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By Zheng Yuanyuan

In the mobile telecom market where competition is fierce, optimum indoor coverage can help operators take the lead in the market and improve user experience. Therefore, indoor coverage grows in importance to operators. 


A key focus for operators 

According to NTT DOCOMO' s statistics, indoor 3G traffic takes up 70% of the total; however, the indoor areas only take up 20% of all covered areas. Based on years of experience in planning and study of 3G data services, Huawei also concludes that 90% of the data traffic takes place indoors. 

High spectrum utilization and throughput are the prominent advantages of WiMAX 16e, and WiMAX has attracted global operators' attention once it was proposed. Considering the non-line-of-sight features of WiMAX brought by high frequencies, most operators select the frequency band of 2-6GHz to realize wireless broadband access. However, higher frequencies mean more transmission loss of radio wave in open space, poorer ability to diffract or bend around obstacles, and greater penetration loss. These require that operators need to design appropriate intervals between base stations and frequency multiplexing before building WiMAX networks. Moreover, through optimization of antenna azimuths and downtilt at outdoor, radio signals can directly penetrate buildings to cover indoor spaces. Structural optimization is also required at later stages to further improve indoor coverage. 


Guarantees from advanced technologies 

A comprehensive indoor coverage solution can not only resolve coverage problems in dense environments, particularly residential areas where high-end users distribute, but also reduce the number and configuration of outdoor base stations. This helps to lower the overall interference level of outdoor networks, improve the average throughput of the entire system, and better satisfy users' requirements on network quality. 

Of course, all these need to be ensured with advanced technologies. 

WiMAX is a combination of advanced MIMO and OFDM technologies, together with flexible configuration of subcarriers and advanced coding and modulation (ACM). WiMAX can realize rates up to 37.44Mbps downlink and 8.06Mbps uplink per sector, in the case of 10M 1×3×3 PUSC with all SC networking and 2:1 of TDD, fully meeting requirements of high speed indoor data services. 

For channel bandwidth, WiMAX allows flexible channel selection among 1-20MHz. This meets operators' requirements on channel bandwidth, avoiding competition for spectrums and promoting harmonious utilization of spectrum resources. Meanwhile, this merit helps operators fulfill needs in special coverage scenarios. For example, operators can provide a bandwidth of 30MHz or above for networking at hotspots with intense traffic, while providing a bandwidth of 5MHz for networking in rural areas to allow basic Internet access. 


Enhanced design based on services 

In recent years, the IEEE802.16 series of broadband wireless access has become gradually matured. Intel and other major WiMAX advocators have set up the WiMAX Forum, which is intended to coordinate the interoperability of network equipment and drive the development of WiMAX. 

However, IEEE802.16 standards define only the air interface, providing the flexibility for upper layer applications. No matter how WiMAX evolves from fixed-nomadic or portable-mobile services, it is always for the last mile coverage. Traditional coverage uses outdoor base stations to meet basic indoor coverage and switching, and depends on further and dedicated coverage for blind points at later stages. 

Compared with 3G technologies, WiMAX is more focused on home and enterprise broadband services. Needs for indoor high speed data services have driven the diversification of coverage modes of WiMAX networks, such as gradual coverage from local hotspots to MANs and even WANs, or simultaneous coverage for both indoor and outdoor areas. 

WiMAX indoor coverage design begins with investigating of coverage targets and capacity requirements. For example, a commercial building has a coffee bar on the top floor, offices on middle floors and a shopping mall on lower floors. The designer first estimates the capacity requirement of each floor, through investigating the layout of each floor, analyzing network access requirements of users, and determining different quality objectives. Then, the designer tests indoor signal environments to identify blind points or weak signal areas, and ensure seamless coverage. 

After that, the designer finds locations where cable laying and hole drilling are allowed, and determines whether existing transmission cables can be used. For indoor coverage design, proper location and management of transmission resources are important, because they are related to not only esthetic considerations but also networking costs. Moreover, it is also important to determine the mounting types, locations and antennas' quantities on each floor based on floor plans of the building. For example, mushroom antennas are favored by operators, thanks to their appealing appearance, low power consumption and low radiation. 

With years of experience in 2G/3G indoor coverage planning, Huawei is able to provide tailored solutions for coverage. By reusing traditional indoor distributed systems of 2G networks, Huawei solutions can help operators maximize their utilization of existing network resources, realize unified network management, authentication and billing, and reduce WiMAX CAPEX. 


Fitting into different scenarios 

Home broadband 

Home broadband is a major market of WiMAX, and the typical scenario is to replace xDSL. If a Wi-Fi network already exists, indoor customer premise equipment (CPE), a terminal only in the size of a modem, can resolve the need for wireless backhaul and avoid high costs due to addition of a new indoor distribution system. In combination with the existing Wi-Fi network, the CPE can realize satisfied indoor coverage, dramatically reduce costs, and shorten the construction period of the network. 

If no existing Wi-Fi network can be utilized, the indoor coverage will be quite different. In the case of a residential area with dense high-rise building, multi-path effects would cause the signals received by a user to be dramatically decreased. In this case, high gain indoor CPE is an effective low cost solution. The equipment can improve uplink quality, allowing users to enjoy continuous services anytime and anywhere. In the case of a residential area with open and flat spaces (suburban), high gain outdoor CPE is an ideal solution, which allows line-of-sight transmission and avoids losses of tens of dBs due to walls or glass windows, thus improving indoor signal quality. 

Enterprise broadband 

For small and medium enterprises, good network coverage is represented by strong indoor signals. Pico base stations, which are light and compact, are the ideal low-cost solution for indoor coverage. Pico base stations may be mounted on ceilings, walls or pillars. Small power base stations can achieve seamless coverage, and they are suitable for indoor scenarios such as apartments, cafes and hotels. 

If there are sufficient floor or vast office spaces, distributed base stations of RRU/BBU+DAS can be used for indoor coverage. Based on Huawei enhanced radio technology, distributed WiMAX base stations are of high capacity, compact and can support networking with multiple frequency bands. The base stations can be easily mounted, dramatically reducing sites, equipment rooms and auxiliary costs. By combining with the data analysis system (DAS) and by properly arranging power dividers, combiners, couplers and other passive devices, the base stations can enable distributed power consumption, insure indoor signal quality in spaces separated by walls, and meet needs for fixed, nomadic and mobile services. 

In summary, WiMAX indoor coverage should take into account costs, indoor wiring, control of outdoor interference and others. To meet increasingly higher requirements on services and network quality, operators can employ advanced technologies and combine different indoor coverage solutions. 

Home and enterprise users who have high requirements on indoor coverage often require high speed services and are valuable users with high ARPU. Along with further improvement of indoor coverage, operators not only can obtain higher revenues but also build outstanding brand images, attracting more high-end users and boosting the steady and continuous growth of WiMAX.

Fig. 1 Indoor traffic of 3G services (Source: NTT DOCOMO statistics)

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