IEEE Wireless Communications • October 2015
90
1536-1284/15/$25.00 © 2015 IEEE
Jingchu Liu is with
Tsinghua University and
Intel Labs.
Shugong Xu is with Intel
Collaborative Research
Institute for Mobile
Networking and
Computing (ICRI-MNC).
Sheng Zhou, Zhisheng
Niu are with Tsinghua
University.
1
This type of link got
“front” in its name since
it is closer to the network
edge compared to back-
haul (BH), which con-
nects different BSs as
well as BSs and core-net-
work elements.
SMART BACKHAULING AND FRONTHAULING FOR
5G NETWORKS
INTRODUCTION
In recent years, cellular networks have witnessed
a tremendous surge in data traffic, which is
largely driven by the widespread adoption of
smart devices such as smartphones and tablets
[1]. While this trend will most likely continue in
the foreseeable future, new challenges are also
emerging with the proliferation of machine-type
communications and real-time cloud services. In
response to these predicted challenges, next-gen-
eration networks are envisioned to provide
1000× capacity, 100× data-rate, and 1 ms latency
compared to fourth generation (4G) Long Term
Evolution (LTE) systems [2]. The enabling tech-
nologies for such supreme performance are
expected to include massive multiple-input mul-
tiple-output (MIMO), ultra-dense networking
(UDN), as well as high-frequency spectrum.
Fronthaul (FH) is an important enabler for
the deployment of these technologies in 5G net-
works. The term FH
1
has its root in the dis-
tributed base station (BS) architecture, in which
the processing functions of a BS are split into
two entities: the remote radio unit (RRU), which
takes charge of radio processing and digital-ana-
log conversion near the antennas, and the base-
band unit (BBU), which handles digital baseband
processing at another location. The classical
form of FH refers to the point-to-point (P2P)
link that transports time-domain complex base-
band radio (a.k.a I/Q) samples between the cor-
responding RRU and BBU. Recently, FH also
finds use in the novel centralized radio access
network (C-RAN) architecture [3]. In C-RAN,
time-domain I/Q samples are aggregated from
scattered antenna sites to a central office for
uplink (UL) processing or sent out in the oppo-
site direction after downlink (DL) processing.
The dominant physical transmission technolo-
gy for classical FH is digital radio over fiber (D-
RoF). Although there are also several competing
technologies such as analog RoF, the longer
transportation range in C-RAN gives D-RoF
great advantages due to its low signal deteriora-
tion. Various specifications have been formed to
support the interoperability between FH prod-
ucts from different manufacturers. For example,
the common public radio interface (CPRI) spec-
ification [4] covers layers 1 and 2 of FH. Its
scope includes the physical topology, line data
rates, framing format, and so on.
Although classical FH has been widely adopt-
ed in distributed BS and C-RAN, it will never-
theless face serious challenges in the face of 5G
networks.
JINGCHU LIU, SHUGONG XU, SHENG ZHOU, AND ZHISHENG NIU
ABSTRACT
The fronthaul is an indispensable enabler for
5G networks. However, the classical fronthauling
method demands large bandwidth, low latency,
and tight synchronization from the transport net-
work, and only allows for point-to-point logical
topology. This greatly limits the usage of fron-
thaul in many 5G scenarios. In this article, we
introduce a new perspective to understand and
design fronthaul for next-generation wireless
access. We allow the renovated fronthaul to
transport information other than time-domain
I/Q samples and to support logical topologies
beyond point-to-point links. In this way, differ-
ent function splitting schemes can be incorporat-
ed into the radio access network to satisfy the
bandwidth and latency requirements of ultra-
dense networks, control/data decoupling archi-
tectures, and delay-sensitive communications. At
the same time, massive cooperation and device-
centric networking could be effectively enabled
with point-to-multipoint fronthaul transporta-
tion. We analyze three unique design require-
ments for the renovated fronthaul, including the
ability to handle various payload traffic, support
different logical topology, and provide differenti-
ated latency guarantee. Following this analysis,
we propose a reference architecture for design-
ing the renovated fronthaul. The required func-
tionalities are categorized into four logical layers
and realized using novel technologies such as
decoupled synchronization layer, packet switch-
ing, and session-based control. We also discuss
some important future research issues.
REDESIGNING FRONTHAUL FOR NEXT-GENERATION
NETWORKS: BEYOND BASEBAND SAMPLES AND
POINT-TO-POINT LINKS
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