While cellular networks have continuously evolved in recent years， the industry has clearly seen unprecedented challenges to meet the exponentially growing expectations in the near future. The 5G system is facing grand challenges such as the ever?increasing traffic volumes and remarkably diversified services connecting humans and machines alike. As a result， the future network has to deliver massively increased capacity， greater flexibility， incorporated computing capability， support of significantly extended battery lifetime， and accommodation of varying payloads with fast setup and low latency， etc. In particular， as 5G requires more spectrum resource， higher frequency bands are desirable. Nowadays， millimeter wave has been widely accepted as one of the main communication bands for 5G. As a result， envisioned 5G research and development are inclined to be heterogeneous， with possibly ultra dense network layouts due to their capability to support high speed connections， flexibility of resource management， and integration of distinct access technologies. In such a heterogeneous 5G structure， a large number of communication scenarios should be fully supported， including special ones involving high mobility （such as vehicular and high speed train communications and networks）.
　　Towards the heterogeneous 5G， the first and foremost hurdle lies in the channel measurement and modeling in the broad and diversified 5G scenarios. This special issue is dedicated to providing a platform to share and present the latest views and developments on 5G channel measurement and modeling issues.
　　This special issue includes five technical contributions from leading researchers in channel measurements and modeling. The first paper entitled “An Overview of Non?Stationary Property for Massive MIMO Channel Modeling” by ZHANG， CHEN， and TANG presents an overview of methods of modeling non?stationary properties on both the array and time axes， which are mainly divided into two major categories： birth?death （BD） process and cluster visibility region （VR） method. The main concepts and theories are described， together with useful implementation guidelines. In conclusion， a comparison between these two methods is made. The second paper is entitled “Measurement?Based Channel Characterization for 5G Wireless Communications on Campus Scenario” by YANG， HE， AI， XIONG， DONG， LI， WANG， FAN， and QIN. It investigates the radio channels of 5G communications below 6 GHz according to the requirements and scenarios of 5G communications. Channel measurements were conducted on campus of Beijing Jiaotong University， China at two key optional frequency bands below 6 GHz. By using the measured data， the key channel parameters at 460 MHz and 3.5 GHz are analyzed， such as power delay profile， path loss exponent， shadow fading， and delay spread. The results are helpful for the 5G communication system design. The third paper， co?authored by ZHANG， WANG， WU， and ZHANG， is entitled “A Survey of Massive MIMO Channel Measurements and Models”. In this paper， the channel measurements and models of massive MIMO in recent years are summarized globally. Besides， their work on related 256 antenna elements with 200 MHz bandwidth at 3.5 GHz， the verification of rationality of measurement method， and the spatial evolution of clusters in mobile scenario are provided. The next paper， co?authored by WANG， GENG， ZHAO， HONG， and Haneda， is entitled “Feasibility Study of 60 GHz UWB System for Gigabit M2M Communications”. In this paper， the feasibility and performance of mm?wave 60 GHz ultra?wide band （UWB） systems for gigabit machine?to? machine （M2M） communications are analyzed. Specifically， based on specifications and channel measurements and models for both line?of?sight LOS and non?LOS （NLOS） scenarios， 60 GHz propagation mechanisms are summarized and 60 GHz UWB link budget and performance are analyzed. The goal of this work is to provide useful information for standardizations and design of 60 GHz UWB systems. The last （but not least） paper “Measurement?Based Spatial?Consistent Channel Modeling Involving Clusters of Scatterers” is co?authored by YIN， ZHANG， WANG， and CHENG. In this paper， the conventional method of establishing spatial channel models （SCMs） based on measurements is extended by including clusters?of?scatterers （CoSs） that exist along propagation paths. Channel models resulted by utilizing this new method are applicable for generating channel realizations of reasonable spatial consistency which is required for designing techniques and systems of the 5G wireless communications.
　　We would like to thank all the authors for choosing this special issue to publish their new research results and all the reviewers for their meticulous review comments and suggestions that help to improve the technical quality and presentation of this special issue. We hope that our readers will enjoy reading the articles and find this special issue helpful to their own research work.