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Optoelectronics & Communications

Passive optical network based on optical-code-division multiple access

A novel network multiplexing approach enables optical-access technology with high security.
6 July 2010, SPIE Newsroom. DOI: 10.1117/2.1201006.002981

Since the Internet and broadband networks were introduced, emerging applications—such as video on demand, teleconferencing, and high-quality audio transmission—have demanded high-throughput optical-access networks with stringent quality-of-service (QoS) capabilities. However, the infrastructure of current access networks suffers from limited bandwidth, high network-management costs, poor flexibility, and low security, which prevent networks from delivering integrated services to their users. Owing to the maturity of optical components and electronic circuits, optical-fiber links have become practical for use in access networks. Passive optical networks (PONs) and different multiplexing technologies have been proposed in this context, including wavelength-division multiplexing (WDM),1 time-division multiplexing (TDM),2 and optical-code-division multiplexing (OCDM).3

PONs have been standardized for fiber-to-the-home solutions and are deployed by network-service providers worldwide. Even though PONs based on TDM (TDM-PON) effectively use fiber bandwidths, they have limitations regarding transmission speed, burst synchronization, security, dynamic-bandwidth-allocation, and ranging accuracy.4,5 WDM technology has also been proposed for PONs. When used in conjunction with PONs (WDM-PON), this emerging technology becomes more favorable as the required bandwidth increases, but it has failed to attract attention from industry because of the high cost of the associated optical components.6 Other schemes for optical access networks are currently under study worldwide.7

Optical-code-division multiplexing access (OCDMA) systems have attracted attention in recent years because of a number of advantages, including their asynchronous access capability, flexibility of user allocation, support of variable bit rates, burst traffic, and security against unauthorized users. OCDMA is an attractive multi-access technique for access systems like local-area networks3 and the first mile, but no detailed network-design schemes have been developed to date. A PON based on code-division multiplexing access (CDMA) has been proposed using pseudo-random and Walsh codes for user identification.8 However, signature processing for multiple access is done in the electrical domain using an application-specific integrated circuit (ASIC), and not in the optical field as we are pursuing.9–11

We have developed the OCDMA-PON, a network structure of PON in conjunction with OCDMA, i.e., a different multiple-access technology from TDM and WDM. OCDMA technology achieves signature processing in the optical rather than the electrical domain8 using an optical encoder/decoder. Figure 1 shows the system's optical-line-terminator (OLT) configuration. For the forward channels, the source is encoded at the OLT and the downstream signal is transmitted at a wavelength of 1550nm. Every user is assigned a unique optical orthogonal code (OOC) and identified by a correlation operation at the optical decoder based on fiber Bragg gratings (FBGs). To reduce multi-user interference (MUI), the nonflattened source spectrum can be compensated by a flatness compensator before entering the FBG encoder. Another benefit of a flattened source is that it relaxes the accuracy requirements regarding the achievable precision with which the phase can be controlled and maintained stably when the number of users increases.12 For the backward channel from the optical network unit (ONU)/optical network termination (ONT) to the OLT, the upstream is transmitted at a wavelength of 1310nm. After upstream traffic passes the optical transceiver, it is sent to multiple decoders, each of which recovers the information for each user.

Figure 1. Optical-line terminator (OLT) of the optical-code-division multiplexing access/passive optical network (OCDMA-PON) system. Rx, Tx: Receiver, transmitter. OOC: Optical orthogonal code. ODN: Optical distribution network. ONU: Optical network unit. ONT: Optical network termination. n: Counter.

Figure 2 shows the ONU configuration of our OCDMA-PON. A stable upstream wavelength is usually required with a stabilized laser source at the ONU's transmitter. The downstream signal from the OLT to the ONU/ONT passes through a circulator to the detector, where the user's information is separated through optical correlation with his/her unique OOC using a balanced receiver.13 The downstream control signal is also obtained and passed to the network-control unit. For the upstream, the signal from the ONU to the OLT is encoded by the OOC for user identification by the optical encoder. It is subsequently transmitted towards the OLT through the fiber link. Our scheme has several advantages. For example, any user may add or drop into the network at random and the network is running asynchronously.

Figure 2. Optical network unit (ONU) of our OCDMA-PON system. i: Counter.

Our OCDMA-PON system is composed of an OLT and an ONU (see Figures 1 and 2, respectively). Every ONU is identified by its own code address. The signal is modulated with both frame information and an address-code sequence. The former is used to accomplish data-load switching, while the latter helps identify different users.

Figure 3 compares the bit-error rate (BER) with the ONU-transmitter input power for two cases, one affected by interference (for example, MUI or from different noise contributions) and a second based on a back-to-back configuration. In this example, we assume that the number of ONUs/ONTs (i.e., the number of active users) is 30 and the bit rate of the downstream traffic is 1.25Gb/s. We find that the system that includes interference exhibits much worse performance than that using the back-to-back configuration, with approximately 6dB penalty at a BER of 10−9. For the OCDMA-PON, we confirm that MUI is the dominant degradation source (see Figure 3) and must be included in the network design.

Figure 3. Performance of the OCDMA-PON for back-to-back configuration and with interference. N-active: Number of active users. R-downstream: Downstream-traffic bit rate.

Figure 4 compares the BER performance as the number of active users increases for the CDMA-PON8 and OCDMA-PON systems. We assumed that the bit rate of downstream traffic is 1.25Gb/s for a fiber-link length of 10km. The OCMDA-PON scheme exhibits a similar performance to the CDMA-PON when the number of users is large. For example, for 30 users and a BER of 10−9, OCDMA-PONs can only increase the number of users compared to that supported by CDMA-PONs by 10%. However, the advantage of OCDMA-PONs over CDMA-PONs becomes obvious when the number of users supported by the OCDMA-PON is small (see Figure 4). Note that the signature is processed in the electrical domain by ASIC with Walsh code for user identification in CDMA-PONs, while the optical domain is used by the FBG encoder/decoder based on source-spectrum flattening and a balanced detector with the prime codes for user identification in OCDMA-PONs. Taking into account the higher bit rate and higher bandwidth offered by OCDMA-PONs based on optical processing of user signatures, we still prefer an OCDMA-PON scheme with intensity modulation. To further improve the performance of the OCDMA-PON, we need to solve the MUI-imposed degradation problems and improve the optical encoder/decoder.

Figure 4. CDMA-PON versus OCDMA-PON comparison as the number of ONUs/ONTs increases. L: Length.

Our OCDMA-PON system is different from classical PONs in that OCDMA is used for multiple-user access instead of TDM or WDM, and the OCDMA-PON scheme combines the advantages of both the PON and OCDMA technologies, including flexible network assembly, fair bandwidth division, differentiated services or QoS in the physical layer, asynchronous access capability, support of variable bit rates and burst traffic, and security against unauthorized users. Based on comprehensive comparisons between the OCDMA-PON and the previously studied CDMA-PON, we find that the former scheme exhibits a better BER performance than the latter for small numbers of users. In summary, OCDMA-PON offers several advantages over CDMA-PON, including higher bit rate, higher bandwidth, and better security against unauthorized users.14 We continue to work towards realizing an experimental demonstration of the OCDMA-PON system.

Chongfu Zhang
University of Electronic Science and Technology of China (UESTC)
Chengdu, China

Chongfu Zhang is an associate professor. He received his MS (2004) and PhD (2009) degrees from UESTC, where has been a faculty member since March 2004. His current research focuses on optical-fiber communications, all-optical networks, and optoelectronic devices.