# Cell Selection Technique for Millimeter-Wave Cellular Systems with Hybrid Beamforming
## 3 Proposed Cell Selection Technique
### Channel Estimation and Cell Selection
Note that the conventional technique (LoS) is better than the analytic result, because the SCM channel model can be approximated as an AWGN channel in the simulation when the beamforming is performed in the LoS direction. In the analytic result (Rician fading), only the array gain (16 \(\times\) 8 in ULA, 64 \(\times\) 16 in UPA) is taken into account. In the conventional technique, BER is measured in cell 1, because cell 1 is selected as the serving cell in the initialization stage. However, the BER is measured in cell 0 in the proposed technique (LoS + NLoS(1), LoS + NLoS(2), LoS + NLoS(4)). As can be seen in Figure 8, an additional maximum-ratio combining (MRC) gain of [1, 2, 3] dB can be achieved in the proposed technique when \(N_{RF}\) is set to [2, 3, 5], respectively. Figure 8: BER performance when the conventional and proposed techniques are used. Figure 7: Achievable channel capacity when the proposed technique is used for varying \(k\)-factor and different antenna configurations. Table 1 compares the processing time and computational complexity required for channel estimation and cell selection when the conventional and proposed techniques are used. Here, \(T_{\text{SS}}\) and \(T_{\text{CBSS}}\) represent the lengths of the synchronization signal (SS) and CBSS symbol, respectively. It is assumed that \(T_{\text{SS}}\) is equal to the length of two OFDM symbols (PSS and SSS), as in LTE systems, and \(T_{\text{CBSS}}\) is equal to one OFDM symbol length. In this example, the number of candidate cells, the number of Tx beams at BS, and the number of Rx beams at MS, are assumed to be \(N_{C}=3\), \(N_{B}=16\), and \(N_{I}=8\), respectively. From Table 1, it can be seen that the processing time required for channel estimation and cell selection with the proposed technique is much smaller than with the conventional technique. In the proposed technique, the processing time decreases proportionally to the number of RF transmit chains. In the conventional technique, the processing time increases linearly with the increase in the number of candidate cells, whereas it does not change in the proposed technique. Next, the computational complexities required for channel estimation are compared. In the conventional technique, CIDs are carried by PSS and SSS, which are transmitted from each beam. Beam sweep is performed at both the BS and MS. However, in the proposed technique, CIDs are carried by PSS and SSS, and BIDs are carried by CBSS in a hierarchical manner. In addition to PSS and SSS, CBSS is also transmitted from each beam simultaneously. Here, beam sweep is performed only at the MS. In the proposed technique, the number of RF transmit chains at the BS is considered to compute computational complexity. Here, \(N\) represents a prime number less than minimum bandwidth in LTE and is set to 61. Table 1 shows that, as the number of RF transmit chains increases, the complexity required for channel estimation decreases because multiple beams can be transmitted simultaneously from the BS with minimal interbeam interference, and channel estimation is performed jointly at the MS.