Abstract—We experimentally demonstrate the transmission

performance of subcarrier multiplexing (SCM) over fiber, where a bulk reflective semiconductor optical amplifier (RSOA) is used as an external modulator. Each subcarrier has the bandwidth around 80 MHz, carrying data rate with 480 Mbps using 64 QAM and 2048 IFFT OFDM. The nonlinear distortion of RSOA external modulator significantly limits the transmission of large signals. A digital predistortion based compensation strategy is also presented for a single carrier case.

Index Terms—Digital predistortion (DPD), distributed antenna systems (DAS), radio over fiber (RoF), reflective semiconductor optical amplifier (RSOA), subcarrier multiplexing (SCM).

I. INTRODUCTION

EXT generation of access networks will aim at the growing demand for the wired and wireless access to provide the

higher data transmission rates, higher radio frequency (RF) transmission bandwidths and higher mobility connections when compared with current networks. An alternative to the current cellular network is to have a distributed antenna system (DAS) [1], which will provide a larger bandwidth considering shorter radius cells. A DAS includes a central unit (CU) and a number of remote antenna units (RAUs). Radio over fiber (RoF) technology has been proposed as a solution to implement optical fiber links between CU and RAUs [2]. The distributed antennas are linked directly to the RAUs in order to enable the greatest flexibility for the accommodation of future radio systems and services. The CU will enable the network to provide heterogeneous wireless systems as well as cooperative transmission between antennas.

Subcarrier multiplexing (SCM) is a scheme where the multiple channel signals are combined in radio frequency (RF) domain and transmitted by a single wavelength [3], [4]. This gives an advantage over a pure wavelength division multiplexing (WDM) access, due to the lower cost of electrical components if compared with an optical multiplexer. A disadvantage of SCM is the limitation imposed by the

nonlinearity of the electro-optical modulators to large driving RF power. As the number of SCM channels increase the power per channel will be reduced and consequently its dynamic range gets worse.

Reflective semiconductor optical amplifiers (RSOAs) will play an important role in future optical communication links and have been investigated recently [5]-[7]. We have already proposed and experimentally validated the RSOA model for single carrier transmission [7]. The good matching and transmission performance were obtained. In this paper we investigate the SCM transmission based on RSOA external modulator.

Degradation of the optical links performance will occur due to the inherent nonlinearity of external modulators [8]. Both predistortion and equalization techniques have been used to compensate for the nonlinear distortion of devices [9]-[12]. A potential issue for an equalizer which is used at the receiver is that it may amplify noise whereas compensation of nonlinear distortion [12], [13]. A predistorter can compensate for the nonlinear distortion at the transmitter before the noise is introduced. Digital predistortion (DPD) is the most commonly used, simple and robust method for improving linearity of nonlinear devices [14], [15]. Memory effects are usually considered in most of the DPD models to improve the linearization performance.

In this paper we demonstrate the transmission performance of four channel SCM signals over fiber using RSOA as the external modulator. In previous FUTON project [16] the intermediate frequency carriers of 300 MHz, 500 MHz, 700 MHz and 900 MHz were assigned to transmit data. We still utilize these frequency carriers for transmission of four channels SCM. The performance of SCM over fiber is evaluated by the error vector magnitude (EVM). We will also show and demonstrate that the DPD can be used to compensate the nonlinear behavior of an optical link based on the RSOA external modulator considering a single channel.

Transmission of Four Channels SCM over Fiber and Nonlinear Compensation for RSOA External

Modulators

Zhansheng Liu, Manuel Alberto Violas, and Nuno Borges Carvalho Instituto de Telecomunicações, Dep. Electrónica Telecomunicaçãoes e Inofrmática

Universidade de Aveiro, Campus Universitário de Santiago Aveiro 3810-193, Portugal

Email:{zhansheng, manuelv, nbcarvalho}@ua.pt

N

First Workshop on Distributed Antenna Systems for Broadband Mobile Communications

978-1-4673-0040-7/11/$26.00 ©2011 IEEE 147

II. TRANSMISSION OF SCM OVER FIBER

A. SCM Measurement Setup

The experimental setup is shown in Fig. 1. The four channel orthogonal frequency division multiplexing (OFDM) signals were first generated in VPI software. Each channel had wideband bandwidth of around 80 MHz. The data rate was up to 480 Mbps. 64 quadrature amplitude modulation (QAM) and 2048 IFFT OFDM were employed. After digital processing, the signals were loaded into Tektronix arbitrary waveform generator (AWG). A commercial RF amplifier (AMP) was used to drive the RSOA. A tunable RF attenuator enabled us to control the input power of the RSOA. Tektronix DPO72004B oscilloscope was used to capture the output signal of the device under test (DUT).

A commercial distributed feedback (DFB) laser, which was biased at 30 mA and had a wavelength of 1550 nm, was used as the seed light of the RSOA. The input optical power of RSOA was set to -7 dBm by adjusting a tunable optical attenuator. The RSOA was biased at 90 mA. The forward and reverse signals of the RSOA were separated by an optical circulator. A photodiode (PD) with a responsivity of 0.8 A/W was used as a detector to convert optical signals to electrical ones.

B. Transmission Performance of SCM over Fiber

The spectrum of the received signal from the oscilloscope is shown in Fig. 2.

In order to evaluate the transmission performance by EVM, several signal processing routine such as alignment, down-conversion, and OFDM demodulation, are first performed to obtain the transmitted symbols and received symbols. T4he EVM can be calculated by

2

1

2

1

( ) ( )

( )

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y n s n

EVM

s n

=

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where ( )s n is the reference transmitted symbol; ( )y n is the

received symbol; N is the number of samples. The measurement EVM results versus the RF input power are

shown in Fig.3. The results show that there is some slight variation with frequency carrier which might be attributed to the relative fluctuation of the frequency response of the RSOA external modulator, the impedance mismatching between the driving amplifier and the RSOA external modulator. The minimum EVM of about 1.9% is obtained when RF input power is about – 2 dBm. The EVM will increase with increasing of RF input power. The nonlinearity of RSOA is dominant.

III. NONLINEAR COMPENSATION

In this section we investigate the nonlinear compensation for the RSOA external modulator based on the DPD techniques. A single carrier case is considered in this work.

A. Digital Predistortion Scheme

The diagram of digital predistortion is shown in Fig. 4. In

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Fig. 4. Scheme of digital predistortion.

Fig. 2. The received four channel SCM signals with 80 MHz bandwidth.

Fig. 3. Measured EVM of four channel SCM versus RF input power.

148

order to extract the model parameters the baseband input and output signals of the DUT (as shown in Fig. 1) should first be measured. As the pth-order pre-inverse transfer function is identical to the pth-order post-inverse transfer function [9], the input and output of the DUT can be regarded as the output and input of the DPD model to inverse the characteristics of the DUT. Thus the coefficients of the model can be simply extracted with an offline process.

B. Generalized Memory Polynomial Model

In order to compensate the nonlinear distortion of the RSOA external modulator in RoF links, we use a generalized memory polynomial [15] as a DPD model, which is given by

( ) ( ) ( )

( ) ( )

( ) ( )

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1 0

1

1 0 1

1

1 0 1

+

+

a a

b b b

c c c

K Lk

klk l

K L Mk

klmk l m

K L Mk

klmk l m

u n a y n l y n l

b y n l y n l m

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−−

= =

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−

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(2)

where ( )u n� and ( )y n� are the complex envelopes of the input

and output of the DUT, respectively; aK , bK and cK are the

order of nonlinearity; aL , bL , cL , bM and cM are the

memory lengths; kla , klmb , and klmc are the corresponding

coefficients of the DPD. As the model is linear in the coefficients, the linear

least-squares error minimization method can be used to estimate the coefficients of the model, which is described by

( )1ˆ H H−

=h Y Y Y u (3)

where ( )H⋅ denotes the complex conjugate transpose; Y and

u are the output signal matrix and input signal vector of the

DUT, respectively; h is the estimated vector of the coefficients, this is, the coefficient vector of the DPD.

In order to obtain the predistorted RF input signal of the DUT, the predistorted baseband output signal of the DPD u can be first obtained by

ˆˆ=u Xh (4) where X is the baseband input signal of the DPD, which is also the desired output signal of the DUT.

C. Experimental Setup for DPD and Results

The experimental setup is shown in Fig. 5. The test baseband signals were designed in Matlab and fed into a vector signal generator (VSG) to be up-converted to RF domain with a carrier frequency of 1 GHz. A commercial broadband amplifier (AMP), ZHL-42W, was used to drive the DUT. A vector signal analyzer (VSA) was used to obtain the baseband input and output signals of the DUT. This was realized by a switch. For instance, the direct connection between the output of AMP and VSA enables us to obtain the measurements of the input signals for the DUT. The VSG and VSA were connected to a computer controller using general purpose interface bus (GPIB) cables. A 10 MHz reference and trigger signals between the VSG and

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Fig. 5. The experimental test bench for DPD.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Normalized input magnitude

Nor

mal

ized

out

put

mag

nitu

de

(a)

without DPD

with DPD

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-40

-30

-20

-10

0

10

20

30

40

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Normalized input amplitude

Pha

se s

hift

(D

eg.)

(b)

without DPD

with DPD

Fig. 6. Dynamic characteristics of RSOA without and with DPD (a) AM/AM; (b) AM/PM.

149

VSA were used in order to synchronize and trigger multi measurement events. Any error in the generation acquisition synchronization could lead to poor repeatability of measurements. For optical link the components were the same within Fig. 1.

In order to extract the coefficients of the DPD and generate the predistorted signal for the RSOA external modulator, two random 64 QAM signals with 20 Msymbol/s, which were filtered with a square root raised cosine (RRC) filter with the roll-off factor of 0.22α= , were generated in Matlab. One of them was used to estimate the parameters of the DPD. The complex baseband QAM signal was fed into the VSG memory to be up-converted to 1 GHz and passed through the DUT. In this case the average RF input power of the DUT was set up to 13 dBm with a peak-to-average power ration (PAPR) of around 5.42 dB, which was the input 1 dB compression point (P1dB) of the RSOA. The complex baseband input and output signals of the DUT were obtained from the VSA. After time alignment and phase correction, 10,000 samples were used to estimate the coefficients of DPD as mentioned in Section 3. In our work we set 5a b cK K K= = = (only odd order considered),

2a b cL L L= = = and 2b cM M= = (30 coefficients total). Once the

coefficients were obtained, they were used to predistort the other random 64-QAM signal according to (3). The predistorted signal had a PAPR of approximately 8.38 dB, which was about 3 dBm more than the original signal’s. The predistorted signal was fed into the VSG and passed through the system. The output of the RSOA with the DPD was obtained from the VSA.

The dynamic AM/AM and AM/PM performances of RSOA external modulator without and with the DPD are shown in Fig. 6. From these figures we can clearly see that the nonlinear distortion and memory effect have been successfully compensated. The normalized power spectral density without and with the DPD and the transmitted signal (output of the AMP) are shown in Fig. 7. The nonlinear distortion was improved with 17 dB with the DPD when compared with the situation without the DPD.

After demodulation process in Matlab, the constellations

without and with the DPD and for the transmitted symbols are shown in Fig. 8. It can also be clearly seen that the nonlinearity effects of the RSOA have been successfully compensated. To evaluate the performance of the DPD, the error vector magnitude (EVM) is used as a figure of merit. The EVMs without and with the DPD are 6.1% and 2.0%, respectively, which were calculated by (1).

IV. CONCLUSION AND FUTURE WORK

We have demonstrated the transmission performance of SCM base on the RSOA external modulator. The optimal transmission for SCM happened when the RF input power was about -2 dBm. The nonlinearity of the RSOA external modulator significantly limits the large signals transmission for SCM.

We have investigated and demonstrated that the DPD linearization technique can be used to compensate the nonlinear distortion of the RSOA external modulator in RoF links, considering a single channel. Experimental results show that the nonlinear distortion and memory effects of the RSOA have been successfully compensated by the DPD based on the generalized memory polynomial model. The nonlinear distortion of the RSOA is improved by 17 dB with the DPD. The EVMs without and with the DPD are 6.1% and 2.0%, respectively.

In the future we will focus on compensating the nonlinear effects of the RSOA external modulator in SCM systems optical links.

ACKNOWLEDGMENT

This work was supported by the project CROWN PTDC/EEA-TEL/115828/2009. Z. Liu is sponsored by the Fundação para a Ciência e Tecnologia (FCT) under Ph.D Grant SFRH/BD/68376/2010, whose support is gratefully acknowledged.

-50 -40 -30 -20 -10 0 10 20 30 40 50-80

-70

-60

-50

-40

-30

-20

-10

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orm

aliz

ed p

ower

spe

ctra

l den

sity

(dB

)

Frequency offset (MHz)

transmitted

without DPDwith DPD

Fig. 7. Normalized output power spectra of RSOA without and with DPD and spectrum of transmitted signal.

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-1

-0.8

-0.6

-0.4

-0.2

0

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0.4

0.6

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In-phase

Qua

drat

ure

Fig. 8. Normalized constellation (blue ‘x’ for without DPD, green ‘•’ for with DPD, and red ‘+’ for the transmitted symbol).

150

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[16] FUTON EU FP7 project.

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