Mobility Aspects in WSN - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 133

In sparse settings, the multi-hop transmission can fail when two nodes that need to
transfer the message to the sink are separated more than their transmission range.
The second approach is based on the principle of Opportunistic Large Arrays
(OLA), introduced in [ 48 ]. OLA is a form of cooperative diversity in which a large
group of simple, inexpensive relays operate without any mutual coordination, but
naturally transmit at approximately the same time in response to energy received
from the single source (or another OLA). In the wave propagation cooperative
transmission approach, each node will repeat a received message once. It will do
this together with all other nodes who have received the same message at the same
time. Thus, the message will propagate through the network like a wave front. The
transmission can terminate before it reaches the sink, if after the first steps, only
few nodes are included in the cooperative transmission, and the distance to the next
nodes is large, thus unachievable.
Accumulative cooperative transmission is a modification of the previous
approach, which goes beyond the previous problem. Here, the nodes that have
received a message will transmit the message several times (number of retransmis-
sions will be set as a system parameter), thus the probability that the message
reaches the sink will be also increased.
The fourth approach proposes alternating the communication between multi-
hop and accumulative cooperative transmission. After the cooperative transmis-
sion, all “new” nodes will try to acquire further partners using multi-hop and, after
this, a new accumulated cooperative transmission will take place with the now
larger group.
The overall coverage can be improved with this mechanism which allows lost
nodes or clusters to establish a connection in a new way, not possible without coop-
erative transmission. Additionally, for the delivery and relaying of packets, it is not
necessary to keep track of connections and paths. Nodes will simply repeat a mes-
sage several times after reception.

6.5 Conclusion

This chapter reviews the most recent proposals utilizing sensor nodes mobility and
data dissemination techniques in WSNs. The mobility of the sensor nodes is an
important aspect in the future WSNs as it may improve the monitoring capabilities
and coverage, enhance communication, and enable new applications. The mobility
of network components can deliver results that are greatly beneficial to the network
performances.
Research on mobility in WSNs evokes a lot of open issues. They comprise
designing novel algorithms and protocols, providing adequate performance metrics
and performance analysis and designing new and appropriate mobility models.
Additional complexity is introduced by allowing different mobility models to have
different influence on the performance metrics. Therefore, the research community
is actively working toward these challenges.