Outage analysis of the hybrid free-space optical and radio-frequency channel

Abstract

Abstract-We study the hybrid free-space optical (FSO) and radio-frequency (RF) channel from an information theoretic perspective. Since both links operate at vastly different carrier frequencies, we model the hybrid channel as a pair of parallel channels. Moreover, since the FSO channel signals at a higher rate than the RF channel, we incorporate this key feature in the parallel channel model. Both channels experience fading due to scintillation, which is slow compared to typical signalling rates. Under this framework, we study the fundamental limits of the hybrid channel. In particular, we analyse the outage probability in the large signal-to-noise ratio (SNR) regime, and obtain the outage diversity or SNR exponent of the hybrid system. First we consider the case when only the receiver has perfect channel state information (CSIR case), and obtain the exponents for general scintillation distributions. These exponents relate key system design parameters to the asymptotic outage performance and illustrate the benets of using hybrid systems with respect to independent FSO or RF links. We next consider the case when perfect CSI is known at both the receiver and transmitter, and derive the optimal power allocation strategy that minimises the outage probability subject to peak and average power constraints. The optimal solution involves non-convex optimisation, which is intractable in practical systems. We therefore propose a suboptimal algorithm that achieves signicant power savings (on the order of tens of dBs) over uniform power allocation. We show that the suboptimal algorithm has the same diversity as the optimal power allocation strategy.

Nearest Neighbor Decoding in MIMO Block-Fading Channels With Imperfect CSIR

“…The outage diversity or the outage SNR exponent, , is defined as the high-SNR slope of the outage probability curve, as a function of the SNR, in log-log scale plot when the receiver has access to perfect CSIR (21) Suppose that is a finite real-valued quantity, which is the case for many fading distributions. Then, we have the following lemma on the strict lower bound to the error probability at high SNR [11].…”

Optimal Power Control for LDPC Codes in Block-Fading Channels

“…Depending on the system parameters and the constraints on the transmit power, optimal power allocation can be determined to minimize the outage/error probability, given that CSI is available at the transmitter [2], [4], [12]- [16]. In this section we further develop the outage/error-region framework to deal with optimal power allocation based on various power constraints.…”

“…The most popular example is an orthogonal frequency division multiplex (OFDM) system, where it is common to assume that the fading coefficient of a single frequency band is constant over a finite number of OFDM symbols. Other examples are frequency-hopping in GSM/EDGE systems, free-space optical systems [3], and hybrid optical-radio frequency systems [4] where the links can be modeled as (possibly correlated) slow-varying fading channels.…”

“…In case CSI is available at the transmitter, the outage probability can be minimized through power allocation, i.e., the transmit power is allocated across blocks as a function of the channel realization subject to certain constraints. Optimal power allocation for delay-limited block-fading channels using continuous or discrete symbol alphabets has been studied in [2], [4], [12]- [17], where shortterm, long-term, and short-to-long-term power ratio (SLPR) constraints have been of particular interest. In some cases, all outages can be removed, showing dramatic performance gains with respect to uniform power allocation.…”

Availability of a Hybrid FSO/RF Link While Using the Link’s Diversity for Packet Scheduling

“…Approaches to widen the applicability and improve the performance of FSO links have been taken in the following directions: adjusting signal intensity by increasing its power in case there are adverse weather conditions [1,2], studying and finding ways to increase the transmission angle so that reception is less of a problem, using systems with multiple receivers and multiple transmitters (MIMO) [3,4], and using hybrid FSO and RF links [1,[5][6][7][8][9] and combinations of two or more of these approaches. Also, multiple recent studies address the performance mixed RF/FSO systems where RF and FSO links are used to bridge separate parts of the transmission path, for example, [10][11][12][13].…”