PUBLISHED PAPERS

Pulse Interval and Width Code Modulation

Z. Ghassemlooy, R. Reyher, E. D. Kaluarachchi, A. J. Simmonds and R. Saatchi

E l e c t r o n i c s & c o m m u n i c a t i o n E n g i n e e r i n g R e s e a r c h G r o u p , S c h o o l O f E n g i n e e r i n g

I n f o r m a t i o n T e c h n o l o g y , S h e f f i e l d H a l l a m U n i v e r s i t y , S h e f f i e l d , U . K .

Abstract

This paper investigates both theoretical and practical aspects of implementation of a new digital pulse time modulation technique based on pulse interval and width code modulation (PIWCM) scheme for transmission of analogue/digital signal. Original expression is presented for power spectral density and the results obtained closely match with simulated and practiacl results. The developed system will have applications in point-to-point fibre optic transmission links and fibre optic broaband networks.

Introduction

Pulse time modulation (PTM) techniques have been proposed for transmission of analogue signals over short to medium haul point-to-point optical fibre communication links. In this type of modulation the transmitted carrier is a binary pulse amplitude, where the pulse width, pulse position, pulse interval, pulse frequency is modulated by an incoming modulating signal. PTM schemes have the advantage that the information can be transmitted at a reduced bandwidth than that required by pule code modulation (PCM) at much reduced cost and complexity. They also have the ability to trade bandwidth overhead for signal-to-noise ratio performance[l-4]

Recently, a digital PTM scheme, known as digital pulse position modulation (DPPM) has been suggested for long-haul point-to-point links over mono-mode fibre, showing substantial improvement in receiver sensitivity compared to PCM under conditions that the fibre bandwidth is not limited. The scheme under consideration utilises the pulse position

3rd International Symposium on Communication Theory and Applications 10-14 July 1995 Charlotte Mason College Lake District, UK

modulation (PPM) [5-7] format where the time interval of Ambits of PCM is subdivided into n - 2M time slots and the information is conveyed by positioning a single pulse in one of the n

time slots. In contrast to its analogue equivalent, digital PPM actually consumes more bandwidth than that required by PCM. It has been shown that digital PPM can offer improvements of between 5-11 dB [8-10] (depending on the coding level , bandwidth and detection technique) in receiver sensitivity when compared to PCM. This represents an increase in point-to-point transmission of between 25-55 km. However, DPPM timing requirements are exceptionally critical to the system performance and far exceed the equivalent PCM timing requirements[ll]. This paper proposes a new digital PTM scheme known as pulse interval and width code modulation (PIWCM) (also known as digital PIWM) offering simplicity and ease of frame synchronisation.

Theoretical

PIWCM is closely related to PIWM in that it employs a waveform in which both mark and space represent the sampled data [3], but with PIWCM these mark and space time interval slots are made discrete. PIWCM is anisochronous PTM technique in which each successive frame length is different and determined only by the sampled value of the modulating signal, not by the choice of the predetermined clock (sampling) period. As a consequence, receiver design complexity is substantially reduced since there is no requirement to extract frame frequency and phase for synchronisation in order to correctly interpret the encoded sampled value. Depending on source connection, PIWCM can carry encoded PCM data or directly sampled signals.

The digital signal converted from analogue signal is coded into PIWCM code by an M bit coder and applied to the modulator, Fig. 1. An Mbits digital signal is divided into two sets of

k bits (here, k is chosen to be equal to half the M bits). The decimal equivalent of binary combination in a given set determines the number of time slots for mark (m) and space (s). At time t equal to zero conversion first takes place for mark followed by conversion for space. To represent zero one is added to each code words. Finally, time slots for mark and space are combined together to produce the desired PIWCM signal. Differentiating the PIWCM

waveform will result in a constant width narrow pulse train known as pulse interval code modulation (PICM), see Fig. 2. This modulation scheme is suitable for long-haul fibre optic communication system where high peak optical power and low average optical power is required. At the demodulator the process is reversed, where the transmitted frame length is determined by simply counting the number of time slots for mark and space, a process which requires frame synchronisation, and converting them to binary digit. PIWCM modulator and demodulator designs can be formulated around state machines. This new scheme offers higher transmission capacity by virtue of fully utilising the time slots for a given frame and illuminating unused time slots. Synchronisation is simply achieved by initiating each frame with marks followed by spaces.

PIWCM frame length will be given by:

0)

where the first and second term represent the number of time slot for mark and space respectively, x is the decimal equivalent of each k bit binary set. The longest and shortest PIWCM frame lengths will be are 2**1 and 2 time slots resulting in average frame length of 2*4-1 time slots. The maximum total time occupied by a PIWCM frame is Tj which is therefore 2k+1 Ts seconds. The expression for time slot duration in PIWCM and PCM in terms of sampling frequency f s can be written as:

T p iw C M = ¥ + T7s

t PCM =

(2)

M /s

PIWCM displays shorter time slot duration compared to PCM (and wider time slot duration compared to DPPM) and hence higher channel bandwidth requirement for values of M greater than 2. For a signal of bandwidth f m Hertz sampled at the minimum Nyquist rate the maximum

k+ 1 ^ •

different frame lengths, the instantaneous sampling frequency (slot rate) of PIWCM changes according to the amplitude of the modulating signal.

The transmission capacity for PCM and PIWCM systems are: £ _ Maximum code length

A verage code length 2 fmL°g2 (Possible number of valid codes)

Thus: CpcM =

lMfm

CpjwCM = ~z

2fmM

2 +1

2 k+1

(3 )