Coping with congestion in unlicensed spectrum.

Introduction: Spectrum use has several dimensions: time, frequency and location. Not the entire spectrum is used all the time. This practice note reviews some of the options available for reducing congestion.

It has been observed that for much of the time, some of the spectrum apparently goes unused. This has led to the proposal that radios be allowed to locate and hop onto temporarily unused pieces of spectrum and remain there until the owner of the spectrum wishes to make a transmission. However, there is still a risk of interference as a result of the ‘hidden terminal’ problem –that a transmitter may not be aware of the location of a competing receiver.  As a result, some form of central management is required to tell terminals whether the spectrum was free and to grant them access. Hence such hopping behaviour will not work without band management, which in turn implies some form of band ownership, rather than unlicensed use.

In addition to transmit power, there are some other factors which can have an impact on the probability of congestion. These are:

  • Restricting the type of equipment which can be used will tend to prevent the band being used for certain applications;
  • Making the equipment more efficient so it uses less of the spectrum resource in transmitting or receiving its message;
  • Making the equipment “polite” so that it does not transmit if doing so would interrupt on-going transmissions.

The first approach essentially blocks a particular application from unlicensed spectrum, or from some of the unlicensed bands. Such a decision would need to be made on the basis that allowing this application would likely reduce the overall utility from the band. In practice, it would be an engineering-based judgement in that allowing the application would result in a high probability of congestion, or excessive interference to existing users.

The last two approaches will tend to make the equipment more expensive, with no apparent gain for the end-user, and so will require regulatory intervention in the form of type approval or similar action. Even so, there may be enforcement problems, particularly if the increase in the price of the equipment is substantial. In this case, users may be tempted to acquire simpler, non type-approved equipment that might perhaps be legal in other countries. Because of the short-range and short-duration nature of most of the transmissions in these bands, enforcement could be difficult.

To date, the key regulatory mechanisms have been to restrict the equipment that can be used and to demand politeness. An extreme example of the former is the DECT band where only DECT equipment is allowed to operate. An example of the latter is the 5GHz unlicensed band where European regulators have required that equipment using this band have dynamic frequency selection (DFS) which seeks a lightly-used frequency within the band before transmitting.

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