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We investigate capacity and achievable rates for transmitter cooperation schemes in ad-hoc wireless networks. In addition to cooperative dirty paper coding, we propose two new cooperative transmission techniques: time-division successive broadcasting and time-division relaying. We show that transmitter cooperation can significantly increase capacity, even if one of the cooperating nodes is halfway between the transmit and receive node clusters. However, the best form of cooperation depends on the relative geometry of the transmit and receive clusters. When the transmitters are close together, cooperative dirty paper coding achieves the highest rates. However, if one of the transmitters is relatively close to the receive cluster, cooperative broadcasting or relaying achieves higher rates than dirty paper coding. That is because, at large separations, the exchange of messages between the transmitters required for dirty paper coding consumes a substantial amount of power. We show that in most cases transmitter cooperation provides a substantial capacity improvement over noncooperative techniques, especially under an equal rate constraint.