The overriding purpose of this research work is to address network sustainability, a key design issue for energy-constrained wireless networks and to reduce the share of wireless in the global carbon footprint brought about by the growing popularity of wireless applications. Simple broadcast protocols with ‘user-defined’ Transmission Thresholds that are based on a form of cooperative transmission called the Opportunistic Large Array (OLA), namely, OLA with Transmission Threshold (OLA-T) and Alternating OLA-T (A-OLA-T) are key contributions of this research. The use of transmission thresholds is an important foundation for future research in wireless sensor networks that do cooperative transmission.
While OLA-T saves energy by limiting node participating in each hop, A-OLA-T balances the load of broadcasting by performing consecutive OLA-T broadcasts using mutually exclusive sets of nodes. Being OLA-based protocols, OLA-T and A-OLA-T offer benefits of spatial diversity and an signal-to-noise ratio (SNR) advantage at the receiver. Thus, the transmit power per sensor is lowered, but the collective signal is strong enough and has enough diversity to enable its reception at a relatively long distance away by the sink node. Necessary and sufficient conditions for sustained network broadcast and performance bounds in terms of system parameters have been derived for two network topologies, namely, the disc- (maximum energy-spill) and the strip-shaped (minimum energy-spill) networks.
Some Key Results:
In Basic OLA , all the nodes in a decoding level form an OLA, which in turn generates the next decoding level. OLA-T  and A-OLA-T  extend Basic OLA  through the introduction of the “Transmission Threshold.” A node that successfully decodes the message (e.g. by passing a cyclic redundancy check (CRC)), compares its received SNR to this threshold and relays only if its SNR is less than the threshold; the nodes that relay are in the best position to participate in the next OLA transmission. By self-scheduling their transmissions using the thresholds, nodes can save significant energy. The OLA-T algorithm has been described in , and when compared to Basic OLA, OLA-T saves up to 32% of the transmitted energy by limiting the number of nodes in each OLA for disc-shaped networks. Figs. 1 (a) and (b) illustrate node consumption during a successful broadcast using Basic OLA and OLA-T, respectively for disc-shaped networks.
The alternating broadcast scheme resulting from the research work, A-OLA-T , is a modified version of OLA-T that optimizes multiple, consecutive OLA-T broadcasts, so that different sets of nodes relay in each broadcast and eventually all nodes relay the same number of times. Since A-OLA-T drains the batteries efficiently and uniformly across the network, it is most appropriate for static networks. Unlike OLA-T and other OLA-based schemes, A-OLA-T optimizes groups of broadcasts instead of a single broadcast. The Transmission Threshold is used to minimize the OLA sizes while maintaining mutually exclusive sets of OLAs on consecutive broadcasts. An illustration of the two-set A-OLA-T for a disc-shaped network is shown in Fig. 2.
In , it is shown that A-OLA-T with two sets extends the network life by a maximum of 17% relative to the Basic OLA when both protocols are operated in their minimum energy configuration. Further, when two-set A-OLA-T is compared to OLA-T, the battery-life of the nodes is doubled. A-OLA-T with m sets, m>>1 extends the network life by a maximum of 44% relative to the Basic OLA when both protocols are operated in their minimum energy configuration .
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 A. Kailas, L. Thanayankizil, and M. A. Ingram, “A simple cooperative transmission protocol for energy-efficient broadcasting over multi-hop wireless networks,” KICS/IEEE Journal of Communications and Networks (Special Issue on Wireless Cooperative Transmission and Its Applications), vol. 10, no. 2, pp. 213-220, June 2008.
 A. Kailas and M. A. Ingram, “Alternating opportunistic large arrays in broadcasting for network lifetime extension,” IEEE Transactions on Wireless Communications, vol. 6, no. 8, pp. 2831-2835, June 2009.
 L. Thanayankizil, A. Kailas, and M. A. Ingram, “Routing protocols for wireless sensor networks that have an opportunistic large array (OLA) physical layer,” Ad-Hoc & Sensor Wireless Networks, vol. 8, pp. 79-117, 2009.
- Aravind Kailas and Mary Ann Ingram, “OLA With Transmission Threshold for Strip Networks”, Military Communications Conference (MILCOM), Boston, MA, October 18-21, 2009. View PDF (1.1MB)
- Aravind Kailas and Mary Ann Ingram, “Investigating Multiple Alternating Cooperative Broadcasts to Enhance Network Longevity,” IEEE International Conference on Communications (ICC), Dresden, Germany, June 2009. View PDF (664 KB)
- Aravind Kailas and Mary Ann Ingram, “Alternating Opportunistic Large Arrays in Broadcasting for Network Lifetime Extension,” IEEE Transactions on Wireless Communication, Vol. 8, No. 6, pp. 2831-2835, June 2009. View PDF (327 KB)
- A. Kailas and M.A. Ingram, “Alternating Cooperative Transmission for Energy-Efficient Broadcasting,” Global Communications Conference (GLOBECOM), December 2008. View PDF (349kb)
- A. Kailas, L. Thanayankizil, and M.A. Ingram, “A simple cooperative transmission protocol for energy-efficient broadcasting over multi-hop wireless networks,”KICS/IEEE Journal of Communication and Networks, Special Issue on Wireless Cooperative Transmission and Its Applications, vol. 10, no. 2, pp. 213-220,July 2008. View PDF (204 kb)
- A. Kailas, L. V. Thanayankizil, and M.A. Ingram, “Power allocation and self-scheduling for cooperative transmission using opportunistic large arrays,” Military Communications Conference (MILCOM), October 2007. View PDF (1.1 MB)