# Enhancing Reliability of Tactical MANETs by Improving Routing Decisions
## 1 Introduction  
In the early 1970s, the first generation of MANETs called packet radio network (PRNET) were designed to provide reliable coverage, especially for voice services [1]. MANETs then gradually evolved to focus on improving their range, survivability and security. Tactical MANETs have become the core technology in digitalized battlefields. The distributed nature of application and connectivity management in MANETs make these networks to be more robust than their centralized counterparts. However, tactical MANETs are usually deployed in environments and conditions which are harsh for radio propagation while they have to cope with the relatively large mobility of network nodes.  
Link quality is one of the main concerns to achieve reliable communications in wireless networks. Typical non-line of sight (NLOS) multipath propagation with shadowing and fading may disrupt all connectivity in infrastructure-less wireless networks, whereas LOS connectivity may still suffer significantly from path loss attenuations. Maintaining sufficient link quality is critical for achieving an effective transmission range of terrestrial radios [2]. Long range military radios operating in very/ultra high frequency (VHF/UHF) bands require a minimum level of SINR in order to achieve the target bit error rate (BER) or packet error rate (PER). Typical minimum SINR values for the target link reliability range from 6 to 23 dBm [3]. Furthermore, tactical communication networks usually exploit link margins as large as 40 dB in order to guarantee connectivity at all times [4]. The minimum required SINR or RSS values are determined by the receiver sensitivity. Unfortunately, sensitivity and selectivity of legacy VHF and UHF receivers is usually poor while also having limited data rates. Consequently, shorter range LOS radios are now being developed for the UHF band 1350-2690 MHz and a new4.4-5 GHz band. This enables having links with much higher capacity and enough gain from small size antennas.  
Modern tactical MANETs carry mostly broadcast and multicast traffic with small proportion of unicast transmissions. Availability of reliable and efficient packet flooding with small protocol overhead is one of the most fundamental services provided in these networks. Several flooding algorithms have been introduced in the literature for general MANETs. Among these algorithms, reactive routing protocols can offer flooding as a route discovery process when searching for feasible routes. The main drawback of such a mechanism is excessive flooding, which can lead to network clogging. A variety of flooding optimization techniques have been proposed to reduce the overhead of route discovery [5]. However, these solutions do not scale well to embrace large scale networks which are often encountered in tactical MANETs. An alternative approach to overcome scaling problem relies on clustering protocols. Hierarchical routing protocols can reduce the impact of excessive or unnecessarily flooding [6]. Proactive routing protocols utilize flooding to advertise routing information. In general, network flooding is a rather common practice in table driven routing protocols where the neighbor information is gathered and propagated throughout the network [7].  
The rest of this paper is organized as follows. Section 2 addresses the issue of connectivity in mobile tactical networks. In Section 3, suitable metrics to make routing decisions are identified, and how these metrics are selected and used is discussed. Section 4 presents a new routing protocol referred to as LQA-AODV. The performance of this protocol is assessed by extensive computer simulations in Section 5. Finally, Section 6 concludes the paper.