In frequency division duplex (FDD) wireless MIMO communications, the receiver conventionally feeds back quantized channel state information (CSI) of the forward link to the transmitter for precoder design. As systems are moving towards multi user and other advanced configurations, it's becoming more important to have accurate CSI at the transmitter (CSIT). In this paper, a double codebook-like mechanism is proposed to enable channel estimation at the transmitter. The novel feedback mechanism and estimation techniques are numerically tested in a time-varying, frequency selective system employing single user MIMO and multi user MIMO. Compared to a LTE Release 8 based scheme, the proposed scheme is seen to a) provide significantly better multi user MIMO performance when single antenna receivers are employed b) allow the transmitter to perform multi user MIMO much more frequently.