Easy WiMAX designs with FPGAs

The explosive Internet growth throughout the last decade has led to an increasing demand for high-speed, ubiquitous Internet access. Broadband Wireless Access (BWA), with a range of one to three miles, is increasingly gaining popularity as an alternative, last-mile technology to DSL lines and cable modems. FPGAs and structured ASICs provide an ideal implementation platform for developing broadband wireless systems such as WiMAX. They also address processor speed challenges, meet flexibility and integration requirements, and help get products to market quickly.

Wireless standards Following the hugely successful global deployment of the 802.11 Wireless Local Area Network (WLAN) standard, Wi-Fi, deployment of the IEEE 802.16d Wireless Metropolitan Area Network (WirelessMAN) standard is currently in progress. This technology supports fixed broadband wireless access to residential and small business applications and enables Internet access in countries without any existing wired infrastructure in place.

Standardization efforts are also under way for the 802.16e version that will provide mobile access to the end user in a MAN environment. In 2003, equipment and component suppliers formed the WiMAX Forum to promote the adoption of IEEE 802.16-compliant equipment and to certify compatibility and interoperability of broadband wireless products.

IEEE 802.16d (IEEE 802.16-2004) standard This revised standard consolidates IEEE standards 802.16, 802.16a, and 802.16c, retaining all modes and major features without adding new modes. This standard specifies the air interface of a fixed, Point-to-Multipoint (PMP) BWA system providing multiple services in a WirelessMAN, with a bit rate of up to 75 Mbps. It also specifies an optional mesh topology enhancement to the Media Access Control (MAC) layer.

The WirelessMAN MAC can support multiple Physical Layer Device (PHY) specifications optimized for the frequency bands of the application. The standard includes PHY specifications applicable to systems operating below 11 GHz and between 10 GHz and 66 GHz. The 10-66 GHz air interface, based on single-carrier modulation, is known as the WirelessMAN-SC air interface. The standard specifies WirelessMAN-SCa, WirelessMAN-OFDM, and WirelessMAN-OFDMA air interfaces for frequencies below 11 GHz. Figure 1 provides an overview of the relevant wireless standards in more detail, and Table 1 provides a comparison.

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Figure 1
Overview of variants within the 802.16 standard

 

802.16

802.16a/802.16d

802.16e

Completed

December 2001

802.16a: January 2003 802.16d: June 2004

Est. mid-2005

Spectrum

10-66 GHz

2-11 GHz

2-6 GHz

Application

Backhaul

Wireless DSL & Backhaul

Mobile Internet

Channel conditions

Line of sight only

Non-line of sight

Non-line of sight

Bit rate

32-134 Mbps at 28 MHz channelization

Up to 75 Mbps at 20 MHz channelization

Up to 15 Mbps at 5 MHz channelization

Modulation

QPSK, 16QAM, and 64QAM

OFDM 256 sub-carriers, QPSK, 16QAM, and 64QAM

Scalable OFDMA

Mobility

Fixed

Fixed

Pedestrian mobility – regional roaming

Channel bandwidths

20, 25, and 28 MHz

Selectable channel bandwidths between 1.5 and 20 MHz

Same as 802.16a with UL sub-channels to conserve power

Typical cell radius

1-3 miles

4-6 miles, maximum range of 30 miles based on tower height, antenna gain, and power transmit

1-3 miles

Table 1

IEEE 802.16e (IEEE 802.16-2005) standard Moving beyond the “d” rev, the IEEE 802.16e standard supports subscriber stations, such as notebook computer and PDA portable devices, moving at vehicular speeds with a bit rate of up to 15 Mbps, thereby specifying a system for combined fixed and mobile broadband wireless access. It also specifies MAC functions to support higher layer handovers between base stations or sectors, such as Macro Diversity Handover (MDHO) and Fast Base Station Switching (FBSS).

The Orthogonal Frequency-Division Multiple Access (OFDMA) PHY layer has also been modified to support scalability between different Fast Fourier Transform (FFT) sizes, or subcarriers, and available channel bandwidths from 1.25-20 MHz. Low-Density Parity Check (LDPC) codes are also specified as an optional Forward Error Correction (FEC) scheme. Operation is limited to licensed bands suitable for mobility below 6 GHz. Fixed IEEE 802.16d subscriber capabilities are not compromised.

PHY layer overview Because of its superior performance in multi-path fading wireless channels, Orthogonal Frequency Division Multiplexing (OFDM) signaling is recommended in the 802.16 standard’s OFDM and WirelessMAN OFDMA PHY layer modes for operation in sub-11 GHz Non-Line Of Sight (NLOS) applications. OFDM technology is also recommended in other wireless standards, such as Digital Video Broadcasting (DVB) and WLAN. Figure 2 provides an overview of the typical PHY layer functions implemented in a WiMAX base station operating in the OFDM/OFDMA modes.

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Figure 2
(Click graphic to zoom by 1.9x)

Apart from such usual functions as randomization, FEC, interleaving, and mapping to QPSK and QAM symbols, the standard also specifies optional multiple antenna techniques. These techniques include Space Time Coding (STC), beam forming using adaptive antenna schemes, and Multiple-Input Multiple-Output (MIMO) configurations that achieve higher data rates.

The OFDM modulation/demodulation is usually implemented by performing FFT and Inverse FFT (IFFT) on the data signal. Although not specified in the standards, other advanced signal processing techniques, such as Crest Factor Reduction (CFR) and Digital Predistortion (DPD), are also usually implemented in the forward path to improve the efficiency of the power amplifiers used in the base stations.

The uplink receives processing functions, such as time, frequency, power synchronization, and frequency domain equalization, along with the remainder of the decoding/demodulation operations necessary to recover the transmitted signal.

Hardware platform for WiMAX implementation Designers of WiMAX systems need to meet a number of critical requirements, such as processing speed, flexibility, and time to market. These stringent requirements ultimately drive the choice of a hardware platform. Some of the major implementation challenges include these requirements as well as a cost reduction path.

Processing speed Broadband wireless systems, such as WiMAX, have throughput and data rate requirements that are significantly higher than those for cellular systems, such as WCDMA and cdma2000. To support such high data rates, the underlying hardware platform must have significant processing capabilities. In addition, several advanced signal processing techniques, such as turbo coding/decoding, and front-end functions, such as FFT/IFFT, beam forming, MIMO, CFR, and DPD, are very computationally intensive and require several billion multiply and accumulate operations per second.

Flexibility WiMAX is a relatively new market and is currently going through the initial development and deployment process. 802.16d has just been standardized, while the 802.16e mobile version is still in the works. Under this current scenario, having hardware flexibility/reprogrammability in the final WiMAX-compliant product is very important. This feature ensures that in-field programmability is possible, alleviating the risks posed by constantly evolving standards.

Time to market Because WiMAX is an emerging technology, time to market is a key differentiator for OEMs looking for early success in gaining market share. Time to market has a direct effect on the development cycle and choice of hardware platform, with designers requiring easy-to-use development tools, software, boards, off-the-shelf Intellectual Property (IP), and reference designs to accelerate the system design.

Cost reduction path Another important requirement to keep in mind while choosing the hardware platform is the availability of a long-term cost reduction path. The evolving WiMAX standard/market is expected to stabilize over time. The history of the Third-Generation Partnership Project (3GPP) highlights that it can often take a significant time to solidify, as this standard is still evolving six years after the initial specification was ratified. Release 7 is currently under development. As the WiMAX standard matures, the final product cost becomes much more important than retaining flexibility. A hardware platform that has a clear cost reduction path and enables a seamless flexibility/cost trade-off is important.

FPGAs – the right solution FPGAs provide an ideal implementation platform for developing broadband wireless systems such as WiMAX. State-of-the-art, high-end, high-performance FPGAs, such as Altera Stratix II FPGAs, are usually used at the heart of high-bandwidth systems to accelerate performance and enable new functionality.

FPGA-based WiMAX system design Each of the implementation challenges associated with WiMAX system design can be effectively addressed with FPGAs. Altera’s Stratix II FPGAs are a good example.

Stratix II FPGAs contain embedded DSP blocks (see Figure 3), the TriMatrix memory architecture, and high-speed interfaces suitable for providing an integrated platform for implementing broadband wireless systems such as WiMAX.

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Figure 3

As noted earlier, hardware flexibility and reprogrammability in the final WiMAX-compliant product is very important due to constantly evolving standards. FPGAs provide the ability to easily evolve WiMAX systems in accordance with changing market demands. Altera’s devices enable remote system upgrade transmission over any communications network, keeping products ahead of the competition and featuring dedicated recovery circuitry to ensure reliable updates.

Using a Stratix II device and flash memory, as shown in Figure 4, a user can perform a remote system upgrade in three simple steps:

  1. Send an update from the development location through a network to the Stratix II device
  2. Store the update in the memory
  3. Update the Stratix II device with the new data
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Figure 4

A user can easily upgrade an FPGA-developed WiMAX system when additional protocol support is required to ensure compatibility with future products or if enhancements or bug fixes are necessary. WiMAX systems that use FPGAs can stave off premature obsolescence because they can support evolving standards and applications that were not known at the time of equipment deployment.

As the standards eventually stabilize, the flexibility of the WiMAX product needs to be traded for a low-cost ASIC implementation. Typical ASIC design today faces increasing product development costs and long design cycles, both due to shrinking process geometries and growing design complexity. Structured ASICs offer a comprehensive alternative to traditional ASICs. Structured ASICs address the pain associated with traditional ASIC design by providing the FPGAs, development tools, intellectual property (IP), and a seamless migration path from the function-verified prototypes to high-volume production devices. Users can also obtain an average of 50 percent power reduction compared to FPGA and guaranteed first-time success in silicon.