What does Sub 6GHz Licensed and Unlicensed mean?
Several vendors and operators use this term: Find out what “Sub 6” means in practice.
What is Sub 6 ?
“Sub 6” means frequencies below 6GHz. Though frequencies from 1GHz up to 6GHz are still classified as microwave frequencies, they are often referred to “radio links”, “microwave links”, “microwave radio links” with these terms used interchangeably.
Why Consider Sub 6GHz?
Typically links below 6GHz are used for longer point-to-point links, or point-to-multipoint links for last-mile access to customers. Frequencies below 6GHz do not suffer significant rain fade. In addition, these lower frequencies can be used for Non-Line-of-Sight Links, in cases where there is no direct Line of Sight between the locations that require connection. The radio propagation characteristics of lower-frequency bands make them ideal for urban areas where radio signals may reflect from buildings and other man-made objects, and can – within limitations – penetrate walls, brickwork and concrete structures.
What does Unlicensed and Licensed mean?
The term Unlicensed in radio technology includes commonly used bands which can be used in many countries without need for a frequency license, such as 2.4GHz and 5.x GHz bands including 5.2GHz, 5.4GHz and 5.8GHz. Please note that in a few countries these frequencies still require licenses, or are not usable by private users.
Unlicensed frequencies have the benefit of not requiring a license to operate (typically, licenses have an annual fee, and are issued by a national regulator or state owned telecom operator). However, unlicensed links can be interfered with by other users, which can cause reduced throughput or complete link outage. Such interference is generally heavier in high density population areas and cities, where 100’s or 1000’s of radios may be competing for the same spectrum in a given region.
Conversely, licensed operation means that the equipment user has to obtain a frequency license before using the band. This can be available on a per-link basis, in which case the regulator allocates specific frequencies for a particular link, holding a central database of all links, or in the case of mobile operator networks, a country-wide license within which the operator self-coordinates the allocation of frequencies and coverage.
The lack of predictability in unlicensed bands is the main reason that operators prefer licensed bands for operation, despite the additional costs of licenses required to operate.
Single Carrier and OFDM Modulation
In the “Sub-6” bands 1-6GHz, a range of Single Carrier, OFDM and OFDM-A technology solutions are available. OFDM and OFDM-A use multiple subcarriers, and can use the properties of this modulation to overcome multipath fading and reflections from hard surfaces present in dense city areas. Conversely, Single Carrier radios use dense modulation with high symbol rates on a single radio carrier. This can give high spectral efficiency and data rates, but limited ability to cope with reflected signals, and hence worse performance in non-LOS situations.
Line of Sight, Non-Line-of-Sight, Near-Line-of-Sight and Radio Propagation
OFDM modulation is generally used in Sub-6 radios and is more suitable to rapidly fading and reflected signals, hence for mobility and non-line-of-sight (non-LOS, NLOS, Near-LOS, nLOS) applications. Generally, the lower the frequency band, the better non-LOS characteristics it has, improving range and in-building coverage and penetration through windows, walls, brickwork and stone.
4G & 5G Mobile and Fixed Networks
Both 4G and 5G technologies defined by the 3GPP use OFDM and OFDM-A technology in the sub-6GHz bands to deliver high speed fixed and mobile data services. These classify as “sub 6” but are rarely referred to as such. MIMO (Multiple Input, Multiple Output) technology is added on top of OFDM to increase throughput still higher. More recently, 5G includes “millimeter wave” bands above 20GHz to add still higher speed services and overcome congestion in lower frequency bands. It is envisaged that users could roam seamlessly between regions with “Sub 6” and “millimeter wave” coverage with suitable handsets or terminal devices.
Managing the Finite Spectrum Available in 1-6GHz
An obvious downside of Sub-6GHz is the limited spectrum available. There is just 5GHz of spectrum available between 1-6GHz which has to be allocated between multiple applications for Telecom Operators, Government and Private networks, utilising signals that can travel 10-50km or more and therefore potentially interfering with each other if inadequately managed. Though most applications are terrestrial, the bands include space for ground-satellite services which again have to avoid interference. Increasingly, frequency regulation is a global issue with international roaming, and huge spectrum demands and pressure on spectrum from Mobile Network Operators who face ever increasing demands for mobile data users worldwide. To meet this demand, spectrum is continually re-farmed and re-allocated between older 2G and 3G services to 4G and 5G services which are capable of delivering higher capacity services. Legacy frequency allocations to Government and Military applications are often released for lease to such operators also.
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