User Density in a Cluster Underwater Acoustic Network

It is well known that a Code Division Multiple Access (CDMA) scheme achieves higher user density in a cellular network with radio frequency signal in a terrestrial environment than either Time Division Multiple Access (TDMA) or (Frequency Division Multiple Access) FDMA could. The concept of a cellular (cluster) network has been introduced for underwater acoustic communications. However, its performance has not been investigated thoroughly. Due to the significant differences between the two propagation environments, the conclusions derived in terrestrial channel could not be extended to an underwater one. This paper investigates the user density difference among CDMA, TDMA and FDMA schemes in a cluster type acoustic network applied to an underwater environment. Close-form formulae are given for both ideal and realistic environments. We find that the user density of CDMA is primarily affected by intra-cluster and inter cluster interference while that of TDMA/FDMA is determined by the total available bandwidth. We also find that when spreading factor is 2, CDMA achieves higher user density than TDMA/FDMA if the required receiving signal quality is below a certain level and guarding time/frequency has to be added into TDMA/FDMA in real underwater environment. This research gives one theoretical direction to design future topology and protocols for underwater acoustic networks. This work is supported by the Office of Naval Research