# Cell Selection Technique for Millimeter-Wave Cellular Systems with Hybrid Beamforming
## Preliminaries  
### Conventional Cell Selection Technique  
A cell search procedure that will be used for an MS in conventional LTE systems can be summarized as follows. The MS first needs to scan by measuring the received signal strength indicators (RSSIs) of all supported carriers for entire bandwidth, each with 20 MHz. The measured RSSIs include the signal strength of the desired cell, interference cells, and noise. Then, RSSIs lower than the predefined threshold (or signal strength sensitivity) are filtered, resulting in a list of surviving candidates. Each surviving candidate is processed by cell identification block until the MS finds a cell on which to camp. Cell identification involves primary synchronization signal (PSS) and secondary synchronization signal (SSS) detection. The SSS carries the information of cell ID group, while the PSS carries the information of cell ID within the cell ID group. A physical cell ID is constructed by combining the cell ID group and cell ID within the cell ID group after PSS and SSS detection.  
However, in mm-wave cellular systems, the selection of the best beam pair, as well as the cell selection, is important because a maximum array gain is obtained in these systems when Tx and Rx beams are perfectly aligned [14; 15]. Especially for systems with narrow beams, a small mismatch or misalignment between Tx and Rx beams may result in a substantial loss in the received power. In mm-wave cellular systems, the TDBS scheme has been used to find the serving cell with the best beam pair in the initialization and handover stages [18; 19]. In the TDBS scheme, individual Tx beams are transmitted from the BS until all the Tx beams are transmitted. The Rx beam sweep is performed at the MS for each Tx beam to measure the SNR for each Tx-Rx beam pair. The measurement of the SNRs for all possible Tx-Rx beam pairs must be performed for all candidate cells to select a serving BS with the best beam pair. Therefore, the serving cell and best beam pair are selected based on the measurement results obtained by all possible Tx-Rx beam pairs of candidate cells. A physical cell ID of the serving cell is obtained by detecting the PSS and SSS transmitted from the serving cell. A beam pair with maximum SNR is selected as the best beam pair and the corresponding cell with the physical cell ID as the serving cell. However, the selected cell and beam pair may not be optimal for hybrid beamforming system because the effect of the multipath channel is not considered in the initialization stage [10; 11; 14].  
Figure 1 shows an example of a mm-wave cellular system with two BSs with different CIDs. Here, it is assumed that a multipath (NLoS) component exists in cell 0 and an MS is located at the cell boundary. It is also assumed that the LoS path is stronger than the NLoS path in cell 0. Here, switched beamforming is used at both the BSs and the MS. In this example, a serving cell will be selected by measuring the signal strengths of LoS paths from two cells. The cell with the higher signal strength of the LoS path will be selected [18; 19]. Here, the multipath (NLoS) component is not used for cell selection. However, in hybrid beamforming systems, the multipath channel is  
used to increase the SNR in power-limited conditions (e.g., cell edge), or to increase user data rates, as long as the multipath channel has sufficiently different propagation paths. Therefore, the serving cell selected in the initialization stage may not be optimal for hybrid beamforming system in the data transmission stage.