  In this article, we propose an index modulation system suitable for optical
communications, based on jointly driving the time and frequency of the signal:
an index-time frequency hopping (I-TFH) system. We analyze its performance from
the point of view of its efficiency in power and spectrum, and its behavior in
terms of error probability for the non-turbulent free-space optical (FSO)
channel. We compare I-TFH with already proposed index modulated systems of the
same nature, but where the amplitude or the number of transmitters are driven
instead of the signal frequency. We derive and compare approximations for the
average symbol and bit error probabilities of all these systems. The simulation
results show that said approximations are tight enough for a wide range of
signal-to-noise ratios and system parameters. Moreover, I-TFH shows to be
better performing in BER and/or power efficiency than the comparative
alternatives, and may offer interesting properties in a variety of contexts.
