nts with high penetration loss. The low-cost UEs inevitably utilize less expensive crystal oscillators that can have carrier frequency offsets (CFOs) as large as 20 ppm. Deployment of NB-IoT in the in-band or guard-band mode introduces an additional raster offset as large as 2.5 or 7.5 kHz, resulting in even higher CFO values. Despite of the large frequency offset, an NB- IoT UE must be able to perform accurate synchronization at low SNR conditions. Time-frequency synchronization in NB-IoT follows the same principles as in LTE; never- theless, it incorporates improvements in the design of the synchronization sequences in 778 Chapter 6 order to mitigate large frequency offset and symbol timing estimation error issues in low SNR regions. As we mentioned earlier, time-frequency synchronization is achieved through NPSS and NSSS signals. The NPSS is used to obtain the symbol timing and correct the CFO, whereas the NSSS is used to obtain the NB-PCID, and the timing within 80 ms block. For UEs operating at low SNRs, an autocorrelation based on a single 10 ms received segment would not be sufficient for detection, thus for more accurate detection, the UE must coherently accumulate several sequences over multiple 10 ms segments. Due to large initial CFO value, the sampling time at the UE is different from the actual sampling time, the difference being proportional to the CFO. For UEs with limited coverage, more number of accumulations might be required to successfully achieve downlink synchronization. Following the synchronization procedure, the UE proceeds to the acquisition of the MIB. The NPBCH consists of eight self-decodable subblocks, and each subblock is repeated eight times. The design is intended to provide successful acquisition for coverage-limited UEs. Subsequent to symbol timing detection and CFO compensation, in the in-band and guard- band deployments, there is still an additional raster offset, as high as 7.5 kHz, which needs to be compensated. The presence of raster offset results in either overcompensatio