Design principles for four branch downlink MIMO for long term HSPA evolution

Several new features are added for the High Speed Packet Access (HSPA) evolution in order to meet the requirements set by the International Mobile Telecommunications Advanced (IMT-A). The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch multiple input multiple output (MIMO), i.e. utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beamforming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high signal to noise ratio (SNR) users and improves the coverage for low SNR users. This paper discusses a few design considerations related to physical layer aspects of four branch MIMO. In the first part of the paper we outline a few design options and show that four branch MIMO can be introduced with modest impact on the existing standards and networks. In the second part of the paper we show by means of system level simulation that significant gains in average user throughput and cell edge user throughput can be achieved with four branch MIMO. Based on these evaluations we conclude that four branch MIMO is an important component in the continued evolution of the HSPA standard.