Optical Fiber Telecommunications Volume VIB by Kaminow Ivan Li Tingye Willner Alan E. & Tingye Li & Alan E. Willner

Author:Kaminow, Ivan, Li, Tingye, Willner, Alan E. & Tingye Li & Alan E. Willner
Language: eng
Format: epub
ISBN: 9780123972378
Publisher: Elsevier Science
Published: 2013-05-20T16:00:00+00:00

Figure 11.17 Distribution of channel capacity at a single frequency for an MDM system with D = 10 modes and an SNR of 20 dB, assuming CSI is not available. The average capacity and outage capacity at outage probability are indicated. Adapted from [30].

Figure 11.13 of Section 11.4.6 shows the correlation coefficients of the modal gains versus normalized frequency separation, illustrating how the gains at nearby frequencies are highly correlated. Figure 11.13 also shows the correlation coefficient of the channel capacity. The capacity is computed as in Figure 11.17, assuming an SNR dB, no CSI available at the transmitter, and equal power allocated to all modes. The normalized one-sided coherence bandwidths of the channel capacity are 0.31 and 0.92 for correlation coefficients of 50% and 10%, respectively. A normalized frequency separation of unity gives a correlation coefficient of 8.8%.

If MDM signals occupy a bandwidth much greater than the coherence bandwidth of the capacity, statistical averaging over frequency should cause the outage channel capacity to approach the average channel capacity. Figure 11.18 shows the outage capacity as a function of SNR, for signals occupying different bandwidths. All parameters are as in Figure 11.13 and Figure 11.17 and the outage probability is . A normalized signal bandwidth is defined as , where Bsig is the signal bandwidth (measured in Hz). In Figure 11.18, signals occupy normalized bandwidths from b = 0 (a single frequency) to b = 8. The average capacity is also shown for comparison. Figure 11.18 shows that as the normalized bandwidth increases, the outage capacity approaches the average capacity.

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