INTRODUCTION
OPTICAL communication plays a significant role in achieving high bit rate and reliable digital communications in networks’ backbone and high-speed local-area networks (LANs) using a fiber-distributed data interface (FDDI) in next-generation network (NGN) subscribers such as fiber-to-the-home/kerb/node (FTTx). Recently, incoherent optical code-division multiple-access (OCDMA) systems have been investigated widely to apply for high-speed LANs,since they allow multiple users’ accessing network resources simultaneously.Asynchronous OCDMA (A-OCDMA) transceivers do not require frame synchronization; hence an optical orthogonal code (OOC) family with a good cross-correlation property is used in such a system . However, it has a few numbers of available sequences as they are code-length and weight dependent.Thus, in A-OCDMA, the number of users is very restricted as the number of available code sequences is very small. An alternative solution to increase the number of users is the synchronous OCDMA (S-OCDMA) where all users are synchronized in time frame.
This means the frame synchronization can be performed by packet synchronization consisting of either a unique or a different data sequence padded to each data sequence stream. Therefore, at the receiver, the synchronization is acquired based on the correlation property of the frame synchronizing sequence. Also, in the S-OCDMA, the number of accommodated simultaneous users can be increased by employing different prime-code families as spreading codes.In a conventional OCDMA, each time-slot is divided into chips that are equals to spreading code-length consisting of 1/0 sequences (depending on spreading codes) addressed to each user. The data are modulated and assigned through optical pulses (OPs) at certain chips of each allocated slot either in on–off keying (OOK) or in pulse-position modulation (PPM) formats.The modulated signal is then transmitted after multiplied by the spreading code in the OCDMA encoder via optical tappeddelay lines (OTDLs), i.e., the output OP in the first chip of a slot is spread in time domain to several chips corresponding to 1s of the spreading codes.
The OP sequences transmitted from users are combined (multiplexed) in the star passive optical network (SPON) couplers as an infrastructure reference and then transmitted over fiber-to-the destination (FTTx). At the receiver, in order to obtain the intended signal from the received signal,despreading is performed in a decorrelator, which consists of OTDL with inverse tap coefficients. The OPs are merged at the last chip in a slot, and the desired data are extracted in the demodulator based on modulation scheme.When the number of simultaneous active users increases, the effect of channel interference also inherently raises in directdetection OCDMA. The reason is that the OPs from the intended user and the interfering users overlap and the bit error rate (BER)tends to have an error floor. Therefore, it is required to reduce the probability of overlapping pulses from interfering users to mitigate the effect of cochannel interference. The probability of overlapping pulses has been reduced by changing the modulation scheme from OOK to -ary PPM. Although the variable pulse positions of pulse occur in PPM, error floor still exists .
On the other hand, interference cancellation techniques using an interference canceller at the receiver have been widely studied with different modulation schemes such as OOK and PPM. When OOK is used, the interference canceller is unable to completely eliminate the channel interference, since the reference signal has the components of the desired user. Also,the PPM using the same interference canceller technique has been proposed in; however, interference elimination is notattained due to the presence of the desired signal components in the reference signal. Alternatively, the interference canceller in which a reference signal does not contain the component of the desired signal has been proposed for the PPM system to improve the BER performance.This was carried out to avoid the cancellation of the desired signal as well as interferences.
OPTICAL communication plays a significant role in achieving high bit rate and reliable digital communications in networks’ backbone and high-speed local-area networks (LANs) using a fiber-distributed data interface (FDDI) in next-generation network (NGN) subscribers such as fiber-to-the-home/kerb/node (FTTx). Recently, incoherent optical code-division multiple-access (OCDMA) systems have been investigated widely to apply for high-speed LANs,since they allow multiple users’ accessing network resources simultaneously.Asynchronous OCDMA (A-OCDMA) transceivers do not require frame synchronization; hence an optical orthogonal code (OOC) family with a good cross-correlation property is used in such a system . However, it has a few numbers of available sequences as they are code-length and weight dependent.Thus, in A-OCDMA, the number of users is very restricted as the number of available code sequences is very small. An alternative solution to increase the number of users is the synchronous OCDMA (S-OCDMA) where all users are synchronized in time frame.
This means the frame synchronization can be performed by packet synchronization consisting of either a unique or a different data sequence padded to each data sequence stream. Therefore, at the receiver, the synchronization is acquired based on the correlation property of the frame synchronizing sequence. Also, in the S-OCDMA, the number of accommodated simultaneous users can be increased by employing different prime-code families as spreading codes.In a conventional OCDMA, each time-slot is divided into chips that are equals to spreading code-length consisting of 1/0 sequences (depending on spreading codes) addressed to each user. The data are modulated and assigned through optical pulses (OPs) at certain chips of each allocated slot either in on–off keying (OOK) or in pulse-position modulation (PPM) formats.The modulated signal is then transmitted after multiplied by the spreading code in the OCDMA encoder via optical tappeddelay lines (OTDLs), i.e., the output OP in the first chip of a slot is spread in time domain to several chips corresponding to 1s of the spreading codes.
The OP sequences transmitted from users are combined (multiplexed) in the star passive optical network (SPON) couplers as an infrastructure reference and then transmitted over fiber-to-the destination (FTTx). At the receiver, in order to obtain the intended signal from the received signal,despreading is performed in a decorrelator, which consists of OTDL with inverse tap coefficients. The OPs are merged at the last chip in a slot, and the desired data are extracted in the demodulator based on modulation scheme.When the number of simultaneous active users increases, the effect of channel interference also inherently raises in directdetection OCDMA. The reason is that the OPs from the intended user and the interfering users overlap and the bit error rate (BER)tends to have an error floor. Therefore, it is required to reduce the probability of overlapping pulses from interfering users to mitigate the effect of cochannel interference. The probability of overlapping pulses has been reduced by changing the modulation scheme from OOK to -ary PPM. Although the variable pulse positions of pulse occur in PPM, error floor still exists .
On the other hand, interference cancellation techniques using an interference canceller at the receiver have been widely studied with different modulation schemes such as OOK and PPM. When OOK is used, the interference canceller is unable to completely eliminate the channel interference, since the reference signal has the components of the desired user. Also,the PPM using the same interference canceller technique has been proposed in; however, interference elimination is notattained due to the presence of the desired signal components in the reference signal. Alternatively, the interference canceller in which a reference signal does not contain the component of the desired signal has been proposed for the PPM system to improve the BER performance.This was carried out to avoid the cancellation of the desired signal as well as interferences.
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