IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
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Optical CDMA Network Codecs Structured With -Sequence Codes Over Waveguide-Grating Routers
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Chao-Chin Yang, Jen-Fa Huang, and Shin-Pin Tseng
Abstract—A compact optical code-division multiple-access (OCDMA) network coder–decoder (codec) of a spectral-amplitude-coding (SAC) scheme is presented. The scheme utilizes -sequence-coded OCDMA codecs structured on arrayed-waveguide-grating (AWG) routers. By using the cyclic properties -sequence codes, the codecs pair can of AWG routers and encode–decode multiple code words simultaneously, thus, each user can share the same hardware for coding process. The configuration not only preserves the ability of multiple-access interference cancellation in SAC-OCDMA system, but also results in a cheap system with reduced system complexity. Index Terms—Arrayed-waveguide-grating (AWG) router, -sequence codes, optical code-division multiple-access (OCDMA), spectral-amplitude coding (SAC).
I. INTRODUCTION
ping, nonzero MAI value degrades system performance when the number of active users is large. In the present study, we propose an AWG router-based optical network codec pair for SAC-OCDMA systems. The information signal employs ON–OFF keying with low cost incoherent sources, and the AWG router is used to control the amplitude spectra of incoherent optical sources. Because of the cyclic properties of both AWG routers and -sequence codes, the proposed codec pair can encode–decode multiple code words of -sequence code while retaining the ability for MAI cancellation. The total system becomes more compact and simple because all users in the network can use a single codec pair. Performance of the proposed system will be evaluated in terms of crosstalk levels in the AWG routers and the influence of crosstalk on beat noise in the detectors.
I
N LOCAL area networks, fiber-optic code-division multiple-access (CDMA) techniques provide a flexible solution for asynchronous high-capacity communication. Early incoherent optical CDMA (OCDMA) systems used pseudoorthogonal sequences to encode signals in the time domain, but the codes were long and multiple-access interference (MAI) limited the number of simultaneous users. Thus, spectral-amplitude-coding of OCDMA (SAC-OCDMA) was proposed to eliminate the influence of MAI and preserve the quasiorthogonality between network users. Several code families can be used in SAC-OCDMA systems, e.g., maximal-length sequence ( -sequence) codes [1], Walsh–Hadamard codes [1], modified quadratic congruence codes [2], etc. Earlier work [2], [3] proposed that such codes use fiber Bragg gratings (FBGs) as encoding–decoding devices in OCDMA networks, but FBG array physical size became impractical when the number of total network users became large. One way to solve the above problem is to use the cyclic property of arrayed-waveguide grating (AWG) routers [4] for coder size reduction. Kim [5] proposed a cyclic optical encoder–decoder (codec) based on AWG routers shared by all users in the OCDMA network. But lengthy fiber delay lines are needed to accommodate large numbers of users. Since the codes used in this system are based on time spreading and wavelength hop-
Manuscript received May 6, 2003; revised October 2, 2003. This work was supported by the Ministry of Education Program for Promoting Academic Excellence of Universities under Grant A-91-E-FA08-1-4. The authors are with the Department of Electrical Engineering, Institute of Computer and Communications, National Cheng Kung University, Taiwan 70010, R.O.C. (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2003.823089
II. SYSTEM CONFIGURATION The proposed OCDMA codec pair is designed with sequence codes using AWG routers. For a total number of network users, only one AWG router is needed for the encoder and only two AWG routers are needed for the decoder. The encoder and decoder are connected by optical star coupler. The information bits of each fiber and a user are encoded optically with different signature sequences by a single encoder. The resulting signal are combined in the star coupler and broadcast to the input ports of a single decoder shared by all users. Each user requires one balanced detector connected to the decoder to extract his information bits. An AWG router consists of input–output waveguides (ports), two focal slab waveguides, and a waveguide array with an optical path difference [4]. The wavelength components of broad-band incoherent light that enter one input port are demultiplexed into all the output ports. However, same-wavelength signals input to different input ports go to different output ports in a cyclic manner. This property can be used to generate all code words in the system with a single AWG router. User code words are designated , where indicates the user number. Let be one unipolar -sequence of length assigned as the code word of ) can user #0; the code word of the th user ( be obtained by cyclically shifting the original sequence (e.g., , where is the operator shifts vectors cyclically to the right by one place). Assume that the link between the th output ports and the star coupler is arranged according to (e.g., the link is connected when is , and disconnected, )th chip of appears in otherwise). It is found that the (
1041-1135/04$20.00 © 2004 IEEE
642
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
Fig. 1.
M
AWG encoder designed with
M -sequence codes of length N = 7.
TABLE I -SEQUENCE CODES OF LENGTH 7 FOR OPTICAL SPECTRAL CODING
Fig. 2. AWG decoder designed with
M -sequence codes of length N = 7.
receiver. The received signal spectrum all users’ transmitted signal spectrum
is the summation of
(1)
*Subscript represents AWG router input port number. the th output port ( denotes modulo- addition). Thus, all chips of each code word are present in the output ports of the AWG encoder. Fig. 1 shows the proposed AWG router-based encoder shared by seven users. In our system, electrooptical modulation is performed when the data bit of each user is used to ON–OFF shift key a broad-band incoherent optical source whose spectrum is filtered for the free spectral range of the AWG router. The resulting optical signals of each user are directed to the corresponding input ports of the AWG router. Because of the cyclic properties of AWG routers and -sequence codes, code words of the active users are generated in the output ports of AWG router and transmitted to the star coupler. The coding process in the AWG encoder shown in Fig. 1 is illustrated in Table I for -sequence code of length . In Table I, the data bits of users #0, #1, #4, and #6 are logical “1,” so only the corresponding code words of these users are produced by the encoder. The signal of user #0 is directed to the with input port #0 of the encoder, so central wavelengths of , , , and appear in output ports #1, #2, #3, and #6, respectively. Due to the cyclic properties of AWG routers and -sequence codes, with central wavelengths of , , , and also appear in output ports #1, #2, #3, and #6. Thus, seven users share the same AWG router as a common encoder. After the encoding, the coded spectral signals are combined in the star coupler and broadcast to the links connected to the
where is the th user’s data bit and belongs to {0, 1}. In the example of Table I, is equal to (2, 3, 3, 1, 2, 2, 3). The AWG router-based decoder to complement the seven-user encoder is shown in Fig. 2. The star coupler is connected to the decoder’s AWG router pair, which distributes received signals to the balanced photodetectors of each user to realize differential decoding [1]. Similar to the connection from the encoder router to the star coupler, connections from the star coupler to the upper and lower AWGs are determined code word and its complement, respectively. The by the will receive from the balanced photodetector of user from the lower AWG. After correlation upper AWG and subtraction is performed in the th balanced photodetector, the th user’s data bit is recovered and other users’ interferences are rejected. Table II shows the wavelength distribution in the upper and lower AWG routers. In the output port #0 of the upper AWG router, the wavelength chips #0, #1, #2, and #5 of received signal are obtained from input ports #0, #1, #2, and #5 of the upper AWG router, and the upper photodiode of user #0 unit energy, as shown in Table II(a). The obtains remaining chips of are obtained from the corresponding input ports of the lower AWG router and the lower photodiode of unit energy, as shown in Table II(b). user #0 obtains Differential detection results in unit energy and user #0 decides that logic “1” bit is transmitted. Similarly, the differential detector of user #2 obtains unit energy and decides that logic “0” bit is transmitted. Each user in the network can, therefore, share a single common decoder (containing two AWG routers) and reject interference from other users. III. SYSTEM PERFORMANCE In SAC-OCDMA systems with flat power spectral density (PSD) of light sources in the coded bandwidth, capacity is mainly limited by phase-induced intensity noise (PIIN) [2]
YANG et al.: OCDMA NETWORK CODECS STRUCTURED WITH
-SEQUENCE CODES OVER WAVEGUIDE-GRATING ROUTERS
643
TABLE II WAVELENGTH DISTRIBUTION IN AWG-ROUTER-BASED DECODER
(a) For upper AWG router.
Fig. 3.
(b) For lower AWG router.
that results from the beating of incoherent light fields in the square-law detectors. Assume each unpolarized light source has with magnitude flat PSD over the optical source bandwidth , where is the effective power from a single source at the decoder. The PSD of the received optical signal in each input port of the AWG router can be written as (2) where
BER versus number of active users for different code lengths.
To obtain (4), the probability of sending bit “1” and also bit “0” is assumed 0.5 for each user, and output signal power is assumed ideal since leaked power is assumed negligibly low. , After employing the expression BER we obtain the relation between bit-error rate (BER) and the number of active users for various code lengths shown in Fig. 3. The crosstalk level is set to 27 dB. We can find that as code length increases from to , the largest number decreases from of simultaneous active users for to . This is because as the number of crosstalk sources increases, the desired signal encounters more interference in the detection process. However, to maximize the number of simultaneous users in the system, the cross-correlation between signature sequences is more important than the crosstalk level of the optical components. IV. CONCLUSION
and is the unit step function. By taking the crosstalk effect into consideration [6] and ignoring the crosstalk-to-crosstalk beat noise, the PSD at the upper and the lower photodiodes for user #0 during one bit period can be written as (3a)
(3b) where is the optical power ratio of each crosstalk component to the expected component in each output port. By using methodology similar to that in [2] and approximating the sumwith the average value , the mation signal-to-noise ratio (SNR) is SNR
(4)
where is the number of active users and is the noise-equivand are the alent bandwidth of the photodetectors, and photocurrents at the upper and lower photodiodes, respectively.
We have presented an AWG-router-based codec pair for compact OCDMA networks. Because of the cyclic properties of AWG routers and -sequence codes, all users can share the same codec pair and require a total of three AWG routers. Low-cost light sources can be used for actual implementation, rendering the networks cheap and compact. When flattened sources are used, transmitted data bit can be recovered without the influence of MAI. REFERENCES [1] M. Kavehrad and D. Zaccarin, “Optical code-division-multiplexed systems based on spectral encoding of noncoherent sources,” J. Lightwave Technol., vol. 13, pp. 534–545, Sept. 1995. [2] Z. Wei, H. M. H. Shalaby, and H. Ghafouri-Shiraz, “Modified quadratic congruence codes for fiber Bragg-grating-based spectral-amplitude-coding optical CDMA systems,” J. Lightwave Technol., vol. 19, pp. 1274–1281, Sept. 2001. [3] J. F. Huang and D. Z. Hsu, “Fiber-grating-based optical CDMA spectral coding with nearly orthogonal -sequence codes,” IEEE Photon. Technol. Lett., vol. 12, pp. 1252–1254, Sept. 2000. [4] H. Takahashi, K. Oda, H. Toda, and Y. Inoue, “Transmission characteristics of arrayed waveguide wavelength multiplexer,” J. Lightwave Technol., vol. 13, pp. 447–455, Mar. 1995. [5] S. Kim, “Cyclic optical encoders/decoders for compact optical CDMA network,” IEEE Photon. Technol. Lett., vol. 12, pp. 428–430, Apr. 2000. [6] H. Takahashi, K. Oda, and H. Toba, “Impact of crosstalk in an arrayedwaveguide multiplexer on optical interconnection,” J. Lightwave Technol., vol. 9, pp. 1285–1287, Sept. 1997.
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