Multihop Wireless Networks: Opportunistic Routing

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 ...