Study On Non-best User Association Scheme For 5G Het Nets

Considering the fact that the current 3G /4G network is unable to satisfy the exponential growth of wireless network data traffic and user demand for higher quality of service due to access of the large number of smart devices,industry and researchers have begun to study the fifth generation mobile communication system(5G).At the same time,in the promising heterogeneous network(Het Nets)regarded as the key technology of future 5G networks,dense deployment of heterogeneous cells causes the decrease of the distance between the transceivers,which can overcome fading wireless transmission and has the advantages of improving the spectrum efficiency and energy efficiency.Generally,in order to make better use of the advantages of Het Nets,it is critical to design a reasonable user association(UA)scheme,by which a typical can select its serving base station(BS)or access point.However,the existing UA schemes have the following disadvantages.Firstly,although the UA scheme based on the strongest downlink(DL)average biased received power(ABRP)can guard the DL quality of service(Qo S)by selecting the best DL access point,but the scheme does not consider the case where the best access point is not available due to the practical application restriction such as schedule,power,and so on.Moreover,the reasonable resolutions have not been provided.Secondly,in the traditional ABRP-based UA scheme,it is difficult to ensure the Qo S of uplink(UL).However,in future 5G networks,besides the DL,the improvement on UL is also required due to the symmetric traffic.Therefore,this thesis focuses the feasible UA scheme based on non-optimal UA and the UL access improvement scheme based on non-optimal access.Firstly,oriented a traditional multi-layer heterogeneous network model,a non-best UA scheme based on DL ABRP is proposed.The core idea of the non-best UA is that when the best access point is unavailable because of practical application constraints,the user can choose to concatenate with the non-best base station or access point from which the DL ABRP is the m-th order statistic.The proposed scheme not only overcomes the problem that the optimal base station is unavailable due to practical application constraints,but also can be used to estimate the performance loss caused by incomplete channel state information.For the proposed m-th ABRP UA scheme,by modeling the positions of network elements in each tier as a homogeneous Poisson point process(HPPP),the thesis provides the analysis of the probability that a typical user is associated with a non-best BS that has the m-th strongest ABRP as well as the corresponding statistical description based on stochastic geometry and order statistics.With the derived statistical description,the DL signal-to-interference-noise ratio(SINR)coverage performance is evaluated.The presented simulation and numerical results validate the derivations.At the same time,the analysis also indicates that the selection of non-best access point imposes great impact of network performance.Secondly,in order to ensure the UL Qo S and improve the throughput of the whole network,by exploiting the promising non-orthogonal multiple access(NOMA)this thesis proposes a decoupling cross layer UL and DL association(DUDA),in which the non-best UA and NOMA technologies are integrated effectively.The key of this cross-tier DUDA scheme is that the two-user NOMA is used in the master and secondary links for both the UL and DL,of which the master access link uses best UA while the secondary access link adopts non-best UA.Based on the DL ABRP,a user can simultaneously access two different layers in the DL.In UL,the distance-based criterion is used,by which a user accesses simultaneously the two different nearest BSs.Due to adopting different UA criterions in UL and DL respectively,the decoupled UL and DL association is achieved in this proposed UA scheme.The advantage is that this scheme can not only provide the asymmetry traffic between UL and DL,but also achieve the multi-target implementation requirement of the user,which are provided by different layers.Then,based on the novel DUDA scheme with non-best UA and NOMA,this thesis investigates all possible decoupling and non-decoupling UA schemes under a more general multi-layer Het Nets case.For each association case,the association conditions are derived as well as the corresponding use association probability,which are given with closed-form expressions.Finally,with the derived association probability and the statistical description,the DL and UL SINR coverage performance and the spectral efficiency of the whole network are studied by means of stochastic geometry and order statistics.