Microwave Engineering Europe - January/February 2009 - (Page 18)

18 4G TECHNOLOGY FOCUS Accelerating global WiMAX adoption: The move to Picocell and Femtocell base stations By Dean Chang, Fujitsu Microelectronics America Emerging alternatives to macro base stations exist that deliver as-needed WiMAX coverage in hard-toreach locations. Micro, pico and femto base stations enable high data rates without over-building the main base station network. The smaller base stations rely on scaled-down versions of WiMAX to minimize initial costs and simplify maintenance. T he high-performance capabilities of large-scale (macro) WiMAX base stations are now providing very good coverage in many locations. But there are also emerging alternatives that deliver asneeded WiMAX coverage in hard-to-reach locations. Micro, pico and femto base stations allow service providers to ensure high data rates to end users without overbuilding the main WiMAX base station network. At the same time, smaller base stations rely on scaled-down versions of WiMAX hardware and ﬁrmware used in macro base stations. This approach minimizes initial costs and simpliﬁes ongoing maintenance. The WiMAX standard typically provides mobile or ﬁxed usage, indoor or outdoor, based on a large macro station that delivers data rates up to 40 Mb per channel and can communicate with several subscriber units. WiMAX macro base stations are similar in size and capability when compared to 3G base stations. The equipment generally consists of a rack-mounted unit that has full baseband redundancy and the ability to hotswap cards when they fail. This approach allows service providers to offer servicelevel agreements (SLAs) with guarantees of minimized down times. A rack of equipment supports multiple sectors and are aggregated through an Ethernet switch or router. The base station’s radios are sometimes mounted near the baseband rack. This arrangement makes it easy to hot-swap radios when they fail but requires expensive, low-loss cables connecting to the antennas on the base station’s tower. The growing trend for WiMAX systems is to mount the radios on the tower with an optical connection to the baseband rack using standards such as the Open Base Station Architecture Initiative (OBSAI) or the Common Public Radio Interface (CPRI). This method supports long optical cables, and the radios lose very little signal power when connected with short cables to the antennas. Macro base station challenges Macro base stations are good at providing blanket coverage, especially with techniques such as beam forming, which adaptively directs radio signals toward receivers. However, some locations will always present challenges: behind buildings and hills or inside buildings, for example. Poor performance in homes or ofﬁces is one of the reasons people change cellular carriers. As users shift to using their mobile phone as their only voice device, demands for better coverage have increased. The exterior wall of any building can signiﬁcantly reduce performance — since every 3 dB reduction represents half of the power, based on the logarithmic decibel scale, 9 dB to 15 dB signal loss means that only one eighth to one thirty-second of the power from the outside makes it inside. Obstructions such as interior walls can easily degrade signal levels enough to result in dropped calls and reduced data rates. Also, higher frequency bands used for broadband wireless compared to traditional second-generation (2G) voice solutions. Typically the higher frequencies have more difﬁculty penetrating exterior walls when operating at comparable power levels. The case for smaller base stations Users want to send and receive virtually any type of media from virtually any location, ● so WiMAX networks need to ensure high data rates over their entire coverage areas. The cost of delivering this coverage can be high if a network uses only macro base stations. The equipment and the backhaul to support it is particularly expensive due to high-performance, carrier-class features such as redundancy, hot-swap capability, highpower radios, and support for thousands of users. In developing countries where average revenue per user (ARPU) is around $10 or less, the cost of the macro base station becomes a barrier to a proﬁtable business model. So, carriers in the 3G business are beginning to take advantage of smaller base stations — most notably pico- and femtostations. WiMAX expands on this concept with an array of choices, each with different performance metrics and price targets. Mico base stations This class of base station is unique to WiMAX. Micro base stations typically consist of a small (usually 1U high) rackmounted device with a tower-mounted radio. Radios typically connect to the rack via an optical cable using OBSAI or CPRI standards. These base stations do not have the carrier-class features of a macro base station and hence are less expensive. Most of the deployments for these base stations are in developing countries or deployments using unlicensed spectrum, where access is otherwise nonexistent and customers are unwilling to pay high ARPUs. Typically, each micro base station supports three sectors, with hundreds of users per sector. To reduce costs, the radio power is not as high as that of a macro base station, typically up to 36 dBm when macro base stations support around 40 dBm. Microwave Engineering Europe ● January/February 2009 www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - January 2009 News Contents Comment Using KPIs to Ensure Quality in a Converging Network Amplifier Error Vector Magnitude Characterisation Using High-Speed Modular PXI Instruments GPS: Making a Play for Femtocells Accelerating Global WiMAX Adoption: The Move to Picocell and Femtocell Base Stations Addressing PA Efficiency for Multi-Mode Wideband Handset Applications Wi-Fi: Mobile Feature or Fundamental RAN? Products Calendar

Microwave Engineering Europe - January/February 2009

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