User:OpticCommunications/Dicode Pulse Position Modulation

Dicode Pulse Position Modulation
Dicode Pulse-Position Modulation (DiPPM) is a Pulse-Position Modulation (PPM) scheme, combination of the Dicode Technique and Digital PPM (DigPPM). Like Digital PPM, it offers greater sensitivity than Pulse Coded Modulation (PCM). However, unlike Digital PPM, sensitivity is achieved with only a twofold increase in speed (in its last form) as it uses only two data slots to code data transitions in a PCM signal. DiPPM is primarily useful for optical communications systems and it can be used in Dense Wavelength Division Multiplexing (DWDM) systems because of low bandwidth. Another advantage of the DiPPM, over most PPM schemes, is the simple and inexpensive construction of its Coder, Decoder and Timing Extraction.

Formatting
In DiPPM, the SET and RESET signals are converted into two pulse positions in a data frame. A PCM transition from zero to one produces a pulse in slot S and a one to zero transition generates a pulse in slot R. No pulse is transmitted when the PCM data is constant at either 1 or 0. As two slots are used to transmit one bit of PCM, and the line rate is two times that of the original PCM, speed is reduced when compared to digital PPM.

In DiPPM scheme there are four symbols and each symbol has a probability of ¼. However, the probability of a no pulse sequence (N) is ½ as it occurs at both 00 and 11 PCM sequence. The S symbol has the same probability as the no pulse (N), because there are only two possible PCM sequences (00 or 01) after an R pulse has been transmitted. A typical Dicode PPM sequence would be S, xN, R with probability of ½,  and ¼ respectively.

Timing Extraction
Timing extraction, slot and frame synchronisation of the DiPPM scheme is simple because of the position of the pulses in the frames. The falling edge of the SET and the rising edge of the RESET pulses are coincident and this generates a strong timing content. Thus, DiPPM appears to be more efficient than previous coding techniques. To achieve clock recovery from a random DiPPM sequence and to synchronise the extracted clock with the DiPPM signal, a second-order Phase-Locked Loop (PLL) system can be used in the DiPPM Timing Extraction circuit.

Spectral Characterisation
The spectrum of the DiPPM deterministic sequence is similar to the PCM deterministic sequence as it is expected because of their sequences similarity. Due to practical limitations and high frequency, the mark to space ratio for the SET and RESET of the DiPPM signals is uneven. This has the effect of moving the nulls in the spectrum to a slightly higher frequency.

The spectrum of the DiPPM signal with a Pseudo-Random Binary Sequence (PRBS) PCM signal, is not concentrated near D.C.. Thus, low frequency external interference will not affect the DiPPM signal. Due to the low-frequency content of the continuous spectrum, DiPPM is suitable for both optical fibre and optical wireless systems communication.

History
In 2003, Martin J.N. Sibley proposed a novel coding scheme for optical communications as a more advantageous format, Dicode Pulse Position Modulation (DiPPM-ISI), with two guard slots to be used to reduce the effects of inter-symbol interference (ISI). When there is minimal ISI, zero guard slots are used. As four slots are used to transmit one bit of PCM, and the line rate is four times that of the original PCM, speed is reduced when compared to digital PPM. In the same year, he analyzed the performance of a PINFET receiver when used with DiPPM-ISI. One year later, in 2004, he published analysis of the performance of a DiPPM-ISI system over dispersive optical channels, while a third-order Butterworth filter has been used as the pre-detection filter.

In 2005, the spectral characterization and the mathematical representation of random DiPPM-ISI have been presented by Robert Anthony Cryan. M. Menon and R.A. Cryan, the same year, presented original results of an optical wireless system employing DiPPM-ISI and a PIN-BJT receiver. At the end of 2005, M.J.N. Sibley described a Maximum Likelihood Sequence Detection (MLSD) algorithm for use with Dicode PPM. Theoretical and mathematical performance of optically pre-amplified DiPPM-ISI has been presented by H. Al-suleimani, Andrew. J. Phillips and M.S. Woolfson in 2008 (Correspondence to Andrew Julian Phillips).

Dicode PPM has been converted at its last form (DiPPM) in 2006 by R.A. Cryan and M.J.N. Sibley. The impact of Inter-Symbol Interference (ISI) on Dicode PPM has been eliminated through the use of central decision detection. DiPPM can be effectively implemented as it uses two slots to transmit one bit of PCM.

In 2006, Romanos Athanasiou Charitopoulos and M.J.N. Sibley presented for the first time practical outcomes of the DiPPM Power Spectral Destiny (PSD) and its differences with software simulation DiPPM PSD outcomes. In 2007, they published original software simulation of the DiPPM and they proposed the use of windowing technique; hence the software and mathematical outcomes will agree with those of practical experiments.

In 2009 R.A. Charitopoulos and M.J.N. Sibley presented for the first time a practical DiPPM coder and decoder and their outcomes. One year later in 2010 they published complete analysis of mathematical representation, software simulation and practical performance of DiPPM. The same year they presented the Timing Extraction and Frame Synchronisation of DiPPM and measurements through optical fibre and Free Space Communication (FSC). In 2010, R.A. Charitopoulos, M.J.N. Sibley and P.J. Mather developed a practical error detector which reaches the expectation of theoretical MLSD (M.J.N. Sibley, 2005). DiPPM MLSD and practical results through optical communication, confirm that DiPPM produces far less errors than previous formats.