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An Experimental Study on Digital Predistortion for Radio-over-FiberLinks

Luis C. Vieira, Nathan J. Gomes, Anthony NkansahBroadband and Wireless Communications Group, University of Kent, Canterbury, UK CT2 7NT

ABSTRACT

Radio-over-fiber (RoF) has been proposed as an enabling technology for broadband networks, such as WiMAX andWiFi. Besides the inherent high bandwidth and reliability of RoF systems, they also allow the reduction of installationand maintenance cost of the remote antenna units (RAUs) and improvement in the coverage area of the basestation/access point. However, the nonlinear distortion of the optical link, which stems mainly from the laser diode, mayimpose serious limitations on the system performance, especially when high PAPR, wideband signals are used. Thus,distortion compensation is a key issue in order to facilitate the application of the RoF technology for broadbandnetworks. In this work, digital predistortion for directly modulated RoF links is experimentally investigated. A memory-

polynomial-based predistorter model is identified from measurements of the baseband OFDM input-output signals andthrough the use of an indirect learning architecture. Then, a predistorted signal is generated and fed to the RoF link forcomparing its output with that of the non-predistorted one. As a result of this compensation technique, an improvementof the static link linearity and the output constellation diagram has been found, with an EVM reduction of 1.73 %.

Keywords: Radio-over-Fiber, digital predistortion, memory polynomial

1. INTRODUCTION

Broadband wireless access networks demand both higher data rates and improved coverage. Radio-over-fiber (RoF) has been proposed as an alternative for distributing RF signals between a base station/access point and remote antenna units(RAUs), being seen as an enabling technology for broadband networks, such as WiMAX [1] and WiFi [2]. Theadvantages of RoF are high available bandwidth, reliability, interference tolerance, transparency, and low attenuation.Additionally, RoF technology allows the reduction of installation and maintenance cost of the RAUs and improvement inthe coverage area of the base station/access point. However, the nonlinear distortion of the optical link, due mainly to thelaser diode, may impose serious limitations on the system performance, especially in a multi-user environment [3]. Also,signal envelopes with high peak-to-average power ratio (PAPR), such as orthogonal frequency-division multiplexing(OFDM) signals, demand large dynamic range. Thus, the use of some nonlinear compensation technique becomes acrucial issue in order to facilitate the application of the RoF technology for transmission of wideband wireless signals.

Predistortion is a common and cost-effective approach to linearization of power amplifiers [4], [5], as well as radio overfibre links [3], [6]. In this technique, if the nonlinear transfer function of the optical transmitter is well known and stable,a circuit with inverse nonlinearity can be inserted before the modulation device, counteracting its distortions. Forcompensation of RoF link nonlinearities, adaptive digital predistortion has been proposed as a more commercially viablesolution compared to optical and/or electro-optical compensation methods [3], [7], [8]. Using this approach, high

performance can be achieved by taking advantage of the accuracy of digital signal processing and the predistorter can beeasily adapted by tracking any device and link operation changes. In addition, the composite effects of the linknonlinearity can be modeled and compensated with no need to focus on specific link components.

Nonlinear systems can be considered as either having memory effects or not. A nonlinear system in which the output atany instant is only dependent on the input at that instant is considered as a memoryless one, whereas a system in whichits signal output also depends on the past input values is said to have memory effects. For RoF links, a memorylessmodel may not be adequate to describe the electro-optic conversion process [6]. Generally, memory effects should betaken into account when wideband OFDM signals are applied to RoF links [9].

In this work, digital predistortion for directly modulated RoF links is experimentally investigated. The behaviouralmodeling approach presented in [9] is applied for extracting the RoF link inverse model. Accordingly, the predistorter

Optical Transmission Systems, Switching, and Subsystems VIII, edited by Yikai Su, Ernesto Ciaramella,Xiang Liu, Naoya Wada, Proc. of SPIE-OSA-IEEE Asia Communications and Photonics, SPIE Vol. 7988,

798828 2011 SPIE-OSA-IEEE CCC code: 0277-786X/11/$18 doi: 10.1117/12.888602

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2.2 Experimental setup

The experimental setup is depicted in Fig. 2. A 64QAM-OFDM input signal is generated in Matlab for the experiments,with a bandwidth of 6.25 MHz and consisting of 2048 subcarriers. The RoF link consists of a 1550-nm Teradian DFBlaser with an optical power of 5 dBm, FC/UPC-FC/UPC fiber patch cord, 10-dB optical attenuator, and AppointechInGaAs PIN photodiode, with bandwidth of 2.5 GHz. The DFB laser has a slope efficiency around 0.13 W/A, thresholdcurrent of 7.8 mA, and relaxation resonance around 3.8 GHz. A current source module is used to bias the laser at 31 mAthrough a surface mount bias tee and a resistance for impedance matching. All the laser module components are enclosedin a metallic box for RF radiation shielding. The photodiode module, mounted on a printed circuit board with a bias tee,is also enclosed in a metallic box.

For the predistorter identification, the OFDM input signal was modulated onto a 50-MHz RF carrier by the AgilentE4438C vector signal generator (VSG) and transmitted through the RoF link, with an average power of +8 dBm (at thelaser input). The link input-output signals were measured by the Agilent E4440A vector signal analyzer (VSA) and pre-

processed by the Agilent Distortion Suite (ADS) software. The ADS performs the precise frequency and time alignmentrequired to find the amplitude and phase distortions. Then, the predistorter coefficients were estimated in Matlab and theresulting predistorted signal was downloaded to the VSG and transmitted through the RoF link for measuring the

predistorted output signal.

Figure 2. Experimental setup for RoF link predistortion. PD = Photodiode, ATT = Optical attenuator.

3. RESULTS

The nonlinearity and memory effect of a RoF link can be described by displaying its dynamic amplitude (AM-AM) and phase (AM-PM) characteristics. In Fig. 3 and Fig. 4, the AM-AM and AM-PM characteristics of the non-predistortedRoF link are compared with those of the predistorted one. Memory effects appear as dispersed points in these graphs. Itcan be seen that the predistorter improves the static link linearity.

The link memory effects have been far from completely eliminated, since the dispersion in the amplitude/phase

characteristics is almost the same for both non-predistorted and predistorted link cases. We suppose that this is duemainly to the memory effects coming from the VSG internal circuits. If these effects are large enough, the predistorterused here cannot compensate for them since the predistorter identification was done based on the RoF link measureddata. Thus, no memory effects from the VSG have been taken into account and compensated for.


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Figure 3. RoF link amplitude (AM-AM) transfer characteristic

Figure 4. RoF link phase (AM-PM) transfer characteristic

The non-predistorted and predistorted link output signals were used in a comparison of the error vector magnitude(EVM) results. The EVM calculations were carried out in Matlab/Simulink. The improvement in the predistorted linkover the non-predistorted system can clearly be observed in their constellation diagrams in Fig. 5 and Fig. 6 and withEVM values of 0.65% and 2.38% respectively.


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Figure 5. Constellation diagram of the non-predistorted RoF link output

Figure 6. Constellation diagram of the predistorted RoF link output

4. CONCLUSION AND FUTURE WORK

This work reports preliminary results on digital predistortion for RoF links using memory polynomials and OFDM inputsignals. The predistorter has improved the static link linearity and the output constellation diagram, with an EVMreduction of 1.73 %. However, the memory effects have not been completely reduced in the amplitude/phase linkcharacteristics, which suggests that some level of memory coming from other sources (e.g. the VSG) should also becompensated for.

Despite the promising results, this methodology still requires more complete validation against other experimental andsimulation data sets. In the future, we plan new experiments using different models and changing the OFDM signal

bandwidth. We also intend to better identify all sources of memory effects in the experimental setup.

ACKNOWLEDGMENTS

This work was carried out within the framework of the European Union Integrated Project FUTON (FP7 ICT-2007-215533). Luis C. Vieira - lecturer at UTFPR and PhD student at the University of Kent - is sponsored by the BrazilianGovernment through CNPq and UTFPR, whose support is gratefully acknowledged.


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REFERENCES

[1] M. Yee; A. Ng'Oma, and M. Sauer, "Performance analysis of IEEE 802.16e WiMAX Radio-over-fiberdistributed antenna system," IEEE MTT-S International Microwave Symposium Digest, Boston, MA, pp.197-200, Jun. 2009.

[2] A. Das, A. Nkansah, N.J. Gomes, I. Garcia, J. Batchelor, and D. Wake, Design of low-cost multimode fiber-

fed indoor wireless networks, IEEE Microwave Theory Tech. , vol. 54, no. 8, pp. 3426-3432, Aug. 2006.[3] X. N. Fernando and A. B. Sesay, Adaptive asymmetric linearization of radio over fiber links for wireless

access, IEEE Transactions on Vehicular Technology , vol. 51, pp. 1576-1586, Nov. 2002.[4] D. R. Morgan, Z. Ma, J. Kim, M. G. Zierdt and J. Pastalan, A generalized memory polynomial model for

digital predistortion of RF power amplifiers, IEEE Transactions on Signal Processing , vol. 54, pp. 3852-3860,Oct. 2006.

[5] A. Katz, Linearization: Reducing distortion in power amplifiers, IEEE Microwave , vol. 2, pp. 3749, Dec.2001.

[6] A. R. Shah and B. Jalali, Adaptive equalisation for broadband predistortion linearisation of opticaltransmitters, IEE Proc. Optoelectronics , vol. 152, pp. 16-32, Feb. 2005.

[7] A, Hekkala, M. Lasanen, L.C. Vieira, N.J. Gomes, and A. Nkansah, Architectures for joint compensation ofRoF and PA with nonideal feedback, in Proc. IEEE Vehicular Technology Conference, VTC-2010 Spring ,Taipei, Taiwan, May 2010.

[8] D5.7 Performance of radio over fibre distribution network, FP7 ICT-2007-215533 FUTON project, Sept.2010. [Online]. Available: http://www.ict-futon.eu/deliverables.aspx[9] L. C. Vieira, N. J. Gomes, A. Nkansah, and F. van Dijk, Behavioral modeling of radio-over-fiber links using

memory polynomials, in Proc. IEEE International Topical Meeting on Microwave Photonics, MWP 2010 ,Montreal, Canada, Oct. 2010.

[10] L. Ding, G. T. Zhou, D. R. Morgan, Z. Ma, J. S. Kenney, J. Kim, and C. R. Giardina, A robust digital baseband predistorter constructed using memory polynomials, IEEE Transaction on Communication , vol. 52, pp. 159-165, Jan. 2004.


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