Behavioral Modeling and Digital Compensation of Nonlinearity in Multi-band
Externally-modulated Radio-over-fiber Links
Chunjing Yin
1
, Jianqiang Li
1
,*, Hao Chen
1
, Qiang Lv
2,3
, Yuting Fan
1
, Feifei Yin
1
, Yitang Dai
1
, and
Kun Xu
1
1
State Key Laboratory of Information Photonics and Optical Communications, Beijing University
of Posts and Telecommunications, Beijing 100876, China
2
CETC Key Laboratory of Aerospace Information Applications, Hebei, Shijiazhuang, 050081,
China
3
The 54th Research Institute of China Electronics Technology Group Corporation, Hebei,
Shijiazhuang, 050081, China
*Corresponding author: jianqiangli@bupt.edu.cn
ABSTRACT
In the well-recognized cloud radio access networks
concept, radio-over-fiber (ROF) links can serve as an
option to make fronthaul connections conveying
multi-band radio signals. High-performance ROF links
covering future higher frequency commonly rely on
external modulation. However, externally-modulated
ROF links suffer from inherent modulator nonlinearities.
In order to address this issue, we investigated the
multi-band nonlinear behavior based on
multi-dimensional memory polynomial model. Based on
this model, multi-dimensional digital predistortion is
experimentally studied for multi-band
externally-modulated ROF links. The experimental
results show that the external modulator instead of
photodetector contributes to the most of nonlinear
distortions among which the cross-band distortions is
dominant with respect to the in-band distortions.
Moreover, the results also suggest us to implement a
memoryless multi-dimensional digital predistortor with
involving nonlinearity orders up to 5 for a practical
multi-band externally-modulated RoF system. With the
suggested parameters, the proposed multi-dimensional
DPD technique is able to improve the RF input power
tolerance by greater than 3 dB for each band in a
two-band case.
Keywords: Behavioral modeling; digital predistortion;
multi-band; nonlinearity; radio-over-fiber
1. INTRODUCTION
Recently, a new radio access network (RAN) concept of
C-RAN has been envisioned by China Mobile and well
recognized by the wireless communications community
[1]. In its broad sense, the letter “C” of “C-RAN” has at
least four representations including centralized
processing, collaborative radio, cloud infrastructure,
and clean networks [1]. The essence of C-RAN is to
provide an open and unified network platform for
flexible upgrade and green evolution. In the context of
C-RAN, the fronthaul links are also preferred to
simultaneously carry multiple radio signals for
multi-band multi-standard coverage in one cell.
Therefore, it is necessary to investigate fronthaul links
for C-RAN in a multi-band multi-standard environment.
On the road toward C-RAN, radio-over-fiber (ROF)
links can serve as another promising option to make
fronthaul connections between central unit (CU) and
remote antenna units (RAUs) [2-4]. ROF links might
outperform the links based on Common Public Radio
Interface (CPRI) or Open Base Station Architecture
Initiative (OBSAI) due to simplified RAU design,
relaxed bandwidth requirement on fronthaul links,
radio-technology transparency, inherent multi-standard
multi-band compatibility, and good scalability. It is
straightforward for ROF links to convey multi-band
radio signals by subcarrier multiplexing (SCM), as long
as the bandwidth of analog optical transceivers covers
the carrier frequencies of these signals.
High-performance ROF links are often enabled by
external modulation instead of direct modulation due to
reduced relative intensity noise, lower chirp, and higher
modulation bandwidth up to potential millimeter-wave
band. However, the inherent nonlinearity from external
modulators results in both in-band and cross-band
nonlinear distortions, ultimately limiting the ROF link
design and RF power transmit efficiency. Unfortunately,
new wireless communication standards suggest use of
advanced modulation formats which feature very high
peak to average power ratio (PAPR), ty
pically on the
order of 10 dB. The RF signals with higher PAPR tend
to feel stronger nonlinearities from limited range of
electro-optic modulation response in external optical
modulators.
The behavioral model and digital compensation of
nonlinearity in directly-modulated ROF links have been
well studied [5-8]. The digital predistortion in
multi-band externally-modulated ROF links was
preliminarily studied in [9]. In this paper, we focus on
2016 25th Wireless and Optical Communication Conference (WOCC)
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