In this paper, we consider the downlink of multipoint joint transmission in a multi-cell environment, where all of the base stations involved will be subject to different timing errors. Because a single UE can only be guaranteed synchronization with the transmitter of its serving cell accurately, targeting a practical solution, we consider a measurement based optimization for this problem. In this paper, we firstly analyze the impact of multiple timing errors from the different cooperative cells on the data demodulation performance of the CoMP UEs, especially for edge users. Then we propose a FFT window search method with cognitive measurements for joint transmission, provided with the accurate knowledge on all involved local channel estimation. Simulation results have been provided and show it improves the SINR behind demodulation about 2dB for downlink multipoint joint transmission.