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Teletraffic Capacity Performance of WDM/ Direct Sequence-OCDMA PON

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Teletraffic Capacity Performance of WDM/ Direct Sequence-OCDMA PON Teletraffic Capacity Performance of WDM/ Direct Sequence-OCDMA PON Mohammad GHARAEI, Catherine LEPERS, Member, IEEE, Olfa AFFES, and Philippe GALLION, Senior Member, IEEE multiple optical network units (ONUs) on the subscriber side Abstract— Passive optical network (PON) has been widely via a non active star coupler (SC). Time division multiplexing accepted to provide broadband services to end users of the access PON (TDM PON) is the commonly accepted PON for network. In this paper, we study teletraffic capacity performance network operators, sharing network bandwidth temporally of a WDM/DS-OCDMA PON. This hybrid configuration is between users. Though TDM PON benefits low installation proposed to increase the user multiplexing capacity and to be a cost effective method per wavelength of a WDM PON. The and maintenance cost, it does not exploit the huge bandwidth architecture of the WDM/DS-OCDMA PON is presented and its of optical fiber. In order to increase optical bandwidth per capacity performance is analyzed using different direct sequence user, wavelength division multiplexing PON (WDM PON) optical code division multiple access (DS-OCDMA) codes. Since was proposed to create point to point links between OLT and multiple access interference (MAI) noise is dominant noise, the users [2]. Each user is assigned a dedicated wavelength to number of available WDM/DS-OCDMA channels is highly enjoy a large bandwidth for communication. WDM PON also dependent to the code correlation properties. For each code type, an appropriate simultaneous capacity threshold has been benefits guaranteed quality of service (QoS), protocol measured in a manner to ensure good transmission performance transparency and channel independency even though, provided (BER~10-9). As this capacity threshold is inferior to the nominal bandwidth by WDM PON is too large for user applications. multiplexing capacity of the considered code, a soft blocking of Moreover WDM PON cost is very high compared with TDM the system has been determined even if resources in the system PON. Therefore hybrid WDM/TDM PON was proposed to are still available. The teletraffic capacity of the WDM/DS- exploit optical bandwidth efficiently with sharing the cost OCDMA system has been then analyzed under the maximum of soft blocking probability constraint for different prime codes. between access network subscribers [3], [4]. In other words, Finally, using extended quadratic congruent code (EQC) has bandwidth sharing technique leads to share the optical been demonstrated to lead to better teletraffic capacity in a resources cost between users. Temporal multiplexing per WDM/DS-OCDMA PON. wavelength leads to increase the number of end users in access network. Yet WDM/TDM PON does not supply with high Index Terms—Code-division multi-access (CDMA), optical data rate per user. Also it requires a contention management fiber communication, queueing analysis. protocol to avoid the situation in which more users access the channel at the same time. To resolve this contention situation, optical code division multiple access (OCDMA) was proposed I. INTRODUCTION [5]-[7]. Using asynchronous OCDMA as a quasi HE conventional data, voice and the new video services contentionless multiple access technique prevents from Tsuch as video sharing, teleteaching, and also Internet contention management protocols [8]. Various OCDMA access with high data rate encourage network operators to techniques have been proposed such as temporal encoding, build up a next step toward a future access network which is known as direct sequence encoding (DS-CDMA) [9], generation. Passive optical network (PON) has been largely spectral amplitude coding (SAC-OCDMA) [10] and/or phase recognized as an attractive solution to the last mile bottleneck, encoding [11]. providing broadband access networks to end users [1]. PON is Taking into consideration the compatibility of another a point to multipoint optical network, connecting an optical dimension to be added by wavelength in WDM system, DS- line terminal (OLT) on the service provider side to the OCDMA is a convenient technique which uses temporal encoding. WDM/DS-OCDMA is a multiplexing technique Manuscript received February 3, 2009. This work was supported in part by which each user data bit is encoded with a given sequence of ARAL project at Institute Telecom. M. Gharaei and P. Gallion are with the Telecom ParisTech, Ecole pulses in temporal axes. Then, for a given wavelength there Nationale Supérieure des Télécommunications, CNRS LTCI, 46 rue Barrault, are different DS-OCDMA encoders/decoders who 75634 Paris Cédex 13, France (phone: +33-145-818-085; fax: +33-145-804- communicate concurrently with a huge optical bandwidth. 036; e-mail: [email protected]; philippe. [email protected]). This approach is deployed for all wavelengths to establish a C. Lepers is with Telecom & Management SudParis, 9 rue Charles Fourier, hybrid WDM/DS-OCDMA network. 91011 Evry Cedex, France (email: [email protected]). We study here, WDM/DS-OCDMA PON where each user O. Affes was with the Telecom Lille1, Cité Scientifique - Rue Guglielmo Marconi, BP 20145 - 59653 Villeneuve d'Ascq Cedex, France. (email: gains the whole bandwidth usage per wavelength which [email protected]). Fig. 1 Architecture of hybrid WDM/DS-OCDMA PON in downstream direction. acknowledges efficient utilization of optical resources. In on how a network operator should control data admission to other words, data rate per bandwidth utilization or spectral the network so as to ensure a more robust quality of service. It efficiency of WDM/DS-OCDMA PON is superior to demonstrates also foreseeable capacity measurement with WDM/TDM PON. Furthermore the number of users is different probability rates. Degradation of transmission quality extremely increased by bringing into play the higher capacity affects the teletraffic capacity of CDMA systems [15]. The codes. Thus WDM/DS-OCDMA is considered as a well- teletraffic performance of OCDMA system has been already matched technique in physical layer to increase the user determined and compared with WDM system by Goldberg et capacity of access networks. al [16]. The authors have demonstrated that OCDMA is well However, transmission performance of WDM/OCDMA suited to applications where conventional hard blocking is system is limited by optical source noise, channel undesirable. nonlinearities and channel noise including multiple access In this paper, we investigate that the teletraffic capacity interference (MAI) [12], [13]. These noises degrade optical performance of OCDMA system is dependent to the code bit-error-rate (BER) performance in the system. MAI noise is family type. We then analyze teletraffic capacity performance considered as the dominant cause of BER degradation in of WDM/DS-OCDMA PON using different prime codes. OCDMA systems. Initially, we measure the number of simultaneous active users With the request of high broadband services by huge employing different prime codes in DS-OCDMA under number of users, determination of system user capacity plays system conventional . This measurement leads to an important role in designing network architecture. bring out the notion of simultaneous capacity threshold. Since Generally, nominal resource capacity is defined as maximum different prime codes have different correlation properties; number of resources in the different systems; these are they support different simultaneous capacity threshold value. wavelengths in WDM system and assigned codes in OCDMA Afterward, these values for different prime codes are used in system. On the other hand, transmission performance of teletraffic theory to compare the maximum value of teletraffic WDM/DS-OCDMA is limited by MAI noise which makes its capacity of WDM/DS-OCDMA system. This results in nominal resource capacity unachievable. Thus we define designing proposed system with a defined grade of services simultaneous user capacity in OCDMA system as maximum and a high system capacity as cost effectively as possible. The simultaneous active users at acceptable BER performance. teletraffic capacity of proposed system architecture Generally lack of resources in a system causes blocking in demonstrates the performance measurement of WDM/DS- that system. It means that for a new user arrival, there would OCDMA system using different prime codes. At last, be no resource any more for transmission. In WDM/DS- comparing this result with WDM PON system leads to build OCDMA PON, there are two types of blocking probability. another step toward the future broadband access network. The first one is hard blocking probability which represents the The rest of this paper is organized as follows. In section II, determined blocking such as reaching to nominal resource the proposed network architecture is introduced and capacity in WDM system. The second is soft blocking simultaneous capacity performance of OCDMA system using probability which demonstrates the flexible blocking as it is different prime codes is defined. In section III, soft blocking observed in OCDMA systems when calculating simultaneous probability of DS-OCDMA is measured for different codes. In user capacity. section IV, by exploiting the physical limits of OCDMA To make a relationship between system capacity and grade- system in teletraffic capacity, we compare the teletraffic of-service, teletraffic becomes a tool by which investments capacity of proposed hybrid WDM/DS-OCDMA PON using can be planned [14]. Teletraffic capacity provides information different prime codes. Finally we conclude this paper in to the code family. Accordingly, we compare the properties of section V. different DS-OCDMA prime codes. First of all, the prime codes are defined as (p,ω,L) where p is the prime number, ω is II. SYSTEM CAPACITY PERFORMANCE OF WDM/DS-OCDMA PON the weight of the code and L denotes the length of the code sequence. As prime number p is increased in a code type, the A. Architecture of WDM/DS-OCDMA PON weight ω and subsequently nominal user multiplexing Hybrid WDM/OCDMA PON approach profits building capacity of code N is respectively enlarged. Prime codes overlay on top of existing WDM PON.
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