Wireless Network Cocast: Location-Aware Cooperative Communications with Linear Network Coding

This technique enables power distribution and transmission in uniformly distributed networks.

In wireless networks, reducing aggregate transmit power and having even power distribution increase the network lifetime. The conventional direct transmission (DTX) scheme results in high aggregate transmit power and uneven power distribution. In conventional DTX, where mobile units directly transmit their information to a common destination, the distant mobile units require more transmit power to provide a comparable quality of service (QoS) to that of the closer ones. Consequently, high aggregate transmit power (the sum of all transmit power of individual mobile units) and uneven power distribution among the units exist in the network. These two issues result in low network lifetime, which is defined as the time until the first mobile unit dies. It is wellknown that diversity techniques such as time diversity, frequency diversity, and spatial diversity result in reduction of transmit power and thus can be used to improve network lifetime. Three location- aware cooperation-based schemes considered in this work are immediateneighbor cooperation (INC), maximal cooperation (MAX), and wireless network cocast (WNC) that achieve spatial diversity to reduce aggregate transmit power and even power distribution. Cooperative communication makes use of the broadcast nature of wireless transmission. Nodes in a network acting as relays can retransmit overheard information to a destination, where the intended information from the source signal and the relay signals is jointly detected. The distributed antennas among the relays are used to provide spatial diversity without the need to use multiple antennas at the source. Various cooperative diversity protocols have been proposed and analyzed. In decodeand- forward (DAF) protocol, each relay decodes the overheard information from the source, re-encodes it, and then forwards it to the destination. In amplify- and-forward (AAF) protocol, each relay simply amplifies the overheard signal and forwards it to the destination.

Practical networks are asymmetric in nature. Node distances to a common destination vary based on node locations, and thus some nodes are disadvantageous in their transmission in comparison with others. Therefore, the node locations, which can be obtained using network-aided position techniques, should be taken into consideration to improve network performance. The first proposed scheme, immediateneighbor cooperation (INC), utilizes two-user cooperative communication in a network. In INC, each mobile unit, except the closest node to the destination, is assigned a single relay — its immediate neighbor toward the destination — and thus a fixed diversity order of two is achieved. Consequently, INC achieves good reduction in aggregate transmit power with the expense of (2N - 1) time slots for a network of N mobile units. However, distant users still require more power than the closer ones and power distribution is still uneven as in DTX.