February 6, 2006
Today's wireless network equipment manufacturers (NEMs) face
constant pressure to reduce the rising cost of base stations.
Resolving this challenge is all the more critical since the
growth of 3G base stations is predicted to increase dramatically
in the coming years. This growth is being driven by the lack
of recent investment in infrastructure, the recent commercialization
of 3G and anticipated network expansion to meet market needs.
One of the industry's primary focuses in addressing this
challenge involves lowering the cost of base station RF modules
and power amplifiers (PAs), as these components account for
nearly 50 percent of the base station cost. Consolidation
of hardware offers one possible solution (Figure 1). By combining
the PA, the frequency up- or down-converter, and the digital-to-analog
converter (DAC) or analog-to-digital converter (ADC), the
overall product and operational costs of the base station
can be reduced without compromising equipment performance.
This reduction is achieved in a number of ways:
- The design lends itself to the use of a digital pre-distortion
amplifier. This reduces tuning time and allows the use of
lower cost components while maintaining comparable performance.
- The consolidation of components allows for greater power
efficiency which results in reduced power costs.
- The consolidation of components substantially reduces
required test time.
FIGURE 1: Evolution
of base station design using consolidated hardware.
The separation of the RF radio head, from the baseband controller
offers another possible means of reducing cost as it enables
multiple remote radio heads to be controlled by a single baseband
controller. This architecture reduces both the power requirements
of the base station and its associated cooling system, as
well as the number of carriers to a single T1 for multiple
radio heads.
These trends, along with a desire for interoperability between
vendors, are now prompting a move toward standardization;
in direct contrast to traditional internal base station interconnects
that utilize proprietary interfaces. This transition will
provide a number of benefits including faster time-to-market,
broader and more flexible solutions for operators and improved
efficiency of network deployment. Additionally, it will enable
test equipment vendors like Agilent Technologies to provide
more efficient, lower cost commercial test solutions. As a
result, customers will benefit in terms of set-up time, flexibility,
speed of test (i.e. throughput), and support.
Moving Toward Standardization
The Common Public Radio Interface (CPRI) Initiative was formed
in June of 2003 in direct response to the industry's call
for standardization. Jointly founded by Ericsson, Huawei,
NEC, Nortel Networks and Siemens, which together represent
more than 50 percent of the base station market, its goal
was to define a publicly available specification for the radio
interface between the RF and baseband controller blocks.
In September 2003, the CPRI Initiative introduced the CPRI
specification which focuses on a 3G base station design that
divides the radio base station into radio equipment (RE) and
radio equipment controller (REC). It provides a new standardized
interface for the interconnect point within the base station.
The CPRI specification allows for flexible architectures and
is not limited by module dimensions or a pre-defined function
split. Using this openly available interface allows each part
of the base station to better benefit from the technology
evolution in its respective area. Network operators gain the
availability of a wider radio base station portfolio with
adaptability to all deployment scenarios and a shorter time
to market. Base-station manufacturers and component vendors
are able to focus their research and development efforts on
their core competencies. By providing an open interface, the
industry and its customers - the operators - can gain the
economies of scale in components and modules. More importantly,
use of the CPRI specification reduces the cost of base station
infrastructure, as well as associated operating expenses.
CPRI Basics
The CPRI specification is an open serial digital I/Q standard
that offers line bit rates up to 2457.6 Mbps between the RE
and the REC of a radio base station (Figure 2). It encapsulates
information on transport, connectivity and control, including
user plane data, control and management (C&M) plane transport
mechanisms and means for synchronization. CPRI supports two
types of C&M channels, fast and slow. The fast channel
is based on Ethernet, while the slow one is based on the High-Level
Data Link Control (HDLC) protocol.
FIGURE 2: The CPRI
digital interface is an open standard which is narrowly focused
on one key interface, thereby leaving more room for flexibility.
The specification covers Layers 1 and 2 of the Open System
Interconnect (OSI) stack. Layer 1 supports both an electrical
interface, used in traditional base stations, as well as an
optical interface for base stations with remote radio equipment.
All data, such as C&M plane HDLC, L1 inband protocols,
vendor-specific data, and user plane data, is time multiplexed
together and transmitted or received via one of these interfaces.
Layer 2 supports flexibility and scalability.
CPRI allows three line bit-rate options. It is mandatory
for REC and RE to support at least one of these options, which
include: 614.4, 1228.8 and 2457.6 Mbps.
CPRI does not have a mandatory physical-layer protocol. But
the protocol used must meet the bit-error-rate (BER) specification,
as well as the clock stability and noise requirement. For
an optical transceiver, Gigabit Ethernet, 10 Gigabit Ethernet,
fibre channel, and InfiniBand are recommended.
The CPRI specification complements current activities in
existing formal standardization bodies such as Third Generation
Partnership Project (3GPP) and 3GPP2. Version 1.3 and 2.0
of the CPRI specification are now available for download at
www.cpri.info. Version 2.0 is similar to version 1.3, but
contains support for networking.
The development of the CPRI specification creates a need
for systematic testing to ensure CPRI modules are interchangeable
and compliant to the specification. At the same time, stringent
requirements for BER performance, timing characterization
and modulation quality are driving a need for high-performance
test equipment. As the digital plane moves closer to the antenna,
an inherent shift is also occurring in the way testing is
performed, creating a need for RF-to-baseband and/or baseband-to-RF
measurements using different types of test devices. These
requirements are applicable both in the R&D and production
test environments.
CPRI Versus OBSAI
Like CPRI, the Open Base Station Architecture Initiative (OBSAI)
was created to address the constant cost pressure facing wireless
NEMs and the need for standardization. It was launched in
October 2002 by a group of leading base-station, module and
component vendors, to create an open specification for wireless
base station architectures. The resulting OBSAI specification
defines a high-level, open framework architecture that enables
designers to choose from a variety of interoperable components.
As a result, advanced products can be brought to market quickly.
The OBSAI specification defines standard interfaces for four
main modules in next-generation base stations: the radio module,
including RF transceivers, amplification and conversion between
digital baseband and analog RF signals; the processing module,
including channel models and the baseband processing for the
air interface; the control module; and the transport module
which handles adaptation between external network and internal
interfaces.
Both CPRI and OBSAI standards have a link layer to support
special requirements in terms of latency and timing synchronization.
Each has a physical layer that is based on already existing
electrical standards from Ethernet 10 Gigabit Attachment Unit
Interface (XAUI) and Gigabit Ethernet. Both also allow different
data rates to implement the various market requirements in
terms of carriers and sectors. Additionally, the specifications
offer a way to commoditize the components of the radio network
and complement existing standardization efforts.
While the CPRI and OBSAI interface standards share many similarities,
their main difference helps explain why CPRI has penetrated
the market more quickly. In striking contrast to OBSAI, CPRI
is characterized by a much more narrow focus. It targets only
the interface between the base-station radio and processing
functions for 3GPP systems. Because CPRI focuses on one key
interface only, it leaves more room for flexibility and is
substantially less complex. As a result, CPRI has pushed ahead
of OBSAI in terms of market timing. With the projected high
growth in network equipment over the next 3 years, it is anticipated
that CPRI will continue to witness ever stronger demand.
Conclusion
Cost pressures within the wireless industry are driving demand
for greater efficiency and standardization of the base station
infrastructure. A number of new technologies and initiatives
are now in process to address these needs. The CPRI standard
offers today's wireless NEMs a standardized internal digital
serial interface that provides both simplicity and flexibility.
More importantly, it contributes to the reduction of base
station infrastructure costs. Vendors like Agilent Technologies
are working to support this standard and the need for systematic
testing that it creates. To date, Agilent Technologies is
the only vendor who offers a commercially-available solution
for CPRI RE test.
About Agilent
Agilent Technologies, Inc. (NYSE:A) is the world's premier
measurement company and a technology leader in communications,
electronics, life sciences and chemical analysis. The company's
27,000 employees serve customers in more than 110 countries.
Agilent had net revenues of $5.1 billion in fiscal year 2005.
Information about Agilent is available on the Web at www.Agilent.com.
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Contacts:
Janet Smith, Agilent
+1 970 679 5397
janet_smith@agilent.com
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