The transition to 5G from 4G could happen much faster compared to the switch from 3G to 4G services.
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Indian government has NOT allocated airwaves for 5G once 5G airwaves(spectrum) is allocated then the race will begin between Jio, Airtel, and Vodaphone-idea.
The government recently said it plans to allocate airwaves to offer 5G services – which can support download speeds 50 to 60 times faster than 4G – by end of 2019.
If sources are to be believed then JIO can have a fully operational 5G network within 6 months of airwaves allocation. so India should get 5G by 2020.
Considering the scale of the project you can add 6 to 12 months, as these projects are prone to delays. so by 2021, we should definitely have 5G. (figures crossed)
if you want to know more about 5G and how it works: http://learnersglobe.com/what-is-5g-%F0%9F%8C%8E%E2%9A%A1-%E2%9D%93%E2%9D%93/