## 4 Simulation Results and Discussion
Optimization of the dispersion map is important not only in the distribution of cumulative dispersion in the entire link, but also in the magnitude of the residual dispersion per span. Figure 5 makes it possible to evaluate the effect of the magnitude of rRD on compensation in the proposed dispersion map in terms of power margin. Figure 5 shows the difference in power margin of the worst channel as a function of the rRD in the dispersion-managed link to each of the 16 different dispersion maps. The power margin difference is the arithmetic subtraction of the power margin obtained when the conventional dispersion map is applied from the power margin obtained when each dispersion map is applied into a dispersion-managed link. In other words, Figure 5 shows how much the power margin in the proposed dispersion maps is improved compared to the conventional dispersion map. As each value shown in Figure 5 increases, the compensation effect can be considered to have increased further. A feature common to Figure 5a,b is that the compensation effect is better improved when the rRD of each fiber span selected for each dispersion map in  
Figure 4: The launch power versus reference residual dispersion that can achieve 1 dB EOP. Figure 2 is moderately small (i.e., around 400 ps/nm). And from the rRD point of view, the dispersion maps that can achieve the improved compensation effect for all considered rRDs over the conventional configuration, albeit to varying degrees, are the fixed configurations of Figure 1(a),b. Although the compensation characteristics for each dispersion map are examined when the TRD of the dispersion-managed link is basically set to 10 ps/nm, the EOP of 1-dB or less can be also obtained at other TRDs. Figure 6 shows the range of TRDs that result in 1-dB EOP at each launch power of the worst channel, which is the example of the above-mentioned feature. In Figure 6, the effective TRD means the upper and lower limits of the TRD that can obtain 1-dB EOP for each launch power. Although the characteristics of launch power versus effective TRD obtained from the conventional dispersion map and the dispersion map of the linear-DG:AD are generally similar, there is a distinguishable difference when the launch power is large. However, it is rather cumbersome to analyze the launch power versus effective TRD characteristics for the 16 dispersion maps in Figure 2 as shown in Figure 6. Looking at Figure 6, it can be known that the effective TRD for the launch power ultimately takes the form of a closed curve. Therefore, one way to numerically analyze the launch power versus effective TRD characteristics of each dispersion map is to evaluate and compare the area of the corresponding closed curve. The area of the closed curve is equivalent to the product of the available launch power and the effective TRD that can yield 1 dB EOP. We will simply call the product of available launch power and effective TRD to 'the product'. As a matter of course, the larger the product, the larger the TRD coverage of the dispersion-managed link, or the larger the launch power of the WDM channel, depending on the link design requirements. Figure 5: The difference power margin versus reference residual dispersion for the worst channel. Figure 6: The effective launch power versus the effective TRD ranges. Figure 7 shows the product of the worst channel as a function of rRD when each dispersion map in Figure 2 is applied to the dispersion-managed link. Figure 7 illustrates the largest and smallest products for each rRD, and shows the corresponding dispersion map resulting in those values. For comparison, the product in the conventional dispersion map is also shown in Figure 7. It can be confirmed that the fixed-DG: DA configuration is most advantageous when WDM channel distortion is compensated for by selecting a relatively small rRD of 400 ps/nm or less. In addition, it can also be known that the fixed-DG: AD or linear-DG: AD configurations are preferable when rRD is selected to be 480 ps/nm or more.