**5. Conclusions**

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. Furthermore, we extend the concepts of diversity and coding gain, which are well known from the RF communication literature [40], to the rate-adaptive FSO systems under study, allowing us to provide simple, insightful, and accurate closed-form approximations for the BER performance at high SNR. Simulation results are further demonstrated to confirm the analytical results. Here, not only rectangular pulses are considered but also OOK formats with any pulse shape, corroborating the advantage of using pulses with high PAOPR, such as Gaussian pulses or squared hyperbolic secant pulses, and concluding the fact that this improvement can be viewed as an additional coding gain, regardless of the rate-adaptive scheme employed or even in absence of rate-adaptive transmission.

In terms of pros and cons of using the adaptive transmission scheme Rep&Sil here analyzed and proposed, we can conclude that one of the pros of this adaptive transmission scheme is the greater simplicity, requiring a lower implementation complexity if compared to alternative rate-adaptive transmission schemes proposed for use in FSO systems [10, 12, 14, 34]. In this sense, the use of OOK signaling with repetition coding implies a hardware implementation of a significant lower complexity if compared to that required by the variable rate turbo-coding scheme as proposed in [34], the adaptive LDPC-coded modulation and transmission scheme that varied both the power and the modulation order of a FSO system with M-ary pulse amplitude modulation as proposed in [12, 14] and the adaptive transmission techniques employing subcarrier phase shift keying intensity modulation as proposed in [10]. Additionally, together with the use of repetition coding, the adaptive scheme Rep&Sil here proposed considers the inclusion of variable silence periods once no more diversity gain is available, providing a higher level of PAOPR and, hence, a better performance, without the need of increasing complexity in hardware implementation. On the contrary, one of the cons of the rate-adaptive transmission scheme here proposed in relation to previous rate-adaptive transmission schemes is the lower capacity achieved since no adaptive modulation is considered, proposing the use of OOK signaling due to its simplicity and low implementation cost and, hence, providing an achievable information rate not higher than 1 bit/channel use.
