Managing 5G Spectrum

Is managing spectrum for 5G fundamentally different? If that is so, then why? Shouldn't 5G follow the same principles for spectrum management as did the earlier versions of technologies? What makes 5G so different? The following note is an attempt to answer these questions (simply as possible) without sacrificing the technical details (important in supporting the rationale). Before I dwell on explaining the nuances it is important to understand the fundamentals of RF system (Radio Frequency)- used for carrying information from one point to another.

How RF System Works?

A typical Radio Frequency System is made up of a set of transmitters and receivers talking to each other using wireless spectrum. The transmitter encodes the information and sends it over the air towards the receiver located some distance away. The receiver decodes it for further consumptions. Both "transmitter" and "receiver" are embedded in every wireless equipment. However, in an FDD system (Frequency Division Duplex), they are always on, as both transmitters and receivers use different blocks of spectrum - separated out by a guard band. Whereas in case of a TDD system (Time Division Duplex), they change roles depending on which one is transmitting and use the same block of spectrum. See figure under.

How RF System Manages Interference?

In FDD system, the transmitters and receivers are always ON. Hence, a "guard band" (of sufficient width) is used to prevent the radio waves from the transmitter spilling into the receiver. Please note that out of band emissions cannot be prevented in a wireless system, as the filters aren't perfect. But sharper the cut-off characteristics of the transmitting filter, the lesser will be the interference.

Whereas in a TDD system, the interference is prevented by synchronising the transmitters and receivers so that they do not operate at the same time, but at predetermined intervals programmed by the network operator. However, there is a problem. Why is the problem relevant for 5G? As most 5G systems operate in TDD spectrum, unlike 4G systems which operate in FDD spectrum. But why? As TDD system works best in the higher frequency spectrum bands where we can find the larger blocks of spectrum - needed by 5G systems to drive higher speeds. But what is this problem? Let's find out.

Managing Coexistence of 5G & 4G systems

Can two TDD system belonging to different operators coexist in adjacent spectrum blocks? Yes, provided the transmitting frames of both the TDD systems are synchronized with each other. This is easily possible in case of two adjacent systems are either 4G or 5G. As they will have identical frames, and doing so will keep the signals from interfering with each other. However, operating 4G and 5G systems adjacents with each other creates a problem as both have different frame structure. See figure below.

Each 4G frame consists of time slots of 1-millisecond width, whereas in case of 5G these slots are of 0.5-millisecond width. Being of unequal size, these frames cannot be synchronised, and the downlink (marked "D" and transmitting) slots will overlap with the uplink (marked "U" and receiving) slots. In absence of a guard band, this interference cannot be prevented. See figure below.

Hence, a guard band of sufficient width will be needed in case a 4G and 5G systems have to coexists with each other. But what is the relevance of this discussion? Let's say that 3.5 GHz band in India is auctioned in blocks of 10 MHz, and an operator decides to buy only two slots (20 MHz) to deploy 4G. But an operator adjacent to him is planning to deploy 5G. Then these two networks will interfere with each other. The only way to prevent this interference is to keep a guard band of sufficient width (could be as high as 20 MHz) - leading to wastage of precious spectrum. Therefore, 3.5 GHz band in India should be auctioned in blocks of 40 MHz or higher to encourage the operators to use it for deploying only 5G.

Managing Border Interference

All TDD system (4G or 5G) suffer from this problem. But why? As both transmitter and receiver use the same block of spectrum. Unlike an FDD system which uses different blocks (spaced out by a guard band). Hold on, did we not manage this issue by synchonizing the transmitter and the receiver? We did, but the problem does not fully evaporate. Why? This is due to the fact that a portion of the signal remains alive (progressing in the air) and interferes with the equipment on the other side as soon as it switches role. To prevent this interference, the transmitter on the other side need to be blocked for the duration of the travelling time of the signal - leading to loss of efficiency. The problem gets magnified in an uncoordinated situation (typically the case when networks are managed by different entities), as the RF signal ends up travelling hundreds of km using the ducts enabled by the atmosphere. See figure below.

Different Operators

Same Operators

Conclusion

This leads us to two major conclusions.

a) 5G spectrum in the 3.5 GHz band should be auctioned on a Pan-India basis and not on a circle by circle basis as was being done for other spectrum bands.

b) 5G spectrum in the 3.5 GHz band should be auctioned with block sizes of 40 MHz to encourage it for being used for 5G and not 4G (aggregating large spectrum blocks using 4G becomes challenging).

Following these approaches will ensure interference-free 5G operations in India - enabling quality service to the end consumer.

(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.