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David Donovan and Martin Streckfuss, Analog Devices
Introduction.
The femto cell market is about to grow dramatically, with most of the leading operators planning to introduce femto cell solutions to their infrastructure. Perhaps the most important reason is the desire for improved in-building coverage, particularly at the higher frequencies of 3G and WiMAX where the RF signal attenuates more quickly. To ensure consumers get good indoor coverage and sustained broadband data rates, one good solution for operators is to add additional home base stations to their network.
Home base stations are also referred to as home access points and femto base stations. While they are functionally the same as the bigger pico base stations, they are designed to support around four voice and broadband data users, rather than the 80 or so voice users supported by a pico base station.
Femto base stations are also built in a smaller form factor than pico base stations, weighing around 1kg not 6kg, and with a lower power consumption of 10mW-100mW rather than around 250mW.
Both femto and pico base stations can support full-rate HSPA (high-speed packet access) at speeds of 14.4Mbps download and 5.6Mbps upload.
Market Expectations and Requirements for Femto Cells
The platform role model for femto cells is the WiFi access point, and manufacturers of femtocells need to set themselves similar goals in terms of cost, manufacturability, integration, QoS, and plug-and-play capabilities.
Looking at the bill of materials (BOM), this should be under $100 in total in 2007, split approximately 80%/20% between the baseband and RF-IF sections. This consists of a baseband processor with an FPGA, with the RF-IF side covered by one IF chip, one transceiver and one power amplifier.
As might be expected, price reduction will be required in the future. By 2010, we expect the target BOM cost to be under $50, with reductions in power consumption also required by the market. The FPGA will no longer be required as an integrated baseband processor absorbs these functions, and a single RF-IF chip will handle the IF and transceiver functions. A separate power amplifier will still be on the board but will likely evolve to a more complex module form factor to support femtocells with multiple radio requirements and band switching.
Further integration in RF-IF will be possible for multi-mode radios. While most development activity at the moment is UMTS, we expect multi mode femtocell activity to increase, adding CDMA or GSM/EDGE to UMTS.
HSPA Support Required
Operators are looking for full-rate HSPA support to get headroom for DSL evolution, providing upgrade paths into ADSL2+. The full-rate capability is required for both download (HSDPA) and upload (HSUPA).
Analog Devices (ADI) has a comprehensive roadmap to a sub-$12 RF Front End with full-rate HSPA support, plus a solution for the digital domain based on TigerSHARC and Blackfin processors. The baseband solution provides 14.4 Mbps HSDPA and 5.6 Mbps HSUPA with the help of a low cost FPGA. We have a complete platform / module level play, including backhaul, base band and radio, providing high integration on a two-chip radio/IF solution plus power amplifier. Further cost reductions on the system level are envisioned.
UMTS Full-rate HSPA Home Base Station Platform Proposal
To provide 3G voice and high-speed data services in the home, Figure 1 shows a proposed RF-IF cost-power reduction roadmap for UMTS HSPA home access points.
Figure 1: Cost reduction roadmap for UMTS HSPA home access points
Figure 2 shows the current 2007-9 platform based on ADI’s TS-201 TigerSHARC DSP with an integrated radio transceiver.
Figure 2: 2007-8 platform with TigerSHARC and integrated radio
Radio Transceiver Implementation
Looking specifically at the radio transceiver, the integrated converter MxFE (Mixed-signal Front End) single-chip radio transceiver can help enable the best cost-performance choice. Examples of suitable components are the AD9860/1/2/3 from Analog Devices, which provide highly integrated dual transmit and receive paths.
Analog Devices also offers a single-chip tri-band 3G radio transceiver for local area base stations, compliant with 3GPP 25.104 release 6 WCDMA/HSDPA and providing WCDMA and GSM monitoring. Monitoring of 2G and 3G bands is a key feature that enables plug & play capability, since the femto base station need to measure the power levels of 2G and 3G macro layers as well as adjacent femto base stations in the femto layer. This eases the system management to avoid interferences and also enables synchronization by measuring radio channels characteristics on the macro layer. This transceiver requires a minimum of external components, and minimizes test time in manufacturing through reduced calibration requirements. It has ultra-low power consumption and a small footprint in a 6x6mm LFCSP package.
Multiple Radio Home Base Stations
Requirements for femto cell home base stations are not limited to the UMTS air standard. Femto cell platforms are emerging that require CDMA2000, GSM and WiMAX air standard support. The requirements for multiple air standard support put additional cost pressures on semiconductor manufacturers and ODMs. Additional IC integrations and packaging options, such as multiple die in one package and modules will be required to meet the longer term BOM cost and space-power requirements.
In addition to the single chip UMTS transceiver for femto cells, Analog Devices currently offers GSM and CDMA2000 RF-IF chipset solutions as well as single chip WiMAX femtocell transceiver solutions. The AD9352/53 RFCMOS direct conversion WiMAX Transceiver combines high integration with high performance, and includes the ADI/QTM digital I/Q interface for a direct connection to a digital baseband. The dual-band AD9352 operates in the 2.3 to 2.7 GHz and 4.9 to 5.9 GHz bands and the single-band AD9353 operates in the 3.3 to 3.68 GHz band.
Digital Base Band: Home Access Points Benchmarks
Let’s look at ADI’s TS-201 TigerSHARC DSP, which is ideal for home access points – see Figure 3.
Figure 3: TS-201 TigerSHARC DSP
The TS-201 is available as a 600 MHz DSP operating at 1.2V, or in a 500MHz version at 1.0V. It offers high core parallelism, and provides 24 Mbits on-chip memory. A 64-bit interface is used to access the high-performance local parallel bus, and four parallel links support up to 32 Gbps of aggregate bandwidth. The TS-201 also provides DMA control, flags, timers, and JTAG.
A benchmark study looked at the TS-201 in a single-DSP implementation of full-rate HSUPA, with an FPGA connected to the DSP via its link port. The TS-201 was able to provide full support of four active users, for any mix of voice and data with dedicated channels. It met full HSDPA release 5 capability, with a 14.4 Mbps theoretical peak rate and 15 codes at category 9 & 10. The FPGA turbo decoder enables full HSUPA capability at a theoretical peak rate of 5.6Mbps.
This implementation is also easy to design, with example designs of both the TigerSHARC link port FPGA and 3GPP turbo decoder available from leading FPGA vendors.
Conclusions
For femto cells, full-rate HSPA support available today enables the end user market, and 25.104 compliance eases network integration.
Analog Devices can offer a technology roadmap for the entire 3G femto cell platform, with a highly integrated radio transceiver, and a high performance digital base band and backhaul.
Looking ahead, there are three key enablers to meet the market requirements for 2008 –2010:
· cost · form factor · power
With a flexible architecture such as that offered by Analog Devices’ TigerSHARC, designers will be able to re-use their existing software investment and move to next–generation platforms that enable them to hit these objectives to 2010 and beyond.
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