Bit Error Rate and Link Power Penalty

The average error probability is the most useful criterion for evaluating the performance of digital communication systems. A decision circuit samples the signal voltage at the midpoint or a determined sampling time of each time slot and 

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. 

Various types of signal impairment that are inherent in optical fiber transmission systems degrade the link performance. Any signal impairment in an optical link reduces the optical power transmitted to the receiver compared to the ideal case. This lower power reduces the SNR of the link compared to the case with no impairment [11]. Because a reduced SNR leads to a higher BER, additional signal power is required at the receiver to maintain the same BER as 

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