Research On Initial Access For Millimeter-Wave Based Cellular Systems

Since current micro-wave spectrum under 5 GHz is fragmented and crowded,people have been more interested in the millimeter wave(mmWave)bands.One the one hand,a large amount of available spectrum resources can meet the high-rate transmission requirements in the wireless initial access network.On the other hand,to overcome the high isotropic path loss of the mm Wave frequencies,outdoor cellular links will require highly directional beam transmissions.This complicates several control layer procedures and is particularly challenging for initial access.In the 3GPP LTE,initial access is mainly composed of the downlink synchronization and the uplink random access.In this paper,we only discuss the sequence design of downlink synchronization signal and the multi-beam search process in the millimeter-wave cellular system.Firstly,the synchronization signals should include more additional information,such as the synchronization signal block index and the beam resource index.In this paper,we design an efficient sequence of tertiary synchronization signal(TSS),carrying the SS block ID(SID)and the beam ID(BID).A combination of a prime-length Zadoff-Chu(ZC)sequence and a polyphase sequence is considered for TSS design.The ZC sequence is often used for preamble design owing to its good correlation property and low peak-to-average power ratio.However,the computational complexity for ZC sequence detection is known to be high.Therefore,a joint sequence design technique is necessary for beam-based cellular communication systems.And a joint detection scheme is proposed,to achieve the trade-off between the complexity and the misdetection probability of the SID and BID detection.The joint detection of SID and BID efficiently maximize cross-correlation value,to find the peak more obviously and accurately and finally decrease the PMD(misdetection probability).Then we introduce several algorithms of beam search,and compare the advantages and disadvantages of each algorithm.The exhaustive search algorithm is good at coverage performance and suitable for edge users.But it is poor at delay.Compared to the exhaustive algorithm,two-level iterative algorithm can shorten the delay.However,each level of beam search needs to calculate the SNR of all the beams,which can increase the complexity.Therefore,this paper proposes the multi-level iterative search algorithm based on threshold,to reduce the number of calculations.By setting the upper and lower thresholds,it is only necessary to calculate the SNR value of one of the weight vectors to determine the position of the optimal beam.Finally,simulation results show that the proposed scheme can provide robust capabilities and good performance,which contributes to the standardization of 5G.PMD is the main performance of synchronization signal transmission and detection.Delay is the main challenge of initial access in the millimeter wave cellular system.For the cell center users,it is better to adopt the typel TSS sequence with the sequence length of 256 and the multi-level iterative search algorithm.For the cell edge users,it is better to consider the type2 TSS sequence with the sequence length of 128 and the exhaustive search algorithm.