Advances in Vehicular Networking Technologies 142 discover a route since the BU started broadcasting the routing information. End-to-end delays are determined using position reporting packets which are sent by the last unit, i.e. the ( h+1)th unit, to the BU for an h-hop network where h varies from 1 to 50. Note that the ( h+1)th unit only starts transmitting the position reporting packets once a route is found to remove the queuing effect due to route discovery. 0 10 20 30 40 50 60 0 5 10 15 20 25 30 35 40 45 Network Diameter (Number of hops) Number of Superframes Route Discovery Delay End-to-End Packet Delay Fig. 15. Route discovery and End-to-End Packet Delay Next, we will analyze the performance of the LAR route selection algorithm. For the analysis, we used a star topology as shown in Fig. 16. Each unit was stationary and spaced at an equidistant of 50m from its adjacent neighbors. BU0, BU1 and BU2 initiated the route construction simultaneously by broadcasting their position packets, and triggered neighboring units to transmit their positioning packets. In the star configuration, the unit at the center, MU0, received position packets from three different neighboring units, namely DU1, DU2 and MU1, see Fig. 16. Consequently, MU0 created three forward routes in its routing table. These routes are referred to as Route 1, Route 2 and Route 3, respectively, as shown in Fig. 16. The hop count of Route 1 and Route 2 is three hops while Route 3 is four. Since hop count is the primary routing metric, the routes with the least hop count would be selected by MU0. In this case, Route 1 and Route 2 were picked by the route selection algorithm of MU0. In the simulations, each unit broadcast position packets at a fixed interval of 4s. Hence, the traffic load was uniformly distributed across the network. In other words, none of the MUs or DUs were more congested than others. Therefore, the route An Ultra-Wideband (UWB) Ad Hoc Sensor Network for Real-time Indoor Localization of Emergency Responders 143 selection algorithm would arbitrarily choose between Route 1 and Route 2. The MU0 was set to transmit position reporting packet at time t = 50s after the BUs started the route construction. Simulation traces show that Route 2 was selected by MU0 for transporting its position reporting packets to BU1. And the end-to-end packet delay is approximately 2 superframes, which conforms to the 3-hop delay in Fig. 15. At t=100s, MU2 was set to send position reporting packets, which introduced extra traffic on to the network. MU2 used Route 2 for transporting its position reporting packets since Route 2 was the shortest. Fig. 17 shows the congestion level seen by MU0. BU 0 DU0 DU1 MU0 BU1 MU1 MU2 DU2 DU4 DU3 BU2 Route 1 Route 2 Ro ute 3 Fig. 16. Star Network Topology Fig. 17 depicts the position reporting packets received by BU0 and BU1. As shown in Fig. 17, initially MU0 selected Route 2 for transporting its position reporting packets until the time was approximately 110s, where it switched to Route 1. The switching occurred when MU0 detected the congestion level on Route 2 was increased to 3. The increase in congestion was caused by MU2 when it started transmitting its position reporting packets at t=100s. Due to congestion, some in-flight packets on Route 2 were experiencing excessive delays and arrived at BU1 later than packets sent on Route 1. The congestion level of both Route 1 and Route 2 continued to rise, and on Route 2, the congestion level reached the maximum at about 150s. When both MU0 and MU2 stopped transmitting position reporting packets at 250s, the congestion level did not drop until t = 350s for Route 2 and t = 410s for Route 1 because of a large number of packets already in the queue. At t = 350s, the congestion level Advances in Vehicular Networking Technologies 144 of Route 2 dropped to 5, which was the same as Route 1. At this point a route change occurred since MU0 selected Route 2 again. All the remaining packets in its queue were sent on Route 2. After time t = 450s, the congestion level of both MU0 and MU2 dropped sharply. 0 100 200 300 400 500 600 0 1 2 3 4 5 6 7 Time (s) Congestion Level Route 1 Route 2 Route 1 Selected Route 2 Selected Route 1 Selected Route 2 Selected Route Change (A) Route Change (C) Route Change (B) Fig. 17. Congestion Level 5. Related work This section reviews the MAC and routing protocols developed for UWB-based ad hoc sensor networks. 5.1 UWB-based MAC protocols for ad hoc sensor networks In the past few years, a number of MAC protocols have been proposed for UWB-based systems. (Legrand et al., 2003) and (Zhu & Fapojuwo, 2005) proposed a modified version of the IEEE 802.15.3 Wireless Personal Area Network (WPAN) MAC protocol, which rely on a centralized controller. These MAC protocols can provide guaranteed Quality of Service (QoS) but are difficult to scale. The WHYLESS.COM project (Cuomo et al., 2002) proposed a distributed UWB MAC, which supports QoS and is scalable but has high complexity. (Chu & Ganz, 2004) described a hybrid MAC for WPAN, which combines the advantages of both centralized and distributed protocols. The MAC protocol assumes that every node in a WPAN is one hop away from every other node. Consequently, the MAC is foreseen to face An Ultra-Wideband (UWB) Ad Hoc Sensor Network for Real-time Indoor Localization of Emergency Responders 145 scalability issues when operating in multi-hop scenarios. Furthermore, a separate control channel is used for signaling purposes, which increases the complexity and is not lightweight for low bit-rate channels. Ultra-Wideband MAC (U-MAC) (Jurdak et al., 2005) is a proactive and adaptive protocol. Similar to (Chu & Ganz, 2004), a separate signaling channel is needed for exchanging a node’s state information with its direct neighbors. (Broutis et al., 2007) and (Benedetto et al., 2005) outlined a multi-channel MAC in which communication between two nodes takes place on orthogonal channels. The complexity and overheads incurred by such a MAC protocol are higher than single-channel MAC protocols. (Merz et al., 2005) proposed a combined Physical and MAC layer for very low power UWB system. No separate control channel is needed. However, the signaling overheads incurred by the MAC can be significant for short data packets and low bit-rate channels. In summary, all of the above-mentioned MAC protocols were not designed for localization application in mind. The IEEE 802.15.4a standard (Karapistoli et al., 2010; IEEE 802.15.4a, 2007) specifies a Physical layer and a MAC layer which support localization. The IEEE 802.15.4a MAC supports two different modes of channel access: beacon-enabled and nonbeacon-enabled. The latter is suited for localization application. Unlike SOC-MAC, the nonbeacon-enabled mode of the IEEE 802.15.4a MAC is based on the classical Aloha scheme or the CSMA/CA scheme. 50 100 150 200 250 300 350 400 450 500 550 50 80 110 140 170 200 Time (s) Application Packet Sequence Number Received by BU0 (Route 1) Received by BU1 (Route 2) Route Change (A) Route Change (B) Delayed in-flight packets Fig. 18. Position Reporting Packets Advances in Vehicular Networking Technologies 146 5.2 Routing protocols for ad hoc sensor networks A large number of routing protocols, e.g. (Kulik et al., 2002; Intanagonwiwat et al., 2000; Schurgers & Srivastava, 2001; Shah & Rabaey, 2002; Lindsey & Raghavendra, 2002; Manjeshwar & Agarwal, 2001), have been developed for ad hoc sensor networks. Although the considered ILS is an ad hoc sensor network, it has some profound distinctions which mean existing ad hoc sensor routing protocols are not directly applicable. Firstly, sensor nodes are generally assumed to have very low mobility after deployment (Al-Karaki & Kamal, 2004) in comparison with ILS. Lastly, the relative size of ad hoc sensor networks is huge in the order from thousands to millions of nodes (Al-Karaki & Kamal, 2004) as compared to ILS. 6. Summary In this chapter, we described the SOC-MAC and LAR protocols that are tailored for indoor localization systems used to track emergency responders. The cross-layer approach is present in the protocol design in order to optimize bandwidth and battery-energy consumption. As a result, SOC-MAC is simple and self-organizing, which is composed of two phases, namely RA-TDMA and reserved TDMA. The former is for initial acquisition of time slots while the latter is for management and maintenance of time slots. In addition to simplicity, LAR is extremely lightweight. No dedicated routing packets are needed. Instead, routing information is carried in the network header of localization packets, which constitutes less than 1% of the total channel capacity. We validated and studied the performance of SOC-MAC and LAR by simulations under varying SOC-MAC and LAR parameters. 7. Acknowledgement The work was partially funded by the IST-004154 EUROPCOM project. 8. References Al-Karaki, J. N. & Kamal, A. E. (2004). Routing Techniques in Wireless Sensor Networks: A Survey, IEEE Wireless Communications Magazine, Vol. 11, No. 6 Benedetto, M G.; De Nardis, L.; Junk, M. & Giancola, G. (2005). (UWB) 2 : Uncoordinated, Wireless, Baseborn Medium Access for UWB Communication Networks, Mobile Networks and Applications (MONET), Vol. 10, No. 5 Broutis, I.; Krishnamurthy, S. V.; Faloutsos, M.; Molle, M. & Forester, J. R. (2007). Multiband Media Access Control in Impulse-based UWB Ad Hoc Networks, IEEE Transactions on Mobile Computing , Vol. 6, No. 4 Chu, Y. & Ganz, A. (2004). MAC Protocols for Multimedia Supporting UWB-based Wireless Networks, Proceedings of 1st Int’l Conference on Broadband Networks (BROADNETS) Cuomo, F.; Martello, C.; Baiocchi, A. & Fabrizio, C. (2002). Radio Resource for Ad Hoc Networking with UWB, IEEE Journal on Selected Areas in Communications, Vol. 20, No. 9 An Ultra-Wideband (UWB) Ad Hoc Sensor Network for Real-time Indoor Localization of Emergency Responders 147 Frazer, E. L. & Tee, D. (2004). A Comparison of UWB Technologies for Indoor Positioning as an Augmentation to GNSS, Proceedings of 2nd European Space Agency (ESA) Workshop on Satellite Navigation User Equipment Technologies (NAVITEC), Noordwijk, The Netherlands, 2004 Harmer, D. (2008). EUROPCOM: Ultra-WideBand Radio for Rescue Services, Proceedings of 2nd Int’l Workshop on Robotics for Risky Interventions and Surveillance of the Environment (RISE), Benicassim, Spain, 2008 Harmer, D., et al. (2008). EUROPCOM: Emergency Ultra-WideBand (UWB) Radio for Positioning and Communications, Proceedings of IEEE International Conference on Ultra-WideBand (ICUWB), 2008 Hofmann-Wellenhof, B.; Lichtenegger, H. & Wasle, E. (2008). GNSS – Global Navigation Satellite Systems: GPS, GLONASS, and more, Springer, Vienna IEEE 802.15.4a (2007). Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (WPANs) Intanagonwiwat, C.; Govindan, R. & Estrin, D. (2000). Directed Diffusion: a Scalable and Robust Communication Paradigm for Sensor Networks, Proceedings of ACM MobiCom, Boston, MA, 2000 Irahhauten, Z.; Janssen, G. J. M., Nikookar, H., Yaravoy, A. & Lighart, L. P. (2006). UWB Channel Measurements and Results for Office and Industrial Environments, Proceedings of Int’l Conference on Ultra-WideBand (ICUWB), MA, 2006 Jurdak, R.; Baldi, P. & Lopes, C. V. (2005). U-MAC: A Proactive and Adaptive UWB Medium Access Control Protocol, Wiley Wireless Communications and Mobile Computing, Vol. 5, No. 5 Karapistoli, E.; Pavlidou, F.; Gragopoulos, I. & Tsetsinas, I. (2010). An Overview of the IEEE 802.15.4a Standard, IEEE Communications Magazine, Vol. 48, No. 1 Kulik, J.; Heinzelman, W. R. & Balakrishnan, H. (2002). Negotiation-based Protocols for Disseminating Information in Wireless Sensor Networks, Wireless Networks, Vol. 8 Legrand, J.; Bucaille, I.; Hethuin, S.; De Nardis, L.; Giancola, G.; Di Benedetto, M.; Blazevic, L. & Rouzet, P. (2003). U.C.A.N.’s Ultra Wideband Medium Access Control Schemes, Proceedings of Int’l Workshop on Ultra Wideband Systems (IWUWBS), 2001 Lindsey, S. & Raghavendra, C. (2002). PEGASIS: Power-efficient Gathering in Sensor Information Systems, Proceedings of Aerospace Conference, 2002 Manjeshwar, A. & Agarwal, D. P. (2001). TEEN: a Routing Protocol for Enhanced Efficiency in Wireless Sensor Networks, 1st Int’l Workshop on Parallel and Distributed Computer Issues in Wireless Networks and Mobile Computing, 2001 Merz, R.; Widmer, J.; Le Boudec, J. Y. & Radunovic, B. (2005). A Joint PHY/MAC Architecture for Low Radiated Power TH-UWB Wireless Ad Hoc Networks, Wiley Wireless Communications and Mobile Computing, Vol. 5, No. 5 Mobility Framework, http://mobility-fw.sourceforge.net OMNeT++, http://www.omnetpp.org/ Rappaport, T. (2001). Wireless Communications, 2nd edition, Prentice Hall Schurgers, C. & Srivastava, (2001). Energy-efficient Routing in Wireless Sensor Networks, MILCOM Proceedings on Communications for Network-Centric Operations: Creating the Information Force, McLean, VA, 2001 Advances in Vehicular Networking Technologies 148 Shah, R. C. & Rabaey, J. (2002). Energy Aware Routing for Low Energy Ad Hoc Sensor Networks, Proceedings of WCNC, Orlando, FL, 2002 Zhu, J. & Fapojuwo, A. O. (2005). A Complementary Code-CDMA-based MAC Protocol for UWB WPAN System, EURASIP Journal on Wireless Communications and Networking, Vol. 2005, No. 2 8 Hybrid Access Techniques for Densely Populated Wireless Local Area Networks J. Alonso-Zárate 1 , C. Crespo 2 , Ch. Verikoukis 1 and L. Alonso 2 1 Centre Tecnològic de Telecomunicacions de Catalunya (CTTC),Castelldefels, Barcelona 2 Universitat Politècnica de Catalunya (UPC), Castelldefels, Barcelona Spain 1. Introduction The IEEE 802.11p Task Group has recently released a new standard for wireless access in vehicular environments (WAVE). It constitutes an amendment to the 802.11 for Wireless Local Area Networks (WLANs) to meet the requirements of applications related to road- safety involving inter- and intra-vehicle communications as well as communications from vehicle to the roadside infrastructure. Indeed, the importance of the targeted applications has forced authorities to allocate some dedicated bandwidth (nearby the 5.9GHz) to ensure the security of the communications. However, despite the suitability of this standard for use in high-speed vehicular communications, it is not possible to pass over the unprecedented market penetration of the popular 802.11 networks, the so-called WiFi networks. Before we can see a world where all the cars are equipped with 802.11p devices, current and near- future applications might probably run on the original 802.11. Moreover, interaction between humans and vehicles will probably be carried out by means of the 802.11, which is the standard that is flooding most of personal tech devices, such as laptops, mobile phones, gaming consoles, etc. Therefore, it is important to keep on working in the improvement of the 802.11 Standard for its use in, at least, some vehicular applications. This is the main motivation for this chapter, where we focus on the Medium Access Control (MAC) protocol of the 802.11 Standard, and we propose a simple mechanism to improve its performance in densely populated applications where it falls short to provide users with good service. Envisioned applications include those were a high number of vehicles and pedestrians coexist in a given area, such as for example, a crossing in a city where all the cars share information to coordinate the drive along the crossing and prevent accidents. Into more detail, the Distributed Coordination Function (DCF) is the mandatory access method defined in the widely spread IEEE 802.11 Standard for WLANs [1]. This access method is based on Carrier Sensing Multiple Access (CSMA), i.e., listen before transmit, in combination with a Binary Exponential Backoff (BEB) mechanism. An optional Collision Avoidance (CA) mechanism is also defined by which a handshake Request to Send (RTS) – Clear to Send (CTS) can be established between source and destination before the actual transmission of data. This CA mechanism aims at reducing the impact of the collisions of data packets and to combat the hidden terminal problem. The DCF can be executed in either ad hoc or infrastructure-based networks and is the only access method implemented in most commercial hardware. Despite the doubtless commercial success of the DCF, the simplicity Advances in Vehicular Networking Technologies 150 of a CSMA-based protocol comes at the cost of a trial-and-error approach where a transmission attempt can result in a collision if several users contend for the access to a common medium as the traffic load of the network increases. Therefore, those networks based on the 802.11 suffer from really low performance when either the number of users or the traffic load is high. In this chapter, we introduce the idea of combining the DCF with the Point Coordination Function (PCF), also defined in the 802.11 Standard, to overcome its limitations under heavy load conditions. The PCF is defined as an optional polling-based access method for infrastructure-based networks where there is no contention to get access to the channel and the access point (AP) polls the stations of the network to transmit data. Therefore, collisions of data packets can be completely avoided and the performance of the network can be boosted. The hybrid approach of combining distributed access with reservation or polling-based access has been already used in the context of infrastructure-based networks [2]-[6] combining static Time Division Multiplex Access (TDMA) with dynamic CSMA access. Most of these works propose different alternatives to use the empty slots of TDMA in the case that the user allocated to a given slot has no data to transmit. However, to the best knowledge of the authors, there are very few works in the literature dealing with this approach in a distributed manner, i.e., for ad hoc networks without infrastructure. This is the main motivation for the work presented in this chapter, where we define the Distributed Point Coordination Function (DPCF) as a hybrid combination of the distributed access of the DCF and the poll-based access of the PCF to achieve high performance in highly populated networks with heavy traffic load. Indeed, the work presented in this chapter has been motivated by the successful results presented in [7]. In that paper, a spontaneous, temporary, and dynamic clustering algorithm has been integrated with a high-performance infrastructure-based MAC protocol, the Distributed Queuing Collision Avoidance (DQCA) protocol, in order to extend its near-optimum performance to networks without infrastructure. Upon the conclusion of that work, we realized that the same approach could be applied to the IEEE 802.11 Standard access methods and thus be able to extend the high-performance of the PCF under heavy load conditions to the distributed environments where the DCF runs. We have observed that there are very few works dealing with the PCF, which can indeed potentially achieve better performance than the DCF under heavy traffic conditions. Some contributions related to the PCF improve the overall network performance through novel scheduling algorithms [8]-[12] or by designing new polling mechanisms that can reduce the overhead associated to the polling process [13]. However, there have been almost no efforts in extending the operation of the PCF to ad hoc networks in order to provide them with some degree of QoS. The only exception can be found in [14] where a virtual infrastructure is created into a MAC protocol called Mobile Point Coordinator MAC (MPC-MAC) in order to achieve QoS delivery and priority access for real time traffic in ad hoc networks maintaining both the PCF and the DCF. In summary, a clustering based mechanism is used to achieve the correct operation of the PCF in a distributed environment. The duration of the PCF and DCF periods and the criterion upon which a terminal is chosen to be the MPC (acting as AP) are fixed and they are determined by the MAC protocol configuration. This approach works well in low dynamic environments where the topology does not vary frequently. In this situation the overhead associated to the “hello” messages required for the clustering mechanism can be kept to a minimum. However, it may not be convenient for spontaneous and highly dynamic environments, such as those present in some vehicular [...]... as in the previous section for the single-hop evaluation  162 Advances in Vehicular Networking Technologies 10 DPCF Throughput to destination (Mbps) 9 DCF 8 7 80% 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 Total Offered Traffic Load (Mbps) 9 10 11 12 Fig 13 Throughput to Destination of DPCF in a Multi-hop Network Average Packet Transmission Delay (ms) 300 270 240 210 180 150 120 90 60 DPCF 30 DCF 0 1 2 3 4 5 6 7... scheme is a particular case of the NAF scheme in which the source does not transmit simultaneously with the relay in the second slot (i.e.,h2,2 = 0) 170 Advances in Vehicular Networking Technologies Modulation and Coding Combined with the Hybrid Cooperation protocol proposed in (E Calvanese Strinati and S Yang and J-C Belfiore, 2007; E Calvanese Strinati and Luc Maret, 2008) In a third part of the section,... implicit mechanism to provide with some incentive to stations to become master despite the extra actions they must carry out and the corresponding increase in energy consumption Probability of transmitting when being polled 1,1 Stations in PCF network Stations in DPCF network AP in PCF network 1 0,9 0,8 0,7 0 ,6 0,5 0,4 0,3 0,2 0,1 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 Total Offered Traffic... of Transmitting when Being Polled in a Single-hop Network Average Packet Transmission Delay (ms) 200 180 160 140 120 100 80 60 40 DCF PCF DPCF 20 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Total Offered Traffic Load (Mbps) Fig 11 Average Packet Transmission Delay in a Single-hop Network 28 30 32 34 Hybrid Access Techniques for Densely Populated Wireless Local Area Networks 161 The performance in terms of... operate on distributed infrastructureless wireless ad hoc networks 154 Advances in Vehicular Networking Technologies As already mentioned before, the main idea is to use the DCF to create spontaneous and temporary clusters wherein the PCF can be executed, having a station acting as the AP for the life time of each cluster We consider a set of terminals equipped with WLAN cards forming a spontaneous ad... QoS Using IEEE 802.11 PCF, in Proc of the IEEE INMIC’05, pp.1 – 6, Dec 2005 [10] D Ping, J Holliday, A Celik, Dynamic scheduling of PCF traffic in an unstable wireless LAN, in proc of the CCNC 2005, pp 445 – 450, Jan 2005 [11] K Byung-Seo, K Sung Won, W Yuguang Fang, Two-step multipolling MAC protocol for wireless LANs, IEEE Journal on Selected Areas in Communications, vol 23, no 6, pp 12 76 – 12 86, Jun... so-called cooperative diversity (E Erkip A Sendonaris and B Aazhang: Part I, 2003; E Erkip A Sendonaris and B Aazhang: Part II, 2003) techniques where a source terminal cooperates with several relays to exploited the spatial diversity in a distributed manner From a physical 166 Advances in Vehicular Networking Technologies layer viewpoint, cooperation drives to improved transmission diversity and consequent... periodically, the unbalanced access of the AP in the PCF network is shared in the DPCF network Every time a station is set to master it can transmit all its backlogged data packets and thus take advantage of the prioritized access to empty its data buffers while operating as master Indeed, the fact that a station 160 Advances in Vehicular Networking Technologies operating in master mode has more channel access... after a Short Inter Frame Space (SIFS) This SIFS is necessary to compensate propagation delays and radio transceivers turn around times to switch from receiving to transmitting mode It is worth noting that since a SIFS is shorter than a DIFS, acknowledgments have more priority than regular data traffic 152 Advances in Vehicular Networking Technologies DIFS SIFS DIFS DATA Source CW ACK Destination Others... presented in this chapter are rather promising and, in fact, future work will be aimed at theoretically evaluating and optimizing the design of DPCF and at implementing the protocol in a testbed to evaluate its actual performance in a real environment Ongoing work is being carried out to evaluate the coexistence feasibility of this new approach with legacy implemented networks based on the 802.11  164 Advances . Advances in Vehicular Networking Technologies 142 discover a route since the BU started broadcasting the routing information. End-to-end delays are determined using position reporting. Change (B) Delayed in- flight packets Fig. 18. Position Reporting Packets Advances in Vehicular Networking Technologies 1 46 5.2 Routing protocols for ad hoc sensor networks A large number of routing. Routing in Wireless Sensor Networks, MILCOM Proceedings on Communications for Network-Centric Operations: Creating the Information Force, McLean, VA, 2001 Advances in Vehicular Networking Technologies