| With the development of commercialization of the Fifth Generation(5G)communication system,the concept of "the Internet of everything" is gradually being realized.Narrowband Internet of Things(NB-IoT),which can effectively support massive machine type communication(m MTC),has been officially incorporated into the 5G standard,indicating that NB-IoT will play an important role in future communication.In NB-IoT,narrowband physical random access channel(NPRACH)is the first step to realize uplink communication resource authorization and synchronization.The successful access rate of NPRACH will directly affect the efficiency of the uplink communication.In this thesis,focusing on the key techniques of NPRACH,a cyclic prefix-free(CP-free)receiver scheme and an effective Time of arrival(ToA)estimation algorithm are proposed.First of all,this thesis proposes a scheme of NPRACH waveform without cyclic prefix in order to improve spectral efficiency and signal to noise ratio at the receiver.The core idea of this method is to use the characteristics of repeated transmission of NPRACH directly to transmit repeated symbols without inserting the cyclic prefix.Then,the receiver intercepts the front-end signal of the subsequent symbol and replaces the front-end signal of the first symbol to construct the circular convolution structure to achieve the same effect as the traditional method with CP inserted.In order to evaluate the performance of the proposed method,the expression of receiving signal-to-noise ratio is derived.In addition,this thesis also uses single tap channel equalization to simplify the receiving process.Numerical simulation results show that the proposed cyclic prefix-free method has performance gain compared with the traditional method,and the spectral efficiency is increased by 20%.Secondly,this thesis presents a ToA estimation method for NPRACH.The difference of frequency hopping distance has directly influenced the estimation error,and the larger frequency hopping distance can reduce the estimation error.This thesis analyzes the factors that affect the results of ToA estimation,and then proposes a receiving algorithm for ToA estimation.Considering that the NB-IoT standard has been frozen,the proposed algorithm does not need to change the original frequency hopping pattern,but makes full use of the fixed frequency hopping pattern by taking two traditional basic units(BU)as a whole unit to construct new hopping distance,and then reduces the estimated error range according to the novel constructed frequency hopping distance.In addition,in order to adapt to the low cost of Internet of Things devices,some frequency-hopping distances are discarded in the proposed algorithm,which has lower complexity than traditional methods.Extensive numerical results show that when Maxmum Coupling Loss(MCL)is large and the number of repeated transmission of the basic unit is large,the ToA estimation algorithm proposed in this thesis is capable of improving the performance gain compared with the traditional method.Then,this thesis integrates the above two methods and proposes an efficient ToA estimation algorithm with CP removed.When the maximum delay extension of the receiver is low,the signal-to-noise ratio of the receiver can be effectively improved to reduce the impact of noise.In order to improve the performance of ToA estimation,the symbol demodulation process in the proposed ToA estimation method is replaced by the CP-free design in this thesis.Simulation results are carried out to validate that when residual carrier frequency offset(RCFO)is ideal,using the CP-free design can further improves the performance of ToA estimation. |
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