  Time Divison Duplex (TDD) wireless communication systems are inherently
bidirectional, which facilitates exploiting channel reciprocity for pilot based
channel estimation of both uplink and downlink. However, there exists a gross
asymmetry in channel estimation complexity for the uplink and downlink,
particularly for Multiple Input Multiple Output (MIMO) TDD systems. Usually,
Base Stations (BS) with more antennas need to estimate fewer parameters from
each antenna, whereas the estimation requirement is disproportionately higher
at the User Equipment (UE). Unlike the UE, the BS has powerful hardware,
computational resources and energy to accurately estimate and track channel
profiles. To overcome this asymmetry, we propose a solution for MIMO-TDD
downlink communication, wherein the post-coder part of the channel matrix is
quantized at the BS, and is communicated to the UE via a low-rate channel.
Using asymptotically tight lower bounds on the downlink achievable rates, we
quantify the performance of the proposed scheme. Simulations reveal that a
moderate number of quantization bits are sufficient to achieve rates close to
the the link capacity. We further show that, when the BS has many more antennas
at than the UE, the channel can be compensated by appropriate transmit domain
precoding without post-coder knowledge at the UE.
