Abstract-Optical wireless communications (OWC) is a promising technology for closing the mismatch between the growing number of connected devices and the limited wireless network capabilities. Similar to downlink, uplink can also benefit from OWC for establishing connectivity between such devices and an optical access point. In this context, the incoherent intensitymodulation and direct-detection (IM-DD) scheme is desirable in practice. Hence, it is important to understand the fundamental limits of communication rates over an OWC uplink employing IM-DD, i.e., the channel capacity. This uplink, modeled as a Gaussian multiple-access channel (MAC) for indoors OWC, is studied in this paper, under the IM-DD constraints which form the main difference with the standard Gaussian MAC commonly studied in the radio-frequency context. Capacity region outer and inner bounds for this channel are derived. The bounds are fairly close at high signal-to-noise ratio (SNR), where a truncatedGaussian input distribution achieves the capacity region within a constant gap. Furthermore, the bounds coincide at low SNR showing the optimality of on-off keying combined with successive cancellation decoding in this regime. At moderate SNR, an optimized uniformly-spaced discrete input distribution achieves fairly good performance.

This paper provides precise performance analysis of the dual-hop mixed radio frequency (RF)/unified free space optical (FSO) decode-and-forward (DF) relaying system, in which the heterodyne detection and the intensity modulation-direct detection (IM-DD) are taken into account for FSO detection. To this end, we derive closed-form expressions for the outage probability, average bit error rate (BER), and the ergodic channel capacity of this system. In this analysis, we utilize, for the first time as per our knowledge, a precise channel capacity result for the IM-DD channel. Moreover, this is the first time that not only the (IM-DD input-independent) but also the (IM-DD cost-dependent) additive white Gaussian noise (AWGN) channel is considered in such system analyses. Additionally in this study, we assume that the first hop (RF link) follows Nakagami-m fading, and the second hop (FSO link) follows Málaga (M) turbulence with pointing errors. These fading and turbulence models contain other models (such as Rayleigh and Gamma-Gamma) as special cases, thus, our analyses can be seen as a generalized one from the RF and FSO fading models point of view. Also, in BER derivation, we assume that the modulation schemes in the two hops can be different, since not all modulation schemes are suitable for IM-DD FSO links. In addition, the system performance is investigated asymptotically at high signal to noise ratios. This investigation leads to new nonreported coding gain and diversity order analyses of such system. Interestingly, we find that in the FSO hop, at high transmitted powers, all the considered FSO detectors result in the same diversity order. Furthermore, we provide simulation results that verify the accuracy of the obtained analytical and asymptotic expressions.

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This paper considers a dual-hop decode-and-forward (DF) mixed radio frequency (RF)/free space optical (FSO) relaying scheme. In this scheme, the first hop is a multiple-input multiple-output (MIMO) RF interference channel (IC), while the second hop is an FSO channel in which heterodyne detection (HD) as well as intensity modulation-direct detection (IMDD) are considered in a unified analysis manner. It is assumed that the MIMO RF links follow Rayleigh fading, whilst the FSO link encounters Málaga (M) fading with pointing errors. In this work, the Interference alignment (IA) technique is utilized in the MIMO RF IC hop to eliminate the interference, and hence this leads to enhance the multiplexing gain in this hop, and accordingly the overall system performance is improved. The design of the IA precoding matrix and receiving interference eliminating matrix is performed by using adapting orthogonalized minimum leakage (OR-ML) algorithm. For the considered scheme, exact closed-form expressions are derived for the outage probability and ergodic capacity, in these expressions the best-known FSO channel capacity results are utilized. Moreover, not only the IMDD input-independent AWGN but also the IMDD cost-dependent AWGN channel is analyzed in these investigations. In addition, the system performance is studied asymptotically at the high SNRs region, where the diversity order and coding gain analyses are illustrated. Further, we allocate simulation results that verify the deduced analytical and asymptotic expressions.