Efficient Testing of Wireless Radio Equipment Including RF and CPRI

Table of Contents

  1. Abstract
  2. Introduction
  3.  What Is CPRI?
  4. What Is A Remote Radio Unit/Head (RRU/RRH)?
  5. Testing Network Components With CPRI
  6. NI’s Solution - PXIe-6592R With CPRI 
  7. NI PXI RF Equipment - The Perfect Complement To CPRI
  8. Conclusion
  9. Additional Resources

Abstract

This white paper addresses the challenges of testing base station equipment such as a remote radio unit (RRU), considering the necessary support of RF and digital interfaces and protocols. It also provides insight into contemporary base station architectures built from two main components and the Common Public Radio Interface (CPRI) digital protocol used for communication between them. With conventional test setups and methods, test engineers are more frequently responsible for only the RF portion of testing. Increasingly, however, test engineers need a complete, cost-effective and future-proof approach that includes the digital interface as well. NI’s CPRI interface module solves this challenge through interfacing directly to the RRU. Using this module, engineers can combine a compact, scalable, and cost-effective test setup for RRU testing.

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Introduction

This document is principally concerned with the Common Public Radio Interface (CPRI) protocol and test challenges involved. The introduction briefly describes technological changes in base station architectures before going into details about the CPRI protocol and base station testing.

In a cellular radio access network (RAN), a traditional base station is a single entity. Such base stations integrate the digital as well as RF processing for the site into a single unit. These units are usually located in a room of a building or in a container at ground level while the antennas are mounted on top of a tower or mast. In some cases the base station also includes a switched in tower mounted amplifier (TMA). Long RF cables are necessary to connect the antennas or TMAs with the base station. The base station itself is linked with other base stations and the core network through copper or fiber-optic cables. Other interfaces inside the base station are inaccessible. The appropriate standardized tests and requirements concern the well-defined outer interfaces of a base station. Shown in Figure 1, the RF interface connects the base station to the antennas. S1 and X2 are protocols over the digital interface to the back haul network where S1 is a link to a so-called mobility management entity (MMW) and X2 a direct link to another base station.

Figure 1. RAN With Standard Interfaces

Over the last several years, the base station architecture has evolved. One of these evolutions has been the physical separation of the digital and radio circuits. Test equipment vendors and the users of those devices now face the challenge of evaluating and testing the radio part separated from the digital part of a base station. This means that the test system has to support not only the RF but also the digital interface by simulating the digital part of a base station. It must act as the controlling and communication partner of the device under test (DUT) sending and receiving data according to the specification of the digital interface. A widely used communication protocol for this interface is CPRI.   

Figure 2. Comparison of DUTs With and Without CPRI

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 What Is CPRI?

CPRI is an industry cooperation between five leading radio base station vendors. It defines and specifies the publicly available interface between building blocks of a contemporary radio base station.

CPRI is a digital protocol used for serial high-speed data transfer between the two parts of contemporary base stations, namely the radio equipment control (REC) also known as a baseband unit (BBU) and the radio equipment (RE) also known as remote radio unit (RRU) or remote radio head (RRH). Although an electrical and optical interface is specified, the physical connection between the two base station parts is realized by a fiber-optic cable in the majority of cases. The fiber-optic connection renders the previously used long RF cables between base station and TMA or antennas redundant. Only short RF cables between RRU and antennas remain.

Those shorter RF cables and the thereby resulting reduction of path loss and overall power consumption were the main reasons to split up a base station into a ground-based and a remote RF unit near the antenna.

Figure 3 illustrates the architecture showing the two basic subsystems (REC and RE) of a radio base station including type of information and interfaces. The architecture allows the use of multiple REs (RRUs/RRHs) controlled by one REC (BBU).

Figure 3. System and Interface Definition (source www.cpri.info)

The CPRI interface can be characterized by a full-duplex, synchronized, and steady transfer of digital baseband data that guarantees high bandwidth and high throughput with low latency. Less time-critical data such as control information—for example, for link setup—as well as time-aligned data such as for Rx and Tx gain control are transferred in addition to the user information.

The clock and timing control ensures that the REC (master) and RE (slave) are synchronized. The timing information is included in the baseband data. A frame structure with control words (CWs) provides the basis for the transfer of that information. The slave port side (RE) synchronizes its clock and frame timing to the master reference (clock recovery). That is essential to map or demap and code or decode the digital data correctly as well as to resend CPRI data to another RRH in a chain topology.

The CPRI protocol defines layer 1 (PHY) and layer 2 (MAC) of the Open System Interconnection (OSI) model. Higher layers are not specified by CPRI but defined and implemented by the vendors of the REC and RE. In other words, there are many possibilities for using CPRI data containers for individual users, as well as control and management information.

The user (baseband IQ), control and management, and synchronization data streams are multiplexed over the same physical interface. Figure 4 gives an overview of CPRI protocol elements in layers 1 and 2.

Figure 4. CPRI Protocol Overview (source www.cpri.info)

IQ data is transferred in the form of a frame structure with basic, hyper, and radio frames. A basic frame consists of 16 words. The first word (W0) of each frame is a CW. The remaining words (W1–15) are used for user IQ data (IQ data block). An IQ data block structure for a 1,228.8 Mb/s line rate is exemplary shown in Figure 5.

Figure 5. Basic Frame Structure for 2 x 614.4 = 1,228.8 Mb/s CPRI Line Bit Rate (source www.cpri.info)

 

A CPRI link length can range from few meters up to more than 80 km. The data transmission rate starts at 614.4 Mb/s (line bit rate option 1) and goes up to 12,165.12 Mb/s (line bit rate option 9) according to the CPRI specification V6.1.

How is CPRI different from standards like PCI express, Gigabit Ethernet (GbE), or Serial RapidIO (sRIO)?

There are several standards for high-speed data transfer, including PCI Express, GBE, and SRIO. These standards, however, are not ideal for communicating with RRHs.  Each of these standards can offer high data rates, but, as asynchronous packet-oriented protocols, they do not natively solve the timing requirements. Moreover, because of the network architecture that includes switches and routing entities, those standards cannot guarantee throughput and latency, as needed for front haul base station equipment. As a result, CPRI is the primary protocol used for data transfer between the REC (BBU) and the RE (RRU/RRH).

Compared with the above mentined protocols, CPRI is a point-to-point connection where both endpoints are running at the same clock speed. It is a link-oriented protocol with a continuous data transfer stream without packets or bursts and cycle slips. CPRI provides a transparent channel in the form of data containers used for baseband IQ data with mappings for LTE, WiMAX, UMTS, and so on with different bandwidth and different word bit width for single input, single output (SISO) and multiple input, mulitple output (MIMO) configurations. The CPRI protocol also contains internal control information for link management and signaling.

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What Is A Remote Radio Unit/Head (RRU/RRH)?

As explained above, a contemporary base station is subdivided into two main parts, the REC or BBU and the RE or RRU/RRH.

Figure 6. Comparison of a Traditional and a Contemporary Base Station

The RRH consists of the base station RF unit, filters, analog-to-digital converters, digital-to-analog converters, and up/down converters. The connection between an RRH and the rest of the base station (BBU) is mainly realized through fiber-optic links.

The communication, meaning the transmission of control and management, synchronization, and I/Q data, is based on the CPRI protocol. As a result, test systems designed to test RRHs must support both the RF and CPRI interface to fully characterize the DUT.

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Testing Network Components With CPRI

A complete RRH test and evaluation requires transmitter (Tx) and receiver (Rx) tests. Tx tests analyze the transmitting path of the RRH also known as the downlink (DL). When testing the DL portion, the test system must send digital CPRI signals to the RRH and measure RF signals acquired from the RRH. Alternatively, Rx test results provide information about the uplink (UL) receiver behavior of the RRH. Here, the test system must generate an RF signal to the RF RRH and a digital CPRI interface reads information from the RRH. Figure 7 illustrates the interfaces and signal directions in DL and UL.

Figure 7. RRH With Interfaces and Signal Directions in DL and UL

 

ETSI or 3GPP

Tests in validation and verification (V&V) or production are usually performed in compliance with the corresponding ETSI or 3GPP specifications. The following specifications cover the different major radio standards.

    • LTE: ETSI TS 136.141/3GPP 36.141
    • WCDMA: ETSI TS 125.141/3GPP 25.141
    • GSM: ETSI TS 151.021/3GPP 51.021

These specifications cover the Tx (DL) and Rx (UL) characterization including signal generation and analysis as well as performance tests.

As an example, LTE Tx and Rx test cases that are relevant for an end of production line test are listed below.

Transmitter (Tx) Tests:

  • Signal generation (CPRI)
    • E-UTRA test model (E-TM) 1.1, 1.2, 2, 3.1, 3.2, 3.3 for calibration and/or verification
  • Data analysis (RF)
    • Base station output power
    • Transmit ON/OFF power (PvT, only applied for E-UTRA TDD BS)
    • Frequency error (Average carrier frequency offset)
    • Error vector magnitude (EVM)
    • Occupied bandwidth (OBW)
    • Adjacent channel leakage power ratio (ACLR)
    • Operating band unwanted emissions
    • RS TX power
    • Receiver (Rx) Tests:
  • Signal generation (RF)
    • UL fixed reference signals according to FRC A1-3
  • Signal analysis (CPRI)
    • Reference sensitivity level

TEST Challenges

In practice, many engineers separate RRH DL and UL testing into two test stations because most test vendors do not offer a complete test system. Base station vendor-specific modifications and IP protection thoughts could also play a role in case of test setup selections. As a result, many engineers use either self-built or customized CPRI transceivers. Because these customized CPRI transceivers are not easily automated and often lack synchronization capabilities, designing a complete test system around them can be a significant challenge.  In addition, the requirement for robustness and ruggedness in high-volume manufacturing applications often drive the requirement for instrument-grade CPRI interfacing.

CPRI test devices have to neccesarily deal with the following hardware- and software-related challenges.

    • Physical interface in form of optical SFP+ transceivers (multiple for multi-DUT tests)
    • Sending and receiving digital baseband data through CPRI
    • Streaming and processing data with rates up to 10 Gb/s or more
    • Master and slave configuration
    • Mapping and demapping according to the specification and vendor-specific features
    • Clock generation and recovery
    • Handling control and management data including individual vendor information
    • Dealing with different vendor flavors

A second challenge when testing RRH modules is DUT control. Ideally it should be implemented in the test system providing at least a transparent channel for the DUT control data through CPRI (Ethernet over CPRI). That is not to be taken for granted considering available test systems.

Figure 8 shows a common test setup with separated control and measurement devices for DL or UL and for RF or CPRI signal generation and analyses, respectively. Typically, the CPRI interface module and the controlling PC are completely independent and only an RF analyzer is part of the standard test setup. Because DUT control isn’t considered a part of a standard test setup, engineers are typically expected to take care of this on their own.

Figure 8. Test Setup With Different Test Boxes From Different Vendors and External DUT Control

The ideal RRH test platform must be flexible, expandable, and scalable enough to support both DL and UL tests.  In addition, it must include synchronized RF and CPRI signal generation or acquisition as well as DUT control functionalities for several types of RRHs. Note that these requirements apply whether the test system is used in either in a validation/verification application or in high-volume production test.

The NI solution addresses these requirements with an open and flexible PXI approach. PXI test systems include a PXI chassis and controller. In addition, they can combine RF instruments and an instrument-grade CPRI interface module into a single system. This system can meet all requirements of an RRH characterization, validation/verification, and even high-volume manufacturing test.  Figure 9 illustrates NI’s compact RRH test platform with an embedded controller, RF and CPRI transceiver modules, as well as integrated DUT control.

Figure 9. Test Setup With NI’s Compact PXI Platform With Built-In DUT Control (Ethernet over CPRI)

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NI’s Solution - PXIe-6592R With CPRI 

The PXIe-6592 With CPRI Bundle is designed for engineers who need to validate and test products that communicate through the CPRI protocol. This bundle is a specially tailored package of software and PXI hardware needed to communicate with the DUT on the CPRI side. It consists of the multiport, FPGA programmable PXIe-6592R high-speed serial module, and the LabVIEW 2014 Instrument Design Libraries for CPRI application software.

CPRI Hardware

The PXIe-6592R high-speed serial instrument consists of a Xilinx Kintex-7 FPGA with an onboard DRAM. It is programmable in LabVIEW FPGA for maximum application-specific customization and reuse.

This instrument takes advantage of FPGA multigigabit transceivers to support rates up to 10.3 Gb/s and up to four Tx and Rx lanes.

As part of the PXI platform, the PXIe-6592R benefits from PXI clocking, triggering, and high-speed data movement capabilities, including streaming to and from disk as well as peer-to-peer streaming, at rates up to 3.2 GB/s.

CPRI Software

The application software LabVIEW Instrument Design Libraries for CPRI offers the possibility to use the CPRI interface module in master or slave mode. In addition, you can change various interface settings such as CPRI port, line bite rate, channel bandwidth, antenna carrier, and others. The following detailed list of features give the module the ability to control a nearly universal list of CPRI devices.

 

    • CPRI line bit rate option 1–7 (9.83 Gb/s max.)
    • Master/slave mode configurable
    • Real-time I/Q data record and playback from DRAM
    • I/Q data streaming to HOST
    • LabVIEW FPGA programmable
    • Multiple ports (2xCPRI+2xEthernet implemented)
    • Generic software I/Q (de)mapping (method 1, sample rate from 3.84–30.72 MHz)
    • Access to vendor-specific control channel
    • Fast C&M channel (Ethernet over CPRI)
    • Ready-to-use sample project with front panel

Besides the primary functions of hardware device and reference clock in/out configuration, the front panel provides access to a variety of CPRI monitoring and setting options.

CPRI link functions:

    • Configure the link parameters (line speed, C&M plane settings)
    • Initiate (enable)/abort (disable) the CPRI link
    • Observe the layer 1 link status and failure conditions

Generation and analysis functions:

    • Configure data generate and capture parameters (for example, triggers and length)
    • Upload generation data to onboard DRAM
    • Configurable trigger/marker signals to synchronize to external equipment
    • Initiate/abort data generation or capture
    • Fetch captured data (buffered in onboard DRAM or streaming)

You can use the NI CPRI interface module either as a stand-alone solution for CPRI interfacing or in combination with NI PXI RF equipment as a flexible testing platform for evaluation and verification of RRHs, active antenna systems (AAS), and wireless front haul equipment.

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NI PXI RF Equipment - The Perfect Complement To CPRI

The forenamed completion of NI’s CPRI testing solution by powerful PXI RF test equipment modules offers a wide range of test possibilities.

Test engineers can extensively characterize RRHs, for example, in DL as well as UL direction, by adding a highly efficient NI vector signal transceiver (VST), a PXI-5695 programmable attenuator, and a corresponding software package to the available CPRI system.

NI Vector Signal Transceiver (VST)

The NI VST is part of a new class of instrumentation that combines a vector signal generator and vector signal analyzer with FPGA-based real-time signal processing and control. Because of this software-designed approach, a VST features the flexibility of a software defined radio architecture with RF instrument class performance.

The VST combines the fast measurement speed and small form factor of a production test box with the flexibility and high performance of R&D-grade box instruments. It is characterized by the following main features.

    • Vector signal analyzer and generator
    • 9 kHz to 6 GHz frequency range
    • 1 GHz instantaneous bandwidth
    • 60 MHz, 8 port high-speed parallel and 12 Gbps, 4 port high speed serial Digital I/0
    • FPGA extensions
    • -50 dB EVM (802.11 ax, Loopback, external LO)
    • Optional baseband I/Q interface
    • Ref in/out

Programmable 8 GHz RF Attenuator PXI-5695

The programmable RF attenuator PXI-5695 offers an easy-to-use way to reduce signal power in 0.5 dB steps. Especially receiver sensitivity or block error rate (BLER) measurements often require several levels of low-power RRH input signals up to -110 dBm or even lower to approach and finally determine RF receiver and baseband processing limits.

    • 50 MHz to 8 GHz frequency range
    • Two attenuator channels: fixed and programmable
    • Max. input power +33 dBm
    • Cascade channels for up to 72 dB attenuation
    • 0.5 dB attenuation resolution from 12 dB to 72 dB
    • 1.2:1 typical voltage standing wave ratio (VSWR)
    • Programmable with LabVIEW

Test engineers can perform 3GGP-compliant base station tests thanks to the opportunity of CPRI and RF interface combinations. NI’s PXI platform supports common RRH DL and UL test such as output power, frequency error, error vector magnitude, occupied bandwidth, ACLR, and receiver sensitivity (BLER).

NI’s test executive software TestStand provides the basis for automated tests in volume production.

Figure 10 shows a test setup example covering PXI hardware and software modules needed for the previously described RRH test application.

 

 

Figure 10. Setup Example of RRH Testing

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Conclusion

Today’s test engineers face the challenges of an extensive characterization of base station units in R&D and fast, cost-effective testing in high-volume production applications. The solution is a flexible and future-proof test platform that includes not only RF measurement capability but also digital interfacing using the CPRI protocol. NI PXI test systems that include the CPRI interface module provide a solution that helps engineers to address both requirements. Using this solution, test engineers reap the benefits of:   

    • One compact test system that covers RF measurements and CPRI communication
    • A scalable system for multi-DUT tests
    • The ability to easily add PXI modules to extend test coverage to other RF or non-RF tests
    • An open platform that can adapt to specific needs
    • A universally applicable vendor-independent solution

 

In addition, test managers gain several key benefits:

    • One solution for different needs/demands in R&D/V&V and production
    • Cost-effective measurements
    • Future-proof and scalable test platform
    • Ready for next generation of communication protocol Open Radio equipment Interface (ORI)

 

For more information on NI’s RRH test solutions, contact your local NI sales representative directly.  For more information on NI’s PXI product offering, see Additional Resources below.

 

 

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