tailieunhanh - Topology Control in Wireless Ad Hoc and Sensor Networks phần 3

kể, sự quan tâm của cộng đồng khoa học và công nghiệp trong lĩnh vực viễn thông gần đây đã chuyển sang kịch bản thách thức trong đó một nhóm các đơn vị điện thoại di động được trang bị với các đài phát thanh thu phát giao tiếp mà không có bất kỳ cố định cơ sở hạ tầng. | 28 TOPOLOGY CONTROL Figure The case for multihop communication node u must send a packet to v which is at distance d using the intermediate node w to relay u s packet is preferable from the energy consumption s point of view. Since w e C implies that cos Y 0 we have that d2 d2 d22. It follows that from the energy-consumption point of view it is better to communicate using short multihop paths between the sender and the receiver. The observation above gives the first argument in favor of topology control instead of using a long energy-inefficient edge communication can take place along a multihop path composed of short edges that connects the two endpoints of the long edge. The goal of topology control is to identify and remove these energy-inefficient edges from the communication graph. Topology control and network capacity Contrary to the case of wired point-to-point channels wireless communications use a shared medium the radio channel. The use of a shared communication medium implies that particular care must be paid to avoid that concurrent wireless transmissions corrupt each other. A typical conflicting scenario is depicted in Figure node u is transmitting a packet to node v using a certain transmit power P at the same time node w is sending a packet to node z using the same power P. Since 8 v w d2 8 v u d1 the power of the interfering signal received by v is higher than that of the intended transmission from u 1 and the reception of the packet sent by u is corrupted. Note that the amount of interference between concurrent transmissions is strictly related to the power used to transmit the messages. We clarify this important point with an example. Assume that node u must send a message to node v which is experiencing a certain interference level I from other concurrent radio communications. For simplicity we treat I as a received power level and we assume that a packet sent to v can be correctly received only if the intensity of the received .