Utilization of the SOA Deep Saturation and Power Averaging Effet to Counterat Intra-Channel Crosstalk in DWDM System Paper Fryderyk M. Dy, Paweł Mazurek, and Jarosław P. Turkiewiz Faulty of Eletronis and Information Tehnology, Warsaw University of Tehnology, Warsaw, Poland Astrat The Semiondutor Optial Amplifier (SOA) is a key omponent of ost-effetive short/medium range transmission systems. However it an introdue signal distortions. In this paper the authors investigate the possiility to redue the signal distortions in SOA operating with the multiple wavelength hannels. Using numerial simulations the negative influene of the nonlinear effets, namely ross-gain modulation (XGM) and the patterning effet an e redued in deep SOA saturation regime. The self-healing effet is pronouned for the or more wavelength hannels and the transmitted symol length longer than doule of the SOA reovery time. Keywords ross gain modulation, Dense Wavelength Division Multiplexing, Semiondutor Optial Amplifier.. Introdution The inreasing demand for the short and medium range high apaity optial transmission systems, utilized in, e.g., Loal Area Networks (LAN), Metropolitan Area Networks (MAN), and data/storage enter interonnetions, has reently aused the growth of interest in Semiondutor Optial Amplifiers (SOAs). Main advantages of SOA are: low ost, possiility of the photoni integration with other omponents like lasers or modulators, relatively high gain, and wide amplifiation andwidth. On the other hand, the main SOA disadvantages are high noise figure (6 db or more) and introdution of the nonlinear effets like the inter-hannel rosstalk aused y the ross gain modulation (XGM) effet. XGM is aused y the derease of the arrier density in the ative region of the SOA. Moreover, the SOA arrier reovery time (t ) in the range of 0 to 00 ps auses patterning effet for the signal with the symol it rate over G/s [], []. Those two effet ontriute to the intra-hannel rosstalk in the Dense Wavelength Division Multiplexed (DWDM) systems. Therefore tehniques are needed to ounterat signal distortions in the SOA. So far, the following tehniques have een used to mitigate the SOA XGM and the patterning effet: utilization of the gain-lamped SOA [3], keeping the SOA in the shallow saturation state [], utilization of the ontinuous wavelength reservoir hannel to suppress the power flutuations in the SOA [5], introdution of the additional dummy signal with inverted polarization to ahieve the onstant intensity of the output signal [6], dispersion management [7], modulation of the SOA injetion urrent in aordane with the transmitting it sequene [], feeding the SOA with many hannels to ahieve the power averaging effet while keeping the SOA in the shallow saturation state [9], utilization of the onstant envelope modulation format [0], utilization of the optial equalizers at the output of the amplifier [], [] or ative ontrol of the deision threshold in the reeiver [3]. The aovementioned methods of ounterating the nonlinear effets have following drawaks: high power penalty [5], [6], poor utilization of the availale SOA output power level [], [9], high system omplexity [7], [], [0], [] or the neessity to replae the urrently installed equipment [3]. In the artile, the authors analyze the possiility to ounterat negative influene of XGM and patterning effet, y driving the SOA into deep saturation. The proposed method allows the mitigation of the negative SOA on the signal quality influene, while avoiding disadvantages of the mentioned methods. In partiular, the impat of the DWDM hannel numer, signal line rate, signal extintion ratio and the depth of the SOA saturation on the amplified signal quality is investigated. The onduted simulations show that redution of the signal distortions and smallest power penalty introdued y the SOA our for the numer of hannels or more and for the line rates, for whih the ratio of the arrier reovery time to symol duration (t /T ) is greater or equal. For a typial SOA, for whih the arrier reovery time is 50 ps [], this orresponds to the it rate over G/s. The signals with high extintion ratio show overall etter signal quality. The method proposed in the artile an e suessfully utilized to inrease the performane of, e.g., the ost-effetive 00G and 000G Ethernet systems.. Semiondutor Optial Amplifier A Semiondutor Optial Amplifier is an optoeletroni devie apale of amplifying the optial signal. Its struture is similar to a semiondutor laser. The amplifiation takes plae in the ative region of the amplifier after otaining
Utilization of the SOA Deep Saturation and Power Averaging Effet to Counterat Intra-Channel Crosstalk in DWDM System the arrier population inversion. In the SOA amplifier, the inrease of the optial output power leads to the derease of the arrier density (or arrier numer), whih in turn leads to the derease of the gain (saturation effet). The SOA gain reovery time varies from 0 to 00 ps, therefore the signal amplifiation depends on the previous signal levels. Impat of the saturation effet and the amplifier s memory (patterning effet) on the transmitted signal was studied in [], [3]. The operation of the amplifier an e modeled with the following rate equations []: n(z,t + t) = n(z,t)+ { I ev n(z,t) a [n(z,t) n o ]I sig (z,t) τ hν } t, () Carrier density [/m ] 3 x0.0. SOA reovery time 50 ps 0 05 355 500 Time [ps] Fig.. The SOA arrier density in the funtion of time after single impulse amplifiation. I sig (z,t + t) = { z I sig,in (z,t + t)exp a [n(z,t + t) n 0 ]dz }, () 0 I sig,in (t) = ΓP in(t) A, (3) A = d W, () where n is the arrier density in the SOA ative region, t is the time, z is a distane from the eginning of the SOA ative region, I is the injetion urrent, e is the elementary harge, V is the volume of the SOA ative region, τ is the arrier life time, a is the differential gain fator, n 0 is the arrier density in the SOA transpareny point (state when the losses within the SOA are ompensated y the SOA gain), I sig is the optial signal intensity, h is the Plank s onstant, ν is the frequeny of the optial signal, Γ is the optial onfinement fator, P in is the instantaneous input optial power, A is the ross-setion area of the SOA ative region, d is the height of the SOA ative region, W is the width of the SOA ative region, and L is the length of the SOA ative region. The utilized model does not take into aount the wavelength dependeny of the gain profile. The presented aove SOA model takes into Gain [db] 0. 7. P sat -0 7.0 0 Output power [dbm] 3 db Fig.. The SOA gain in the funtion of the output power. aount the saturation effet, the XGM effet and the arrier reovery time. The applied in simulations SOA amplifier had the nominal small signal gain G of 0. db and saturation output power P sat of 7.07 dbm (Fig. ). The arrier reovery time t measured with 0%-0% method was equal to 50 ps (Fig. ). 3. Numerial Simulations 3.. Simulation Setup Blok Diagram The lok diagram of the simulation setup is shown in Fig. 3. The SOA amplifier is fed with the multi-wavelength signals haraterized y the extintion ratio ER, line rate B, Eye diagram Transmitter Mod Mod Mux Fig. 3. The simulation setup. SOA Att. Att. Filter Reeiver PIN Demux numer of hannels Ch, and input optial power per hannel P in. The utilized pseudo-random it sequene had the length of 5. The it sequenes and the initial phases of the DWDM signals were random for eah hannel, whih resulted in the signal deorrelation. The numer of OOK modulated hannels Ch was hanged from to and the extintion ratio ER had values of 0 db and 30 db, whih orresponds to the typial values of two major types of the 3
Fryderyk M. Dy, Paweł Mazurek, and Jarosław P. Turkiewiz optial modulators: the eletro-asorption modulators and the Mah-Zehnder modulators, respetively. The utilized wavelength multiplexer and demultiplexer were ideal without any losses and intra-hannel rosstalk. In the reeiver, only the thermal noise was taken into aount and its level was independent of the signal power []. The eletrial filter used in the reeiver was the 5th order Bessel filter. Moreover, in the reeiver an ideal eletrial amplifier was used. Based on the eye diagram of the reeived signal, the signal quality was estimated. 3.. Signal Quality Measure The most important signal quality measure used in teleommuniation systems is the it error rate (BER). There are various methods of determining the BER. The most popular of whih are the diret approah of ounting the errors and the Gaussian approximation method. In the ommerial teleommuniation systems the required BER is around 0 0 3. Determining this value in simulations using the diret method is impossile due to the very long simulation time, as this approah would require the transmission (simulations) of at least 0 its. Gaussian approximation is an analytial method taking into aount only the mean values of 0 and it levels and their variations [], whih is why this method is roadly utilized. However, it is required that the values of the reeived signal samples have the Gaussian distriution and that is why it is not useful in the ase investigated in the paper. Figure shows the differenes etween the atual distriutions of reeived signal samples and orresponding Gaussian distriutions. It an e seen in the Fig. the distriutions differ muh from Gaussian distriution. As a result, the indiated BER value is relatively high despite the wide eye opening. This means that in the ase of analyzing the signal amplified y the SOA driven into deep saturation, the Gaussian approximation method is ineffetive as it would ring unreliale results. In general, it is possile to determine the BER value in the analytial way if the proaility density funtion of the reeived signal samples is known. Unfortunately, the distriution of the signal samples after the amplifiation in the SOA has not een determined yet. However, the lowest value of it does not generally fall elow some onstant level, while the highest value of it may vary in the wide range [], [7], []. This lowest value of it as I th whih an e seen in Fig.. Similarly we denote the highest value of 0 it as I 0th. Therefore, as the reeived signal quality measure the eye opening of the signal was taken defined as I th I 0th. Sine it diretly reflets the signal quality, the eye opening width an e onsidered to e diretly related to the BER. Bit 3.3. Simulations () Threshold alulated in the Gaussian approah Proaility density EO Bit 0 The simulations were arried out as follows: the SOA amplifier was fed with the signals with defined values of parameters: the extintion ratio ER, the line rate B, the input optial power per hannel P in and the numer of hannels Ch. Numer of transmitted its was 096. Based on Eqs. () (), the SOA output signal was alulated. The total output power P outtot and the output power per hannel P out was measured. Next, the investigated hannel was filtered out in the wavelength demultiplexer and attenuated or amplified to ahieve the optial power of 5 dbm. The optial signal was onverted into eletrial domain and the orresponding eye diagram was otained and analyzed. Finally, in the middle of the it duration, the eye opening width was measured. BER Proaility density Fig.. Distriutions of reeived signal samples and orresponding Gaussian distriutions: SOA in shallow saturation, () SOA in deep saturation. I th EO I 0th 3.. Results To make the results independent of the SOA harateristis, the results were normalized in the following way: the optial signal power was normalized with respet to the SOA saturation output power P sat with the relationship P P out [db] and the line rate B was normalized with respet to the SOA arriers reovery time t with relationship t /T, where T denotes the it duration. The eye opening width was expressed in the amplitude units. The results of simulations showed that depending on the extintion ratio, line
Utilization of the SOA Deep Saturation and Power Averaging Effet to Counterat Intra-Channel Crosstalk in DWDM System rate, the numer of hannels, and the depth of the SOA saturation, the reeived optial signal had different eye opening widths. Those widths varied in the range of 30 to 0 amplitude units (hange of.5 db). The eye diagrams presented in the Fig. 5 show the eye diagram shape hanges depending on the system parameters. t / T = 0 t / T =.5 t / T = 0.5 hannel hannels hannels Fig. 5. The signal eye diagrams otained for different values of line rate and hannel numer (shown optial power values are normalized). In general, in the analyzed SOA amplifier, if different signals reeived with the same optial power generated eye diagrams with the different eye opening widths, then the signal whih generated the eye diagram with wider eye opening was less distorted in the SOA. In the Fig. 5 it an e seen that the width of the eye opening inreases with the inrease of the hannel numer. The improvement an e explained y the power averaging effet. With the inrease of the hannel numer, the total input signal power shows lower flutuations around the mean level and therefore the flutuations of the SOA arrier density and the SOA gain are also smaller. Inrease of the line rate shortens the duration time of symols and therefore redues the gain variations within a given symol or symol group. In the third olumn, presenting the results otained for hannels system, it an e seen that inreasing the normalized hannel line rate from 0.5 to 0 aused the inrease of the eye opening width y 0 amplitude units. Along with that inrease the onentration of optial power near the I th level also inreased. The values aove the I th level are unwanted, as the deision threshold in the reeiver must e set aording to the I th and I 0th levels [3]. The inreased onentration of it optial power near the I th level means the derease of the power penalty, i.e., eye opening inrease, as ideally the whole optial power of it should e onentrated in the I th level. Desried hange indiates the signal quality improvement. Inreasing the output optial power, means driving the SOA amplifier into the deeper saturation. In the Fig. 6 it an e I th I 0th EO seen that as the hannel numer inreases the ahievale eye opening width also inreases. In other words, inreasing the hannel numer leads to the derease of the signal degradation. 0 log 0 (EO) 0 log 0 (EO) 0 log 0 (EO) 0 9 7 5-0 0 9 7 5-0 0 0 0 9 7 5-0 0-0 0 s and their orresponding olors: t / T = 0.5-0 t / T =.5-0 t / T = 0 () 0 0 Z axis: 0 log (EO), olor sale: 0 Fig. 6. The eye opening width as a funtion of the total output power and hannel numer for the extintion ratio of 0 db. What is more, the eye opening width inreases with the inrease of the line rate, what an e partiularly seen for the ases of and hannels. In the ase of hannels the improvement in the eye opening width reahed approximately 3 db. This is evidently the result of the power averaging effet. The iggest inrease in the eye opening width an e ahieved for the normalized output optial power of 7 db, in the SOA deep saturation. In the graphs in the olumn of the Fig. 6, it is learly seen that in the whole range of the output optial powers inreasing the hannel numer leads to the inrease of the eye opening width. The iggest improvement is ahieved in the deep saturation of the SOA and it reahes 3 db. In the Fig. 7 in the a olumn, it an e seen that even for high hannel numer it is possile to ahieve high output optial power per hannel. The inrease of the hannel numer leads to the redution of the signal degradation. In the graphs in the olumn it is visile that inrease of the hannel numer (with onstant optial power per hannel) 5
Fryderyk M. Dy, Paweł Mazurek, and Jarosław P. Turkiewiz 0 log 0 (EO) 0 log 0 (EO) 0 log 0 (EO) 0 9 7 5-5 -0-5 0 9 7 5-5 -0-5 0 0 0 9 7 5-5 -0-5 0-5 -0-5 0 s and their orresponding olors: t / T = 0.5-5 t / T =.5-5 t / T = 0 () -0-5 0-0 -5 0 Z axis: 0 log (EO), olor sale: 0 initially leads to the eye opening width redution. However, eyond a speifi hannel numer the eye opening egins to inrease, in the investigated ase of hannels. The hannel numer, aove whih the improvement is oservale, ours at lower hannel numer for the high values of the output optial power per hannel. Again, the improvement is pronouned for the hannels with the short symol duration. Figure shows the results of the simulations for two extintion ratios. The 30 db extintion ratio is ahieved in the Mah-Zehnder modulator and the 0 db one in the eletro-asorption modulator. The signals with the 30 db extintion ratio have overall etter signal opening than signals with the 0 db extintion ratio. For the high extintion ratio signals, the improvement in the signal quality ours for lower value of the output power. Extintion ratio [db] 30 0 hannels, PoutCh - P sat = 0 db Fig. 7. The eye opening width as a funtion of the output power per hannel and hannel numer for the extintion ratio of 0 db. 0 9 7 5-5 -0-5 0 () t / T =.5 ER = 0 db 0 log 0 (EO) 0 log 0 (EO) t / T =.5 ER = 0 db 0 9 7 5-5 -0-5 0-5 -0-5 0 s and their orresponding olors: -5-0 -5 0 Z axis: 0 log (EO), olor sale: 0 Fig.. The eye opening width as a funtion of the output power per hannel and hannel numer, for the onstant line rate and two extintion ratio values: 0 db and () 30 db. 0.5 0.5 t/ T () hannels, PoutCh - P sat = 0 db Extintion ratio [db] 30 0 0.5 0.5 t/ T Z axix: 0 log0 (EO), olor sale: Fig. 9. The eye opening width as a funtion of the normalized line rate and the signal extintion ratio for: hannels and () hannels, and for the onstant normalized output power per hannel. Figure 9 shows the eye opening width as a funtion of the extintion ratio and the line rate for and hannels. Normalized output optial power per hannel was onstant 6
Utilization of the SOA Deep Saturation and Power Averaging Effet to Counterat Intra-Channel Crosstalk in DWDM System and equal to 0 db, therefore in the seond ase the total output optial power was 6 db higher than in the first ase, so the SOA was operating in muh deeper saturation. In oth analyzed ases the improvement of the eye opening width an e ahieved y the inrease of the line rate (up to db improvement). The maximal improvement aused y the line rate inrease is otained for the normalized line rate values higher than 3. In hannels ase, in the whole range of parameters the maximal improvement oserved is approximately db. Figure 0 presents the eye opening width as a funtion of the line rate and hannel numer, for the onstant value of total output optial power. In the Fig. 0a, the output optial power was equal to 0 db and in the Fig. 0 it was equal to 6 db (deeper saturation). In oth analyzed ases the improvement of the eye opening width an e ahieved y the inrease of the hannel numer (up to 3 db improvement) as well the inrease of the line rate (up to db improvement). For the SOA operating in the deep saturation (Fig. 0) the eye opening widths are db ER = 0 db, - P sat = 0 db () 0.5 0.5 t/ T ER = 0 db, - P sat = 6 db 0.5 0.5 t/ T Z axix: 0 log0 (EO), olor sale: Fig. 0. The eye opening width as a funtion of the normalized line rate and the hannel numer, for the onstant normalized total output power: 0 db and () 6 db. smaller. It has to e noted, though, that in the () ase the total output optial power is 6 db higher what is desired in the DWDM system to ahieve longer transmission distane. It an also e seen that higher improvement an e ahieved y inreasing the line rate and hannel numer in the deep SOA saturation. The results of simulations show that for shallow SOA saturation the eye opening widths of the reeived signals are high and therefore the introdued signal distortions are small. When the SOA is driven into deep saturation, the system with small hannel numer experienes muh higher redution of the eye opening, while for the system with the high hannel numer this redution is lower. For the system with hannels the eye opening width redution is almost negligile.. Conlusion The authors investigated the possiility of ounterating the inter-hannel rosstalk and related power penalty in the DWDM system with the utilization of the SOA deep saturation and power averaging effet. Results of onduted simulations show that power averaging effet aused y the inrease of the line rate, as well as the hannel numer in the SOA amplifier, has the strongest positive impat on the signal quality when the SOA amplifier is driven into deep saturation. To maximize the power averaging effet, the SOA should operate with high total output optial power and with many high line rate hannels. It is also shown that even for the large hannel numer it is possile to keep the high output optial power level per hannel. The iggest redution of the signal distortions is oserved for the hannel numer over and with line rate for whih the ratio t /T was more than. Aknowledgements This work was supported y the Polish National Siene Centre NCN under the ontrat UMO-0/03/D/ ST7/097. Referenes [] K. Inoue, Waveform distortion in a gain-saturated semiondutor optial amplifier for NRZ and Manhester formats, IEE Proeedings Optoeletronis, vol., no. 6, pp. 33 33, 997. [] IEEE Standard 0.3a-00,.09.03 [Online]. Availale: http://standards.ieee.org/aout/get/0/0.3.html [3] M. J. Conelly, Semiondutor Optial Amplifiers. New York: Springer, 00. [] A. A. M. Saleh, Nonlinear models of travelling-wave optial amplifiers, Eletron. Lett., vol., no., pp. 35 37, 9. [5] A. Ghazisaeidi, F. Vaondio, A. Bononi, and L. A. Rush, Bit patterning in SOAs: Statistial haraterization through Multianonial Monte Carlo simulations, IEEE J. Quantum Eletron., vol. 6, no., pp. 570 57, 00. [6] P. Doussiere et al., Clamped gain travelling wave semiondutor optial amplifier for wavelength division multiplexing appliations, in Pro. th IEEE Int. Semiond. Laser Conf., Miami, HI, USA, 99, pp. 5 6. 7
Fryderyk M. Dy, Paweł Mazurek, and Jarosław P. Turkiewiz [7] L. H. Spiekman et al., Transmission of DWDM hannels at 0 G/s over 0 km of standard fier using a asade of semiondutor optial amplifiers, IEEE Phot. Tehnol. Lett., vol., no. 6, pp. 77 79, 000. [] Y. Sun et al., Error-free transmission of.5 Git/s DWDM hannels over 5 km using asaded in-line semiondutor optial amplifiers, Eletron. Lett., vol. 35, no., pp. 63 65, 999. [9] H. K. Kim and S. Chandrasekhar, Redution of ross-gain modulation in the semiondutor optial amplifier y using wavelength modulated signal, IEEE Phot. Tehnol. Lett., vol., no. 0, pp., 000. [0] A. H. Guan, H. Yin, and H. L. Fu, A dispersion management sheme for reduing SOA indued rosstalk in WDM links, Guangzi Xueao/Ata Photonia Sinia, vol. 3, no. 7, pp. 790 793, 009. [] C. R. Doerr, C. H. Joyner, M. Zirngil, L. W. Stulz, and H. M. Presy, Elimination of signal distortion and rosstalk from arrier density hanges in the shared semiondutor amplifier of multifrequeny signal soures, IEEE Phot. Tehnol. Lett., vol. 7, no. 0, pp. 3 33, 995. [] J. Jennen, H. De Waardt, and G. Aket, Modeling and performane analysis of WDM transmission links employing semiondutor optial amplifiers, IEEE J. Lightw. Tehnol., vol. 9, no., pp., 00. [3] Z. Li, Y. Dong, J. Mo, Y. Wang, and C. Lu, 050-km WDM transmission of 0.709 G/s DPSK signal using asaded in-line semiondutor optial amplifier, IEEE Phot. Tehnol. Lett., vol., no. 7, pp. 760 76, 00. [] C. R. Doerr et al., Simple multihannel optial equalizer mitigating intersymol onterferene for 0-G/s non-return-to-zero signals, IEEE J. Lightw. Tehnol., vol., no., pp. 9 56, 00. [5] K. Inoue, Tehnique to ompensate waveform distortion in a gainsaturated semiondutor optial amplifier using a semiondutor saturale asorer, Eletron. Lett., vol. 3, no., pp. 376 37, 99. [] A. A. M. Saleh and I. M. I. Haa, Effets of semiondutor optial amplifier nonlinearity on the performane of high-speed intensity-modulation lightwave systems, IEEE Trans. Commun., vol. 3, no. 6, pp. 39 6, 990. [7] M. J. Connelly, Wideand semiondutor optial amplifier steadystate numerial model, IEEE J. Quantum Eletron., vol. 37, no. 3, pp. 39 7, 00. [] G. P. Agrawal, Fier-Opti Communiation System, th ed. New York: Wiley, 00. [9] R. M. Jopson et al., Measurement of arrier-density mediated intermodulation distortion in an optial amplifier, Eletron. Lett., vol. 3, no. 5, pp. 39 395, 97. [0] K. Inoue, Crosstalk and its power penalty in multihannel transmission due to gain saturation in a semiondutor laser amplifier, IEEE J. Lightw. Tehnol., vol. 7, no. 7, pp., 97. Fryderyk M. Dy reeived the B.S. degree in 0 and M.S. degree in 03, oth in Teleommuniations, from the Faulty of Eletronis and Information Tehnology, Warsaw University of Tehnology, Poland. He ompleted also postgraduate studies in Management at Warsaw Shool of Eonomis. His professional interests fous mainly on omputer siene and finding out new ways how tehnology an improve life. E-mail: fryderyk.dy@gmail.om Institute of Teleommuniations Faulty of Eletronis and Information Tehnology Warsaw University of Tehnology Nowowiejska st 5/9 00-665 Warsaw, Poland Paweł Mazurek reeived the B.S. degree in Data Communiations and Teleommuniation Management (0) and the M.S. degree in Teleommuniations (0), oth from The Faulty of Eletronis and Information Tehnology, Warsaw University of Tehnology, Poland. Currently he is a Ph.D. student at WUT. His researh interests inlude high speed and apaity transmission and digital signal proessing. E-mail: p.mazurek9@gmail.om Institute of Teleommuniations Faulty of Eletronis and Information Tehnology Warsaw University of Tehnology Nowowiejska st 5/9 00-665 Warsaw, Poland Jarosław P. Turkiewiz reeived the M.S. degree in Teleommuniations from the Warsaw University of Tehnology, Warsaw, Poland, in 99 and Ph.D. degree in Optial Communiation from the Eindhoven University of Tehnology, Eindhoven, The Netherlands, in 006. From 007 he is a researh expert at Orange Las Poland, Warsaw, Poland as well as an assistant professor at Warsaw University of Tehnology, Poland. He pulished over 70 peer reviewed papers and ontriuted and led several national and international researh projets. He ats as a reviewer for IEEE PTL, IEEE JLT, Optial Fier Tehnology, Eletronis Letters as well as was a memer of Tehnial Program Committee of 39th European Conferene on Optial Communiations (ECOC) 03. His sientifi interests inlude high speed optial signal transmission and swithing. Dr. Turkiewiz was awarded IEEE LEOS Graduate Student Fellowship in 005 as well as three Warsaw University of Tehnology awards for sientifi and eduational ahievements. E-mail: jturkiew@tele.pw.edu.pl Institute of Teleommuniations Faulty of Eletronis and Information Tehnology Warsaw University of Tehnology Nowowiejska st 5/9 00-665 Warsaw, Poland