# Design and Performance Analysis of Hardware Realization of 3 GPP Physical Layer for 5G Cell Search
## 4 UE Receiver PHY for CS  
The downlink, i.e., UE, receiver PHY for CS needs to blindly detect the SS and identify the transmitted PCI, \(N_{ID}^{Cell}\), and the index of the detected SS block, \(SS_{i}\). It also helps the UE to locate the starting sample of each frame, sub-frame, slot, and symbol which is critical for receiving the MSI and establishing the uplink communication with the gNB.  
The block diagram of the receiver PHY is shown in Figure 6. The first step is to identify the presence of CP so that the receiver can group the received samples in a packet of 4096 samples comprising a single OFDM symbol.  
In this work, we assume ideal synchronization between BS and UE [50]. This is done via a conventional auto-correlation-based approach where the received signal is auto-correlated with its delayed version since CP is the initial section of the OFDM symbol appended at the end. The CP detection enables the identification of symbol boundaries so that 4096 complex symbols corresponding to a single OFDM symbol are extracted for the PSS search task. Since CP detection is a well-known task in wireless PHY, we skip the discussion on its architecture. Note that the CP detection is an optional step in CS PHY but it helps to reduce the number of samples to be processed by the PSS search block thereby speeding up the PSS search. The CP detection may fail when SNR is poor and in such cases, all the samples are forwarded to the PSS search block.  
Figure 5: Architecture for generating the 3 GPP PBCH-DMRS.  
Figure 6: Block diagram of the CS receiver PHY.  
The next step is to identify the OFDM symbols containing the SS block. Though SS is transmitted in pre-defined OFDM symbols, the receiver is not aware of the index of the detected OFDM symbol, and hence, blind detection of SS is needed. We first detect the presence of the PSS using the PSS Search block and identify the detected PSS sequence, \(\text{PCI\_2}\in\{0,1,2\}\). The PSS detection also enables the extraction of symbol boundaries whenever CP detection fails. The OFDM symbol containing PSS can also be detected using the \(frame\_tick\) signal generated by the boundary search block. However, in the beginning, \(frame\_tick\) signal may not be available and hence, accurate and fast detection of PSS is critical for CS PHY. The SS OFDM Symbol Extractor block extracts the four OFDM symbols containing one instance of SS and forwards them to the OFDM demodulator containing the CP removal and 4096-FFT. After FFT, 960 symbols of the detected SS instance are extracted. Among them, samples belonging to SSS in the third OFDM symbol are sent to the SSS search block to identify the SSS sequence, \(\text{PCI\_1}\in\{0,1,\ldots 335\}\). Using the PSS and SSS search, we can estimate the PCI as 3*PCI\_1 + PCI\_2. The extracted 144 DMRS symbols are forwarded to the DMRS search unit to identify the SS index, \(SS_{i}\in\{1,2,\ldots,8\}\). In the end, the boundary search block exploits the timing information of the multiple SS blocks to identify the frame, sub-frame, and slot boundaries. Next, we present the architecture of the PSS search, SSS search, DMRS search, and boundary search blocks of the receiver CS PHY.