Introduction to In-Building Solutions
The Indoor Coverage Challenge:
The demand for the Capacity of modern mobile communication is growing year by year. People all over the world are using voice call services along with data transfer services with their mobile handsets. Many of these services, including Web surfing, downloading emails, video streaming and video calls require high speed connections and generate large amounts of data traffic to the network. In the near future, the wireless service providers along with mobile terminals will have to provide the bit rates comparable to the present fixed internet connections.
Coverage is one of the serious notable issues related to mobile telecom networks, especially in the rural areas due to the long distance between base stations and in indoor and underground locations due to the wall attenuations.
In cellular networks, round about 2/3 of calls and above 90% of data services occur indoors. So, it is extremely important for cellular operators to provide good indoor coverage for both voices as well as data services. However, some surveys show that 45% of households and 30% of businesses experience poor indoor coverage problem. Hence, how to provide good indoor coverage, in particular, for high speed data services, is a big challenge for operators.
3G networks are one of the solutions towards achieving high data rates but to do so, stronger received signal strength is demanded. As a result, providing better coverage and capacity inside buildings is a requirement.
Indoor Coverage Problems due to Macrocells:
One of the most conventional approaches of providing indoor coverage is to use outdoor macrocells, but this approach has a number of drawbacks:
- For an in building environment, free space losses as well as signals to and from in-building users covered from macro cells face penetration losses. Furthermore, the modern building infrastructure, and the metal-coated windows, makes it even more difficult for the signals to penetrate into the building.
- There are also some issues related to the relative distance between the user and the macro-cell antenna. Due to the additional penetration loss, users located at the far side of the building, away from the macro antenna, suffer more from bad coverage. This affects signals traveling both to and from the users. Similarly, more power is required for providing communication links between the BS and the mobile user.
- Buildings with macro antennas on the roof may have coverage problems at their base. This lack of coverage is typically caused by the lack of reflections from adjacent buildings.
- The use of the "outside in" approach is not economically efficient. As in UMTS, an indoor user will require higher power drain from the base station in order to overcome high penetration loss. This will result in less power to be used by other users and lead to reduced cell throughput. This is because the power used by indoor users is not efficient in terms of generating capacity and in UMTS capacity is linked to power.
- For a high capacity network, large no. of base stations are required, the acquisition of which has become very challenging in densely populated areas.
- It is not a practically applicable approach for a high capacity network, due to issues like interference and higher power drain from base stations to serve indoor users from outdoor macrocells, etc.
- In case of a dense cellular network structure, the network planning and optimization becomes a major issue. For example, in GSM/GPRS/EDGE networks, the frequency planning and in CDMA based networks, the planning of soft handover regions, etc.
- Turbo 3G, also known as HSDPA and HSUPA, offer substantially higher data rates. However, these services demand excellent RF-link conditions, which are typically not achieved when using macro cells to reach indoor users.
- Similarly, to achieve higher data rates, higher modulation and coding schemes are needed. These require better channel conditions for HSDPA, WIMAX and LTE, which can only be met near those windows facing macrocell sites.
- In case of UMTS, as it utilizes the soft handover, On the top floors of high-rise buildings, the problem of pilot pollution can occur. This happens when a mobile receives a pilot signal from a distant BS that is not listed as a neighbor cell, and therefore cannot take part in soft handover.
- Buildings served by more than one macro antenna usually have better coverage. However, problems can occur when UMTS users connects to more than one BS and enters in soft handover condition, which increases the interference and the load on the network.
- As 3G and future networks will normally operate at 2 GHz or above, the building penetration will be a great challenge for networks operating above 2 GHz.
Good in-building coverage has an important role in attracting and providing satisfactory service to mobile subscribers. This means that the coverage from the macro network extending into the building should be combined effectively with the dedicated in-building system. Those networks which are upgrading themselves to 3G has to meet the challenges like Greater data capacity and provide high-speed data services, which increase the demands put on the cellular network. Subscribers have great expectations regarding third-generation services. But, the problem related to the present 3G infrastructure is that it is primarily designed for coverage in outdoor environments-not inside buildings. This leads us to pay our attentions on the In-building solutions and apply them accordingly to get high data rata and capacity.
This leads us to pay our attention towards some successful and attractive indoor solutions such as DAS (Distributed Antenna Systems) and Pico-cells which are becoming an attractive and viable business proposition in hotspots such as large business centers, office buildings and shopping malls. These indoor building systems are deployed by different network operators. These indoor solutions will improve the performance of the radio links which in result provide us with the benefits like in-building coverage, offload traffic from outdoor macrocells, enhance service quality and high data rate services. In UMTS, the orthogonality can be improved by deploying indoor solutions, which will result in high throughput. In HSPA/LTE or WiMAX, the better channel conditions will enable high modulation and coding scheme to be used and thus deliver advance services that further drive demand.
The indoor coverage problems are typically solved with the help of dedicated in-building solutions. Dedicated in-building solutions can be implemented using different approaches. These approaches are used to analyze different performances regarding radiation, coverage and quality. A brief introduction is given below.
1) DAS (Distributed Antenna System) solutions:
A Distributed Antenna System (DAS) is one of the methods/technique used for providing better indoor coverage. It consists of a series/network of antennas distributed throughout a building to provide dedicated in-building coverage. These systems can be passive or active.
The working principle of DAS technique is to subdivide one area into smaller areas so that each DAS antenna can cover that area. A DAS solution also increases the overall network capacity by providing operators with better control and confine coverage within a building and by reducing interference to and from the macro network.
DAS shows performance improvement in terms of coverage area, path loss and less Tx power from BS and mobile phone.
Coverage: DAS technique uses the Omni-directional antennas with equal coverage radius, Thus improvement in total coverage area is observed as compared with the single antenna.
Reduction in Tx Power: Using DAS, Mobile transmits with less power as compared to single antenna system, because coverage is being provided by multiple antennas at a particular spot.
Path loss: Using DAS Path loss is decreased.
2) Repeater Solutions:
Another way to provide good indoor coverage is to use the repeaters. They are widely used to provide coverage in environments like tunnels, roads etc. The basic operation of the repeater is to amplify the received signal from a base station, and re-transmit it to the coverage area, this result in a better signal strength reception at the user end. Repeaters can also be used in a combination with DAS to provide indoor coverage. In this approach the repeater receives the signal from macro antenna and after amplification, provides signals to indoor users through DAS antennas. This Repeater-to-DAS configuration provides us with good coverage for indoor environment, similarly capacity gain, SIR improvement and reduction in DL and UL power is observed for 3G.
3) Picocell Solutions:
Picocell technique is also another solution to the indoor coverage problems. Apicocellis a wireless communication system used for providing coverage to a small area, such as in-building (offices, shopping malls, train stations, etc.), or in-aircraft. A picocell issimilarto aWi-Fiaccess point.
n cellular networks, the basic use of the picocells is to extend coverage to indoor areas where outdoor signals do not reach well, or to enhancenetwork capacity in dense populated areas having large no. of phone users, such as train stations.
The picocell base station is characteristically a low cost, small and simple unit that connects to aBase station Controller (BSC).A single BSC is used to provide link between Multiple picocell 'heads'. The BSC operation includes, radio resource management, hand-over management, and data flow control towards theMobile Switching Centre (MSC)andGPRSSupport Node (GSN).
Advantages of in building solutions:\
- The indoor solutions will enhance service quality and facilitate high data rate services due to the improved performance of radio links and offload traffic from outdoor macrocells.
- For those Operators, who provide dedicated in-building coverage in key locations, such as airports and train stations, they can also take the benefit of providing roaming services to subscribers as most roaming subscribers have automatic public land mobile network (PLMN) selection, which means that if the regular connection is lost, their phone automatically selects the best available PLMN.
- The implementation of dedicated in-building coverage in CDMA-based networks also offloads the macro system, thereby increasing overall system capacity. In other words, operators can continue to use existing networks to serve a growing number of subscribers without any major increase in the budget.
- With indoor solutions, In UMTS, high throughput can be achieved by improving orthogonality, which can only be achieved by deploying indoor solutions.
- In technologies like HSPA/LTE or WIMAX, the demand of the system can be increased by introducing some advance rich services, and these services can be achieved by providing better channel conditions(depends upon high modulation and better coding scheme).
- As the indoor building solutions increase the capacity of the system, this shows that we can exclude the cell splitting techniques which as a result will reduce the infrastructure cost of the network expansion. In particular, it is more beneficial to deploy dedicated in-building coverage systems in third-generation CDMA-based networks than in time-division multiple access (TDMA) networks.
- The overall cost of the cellular network is reduced by deploying indoor building solutions as several parameters results in enhancing the gain of the system e.g lowering transmission costs, increasing trunking gain, and by sharing RBS equipment.
- The in-building systems have the potential to increase the network capacity up to 400 percent if deployed at its full peak. Furthermore, cost per subscriber, including the cost of the in-building system, can be reduced by almost 67%.
Despite the fact that the above mentioned indoor solutions are more cost effective as compared to the use of macrocell network for indoor coverage for services like voice and high speed data transfer, such solutions are still too expensive to be used in some scenarios such as SOHO (Small Office and Home Office) and home users (for personal communications and entertaining, etc.).Recently, the development of femtocells provides a good opportunity for low cost indoor solutions for such scenarios. Unlike picocells, femtocells are deployed by users.