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November 2007

First Pass Accuracy with Momentum GX for WiMAX Design
By William Clausen and Mounir Adada, Agilent Technologies

The IEEE 802.16e standard, often referred to as mobile WiMAXTM, specifies air interfaces for broadband wireless access (BWA) systems. It uses roaming and handoff to enable laptop and mobile phones to operate. The technology is expected to energize the BWA industry by opening up new opportunities to deploy systems in applications that were previously cost-prohibitive.

Multiple-Input Multiple-Output (MIMO) technology - a capacity enhancing, multi-antenna technology which is a fundamental component of mobile WiMAX – is today addressing some of the issues associated with mobility by extending high throughput and improving network capacity. Despite these benefits, cost and power consumption will continue to be a challenge for designers, especially as they attempt to achieve optimal system performance. Luckily, Agilent Genesys users now have access to an industry-proven, 3D-Planar electromagnetic (EM) simulation technology which is specifically designed to address such challenges. Referred to as Momentum GX, this solution helps designers optimize and verify the physical performance of high-frequency WiMAX circuits and interconnects within the Genesys design flow.

WiMAX: The Basics
To better understand the role of Momentum GX in the design and verification of WiMAX circuits, it is first crucial to have a clear understanding of WiMAX, or as it is also known, the Worldwide Interoperability for Microwave Access standard. As opposed to a Local Area Network (LAN) or a Wide Area Network (WAN), WiMAX is a Metro Area Network (MAN) based on the IEEE 802.16 specification and supports both licensed and unlicensed bands. It provides interoperable broadband wireless connectivity to both fixed and portable users, at a distance of up to 30 miles or 50 kilometers of service without the need of line-of-sight. Also, it can achieve data rates as high as 75 megabits. WiMAX has sufficient bandwidth to service hundreds of businesses and homes with a single base station at T1 rates.

What makes WiMAX different is its modulation method and adaptability (see Figure 1). It uses a technique called Orthogonal Frequency Division Multiplexing (OFDM) in which data is transmitted on multiple subcarriers for each symbol. The subcarriers are spaced so that interference with each other is minimized, thereby creating an orthogonal set of subcarriers. Transmitting on multiple subcarriers also enables signals to be transmitted at lower symbol rates. In other words, the designer can realize the same throughput (e.g., bits per second), with less susceptibility to interference from narrow-band interference or other transmitters, as well as multipath. Note that good multipath reception is critical for mobile service.

WiMAX also provides an Adaptive Modulation and Coding (AMC) technology which enables it to overcome multipath issues by slowing the symbol rate. Essentially, it allows the WiMAX system to adjust the signal modulation scheme depending on the signal to noise ratio (SNR) condition of the radio link. When the radio link is high in quality, the highest modulation scheme is used, giving the system more capacity. During a signal fade, the WiMAX system can shift to a lower modulation scheme to maintain the connection quality and link stability. Consequently, when the system is close to a base station, it might be able to send and receive at 256QAM. As the system moves further away though, or the SNR decreases, it reverses to QPSK or BTSK.

WiMAX Design Challenges for RF Engineers
Designing WiMAX systems poses a number of challenges for today’s RF engineer (see Figure 2). Consider, for example, that WiMAX provides support for either Time Domain Duplex (TDD) or Frequency Domain Duplex (FDD) schemes. In some cases, it supports both duplexing schemes – a fact which can significantly complicate the designer’s circuitry and structure, and may even require the use of switching networks. WiMAX also operates at a higher frequency band, between 2 and 11 GHz. At these frequencies, the designer faces a host of problems including increased parasitic effects, coupling of smaller circuits between networks which degrades signal integrity, and non-ideal behavior of lumped and distributed components.

While closed-form design solutions exist that allow the designer to determine element values and their influence on the circuit, such solutions fail to provide the degree of accuracy required at the higher frequencies in which WiMAX systems operate. And, because many of today’s WiMAX designs are mobile in nature and therefore powered by batteries, there is a whole slew of other problems and issues relating to power consumption that the designer must contend with. Further complicating matters, designers must also deal with radiation effects, cost constraints and manufacturability.

Effectively addressing these technical issues requires a top-level RF system design tool such as Spectrsys, available in the Genesys environment, which can plan, optimize and specify component parameters prior to manufacture. It also requires a synthesis platform to help speed the development of all system components such as filters, mixers and oscillators. Both linear and non-linear simulation technology is needed to verify things like noise figure, phase noise, power, intermods, spurious effects, and so forth. And, to ensure the product is manufacturable and that the desired yields can be achieved, designers must utilize yield analysis tools. More importantly, they need an accurate EM design tool to help optimize and verify the performance of the design prior to going to manufacturing.

An Effective Solution
The Agilent Genesys solution, with its full suite of tools, fully supports the requirements of today’s RF engineers tasked with designing WiMAX systems. In addition, it also now offers Momentum GX - an industry-proven technology for simulating complex multi-layer 3D-planar networks anywhere from DC to light.

Momentum GX works seamlessly with Genesys to compute and produce the S, Y or Z parameters. It is a 3D EM simulator that is two dimensional with Z-directed currents for multi-layer board analysis. Based on a method of moments, it enables RF and microwave engineers to expand the range of accuracy for their passive models. A conformal geometry meshing system ensures that designer gets optimal coverage from both very small and very large components over varying distances, such as from matching elements, and takes into account parasitic effects.

In addition to conformal meshing, Momentum GX offers two EM solvers: RF mode and a full-wave microwave simulation mode - both in 32-bit and 64-bit versions. The RF mode is a quasi-static mode which offers designers a much faster method of simulating. It does not perform a full-wave analysis, but in cases where circuit dimensions are less than half of a wavelength, a full-wave analysis is not necessary. When the designer does need to account for dispersive effects, radiation effects, or when the circuit starts to approach something larger than a half of a wavelength, the full-wave microwave mode can be utilized.

Momentum GX also offers a Genesys co-simulation technology which allows the designer to embed components in an EM simulation and examine the results as a combination. An Adaptive Frequency Sweep (AFS) function provides the designer broadband frequency information with much fewer points. It also accounts for thick metal modeling for accurate coupling between circuits and traces.

Because of its functionality, Momentum GX is suitable for designing passive matching structures or filters, and component modeling. The close-form models typically used for microstrip elements are not as accurate at higher frequencies. Momentum GX can also be used for layout verification to help the designer put components on pads and analyze how those pads effect the ultimate response, as well as for signal integrity. In the latter case, Momentum GX enables the designer to determine how much coupling occurs between different layers within the board. The solution can also be used for antenna design – a task which is very important, especially in WiMAX, and for designing and verifying Low Temperature Co-Fired Ceramic (LTCC) material designs.

Other uses for Momentum GX include:
• New topologies, where there are no closed-form solutions or equations to describe a
  network, a tool like Momentum GX becomes essential for modeling and analyzing the
  network.
• Manufacturability. Momentum GX can be used to look at the effect of changes in substrate
  permittivity variations and their effects on the final design.
• Reducing component count. At higher frequencies very small capacitors and inductors
  can be replaced with printed elements, but the models that describe those elements
  are few and usually not very accurate. Momentum GX generates EM accurate models.
• Improved passive models for standard structures such as steps, tees and open
  grounds. In many multi-layered designs, the designer winds up with an open in
   the ground that either a signal line or power line can go through.

The 3D-planar EM technology in Momentum GX differs greatly from the typical grid-based EM solver, as represented in Figure 3, which relies on the use of grid dimensions as opposed to those of the component values. Consider, for example, a 115 mil-wide line and a grid with 20 mil grid cells. In this scenario, the designer would only be allowed to snap to, or to analyze, a situation where the line was either 100 or 120 mils. In order to see 115 mil, the designer would have to change the grid dimensions to 5 mils, thereby allowing the tool to snap on a grid point. But changing the grid dimensions increases the size of the matrix, along with the time it takes to solve the problem. In some cases, the designer may be unable to solve it at all, in part because the solution space might be aimed at the multi-gigabyte memory.

Typical grid-based EM solvers also suffer from the “big/small” problem in which there are large distances between small elements or different-sized elements. In Figure 3, this problem can be witnessed with the spiral inductor. Clearly, too coarse of a mesh has been used and it is not meshed accurately enough. As a result, the simulator might easily resolve it as a solder blob when in reality it is actually a spiral inductor. The same issue holds true for the meander line in Figure 3. Essentially then, if the designer creates a grid to achieve accuracy with the smallest element, he may wind up with a very big solution space. When solving matrices, a general rule of thumb is that as the solution space increases, so to does the number of required calculations (e.g., by a factor of two). Put simply, the “big/small” problem can be a very daunting challenge for the designer.

Thanks to its unique conformal meshing technique, Momentum GX is much more accurate for all geometries – even small ones – essentially overcoming the “big small” problem. Figure 4 shows the same network as pictured in Figure 3, only this time it is meshed in Momentum GX. Note that there is no accuracy lost in the smallest element. On the contrary, even with the small mesh that occurs it is clear the spiral inductor has been uniquely meshed and provides enough information for accurate results. The same is true for the meander line and the microstrip components. They are meshed a little bit larger, allowing the designer to get accurate simulations for the various geometries. Far less memory is required with this approach and it also provides a faster solution. If the physical length of this circuit is small compared to the wavelength of the operating frequency the designer can even further increase the speed of the process by using Momentum GX’s RF mode.

Using Momentum GX
Momentum GX can prove especially useful when it comes to designing WiMAX circuits. To better understand that usage, consider the design example of a simple three-pole, Butterworth microwave filter operating in the 3.4 to 3.6 GHz range. The Genesys synthesis tool is used for fast and easy filter design.

To begin, the designer must first step through a process of tabs and options to select the filter type, shape and subtype that are going to be used in the design (see Figure 5). The type of filter selected depends on several factors, including: design size, what the designer can live with in terms of size and ease of manufacturing. Some filters require tighter spacing tolerances than others. The filter’s “cost to produce” is also an important factor as is its recurring frequency response. Note that all distributed networks exhibit a recurring frequency response at multiples of either a quarter-wave or half of the operating frequency. A cone filter, for example, offers the most compact size relative to the others and allows the designer to choose where the recurring band pass occurs. Unfortunately though, it also costs more because it requires the addition of more components such as capacitors.

Once the filter is selected - in this case, an edge-coupled filter – the designer must set the reference impedance which could be 50 ohms or some other impedance value. If the filter is embedded in another design, then the designer may want to select either a higher or a lower reference impedance. Next, the attenuation at the band edges, the order of the filter and the frequencies are selected, giving the designer control over the resonator, Zo, and the ability to optimize the unloaded queue of the resonator. As an option, the designer can also specify tapped input and the length of the input line. Following filter set up, a layout is selected and generated. The manufacturing process is also selected (e.g., coplanar, strip line, inverted strip line, microstrips, lab line, etc.) and the design readied for analysis via Momentum GX.\

Figure 6 depicts the results of a Momentum GX simulation, including the Momentum GX generated mesh for the selected filter. Note that the results show a shift in the filter response. The red trace represents the response of the linear network that was synthesized and includes losses. The green trace represents the Momentum GX simulation or filter and shows that a shift higher in frequency has taken place and that the bandwidth is slightly less. In other words, the filter is slightly undercoupled and its lengths must therefore be slightly lengthened. This is problem which can be easily fixed, but would likely not have been found were it not for Momentum GX’s ability to accurately indicate filter performance.

In some filter cases, especially those with bandwidth of better than 10 percent (e.g., 15 percent or so), the input and output coupling on, for instance, an edge-coupled filter can become very narrow. It may be too difficult therefore, to hold this level in manufacturing tolerances. Genesys features a tapped model which offers a work around to this issue, but the closed-form tapped models are limited in terms of accuracy and frequency (see Figure 7). Momentum GX assists with this problem by re-tuning or repositioning the taps in order to bring the filter back into compliance. It then further helps by improving the model that is used to design the filter.

At this point, the design is almost ready to be released to manufacturing - pending verification for manufacturability. This is a very important step – as it would be highly embarrassing to get a board back only to find out that it is not what was expected. To minimize this risk, the permittivity for the board must be swept. Figure 8A shows the Momentum GX swept simulation results of Er from 4.1 to 4.9 GHz. Note that the center frequency has shifted from 3.3 to 3.6 GHz. With this information, the designer can specify the tolerances on the manufacturing process or in the permittivity of the substrate. Figure 8B shows a Monte Carlo linear analysis based on the spacing dimensions and the width of the lines for the filter in question.

There are a number of key lessons which can be gleaned from this example. These lessons include:

• Closed-form distributed models are limited. The radiation effects and loss modeling
  is limited in terms of frequency and accuracy.
• Adjacent conductor coupling does not account for thick metal. In other words, there are
  really no zero-thickness lines.
• Via models are usually limited to lumped equivalents, and non-adjacent coupling
  does not exist. The models for coupling between lines are only for single-pair coupled
  lines. They don't take into account the third or fourth resonator over - which does and can
   affect the shape.
• Swept analysis is necessary to ensure manufacturability.

Conclusion
WiMAX – and especially mobile WiMAX – is today an emerging telecommunications market. Its proliferation will be determined in part by the designer’s ability to overcome the various design challenges involved in meeting current market demands. Momentum GX, an addition to the Genesys suite of products, now offers designers the functionality and flexibility they need to significantly expand the range and accuracy of their passive circuits and circuit models. With its unique meshing technology and ability to analyze arbitrary shapes, on multiple layers as well as to consider real-world design geometries when simulating coupling and parasitic effects, Momentum GX is an indispensable and necessary tool for ensuring first pass design success.

WiMAX,” the a trademark of the WiMAX Forum.

AGILENT TECHNOLOGIES
www.agilent.com
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