Outage and Error Analysis of Dual-Hop TAS/MRC MIMO RF-UOWC Systems

This paper gives an insight on the performance of mixed dual-hop radio-frequency (RF)-underwater optical wireless communication (UOWC) systems. The system consists of multiple-input multiple-output (MIMO) RF hop employing Nakagami-
$m$
fading channel on the source (
$S$
) node communicating with a destination node (
$D$
) considered as the legitimate receiver via an amplify-and-forward (AF) relay (
$R$
) node equipped with multiple RF antennas for reception. It considers transmit antenna selection (TAS) scheme for communication in the MIMO RF hop while the information is transmitted from the
$S$
node to the
$D$
node, i.e. submarine etc., via the UOWC hop. Specifically, the
$R$
node receives incoming information messages from
$S$
node via MIMO RF links, applies maximal-ratio combining (MRC) technique, amplifies the output combined signal, and subsequently forwards it to the destination utilising a variable gain relaying (VGR) via an UOWC link. We derive exact closed-form expressions for the system's end-to-end (E2E) statistical channel characteristics. Our derived analytical expressions present an efficient technique to depict the impact of our system and channel parameters on the performance, namely the varying number of increasing antennas
$N_t=N_r=2,3,4$
or more from the
$S$
node towards
$R$
node and the involvement of underwater detection techniques of
$r = 1$
for heterodyne detection and
$r = 2$
for intensity modulation/direct detection (IM/DD) in the underwater turbulence severity of the UOWC link. Outage probability (OP) and average bit error rate (BER) closed-form expressions for the varying bubble levels (BL) (L/min) for different scenarios, varying temperature gradients (TG) (°C cm
⁻¹
), different fresh and saline waters, and various binary modulation techniques have been accurately validated for the E2E system presented in this work along with the tightness of their respective high-end asymptotes.