# Cell Selection Technique for Millimeter-Wave Cellular Systems with Hybrid Beamforming
## 1 Introduction
However, the conventional cell selection technique, in which the serving cell is selected based on the measurements of Tx-Rx beam pairs for all possible links (candidate cells), may not be optimal in multipath environments. For example, in a multipath channel where one line-of-sight (LoS) and multiple non-line-of-sight (NLoS) paths are available in candidate cells, only a single path providing the highest SNR (possibly LoS) will be used to select the serving cell in the conventional technique. However, the selected cell in the initialization stage may not be optimal in the data transmission stage where hybrid beamforming is employed. So far, to the best of our knowledge, there has been no attempt to select a serving cell in the initialization stage by considering the hybrid beamforming structure to be used in the data transmission stage. In this paper, a cell selection technique for mm-wave cellular systems in the initialization stage is proposed for hybrid beamforming structures in data transmission stage. To select a serving cell, taking into consideration the hybrid beamforming structure in the initialization stage, we need to estimate angles of arrival (AoA) and angles of departure (AoD) for all candidate cells. The processing time required for channel estimation increases proportionally to the product of the number of Tx beams, the number of Rx beams, and the number of candidate cells [20]. This long processing time will create significant overheads for a moving MS because the channel estimation should be performed periodically for a possible handover. A new channel estimation technique is proposed by transmitting multiple beams simultaneously, where each beam is mapped with a unique proposed cell and beam synchronization signal (CBSS), all in the physical layer. In the proposed CBSS, the beam ID (BID) is designed in conjunction with cell ID (CID), because the BID must be detected in a multicell environment. It is shown that multiple beams with different BIDs can be transmitted simultaneously with minimal inter-beam interference in a multicell environment, resulting in a significant reduction in processing time for channel estimation and cell selection. The hierarchical structure of CBSS gives the advantage of reusing the same BIDs in adjacent cells, allowing us to generate a large set of sequences. In the proposed cell selection technique, the serving cell is selected based on the maximum channel capacity of the hybrid beamformer with the estimated channel information and optimum precoder. Moreover, the serving cell and the corresponding AoDs and AoAs are determined by taking into consideration the number of RF chains that will be used in the data transmission stage. The remainder of the paper is organized as follows. Section 2 summarizes the conventional cell selection technique for mm-wave cellular systems and the system model that will be used in this paper. In Section 3, the cell selection technique for mm-wave cellular systems with hybrid beamforming is proposed, after describing the properties of CBSS and channel estimation technique using the CBSS. In Section 4, the performance of the proposed technique is verified by computer simulation using a simple model of a mm-wave cellular system. Finally, conclusions are drawn in Section 5. The following notations are used throughout the paper: **A** is a matrix, **a** is a vector, and \(\mathcal{A}\) is set. \([\cdot]^{*}\), \((\cdot)^{\prime}\), \([\cdot]\), \([\cdot]\), and \(<\) \(>\) are conjugate, conjugate transpose, floor operation, ceiling operation, and Jacobi symbol, respectively. \(N_{TX}\) and \(N_{B}\) denote the number of antennas and beams at the BS while \(N_{RX}\) and \(N_{I}\) denote the number of antennas and beams at the MS, respectively. \(N_{S}\) is the number of data streams and \(N_{RF}\) is the number of RF transmit chains. \(c\) is the cell ID, \(\overline{c}\) is reference cell ID, \(b\) is the beam ID, and \(\overline{b}\) is reference beam ID. **I** is the identity matrix and \(\mathbf{1}_{N}\) is the \(N\)-length all-ones vector.