**Acknowledgments**

16 Will-be-set-by-IN-TECH

be observed that the rate-adaptive scheme Sil does not take advantage of the potential time-diversity order available in the turbulent channel. The achievable information rate in (26), (27) and (28) is depicted in Fig. 5, corresponding to the rate-adaptive transmission schemes Sil, Rep and Rep&Sil, respectively, for two different target BER requirements, *Pb* <sup>=</sup> <sup>10</sup>−<sup>4</sup> and *Pb* <sup>=</sup> <sup>10</sup>−8, and TDO={2, 4}. It can be noted that obtained results are in excellent agreement with previous results presented in terms of BER performance in Fig. 2. In this sense, it can be observed that the required value of SNR to satisfy a target BER is greater when the impact of pointing errors is more severe, even when the same level of atmospheric

From this figure, it can be deduced not only the superiority of the rate-adaptive transmission scheme based on the joint use of repetition coding and variable silence periods but also that an adaptive transmission design approach based on taking advantage out of the potential time-diversity order available in the turbulent channel is required, corroborating the fact that the rate-adaptive transmission scheme only based on variable silence periods implies a remarkably inefficient performance from the point of view of information theory. In this way, it can be observed that even when the available time-diversity order is low (TDO=2) a relevant improvement in achievable information rate is obtained, especially when a lower target BER is demanded. In spite of high values of TDO cannot be possible because of the latency introduced by the interleaver, to achieve a time diversity order available of TDO=2, perfect interleaving can be done by simply sending the same information delayed by the expected fade duration, as shown experimentally in [32] for a rate reduction of 2. It must be commented that the rate-adaptive transmission scheme proposed is not based on an adaptive signal constellation where more complicated modulation techniques can be defined, being assumed the use of OOK signaling due to its simplicity and low implementation cost, and, hence, an achievable information rate not higher than 1 bit/channel use can be achieved, since this is determined by the signal constellation and how the coding technique is able to take advantage of it. At the expense of a greater simplicity in hardware implementation, lower values of capacity are achieved if compared to rate-adaptive transmission schemes based on adaptive modulation or coding techniques more sophisticated than repetition coding and the

In this chapter, a simple rate-adaptive transmission scheme for FSO communication systems with intensity modulation and direct detection over atmospheric turbulence channels with pointing errors is analyzed. This scheme is based on the joint use of repetition coding and variable silence periods, exploiting the potential time-diversity order available in the turbulent channel as well as allowing the increase of the PAOPR, which has shown to be a favorable characteristic in IM/DD FSO links [19, 24, 26]. Here, repetition coding is firstly used in order to accomodate the transmission rate to the channel conditions until the whole time diversity order available in the turbulent channel by interleaving is exploited. Then, once no more diversity gain is available, the rate reduction can be increased by using variable silence periods in order to increase the PAOPR. Novel closed-form asymptotic expressions are derived when the irradiance of the transmitted optical beam is susceptible to either a wide range of turbulence conditions (weak to strong), following a gamma-gamma distribution of parameters *α* and *β*, or pointing errors, following a misalignment fading model, as in [16–18, 38], where the effect of beam width, detector size and jitter variance is considered.

turbulence strength is considered.

inclusion of variable silence periods [10, 12, 14].

**5. Conclusions**

The authors are grateful for financial support from the Junta de Andalucía (research group "Communications Engineering (TIC-0102)").
