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Multicast has been known as an efficient transmission technique for group-oriented applications such as multi-party video conferencing, video streaming for paid users, online gaming, and social networking. In this paper, we investigate physical-layer multicasting in mobile cellular downlink systems, where the antennas at base station are employed to transmit common signals to multiple users simultaneously. A central design problem of downlink physical-layer multicasting is the search for the optimal beamforming vector that maximizes the multicast rate. Traditionally, the problem has been formulated as a quadratically constrained quadratic programming problem and shown to be NP-hard in general. In this paper, starting from examining the Karush-Kuhn-Tucker stationary conditions, a new method based on two-user approximation is proposed for the search for the optimal beamforming vector. The method is able to achieve a much higher multicast rate than the existing methods and provides an attractive trade-off between performance and complexity, especially for the case of using a large number of antennas. Using a large number of antennas at base station, also known as the large-scale multiple-input and multiple-output technique, has been regarded widely as one of the most promising technologies to increase system capacity, coverage, and user throughput for future generations of mobile cellular systems.

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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%.

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