FPGAs compete with ASSPs for wireless backhaul applications

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FPGAs compete with ASSPs for wireless backhaul applications

With the growing adoption of all sorts of mobile devices, from smartphones to tablets to e-readers, we often hear dire warnings that a shortage of wireless spectrum to support seemingly insatiable user demand for consuming data is imminent. The FCC has been focusing on this issue by developing a plan to free up 300 MHz of licensed and unlicensed spectrum by 2015, and a total of 500 MHz by 2020. Less well publicized, but just as critical, is how that massive volume of data gets to the cell tower in the first place so that it can be transmitted over cellular networks to the devices in our hands. This is where mobile backhaul comes in – the so-called “last mile” data link between wireless base stations and the carrier aggregation networks.

Network operators employ either high-bandwidth fiber networks or wireless links, most often microwave, for their backhaul. At lower frequency bands, 2.4-5.8 GHz, the wireless link can be Non-Line-of-Sight (NLoS). At higher microwave frequencies, such as 60 GHz V-band, a direct Line of Sight (LoS) is required. To address the high capacity and throughput needs for 4G connectivity on the user side, operators are also deploying more heterogeneous networks that combine small coverage area pico or femto cells with the traditional large metropolitan macro cells. Outdoor picocell basestations will be mounted on light poles or the sides of buildings where fiber networks connections are not likely to be available. This will make low-cost, low-power microwave backhaul even more important, which has attracted the attention of semiconductor manufacturers of both Application-Specific Standard Products (ASSPs) and FPGAs. The wireless infrastructure market is becoming another battleground for standard versus field-programmable logic Integrated Circuits (ICs).

Taking action for future wireless infrastructure advantages

To address this opportunity, in April 2011 Broadcom acquired Provigent Inc., a developer of mixed-signal SoCs for microwave backhaul systems. Six months later the company announced the BCM85620, which they claimed to be the “first Gigabit SoC for microwave backhaul.” The BCM85620 combines the functionality of up to ten individual ICs, including a modem and CPU, network processor, switch, and the DACs and ADCs needed to convert radio signals from analog to digital for DSP operations. Broadcom emphasized the algorithmic innovation employed in their SoC, which affects baseband LTE modem functions across the board and meets the low-latency requirements of 4G networks with robust error correction.

Then, in August 2012, Qualcomm moved further into the small cell market with their acquisition of DesignArt Networks. DesignArt will become part of Qualcomm-Atheros, adding to Qualcomm’s recent introduction of a Femtocell Station Modem (FSM) at the 2012 HotChips Conference. In an interview regarding the acquisition, Nick Karter, Business Development Product Manager at Qualcomm, cited DesignArt’s "pioneering work" on both LoS and NLoS low-latency wireless backhaul.

On September 17, Xilinx joined the competition with their acquisition of a small company based in Estonia, Modesat Communications. Modesat also has an office in San Diego, the well-known home of Qualcomm. Sunil Kar, Senior Director for Wireless Communications at Xilinx, says that with the Modesat acquisition, Xilinx will address the large backhaul market opportunity where wireless infrastructure companies such as Ericsson, NEC, Huawei, Alcatel-Lucent, and ZTE currently have limited options. Modesat has produced backhaul products for Point-to-Point (P2P) microwave and millimeter wave radio subsystems using FPGA technology, so they bring IP to Xilinx that can immediately be added to their wireless communication portfolio. Kar says that they also have a roadmap for developing high-capacity NLoS modems for picocell backhaul.

Also emphasizing the value of algorithmic innovation, Modesat describes their PilotSync technology as implementing proprietary DSP algorithms for parallel processing of carrier and symbol frequencies. Modesat says that operators can take advantage of their DSP techniques to improve signal quality and extend their signal coverage area. The Modesat software-defined, FPGA-based solutions enable “on-the-fly” upgrading and adaptability to different radio environments. Modesat has also provided solutions for High Frequency Trading applications, which share the requirement for low latency that differentiates LTE networks.

The Xilinx acquisition of Modesat will complement their 2011 purchase of Modelware, a developer of semiconductor IP for data traffic management and packet processing. By combining their communications IP with the dual ARM core processor architecture in Zynq, Xilinx will have an FPGA microwave backhaul solution that is architecturally very similar to the Broadcom-Provident solution. Kar sees the ability to offer a one or two-chip solution that reduces Bill of Materials (BOM) and power consumption compared to existing solutions. He says that Xilinx is targeting Q1 of 2012 for a demonstration of microwave backhaul on a Zynq 7045, likely at the Mobile World Congress in Barcelona.

FPGAs have an edge over the competition

Kar says that implementation of a modem for microwave backhaul is not as complex as for the Radio-Access Network (RAN) functions of a wireless base station, and a design can be fit into a “small” FPGA, which he claims will be cost-competitive to ASSPs. He also points to a time to market advantage for an FPGA solution, saying that new ASSP development, such as Qualcomm-Atheros’ plan to integrate DesignArt’s technology with the Snapdragon processor, can take as long as two years. According to Kar, 80 percent of the implementation of a backhaul solution is foundation IP, identical from one vendor to the next, while 20 percent is customized to enable Tier 1 OEMs to differentiate their products. The programmable logic in an FPGA-solution will more readily enable that customization, while ASSP solutions are restricted to the semiconductor vendor’s IP.

As with all ASIC/ASSP versus FPGA arguments, it remains to be seen if the cost of a Zynq-based microwave backhaul solution will be cost competitive. While flexibility and configurability offer clear advantages for FPGAs, as the volumes for small cells ramp up rapidly over the next few years vendors will be directly competing with more broad-based and higher-volume wireless offerings from companies such as Broadcom and Qualcomm. Xilinx will be partnering with their ecosystem vendors for components such as data converters, Power Amplifiers (PA), and RF components. Qualcomm-Atheros and Broadcom design such components in-house, and are integrating them into their single-chip ASSP solutions.

For more information, contact Mike at mdemler@opensystemsmedia.com.