A Bit-Adaptive PMI Feedback Mechanism

A bit-adaptive precoding matrix index (PMI) feedback mechanism is proposed in Long Term Evolution-Advanced (LTE-A). In Coordinated Multipoint Transmission (CoMP) scenarios, the number of feedback PMI bits for all channel state information (CSI) processes at the same time may be larger than that the current LTE-A system can support. However, in some environments like slow-fading channels, the number of feedback PMI bits is thus reduced at the cost of limited performance loss without changing the codebook in LTE-A. To carry out this concept, we build up a candidate set for each PMI in the codebook of LTE-A. Based on the previous selected PMI, the current selected PMI belongs to the candidate set of the previous selected PMI. The candidate set is defined as a subset of precoder indices in the codebook of LTE-A. Since the number of elements in the candidate set is smaller than that of all precoders in the codebook, the number of feedback PMI bits can be reduced. Simulations show that the feedback PMI overhead can be reduced at the cost of limited performance loss. When the mobility speed is 20 km/h, the SNR loss is about 0.25 dB for block error rate is 10^-2 and the overhead reduction ratio is 25%.