Worldwide Interoperability for Microwave Access (WiMAX) is a wireless digital communication technology, based on the IEEE 802.16 and ETSI HiperMAN wireless metropolitan area network (MAN) standards. It can provide broadband wireless access (BWA) up to 50 km for fixed stations (e.g., desktop PCs), and 5-15 km for mobile stations (e.g., notebooks, computers, mobile phones, personal media players, and PDAs). The newest version of the IEEE 802.16 standard, dubbed 802.16m or Mobile WiMAX™ 2.0, could drive mobility up to 350 km/hr and push the data transfer speed up to 1 Gbps. Draft one of 802.16m is expected to deliver performance of over 300 Mbps in 4x4 MIMO configurations using 20-MHz channels and will likely be finalized in 2011.

As compared to a wireless technology like Wi-Fi, WiMAX is more immune to interference, allows more efficient use of bandwidth and is intended to allow higher data rates over longer distances. Because it operates on licensed spectrum, in addition to unlicensed frequencies, WiMAX provides a regulated environment and viable economic model for wireless carriers. These benefits, coupled with the technology’s global support (e.g., ongoing worldwide deployments, spectrum allocation and standardization), make it the popular choice for quick and cost-effective delivery of super-fast broadband wireless access to underserved areas around the world.

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The fixed version of WiMAX, provides non-line-of-sight (NLOS) transmission to stationary devices using the 2-11 GHz frequencies. Higher frequencies require line of sight. Fixed WiMAX™ provides a high throughput broadband connection at speeds up to 75 Mbps over a distance as far as 30 miles. It is based on orthogonal frequency division multiplexing (OFDM), uses multiple pilot tones and supports modulations ranging from BPSK to 64 QAM. WiMAX systems can use variable bandwidths from 1 to 28 MHz with 256 subcarriers (192 data subcarriers) in either licensed or unlicensed spectrum. It can be used for a variety of applications including a Last Mile broadband connection, hotspot and cellular backhaul, and high-speed enterprise connectivity for businesses. The mobile version of WiMAX is an extension for mobile use in the 2-6 GHz band.(See Figure 1.) It allows WiMAX technology to be built into notebook computers and other mobile devices.

Figure 1. Comparison of 802.16 wireless technologies.

Market Impact
While WiMAX technology is increasingly being embraced by countries worldwide, there is little doubt that the global economic downturn has had an impact on its deployment, in some cases slowing down or even delaying planned roll outs. Nevertheless, the Telecommunications Industry Association (TIA) projects a bright future for the technology with an annual growth rate that will increase 34 percent in the next 3 years alone. (See Figure 2.)

Figure 2. This graph, courtesy of Infonetics, highlights the growth in WiMAX equipment revenue through 2013. In the first quarter of 2009, EMEA (Europe, the Middle East, Africa) and Asia Pacific were the hotbeds of WiMAX activity, led by the Middle East, India, and particularly Africa. Graph © Infonetics Research, WiMAX Equipment, Devices, and Subscribers Quarterly Worldwide and Regional Market Share, Size, and Forecasts, June 2009.

WiMAX allows for infrastructure growth in underserved markets and is today considered the most cost-effective means of delivering secure and reliable bandwidth capable of supporting business critical, real-time applications to the enterprise, institutions, and municipalities. It has proven itself on the global stage as a very effective Last Mile solution.

In the United States though, licensed spectrum availability and equipment limitations have held up early WiMAX adoption. In fact, while there are currently 1.2+ million WiMAX subscribers worldwide, only about 11,000 of those are from the United States.

Future growth in this market will be driven by wireless ISPs like Clearwire who intends to cover 120-million covered POPs in 80 markets with WiMAX by the end of 2010. Growth will also be driven by the availability of the 3.65-GHz spectrum that the FCC opened up this past year.

Although the potential for WiMAX in the U.S. market looks good, analysts believe that its real opportunity lies in emerging markets like India and Pakistan where the mobile phone has achieved greater than 50 percent penetration. Pakistan is currently among the first countries in the world to roll out a functional WiMAX service. India’s planned rollout, expected some time in 2010, comes in response to a government requirement that 20 million broadband lines be in service by 2010. By 2012 alone, Springboard Research estimates that there will be 15.8-million WiMAX subscribers in India -- close to 47 percent of total subscribers in the entire Asia Pacific region.

Perhaps some of the biggest planned deployments of WiMAX though will come from South Asia. According to Springboard Research, WiMAX services revenues in South Asia will grow from an estimated $58 million in 2007 to approximately $5.5 billion by 2012. Likewise, the estimated number of Fixed and Mobile WiMAX subscribers is expected to grow from 230,000 in 2007 to 33.9 million by 2012, with Mobile WiMAX services garnering the majority of revenues and subscribers.

Challenges Ahead
Despite its growing market approval, many factors conspire to make WiMAX system design challenging, including consumers, who today demand greater range, faster data rates and lower price-points. The 802.16 specification itself also poses challenges as it forces engineers to face many different system considerations in terms of RF requirements and architectures. Will the WiMAX system be deployed as Time Division Duplexing (TDD), Frequency Division Duplexing (FDD) or half-duplex FDD? Will it be based on a superheterodyne or direct-conversion RF architecture? Ineffectively addressing any one of these challenges threatens to hamper the proliferation of WiMAX and directly ties into the success of a WiMAX product.

Seven key engineering challenges that currently exist include:

Challenge #1: Conformance Test
Interoperability issues commonly plague new technology at introduction. Conformance testing -- whether Protocol Conformance Test (PCT), Network Conformance Test (NCT), Radiated Performance Test (RPT), or Radio Conformance Test (RCT) -- plays a critical role in addressing this challenge. Not only does it help alleviate interoperability issues with other WiMAX equipment, it also ensures a positive end-user experience for consumers. NCT, for example, like PCT, RCT, and RPT, it is a vital part of the WiMAX Forum certification process, testing conformance above the MAC layer to verify Internet Protocol layer signaling and messaging to and from the subscriber device. The availability of rigorous and efficient network test solutions is therefore a must for ensuring successful delivery of WiMAX technologies. The key, of course, is in having early access to conformance tests well in advance of commercial service.

Challenge #2: Orthogonal Frequency Division Multiple Access (OFDMA) Complexity
OFDMA is the digital modulation scheme employed by Mobile WiMAX. It uses OFDM technology in an innovative way to allocate the RF spectrum more effectively to more users. While this provides a very “granular” way to dissect bandwidth and charge for service, the standard and signal structure are extremely complex, creating a number of challenges.

To begin with, as the expected and required services from WiMAX networks expand, ever greater demand will be placed on the processors in both the base and mobile stations. Additional processor strain will come from service providers using dynamically allocated subcarriers to allocate spectrum resources. And, while WiMAX’s complex signal structure provides network operators with the flexibility they need, its associated cost puts enormous strain on the power amplifier (PA). Another challenge stems from WiMAX’s utilization of non-traditional frequency re-use schemes which can create inter-cell interference, especially at the edges of the cell -- just when a critical hand off needs to occur.

Challenge #3: Multiple-Input Multiple-Output (MIMO) Complexity
MIMO technology uses multiple antennas at both the transmitter and receiver to improve communication performance. Because it allows more bits/hertz to be transmitted in a given bandwidth, the technology improves spectral efficiency which, in turn, allows service providers to flexibly configure communication services and not just peak data rates. While MIMO offers increased signal robustness and capacity improvements, those benefits come at the cost of increased complexity for both the base station and the mobile station, placing large demands on processing power and antenna design.

Challenge #4: A Tight EVM Requirement
The 802.16 standard specifies that Error Vector Magnitude (EVM) be held to -31 dB, based on a 1% packet error rate. While this error rate, and a stringent receiver noise figure (7 dB maximum), help contribute to WiMAX's longer range, having to meet the EVM target has a number of implications. For example, all system blocks must be more linear and phase noise must be considerably better than in an 802.11 design, impacting the synthesizer and resulting in a longer settling time.

The PA is also impacted by the tight EVM requirement. In WiMAX systems, PAs must deliver more power, be more linear, and be able to handle a high Peak-To-Average Power Ratio (PAPR) -- about 10 dB. Consequently, they consume more power and are less efficient. As a result, considerable effort must be made to develop higher efficiency, more linear PAs, especially for mobile applications where power consumption is critical.

Challenge #5: Receiver Performance
Nearly all WiMAX-enabled devices have two receivers and many are capable of MIMO reception in several frequency bands, commonly 2.3 GHz and 2.5 GHz. This added complexity challenges the designer to find space to adequately separate two receive antennas in the mobile station (MS) and thereby ensure signal recovery. Also, the need to differentiate the multiple data streams from the received signals places increased demands on the processor. WiMAX designs must therefore be thoroughly evaluated to ensure success in the conformance test process, and to verify the device will operate in the electromagnetically harsh and dynamic world in which it will work. Unfortunately, the wide range of wanted and unwanted signals, combined with the many nested feedback loops, make receiver design one of the most difficult challenges in the Mobile WiMAX cellular system. In fact, many consider this task one of the hardest in modern radio design, forcing designers to deal with everything from hardware performance and current consumption to memory and algorithm complexity constraints, while recovering some highly complex MIMO signals suffering from linear and non-linear distortions.

One of the specific challenges designers face results from the 802.16 specification's support of subchannelization. This means that instead of transmitting on all 192 data subcarriers, the base station (BS) can transmit on just a subset for a given user. Using the same amount of power over fewer carriers can result in greater range for the system, but because the subcarriers are spaced more closely together, tighter requirements exist for phase noise and timing jitter. Also, higher-performance synthesizers must be utilized.

Another challenge stems from the fact that WiMAX systems rely on multipath to provide NLOS coverage. The receivers used in the system are especially susceptible to phase noise, timing jitter and frequency mismatch/synchronization. This can create a challenging situation, given the already tight requirement for phase noise and jitter as specified in the standard. Improving the receiver's performance will help by also improving its range and data rate. If receiver performance is not adequately addressed, the range and data rate of the WiMAX system will be adversely impacted, as will the price.

Challenge #6: Spectral Efficiency/Latency
The overall complexity of Mobile WiMAX, along with its highly dynamic air interface requires a significant amount of optimization and management. Enabling the MS to make some decisions about engaging on the network, for example, can reduce latency and free the base station to manage more of the additional, complex tasks associated with network management. Also, the flexibility to dynamically allocate spectrum comes at the price of tremendous signaling complexity. As a result, the base station must manage the constantly changing data requirements of multiple antennas (for MIMO) and a wide range of mobile stations traveling at potentially high speeds. The signaling must be flexible enough to manage these issues. Further, the designers and service providers must be able to verify the performance of the network under realistic traffic conditions both before and after deployment.

Challenge #7: Integration with Legacy Systems
WiMAX networks must be able to integrate into existing cellular networks, providing seamless connectivity and user experience. Of course designing this into the network operation, base station and mobile station is a challenge task since WiMAX's modulation scheme is so very different from legacy systems.

Class Dismissed
WiMAX is intended to make point-to-multipoint broadband network access widely available, without the expense and distance limitations associated with wired options. Critical to this emerging technology and the overall success of WiMAX applications is the ability to generate, detect, demodulate, and troubleshoot PHY layer signals.

Endnotes:
Springboard Research www.springboardresearch.com
Telecommunications Industry Association (TIA) www.tiaonline.org

“WiMAX,” “Fixed WiMAX,” “Mobile WiMAX,” “WiMAX Forum,” the WiMAX Forum logo, “WiMAX Forum Certified,” and the WiMAX Forum Certified logo are trademarks of the WiMAX Forum. All other trademarks are the properties of their respective owners. For more information, visit the WiMAX Forum at www.wimaxforum.org.

Agilent Technologies delivers a broad range of solutions for addressing the unique challenges brought about by the Fixed and Mobile WiMAX specifications. Agilent was first to market with R&D-based WiMAX solutions and is an active member of the WiMAX Forum. With its ongoing commitment to supporting WiMAX, the company will continue to develop and introduce solutions as the market grows and the technology advances. For more information, visit www.agilent.com.

About the Author
Jennifer Stark started her career with Agilent Technologies, Inc. (then Hewlett-Packard) as a product marketing engineer in 1998. Jennifer currently leads the Wireless Connectivity Business Team in Agilent’s Electronic Measurements Group. For more information, visit www.agilent.com or email jennifer_stark@agilent.com.

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