The　fifth　generation　(5G)　mobile　communication　systems　are　being　widely　deployed　and　tested　worldwide,　and　the　key　techniques　are　gradually　maturing　in　2021.　Considering　the　general　needs　in　the　future　of　the　explosive　growth　of　data,　researchers　around　the　world　are　beginning　to　study　the　next-generation　system,　i.e.,　the　sixth　generation　(6G)　communication　systems,　which　will　be　deployed　in　the　future　network　of　2030.　This　review　begins　with　the　historical　development　of　communication　systems,　and　elaborates　on　the　current　and　future　innovations　of　communication　technologies.　The　key　technology　of　5G,　massive　multiple-input　multiple-output　(MIMO),　is　introduced,　especially　into　the　advantages　and　challenges　to　the　deployment　of　5G.　Meanwhile,　the　potential　key　technology　in　6G,　intelligent　reflecting　surface　(IRS)　is　also　presented,　the　concept　of　IRS　and　the　advantage　in　the　aspect　of　energy　efficiency　is　given　in　the　paper.　©　2021　IEEE.