and measuring equipment for base stations. The RE and REC are now to be tested independently of one another, since an RE must function with every REC, and vice versa. The measuring equipment that is used must therefore be capable of sending or receiving suitable test signals to and from the device under test via the CPRI interface.

The EX-IQ-Box provides versatile digital baseband inputs and outputs for Rohde & Schwarz measuring instruments, e.g. for the R&S SMU200A and R&S SMBV100A vector signal generators (shown above), as well as for the R&S FSQ and R&S FSV signal and spectrum analyzers. New options allow the box — together with these measuring instruments or even independently — to perform tests on the base station modules via the CPRI interface. The R&S EXBOX-B85 option provides the necessary hardware as a CPRI-compliant breakout board.

The R&S EXBOX-K10 and -K11 options include the functionality needed for testing REs and RECs. The R&S EX-IQ-Box currently supports version 4.0 of the CPRI standard with line bit rates of up to 3072 Mbit/s. Ready-to-use interface settings are available for the 3GPP FDD / HSPA / HSPA+, 3GPP LTE and WiMAX standards primarily supported by CPRI. However, the box also enables user-defined configurations, providing users with maximum flexibility.

Test setup for tests on CPRI radio equipment: Fully duplex-capable, the uplink and downlink can be tested simultaneously or consecutively.

The illustration above shows the test setup for comprehensive tests on the RE, comprising an R&S EX-IQ-Box with corresponding options for CPRI as well as vector signal generators (R&S SMU200A) and signal analyzers (R&S FSQ) with options for generating and analyzing baseband or RF signals. Transmitter tests on the RE (downlink direction), for example, can be carried out in the same way as on a complete base station. The baseband section of the generator generates the required digital I/Q signals, and therefore plays the role of an "ideal" REC. The R&SEX-IQ-Box acts as the CPRI interface of the REC and sends the I/Q data, embedded in the CPRI protocol and together with the necessary CPRI control data, from the generator to the RE.

All of the box parameters are conveniently configured using the R&S DigIConf PC software (shown at right). The software can be remote-controlled via LAN in the same way as the measuring instruments so that the complete system can be embedded in existing test environments. The quality of the RF signals output by the RE can then be measured in the same way as with a complete base station using a suitable signal and spectrum analyzer such as the R&S FSQ.

For tests in the uplink, the R&S SMU200A generator sends RF test signals to the RX port of the RE. The resulting digital I/Q signals are sent from the RE to the R&S EX-IQ-Box via the CPRI interface. The signal and spectrum analyzer is connected to the box via its optional digital baseband interface and analyzes the I/Q signals. The performance of the RE can be determined via measurands such as noise figure or EVM.

The major advantage of this concept is that the same measuring instruments — and for the most part, the same measurement methods — can be used as are also employed for testing the complete base station. This makes expensive new investments superfluous in many cases. Plus, measurements on the modules and on the complete base station are easier to compare — which can significantly speed up DUT optimization and troubleshooting.

In addition, users are able to tailor the test setup precisely to their requirements by simply selecting the suitable instruments from Rohde & Schwarz.

The new options for the R&S EX-IQ-Box digital interface module make it possible to perform versatile tests on baseband and RF modules via the CPRI interface. The box can be used both independently as well as in combination with Rohde & Schwarz vector signal generators and signal and spectrum analyzers, and allows comprehensive characterization of REs and RECs.

Dr. Rene Desquiotz is with Rohde & Schwarz. This article was excerpted from R&S NEWS 202/10 available at http://ow.ly/4Ryg4.
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