Intersymbol interference

Before leaving the subject of pulse modulation formats, we discuss one more principle involved in choosing pulse shapes with bandwidth occupancy in mind, known as Nyquist s pulse-shaping criterion. The idea is to find pulse shapes p t), withbandlimited spectra, that have the zero intersymbol-interference... 

Intersymbol interference Interference from preceeding or suceeding pulses being smeared into a pulse of interest during a sampling process, usually at the receiver. 

NyquisTs pulse-shaping criterion A condition placed on the spectrum of a pulse shape function that guarantees zero intersymbol interference when samples are taken at a proper sampling rate. 

Various efforts to improve the signal-to-noise ratio in channels with additive noise and intersymbol interference, which is characteristic of severely bandlimited channels (e.g., the telephone channel), led to the original development of set partitioning techniques that culminated in Ungerboeck s development (1982) of trelliscoded modulation (TCM). 

The bandwidth of an optical fiber strongly depends on the modal dispersion. The modal dispersion can be controlled by controlling the refractive index profile (RIP), which has a decisive effect on the bandwidth (see Chapter 3) and is a basic and important parameter of optical fibers. The pulse-broadening caused by modal dispersion seriously limits the transmission data rate of multimode fibers (MMFs) because overlapping of the broadened pulses induces intersymbol interference and disturbs correct signal detections, increasing the bit error rate [1]. 

However, in practical systems, the signal is degraded by various types of noise, interference from adjacent pulses, and the finite extinction ratio ofthe light source. If an optical transmission system has no optical amplifier, thermal and shot noises are the dominant factors in the receiver [6]. Thermal noise is independent ofthe incoming optical power in contrast, shot noise depends on the received optical power. A critical error source is intersymbol interference (ISI), which arises from pulse spreading due to dispersion [7]. The pulse broadening causes the received signal to spread into the adjacent bit period. Thus, errors in the bit decision occur. 

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