Radio over Fiber Systems for Small Cell Wireless Communications

Ajung Kim

, K. Cho

and J. Choi

Department of Ele. Eng., Sejong University, Seoul, Korea

Actus Networks, Ansan, Korea

Department of Computing, Hanyang University, Seoul, Korea

akim@sejong.ac.kr

Keywords: Optical OFDM, Fronthaul Networks, Optical Microwave Signal Transmissions.

Abstract: We established a radio over fiber system applicable to broadband wireless communications for small cell

applications and to mobile fronthaul network segment in cloud-radio access networks (RAN). Optical radio

signals are transmitted over a fronthaul fiber using orthogonal frequency division multiplexing (OFDM)

techniques and subcarrier frequency division multiple access (SC-FDMA). System parameters are evaluated

for various subcarrier modulations and the results of link performance measurements are analyzed.

1 INTRODUCTION

Recently strong demands for broadband wireless

communication systems to support the applications of

multimedia data transmissions have been growing.

The micro/mm wave band is considered to be a

promising solution owing to its spectrum availability,

compact size of radio frequency (RF) devices, and its

severe atmospheric attenuation characteristic.

In transmission of high speed data up to several

hundreds of Mbps via the broadband RF wireless

channel, error performance is considerably degraded

by intersymbol interference (ISI) due to multi-path

fading phenomena. To reduce the degree of ISI,

preferred are parallel signal transmission techniques

which provide much longer symbol duration than the

spread of a fading channel impulse response (ETSI,

1999). Especially, an orthogonal frequency division

multiplexing (OFDM) technique is being widely used

for broadband wireless access applications (Shieh,

2008).

The RF wave signals, however, suffer from severe

loss along the transmission line as well as

atmospheric attenuation. Low-attenuation, EMI-free

optical fiber transmission is considered attractive for

transport of wireless signals in the micro/mm wave

band. Recently, vigorous research activities have

been in progress to develop optical network systems

supporting the broadband wireless communication

systems (Pizzinat, 2015).

Optical RF and microwave signals find applications

in fronthaul networks and small cell wireless

communications (ChihLin, 2015).

This paper includes the followings. Section 2

presents system design and requirements for

fronthaul networks and small cell wireless

communication application. Section 3 suggests the

configuration of a hybrid fiber radio system for RF

broadband wireless communications adopting OFDM

and SC-FDMA techniques. Section 4 shows the

results of experiments to measure its link

performances. Finally, a summary and concluding

remarks are given in Section 5.

2 SYSTEM DESIGN OF OPTICAL

RADIO LINKS

Traditional base stations (BS) are composed of a

digital unit (DU) or Baseband Unit(BBU), and a radio

unit (RU) or remote radio head(RRH). DU performs

digital signal processing and RU contains the RF

components and is connected to the antenna. To

support a steep increase of wireless traffic, the

network architecture has evolved into small size cell

architecture and a cloud radio access network (C-

RAN) (Pizzinat, 2013). Fronthaul is a network

segment between DU and RU that appears in C-RAN

architecture (Pizzinat, 2015).

(1)

Actual In fiber

(2)

Figure 1: (1) System architectures of D-RAN (a) vs. C-

RAN (b) and (2) Frequency allocation in backhaul networks

for WLL (wireless local loop) and LMDS (local multipoint

distribution service).

The comparison between distributed RAN (D-RAN)

and C-RAN is shown in Figure 1. While the internal

interface between RU and DU has been defined as the

result of the digitization of the radio signal according

to common public radio interface (CPRI)

(CPRI,v.6.1), an option of optical analog RF wave

transmissions requires much less bandwidth

compared to the CPRI option.

In addition, wireless micro/mm wave systems in

conjunction with optical systems in backhaul

networks have several options for band conversion

between radio and carrier frequencies. Among

system options of baseband, intermediate frequency

(IF), and optical micro/mm-wave transmission, the IF

around 2GHz option is considered as a cost-effective

and efficient solution as shown in frequency

allocations Figure 1 (b).

For wireless systems where signal fading is fatal,

OFDM and SC-FDMA techniques are parallel

transmission schemes modulating data symbols on

multiple carriers simultaneously through an FFT

procedure (Armstrong, 2009). The resulting OFDM

symbol has much longer symbol duration than

temporal dispersion of fading channel. Consequently,

OFDM-based systems are robust to ISI due to multi-

path fading phenomena, and have an additional

advantage of low complexity in implementing an

equalizer.

With all analysis, we adopted a radio over fiber

system in a RF band of a center frequency 2.1GHz

employing OFDM and SC-FDMA techniques.

Subcarrier multiplexing (SCM) was used for

interfacing the optical fiber link with the wireless

system for high speed transmission of signals.

3 SYSTEM CONFIGURATION

Considering downlink application in fronthaul

networks, we elected 16QAM-OFDM and employed

forward error correction (FEC) and pilot tones for

spectral efficiency. Accumulated synchronization

errors were correctable by using some of the

subcarriers in the signal band as pilot tones. The FEC

coding rate was 2/3 and the errors of the sampling

clock in the FEC were less than 10

-4

. The data loaded

subcarriers and the pilot subcarriers were 176 and 24,

respectively.

Signals modulated with 16 QAM scheme and

coded with FEC were transmitted at a data rate of 150

Mbps. Trellis coded modulation (TCM)-OFDM with

an FFT size of 512 was used in multiplexing RF

signals. Spectral efficiency was greater than 2

bits/sec/Hz.

The system configuration is depicted in Figure 2.

The system mainly consists of OFDM/SC-FDMA

modems, baseband signal processing modules, RF

up/down converters, an optical single mode fiber link.

The upper part in Figure 2 corresponds to a DU in a

fronthaul network or a Tx in a backhaul network, and

the lower part does to a RU or a Rx. The system

mainly consists of OFDM/SC-FDMA modems,

baseband signal processing modules, RF up/down

converters, an optical single mode fiber link. The

baseband signals were up converted to RF signals of

2.1GHz by mixing signals from the local oscillator

(LO). The OFDM signals at 2.1 GHz of a center

frequency with the signal bandwidth of 100 MHz

modulate a DFB-LD (distributed feedback laser

diode) with a wavelength of 1550nm and are fed to

20 km of a G.652 fiber. For the system analysis, the

signals were O/E converted in a PD (photo detector)

and then down-converted to the baseband by a LO

and equalized with an automatic gain control (AGC)

mechanism in baseband signal processing.

Digital-to-

analog (D/A) and analog-to-digital (A/D) conversion

were carried out as well. In the baseband processing

in the Tx part, S/P (serial to parallel) conversion,

QAM modulation, IDFT (inverse discrete Fourier

transform), GI (guard cyclic prefix insertion) are

carried out in order, and in the Rx part, the inverse

process are performed in reverse order.

Figure 2: Configuration of a radio over fiber system for

OFDM signals.

For uplink application in fronthaul network, we

also built a 16 QAM SC-FDMA system, where an

additional DFT process is inserted after QAM

modulation in OFDM signal process.

4 RESULTS AND ANALYSIS

System evaluations were performed for 16 QAM

OFDM and SC-FDMA systems. The waveform of

OFDM signal has noise-like envelopes with very high

peak-to-average power ratio (PAR), which cause

nonlinear distortions in amplifying the signal at a

power amplifier (PA). Those distortions degrade the

performance of an OFDM-based system so severely

that a large value of output back-off (OBO) should be

assigned to a PA.

The power measured before the S band down-

converter was not amplified with the AGC. The

attenuation level was adjusted to 37dB for input

power of 7 dBm on the RF link. For lower attenuation

or higher input power, system degradation ensued as

the power was carried over into the nonlinear region

of the amplifier and so nonlinear effects became

significant. Figure 3 shows s

pectrum of the received RF

signals with OFDM and with SC-FDMA.

Transmission over a G.652 fiber was performed

without dispersion compensation and amplification,

as fiber dispersion is not significant in 20 km

transmission of the 2.1 GHz directly modulated

signals (Schmidt, 2008).

Signal constellation of the received 16 QAM

OFDM signals and 16QAM SC-FDMA signals are

shown in Figure 4. An EVM (error vector magnitude)

value was measured as 6.7% for 16 QAM OFDM

signals with a CNR (carrier to noise ratio) of 23.5dB.

Although EVM requirements has not been

standardized yet for fronthaul networks, the 3GPP

Figure 3: Spectrum of the received RF signals with OFDM

(above) and with SC-FDMA (below).

Figure 4: Signal constellation of the received 16 QAM

OFDM signals (above) and SC-FDMA signals (below).

generation partnership project) specification puts

a limit on EVM as less than 12.5% for 16 QAM

OFDM signals. Given that this requirement

corresponds to eNodeB case, the coverage of which

includes fronthaul segment, the fronthaul segment

itself requires stricter values on EVM than those for

eNodeB. Considering a system margin of 5dB on CNR

in fronthaul segment, a tolerable CNR value is

estimated as 23dB, from which the maximum EVM

value allowed for fronthaul segment is obtained as 5%.

For optical OFDM transmission outperforming

optical RZ-OOK transmission in system reach, it was

analysed that optical QPSK-OFDM can be employed

in long reach systems for error performance

enhancement, and optical 16 QAM-OFDM in short

reach for spectral efficiency improvement. The link

penalty of 16 QAM-OFDM can be compensated by

coding with the synchronization and channel

estimation algorithm and forward error correction

coding.

5 CONCLUSIONS

We established radio over fiber systems applicable to

broadband wireless communications for small cell

applications and to mobile fronthaul network segment

in C-RAN. Optical radio signals are transmitted over

a fronthaul fiber using OFDM techniques or SC-

FDMA techniques.

The proposed system design of radio over fiber

systems allows utilizing the legacy infrastructure

readily connected with optical backbone systems and

further expands the range of deployment. The

resulting system can be utilized in applications of

small cell mobile systems, high speed wireless LANs

and home networking system and provide a

promising solution for future mobile evolution towar

d 5G and beyond.

ACKNOWLEDGEMENTS

This research was supported by the MSIP and

supervised by IITP (

R0127-15-1048,

IITP-R0992-15-

1017).

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