Licensed and Unlicensed Spectrum
- (Salem, Massachusetts - ROC (Taiwan) Student Association of MIT)
Spectrum Bands: Licensed vs. Unlicensed
All wireless technologies use the airwaves to transmit and receive information. In order for many different technologies to use the airwaves simultaneously, wireless spectrum is carved up into chunks called frequency bands. Licensing these frequencies is a way of ensuring Mobile Network Operators (MNOs) do not interfere with each other's transmissions.
Radio Spectrum, in general, can be categorised into two types, (a) licensed - assigned exclusively to operators for independent usage, ( b) unlicensed - assigned to every citizen for non-exclusive usage subject to some regulatory constraints, like restrictions in transmission power etc.
Spectrum is a powerful resource that we can leverage as the industry develops. It is more important than ever. The world has gone mobile, and as the demand for wireless services increases, our dependence on spectrum will only grow greater. As things stand, the licensing model limits our ability to grow our use of wireless broadband, so the differences between licensed and unlicensed spectrum, and the way this is approached, will become increasingly important.
There are a number of “licensed frequencies” available. Licenses can be obtained from FCC, and most of the licensed equipment vendors will assist in obtaining a spectrum license for your link (most licenses are per-link; per-area licenses are typically very expensive, if they are available at all – the major cellphone companies own the majority of the licensed spectrum for wide-area coverage).
Advantages and Disadvantages To Using Licensed Spectrum
- Advantages To Using Licensed Spectrum
The advantages to using licensed spectrum include:
- No Congestion. If you have a license, then the spectrum covered by the license is yours. If interference appears on your link, you have the right to involve the authorities in removing the interfering parties from your licensed area.
- Performance. Many licensed links out-perform unlicensed links, mostly because of the lack of congestion (above), but also because some of the “better” frequencies are only available with a license.
- It can be used more efficiently. The mobile network operators (MNOs) can exercise more flexibility in deploying their cellular networks to manage interference. The base stations (transmitting signals to devices) can be spaced apart, thereby optimising network costs - making services affordable.
- Disadvantages To Using Licensed Spectrum
There are also significant disadvantages to using licensed spectrum:
- Equipment Cost. Equipment for licensed spectrum is often several orders of magnitude more expensive than equipment for unlicensed spectrum. The majority of the buyers are large, so there is no impetus for equipment vendors to aggressively reduce pricing.
- License Cost. Purchasing licenses can be expensive, especially if your network is large and requires many licensed links. Additionally, maintaining spectrum licenses can require FCC attorney assistance (or your regional equivalent), which can be a very large expense in and of itself.
Most start-up WISPs (Wireless ISPs) operate entirely in unlicensed frequencies. As they grow, licensed options can become attractive for heavily utilized “backhaul” [trunk routes between towers] sites. It is therefore recommended that WISPs remain mindful of licensed options for the future, but start out in unlicensed territory to reduce start-up costs.
Advantages and Disadvantages To Using Unlicensed Spectrum
- Advantages To Using Unlicensed Spectrum
- It can be used more efficiently. The mobile network operators (MNOs) can exercise more flexibility in deploying their cellular networks to manage interference. The base stations (transmitting signals to devices) can be spaced apart, thereby optimising network costs - making services affordable.
- There are no fees associated with using it.
- Equipment for the popular unlicensed spectrum bands is widely available, at a reasonable cost.
- There is considerable practical knowledge to be found on the properties of unlicensed spectrum, because of its widespread use.
- Disadvantages To Utilizing Unlicensed Spectrum
There are also disadvantages to utilizing unlicensed spectrum. These include:
- Congestion. Some frequency bands are heavily utilized, particularly in cities, and may be rendered useless by other mobile network operators (MNOs). This is particularly true of the 2.4ghz spectrum band, which is commonly used by home and business Wi-Fi. There are, however, a number of tools available to help you make use of this spectrum even in crowded areas – and with proper insight, planning and monitoring it is entirely possible to co-exist with other systems.
- Regulatory Limits. Many frequency bands operate under regulatory limits on effective radiated power (“EIRP” - Effective Isotropic Radiated Power). It is necessary to be aware of the regulations affecting your region, and to comply with these restrictions.
- Managing Interference Between Adjacent Users
Without licensing, interference would cause problems on the transmissions on both sides. It's a good way to ensure that every provider knows where the others' signal is and to ensure that users get the best quality because there is a dedicated frequency for their use. The downside of the unlicensed frequencies or bands is that anyone else can use the same frequency ranges, which can cause interference, so when using an unlicensed technology like WiFi, users may have to make adjustments to avoid this interference. Regulations exist around these bands, so using unlicensed radio bands is not a free-for-all. The big difference between licensed and unlicensed bands is that the licensed bands are allowed to be used only by the company that licensed them, whereas the unlicensed bands are used by anyone who wants to use them.
It can be done using the following strategies:
- Reducing transmitting power - Radio waves will die quickly before its collides with adjacent users.
- Use higher frequency bands - Radio waves travelling at higher frequency bands die much faster (at an exponential rate) compared with those travelling at lower frequency bands.
- Use frequency hopping - Transmitting equipment are empowered with intelligence to select a specific block of spectrum (out of a large number) which is relatively interference free than others. This ends up wasting a lot of spectrum, as only a fraction of the total assigned is used at a time.
[More to come ...]
