OCDMA TECHNOLOGY

Introduction

OCDMA is a category of multiplexing and inter networking technologies thatencodes/decodes signals through employing simple and cost-effective passiveoptical components (not requiring optical logic components) such that the signalmultiplexing, routing and switching can be implemented smoothly. It has manyadvantages, such as asynchronous random access, simple management, flexiblenetworking, good compatibility with WDM and TDM, suitable for busty traffic, supporting multiple services and differentiated QoS, while providing some confidentiality of data transmission. It is a very important technology to implement optical access networks, metropolitan area networks and optical signal multiplexing and switching in backbone networks.

OCDMA has a twenty-year history from its first proposal and its firstexperimental demonstration in principle. However, since the information capacity demands were so much smaller and the development scale of communication networks was so limited in the past, the current higher functionalities of networks were not required. At the same time, WDM technology provided the transmission tunnels and wavelength switching for ultrahigh speed data, which could then meet the earlier demands of optical network functionalities. Therefore, OCDMA has remained outside the mainstream of optical communication research for a long time. However, at present, with the advent of the knowledge economy and the global reach of the Internet, the disharmony between the service provisions of transport networks and access networks is becoming a serious issue. Meanwhile, there exist many issues in data backbone networks, such as the electronic node bottlenecks, clumsy and low-efficient granularity of traffic, and so on. To solve these current issues, it seems that the ability of WDM and TDM technologies is inadequate while, the high networking flexibility of OCDMA and the very good complementary properties of OCDMA with WDM and TDM are now recognized. Simultaneously, due to the rapid advancement of optical component technologies, all aspects of OCDMA technologies have become research hotspots, which should boost OCDMA development.

Technique Characteristics and System Classifications of OCDMA

OCDMA has become a promising technology to implement truly all-opticalcommunication and networking that uses optical signal processing directly,combining the advantages of electrical CDMA with the bandwidth predominance of fiber-optic and optical-signal processing devices. The passive optical-access networks, LAN and WAN, can be built up by using OCDMA technology. The combination of OCDMA with WDM or/and TDM can enhance signal multiplexing and label switching through the combination of OCDMA with WDM or/and IP over WAN so that the transmission and switching capacity of network can be improved, the flexibility of network can be enhanced and the performance of the communication network can be heightened.

Technique Characteristics of OCDMA

(1) OCDMA can implement high-speed transmission, switching and add/drop ofdata through using all-optical signal processing and thus it can realize all-opticalcommunication and all-optical networking and overcome the effect of electronicbottleneck, which exists in the electronic node in the traditional network.

(2) Subscribers can access the network at random and the network has softcapacity. Meanwhile, the pattern of networking is very flexible.

(3) OCDMA doesn’t need buffering in queue because it uses the tell-to-goprotocol.

(4) OCDMA networks can assign the bandwidth dynamically and implementthe bandwidth assignment with different granularities deftly, and use opticalnetwork bandwidth effectively.

(5) The traffic, protocol and network topology are transparent in the OCDMAnetwork. OCDMA can support variable bit-rate traffic and bursty traffic andimplement differential QoS according to demand. OCDMA networks can bereadily upgraded and extended.

(6) An OCDMA network is somewhat secure and cryptic for the transmissioninformation.

(7) An OCDMA network needs fewer devices than a WDM network and itsequipment is simple, and the implementing cost of OCDMA networks is low. ADWDM network needs accurate wavelength control and conversion. Furthermore, OCDMA is highly compatible with DWDM and TDM.

(8) OCDMA networks employ distributed management, which is simple and itis convenient to locate network failure and protect and recover. Because of the advantages mentioned above, OCDMA can support multimedia including voice, data, video, including IP traffic, video-on-demand, streaming media, interactive applications, etc. And it also offers many kinds of QoS and differentialdegrees of security according to different services and user’s requirements.Meanwhile, it can overcome the shortcomings of asymmetric uplink and downlink in current access networks and supports FTTH of the peer-to-peer traffic.

As the transmission media of communication, optical fibers have manyadvantages.

(1) Enormous communication capacity.

(2) Low transmission loss.

(3) Small size and weight.

(4) Immunity to electromagnetic interference and high signal security intransmission.

(5) Rich resource and potential low cost.

Classifications of OCDMA Systems

Many types of OCDMA systems have been proposed as the result of intensive research on OCDMA in the past 20 years. If we classify them in terms of the nature of the superposition of the optical signal, they can be divided into coherent OCDMA systems and incoherent OCDMA systems. The coherent OCDMA system makes use of the coherent property of light and implements bipolar encoding of the optical signal, i.e., encoding the phase of optical signals, with the phase of light detected at the receiving terminals. The form of signal addition is the superposition of light signal amplitudes. This kind of OCDMA system needs to use ultra short broadband light pulse sources. The incoherent OCDMA system employs the presence of light signal or absence of light signal to represent the binary “1” and “0” respectively, which is unipolar encoding, where the light signals are detected with the square-law devices at the receiving terminals. This form of signal addition is the superposition of light powers. This kind of OCDMA system may use incoherent light sources, such as amplified spontaneous emission (ASE), light-emitting diode (LED), etc.

If we categorize them depending on the differences of coding approaches foroptical signals, there are six kinds of OCDMA systems:

(1) direct-sequence or temporal encoding OCDMA systems, also known as spread-spectrum encoding OCDMA systems;

(2) spectral amplitude encoding OCDMA systems;

(3) spectral phase encoding OCDMA systems;

(4) temporal phase encoding OCDMA systems;

(5) two-dimensional spatial encoding OCDMA systems, also known as spread space encoding;

(6) hybrid encoding OCDMA systems. This kind of system uses a combinationof the encoding approaches mentioned above. We can acquire two-dimensionalencoding, for instance, wavelength-hopping/time-spread (WH/TS) encoding, through using the combination of spectrum encoding with temporal encoding. If space encoding is combined with WH/TS encoding again, space-spread/ wavelength hopping/ time-spreading encoding (SS/WH/TS)[40] can be obtained and the other options may be deduced by analogy.

Options (1), (2) and (5) refer to incoherent OCDMA systems, (3) and (4) arecoherent OCDMA systems, and (6) may be either. If we sort them according to the amount of resources of time, wavelength and space used, they can be divided into one-dimensional systems, two-dimensional systems and three-

dimensional systems. The aforementioned types of (1), (2), (3) and (4) belong to one-dimensional systems, (5) is two-dimensional system, and the systems with greater than two dimensions can be implemented by (6). If the polarization is also taken into account, the four-dimensional systems can be obtained.

The working principle of the coherent time spreading (TS)OCDMA

At the receiver, the OCDMA decoder recognizes the OCs by performing matched filtering, where the auto-correlation for target OC produces high level output, while the cross-correlation for undesired OC produces low level output. Finally, the original data can be recovered after electrical thresholding. Recently, coherent OCDMA technique with ultra-short optical pulses is receiving much attention for the overall superior performance over incoherent OCDMA and the development of compact and reliable en/decoders (E/D) [7~15]. In coherent OCDMA, encoding and decoding are performed either in time domain or in spectral domain based on the phase and amplitude of optical field instead of its intensity.

The working principle of the coherent time spreading (TS) OCDMA, where the encoding/decoding are performed in time domain. In such a system, the encoding is to spread a short optical pulse in time with a phase shift pattern representing a specific OC. The decoding is to perform the convolution to the incoming OC using a decoder, which has an inverse phase shift pattern as the encoder and generates high level auto-correlation and low level cross correlations.

Block diagram of OCDMA network.

Arrayed waveguide gratings (AWG) are commonly used as optical (de)multiplexers in wavelength division multiplexed (WDM) systems. These devices are capable of multiplexing a large number of wavelengths into a single optical fiber, thereby increasing the transmission capacity of optical networks considerably.

The devices are based on a fundamental principle of optics that light waves of different wavelengths interfere linearly with each other. This means that, if each channel in an optical communication network makes use of light of a slightly different wavelength, then the light from a large number of these channels can be carried by a single optical fiber with negligible crosstalk between the channels. The AWGs are used to multiplex channels of several wavelengths onto a single optical fiber at the transmission end and are also used as demultiplexers to retrieve individual channels of different wavelengths at the receiving end of an optical communication network.

Operation of AWG devices

The incoming light (1) traverses a free space (2) and enters a bundle of optical fibers or channel waveguides (3). The fibers have different length and thus apply a different phase shift at the exit of the fibers (3) the light traverses another free space (4) and interferes at the entries of the output waveguides (5) in such a way that each output channel receives only light of a certain wavelength. The orange lines only illustrate the light path. The light path from (1) to (5) is a demultiplexer, from (5) to (1) a multiplexer.

Conventional silica-based AWGs schematically shown in the above figure, are planar lightwave circuits fabricated by depositing doped and undoped layers of silica on a silicon substrate. The AWGs consist of a number of input (1) / output (5) couplers, a free space propagation region (2) and (4) and the

grating waveguides (3). The grating consists of a large number of waveguides with a constant length increment (ΔL). Light is coupled into the device via an optical fiber (1) connected to the input port. Light diffracting out of the input waveguide at the coupler/slab interface propagates through the free-space region (2) and illuminates the grating with a Gaussian distribution. Each wavelength of light coupled to the grating waveguides (3), undergoes a constant change of phase attributed to the constant length increment in grating waveguides. Light diffracted from each waveguide of the grating interferes constructively and gets refocused at the output waveguides(5), with the spatial position, the output channels, being wavelength dependent on the array phase shift.

a) TS of OCDMAb) SPETS of OCDMA

Proposed SPE O-CDMA Scheme

Phase OCDMA system, having following components

1) Lasers (mode locked laser requited to produce 4 wavelength signal)

2) Encoders consisting of required components like fiber delay lines, PRBS, External Modulator, multiplexers

3) Multiplexers

4) Optical fiber of 60 km length

5) De multiplexers

6) Decoders corresponding to each encoder

7) Receiver etc

Block Diagram of SPE O-CDMA

Simulation of SPE O-CDMA system for N users

After modulation an encoder is used for encoding the signal. The modulated signals are distributed to the respective encoders, which have been assigned a unique W/T code respective to each encoder. The encoded data from all users are multiplexed by Optical MUX and then passed through a 60 km span of standard single mode optical fiber followed by a loss compensating optical amplifier which is OptAmp.

The output signal from a fiber span is then passed through OptSplit1 to split the signal and routed to the user’s decoder. The decoder uses optical filters and inverse delay line arrays providing delays in terms of integer multiples of chip times.

The decoded signal finally arrives at optical receiver (Receiver), BER Test and Eye Diagram. Eye diagram analyzer has been used to take the plot of Eye pattern at the receiver end. Bit error rate values for different number of transmitting users have been taken from BER Tester. The system has been redesigned for different number of users. In spite of the use of orthogonal codes, the main effect limiting the effective signal-to-noise ratio of the overall system is the interference resulting from the other users transmitting at the same time, which is called Multiple Access Interference (MAI). MAI is the major source of noise in OCDMA systems.

ConclusionComparing to conventional OCDMA, using other advance modulation techniques, such as DPSK, DQPSK and CSK in OCDMA system has advantages of

(1) Improved receiver sensitivity

(2)Better tolerance to beat noise and MAI noise

(3)No need for optical thresholding;

(4)No need for dynamic threshold level setting; and

(5) Enhanced security. High capacity, spectral efficient OCDMA systems have been demonstrated using these techniques.

References Optical Code Division Multiple Access Communication Networks

Theory and Applications Hongxi Yin, David J. Richardson

Design and simulation of a novel spectral phase-encoded OCDMA systemSavita R.Bhosale, S.B.Deosarkar and S. L. Nalbalwar

K. Kitayama, X. Wang, and H. Sotobayashi, “State of the art andapplications of optical code division multiple access (Invited),”in ECOC’04, (Stockholm, Sweden, 2004), Tu4.6.

Advanced Modulation Techniques in OCDMA SystemXu Wang, N. Wada, T. Miyazaki, G. Cincotti, and K. Kitayama

X. Wang et al, JLT, 22(2004), 2226-2235.

Z.Jiang et al, IEEE PTL, 17(2005), 929.