| This dissertation addresses communications in the physical layer among wireless sensor networks, where a clock and an antenna are associated with one sensor. In this configuration, to achieve better performance, we can organize multiple sensors to communicate cooperatively. Research issues include synchronization, sensor motion, and local communication. In the scenario with multiple transmit antennas and one receive antenna in the AWGN channel, the coherent transmission approach adjusts the phase of transmit clocks so that signals combine coherently in the medium at the receive side. This requires phase-level synchronization, pre-compensation and its estimation. We propose a system to achieve this. The distribution of the phase offset for perfect combining is determined. We show the benefit of coherent combining, and the optimal power distribution between synchronization overhead and data transmission. We also devise a scheme to deal with motion of the receive antenna. In the analysis, we include the effects of the Doppler, forward and backward medium disturbances. The time correlation of the phase process associated with combined signals is explored. It is shown that coherent transmission is still beneficial.; When there are many stationary unsynchronized transmit antennas, the perceived channel depends on the phase relationships. We relate the channels having different phase relationships, and show the channel capacities are identical. The channel decomposition and the water-filling algorithm to achieve this capacity can be fixed as long as the phases of signals are compensated at baseband. The relationship between two channel decompositions with different phase statuses is also derived.; For the 3-node wireless network, we consider all communication power consumption. Initially, each information stream is divided into the relay-path and direct-path substreams. Two concepts are used in the derivation of achievable rate regions. Scenarios considered include: two sources and one common destination, one source and two destinations, and two sources and two destinations with four streams, for various normalized noise power conditions. Generally speaking, for a specific stream, if the third node is closer to the source, it should help transmission of the direct-path substream using coherent combining. |
