Research On Media Access Mechanism For Sensor Nodes Under WiFi Interference

In recent decades, we are seeing the wide applications of wireless sensor networks(WSNs), and more and more sensor products going into our daily life. For example, a large number of wireless sensor products have been used in medical monitoring, game control, and so on. Though WSNs bring much convenience to us, they usually suffer from serious collision caused by Wi Fi signal when the wireless sensors co-exist in the same inviroment with Wi Fi.Currently, many solutions have been proposed to improve the throughput of sensor nodes under Wi Fi interference. These solutions can be divided into two main categories: one is based on collision-avoidance idea, and another is based on collision-recovery idea. These solutions cannot efficiently utilize the fractional idle channel under the high dynamic distribution of Wi Fi interference, and their performance will highly deteriorate when the channel is full with high Wi Fi load. In order to protect the sensor packets from Wi Fi interference, collision-avoidance based schemes usually wasted more short free interval of channel. Though collision-recovery based schemes can utilize the short free interval of channel, they can not quickly adapt their overhead to the highly dynamic distribution of Wi Fi interference. Aiming at two cases of Wi Fi interference, i.e., highly dynamic distribution of Wi Fi interference and high Wi Fi interference load, this paper repectively proposes two channel access schemes for sensor nodes under Wi Fi interfence.For the first case of Wi Fi interference, we propose a realtime(<5ms) lightweight adaptive coding scheme compatible for sensor nodes. The main idea is to establish the realtime(100ms window size) model for the occupation of Wi Fi channel, which helpes sensor nodes to always adopt appropriate coding technology according to the current Wi Fi interference distribution.For the second case of Wi Fi interference, we study the anti-Wi Fi packet transmission of sensor nodes in view of bit error rate(BER) in physical layer. We find that, when setting lower transmission rate of sensor nodes, their channel bandwidth will become smaller, thus the power of Wi Fi interference within the channel bandwidth will decrease. Hence, signal of sensor nodes with low transmission rate can have high signal noise interference ratio(SNIR). Thus, with the low BER due to the high SNIR, the packet transmission of sensor nodes could survive from the high Wi Fi interference load. Our target is to find the optimal parameters in the packet transmission of sensor nodes(including transmission rate, modulation mode, packet length) when given the power of Wi Fi interference, so as to achieve the highest throughput of sensor nodes under the Wi Fi interference.This paper not only gives the theoretical analysis on the two proposed schemes, but also verifies the performance with a series of experiments. According to the experiment results, the proposed schemes can remarkably improve the throughput of sensor nodes under Wi Fi interference, thus being of high practical significance.