Soft-Stacked PON for Soft C-RAN
Weisheng Hu, Lilin Yi, Hao He, Xuelin Yang, Zhengxuan Li, Meihua Bi, Kuo Zhang,
Haiyun Xin, Yuan Liu, and Weijia Du
Abstract—Cloud radio access networks (C-RANs) feature
central pooling baseband processing units (BBUs) and low-
cost remote radio headends (RRHs) with fiber-connected
fronthauls between them, and they are promising for 4G.
Researchers are redesigning the C-RAN with different split
approaches to relax the fronthaul capacity for 5G with mas-
sive antennas, which is termed soft C-RAN. In this paper,
we propose a soft-stacked passive optical network (PON)
to support the soft C-RAN as an integrated solution. Two
approaches are investigated in this paper. The first one
is realized by employing an arrayed waveguide grating
router (AWGR) and directly modulated tunable lasers,
and it is promising for digital fronthaul and midhaul trans-
mission and switching. The second is realized by employing
wavelength-selective switches (WSSs) to allocate one-to-
one and many-to-many connections, and it is promising
for analog fronthaul transmission and switching. Their per-
formances are analyzed and measured with directly modu-
lated lasers (DMLs) that are either fixed or tunable. A delay
interferometer is used for the DMLs’ chirp and fiber
dispersion management. The result is significant for future
soft C-RANs and stacked PON.
Index Terms—Base stations; Optical fiber networks;
Radio access networks; Wavelength division multiplexing.
I. INTRODUCTION
W
ith the boom of ubiquitous mobile Internet and
various smart devices, the population and band-
width accessing the Internet are increasing continuously.
Today’s 4G provides 100 Mbps to 1 Gbps bit rate, which
is projected to 10 Gbps in the future 5G era [1–4].
Meanwhile, fiber-to-the-home (FTTH) is being deployed
worldwide to provide access capacity of 10 Gbps, which
is projected to 40–100 Gbps in next-generation passive op-
tical networks (NG-PONs) [5–7]. It is regarded that 4G con-
sumes more power than FTTH at higher bit rates and
longer distances. Therefore, it is significant to design
and deploy both together to reduce the cost and power con-
sumption. In an FTTH global conference, one hot topic was
entitled “The mobile and fibre convergence—Why FTTH
loves mobile and mobile needs FTTH” [8].
The central office is the hub for the mobile and fiber
access network. Currently, operators are redesigning
their central offices and, for example, demonstrating a
joint project called Central Office Re-architected as a
Datacenter (CORD). The mission is to bring datacenter
economies and cloud agility to service providers for resi-
dential, enterprise, and mobile customers using an open
reference implementation, which is built from commodity
servers, white-box switches, disaggregated access technol-
ogies, and open source software [9]. Both radio access net-
works (RANs) and PONs are being reshaped as soft clouds.
In this paper, we will report a soft-stacked PON for a soft
cloud radio access network (C-RAN), which is a solution to
integrate the access network between 5G and NG-PON. In
Section II, we will review the soft C-RAN and focus on the
long and short fronthual and midhaul and their require-
ments for the fiber transmission. In Section III, we will de-
scribe the soft-stacked PON with optical transceiver
pooling. In Section IV, we will report a soft-stacked PON
using an arrayed waveguide grating router (AWGR) and
tunable lasers. In Section V, we will report another soft-
stacked PON using wavelength-selective switches
(WSSs) and parallel signal detection (PSD). Finally, a sum-
mary is given in Section VI.
II. REQUIREMENTS OF SOFT C-RAN FOR OPTICAL
TRANSMISSION AND TRANSPORT
The left part of Fig. 1 shows the conventional C-RAN ar-
chitecture, which is a centralized, collaborative, cloud,
clean RAN [1–4]. The digital baseband processing units
(BBUs) are pooling in the central office to serve a large
group of remote radio heads (RRHs). The link between
them is termed a long fronthaul in the range of 10–
20 km. The common public radio interface (CPRI) specifies
the transmission of digitized radio signals (I/Q) across this
interface along with control and management, synchroni-
zation, and other auxiliary information [10]. One CPRI link
contains one or multiple I/Q flows, each carrying the data of
one antenna for one carrier. The total bit rate of the CPRI
link is proportional to the number of sectors and antennas
per sector, the sample rate and the number of bits per sam-
ple, the bandwidth of the radio channel, and the overhead
of the framing, including the synchronization and control
[1–4]. Current analysis shows that the bit rates of the cur-
rent 3G and 4G systems are usually no more than 10 Gbps
http://dx.doi.org/10.1364/JOCN.8.000B12
Manuscript received June 1, 2016; revised September 3, 2016; accepted
September 5, 2016; published October 4, 2016 (Doc. ID 267495).
Weisheng Hu (e-mail: wshu@sjtu.edu.cn), Lilin Yi, Hao He, Xuelin Yang,
Kuo Zhang, Haiyun Xin, Yuan Liu, and Weijia Du are with the State Key
Laboratory of Advanced Optical Communication System and Networks,
Department of Electronic Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China.
Zhengxuan Li is with the School of Communications and Information
Engineering, Shanghai University, Shangha i 200072, China.
Meihua Bi is with the College of Communication Engineering, Hangzhou
Dianzi University, Zhejiang Province, Hangzhou 310018, China.
B12 J. OPT. COMMUN. NETW./VOL. 8, NO. 11/NOVEMBER 2016 Hu et al.
1943-0620/16/110B12-09 Journal © 2016 Optical Society of America