| Nowadays,we have stepped into an era of information,and have been experiencing explosive mobile data traffic increase.To meet the future mobile data traffic demand,both academia and industries are seeking for effective methods to promote system capacity.Therefore,many spectrum efficiency enhancement strategies,such as adaptive modulation and coding(AMC),multiple-input multiple-output(MIMO),coordinated multi-point transmission(CoMP),etc.,and dense small cell networks to increase space utilization of spectrum have become research focus.However,only relying on those system capacity enhancement technologies still cannot catch up with the tremendous data traffic growth,meanwhile deployment of those technologies may bring in some negative impacts,such as high complexity,high hardware requirements,and hard network management.Thus,as a most straightforward and effective way to raise capacity,spectrum extension has attracted more and more attentions.Unfortunately,the electromagnetic radio spectrum is scarce and non-renewable national strategic resources.Almost all the spectrum suitable for wireless communications has been allocated for dedicated use,which makes wireless communication operators may be not able to get more licensed spectrum even with huge economic price paid.Despite shortage,according to relevant investigations and studies,many spectrum resources are still underutilized,which spurs federal communications commission(FCC)to develop dynamic spectrum access rather than static way.Accordingly,enormous spectrum from low frequency bands,like 800 MHz,2.4 GHz,and 5.8 GHz,to high frequency bands even including millimeter wave,like 24 GHz and 60 GHz,is opened up for unlicensed use.As a result,the development and research for wireless communication systems using unlicensed bands are urgently needed.Thus,with the evolution of mobile networks from fourth generation(4G)to fifth generation(5G),this thesis turns research focus from traditional LTE in unlicensed bands(LTE-U)system to C-plane/U-plane(C/U)decoupled LTE-U system.Finally,based on promising technologies of 5G mobile networks,a 5G licensed and unlicensed spectrum interoperable network will be designed,and corresponding optimizations will be proposed to improve performance.The detailed research work in this thesis could be presented from two main aspects.First,for feasiability and engineering implementation,technical details of unlicensed spectrum wireless communication systems are thoroughly designed.Second,to realize high-performance wireless transmissions on unlicensed spectrum,solutions and optimation algorithms are proposed to solve the problems that may encounter in practice use.The main innovative achievements in this thesis are described as follows.1)The standard of traditional LTE is established to support wireless transmissions on low frequency bands below 5 GHz,while unlicensed bands are normally distributed over high frequency bands.Hence,the key technologies of physical layer of LTE-U are resigned,including frame structure,physical channels,reference signal distribution,etc.,to let them applicable for high frequany bands transmissions.On unlicensed bands,LTE-U devices may suffer from uncontrollable interference from other wireless systems using unlicensed bands.Due to normally no cooperation between different systems,they need to compete with each other to gain spectrum access.If power control is employed in non-cooperative unlicensed bands sharing,each system involved in competition may raise transmit power to conquer interference and cause severer interference for other systems.To avoid this vicious competition,from the basic idea of multiuser divisity,we put forward a Hungarian method based unlicensed channel allocation algorithm to carry out interference coordination.The simulation results show that this alogithm has excellent interference coordination performance and can let LTE-U obtain high throughput on unlicensed bands with users fairness guaranteed.2)We apply C/U decoupled architecture in LTE-U networks to tackle some inherent issues of LTE-U networks,such as high system complexity and hard resource management.As a novel and advanced architecture,there is rare research regarding technical details of C/U decoupled architecture.Therefore,the network configurations and system procedures of C/U decoupled LTE-U system,incluing network architecture,protocol,frame structure,measurement,system broadcast,and handoff,are analyzed and designed herein so that practicability could be maintained.However,due to particularity of unlicensed bands and inherent drawback of C/U decoupled architecture,LTE-U system will suffer from more serious handoff problems,especially in high mobility scenario.Because of high propogation loss and strict transmit power constraints,dense small cell networks are necessary for unlicensed spectrum coverage,which will exacerbate frequent inter-microcell handoff issue.Moreover,in a C/U decoupled architecture,once inter-macrocell handoff will experience twice handoff execution procedures,namely macro-macro and micro-micro handoffs.This may result in handover trigger hysteresis and overlapping region shortage,finally causing low handoff success probability.To cope with them,we first put forward a seamless inter-microcell handoff scheme,in which macro-eNB replaces micro-eNB to transmit data on licensed bands when inter-microcell handoff is executed to avoid data transmission interruption caused by hard handoff and keep data transmissions during whole inter-microcell handoff.Furthermore,based on GM(1,n)model of the grey system theory,a handoff trigger decision algorithm is proposed to tackle handoff problem in high mobility scenario(high-speed railway).The simulation studies show that the proposed algorithm could trigger handoff in advance when train drive to an approricate location,and significantly improve handoff success probability.3)Most unlicensed spectrum access mechanisms for LTE-U are similar with carrier sense multiple access with collision avoidance(CSMA/CA),such as dynamic spectrum selection and listen-before-talk.Even through this type of mechanisms are capable of protecting the fairness of Wi-Fi devices,the throughput of LTE-U on unlicensed bands is exorbitantly sacrificed,especially in crowded Wi-Fi devices scenario.Thus,a novel spectrum access mechanism is proposed for LTE-U system,in which spectrum access parameters of LTE-U could be dynamically adjusted based on the parameters of Wi-Fi devices.For feasibility,a LTE-802.11 fusion protocol stack is designed to allow LTE-U to read the control information contained in 802.11 physical frame sent by Wi-Fi devices.For throughput maximization,an optimization problem that maximizes ergodic capacity of LTE-U is formulated to obtain optimal transmit power and occupation time.The simulation results indicate that with our proposed scheme,the LTE-U throughput can be improved significantly with the fairness of Wi-Fi systems guaranteed.4)Based on could radio access network(C-RAN),we present a framework of 5G unlicensed and licensed spectrum interoperable networks,which possesses lower complexity,higher flexibility,more efficient signal processing and computation,as well as more efficient network-level management.As for cost and reliability performance in our designed networks,improving of one of them always results in sacrificing the other.The reason is that low cost always means more unlicensed spectrum usage with low reliability,while high reliability brings in high cost due to more licensed spectrum needed.To find an excellent balance between cost and reliability,we first investigate a cost-reliability tradeoff by a cost minimization problem under a given reliability constraint.However,from simulation results,it can be seen that although cost-reliability tradeoff could be comprehensively analyzed by this method,reasonable balance points can not be found.Hence,we define a QoE efficiency utility,which is a complementary measure to characterize cost and reliability,offering an inherent tradeoff between them.Then,a QoE efficiency maximization problem is formulated to obtain optimal unlicensed and licensed joint reousece allocation strategies.The simulation studies show that this method could provide an excellent balance between cost and reliability by maximizing network QoE performance.Moreover,to alleviate the inaccuracy of unlicensed channel measurement caused by abrupt interference and measurement feedback delay,an interference power estimation method based on Grey-Markov prediction model is introduced,the simulation results of which demonstrate its effectiveness. |
PDF Full Text Request