Capacity Enhancement of RF Wireless Mesh Networks Through FSO Links

Farshad Ahdi; Suresh Subramaniam

doi:10.1364/JOCN.8.000495

Journal of Optical Communications and Networking
Vol. 8,
Issue 7,
pp. 495-506
(2016)
•https://doi.org/10.1364/JOCN.8.000495

Capacity Enhancement of RF Wireless Mesh Networks Through FSO Links

Farshad Ahdi and Suresh Subramaniam

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Farshad Ahdi and Suresh Subramaniam, "Capacity Enhancement of RF Wireless Mesh Networks Through FSO Links," J. Opt. Commun. Netw. 8, 495-506 (2016)

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Abstract

RF-based technologies are often used to address the last-mile problem. However, RF technologies generally create a bottleneck for the ever-increasing demand for bandwidth, causing the under-utilization of core network resources. Enhancing the capacity of wireless mesh networks can be done efficiently and cost effectively through boosting the capacity of some strategically located links. In this paper, we propose link augmentation via free-space optics (FSO) links. The high capacity of optical links is the key in link capacity enhancement in the resulting hybrid network. Adopting a TDMA-based framework, we formulate the problem of transceiver placement and the scheduling of RF links to maximize network capacity. To avoid the complexity of the ILP, we also propose a randomized greedy heuristic algorithm. In addition, simulated annealing is used to provide a baseline for comparison and to show the efficiency of our heuristic. We show by simulations that our heuristic, which can be implemented efficiently, achieves a high fraction of the optimal network capacity.

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Symbol	Description
$G$ , $G$ , $V$ , $E$	Graph, gateways, transceivers, links
$r_{u}$ , $r_{d}$	Traffic demand: uplink, downlink
$e_{t}$ , $e_{r}$	Transmitter, receiver node of link $e$
$S$ , $S (e)$	Schedule: matrix, vector for link $e$
$π (e)$	Avaiabilty of FSO link $e$
$T$ , $M$	Number of: time slots in a frame, FSO links
$R$ , $Λ_{u}$ , $Λ_{d}$	Network capacity, traffic flow: uplink, downlink
$E_{o} (v)$ , $E_{i} (v)$	Links from and to node $v$ : outgoing, incoming
$V_{o}$ , $V_{i}$ , $V_{o i}$	Link matrix: outgoing, incoming, outgoing–incoming
$c_{F}$ , ${\bar{c}}_{R}$	Link capacity: FSO, RF (effective time averaged)
$X (v, t)$ , $Y (v, t)$	Node $v$ status at time slot $t$ : transmitting, receiving
$I_{e}$ , $I$	Interfering links set of $e$ , interference matrix
$C_{R}$ , $C_{F}$	Nominal link capacity: RF, FSO
$p_{t} (v)$ , $p_{r} (v)$	Node $v$ status probability: transmitting, receiving
$σ$ , $\hat{σ}$	Time-averaged schedule vector: relaxed ILP, PGS
$α$ , $η$	Projection of $\hat{σ}$ on $σ$ , link probability update factor
${\bar{c}}_{R}$ , ${\bar{\hat{c}}}_{R}$	Time-averaged RF capacity: relaxed ILP, PGS
$ρ (t)$	Link activation probability at time slot $t$
$θ_{0}$ , $θ_{f i n}$ , $θ$ , $β$	Initial, final, current temp., decay factor (SA)
$A$ , $L$	The set of active links, remaining links
OPT, PGS, SA	Optimal Alg., rand greedy Alg., simulated annealing

$capacity = \max R$
Flow	$V_{i} (v) \cdot Λ_{u} = 0 V_{o} (v) \cdot Λ_{d} = 0$	$v \in G$
	$V_{o i} \cdot \frac{Λ_{u}}{r_{u}} = - V_{o i} \cdot \frac{Λ_{d}}{r_{d}} = R$	$v \in V ∖ G$
	$Λ_{u} + Λ_{d} \leq C_{F} S_{F} π_{F} + C_{R} (\frac{1}{T} S_{R} \cdot 1)$
Schedule	$V_{i} \cdot S_{R} = Y V_{o} \cdot S_{R} = X$
Schedule	$X + Y \leq 1 (I + \| E \|) \cdot S_{R} \leq \| E \|$
Location	$S_{F} \cdot 1 \leq M$

$UB = \max R$
Flow	$V_{i} (v) \cdot Λ_{u} = 0 V_{o} (v) \cdot Λ_{d} = 0$	$v \in G$
	$V_{o i} \cdot \frac{Λ_{u}}{r_{u}} = - V_{o i} \cdot \frac{Λ_{d}}{r_{d}} = R$	$v \in V ∖ G$
	$Λ_{u} + Λ_{d} \leq C_{F} S_{F} π_{F} + C_{R} σ$
Schedule	$V_{i} \cdot σ = p_{r} V_{o} \cdot σ = p_{t}$
Schedule	$p_{r} + p_{t} \leq 1 (I + \| E \|) \cdot σ \leq \| E \|$
Location	$S_{F} \cdot 1 \leq M$

Method	OPT	PGS	SA
$T = 10$	$3.536 \times 10^{4}$	NA	NA
$T = 20$	$8.121 \times 10^{4}$	$1.142 \times 10^{2}$	$1.519 \times 10^{2}$
$T = 50$	NA	$2.345 \times 10^{2}$	$3.211 \times 10^{2}$

Symbol	Description
$G$ , $G$ , $V$ , $E$	Graph, gateways, transceivers, links
$r_{u}$ , $r_{d}$	Traffic demand: uplink, downlink
$e_{t}$ , $e_{r}$	Transmitter, receiver node of link $e$
$S$ , $S (e)$	Schedule: matrix, vector for link $e$
$π (e)$	Avaiabilty of FSO link $e$
$T$ , $M$	Number of: time slots in a frame, FSO links
$R$ , $Λ_{u}$ , $Λ_{d}$	Network capacity, traffic flow: uplink, downlink
$E_{o} (v)$ , $E_{i} (v)$	Links from and to node $v$ : outgoing, incoming
$V_{o}$ , $V_{i}$ , $V_{o i}$	Link matrix: outgoing, incoming, outgoing–incoming
$c_{F}$ , ${\bar{c}}_{R}$	Link capacity: FSO, RF (effective time averaged)
$X (v, t)$ , $Y (v, t)$	Node $v$ status at time slot $t$ : transmitting, receiving
$I_{e}$ , $I$	Interfering links set of $e$ , interference matrix
$C_{R}$ , $C_{F}$	Nominal link capacity: RF, FSO
$p_{t} (v)$ , $p_{r} (v)$	Node $v$ status probability: transmitting, receiving
$σ$ , $\hat{σ}$	Time-averaged schedule vector: relaxed ILP, PGS
$α$ , $η$	Projection of $\hat{σ}$ on $σ$ , link probability update factor
${\bar{c}}_{R}$ , ${\bar{\hat{c}}}_{R}$	Time-averaged RF capacity: relaxed ILP, PGS
$ρ (t)$	Link activation probability at time slot $t$
$θ_{0}$ , $θ_{f i n}$ , $θ$ , $β$	Initial, final, current temp., decay factor (SA)
$A$ , $L$	The set of active links, remaining links
OPT, PGS, SA	Optimal Alg., rand greedy Alg., simulated annealing

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Equations (14)

Journal of Optical Communications and Networking