Research On Power Allocation In OTFS-NOMA Cross-domain Transmission System

With the development of the 5th Generation Mobile Communication(5G),spectrum resources are becoming increasingly scarce.Non-Orthogonal Multiple Access(NOMA)can support large-scale user access,which can greatly improve spectrum efficiency and become a key technology for 5G.At the same time,the rapid development of high-speed traffic,the user channel in the high-speed scene has a Doppler shift that cannot be ignored,and the traditional Orthogonal Frequency Division Multiplexing(OFDM)technology can no longer meet the actual needs.Orthogonal Time-Frequency Space(OTFS)can convert a fast time-varying channel into a slow-varying channel in the Delay-Doppler domain to obtain a near-constant channel gain.At the same time,it can use the full range of time and frequency.Channel diversity can effectively resist the negative effects of Doppler and multipath effects.This thesis combines NOMA and OTFS,considers the OTFS-NOMA cross-domain transmission system,and studies the signal transmission between users with different Doppler shifts.Among them,high-speed user signals with high Doppler frequency shift are transmitted in the Delay-Doppler domain,low-speed user signals with low Doppler frequency shift are transmitted in the time-frequency domain.This thesis conducts research on user power allocation.The main work is reflected in the following two aspects:(1)OTFS-NOMA cross-domain transmission system downlink power allocation.Aiming at the downlink scenario of the OTFS-NOMA cross-domain transmission system,a cross-domain hierarchical power allocation strategy based on OTFS-NOMA is proposed.In this strategy,high-speed user signals are transmitted in the Delay-Doppler domain,low-speed user signals are transmitted in the time-frequency domain,and hierarchical power allocation is performed at the base station.First,in the Delay-Doppler domain,the high-speed user and the low-speed user group are regarded as a pair of NOMA users,according to the transmission rate guarantee of the high-speed user and the low-speed user group can achieve the successive interference cancellation(SIC)requirements,to complete high-speed user power allocation;Secondly,after completing the SIC of the Delay-Doppler domain signal,it is transformed to the time-frequency domain where the low-speed user group signal is located,and the user power is allocated according to the low-speed user channel state conditions to achieve the maximum reach sum rate.The simulation results show that the proposed cross-domain hierarchical power allocation strategy can provide higher reach and rate compared with the traditional power allocation strategy,and can guide the actual transmission of users with different mobility.(2)Uplink power allocation for OTFS-NOMA cross-domain transmission system.Construct the uplink scenario of the OTFS-NOMA cross-domain transmission system.In this scenario,not only the high-speed user and the low-speed user group act as a pair of NOMA users,but the users in the low-speed user group also transmit signals in the form of NOMA.For this scenario,two power allocation strategies are proposed.The first is to consider the power allocation strategy for the throughput of the cross-domain transmission system.In this strategy,the power allocation between users is obtained by maximizing the throughput of high-speed users and low-speed users;Next is the power allocation strategy for the fairness of low-speed users in the cross-domain transmission system,and power allocation is performed with the goal of maximizing the minimum low-speed user data transmission rate.The simulation results show that the proposed power allocation strategy is better than the traditional power allocation strategy and can achieve better system performance.