# Hybrid Beamforming for Reduction of Inter-Beam Interference in Millimeter-Wave Cellular Systems
## 1 Introduction
In the data transmission stage, the beamforming antennas allow each \(BS\) to serve a number of \(MS\)s simultaneously, providing a substantial gain in system capacity. The challenge pertaining to space-division multiple access (SDMA) systems is the inter-beam interference (IBI) caused by the beams formed by the \(BS\) for multiuser service. In the case of conventional low-frequency systems, digital precoding techniques are used to reduce the co-channel interference of other users who utilize full channel information in the baseband [19]. However, in mm-wave systems, the implementation of a full digital beamformer with precoder is difficult because of the associated high cost and large power consumption. In addition, it is difficult to obtain full channel information at the \(BS\) because of the large training and feedback overhead that are required in mm-wave systems with a large number of antennas. In [20; 21; 22; 23], hybrid precoding techniques for multiuser mm-wave systems are proposed to reduce computational complexity in the design of a digital precoder using sparse characteristics of the mm-wave channel as well as to reduce beam training and feedback overhead. However, to date, the design of hybrid precoders has been focused on the reduction of IBI in single-cell multiuser mm-wave systems. In this paper, we propose precoding techniques for mm-wave systems with a hybrid beamformer in multicell multiuser environments. Unlike single-cell multiuser environments, an \(MS\) in a multicell environment may receive IBI not only from a serving cell but also from neighboring cells. The beams formed toward \(MS\)s in the serving cell may generate a large amount of interference in the direction of \(MS\)s in neighboring cells, especially for \(MS\)s at the cell boundary. In this paper, we propose four different digital precoding techniques (Type-1, Type-2, Type-3, and Type-4) to reduce IBI in mm-wave cellular systems with hybrid beamforming. Then, we analyze pros and cons of the proposed precoding techniques for reduction of IBI in multicell multiuser environments. We compare the performances of the four proposed precoding techniques in terms of the achievable rate and bit error rate (BER). In addition, for the design of the proposed precoders, we need to estimate the channels between the \(MS\) and neighboring \(BS\)s as well as the channel between the \(MS\) and the serving \(BS\). The channel estimation technique which can reduce the processing time in multicell environment is discussed for the design of the proposed precoders. In Section 2, we propose four different digital precoding techniques for mm-wave systems with a hybrid beamformer in multicell multiuser environments. We derive achievable rates of the four different precoders, and compare pros and cons of the precoders for reduction of IBI in multicell multiuser environments. Also, a channel estimation technique appropriate for the design of the proposed precoders is discussed. Then, in Section 3, we verify the performances of the proposed techniques by performing computer simulations using a simple model of an mm-wave cellular system. Finally, we conclude the paper in Section 4.