Widespread variations in national and international regulations for Urban Small Cells are one of the hurdles holding back faster deployment. What can be done to standardise and expedite this? Here we consider aspects of RF transmit power rather than physical planning/zoning..
While there have been some 3G urban small cell trials, most of the activity of late has been around LTE only primarily to add capacity. So although the RF transmit power levels are independent of generation (2G, 3G or 4G), I’ve focussed on LTE.
RF power vs physical format
Firstly, there’s an important distinction between RF radiation from an outdoor small cell and its physical appearance. EIRP (Effective Isotropic Radiation Power) is the criteria applied. You could expect national regulators will set constraints for the power levels and proximity to the public based on health considerations. These should follow international guidelines.
The physical appearance (size, colour, weight etc.) are often determined by local planning authorities, who may have stronger views in more historic areas. As with larger tower sites, national regulators could define parameters below which planning approval is simplified or even exempted.
Defining RF power classes
There are two international standards regulating human exposure to base station RF (height, power limits):
ITU-T K.100, which has been adopted by India
IEC 62232, which supersedes EN 50400, and will be adopted throughout Europe
Both address the same issue, both are evolving, being simplified and becoming more closely aligned. Countries may choose to adopt either of these standards, with or without modifications.
There are two particular RF power classes of most relevance:
|E2||<2 Watts||Few centimeters from public areas|
|E10||<10 Watts||2.2 meters above public walkway|
Examples of some regional variations relates to the maximum power level where an exemption or simplified applies procedure applies.
Japan 20mW, Malaysia 2W, France up to 5W, Germany 10W
The United States nationally uses a combination of height and RF power, with local/state regulators adding their own criteria.
When considering higher transmit power, you also have to consider the uplink. Typical handsets today can achieve 1W at most (and would usually operate at much lower levels), so blasting a stronger signal doesn’t help unless you have a large antenna to receive the weaker signal on the return path. The physical size of small cell installations usually constrain antenna size and thus the value of higher output power.
Multiple-Input/Multiple-Output is a popular feature, especially for LTE. This requires two separate radios for each frequency connected to different antennas. It’s also possible to use a combined antenna which polarises the signals horizontally and vertically, directing each to a different path.
In this case, you have to add the total power of both radios. Thus 1W total might comprise 2x500mW radios.
Many operators have been rolling our Carrier Aggregation, combining two or even three separate frequencies to achieve faster data rates. This doesn’t add extra capacity per se. There is some argument that there are slight efficiency improvements compared to serving different users on independent frequencies, but it is doubling the spectrum that provides the biggest benefit.
Where dual carrier small cells are installed, the combined RF power must be considered. This could be 1W+1W for an E2 class installation. Where MIMO is used, this would require four radios each of 500mW.
Higher data speeds can also be achieved by paralleling data streams with a macrocell, using Carrier Aggregation or Dual Connectivity – both standard LTE features.
You will often see headline peak data rates quoted in press announcements, claiming anything up to 1Gbps or more. In areas of high traffic capacity serving many users, you would rarely find this is achievable at busy times. The very high modulation rates of 256QAM fall back to lower levels, reducing the peak rates possible. Small Cells located closer to the end user can improve signal quality and thus regain higher speeds through higher QAM.
There is therefore an argument that dual carrier small cells are useful but not mandatory.
Shared Small Cells
As for larger cell towers, urban small cell sites could also be shared between network operators. In the case, the combined RF power of an installation would also need to fit within the power classes. Typically this would either be 1W or 5W each.
I’ve heard of several countries where operators are considering some site sharing arrangements, typically with two operators per site and not more. In most countries with a total of four operators, this would mean two parallel sets of deployments rather than four.
One approach is to install two independent small cells housed within the same physical case, sharing backhaul and power. Following on from the previous assertion, carrier aggregation and MIMO would require eight radios in total which is unlikely. My guess would be to prioritise MIMO first. Network sharing standards such as MORAN, which allow a single basestation unit to be shared between operators (but with each transmitting on their own spectrum), would save product cost.
Another approach for closer network integration uses the MOCN standard. A single small cell, using frequencies assigned to a single network operator, is shared between two or more networks. A potential benefit here is that the RF power available at each site can be fully utilised for each individual subscriber, regardless of their home network.
A third approach is to use DAS (distributed antenna systems), which requires dedicated dark fibres to every site. Centralised basestation hotels with equipment from each mobile network drive the system, sharing power, backhaul and RF heads at every point. My concerns would be that the cost of the RF heads is unlikely to be much less than a Small Cell, and the cost and availability of dark fibre won’t be viable in some countries.
Clearly the impending updates to ITU-T and IEC specifications will set a worldwide benchmark for individual regions and countries to adopt. I would think the 2W and 10W classes are most relevant, and would be widely adopted.
It’s important to have as few variants worldwide as possible, in order to allow manufacturers to design and build the smallest range. This helps achieve lower cost, shorter time to market and operational simplicity for the benefit of all.
We should encourage national regulators to agree and adopt the simplest but most commonly appropriate criteria.
Setting a common goal
It would be helpful for the industry to set some direction and define a subset of likely options/product requirements. I've not seen this yet.
My expectation for urban outdoor small cells would be for either
- LTE dual carrier with MIMO comprising four radios of 500mW each.
- LTE single carrier with MIMO or dual carrier non-MIMO with two radios of 500mW each.
and support for a variety of different frequency bands, mainly between 1800 and 2600MHz, using either TD-LTE or FDD mode as appropriate.