Error-performance degradation can be classifyed in two group. The first one is due to a decrease in received signal power or an increase in noise or inteference power, giving rise to a loss in signal-to-noise ratio EB/N0EB/N0 size 12{E rSub { size 8{B} } /N rSub { size 8{0} } } {}. The second one is due to signal distortion such as ISI.

Figure 1 Bit error probability

Suppose that we need a communication system with a bit-error probability PBPB size 12{P rSub { size 8{B} } } {} versus Eb/N0Eb/N0 size 12{E rSub { size 8{b} } /N rSub { size 8{0} } } {} characteristic corresponding to the solid-line curve plotted in Figure 1. Suppose that after the system is configured, the performance dose not follow the theoretical curve, but in facts follows the dashed line plot (1). A loss in Eb/N0Eb/N0 size 12{E rSub { size 8{b} } /N rSub { size 8{0} } } {} due to some signal losses or an increased level of noise or interference. This loss in EB/N0EB/N0 size 12{E rSub { size 8{B} } /N rSub { size 8{0} } } {} is not so terrible when compared with possible effects of degradation caused by a distortion mechanism corresponding to the dashed line plot (2). Instead of suffering a simple loss in signal-to-noise ratio there is a degradation effect brought about by ISI. If there is no solution to this problem, there is no a mount of EB/N0EB/N0 size 12{E rSub { size 8{B} } /N rSub { size 8{0} } } {} that will improve this problem. More EB/N0EB/N0 size 12{E rSub { size 8{B} } /N rSub { size 8{0} } } {} can not help the ISI problem because a incresing in EB/N0EB/N0 size 12{E rSub { size 8{B} } /N rSub { size 8{0} } } {} dose not make change in overlapped pulses.