DPSK Modulation OTDM System – A Survey

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DPSK Modulation OTDM System – A Survey

Darshan N

Student, Dept. of ECE

N I E Institute of Technology Mysuru, Karnataka, India

Harish S V

Assistant Professor, Dept. of ECE N I E Institute of Technology Mysuru, Karnataka, India

Dilip Kumar

Student, Dept. of ECE

N I E Institute of Technology Mysuru, Karnataka, India

Abstract Optical fiber is the trending technology which drags major attention on multiple data multiplexed transmission. There is a congestion in sending data over a shared channel. OTDM (Optical TimeDivision Multiplexing) is an important technique to overcome the electronic bottleneck and achieve a single channel system. In this paper we suggest a point to point communication using OTDM system and that uses one of the PSK modulation scheme for high transmission rate.

Keywords Optical fiber communication; MachZehnder Modulator; Optical TimeDivision Multiplexing (OTDM); Phase Shift Keying (PSK);


    The different users send the data in bits. These bits are transmitted over the single shared, medium optical fiber. These data are recovered at receiver. Figure 1 shows block diagram of the OTDM System along with MachZehnder modulator and DPSK modulator.


    The optical fibers are next generation future in Physical layer of computer networks and telecommunication. The range of usage of optical fiber in communication changes the speed of transmission over cannel. Signals are multiplexed and sent over the single channel. But use of optical fiber as core or at the back bone of the network leads to poor efficiency and low bit rate. So this leads to optical modulation techniques and Phase Shift Keying (PSK) Modulations.

    To multiplex signal simple technique is TimeDivision Multiplexing (TDM). TimeDivision Multiplexing is a system in which each signal appears at the shared medium only a fraction time in alternating pattern by the means of synchronized switches [1].

    A special electrooptical used to modulate optical signal with electrical signal. MachZehnder is a special kind of electrooptical modulator which is used in this paper. An optical signals intensity is modulated accordance with the input electrical signal [2][4].

    Phase Shift Keying is one of the effective way of modulation. Compared to onoff keying(OOK) and Amplitude shift keying (ASK), PSK modulation exhibits high efficient modulation. Differential Phase Shift Keying (DPSK) is used in this paper for good performance.

    In this paper we suggest an Optical TimeDivision Multiplexing and optical modulation called MachZehnder Modulation along with PSK, specifically Differential PSK.

    Figure 1 Block Diagram of OTDM System.

    1. DPSK Modulation and DeModulation

      DPSK is a digital modulation technique used to modulate phase of the carrier signal in accordance with the present and previous data. This modulation technique is used at the sender end. When a system receives data bit stream, it modulates with this technique.

      DPSK encodes carrier signals phase with the modulating signal. The serial data input is given to the XOR gate and output is again fed back to the other input through 1bit delay. The output of the XOR gate along with the carrier

      signal is given to the balance modulation, to produce the DPSK modulated signal. Figure 2 shows block diagram of DPSK modulator.

      Figure 2 Block Diagram of DPSK

      The coding rule of differential coder is as below equation.

      bn=an bn-1

      where bn is current output, an is current input and bn1 is previous output. For example, consider the input binary signal 01011001. The TABLE I below shows the phase in the carrier.




























      At receiver end the same data is demodulated to retrieve the bit stream sent by the sender. The below figure 3 shows block diagram of the DPSK DeModulation. The input electrical signal is split into two and this are multiplied with carrier and inverse of carrier which is carrier phase shifted by 900 as shown in figure 3. These signals are compressed and change to Mary format using. These signals are compared and coded using DPSK Decoder.

      Figure 3 Block Diagram of DPSK Demodulation

    2. MachZehnder Modulator

      MachZehnder is a special find of electrooptical modulator. There are several electrooptical modulators such as Electro Absorption Modulator (EAM), Amplitude Modulator, Phase Modulator and etc. in which Mach Zehnder is one of the kind. Waveguide LiNbO3 or Mach Zehnder modulator which accepts optical carrier and electrical modulation signal and gives modulated optical signal.

      The MachZehnder has an input optical branch which splits the incoming optical signal in to two arms. These two arms have the signal out of phase to each other. These arms contain electrodes as shown in figure 4. These electrodes are applied with electrical input signal. Application of electric signal to one or both of the optical arms through electrodes controls the degree of interference. Then the output optical signal is intensity varied input in accordance with electrical signal. Figure 4 shows the Schematic diagram of Mach Zehnder Modulator.

      Figure 4 Schematic Diagram of MachZehnder

    3. Optical TimeDivision Multiplecing (OTDM)

      The TimeDivision Multiplexing technique in which the inputs and output will be of optical signal. N number of optical signals are fed to the OTDM and all signals are combined to a single optical signal. This process is called Optical TimeDivision Multiplexing. Figure 4 shows the simple timing diagram of four input channel OTDM system


      Figure 4 Simple Timing Diagram.

      OTDM is made up of time delayer and power combiner. Delay for each channel is varied. If there are N channels to be multiplexed, then the delay of nth channel (d) is given by below formula.

      d = 1/ ((Bit rate)) × (nth channel 1)/ N

      Time delayer adds time component to the optical signal. For example, let us consider four input channel. Lets consider the bit rate of the system is 500Mbps. So for first channel there will be no delay, that is 0s. For second channel there will be delay of 0.5ns. For third channel there will be delay of 1ns. For fourth channel there will be delay of 1.5ns.

      Finally, the multiplexed data stream propagates in fiber link. Figure 5 depicts the input OTDM system.

      Figure 5 Block diagram of OTDM System

    4. Optical TimeDivision DeMultiplexing

      Multiplexed signal at the sender end is demultiplexed at receiver end. This process takes input optical signal from a single shared channel and splits into N channels. The Optical TimeDivision DeMultiplexer consists of Power splitter cascaded with Optical Time Delayer and MachZehnder Modulator.

      Power splitter splits data stream into identical data stream. These data streams are added with respective optical delay. Delay(d) for each arm is calculated by the below formula.

      d = 1/ ((Bit rate)) × (N nth channel 1) / N/p>

      These optical signals are fed to input optical branch of MachZehnder Modulator. A high logic Return to Zero format signal is fed to electrodes of MachZehnder Modulator to recover the optical signal sent by the particular user at particular channel. The Figure 6 shows the Block Diagram of Optical TimeDivision Multiplexing.


    The major advantages of this proposal is High bit rate transmission. The data recovery become easy as Mac- Zehnder Modulation is used. As we modulate the users data with optical signal then these optical signals are multiplexed, this increases bandwidth and efficiency of the system.

    This system has lots of scope in Date base, Servers and Date Storage systems where fast transmission is required. In Industrial commercial where the data need to be accurate and fast transmission this system is used. In Broadcasting and HDTV connection these systems are helpful for reliable transmission. It as scope in defense system and sonar vehicles sensors and actuators.


With the content in this paper, we propose a DPSK Modulation OTDM system. It has large scope in future Optical Communication Systems. As Optical signals from individual channels are multiplexed, the efficiency and Data recovery is high. This system is more useful because it improves bitrate of transmission and bandwidth of the system.


Figure 6 Block Diagram of Optical TimeDivision DeMultiplexing

  1. Photodetector

To convert an optical signal into electrical signal Photodetectors are used. These devices have capacity to absorb light and converts the optical energy to electrical energy. The Photodiode is used to convert an optical signal into an electrical current based on the devices Responsivity. When an optical signal strikes the diode, it generates electrical current corresponds to optical intensity by the electronhole pair formation. This current is called photocurrent. By the means of this device we convert optical signal into corresponding electrical signal.

The authors would like to express sincere gratitude to Harish S V, Assistant Professor, Dept. of ECE, N I E Institute of Technology, Mysuru for their guidance and constant encouragement. I take this opportunity to record my sincere thanks to all the faculties of the Dept. of ECE, N I E Institute of Technology, Mysuru for their help and encouragement.


  1. K. Merzouk*, Y. Le Guennec, B. Cabon Low Cost 10 Gb/s OTDM System IEEE 2007.

  2. D.T. Neilson, C. R. Doerr, L. Zhang, and L. L. Buhl EAMbased InP MZ modulator for 40Gb/s PSBT using 20Gb/s tributaries ECOC 2008, 2125 September 2008, Brussels, Belgium.

  3. Luo Wen, Guan Yalin A DPSK Modulation OTDM System at 40Gbps at 2016 IEEE International Conference on Electronic Information and Communication Technology (ICEICT 2016).

  4. Giuseppe Scotti, Pasquale Tommasino, AlessandroTrifiletti, and Antonello Vannucci A MachZehnder ModulationModel for the Digen of OpticalFiber Anolog Transmission IEEE 2011.

  5. Arpana Mishra, Priyesh Mishra Optical Communication with Time Division Multiplexing (OTDM) and Hybrid WDM/OTDM PON International Journal of Science and Research (IJSR 2016).

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