Reductions of peak-to-average power ratio and ...

Photon Netw Commun DOI 10.1007/s11107-013-0405-3

Reductions of peak-to-average power ratio and optical beat interference in cost-effective OFDMA-PONs A. Tsokanos · E. Giacoumidis · G. Zardas · A. Kavatzikidis · N. P. Diamantopoulos · I. Aldaya · I. Tomkos

Received: 29 May 2012 / Accepted: 3 June 2013 © Springer Science+Business Media New York 2013

Abstract The peak-to-average power ratio (PAPR) and optical beat interference (OBI) effects are examined thoroughly in orthogonal frequency-division multiplexing access (OFDMA)-passive optical networks (PONs) at a signal bit rate up to ∼ 20 Gb/s per channel using cost-effective intensity-modulation and direct-detection (IM/DD). Singlechannel OOFDM and upstream multichannel OFDM-PONs are investigated for up to six users. A number of techniques for mitigating the PAPR and OBI effects are presented

and evaluated including adaptive-loading algorithms such as bit/power-loading, clipping for PAPR reduction, and thermal detuning (TD) for the OBI suppression. It is shown that the bit-loading algorithm is a very efficient PAPR reduction technique by reducing it at about 1.2 dB over 100 Km of transmission. It is also revealed that the optimum method for suppressing the OBI is the TD + bit-loading. For a targeted BER of 1×10−3 , the minimum allowed channel spacing is 11 GHz when employing six users.

A. Tsokanos (B) Faculty of Science and Technology, School of Computing and Mathematics, Plymouth University, Drake Circus, Plymouth PL4 8AA, UK e-mail: [email protected]

Keywords Orthogonal frequency-division multiplexing (OFDM) · Passive optical network (PON) · Direct-detection

E. Giacoumidis · A. Kavatzikidis · N. P. Diamantopoulos · I. Tomkos Athens Information Technology (AIT) Center, 19.5km Markopoulo Ave., 19002 Peania, Athens, Greece e-mail: [email protected]

1 Introduction

E. Giacoumidis · I. Aldaya Département Communications & Électronique, Telecom Paris-Tech, 46 rue Barrault, 75013 Paris, France A. Kavatzikidis iKnowHow S.A., Lefkados 3, Glyka Nera, Athens, Attica, Greece N. P. Diamantopoulos School of Engineering and Applied Science, Aston University, Aston Triangle, Birmingham B4 7ET, UK N. P. Diamantopoulos Graduate School of Engineering, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan G. Zardas Department of Informatics, Technological Educational Institute (TEI) of Lamia, 3rd Km Old National Road Lamia-Athens, C.P. 35100 Lamia, Greece

The orthogonal frequency-division multiplexing (OFDM) technique is used widely in wireless and wired communications since it provides immunity to interference caused by a dispersive channel [1]. Despite this, widespread use of OFDM technology has only recently been applied to optical communications. This is mainly because new developments in digital signal processing (DSP) technology make processing at optical data rates feasible [1–3]. With resilience in dispersion and its spectral efficiency feature, optical OFDM (OOFDM) has emerged as an interesting research topic that has attracted much attention recently for the future optical transmission systems [2,3]. On the other hand, modern bandwidth-hungry applications such as high-definition TV (HDTV), video-on-demand (VoD) and video conferencing demand greater channel capacities for the broadband residential services. To meet this bandwidth demand, passive optical networks (PONs) [4] using OOFDM technology have very recently gained overwhelming research and development

123

Photon Netw Commun

interests in next-generation access networks (NGN) [5]. In an OFDM-PON [5], multiple optical sources are transmitted simultaneously in the upstream direction. When two or more lasers are very close in wavelength, a considerable mixing of the optical fields is produced. In the photodetection process, this produces beat signals which can overlap an active subcarrier channel (cross-mixing terms at the difference frequencies corresponding to each pair of optical fields) producing additional noise: This effect is known as optical beat interference (OBI) [6–8]. OBI is also caused by temperature fluctuations and phase noise effects which add to the random nature of these non-preselected sources. Besides, when detected by a single photodiode, unwanted missing products are produced and interfere with the OFDM data frequencies [9,10]. OBI may be controlled and utilized for communication in systems with coherent detection, but unfortunately in a subcarrier multiple access (SCMA) PON, it may cause unwanted disturbing products in the electrical frequency range used for the subcarriers. It is then denoted optical beat noise (OBN) [11]. Furthermore, together with the OBI problem, another challenge still remains in the application of OOFDM-PON systems: the high peak-to-average power ratio (PAPR). A high PAPR causes distortions coming from transmitter nonlinearities and results in two major impairments: out-of-band power and in-band distortion deteriorating the nonlinear impairments in optical fibers [1,12]. A method to reduce the PAPR is the simple clipping technique which unfortunately leads to distortions within and out of the signal bandwidth [13]. In addition, the utilization of adaptive-loading algorithms such as the bit-loading or power-loading techniques [14] has led to significant transmission performance improvements by improving the tolerance to a number of linear and nonlinear effects such as the distributed feedback (DFB)-laser chirp effect and fiberassociated four-wave mixing (FWM)/cross-phase modulation (XPM) effects for OOFDM-PON systems by lowering the signal’s PAPR [14]. On the other hand, from authors’ recent investigations on OFDMA-PONs [15], an effective method to suppress the OBI effect is the thermal detuning (TD) of the lasers. The present paper is a detailed presentation and a significant extension of authors’ conference paper reported in Ref. [15] related to PAPR and OBI mitigation techniques in OFDM-PON systems. In Ref. [15], two techniques for mitigating the PAPR were evaluated, namely the bit-loading algorithm and clipping. For combating the OBI effect, the power-loading with TD was applied. In the current paper, some important issues are added to the aforementioned OBI investigations: (1) The OBI effect is investigated over six users for a multichannel OOFDM-PON system, and (2) the bit-loading algorithm is also explored for the OBI suppression.

123

The current work firstly examines the PAPR reduction techniques by comparing them with a conventional modulation technique which distributes identical powers/bits on OFDM subcarriers: (a) the bit-loading algorithm using OOFDM or simply the adaptively modulated OOFDM (AMOOFDM), where individual subcarriers within a symbol are manipulated in the frequency domain by using different signal modulation formats according to the frequency response of the given transmission link [14,16] and (b) a nonlinear distortion technique, where an optimum clipping ratio (defined in Ref. [14]) is applied on the amplitude distribution of an oversampled adaptation of the digital signal [1,14,16]. It should be noted that the results for the PAPR investigation are limited for a point-to-point OOFDM system with no split losses that will present the worst-case scenario. Nevertheless, the PAPR investigations are used as baseline for any OOFDM-PON system. Afterward, the OBI effect is investigated in multichannel OOFDM-PON for the upstream direction using three techniques: the bit-loading/power-loading algorithms and the TD of the lasers. In particular, the study on OBI is carried out to identify the minimum channel spacing that provides OBI-free transmission for six optical network units (ONUs) and optical line terminals (OLTs) using these methods. It is shown that the bit-loading algorithm is a very efficient PAPR reduction technique. In particular, the PAPR effect is reduced by 1.2 dB over 100 Km of transmission. On the other hand, the clipping technique impact on reducing the PAPR is not significant as expected. For the OBI problem, it is revealed that the optimum method for suppressing the OBI is the TD + bit-loading. For a targeted BER of 1 × 10−3 , the minimum allowed channel spacing is 11 GHz when employing six users, respectively, entailing to a minimum required bandwidth of 61.5 GHz for the OLT photodetector.

2 System models and OOFDM-PON parameters Two system models are developed in this work, one for the PAPR study and the other one for the OBI investigation. For both system models, cost-effective intensitymodulation with direct-detection (IM/DD) transceivers are employed for OOFDM transmissions being developed in a Matlab/Virtual Photonics Inc. (VPI)-transmission-Maker co-simulated environment. The DSP for the developed OOFDM-PON system is based on the work reported in Ref. [14]. The generation and detection of the electrical OFDM signals are developed in Matlab platform, while the standard VPI-transmission-Maker modules are used for the optical components and the standard single-mode fiber (SSMF). The reference network architecture (RNA) of our proposed

Photon Netw Commun

Fig. 1 Reference network architecture (RNA) schematic diagram incorporating the proposed PON system using OFDM technology. Inset The DSP processors acronym descriptions for the OLT/ONUs (OFDM configuration)

system is depicted in Fig. 1 incorporating the OOFDM-PON technology. 2.1 System model for PAPR For the PAPR study, a single-channel, point-to-point system is depicted as illustrated in Fig. 2 for the purpose of isolating the PAPR effect. In addition, a chirpless ideal-amplitude modulator is considered to investigate the PAPR effect coming only from fiber nonlinearities without considering the nonlinearities of the CW-laser. In the transmitter, a serial-to-parallel converter truncates the encoded data into a large number of sets of closely and equally spaced narrowband data, the subcarriers. An inverse fast Fourier transform (IFFT) is then applied to the subcarriers arranged to fulfill the Hermitian symmetry property in order to generate real-valued OFDM symbols. These symbols are serialized to form a signal sequence, to which signal clipping is applied for the purpose of limiting the signal power within a predetermined range and reduce the transmitted PAPR of the OFDM signal. At the final stage, a digitalto-analogue converter (DAC) is used to convert the digital data sequence into an analogue signal waveform, which will drive the CW-laser at 1,552.524 nm. It should be noted that no optical amplification is incorporated to the system, thus preventing degradation due to the associated amplified spontaneous emission (ASE) noise and reducing the link cost.

Finally, the launched optical power for the single-channel is fixed at 0 dBm using a variable optical attenuator (VOA). In the receiver, after passing through a low-pass filter (LPF), the converted electrical signal encounters the inverse of that described in the transmitter, where an analogue-to-digital converter (ADC) module converts it back to its digital form, and afterward, a fast Fourier transform (FFT) is employed to decode the signal to its original sequence. The targeted BER is 1.0 × 10−3 which in combination with an appropriate forward-error-correction (FEC) leads to the values of < 1.0 × 10−9 [14]. For the real-valued signal waveforms, the encoder creates the OFDM symbols with the original data in the positive frequency bins, while the complex conjugate of the data in the negative frequency bins. In addition, no power is contained in the first subcarrier close to the signal baseband. From the 64 employed subcarriers, the 31 carry real data, 1 contains no power, and the remaining 32 are the complex conjugate of the rest subcarriers. 8-bit resolution DAC/ADC is used at a sampling rate of 12.5 GS/s. The length of the cyclic prefix (CP) was chosen to be very long, i.e., 25 %, in order to avoid virtually all inter-symbol interference (ISI) and inter-carrier interference (ICI) effects arising from the inter-subcarrier intermixing of the narrowband OFDM subcarriers through the chromatic dispersion (CD) for isolating the PAPR effect. Finally, the clipping level is fixed at 13 dB. A summary of the transceiver parameters for the PAPR study is given in Table 1 (left).

123

Photon Netw Commun

Fig. 2 a Point-to-point IM/DD OOFDM transmission-link diagram for the PAPR investigations; b upstream IM/DD OOFDM-PON transmissionlink diagram for the OBI investigations. The electrical domains are generated in Matlab and the optical domain in VPI

Table 1 Transceiver parameters for the PAPR study (left) and the OBI study (right) Transceiver parameters for PAPR study

Transceiver parameters for OB1 study

Number of subcarriers

31

Number of users

6

DAC/ADC quantization bits

8

Number of subcarriers per user

31

DAC/ADC clipping ratio

13 dB

DAC/ADC quantization bits

8

DAC/ADC sampling speed

12.5 GS/s

DAC/ADC clipping ratio

13 dB

Cyclic prefix (CP)

25 %

DAC/ADC sampling speed

6.5 GS/s

Signal bit rate

19,375 Gb/s

Cyclic prefix (CP)

25 %

Optical modulator

Ideal-amplitude modulator

Signal bit rate per user

10,4 Gb/s

Initial laser frequency

193.1 THz

ONU optical filter insertion loss

3 dB

123

Optical intensity modulator

DFB laser

Initial laser frequency

192 THz

Photon Netw Commun

Fig. 3 Signal peak occurrence probability density versus PAPR for adaptive modulation (AMOOFDM) and identical modulation OOFDM over A. 20 Km, B. 60 Km and C.100 Km

Initially, no bit-loading (adaptive modulation) is applied to the signal; therefore, the powers of all OFDM subcarriers are assumed to be identical [14,16]. For the adaptive modulation, at the establishment stage of a connection over the SSMF-link, negotiations between transmitter and receiver define the highest signal modulation format that should be taken to each subcarrier as shown in Fig. 2a. The signal modulation formats used for the numerical simulations are dependent on the frequency response of a given transmission link and include differential binary phase shift-keying (DBPSK), differential quaternary PSK (DQPSK) and 8-quadrature amplitude modulation (8-QAM) up to 256-QAM. It should be noted that even though in the adaptive modulation, the signal modulation format to each subcarrier is changing, the net signal bit rate remains unchanged. In addition, since power normalization is applied, the optical spectrum of the OFDM signal after bit-loading does not change. The signal bit rate for the single-channel, point-to-point system is fixed at 19.375 Gb/s. Finally, transmission distances of up to 100 km are investigated. 2.2 System model for OBI Low-cost light sources, namely DFB-directly modulated lasers (DMLs), are employed for the OBI investigations without considering external modulation and optical amplification. For simulating the performance of the DFB-based DML, the DFB lasers operate in a bias current of 30 mA (a threshold bias current of 4.2 mA) and a peak-to-peak drive current range of 15 mA [14]. The resulting signal extinction ratio is 2 dB and the adiabatic frequency chirp 5 GHz. It should be pointed out, in particular, that under such operating conditions, the DML frequency chirp effect has been found to be minimum on the transmission performance of the OOFDM/AMOOFDM signals in SSMFs [14]. It has been shown [17] that the downstream direction of an OOFDM-PON system can be supported by data rates as high as 40 Gb/s per wavelength, while the upstream signal bit rate is limited mainly due to the presence of the OBI effect

Fig. 4 Comparison of PAPR reduction difference versus reach performance for A. identical modulation OOFDM versus identical modulation OOFDM with clipping, B. identical modulation versus adaptive modulation (AMOOFDM) and C. identical modulation OOFDM versus adaptive modulation (AMOOFDM) with clipping

[6–8], and therefore, 10.4 Gb/s per wavelength are transmitted in this section. As shown in Fig. 2b, the OOFDMA IMDD PON consists of 6-ONUs: a single OLT including a PIN-FET detector with a responsivity of 1A/W (taking into account both thermal and shot noises) and an optical coupler connecting the 6-ONUs and the SMF. The PON architecture is outlined as follows: The ONUs are composed of OFDM transmitters, a DFB laser and a Butterworth band-pass optical filter with bandwidth twice the ONU band and insertion loss of 3.5 dB. The reason for optical filtering at the transmitter is to mitigate crosstalk with other ONUs. The OFDM generation on the transmitter is similar to that described in Sect. 2.1, using a fixed signal modulation format of 16-QAM (10.4 Gb/s). The launched optical power per channel is fixed at 0 dBm using a VOA. It should be noted that the SSMF noise has been turned-off in VPI for 20 Km of transmission (loss remains at 0.2 dB/km) to isolate the OBI effect and the CP length remains at 25 % to compensate virtually all CD. A summary of the transceiver parameters for the PAPR study is given in Table 1(right). At this point, it should be marked that for the upstream OFDM signals, since a single photodetector in the OLT receives the aggregated signals,

123

Photon Netw Commun Fig. 5 Electrical spectrum of the six ONUs for the upstream IM/DD OOFDM-PON

2.5 3 2 2

Imag

1

1.5

0 1

-1 -2

0.5 -3 -4

0 -4

-3

-2

-1

0

1

2

3

4

Real

Fig. 6 left BER distribution of the six ONUs with a channel spacing of 18 GHz using the TD method for the upstream IM/DD OFDM-PON; right Received constellation diagram of user #3 of the 16-QAM OFDM-PON at a BER = 4 × 10−4

therefore, OBI noise occurs within the OLT bandwidth causing the performance degradation. It is also worth mentioning that since the target of Sect. 2.2 is the study of OBI, the photodetector bandwidth limitation is not considered. Two adaptive-loading algorithms are developed to suppress the OBI effect, the bit-loading (AMOOFDM) and the power-loading algorithms [18]. While the procedure for the bit-loading algorithm is described in Sect. 2.1, the powerloading algorithm for the OBI mitigation is outlined as follows [18]: A fixed signal modulation format (16-QAM) is taken on all subcarriers and the individual subcarrier powers are optimized according to the system frequency response. Experimentally, the TD technique is done by variation of the temperature, usually by heating. Temperature control allows for approximately 0.1 nm/◦ C tuning for a DFB laser [19]. Temperature control can be done by using a Peltier cell. Practically, the maximum temperature forced variation can be in the range of 1-2nm. This depends on the installation conditions and the specific design of the laser module. In this

123

work, the TD is simulated by simply drifting the frequency spacing of the lasers.

3 Papr reduction results using bit-loading (AMOOFDM) and clipping In this section, the bit-loading algorithm and the clipping are applied in a single-channel OOFDM-PON system for reducing the PAPR effect. According to Fig. 3 and Fig. 4, it is shown that the bit-loading algorithm (or AMOOFDM) can produce a signal PAPR lower than that corresponding to an identical modulation OOFDM. This statement is validated by measuring the signal peak occurrence probability density [14]. In Fig. 3, the signal peak occurrence probability density versus PAPR is plotted for AMOOFDM and identical modulation over A. 20Km, B. 60Km and C. 100Km. While in 20Km, their performance is almost the same, as the transmission distance increases, AMOOFDM produces more signal peaks in a

Photon Netw Commun

Fig. 7 BER versus channel spacing using thermal detuning (TD), TD + bit-loading and TD + bit-loading for 6 users (channel#3 at 10.4 Gb/s)

low-PAPR region of 4 dB. This is because AMOOFDM decreases the probability of independently modulated subcarriers being added up coherently by the IFFT [14]. It should be noted that when the clipping technique is applied to AMOOFDM or identical modulation OOFDM, the signal’s PAPR is reduced by only ∼ 0.1 dB as revealed in Fig. 4, and therefore, the clipping technique impact on reducing the PAPR is not significant.

4 OBI suppression results using thermal detuning, power- and bit-loading The target in this section is to investigate the OBI effect over a typical multi-channel IM/DD OOFDM-PON system for the upstream direction using the TD of the lasers and the power/bit-loading algorithms at a signal bit rate per ONU of 10.4 Gb/s. Six channels are transmitted with the 3rd middle OOFDM channel set at 1,552.524 nm, which is assumed to be the baseband wavelength channel, while the rest of the OOFDM channels are thermally detuned (TD method), and afterward, power- and bit-loading algorithms are applied according to the related procedures described in Sect. 2. The electrical received spectrum for the six ONUs at 11GHz spacing is depicted in Fig. 5 where it is clearly identified the OBI peak at the frequency reference of 45 GHz and the 6 OFDMA channels at much lower frequencies. In Fig. 6 (left), the BER distribution for the six users is presented when TD is only applied for 18 GHz of channel spacing (defined as the distance within two neighbor lasers optical carriers), and in Fig 6 (right) the 16-QAM received constellation diagram of user #3 is depicted at a BER of 4×10−4 . It is shown, from Fig. 6, that

all users have an acceptable BER performance of < 1×10−3 . The effect of intra-channel crosstalk is also shown, where the central channels have higher BERs compared to the initial and last channels. In Fig. 7 the three methods of TD, TD + power-loading, and TD + bit-loading, for mitigating the OBI effect are compared as a function of BER for channel #3 for the 6-ONU OOFDM system over different channel spacing. The minimum allowed channel spacing is about 12.8 GHz for TD, 12 GHz for TD + power-loading and 11 GHz for TD + bitloading. The difference in the minimum channel spacing between bit- and power-loading can be explained as follows: When bit-loading is considered, the level of signal modulation format can be decreased (increased) on subcarriers with low (high) signal-to-noise ratio (SNR) leading to the improvement of the subcarriers SNR and consequently of the BER. On the other hand, when PL is employed, the occurrence of a large number of detected errors for some subcarriers cannot be tolerated since the subcarriers SNR is not significantly improved. 5 Conclusions The PAPR and OBI problems have been examined comprehensively in IM/DD single-channel OOFDM-PON and multi-channel OOFDM-PON with six users for the upstream direction. Four techniques for mitigating the PAPR and OBI effects were evaluated: the bit-loading algorithm (or AMOOFDM) and clipping for the PAPR, the TD and bit-loading/power-loading algorithms for the OBI. It was shown that the bit-loading algorithm is a very efficient PAPR reduction technique because it decreases the probability of independently modulated OFDM subcarriers being added up coherently by the IFFT. In particular, the PAPR effect was reduced by 1.2 dB over 100 Km transmission. On the other hand, the clipping technique impact on reducing the PAPR was not significant. For the OBI effect, the TD, TD + power-loading and TD + bit-loading were explored and compared in an upstream multi-channel IM/DD OOFDM-PON system for six users. It was shown that the optimum method for suppressing the OBI was the TD + bit-loading. For a targeted BER of 1 × 10−3 , the minimum allowed channel spacing was 11 GHz. Acknowledgments This work was supported by the ICT EU FP7 project ACCORDANCE.

References [1] Armstrong, J.: OFDM for optical communications. IEEE J. Lightwave Technol. 27(3), 189–204 (2009) [2] Qian, D., Hu, J., Ji P., Wang, T., Cvijetic, M.: 10-Gb/s OFDMAPON for Delivery of Heterogeneous Services. In: Optical Fiber

123

Photon Netw Commun

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OWH4 (2008) Wei, W., Hu, J., Qian, D., Wang, T.: Optical frequency division multiple access (OFDMA)-based Optical Access/Metro Ring Networks. In: National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper JWA123 (2008) Kim, D., Xing, P. Y., Won, Y., Park, S., Han, S.: Bidirectional 1.25G b/s Colorless RSOA Based WDM-PON Using Suppressed Optical Carrier and Polarization Beam Splitter. In: National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper JThA97 (2008) Liu, B., Xin, X., Zhang, L., Yu, J., Zhang, Q., Yu, C.: A WDMOFDM-PON architecture with centralized lightwave and PolSKmodulated multicast overlay. OSA Opt. Express 18, 2137–2143 (2010) Soerensen, S.: Optical beat noise suppression on power equalization in subcarrier multiple access passive optical networks by downstream feedback. IEEE J. Lightwave Technol. 18(10), 1337–1347 (2000) Desem, C.: Optical interference in subcarrier multiplexed systems with multiple optical carriers. IEEE J. Select. Areas Commun. 8(7), 1290–1295 (1990) Jin, X.Q., Tang, J.M.: Experimental investigations of wavelength spacing and colorlessness of RSOA-based ONUs in real-time optical OFDMA PONs. IEEE J. Lightwave Technol. 30, 2603–2609 (2012) Cano, I., Santos, M.C., Polo, V., Prat, J.: Dimensioning of OFDMA PON with non-preselected independent ONUs sources and wavelength-control. In: Proceeding of European Conference on Optical Communication (ECOC), Geneva, Switzerland, 2011, paper Tu.5.C.2 Cano, I., Santos, M., Escayola, X., Polo, V., Kachris, C., Giacoumidis, E., Tomkos, I., Prat, J:, An OFDMA-PON with nonpreselected independent ONU sources and centralized feedback wavelength control: Dimensioning and experimental results. IEEE International Conference on Transparent Optical Networks (ICTON), Coventry, UK, paper Tu.B3.3 (2012) Soerensen, S.: Optical beat noise suppression and power equalization in subcarrier multiple access passive optical networks by downstream feedback. IEEE J. Lightwave Technol. 18, 1337–1347 (2000) Sung, M., Kang, S., Shim, J., Lee, J., Jeong, J.: DFT-precoded coherent optical OFDM with Hermitian symmetry for Fiber nonlinearity mitigation. IEEE J. Lightwave Technol. 30, 757–2763 (2012) Goebel, B., Hellerbrand, S., Haufe, N., Hanik, N.: PAPR reduction techniques for coherent optical OFDM transmission. In: IEEE International Conference on Transparent Optical Networks (ICTON), Munich, Germany, paper Mo.B2.4 (2009) Giacoumidis, E., Wei, J.L., Yang, X.L., Tsokanos, A., Tang, J.M.: Adaptive-modulation-enabled WDM impairment reduction in multichannel optical OFDM transmission systems for nextgeneration PONs. IEEE Photonics J. 2(2), 130–140 (2010) Giacoumidis, E., Kavatzikidis, A., Cano, I., Santos, M. C., Tang, J.M., Prat, J., Tomkos, I.: Mitigation techniques for peak-toaverage power ratio and optical beat interference in OFDMAPONs. In: IEEE International Conference on Transparent Optical Networks (ICTON), Stockholm, Sweden, paper Tu.C6.3 (2011) Tang, J.M., Shore, K.A.: 30-Gb/s signal transmission over 40km directly modulated DFB-laser-based single-mode-fiber links without optical amplification and dispersion compensation. IEEE J. Lightwave Technol. 24(6), 2318–2327 (2006) Reichmann K. C., Iannone P. P., Brinton C., Nakagawa J., Cusick T., Kimber M., Doerr C., Buhl L. L., Cappuzzo, M., Chen,

123

E.Y., Gomez, L., Johnson, J., Kanan, A.M., Lentz, J., Chang, Y.F., Pálsdóttir, B., Tokle, T., Spiekman, L.: A symmetric-rate, extended-reach 40 Gb/s CWDM-TDMA PON with downstream and upstream SOA-Raman amplification. IEEE J. Lightwave Technol. 30(4), 479–485 (2012) [18] Giacoumidis, E., Jin, X.Q., Tsokanos, A., Tang, J.M.: Statistical performance comparisons of optical OFDM adaptive loading algorithms in multimode fiber-based transmission systems. IEEE J. Photonics 2, 1051–1059 (2010) [19] Uchiyama, S., Yokouchi, N., Nimomiya, T.: Continuous-wave operation up to 36 degrees C. IEEE Photonics Technol. Lett. 9(2), 141–142 (1997)

Author Biographies A. Tsokanos works as a lecturer in computer networks at Plymouth University. Also, he has worked as a lecturer for Lamia Institute of Technology, Greece, where he used to teach high-speed networking and network design. Previously, he was employed as a technical specialist in JA.NET UK, which is the UK’s education and research network. He holds a Ph.D. in optical networking and an M.Sc. in mobile and satellite communications both from Bradford University. His bachelor’s degree was in electrical engineering from Lamia Institute of Technology. His research interests include OFDM, ROADMs, traffic modeling as well as various optical networking technologies. E. Giacoumidis joined the Optical Communications Research Group at Bangor University (2006–2010) for a Ph.D. degree with a full EPSRC scholarship working simultaneously as a teaching assistant for undergraduate Electronic Engineering Design and Applications. In July of 2010, he joined the High-Speed Networks and Optical Communications (NOC) Research Group of Athens Information Technology (AIT) research center for a Senior Research Scientist position. Since November of 2011, he works in Telecom Paris-Tech as a Research Associate for the Optical Communications Group. He is also a “visiting and collaborating researcher” for both Bangor University and AIT center. He has authored and coauthored more than 30 papers that appeared in international peerreviewed journals and conferences. His current research involves balanced theoretical and experimental exploration in the areas of highspeed communications systems and optical networking with specialization in DSP advanced modulation formats/techniques such as optical orthogonal frequency-division multiplexing (OOFDM), fast-OFDM, polarization-division multiplexing (PDM), equalization techniques for passive optical networks (PONs), LANs, MANs and long-haul transmissions. G. Zardas works as a lecturer for Lamia Institute of Technology, Greece, where he teaches management information systems and database design. He holds a Ph.D. in Applied Informatics, from the University of Macedonia, Greece, as well as a Master of Business Administration in Management Information Systems from State University of New York at Albany. His research interests

Photon Netw Commun include E-learning, adaptive learning systems, management information systems and Internet technologies A. Kavatzikidis is a very experienced research scientist and technical project manager with a deep knowledge in wireless communication systems, networks and engineering project management. He has more than 5 years of experience on high level R&D projects both in technical development and project management. He was awarded a full scholarship for his PhD studies in “Measurement & Modelling of the Spatio-Temporal Ultra-Wideband Radio Channel” from the Engineering and Physical Sciences Research Council (EPSRC) at the Department of Engineering Science, University of Oxford. N. P. Diamantopoulos was born in Athens, Greece, in 1988. He received the B.Sc. degree in Telecommunications Science and Technology from University of Peloponnese, Greece, in 2009. He is currently working toward the master’s egree in Photonic Networks Engineering under the ERASMUS MUNDUS MAPNET consortium [Scuola Superiore Sant’Anna Pisa (Italy), Aston University (UK), Osaka University (Japan), Technische Universitat Berlin (Germany)]. In 2011, he joined the “High Speed Networks and Optical Communication (NOC)” group in Athens Information Technology, Greece.

I. Tomkos (B.Sc., M.Sc., Ph.D.) has been with the Athens Information Technology Center (AIT) since September 2002. In the past, he was a senior scientist (1999–2002) at Corning Inc., USA, and a research fellow (1995–1999) at the University of Athens, Greece. At AIT, he founded and serves as the Head of the “High Speed Networks and Optical Communication (NOC)” Research Group that was and is involved in many EU funded research projects within which he is representing AIT as Principal Investigator and has a consortium-wide leading role (e.g., Project Leader of the EU ICT STREP project DICONET, Project Leader of the EU ICT STREP project ACCORDANCE, Technical Manager of the EU ICT STREP project SOFI, Technical Manager of the EU IST STREP project TRIUMPH, Chairman of the EU COST 291 project, WP leader in many other projects). He is also the chairman of the Working Group on “Core Network Design and Transmission” within the GreenTouch Initiative/Consortium, an industry-driven initiative focusing on improving the energy efficiency of telecom networks, and a member of the industry-driven fiber to the Home Council Europe, which aims to promote the deployment of FTTH in Europe. Together with his colleagues and students, he has authored over 460 peer-reviewed archival articles (an updated list may be found using the “Publish or Perish” tool; over 200 IEEE sponsored items may be found through IEEE Xplore), including about 110 journal/magazine/book publications and 280 conference/workshop proceedings papers.

I. Aldaya was born in 1980. He received the B.E. from the Public University of Navarre (UPNA), Pamplona, Spain, in 2005. Since 2008, he is a full time doctorate student at the School of Electronics and Information Technologies, Monterrey Institute of Technology and Higher Education (ITESM), Monterrey, Mexico, where he is part of the optical communications research group. In 2009, he spent a two-month research internship at the University of Bologna, Bologna, Italy. His main areas of research are radio over fiber networks, optical injection locking of semiconductor lasers, and physical modeling of optical routers.

123