This section aims to demonstrate the effects of source density on transmission current consumption. A distance of 50m shown in Figure 6.10 is used as it demonstrates the indoor range of [Tmote]. In total seven different sources are uniformly scattered over a line topology which spans a distance of 50m. Each source transmits its packet to the base station directly in the single hop scenario.
Figure 6.10: A 50m-distance line topology
Each source sends a packet which has to be received by all intermediate nodes in the case of multi- hop. The numbers of sources,n, are set to 1, 2, 5, 10, 20, 50 and 100. The corresponding densities are defined as the number of sources per metre; therefore the densities are 0.02, 0.04, 0.1, 0.2, 0.4, 1 and 2. Both the minimum current based upon the experimental results providing at least 95% PRR and maximum current based upon the full transmission power capability are calculated and shown in Table 6.3.
Table 6.3: Minimum current consumption required for transmissions at 7 source densities
No. of
Sources (nodes/m)Density Spacingd(m)
Single-hop Multi-hop
Min. Current
(mA) No. of
Transmissions
Min. Current (mA)
Total SourcePer By EachSource Total SourcePer
1 0.02 50 12.5 12.5 1 12.5 12.5 12.5 2 0.04 25 25 12.5 3 12.5 37.5 18.75 5 0.1 10 61.2 12.24 15 11.2 168 33.6 10 0.2 5 121.1 12.1 55 9.9 544.5 54.45 20 0.4 2.5 240.8 12.04 210 8.5 1,785 89.25 50 1 1 597.3 11.95 1275 8.5 10,837.5 216.75 100 2 0.5 1,193.3 11.93 5050 8.5 42,925 429.25
The required transmission power depends upon the distance between source and base station in the single-hop case. In multi-hop, it depends upon the distances between two neighbours. The total current consumption is equal to the summation of the required current for all sources. The current per source is the total divided by the number of sources. In the multi-hop case, the total number of transmissions is also given. The maximum current consumption is computed based upon 0 dBm power. According to Table 6.3, in the single-hop case, the total current consumption increases with the numbers of sources. The total current per source insignificantly decreases as a lower
transmission power can be used by the sources located closer to the base station. Therefore, the total current does not constantly increase with the number of sources. In the multi-hop case, the number of transmissions significantly increases with the number of hops. Figure 6.11 shows minimum and maximum current consumption of both single and multi-hop communications in the line topology.
The results demonstrate a significant benefit of direct communication over the multi-hop scenario. The benefit is higher for a denser network in comparison to the required message forwarding of a multi-hop network. However, a source may not be able to conduct direct communication at 50m. According to Table 6.2, a power setting of -7 dBm or lower can be used for a 10m range which means that 5 sources are required in the topology. Each source consumes approximately 12.24 mA compared to the 33.6 mA used by the multi-hop. Hence, almost two-thirds of the transmitting current can be conserved.
Figure 6.11: Minimum and maximum current consumption in single and multi-hop
6.4.5 Summary
This section aims at investigating feasible communication ranges of sensors and conducting comparative studies between single and multi-hop communications. The free space propagation model is analysed and a set of ideal communication ranges up to a kilometre is obtained. The results are validated by looking at the measurements in the previous chapter and associated literature. Logarithmic curve estimation gives the highest R-square and is therefore used to develop regression lines to estimate RSSI and range. A 96m indoor range can be obtained if the maximum power is set whilst a 10m range is obtained for the minimum power setting. Estimated indoor ranges for all feasible transmission power settings in CC2420 are obtained by linear regression analysis.
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A similar study is applied to the outdoor measurements in [SKPP07]. Up to a range of 450m is obtained for the maximum power. Much lower ranges are obtained in [LZZ+06]. This confirms the variability in signal propagation and so the measurement-based approach should be used to determine the current link quality. A -85 dBm RSSI which often produces the PRR of nearly 100% is also used to estimate indoor and outdoor communication ranges. A 38m indoor range can be obtained if the maximum power is set whilst a 2.5m range is obtained for the minimum power setting. Up to an outdoor range of 143m is obtained for the maximum power setting. Hence, direct communication is possible in a significant proportion of scenarios.
The distances between sources and their densities are varied in the comparative studies. Line topology demonstrates the data forwarding in the multi-hop communication. The current consumption per source increases with increasing distance between the sources for both single and multi-hop cases. According to the effect of densities, significant increases in the consumption are observed in the multi-hop when more sources are used for message forwarding. However, the current consumption in the single-hop slightly decreases.
The sources significantly benefit in the single-hop case when the source density is high. In the case where 5 sources are uniformly distributed over a 50m line topology, almost two-thirds of the transmitting current can be conserved if single-hop is used. More current will be saved in a higher density network as the number of message forwarding increases.