Towards higher scalability of hybrid optical CDMA network
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A novel approach for improving the number of simultaneous users in a hybrid OCDMA-OTDMA network is proposed and analysed. OCDMA system is based on two-dimensional wavelength-hopping time-spreading codes with multi-wavelengths picosecond carriers. The scalability increase was achieved by adding a third dimension to separate OCDMA user groups within OTDMA time slots by assigning them into different wavelength bands. We have shown this will scale-up the system capacity proportionally to the number of assigned wavelength bands. A self-clocking all-optical time gate was then demonstrated as an effective means for suppressing the growing detrimental multi access interference noise resulted from this capacity increase.
KeywordsOCDMA OTDMA All-optical clock recovery Scalability
Recent advances in information technology have increased the demand for large communications bandwidth (Berthold 1998). Optical code division multiple access (OCDMA) has been widely investigated as a potential alternative to orthogonal frequency division multiple access (OFDMA) (Ergen 2009), wavelength division multiplexing (WDM) (Glesk et al. 2008) and optical time division multiplexing (OTDM) (Hao et al. 2013). Several implementations of incoherent (Glesk et al. 2008) or coherent OCDMA (Amaya et al. 2011) have been reported and investigated in the literature with the ultimate aim of achieving a highly scalable system that is capable of handling large number of simultaneous users without compromising system performance. In particular, the use of multiplexing (WDM-OCDMA Kitayama et al. 2006, TDM-OCDMA Yoshima 2010) schemes to form hybrid designs are recently being proposed in order to exploit the advantages of each of the individual techniques.
Osadola et al. (2012) proposed and demonstrated a novel architecture for increasing the scalability of an incoherent OCDMA system by employing the technique of OCDMA code reuse that enables different groups of OCDMA users to transmit in separate OTDMA channels. We showed that the scalability of such hybrid system is increased by a factor of M × N where N is the number of OTDM channels and M is the number of simultaneous OCDMA users per the OTDM channel.
In this paper, we propose and analyze a novel approach that will further increase the scalability of the OCDMA-OTDMA system we originally reported in Osadola et al. 2012. In this new approach, a third dimension is being added to the original OCDMA-OTDMA architecture by allowing multiple OCDMA user groups assigned to different wavelength bands to transmit simultaneously within each OTDM channel. (Note that in general, it is possible to implement coherent or incoherent OCDMA schemes in any channel of the OTDM system). Our analysis and discussions in this paper assumes the use of multiwavelength two dimensional wavelength-hopping time-spreading (2D-WH/TS) OCDMA codes in all OCDMA sub-systems operating in OTDM channels. Our OCDMA implementation and the overall system performance analysis also reflect using 2D-WH/TS codes based on multiwavelength picosecond pulses (Yang and Kwong 2002). In support of building such system we also demonstrate an all-optical self-clocking method for multi-access interference suppression by using an all-optical time gating technique with a semiconductor optical amplifier -based Mach–Zehnder interferometer (MZI-SOA).
2 Design of a highly scalable OCDMA over OTDMA hybrid system
Wavelength pulses (OCDMA code carriers) used by any given 2D-WH/TS OCDMA in ΔΛi in Fig. 3 where i = (1, 2, … P) occupy a wavelength interval ΔΛi. In general, ΔΛi = Λ/w where ΔΛi is a wavelength spectrum assigned to the 2D-WH/TS OCDMA sub-system and w is the number of wavelengths (code carriers) used (the code weight). As a result of this architecture, a P number of M-Users OCDMA ΔΛi can coexist together in the given spectral space Λ without interfering with each other. Please note that different code-sets will not share any wavelength. The resulting traffic from P × M-Users OCDMA sub-systems is now time division multiplexed into an OTDM frame. The OTDM channel duration is set not to exceed the longest code-set of any of the M-Users OCDMA (see Fig. 3). The time multiplexing is done using N × 1 power coupler with an appropriate delay N × tCh, where N is the OTDMA channel number. The combined resulting OTDM traffic is then launched into a fiber optic transmission link. In case that each group of M-Users OCDMA uses equal bandwidth the number of simultaneous users k in this new architecture will be P × M-Users per OCDMA × number of OTDMA channels N.
Depending on the application, the code-set used by individual OCDMA system groups across different OTDMA time slots can either be the same or can vary to meet the specific application needs. If kept the same, then a hardware reuse will be possible leading to cost savings. As previously stated, the analysis and discussions in this paper will assume the reuse of multi-wavelengths 2D-WH/TS OCDMA code sets in OTDMA time slots.
In addition, in Sect. 4, we describe an experimental demonstration of ultra-fast optical time gating to be used in OCDMA receivers for suppressing the multiple access interference that might occur as a result of increased traffic in this highly scalable hybrid system.
3 Scalability calculations
4 Experimental demonstration of multi access interference suppression by using self-clocked all-optical picosecond gate in the OCDMA decoder
As a result of the increased number of simultaneous users in the described system, multiple access interference (MAI) will be increased thereby resulting in the bit error rate (BER) deterioration. Also, aside the detrimental effects of MAI, at high data rates, the received signal may also suffer from timing jitter. The effect of timing jitter will cause significant problems to achieve receiver synchronization. To achieve a high performance OTDMA, a precise timing control and synchronization will be required. The synchronization can be achieved using a clock signal that is either generated from the incoming signal or by sending a separate clock signal from the transmitter. In order to achieve conditions required for using Eq. 1, sub picosecond sampling of the auto-correlation peak is necessary. Therefore extracting the clock signal from the incoming signal will supress the timing jitter and provide clock pulses that are ‘fast’ enough to create the needed sampling window. An all-optical clock recovery from OCDMA signal has been previously demonstrated in Idris et al. (2013a). It was shown that it is possible to extract an optical clock from the auto-correlation output of an OCDMA decoder.
We have proposed a novel architecture to improve the scalability of the previously demonstrated hybrid OCDMA–OTDMA system (Osadola et al. 2012). To increase a number of simultaneous users, a third dimension was added to the original OCDMA-OTDMA scheme by allowing multiple OCDMA sub-systems to reside in different wavelength bands before being transmitted simultaneously into OTDM time slots (channel). To do these, 2D-WH/TS codes based on multi-wavelength picosecond pulses are design to occupy P wavelength bands. Each band supports one of the P M-Users OCDMA systems who then simultaneously operate in a given OTDMA channel. Approach can also support hardware reuse and can therefore offer some cost savings. Our OCDMA-OTDMA system scalability analysis based on P = 8, N = 4, M = 17 shows that up to 544 users can broadcast simultaneously. (Compare to only 68 simultaneous users in the original hybrid scheme, P = 1).
The conducted experiment has confirmed the effectiveness of the all-optical picosecond time gate self-clocking as a means of suppressing the multi access interference noise resulted from the accumulation of timing jitter during transmission that would be detrimental to system performance.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 734331.
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