Grant-free Mechanism Design For Non-orthogonal Multiple Access In 5G

Massive machine type communication(mMTC)is one of the key scenarios in 5G,which is to meet the requirements of massive connection and low power consumption.And the characteristics of small packets,sparse active users and random data arrival make the contention based grant-free nonorthogonal multiple access technology to be the ideal scene in uplink communication of mMTC.In order to increase the number of connection,the CTU(contention transmission unit)is usually shared by multiple UEs(user equipment).Once data arrives,the UE can transmit it in the corresponding radio resources immediately.Collisions will occur when several UEs who shared the same radio resources transmitted data simultaneously.In the existing grant-free access mechanism,CTUs are allocates by UE ID.The mapping relationship between UE and CTU is relatively fixed,which is leading to the unfairness among UEs.Moreover,when UE retransmits data by taking the random back-off mechanism,they may collide again.Therefore,taking the feature of sparse active UEs in mMTC,we make the retransmission UE to be allocated an exclusive CTU,and the other UEs will share the rest of CTUs.In this way,the average collision probability will be reduced.When the transmission traffic varies from UE to UE,the greedy algorithm is applying in the UE-to-CTU mapping problem.To reduce the computational complexity,we simplify the expression of each iteration in the algorithm.Through the discussion of the optimal value of the competition window,the mapping scheme which is suitable for scenarios where the user traffic varies is given.The author derives the theoretical formulas for the collision probability of these two proposed UE-to-CTU mapping schemes and the controlled schemes,then carries out the simulations.Finally,the simulation results are consistent with the theoretical analysis,which proves that the resource mapping scheme described in this paper can achieve a lower average probability of collision.