5G Spectrum in 3.5 GHz band should be assigned "Pan-India" and not "Circle by Circle", but why?

The reason is simple. 3.5 GHz band is TDD (time division duplex) band, and such spectrum if assigned on circle basis (with different operators occupying the same spectrum blocks in the adjacent circles), is likely to severely interfere with each other. The reason for such interference is explained below.

Interefence in TDD Spectrum

The concept is a little difficult to understand, but let me make an attempt. FDD spectrum (unlike TDD) is like the double road with a divider, i.e one side of the road supports traffic in only one direction and the other side support traffic in the opposite direction (traffic here is used as a metaphor for radio waves). Whereas TDD (Time Division Duplex) is like a road without a divider, where both sides of the road are used simultaneously for transmitting signals in either direction. The TDD receiver has to be synchronized such that it opens up to receiving signals only when the transmitter (on the other side) is radiating - unlike FDD which is always on. This creates a problem for TDD (especially at the lower frequencies). As at these frequencies, the radio waves travel far, and therefore the receivers (which are spaced apart) have to remain blocked for a duration of time to take care of the "travelling time" of the transmitting signal to reach the receiver (to prevent interference). This results in inefficiencies (higher the blocking time more is the inefficiency), compared to FDD where no such synchronization is required. Also, such interference in the TDD spectrum travel hundreds of km across the border compared to the FDD spectrum. That is the reason why spectrum in TDD band is best assigned on a Pan India basis, then on a circle by circle basis.

Interference in the 2300 MHz band

2300 MHz band was auctioned on a circle by circle basis in the year 2010. This resulted in different operators occupying the same spectrum block in the adjacent circles. One such scenario is laid out in the following map (2352.5 to 2355 MHz).

For other assignments in the 2300 MHz across all circles, please refer to the following movie. Each slide in the frame describes a spectrum block of 2.5 MHz.

Harmonization of 2300 MHz band

Hence, the 2300 MHz band needs to be harmonized. The process is described in my earlier note titled "Harmonizing the 2300 MHz Band". However, post-auction, it is a challenge to steer coordination between rival operators - making the process difficult to execute. Option 1 described in the note is the ideal plan. However, a fallback option 2 will have border interference described in the following movie. Each slide in the frame describes a spectrum block of 2.5 MHz.

Harmonization More Difficult in 3.5 GHz band

Spectrum in the 3.5 GHz band will be more difficult to harmonize compared to the 2.3 GHz band. This is due to the fact that unlike the 2.3 GHz, spectrum in 3.5 GHz band will be used for 5G - requiring each operator to hold at least 100 MHz of spectrum, i.e 3 to 4 times that of the 2.3 GHz band. The situation will further aggravate if the block size is kept small, i.e 20 MHz (at par with 2.3 GHz band). As such auction design will easily fragment the band - making the harmonization process extremely difficult if not impossible to execute.

Conclusion

Hence, 3.5 GHz band should be auctioned on a Pan-India basis and Not on a circle by circle basis as was being done in past. This will significantly increase the operational efficiency of 5G networks in India - enabling the consumers to experience interference free 5G services.

(Views expressed are of my own and do not reflect that of my employer)

PS: Find the list of other relevant articles in the embedded link.