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Victor C. M. Leung is a professor of electrical and computer engineering and holder of the TELUS Mobility Research Chair at the University of British Columbia （UBS）， Canada. He has co?authored more than 1000 technical papers in the areas of wireless networks and mobile systems， in addition to 37 book chapters and 12 book titles. He is Fellow of the Royal Society of Canada， the Canadian Academy of Engineering and the Engineering Institute of Canada. He is also a Fellow of IEEE.
ZHANG Haijun is currently a full professor at University of Science and Technology Beijing， China. He was a Postdoctoral Research Fellow at Department of Electrical and Computer Engineering， the University of British Columbia （UBC）， Canada. He serves as editors of IEEE Transactions on Communications and IEEE 5G Tech Focus and leading guest editors for IEEE Communications Magazine and IEEE TETC. He received the IEEE ComSoc Young Author Best Paper Award in 2017. He is a Senior Member of IEEE.
With the continuous enrichment of mobile communication application scenarios in the future， the traditional macro?cellular?based mobile communication network architecture will be difficult to meet the explosive growth in demand for communications services. A promising solution is the deployment of ultra dense networks （UDNs） comprising flexibly deployed low?power small base stations （BSs）， such as microcell BSs， picocell BSs and femtocell BSs. In 5G， ultra dense networks that are deployed with low?cost and low?power small cells are expected to enhance the overall performance of the network in terms of energy efficiency and load balancing. The essence of ultra dense cell deployment is to shorten the physical distance between the transmitter and the receiver， so as to improve the performance of the system.
For this feature topic， academic and industrial researchers have been invited to discuss technical challenges and recent results related to future mobile networks employing UDNs. After a rigorous review process， five papers have been selected for inclusion of this feature topic.
In the first article， LI Tongxin et al. discuss the intrinsic features and potential benefits of unmanned aerial vehicles （UAVs） and introduce the architecture of a multi?layer heterogeneous wireless network （MHetNet）， in which traditional wireless network is assisted by UAVs. Then， an explicit discussion on the factors that limit the performance of MHetNet is presented. Simulations show that the altitude of UAV is a limiting factor that should be optimized to improve the spatial throughput （ST） of MHetNet. The second article by JIN Yaqi et al. proposes a multi?QoS guaranteed resource allocation algorithm for multi?services based on opportunity cost. This algorithm can achieve a well?done balance between user satisfaction and system fairness. The authors first formulate a unified utility function with effective capacity to describe the multi?QoS metrics of different services. Then they introduce the theory of opportunity cost in economy to form the concept of opportunity cost applying. Finally， the simulation results show that the algorithm can achieve superior overall user satisfaction.
In the third article， FENG Hong et al. propose a wireless backhaul algorithm to find an effective backhaul method for the densely?deployed small base stations （SBSs） and to maximize the energy efficiency of the system. They put forward adaptive backhaul methods of indirect and direct modes. At the same time， the algorithm allocates network resources， including the power of SBSs and system bandwidth， to solve the serious interference problem in UDNs. Simulation results show that the proposed wireless backhaul algorithm has desired performance to achieve higher energy efficiency with a required data rate.
There are transport networks with different performance that varies from high transport latency to low transport latency in the real deployment； in order to cater for these various types of transport networks and realize multi?vendor centralized unit and distributed unit （CU?DU） operation， the radio access network （RAN） architecture in New Radio （NR） is split into two kinds of entities， i.e.， CU and DU. In the fourth article， GAO Yin et al. introduce the general aspects of CU?DU split architecture， including the split method， interface function， mobility scenarios and other CU?DU related issues.
In the fifth article， GAO Tengshuang et al. propose a linear interpolation method for time?division duplex coordinated multiple point transmission （TDD?CoMP） systems to estimate the uplink channel at the receiver， which can reduce the channel difference caused by time delay and decrease the probability of codeword mismatch between both sides. Moreover， to mitigate severe inter?cell interference and increase the coverage and throughput of cell?edge users in UDN， a two?codebook scheme is used to strengthen cooperation between base stations （BSs）. Simulations show that the proposed scheme can significantly improve the link performance compared to the global precoding scheme.
This special issue covers the network architectures， key technologies， challenges， and methodologies of UDN， and it gathers the researchers of the related areas to analysis the future development of UDN in detail.
ZHANG Haijun is currently a full professor at University of Science and Technology Beijing， China. He was a Postdoctoral Research Fellow at Department of Electrical and Computer Engineering， the University of British Columbia （UBC）， Canada. He serves as editors of IEEE Transactions on Communications and IEEE 5G Tech Focus and leading guest editors for IEEE Communications Magazine and IEEE TETC. He received the IEEE ComSoc Young Author Best Paper Award in 2017. He is a Senior Member of IEEE.
With the continuous enrichment of mobile communication application scenarios in the future， the traditional macro?cellular?based mobile communication network architecture will be difficult to meet the explosive growth in demand for communications services. A promising solution is the deployment of ultra dense networks （UDNs） comprising flexibly deployed low?power small base stations （BSs）， such as microcell BSs， picocell BSs and femtocell BSs. In 5G， ultra dense networks that are deployed with low?cost and low?power small cells are expected to enhance the overall performance of the network in terms of energy efficiency and load balancing. The essence of ultra dense cell deployment is to shorten the physical distance between the transmitter and the receiver， so as to improve the performance of the system.
For this feature topic， academic and industrial researchers have been invited to discuss technical challenges and recent results related to future mobile networks employing UDNs. After a rigorous review process， five papers have been selected for inclusion of this feature topic.
In the first article， LI Tongxin et al. discuss the intrinsic features and potential benefits of unmanned aerial vehicles （UAVs） and introduce the architecture of a multi?layer heterogeneous wireless network （MHetNet）， in which traditional wireless network is assisted by UAVs. Then， an explicit discussion on the factors that limit the performance of MHetNet is presented. Simulations show that the altitude of UAV is a limiting factor that should be optimized to improve the spatial throughput （ST） of MHetNet. The second article by JIN Yaqi et al. proposes a multi?QoS guaranteed resource allocation algorithm for multi?services based on opportunity cost. This algorithm can achieve a well?done balance between user satisfaction and system fairness. The authors first formulate a unified utility function with effective capacity to describe the multi?QoS metrics of different services. Then they introduce the theory of opportunity cost in economy to form the concept of opportunity cost applying. Finally， the simulation results show that the algorithm can achieve superior overall user satisfaction.
In the third article， FENG Hong et al. propose a wireless backhaul algorithm to find an effective backhaul method for the densely?deployed small base stations （SBSs） and to maximize the energy efficiency of the system. They put forward adaptive backhaul methods of indirect and direct modes. At the same time， the algorithm allocates network resources， including the power of SBSs and system bandwidth， to solve the serious interference problem in UDNs. Simulation results show that the proposed wireless backhaul algorithm has desired performance to achieve higher energy efficiency with a required data rate.
There are transport networks with different performance that varies from high transport latency to low transport latency in the real deployment； in order to cater for these various types of transport networks and realize multi?vendor centralized unit and distributed unit （CU?DU） operation， the radio access network （RAN） architecture in New Radio （NR） is split into two kinds of entities， i.e.， CU and DU. In the fourth article， GAO Yin et al. introduce the general aspects of CU?DU split architecture， including the split method， interface function， mobility scenarios and other CU?DU related issues.
In the fifth article， GAO Tengshuang et al. propose a linear interpolation method for time?division duplex coordinated multiple point transmission （TDD?CoMP） systems to estimate the uplink channel at the receiver， which can reduce the channel difference caused by time delay and decrease the probability of codeword mismatch between both sides. Moreover， to mitigate severe inter?cell interference and increase the coverage and throughput of cell?edge users in UDN， a two?codebook scheme is used to strengthen cooperation between base stations （BSs）. Simulations show that the proposed scheme can significantly improve the link performance compared to the global precoding scheme.
This special issue covers the network architectures， key technologies， challenges， and methodologies of UDN， and it gathers the researchers of the related areas to analysis the future development of UDN in detail.