Beam Resource Control And Management Technology For Next Generation Millimeter Wave Networks

With the explosive growth of mobile data demand,future wireless networks [i.e.,the fifth generation(5G)networks and beyond] would exploit new available frequency spectrum to greatly increase communication capacity to overcome spectrum congestion at the conventional microwave band below 10 GHz.Recently,millimeter wave(mmWave)communications,operating in about 30-300 GHz bands,have attracted significant research interest,since the massive available spectrum can potentially provide multiple Gbps(gigabits per second)data rate.In particular,60 GHz band is more appropriate for indoor communications,in which line-of-sight(LOS)or short-distance transmissions are dominant than other mmWave bands.It has been widely studied by several standardization organizations,such as IEEE 802.15.3c and WirelessHD for indoor wireless personal area networks(WPANs),wireless gigabit alliance(WiGig)and IEEE 802.11ad/ay for wireless local area networks(WLANs).Compared with existing microwave systems,mmWave communications are faced with two major challenges: severe propagation loss and sensitivity to blockage.To compensate for high propagation loss,directional beamforming(BF)has been widely used as an essential technique to form a directional beam pattern with high antenna gain.Thanks to the short wavelengths of mmWave radios ranging from 10 mm to 1 mm,massive antenna arrays can be packed into the limited dimensions of mmWave transceivers.Therefore,with directional BF,it is possible to form multiple beams at both mmWave transmitter and receiver sides.That is,the transmitters and receivers are capable of supporting multi-beam simultaneous transmissions in short-range mmWave networks with multiple reflected paths and,meanwhile,can realize spatial spectrum reuse.It can be regarded as a kind of high-dimensional MIMO,which is also described as beamspace MIMO in our study.However,most researches have only considered a single beam scenario,which means that they do not make full potential capacity of mmWave.Furthermore,the connectivity of single beam transmission can easily be blocked.In this context,we are committed to the research on radio resource(e.g.,beam and power)management technology for mmWave communications in future wireless networks,including BF training,inter-beam interference coordination,multi-beam power allocation,and link blockage recovery processing.Moreover,concerning the peculiar propagation characteristics of mmWave,which makes the signal or interference power in mmWave communications highly directional,we would like to know that will the traditional interference mitigation and physical layer security techniques still be efficient or necessary in mmWave networks? To answer the question,we also investigate the performance limits of interference and physical layer eavesdropping immunity in mmWave communications.In particular,our work is mainly based on WLAN protocols for 60 GHz band(e.g.,IEEE 802.11ad/ay).Firstly,we investigate the nonorthogonal beam interference in mmWave communications.An mmWave base station(MBS)can transmit simultaneously to multiple mmWave user equipments(MUEs)with different beams in mmWave networks.However,the beams that serve different MUEs may transmit(almost)in the same direction,especially when MUEs are distributed densely.That is,they are not in perfect orthogonal beams.Due to the leakage of transmission power,the interference among these beams may be severe.To address this problem,typically the MBS may serve these MUEs in time division multiplex(e.g.,TDMA),which will degrade the spectral efficiency.In this context,we investigate the effect of nonorthogonal beam interference and then propose two effective beam scheduling solutions,i.e.,dynamic beam switching and static beam selection.On this basis,an improved downlink multi-user simultaneous transmission scheme is introduced.In the scheme,an MBS can serve multiple MUEs simultaneously with multiple orthogonal and/or nonorthogonal beams.The results show that,compared with the schemes based on time division,the proposed scheme can serve lots of MUEs simultaneously and can guarantee each MUE's Quality of Service(Qo S).Meanwhile,it can effectively reduce the interference and largely improve the achievable rate of the network.Secondly,we investigate the challenges and potential solutions for the beamspace MIMO in single user scenarios(i.e.,beamspace SU-MIMO),including:(i)by utilizing the capability of supporting multiple beams in mmWave transceivers,we improve the DMG BF shceme in 802.11 ad to increase the efficiency of multi-beam selection and,meanwhile,to make it applicable for the beamspace SU-MIMO;(ii)we propose a multi-beam cooperative beam tracking mechanism to mitigate the impact of the link blockage caused by obstacles'(e.g.,human)activity,whose main idea is to restore the broken link through interactions of beam switching signalings using the transmit and receive beam pairs operating on unbroken links;(iii)by setting synchronous and waiting timers and controlling the split ratio of the multiple links,we address the multi-beam synchronization problem to ensure the merging of multiple data streams and to reduce the buckets effect;(iv)in order to obtain the maximum achievable rate,we propose and compare two multi-beam power allocation solutions,i.e.,the average power allocation and the prioritized power allocation.The results show that the beamspace SU-MIMO can largely improve the achievable rate of mmWave systems and,moreover,can greatly enhance the robustness of the network connection.Thirdly,we extend our previous work to multi-user scenarios,namely beamspace MU-MIMO,on the basis of existing research results.We not only consider the differences of the same problem in the different scenarios,but also investigate the new challenges of the beamspace MU-MIMO,including:(i)we utilize the capability of supporting multiple beams both at the MBS and MUEs to detect the quality of multiple links simultaneously,and thus to increase the efficiency of multi-user BF training;(ii)we analyze the inter-user interference to avoid beam selection conflicts and to accomplish simultaneous users' grouping;(iii)different from the traditional trial and error methods,in order to improve the efficiency of resolving blockage events,we first determine the reasons of link blockage through changes in the quality of the unbroken links and then take different blockage control strategies for different reasons;(iv)we consider the fairness of power allocation among simultaneous MUEs and further extend and improve the prioritized power allocation solution proposed in the beamspace SU-MIMO to obtain better system performance.The results show that the beamspace MU-MIMO can further improve mmWave network performance in terms of the transmission rate and the robustness of the network connection.Lastly,we extend the concept of interference immunity in the time domain of UWB technology into the mmWave beamspace MIMO and investigate the performance limits of interference immunity in dense mmWave networks and that of physical eavesdropping immunity under passive/active eavesdropping scenarios.For passive eavesdropping scenario,the potential eavesdropper uses a single directional beam to eavesdrop on the legitimate link.For active eavesdropping scenario,the potential eavesdropper interferes with the legitimate link reception with one beam while eavesdropping the secret messages with another beam.In addition,we consider both non-colluding and colluding eavesdroppers in the above two scenarios.Furthermore,considering the sensitivity of mmWave communications to link blockage,we analyze the impact of eavesdropper blockage to the secure connection of the legitimate link.The theoretical results show that,compared with conventional microwave communications,mmWave communications have inherent interference and eavesdropping immunity when the beamwidth,departure/arrival angles,transmission distance,or transmission power meets certain conditions.That is,some of the existing interference mitigation mechanisms and physical layer security techniques may be simplified or even unnecessary in mmWave networks,and thus the corresponding design and implementation cost of wireless systems can also be reduced.The numerical results also verified this conclusion.