Indoor Base Stations

In the previously described methods for increasing radio coverage, the first ones to be developed aimed at increasing the indoor signal level by using the signal emitted by outdoor base stations (like macrocell, microcells and repeaters), and the later ones aimed at extending the coverage of the base stations inside a building by using radiating cable or distributed antenna systems. A new proposal for increasing both the coverage and the capacity, is to deploy small base stations directly inside the buildings as represented in Figure 2.11. Two main approaches have thus been proposed: the picocells and the femtocells.

2.6.1 Picocells

With the success of IEEE 208.11 standard, also called WiFi (Wireless Fidelity), which allows people to access to their broadband Internet connection via air interface, operators started to think about extending this concept to their mobile networks. In WiFi, the user connects to a device called an Access Point (or AP), which integrates an antenna to make the link between the user, and the connection to the Internet. A picocell is a small base station very similar to an access point. It is usually small (typically A4 paper size, and a few centimetres thick), and integrates an antenna that radiates a low power signal.

Indeed, a picocell is a simplified base station, with low power and lower capacity than microcell or macrocell base stations. It connects to the Base Station Controller (BSC) of the operator. As with standard base stations, the BSC manages the transmission of data between the picocell and the network, and performs the hand-overs between the cells and the allocation of the resources to the different users. The picocell is connected to the core network via standard in-building wiring, fibre optic or Ethernet connection. Usually an omni-directionnal antenna is integrated into the picocell [19].

Advantages

The main advantage of picocells is that they are cheaper than standard base stations, and the installation cost is also lower. They effectively increase indoor coverage because they

Indoor Base Stations 31

BS

BS

BS BS

Figure 2.11 Indoor base station radio coverage of a three-floor building

are installed indoors. The coverage area of the cells is small compared with outdoor base stations, because the radiated power is lower, and also because of the numerous reflections and diffractions due to the walls and other obstacles inside the building. Thus, covering the inside of a building requires the use of many picocells compared with outdoor cells.

This allows the operator to have more cells, and thus increase the capacity of the network inside the building. That is why picocells are deployed in indoor areas containing a high density of users.

Moreover, by installing a picocell inside a building, the operator can have more capacity in the outdoor network because the outdoor cells that are used to cover the building become available just to outdoor users. In many situations, it also gives an opportunity for the operator to reduce the radiated power of the outdoor cells used to cover the building.

That is why picocells not only increase the capacity inside the building, but also increase the outdoor capacity, and reduce the outdoor interference because the overlap between outdoor cells can be reduced. Picocells, because they are small cells, can also be used in scenarios where localization is important [20]. Indeed, Indoor localization using outdoor cells and triangulation methods, is not accurate enough due to the reflections from the obstacles. However, with small cells it can be easily ascertained which building the user is in, and if there are many picocells a more accurate position of the user is known.

Deployment

Like DAS or repeaters, deployment of picocell networks needs special attention. Where the picocells should be located and what are the best parameters is the main challenge. For the

deployment of picocells, an optimal solution would be that where the number of picocells is sufficient to ensure the coverage and capacity requirements, but also a solution where the number of picocells is not too high in order to avoid interference. Picocells are mainly deployed in large areas like commercial centres or airports, thus such an installation can be challenging. When installing picocells another important effect to be minimized is the interference with neighbouring outdoor cells. Indeed, in buildings close to outdoor cells, a part of the signal coming from outside will interfere with the indoor signal. Moreover, a part of the signal emitted by picocells will go outside and interfere with the outdoor cells.

These phenomena mainly occur through windows, because the signal reflects less from glass than from stone or concrete. Thus the main challenge when installing picocells is taking into account the outdoor cells. Hence, combined indoor/outdoor network planning is an important issue.

In typical multifloor buildings, the approach to installing picocells is quite similar to that used when installing DAS. When installing a DAS, antennas connected to the same base station are distributed between the different floors, whereas in a picocell network installation, base stations are directly distributed between the floors. The main challenges with picocells are the interference between the picocells that are at the edge of outdoor cells [21], and managing the handoffs between the picocells [22]. In [23], some simula-tions for overlapping GSM picocells are described, and it is shown that to improve the performance a good solution is to take into account the picocells positions accurately.

This can be done by entering in the Base Station Sub-system (BSS) database the relative positions between the picocells.

Applications

As detailed before, the advantage of picocells is that they efficiently increase both the indoor radio coverage and the capacity. They are a very good way of fulfilling two requirements:

• Filling the macro network coverage holes where the signal level is too low.

• Offloading traffic from the macro network in dense urban areas.

Some examples of applications include business environments and shopping centres. They are also useful in high-rise buildings. Indeed in many cities high-rise buildings are more and more common, but the macrocell signal strength tends to get weaker the higher you go. This is due to the fact that very often, in densely populated areas, operators have to add more outdoor cells. To increase the capacity, the size of these cells has to be reduced, which is why operators tend to minimize the antenna tilt. The immediate effect of this approach is poor coverage at the high levels of many buildings. Picocells placed at the high level of office environments can efficiently solve this issue.

Picocells are also useful in difficult buildings such as historical buildings (huge walls made of stone absorbing most of the signal coming from macrocells) or buildings made of complex shapes and materials (like metal structures or special glass windows for thermal efficiency).

Indoor Base Stations 33

Finally, picocells are also used in vehicular applications where the backhaul connection is ensured by satellite for example. Thus picocells can be used to provide phones for passengers in aircrafts or cruising ships.

Proposition of Small Picocells

With the success of picocells for multi-user indoor environments, in 2002 a group of engineers at Motorola started to develop the smallest UMTS base station. The main idea was to propose a WiFi-like solution but for mobile phone networks to deploy in the home environment. A few years later, the concept of a residential base station appeared, which aimed at a low power indoor solution for the home market. As represented in Figure 2.12, the idea of such small picocells is to cover only one house, which is why the low power should be adapted so that the cell size is between 20 and 30 metres maximum. This figure illustrates well how the cell sizes evolved over time, by reducing the size of the cells to fulfill the networks requirements, which are always more and more capacity demanding.

Such very small cells were called femtocells and will be presented below.

2.6.2 Femtocells

To extend the idea of picocells to home networks only, with an approach more similar to WiFi access points, femtocell base stations have been proposed. The femtocell is a simplified picocell directly installed by the customer in their home. It combines, in the same device, all the functionalities of a picocell and a BSC. Thus, instead of being connected to the operator’s BSC (like a picocell), the femtocell is connected directly to the Internet as represented in Figure 2.13. With femtocells, all the communications go to the operator’s network through the Internet, and there is no need for BSC/MSC infrastructure. Femtocells, because they typically cover a smaller area and have fewer users than picocells, and because they have to be cheap, are limited in output power and capacity (between 10 and 20 dBm, between four and six users). Within femtocell networks, outdoor users connect to the macrocells and when they enter their home they

Femtocell

Picocell Microcell Macrocell

<30 m ~100 m ~500 m >1 km

Figure 2.12 Comparison of cell sizes for different technologies

Internet

Core Network

Figure 2.13 Typical femtocell and macrocell scenario

handoff and connect to their femtocell. This ensures a smooth communication for the user and a maximal coverage is obtained inside the home [24].

Advantages

Femtocells are installed by users inside their homes, so this solution will ensure good coverage for subscribers. This will not only increase the coverage but also the number of cells, and thus the capacity of the network. For the operators, femtocells are not only an efficient solution to increasing the indoor coverage, but also a cheap solution because femtocells are paid for by the customers. The alternative consisting in increasing the indoor coverage by adding more outdoor cells would be a lot more expensive for operators.

Deployment

Femtocells could be deployed in different kinds of scenario. The first market is in the home, and the requirement is access to a broadband Internet connection. In rural envi-ronments, without sufficient macrocell coverage, femtocells can be a suitable approach to allowing new customers to access the mobile network and have a maximal coverage inside their home. Thus, there is a high potential market in the USA where huge areas are still not well radio covered, but where high data rate Internet is largely deployed.

In urban environments, femtocell is a good approach to covering dense buildings areas, where the customers, apartments are rarely in direct view of a macrocell, and where the losses due to the other buildings cause poor indoor coverage.

The second market for femtocells would be office environments, where the low capacity of femtocells would be sufficient to cover a number of offices. In this case, buildings would be covered by a set of femtocells that should be carefully planned, in a similar way as with picocells.

In the home environment, femtocells will probably be deployed in a private access mode. This means that only the subscriber of a femtocell will define the list of allowed users of a femtocell. This is mainly due to the fact that it is very unlikely that subscribers, who pay for a femtocell, will allow everybody free access to their femtocell. In the office

Indoor Base Stations 35

environment, because of moving users, the femtocells would have to be deployed in public access. In this mode all the users will have the right to use the femtocells.

Finally, a future idea would be to deploy femtocells outdoors in small hot spot areas where customers would enjoy a maximal coverage. In this scenario the cheap price of femtocells, and their auto-configurability, would allow the creation of many very small cells with high coverage. An example could be to install femtocells in the bus stops of a city. Of course there are still many challenges to overcome before femtocells can be installed outside. The main challenge is that femtocells will have to configure themselves in order to avoid interfering with macrocells or other femtocells.

2.6.3 Differences between Picocells and Femtocells

Taking into account the previous discussions, a comparison between picocells and femtocells is presented in Table 2.2. To summarize, femtocells are small picocells where the properties have been simplified to reduce the cost and simplify the installation.

Contrary to picocells, Femtocell Access Point (FAP) have the particular following characteristics.

Connection

FAP, unlike picocells, are connected to the operator’s network by the broadband con-nection. The femtocell is a self-contained base station and is linked to the core network using IP. The femtocell is self-configurable and the interface between the femtocell and the core network has to be simple to avoid any action from the operator. Moreover, it is important to standardize this interface because if the operators want to use different femtocells from different manufacturer they have to be compatible.

Installation

The femtocells are installed by the customers inside their home. This is why such instal-lation must be as simple as possible (Plug and Play). A newly installed femtocell has to configure its parameters automatically depending on the surrounding environment, to avoid a negative impact (interference) with the neighbouring macrocells and femtocells.

Ideally, a user should only have to plug in the power supply and connect the femtocell

Table 2.2 Comparison between picocells and femtocells

Parameter Picocells Femtocells

Installation By the operator By the user

Connection to the core network Coaxial or fibre optic ADSL, cable

Price Cheap Very cheap

Capacity 10 –50 users 3 –5 users

Covering range <100 m <30 m

to the broadband connection. This self-configuration is very important, because if many femtocells are deployed, operators cannot afford to optimize the parameters of all the femtocells in order to reduce the interference with their macrocells. However, in an open access scenario, where numerous femtocells are required to ensure radio cover-age, femtocells can be deployed by the operators themselves in an approach similar to picocells.