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 well known 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 immediate-neighbor 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 decode-and-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, immediate-neighbor 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.

The fundamental cause of high aggregate transmit power and uneven power distribution attributes to the dependency of transmit power on the distance between the source and the destination. Therefore, incremental diversity, a measure of diversity order of mobile units that varies incrementally in terms of node location, should be leveraged in a network to provide high diversity orders for distant units to compensate the high required transmit power. The second proposed scheme, maximal cooperation (MAX), provides incremental diversity to a network by means of cooperative communication. Multi-node cooperative communication is utilized in MAX, where each mobile unit is assigned a group of mobile units locating between itself and the destination as relays. Thus the more distant the mobile unit, the higher diversity order to compensate the high required transmit power. Furthermore, the higher transmit power is shared and compensated by the larger group of relaying mobile units. Consequently, MAX with the incremental diversity achieves great reduction of aggregate transmit power and even power distribution.

The major drawback in MAX is the large transmission delay since each relay requires a time slot for its transmission. For a network of N mobile units, MAX incurs a delay of N(N + 1)/2 time slots, which grows quadratically with the network size, defined as the number of mobile units N. Therefore, a novel concept of wireless network cocast (WNC) was introduced to resolve the weaknesses of INC and MAX. Cocast, an analogy to broadcast, unicast, and multicast, is a newly defined transmission method that utilizes linear network coding for a number of nodes to cooperatively transmit their information to the same intended destination. WNC achieves the incremental diversity as in MAX with the low transmission delay of (2N - 1) time slots as in INC. Both DAF and AAF protocols in cooperative communication are considered in WNC, where mobile units acting as relays form unique linearly coded signals from a set of overheard symbols of different sources and transmit them to the destination. At the destination, a multiuser detection technique jointly detects the intended symbols from all received signals in the network.

INC utilizes two-user cooperative communication, resulting in good reduction of aggregate transmit power and low transmission delay; however, power distribution is still uneven. MAX utilizes multi-node cooperative communication, providing incremental diversity to achieve even power distribution and substantial reduction in aggregate transmit power. Transmission delay in MAX, however, grows quadratically with network sizes. As a result, the novel WNC is proposed to achieve incremental diversity

as in MAX and low transmission delay as in INC. Performance evaluation in uniformly distributed networks shows that INC, MAX, and WNC substantially reduce aggregate transmit power, while MAX and WNC also provide even power distribution.

This work was done by Ahmed S. Ibrahim and K. J. Ray Liu of the University of Maryland, and Hung-Quoc Lai of the U.S. Army RDECOM CERDEC. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. ARL-0080


This Brief includes a Technical Support Package (TSP).
Wireless Network Cocast: Location-Aware Cooperative Communications with Linear Network Coding

(reference ARL-0080) is currently available for download from the TSP library.

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