Manchester and Bi-phase Line Coding In Optical System.
Princess Sumaya University for Technology, [email protected]
Abstract – Line coding is a way to represent a digital signal to be transmitted, with a waveform that it is transported through the physical channel and errors are easy to find at the receiver.
This experiment is intended to study different line encoding schemes used in an optical system. An optical communication system uses a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal
Digital data is represented by waveforms. The simplest waveform uses two levels to represent the bits 0 and 1, by giving the 1 a high level and a low level for the 0. For the basic waveforms the level remains the same for the duration of the bit. This type of waveforms is called Non Return to Zero (NRZ). In the receiver the signal is interpreted with a clock signal to determine whether the transmitted bit is a 0 or a 1.
The problem with this type of encoding is that if the transmitted signal has long sequences of 0s or 1s, then it is difficult to decode the signal correctly.
The solution of this problem was to use different coding schemes where the level of the bits change constantly. Manchester and Bi-phase schemes are used in optical systems.
Manchester code is a line coding scheme in which each bit is represented by two levels, a high then a low or vice versa for equal intervals of time. It does not need an external clock or DC bias.
Bi-phase Mark/space code is a differential line coding scheme in which there is a phase change in the end of each bit interval. There is a another phase change in the middle of the bit period if the bit is 1 in Mark Bi-phase and in Space Bi-phase the transition in the middle happens if the bit is 0.
Figure 1: Manchester and Bi-phase encoding
The goal of this experiment is to study Manchester and Bi-phase coding, by making a digital transmission system and applying these coding schemes to 8 multiplexed channels, then inspecting the decoding and demultiplexing of these channels through a fiber optic cable.
The experiment was done in the lab on MCM 40 fiber optic communication trainer board, 820 nm LED was used as a transmitter, 820 nm PIN photo detector as a receiver, and a single mode glass fiber optic was used as a channel, that is for the optical system. For the digital part the board has 8*1 multiplexer which was used to send 8 different bits, which were then encoded using Manchester encoder or Bi-phase encoder then were sent to the LED using digital driver, and at the receiver there is a decoder with a demultiplexer, the received data is shown on 8 LEDs.
Figure 2: Block diagram of the system
RESULTS AND SIMULATION
In the lab the experiment was done seamlessly, no errors were detected and the received data was exactly the same as the transmitted data, an analog oscilloscope was used to observe the signal in various stages. OptiSystem 7.0 software was used to simulate the experiment, the Bi-phase part could not be done using the simulator due to technical issues such as: the simulator does not save any previous data.
The transmitter in OptiSystem was done as in the figure:
Figure 3: Manchester encoding and optical Tx
The transmitted data was 01011011 and was coded using Manchester code.
Figure 4: Encoded Data
The LED converted the electrical signal into optical power
Figure 5: Transmitted Optical Power
unfortunately, we had no means to measure the optical power at the lab.
The optical power was then carried through a single mode fiber, different lengths were used to detect the effect on the transmitted signal.
Figure 6: Optical power using 0.03 Km fiber
Figure 7: Optical Power using 10 Km Fiber
Figure 8: Optical Power using 15 Km Fiber
We notice that with longer distances, optical power is much less, which is expected as attenuation and dispersion are functions of distance.
At the receiver, 820 nm PIN photo detector was used to convert optical power into electrical signal.
Figure 9: Electrical signal after PD
After the data was received, it must be decoded to get the transmitted bits. We met some obstacles at this stage of simulation, as the received electrical signal has very small amplitude and is affected by noise, to solve this problem a threshold detector was used.
Figure 10: Thresholding and Decoding at Rx
Figure 11: Decoded Data (0.03 Km)
Figure 12: Decoded Data (10 Km)
Figure 13: Decoded Data (15 Km)
The attenuation and dispersion affected the signal but that is to be expected from the eye diagram of the system.
Figure 14: Eye Diagram (0.03Km)
Figure 15: Eye Diagram (10 Km)
Figure 16: Eye Diagram (15Km)
Table 1: Comparison Between Different Distance