Future radio systems will make use of cooperative concepts between independent transmitters and receivers to increase the coverage area and the data rate, and/or to decrease the power consumption of the wireless communication network. Therefore, a station wishing to transmit cooperates with reachable stations in its vicinity to achieve a performance gain from the resulting distributed multiple antenna link. The resulting communication situation is related to the common multiple access channel. We consider the simple cooperative concept of amplify and forward, where the cooperating stations retransmit the amplified, received signal. In order to gain knowledge about the properties of the cooperative transmitter system, it is sensible to analyze its capacity. Furthermore, the amplification factors which maximize the system capacity with respect to the total transmission power of all transmit stations will lead to important system design information.