20-12-2012, 03:33 PM
OpenRAN: A New Architecture for Mobile Wireless Internet Radio Access Networks
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ABSTRACT
Cellular telephonynetworks depend on an extensive wired
network to provide access to the radio link. The wired network, called a radio
access network, provides such functions as power control and, in CDMA
networks, combination of soft handoff legs (also known as macrodiversity
resolution) that require coordination between multiple radio base stations and
multiple mobile terminals. Existing RAN architectures for cellular systems are
based on a centralized radio network controller connected by point-to-point
links with the radio base transceiver stations. The existing architecture is
subject to a single point of failure if the RNC fails, and is difficult to expand
because adding an RNC is expensive. Also, although a network operator may
have multiple radio link protocols available, most RAN architectures treat
each protocol separately and require a separate RAN control protocol for
each. In this article we describe a new architecture, the OpenRAN
architecture, based on a distributed processing model with a routed IP
network as the underlying transport fabric. OpenRAN was developed by the
Mobile Wireless Internet Forum IP in the RAN working group. The
OpenRAN architecture applies principles to the radio access network that
have been successful in reducing cost and increasing reliability in data
communications networks.
INTRODUCTION
In this article we discuss a new architecture for mobile
wireless RANs. The architecture, called the OpenRAN, is based
on a distributed processing model with a routed IP network as the
underlying transport fabric. The next section briefly discusses existing
cellular RAN architectures and why a new architecture might be
appropriate for fourth-generation (4G) cellular systems. We then
present the principal requirements that drove the Open RAN
architectural design. In a later section the Open-RAN architecture is
summarized. The architecture consists of acollection of 77 atomic
functions grouped into 16 functional entities with 32 interfaces
between them, so only a brief overview of the architecture is possible
to present here.
GOALS OF OPENRAN
The vision of the OpenRAN architecture is to design radio
access network architecture with the following characteristics:
Open,
Flexible,
Distributed,
Scalable.
Such architecture would be open because it defines open,
standardized interfaces at functions grouped into 16 functional entities with 32 interfaces
between them, so only a brief overview of the architecture is possible
to present here.
MOTIVATIONS
Open RAN work has been motivated by perceived shortcomings in
current RAN architectures, many of the issues raised in this section
may be addressed by compatible changes to existing architectures or
even through particular implementations of existing architectures. The
intent of the OpenRAN work is to see whether all of these issues can be
addressed in a comprehensive fashion, by starting from scratch and
redesigning the RAN completely.
By deploying a radio access network based on the OpenRAN
architecture, public network operators could achieve independence of
their core networks from the access network technology. This is
intended to allow public network operators to leverage their core,
service based network, including support for mobility, across a variety
of access technologies, achieving the potential of a larger market for
their services. Because the OpenRAN architecture is designed to allow
the co-existence of multiple radio technologies within a single RAN
infrastructure, deployment of OpenRAN-based radio access networks is
intended to allow an operator to achieve cost-effective utilization of
their expensive spectrum assets by selecting the most appropriate radio
protocol.
OBJECTIVES
The long term vision of the OpenRAN architecture is to
extend the peer-to-peer and distributed Internet architecture to radio
access networks, so a radio access network becomes just another access
network, like cable, DSL, Ethernet, etc.. The first step toward this goal
is to gather requirements. The second step is to determine the basic
functionality of a radio access network as a collection of functional
entities in a functional architecture, identifying the interfaces between
the functional entities. The third step is to describe which interfaces are
open and how they can be implemented using IETF protocols, or
protocols based on IP but designed specifically for the radio access
network functions. The fourth step is to trace back the architecture to
validate that it does, in fact, meet the requirements. The objectives of
the first version of this report are to address steps one and two in the
above process; future versions of this report will address the remaining
steps.
OpenRAN ARCHITECTURE
The OpenRAN architecture was developed by
partitioning a RAN into atomic functions, then grouping the
atomic functions into functional entities based on the requirements.
Interfaces between the functional entities were then identified, and the
nature of the traffic over those interfaces was characterized as a
first step toward possible protocol development on those interfaces
that are declared open.
Mobile Control:
The Mobile Control functional entity is responsible for all
control functions associated with a particular mobile terminal.
Functions involved in coordination of a mobile’s access to the
radio network are one area managed by Mobile Control, including
allocation of physical and logical radio resources dedicated to a
particular mobile terminal, coordination of admission control for
allowing a new mobile terminal into the network, and setting up
and releasing radio connections. Another area is coordination
involved in maintaining a mobile’s connection with the RAN,
including admission control on packets to the radio network,
management of the radio resource context recording the mobile’s
resource allocations, and uplink outer loop and downlink outer
loop power control. The Mobile Control functional entity also
handles mapping of QoS between the radio and wired parts of the
network, and such diagnostic functions as tracing a mobile terminal’s
activity and measurement of the MAC layer for diagnostic and
administrative purposes. Paging of the mobile on dedicated (as
opposed to common) channels is also handled by the Mobile Control
function, although the Paging/Broadcast functional entity controls
paging overall.
CONCLUSION
The OpenRAN architecture presented in this article is a
first step toward an all-IP radio access network. The architecture
as it currently stands describes how to decompose radio access
network functionality in a way that allows a distributed
implementation and opens the door to implementing functions
common among multiple radio link protocols, namely mobility
management, QoS, and security/AAA, at the transport layer instead of
in separate radio network layer protocols. There is still much work to
be done on key issues involving how to implement common functions
and on a common radio network layer protocol, and on
interoperable network management.