Synchronous Optical CDMA Networks Capacity ... - IEEE Xplore

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 17, SEPTEMBER 1, 2010

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Synchronous Optical CDMA Networks Capacity Increase Using Transposed Modified Prime Codes M. Massoud Karbassian, Member, IEEE, and Franko Kueppers, Member, IEEE, OSA

Abstract—A novel spreading code-set based on the prime code (PC) families, referred to as ‘transposed modified prime codes (T-MPC)’, is proposed for synchronous optical code-division multiple-access (OCDMA) networks. The new code-set is constructed algorithmically to enhance and simplify its implementation. The proposed code-set performance is compared with existing spreading code families in terms of correlations, bit-error rate (BER) and cardinality. The proposed optical spreading code family doubles the cardinality as compared to existing PC families. This also implies that greater number of users can be accommodated in the network. Since there is no longer a time-shift feature in T-MPC like in conventional modified prime codes (MPC), the code is not predictable and thus even more secure. However, the code structure is similar to MPC, thus its employment in a system running MPC will be nondestructive. The code is also compatible with low-weight MPC. The results indicate that the proposed code-set has properties to enhance the OCDMA network capacity remarkably. Index Terms—Multiple-user interference, optical CDMA, optical coding, prime codes, spreading code design.

I. INTRODUCTION

A

HIGH-PERFORMANCE optical code-division multiple-access (OCDMA) network requires that the receiver correctly recognizes the intended user information in the presence of the other interfering users as well as accommodates more subscribers. OCDMA offers the same virtual point-to-point topology as wavelength division multiplexing (WDM), however using a simpler network configuration. In doing so, WDM requires individual wavelength filters at the user-end with associated power loss, while OCDMA requires only the power-splitter and/or correlator. Since WDM systems use multiple wavelengths, the effect of beat noise is very remarkable. Unlike WDM and time division multiplexing (TDM), OCDMA can accommodate a large number of low and high bit-rate users on the same channel supporting multiple services. Such aspects correspond to access traffic patterns are highly desirable since they eliminate electronic grooming as well as supporting bursty traffics like Internet protocol (IP) [1]. Finally, OCDMA exhibits higher levels of security due to the optical signal coding.

Manuscript received February 24, 2010; revised May 18, 2010; accepted June 25, 2010. Date of publication July 08, 2010; date of current version September 01, 2010. This work was supported in part by the National Science Foundation through NSF ERC CIAN under Grant EEC-0812072 and in part by the Science Foundation Arizona under Grant CAA-0218-08. The authors are with the College of Optical Sciences, University of Arizona, Tucson, AZ 85721 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2010.2056356

Hence, it is preferable to employ suitable optical sequence codes which have the best orthogonal characteristic. In terms of the correlation properties, spreading sequences are selected with the features of maximum auto-correlation and minimum crosscorrelation in order to optimize the differentiation between the desired signal and interferences. Various optical coding sequence schemes for synchronous OCDMA communications and networks have been studied, particularly prime code families, including prime codes (PC), modified prime codes (MPC) [2] and various padded MPC (xPMPC) [3]–[6]. However, the previously introduced PC families are still struggling with the code security and the cardinality. Another common family of optical spreading codes is the optical orthogonal codes (OOC) [7], [8] which are extensively employed in asynchronous OCDMA schemes due to their correlation properties. Asynchronous OCDMA however supports less number of subscribers and requires random/medium access protocol to control the interference, collisions and code-assignments which brings complexity to the system implementations to compare with synchronous one [9]. The synchronization is though a challenging task, there are a few methods currently considered for the synchronization [10], [11]. On the other hand, a major problem associated with PC families is their code-weight, , which is always fixed to the number of subsequences and must be a prime number [2]–[6]. To acis commodate more users in an OCDMA system, a larger required, so is the code-weight . Since OCDMA encoders/decoders for PC families use a parallel configuration by active multipliers or passive optical tapped-delay lines (OTDL), shown in Fig. 1, the resulting optical power losses and complexity increases. of an encoder/decoder could also be high when Note that the decoder has similar architecture of Fig. 1 with inversed-delay coefficients. For example, the power loss of an all-parallel encoder (or decoder) is as high as 35.4 dB when [12], and the required number of optical delay lines . Consequently, per encoder (or decoder) equals the encoders/decoders will be bulky and their implementation by integrated optics will be challenging and inefficient. and Other major degrading factors are the autoconstraints and multiple-access intercross-correlation ference (MAI) which affect the overall performance of an OCDMA network. To reduce the effect of MAI, the optical ad(i.e., the weakest MAI) are dress sequences with minimum desirable for synchronous time-spreading OCDMA networks. These issues limit the capacity of OCDMA networks for a given power budget and system cost. Compared with OOCs of , PCs of can use a simple tunable encoder/decoder (i.e., tunable OTDL) to reduce both coding power loss and transmitter cost due to the property of equal-length ‘subsequences’ in each codeword. To further reduce the cost and

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Fig. 1. OCDMA encoder using parallel OTDL architecture; w is the codeweight and T is the chip-duration of code sequence of 100 100 100 100 010 for example.

power loss, we can employ low-weight PCs [12] which have symmetrically distributed ‘pulses’ in any codeword. It is observed that current research on the code enhancement has been only on code-length [3]–[6], [13], code-weight [12], [14], [15] and accordingly correlation values. It is apparent that the correlation properties will enhance when code-length and code-weight grow; while, the longer code-length and the higher code-weight can imply lower data-rate (i.e., throughput), lower spectral efficiency, higher power consumptions and complex transceivers (i.e., impractical). There is an immediate need for code generation algorithms to enhance the code cardinality under the already accepted constraints in order to expand the number of accommodated users due to day by day growth of network users. This paper proposed a novel code family using MPC as a seed referred to ‘transposed-MPC (T-MPC)’. Its performance and properties are compared with existing code families. This paper is organized as follow. Section II is dedicated to T-MPC construction and properties. Its performance analysis is outlined in Section III followed by the discussion of the results in Section IV. Finally, the paper is concluded. II. CONSTRUCTION OF TRANSPOSED-MPC in an Each data bit ‘1’ is encoded into a waveform OCDMA system where consisting of a code sequences or address sequences of chips (i.e., code-length) that addresses the destination of that data bit. Data bit ‘0’ is not encoded in incoherent schemes, while the bits ‘1’ and ‘0’ are phase modulated or wavelength encoded in coherent schemes. Each receiver correlates its own locally stored address with the received signal . The receiver output is then [5]: (1) , If the signal reaches the right destination, then and represents an auto-correlation function; and if represents a cross-correlation function. Maximizing the auto-correlation value and minimizing the cross-correlation

values in order to discriminate the correct address from all other interfering signals is essential at the receiver. This can be accomplished by producing well-designed set of address codes. The PC can be generated from the multiplication of the Gawith and then relois field ducing the product by modulo where is the prime number starts from 3. The number of available sequences is and the length of each code is . MPC is the time-shifted version of the PC sequences [2]. It is noted that time-shifting is allowed in a synchronous OCDMA system, and this leads to an increase in the number of subscribers as an advantage. Each PC sequence can be a seed to produce more MPC sequences. Thus, the available number of sewith code quences (i.e., cardinality) can be extended to sequences in each group, while the length of each code is , and the weight (the number of ones in the sequences) still is . Padded-MPC, new-MPC, double-padded-MPC and enhanced-MPC are all recent spreading codes developed based on PC family and analyzed in [3]–[6] respectively. All previavailable number of sequences ously introduced codes have and focused only on code-length and correlation properties enhancement. The proposed optical signature sequences, T-MPC, are generated in two folds: i) Firstly, full-padded MPC sequences are generated by times concatenating the MPC sequences with the last sequence-streams of the previous MPC sequence in the same group. At every step, one column is produced and padded to MPC sequences and the process continues with new padded MPC. This implies that the last sequence-stream of the previous padded MPC sequence is again padded to the next newly padded MPC sequence. Note that the padding rotates in the same group meaning that the last sequence-stream of the last MPC sequences is padded to the first MPC sequence in the same group, not continuing throughout the code-set. This padding procedure repeats only times since the th time will reproduce the last sequence-stream column of MPC itself. Table I illustrates an example of full-padded MPC when . According to Table I, for example sequences are ‘00100 00010 00001 10000 01000’ as MPC sequences and ‘10000 00001 00010 00100 01000’ as full-padded sequences. As observed, the ‘10000’ sequence-stream in full-padded sequences is the last sequence-stream of previous . As seen, the padding is performed column by column. Moreover, the next sequence-stream (i.e., ‘00001’) in full-padded sequences is the last sequence-stream of padded ; and this diagonal padding keeps going. It is noted that padded sequences are not restricted to the final sequence-stream column of MPC; they can be any sequence-stream column of MPC sequences. This is due to the uniqueness of each MPC sequence-stream that makes each sequence-stream distinctive; however, it is important to maintain the column’s order throughout the padding process; otherwise this would increase the crosscorrelation values. Since the diagonal padding procedure only repeats times, for the th sequence-stream the group sequence-stream is padded to sequences in the same group to finalize the full-padded MPC sequences.

KARBASSIAN AND KUEPPERS: CDMA NETWORKS CAPACITY INCREASE

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TABLE I FULL-PADDED MPC SEQUENCES WHEN P

=5

The generated full-padded MPC has code-weight, available number of sequences with code-length of . The auto- and cross-correlation values for any pair of codes and is given by: (2) where . It is apparent that this new fullpadded code-set suffers from increased code-length and loses the orthogonal sequences to compare with existing PC families [2]–[6]. Secondly, the unique lemma is now to treat the fullpadded MPC sequences, in Table I for example, as a matrix in which every chip (i.e., every ‘0’ and ‘1’) representing the full-padded MPC sequences as the elements of the matrix. By applying a ‘transpose function’ on this matrix, a new matrix will be generated. After rearranging the chips into -sequence-streams, the transposed-MPC (T-MPC) code-set is generated as seen in Table II. The new values of code-weight, code-length and cardinality for the novel T-MPC are and respectively. The codelength and code-weight are now practical and efficient; while, greater available sequences (i.e., doubled) are provided resulting in higher throughput. Since the time-shift feature is no longer valid in order to generate the T-MPC, its predictability much reduces and then the system security enhances remarkably. The in-phase auto- and cross-correlation values for any pair of codes and of T-MPC is then given by:

(3)

Fig. 2. Auto-correlation of T-MPC of C T is synchronization time

for the data stream of 11010 where

(code0length 2 chip0duration).

. It is also observed that the shrink where in the code-length improves the correlation values (see (2)). Based on this correlation function and example of T-MPC from Table II, the auto- and cross-correlation values of the T-MPC for the data-stream of ‘11010’ are displayed in Figs. 2 to 5. at every bit synIn Fig. 2, the auto-correlation values of chronous positions , which is equal to the code-length times chip-duration, are displayed. As expected, the peak value equals . It can be seen in Fig. 3 that the value of the cross-correlation of two sequences from the same group, e.g., and , at every synchronized time is ‘0’, which implies perfect orthogonality. As one of key features of T-MPC, It is worth to mention that the number of groups containing perfect orthogonal sequences has been doubled as well as total number of available sequences in T-MPC as observed from Table II.

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TABLE II TRANSPOSED-MPC SEQUENCES WHEN P

=5

Fig. 3. Cross-correlation of T-MPC of C and C stream of 11010 where T is synchronization time.

(same group) for the data

Fig. 4. Cross-correlation of T-MPC of C and C (different groups hit ‘one’) for the data stream of 11010 where T is synchronization time.

Two pairs of other sequences from different groups, e.g., and are presented in Figs. 4 and 5 yielding a value of ‘one’ and ‘two’ respectively for the same data stream at every synchronized time . It can be seem from Figs. 2–5 that the sequences follow the data stream as a result of CDMA encoding. In case T-MPC sequences are employed in the asynchronous scheme, users would communicate in the network in different time slots which causes out-of-phase and undesirable correlation values; as it is observable from Figs. 2–5 between each synchronized time. Then dynamically complex threshold and analysis would be required. Due to the properties of MPC families, the codes are popular in synchronous schemes.

Fig. 5. Cross-correlation of T-MPC of C and C (different groups hit ‘two’) for the data stream of 11010 where T is synchronization time.

III. LOW-WEIGHT T-MPC CONSTRUCTION AND ANALYSIS It is reported in [2] that the bit-error rate (BER) of incoherent OCDMA systems using PC families is decreased with increasing code-weight . When is large enough (e.g., without optical hard-limiters), the BER becomes very low even if all the users simultaneously transmit data in the network.


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Jian-Guo Zhang et al. [12] introduced a scheme to reduce the code-weight of MPC to achieve the same performance as conventional MPC obtains with higher code-weights. It is expected that we can choose a lower weight than that of the conven) by removing tional MPC to ensure a prescribed BER (i.e., some ‘redundant’ pulses from the original MPC sequences of pulses. The analysis in [12], [16] resulted in the construction of a new family of modified prime (MPR) codes for OCDMA applications. Subsequently here in this paper, we have also employed the same technique to reduce the code-weight of T-MPC and compare the results with previously generated MPR. The different method here is also to remove the pulses from middle subsequences rather than from two sides to increase the decoding slots distance of code family to be more distinguishable as introduced and explained in the following. From MPC of prime number , a T-MPC code-set of length , constant weight , auto-correlation and maximum cross-correlation supporting specific number of active users under the prescribed is constructed as described previously. To generate low-weight T-MPC (LW-T-MPC) the arbitrary ‘ones’ representing the optical pulses are removed from the T-MPC sequences. The remaining ‘ones’ (i.e., ) in the sequences form new LW-T-MPC. The resulting LW-T-MPC preserves the same cross-correlation constraint as original T-MPC. The proof and discussion on how to reasonably choose the weight and length of code-set sequences for OCDMA to optimize the system performance can be found in detail in [12], [16], [17]. We define as the maximum pulse separation in the th code sequence. equals the slot distance between the first and the last ‘one’ chips in the th code sequence of weight . For example, suppose a code-set with three sequences of , and , then we have , and . Note that can also represent the maximum delay-time difference among optical tapped-delay lines in the th optical encoder/decoder, shown in Fig. 1. The physical significance is that the separation of any two optical pulses from either the same code sequence or two successive code sequences larger than does not match any delay-time difference among optical delay lines in the th optical decoder. Therefore, the output of the decoder cannot exceed the maximum cross-correlation value [17]. Similarly, the maximum decoding slots distance of an -user OCDMA code is defined by . For example, the above three-user code-set’s is then equal to 23. This means that any pulse is a safe slot distance of a separation being greater than given spreading/address code due to the fact that safe slot distances do not make the output of the decoders exceed maximum cross-correlation value of specific given code-set (i.e., LW-T-MPC). Now by using the concepts of and , we can give the bounds on the length of the novel code-sets of LW-T-MPC and MPR [16]–[18] as follows: (4)

When the maximum decoding slots distance satisfies:

of a code-set (5)

the code-length can be shortened with not violation in the correlation constraints of a code-set sequences [12]: (6) Thus the reduction of bandwidth expansion, code-length decrement for the OCDMA will be:

, from the (7)

In practice, we can choose for symmetric OCDMA links, i.e.:

to minimize the (8)

successive pulses (i.e., ‘ones’) are removed When the last from an original code-set of size to obtain a new -user lowweight code-set, then the maximum decoding slots distance, , of the resulting code-set will equal . If satisfies , then the code-length for MPR code-set is chosen as: (9) and the is thus equal to . On the other hand, increase in the decoding slots distance is the key for outperformance due to the fact that it makes code sequences better distinguishable at the decoder. Therefore, to increase the maximum decoding slots distance for LW-T-MPC, the ‘ones’ can be removed from the body (middle) of sequence-streams instead of heads or tails of successive ‘ones’ in the code sequences as it is in MPR code-set. This results in , thus is chosen to equal the T-MPC original code-length. The BER of one-dimensional time-spreading signature codes can be calculated by the ‘hits’ possibility between ‘ones’ in code sequences in a simple incoherent intensity modulated on-off keying (OOK) system. According to the research by M. Azizoghlu, et al. [19] and followed by [16]–[18], the calculation is based on the ideally comprised of the possibility of binary stream occurrence, the threshold decoding range and ‘hits’ possibility between ‘ones’ within different signature codes and can be formulated as: (10) where is the code-weight of the code-set and is the number of simultaneous interfering subscribers. More importantly, [12], [16]–[19] varies in different code-set families depending on the maximum decoding slots distance and code-length and respectively under the specific correlation constraints. IV. DISCUSSION OF RESULTS The following data analysis for LW-T-MPC systems is based on the above calculations. For comparison purposes, systems

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Fig. 6. Code cardinality of T-MPC and MPC families against the prime number, P.

Fig. 7. BER performances of T-MPC and MPR against the number of simultaneous users, K when P = 53 and variable code-weights, w.

with employment of MPR have also been considered [12], [16]–[18]. is plotted on the graphs as a reference for comparison purposes to assist the eye. Since the MPR codes are currently the best-performed low-weight MPC family, here the results are only compared to this family and discussed accordingly. The aim of this paper is to analyze the performance of this novel spreading code itself and to compare with existing ones, thus the major degrading factor in the analysis is the ‘hit’ interferences causing the bit-errors. The cardinalities of T-MPC code-set and MPC families are compared in Fig. 6. It can be observed that the number of available sequences of T-MPC against the prime number grows faster and exceeds MPR (and MPC) family size (i.e., cardinality). Fig. 7 illustrates the BER performance of different codes against the number of simultaneous users, under the given condition of and code-weights of 18 and 20. As it can be observed, the LW-T-MPC outperforms MPR codes of various weights. The LW-T-MPC of and tolerates more interferences with lower BER than MPR of and , meaning less power consumption and better performance. It can be seen that MPR of and 20 accommodates 30 and 35 simultaneous users with respectively. While, LW-T-MPC of and 20 accommodates 40 and 45 simultaneous subscribers respectively. This implies significant 30% with system capacity enhancement with same code properties but different maximum decoding slots distance. Fig. 8 presents the BER performance of different codes against the number of simultaneous users, when the code-weight is fixed to 20 and prime number varies from 59 to 71. One might argue that the high values of prime number in this analysis are impractical and make the encoders/decoders complex. It should be noted that the analysis is for the extreme conditions of optical coding, and simultaneous hit occurrences (i.e., interference). On the other hand, the results discussed here should be comparable with the ones presented in [12], [16]–[19], which are under the same given conditions, for making fair argument.

Fig. 8. BER performances of T-MPC and MPR against the number of simultaneous users, K when w = 20 and variable prime number, P.

It is observable from Fig. 8 that the increment in value enhances the system performance dramatically with the cost of complexity of course. The MPR of can support 40 and 45 simultaneous users when and 71 respectively under the prescribed BER of . Whereas, LW-T-MPC of supports 55 and 75 simultaneous users when and 71 . It is apparent that by increasing respectively and the value the decoding slots distance also grows (i.e., to compare with ). Fig. 9 compares the BER performances of different codes against the prime number under various conditions of codeweights and where 5% and 10% of total number of simultaneous users are present when varies. It can be seen that the graphs cross over at certain values. The reason is that the decoding slots distances of LW-T-MPC and MPR become almost equal that make both codes perform likewise. It is observed that the crossover point in Fig. 9 is shifted vertically (i.e., regarding the BER) by changing the number of interfering users or horizontally (regarding or decoding slots distance) by varying the code-weight . As expected, the


Fig. 9. BER performances of T-MPC and MPR against the prime number, P under various conditions.

crossover point will be around which should be considered in the design process. As seen, in higher values , LW-T-MPC outperforms in terms of BER and supporting greater number of subscribers. It is also observable when changes with corresponding number of simultaneous users (i.e., 5% or 10% at every step), the BER of LW-T-MPC increases very slightly, indicating almost independency upon values implying well-suited higher system scalability (i.e., in higher values). V. CONCLUSION The novel optical address code based on prime code families for time-spreading synchronous OCDMA system, hereby referred to as transposed modified prime code (T-MPC) has been proposed and analyzed. Its optimized code-length and codeweight are practical, implementable and efficient with higher throughput and spectral efficiency. The T-MPC structure is similar to the well-established MPC sequences; consequently, employing this novel code is hassle-free and non-destructive to current working systems based on MPC. Rewardingly, the available number of code sequences has been doubled to compare with all existing MPC families. Since the time-shift feature is no longer valid in order to generate the T-MPC, its predictability much reduced and then the security enhanced remarkably. The code generation algorithm is simple and easily implementable for code assignment purposes. The BER results indicated that the T-MPC is capable of accommodating greater number of users to compare with similar featured code families since it provides greater decoding slots distances. This unique matrix-based ‘transpose’ approach can also be applied to other PC families or to even longer full-padded MPC codes or different spreading code families like OOCs that may be generated in the future to increase the code-set cardinality, while the correlation constraints of new code-set are practical. REFERENCES [1] M. M. Karbassian and H. Ghafouri-Shiraz, “IP routing and transmission analysis over optical CDMA networks: coherent modulation with incoherent demodulation,” J. Lightw. Technol., vol. 27, no. 17, pp. 3845–3852, 2009.

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[2] W. C. Kwong, P. A. Perrier, and P. R. Prucnal, “Performance comparison of asynchronous and synchronous code-division mutiple-access techniques for fiber-optic local area networks,” IEEE Trans. Commun., vol. 39, no. 11, pp. 1625–1634, 1991. [3] M. Y. Liu and H. W. Tsao, “Cochannel interference cancellation via employing a reference correlator for synchronous optical CDMA system,” J. Microw. Opt. Tech. Let., vol. 25, no. 6, pp. 390–392, 2000. [4] F. Liu, M. M. Karbassian, and H. Ghafouri-Shiraz, “Novel family of prime codes for synchronous optical CDMA,” J. Opt. Quantum Electron., vol. 39, no. 1, pp. 79–90, 2007. [5] M. M. Karbassian and H. Ghafouri-Shiraz, “Fresh prime codes evaluation for synchronous PPM and OPPM signaling for optical CDMA networks,” J. Lightw. Technol., vol. 25, no. 6, pp. 1422–1430, 2007. [6] A. Lalmahomed, M. M. Karbassian, and H. Ghafouri-Shiraz, “Performance analysis of enhanced-MPC in incoherent synchronous optical CDMA,” J. Lightw. Technol., vol. 28, no. 1, pp. 39–46, 2010. [7] W. Liang et al., “A new family of 2-D variable-weight optical orthogonal codes for OCDMA systems supporting multiple QoS and analysis of its performance,” Photon. Netw. Commun., vol. 16, no. 1, pp. 53–60, 2008. [8] M. S. V. Maric, M. D. Hahm, and E. L. Titlebaum, “Construction and performance analysis of a new family of optical orthogonal codes for CDMA fiber-optic networks,” IEEE Trans. Commun., vol. 43, no. 2/3/4, pp. 485–489, 1995. [9] P. Kamath, J. D. Touch, and J. A. Bannister, “The need for medium access control in optical CDMA networks,” in Proc. IEEE InfoCom, 2004, pp. 2208–2219. [10] A. Keshavarzian and J. A. Salehi, “Multiple-shift code acquisition of optical orthogonal codes in optical CDMA systems,” IEEE Trans. Commun., vol. 53, no. 4, pp. 687–697, Apr. 2005. [11] M. M. Mustapha and R. F. Ormondroyd, “Dual-threshold sequential detection code synchronization for an optical CDMA network in the presence of multi-user interference,” J. Lightw. Technol., vol. 18, no. 12, pp. 1742–1748, Dec. 2000. [12] J. G. Zhang, A. B. Sharma, and W. C. Kwong, “Cross-correlation and system performance of modified prime codes for all-optical CDMA applications,” J. Opt. A: Pure Appl. Opt., vol. 2, no. 5, pp. L25–L29, 2000. [13] Y. H. Lee, Y.-G. Jan, H.-W. Tseng, and M.-H. Chuang, “Performance analysis and architecture design for a smartly generated prime code multiplexing system,” J. Opt. Commun., vol. 28, no. 3, pp. 216–220, 2007. [14] K. Murugesan, “Performance analysis of low-weight modified prime sequence codes for synchronous optical CDMA networks,” J. Opt. Commun., vol. 25, no. 2, pp. 68–74, 2004. [15] L. L. Jau and Y. H. Lee, “Optical code-division multiplexing systems using common-zero codes,” J. Microw. Opt. Technol. Lett., vol. 39, no. 2, pp. 165–167, 2003. [16] J. G. Zhang, W. C. Kwong, and A. B. Sharma, “Effective design of optical fiber code-division multiple access networks using the modifed prime codes and optical processing,” in IEEE WCC-ICCT, 2000. [17] J. G. Zhang, “Address codes for use in all-optical CDMA systems,” Electron. Lett., vol. 32, no. 13, pp. 1154–1156, 1996. [18] J. G. Zhang and W. C. Kwong, “Design of optical code-division multiple-access networks with modified prime codes,” Electron. Lett., vol. 33, no. 3, pp. 229–230, 1997. [19] M. Azizoghlu, J. A. Salehi, and Y. Li, “Optical CDMA via temporal codes,” IEEE Trans. Commun., vol. 40, no. 8, pp. 1162–1170, Aug. 1992. M. Massoud Karbassian (S’06–M’09) received the B.Sc. degree in electrical engineering from K. N. Toosi University of Technology (KNTU), Tehran, Iran in 2001, the M.Sc. degree in electronic and computer engineering and the Ph.D. degree in optical communications and networks from the University of Birmingham (UoB), U.K., in 2005 and 2009, respectively. He is currently with the College of Optical Sciences, University of Arizona, Tucson, as a postdoctoral Research Associate. From 2001 to 2004, he was with Lenze GmbH agent in Iran, as an applications engineer devising digital electronic and control systems. He was also working as teaching assistant at UoB from 2006 to 2009. His current research interests include optical aggregation nodes, optical network architectures, optical CDMA, multiplexing, and signal coding techniques. Dr. Karbassian also serves as a technical reviewer for many prestigious journals in microwave and optical technologies. He was awarded the ‘Best Masters Project Prize’ of the UoB in 2005 and is also an active member of Institution of Engineering and Technology (IET) in the UK, Optical Society of America (OSA) and the International Association of Engineers (IAENG).

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Franko Kueppers received Diploma degrees in electrical engineering and in communications engineering from the University of Applied Sciences GiessenFriedberg and from Kassel University, respectively. The Ph.D. (Dr.-Ing.) degree in optical communications engineering he received from Kaiserslautern University of Technology. He worked at Siemens and was with Deutsche Telekom where he directed the Optical Networks research group and the Photonic Systems department at the Research and Technology Center in Darmstadt, Germany. He is an Associate Professor of Optical Sciences at the College of Optical Sciences at the University of Arizona, Tucson, since 2003. He built and runs the Photonic Telecommu-

nication Systems research group and also serves as the testbed lead for the NSF Engineering Research Center for Integrated Access Networks (ERC CIAN). He published more than 50 scientific articles and is a Guest Editor of the Journal of Lightwave Technology Special Issue on 40 Gb/s Lightwave Systems. Dr. Kueppers has received the National Science Foundation CAREER Award, the Science Foundation Arizona Competitive Advantage Award, and the University of Arizona/College of Optical Sciences Award of Distinction for Outstanding Undergraduate Teaching.