4.2 Impact of Transmission Rate and Forwarding Strategy on Throughput

The impact of the transmission rate on the throughput of OR is twofold. On the one hand, different rates have different transmission ranges, which lead to different neighborhood diversity. A high rate usually has short transmission range. In one hop, there are few neighbors around the transmitter, which presents low neighborhood diversity. A low rate is likely to have long transmission range and therefore achieves high neighborhood diversity. From the diversity point of view, a low rate may be better. On the other hand, although low rate brings the benefit of larger one-hop distance, which results in higher neighborhood diversity and fewer hop counts to reach the destination, it may still end up with a low effective end-to-end throughput because the low rate disadvantage may overwhelm all other benefits. It is nontrivial to decide which rate is indeed better.

We now use a simple example in Figure 4.3 to illustrate that transmitting at lower rate may achieve higher throughput than transmitting at higher rate for OR. In this example, we assume all the nodes operate on a common channel, but each node can transmit at two different rates R and R/2. We compare the throughput from source a to destination d when the source transmits the packets at the two different rates. Figure 4.3(a) shows the case when all the nodes transmit at rate R, and the packet delivery ratio on each link is 0.5. So the effective data rate on each link is 0.5R. There is no link from a to d because d is out of a's effective transmission range when a operates on rate R. Assume the four nodes are in the carrier sensing range of each other, so they cannot transmit at the same time. Assuming b and c are the forwarding candidates of a, and b has higher relay priority than c. Then link lac has effective forwarding rate of 0.25R. By using the formulations in Figure 4.1, we obtain an optimal transmitter schedule such that a, b and c are scheduled to transmit for a fraction of time 0.4, 0.4 and 0.2, respectively. So the maximum end-to-end throughput from a to d is 0.3R. While in Figure 4.3(b), when a is transmitting at a lower rate R/2, it can reach d directly with packet delivery ratio of 0.6. Additionally, we get higher packet deliver ratio from a to b and c as 0.8. In this case, the lower rate achieves longer effective transmission range and brings more spacial diversity chances. Assume d, b, and c are forwarding candidates of a, and with priority d > b > c. Similarly, we calculate the maximum throughput from a to d as 0.36R, which is 20% higher than the scenario in Figure 4.3(a) where the system operates on a single rate.

Figure 4.3 End-to-end throughput comparison at different transmission rates. Reproduced by permission of © 2008 IEEE.

Besides the inherent rate–distance, rate–diversity and rate–hop tradeoffs, which affect the throughput of OR, the forwarding strategy will also have an impact on the throughput. For example, different forwarding candidates may achieve very different throughput, and even for the same forwarding candidate set, different forwarding priority will also result in different throughput, etc. We refer readers to Zeng et al. (2007c) for detail analysis of the impact of forwarding strategy on the OR throughput.