Discussion on Cluster Size

The simulation results in Subsection 5.4.3 and 5.4.4 are based on the cluster with ten nodes. To mitigate the limitation of the results, the effect of cluster size on the restoration error and network energy consumption is investigated below. Fig.5.9 shows the influence of cluster size on network energy consumption with different data aggregation algorithms.

1 2 3 4 5 6 7 8 9 10 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 Cluster Size

Total Energy Consumption (J)

None CR−PCA PR−PCA

Figure 5.9: Network energy consumption of algorithms with different cluster sizes.

It can be seen that the network energy consumption of PR-PCA decreases with the incre- ment in cluster size. The reason is that the sensor data can be further compressed when more sensor nodes within the same cluster. The network energy consumption is even higher than that of CR-PCA when the sensor nodes are not clustered.

The relative restoration error of algorithms with different cluster sizes is listed in Table 5.2. It can be seen that therreof PR-PCA is smaller than CR-PCA. Besides, the relative restoration error of PR-PCA increases when the cluster contains more sensor nodes.

Table 5.2: Relative Restoration Error of CR-PCA and PR-PCA

Cluster Size 1 4 7 10

CR-PCA 0.0598 0.1022 0.1166 0.2291

PR-PCA 0 0.0866 0.0928 0.1289

Based on the discussions on Fig.5.9 and Table 5.1, it can be concluded that in terms of the cluster size, there is a trade-offbetween the network energy cost and the restoration accuracy. In practical scenarios, the specific cluster size is determined by the application requirements.

5.5

Chapter Summary

In this chapter, a new recursive principal component analysis (R-PCA) model is proposed, which recursively updates the transformation basis. As compared to the conventional PCA model and EW-PCA model, the R-PCA model better adapts to the changes of wireless sensor networks and the R-PCA based fault detection method improves the fault detection accuracy. The R-PCA model based multivariate fault detection and data aggregation algorithm consid- ers the multiple physical parameters in WSNs and processes the data based on clusters. In comparison with the conventional local-based algorithm, the proposed algorithm decreases the restoration error and reduces the network energy consumption, because the correlation between sensor readings from neighbor nodes of the same physical parameter is stronger than that of different parameters from the same node.

Chapter 6

Conclusion and Future Work

6.1

Conclusion

In this thesis, three major challenges in wireless sensor networks were investigated in detail, namely, energy efficiency, data fault and data redundancy. In order to overcome these problems in WSNs, three algorithms were proposed in Chapter 3-5, respectively.

In Chapter 3, a novel sensor scheduling mechanism was proposed, aiming at improving the network energy efficiency. The basic principle behind this mechanism was reducing the data transmission based on the highly spatial correlation of sensor data. More specifically, all the sensor nodes within the network were clustered by the proposed adaptive DK-means algorithm based on the spatial correlation of sensor data first. Then the order and duration of sensor nodes working as cluster representatives were determined by the new sensor scheduling algorithm. Instead of all the sensor nodes within the network, only the cluster representative nodes gen- erated and transmitted sensor data at the meantime, so that the energy costed by sensing and transmitting were saved. Simulations conducted in OPNET proved that the proposed sensor scheduling algorithm reduced the energy cost, as compared to the baseline ZigBee protocol.

In order to detect the faulty data, a novel distributed fault detection algorithm based on temporal and spatial correlation of sensor data was proposed in Chapter 4. Since the physi- cal parameters changed continuously in nature, the normal range of the sensor measurements to be collected could be predicted by both its own historical observations and its neighbor sensor readings. Therefore, the abnormal sensor data could be detected by the dramatical

variance from the normal range. Besides, both the result of temporal detection and received signal strength indicator were used as weights in the spatial detection procedure, which further improved the detection accuracy. Simulations based on both practical and synthetic datasets showed that the proposed algorithm improved the detection accuracy indeed, as compared to the distributed fault detection algorithms in the literature.

The last contribution of this thesis was the reduction of the data redundancy. In Chapter 5, a recursive principal component analysis (R-PCA) based data aggregation algorithm was proposed. At the beginning, the R-PCA model was introduced based on the modification of basic PCA model so that the transformation basis could be recursively updated. Based on the analysis of data correlation, the proposed data aggregation algorithm was implemented along the cluster tree instead of the local nodes, since the correlation between the same physical measurements from neighbor nodes was stronger than that of different physical measurements from the same node. For this reason, sensor readings from leaf nodes were aggregated by R-PCA model at cluster head before being forwarded to the sink node. As compared to the conventional local data aggregation algorithm, the proposed algorithm improved the restoration accuracy and reduced the network energy consumption.

In summary, this thesis proposed several solutions to the urgent challenges in WSNs so that the network performance could be improved. With more efforts on performance enhancement in WSNs, the networks will be ubiquitously used soon.