and Hidetoshi Kayama

The merits of 3GPP long term evolution (LTE), such as high spectral efficiency, very low latency, support of variable bandwidth, simple architecture, etc, make it the most competitive candidate for the next generation mobile communications standard. In the first release of LTE, only single transmit antenna is supported in the uplink due to its simplicity and acceptable performance. However, in order to keep its current leading position, LTE needs further evolvement (known as LTE-Advanced (LTE-A)) to provide better performance, including a higher uplink spectrum efficiency. Therefore, multiple transmit antennas must be supported in the LTE-A uplink and one important issue is the uplink demodulation reference signal (DMRS) design, which will influence uplink channel estimation accuracy and eventually determine uplink reliability and throughput. In this study we first briefly review the current status of DMRS in LTE uplink and then different DMRS enhancement schemes are investigated for LTE-A uplink multiple-input multiple-output (MIMO) transmission. Also, two-dimensional channel estimation algorithms are provided to realize accurate uplink channel estimation. With computer simulations, the performances of several candidate LTE-A uplink DMRS design schemes are evaluated and compared. Finally some basic conclusions are provided together with the latest standardization progress.