## 1 Introduction  
In this paper, we present an energy-efficient Rate-Adaptive MAC (RA-MAC) algorithm for long-lived WSNs [1; 2; 3]. The lifetime of a long-lived WSN is typically expected to be on the order of years with node duty cycles less than 1% (e.g., less than 1 sample per minute). (An earlier version of this article appeared as a poster abstract in Proceedings of the 6 th European Conference on Wireless Sensor Networks (EWSN 09) [4]. This article features comprehensive design, analysis, implementation, and evaluation of proposed Rate Adaptive Media Access Protocol.) Performance of radio communication links, in terms of Packet Reception Rate (PRR) for example, are critical to the overall performance of wireless sensor networks, both in terms of network data throughput and energy consumption. Higher layers of the network stack are designed to maximally utilize the communications link and thus require an understanding of link characteristics. A long-term water monitoring network [3] of 8 nodes monitors ground water extraction and salinity in a sugar cane growing region of north-eastern Australia. The network is unusual in that it has long radio links -- in the range from 1 to 1.8 km. In late summer, researchers observed that many of the radio links go down at night, regularly. They observed no direct correlation with local climatic conditions such as precipitation or humidity measured at a nearby weather station, and speculate the problem is due to a layer of moist air above the fully grown cane which is trapping or reflecting the radio waves. In an independent study [5], detailed work shows that the transmission range of Mica2 motes can be reduced by a factor of 5 in high humidity conditions, including fog. [PERSON] and [PERSON] [6] take a theoretical approach to the problem. They model the radio channel, the radio itself, and the noise floor, and show the existence of a transition region, also referred to as a \"gray area\" [7], characterised by highly variable packet reception rates. They show how the transition region limits can be determined, and that these limits are a function of the received Signal-to-Noise Ratio (SNR). Traditional approaches to compensate for poor link quality in WSNs use sophisticated MAC and routing protocols with acknowledgements and retransmissions. However, these techniques introduce extra traffic, increased energy consumption, and increase the size and complexity of the node's software. There has been much less work focused on addressing the underlying link quality. Well known approaches in the communications field include: relay nodes, higher transmit power, delay tolerant routing, frequency diversity, antenna diversity, and spread spectrum modulation techniques. All have difficulties or cost implications. Relay nodes allow for shorter links with higher quality, but are expensive. For example, the transmission range of the Mica2 motes can be reduced by a factor of 5 in high humidity conditions [5]. To compensate for this using relay nodes would require \(5\times 5=25\) times as many nodes to be deployedover the same area. Along with the extra cost of the hardware involved, it has been noted (e.g., [2]) that multi-hop transmissions may be more energy expensive compared with single-hop transmissions. Transmission power is limited by regulation and most nodes already operate at the maximum permissible. Delay tolerant architectures can buffer data until the link quality improves but at the expense of increased latency and energy issues (perhaps extra flash read/write operations [2]). Increasing the channel gain can be achieved by larger, or more directional antennas. However, large antennas go against the ultimate goals of wireless sensor networks, in general, and their increased cost can introduce scalability issues. Moreover, directional antennas do not suit many network topologies. Coding and data bit rates are the _only remaining degrees_ of freedom in this problem.