Spectrum is the life-blood of a telecom operator – it’s literally a range of electromagnetic waves, and when we talk about telecom spectrum, we’re talking about the frequencies that are used to transmit sound and data across the country to our phones. Every telecom operator has been assigned certain portions of spectrum to use in India, through auctions and administrative allocations. Essentially, you have spectrum “bands”, and frequencies around a particular band are then auctioned off.
The allotment of spectrum is either paired or unpaired. In case of paired spectrum, different sets of frequencies are allocated for uplink and downlink. For example in case of 900MHz spectrum, the DoT allocated frequencies between 890MHz – 915Mhz for uplink, and between 935MHz – 960Mhz for downlink in each circle. Source