Link allocation, routing and scheduling of FSO augmented RF wireless mesh networks

Abstract

Wireless Mesh Networks (WMNs) are designed to provide broadband wireless services; however, capacities of WMNs are fundamentally limited in bandwidth because of the scarcity of RF spectral resources. A solution is to design a WMN using multiple wireless technologies. Since the optical spectrum remains unlicensed and under-utilized, we propose to augment RF WMNs with free space optical (FSO) systems. FSO links do not interfere with RF links and have higher capacity than RF links. In this paper, we study the throughput improvement of FSO-augmented RF WMNs. We address two questions. First, given a fixed number of FSO links, where should we install them to achieve the maximum improvement of network throughput for given traffic demands? Second, how do we route and schedule the flows in the hybrid FSO/RF network to achieve this throughput? We jointly formulate the above problems as one mixed integer linear programming problem. The results show that the throughput of the network increases dramatically by properly allocating FSO links. We also provide a computationally efficient algorithm to evaluate upper and lower bounds on the throughput of this hybrid FSO/RF network. The network administrator can use these bounds to determine the tradeoff between the number of FSO links (which are expensive) and the throughput improvement. I. INTRODUCTIONWireless mesh networks (WMNs) have emerged to meet the growing demand for broadband wireless services. However, the performance of WMNs is limited by the cluttered RF spectrum. Free space optical (FSO) technology, using the unlicensed optical spectrum, is an alternative to RF technology for broadband wireless networks. FSO links can provide much higher data rate than RF links and do not interfere with RF links in the network. Among various wireless mesh network architectures [1], we are interested in infrastructure WMNs, as illustrated in Figure 1. Such WMNs consist of mesh routers (MRs) and mesh clients (MCs). The MRs in this infrastructure WMN form a backbone for MCs and are generally stationary. Thus, FSO technology, difficult to use in mobile environments but well adapted to stationary systems, can be added to this network to improve the network performance. Two major questions addressed in this paper are: (1) given a fixed number of FSO links, what is the maximum throughput of the network achievable by adding those FSO links? (2) how do we achieve this throughput by properly allocating the FSO links and choosing routing and scheduling schemes?There have been several studies on hybrid FSO/RF networks using different models based on their particular applications.

Wireless Energy and Information Transmission in FSO and RF-FSO Links

“…An efficient method for increasing the system availability is to complement the FSO link with an additional RF link, creating a hybrid RF-FSO communication system. Here, along with joint transmission, e.g., [4], particular attention is paid to serial RF-FSO setups where the RF link acts as a backup when the FSO link is down, e.g., [5].…”

“…From another perspective, free-space optical (FSO) systems are powerful tools for transferring data over short distances [3]- [5]. FSO systems provide fiber-like data rates through the atmosphere using lasers or light emitting diodes (LEDs) with considerably lower cost, compared to fiber links, and without extensive installation needs such as digging.…”

“…For these reasons, and also because of the line-of-sight (LOS) propagation nature of the FSO link with narrow beams, it is an attractive candidate for wireless energy and information transmission setups, as recently considered in industry, e.g., [6]. Also, the combination of FSO and radio frequency (RF) links in a hybrid RF-FSO setup is considered as a strong candidate to improve the network outage probability because each link can compensate for the other link when it is down [4], [5].…”

On the performance of millimeter wave-based RF-FSO links with HARQ feedback

“…The performance of RF-FSO systems is studied in different papers, e.g., [1]- [3], where the RF and the FSO links are considered as separate links and the RF link acts as a backup when the FSO link is down. On the other hand, in [4]- [8] the RF and the FSO links are combined to improve the system performance.…”

Hybrid Free Space Optical and RF Wireless Communication

“…A possible solution is to combine our method with the rateless code described in [36]. Since the optical spectrum remains unlicensed and under-utilized, hybrid free space optics (FSO)/RF technology has been introduced in [29,64,30,31,32,33] to improve the network throughput. FSO links do not interfere with each other due to their narrow beams, nor with RF links, and typically have a higher capacity; thus, already deployed networks can be upgraded by installing FSO links without compromising the existing systems.…”