Challenges of Wimax

8/11/2019 Challenges of Wimax

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RF Chipset Design Challenges For WiMAX

by Russell Hoppenstein, RF Applications Engineer, Texas Instruments Incorporated

In the communications industry, technology advancements seem to come in waves. The

next crest that is hitting is the 802.16 standard, commonly referred to as WiMAX.Worldwide Interoperability for Microwave Access (WiMAX) has taken the industry by

storm, quickly generating a flurry of activity and interest from small rural operators to

major service original equipment manufacturers (OEM) and providers.

The WiMAX standard was originally intended for fixed station broadband

communication deployment. The rising popularity of these applications has morphed

WiMAX into point-to-multipoint (ie last mile deployment), cellular backhaul and highspeed area networks for business or college campuses. Though many technologies that

preceded WiMAX can perform similar functions, the pivotal selling point of this

particular technology is the interoperability feature. The fact that WiMAX is tied to the

802.16 standard ensures that compliant systems can maintain interoperability with eachother. This offers greater flexibility by WiMAX providers as they are assured that they

can build their system with a variety of suppliers without the fear of availability issues,

incompatible upgrade enhancements or procurement delays. From the suppliersperspective, interoperability has opened the door for smaller companies to vie for a

portion of the business. Without the security of interoperability, only large corporations

would be sufficiently trusted to supply system components. The advantage ofinteroperability capabilities however is that smaller companies can also leverage their

internet protocol (IP) for the provider since the provider will assume little risk if they are

forced to go with a different vendor.

While there is an interoperability standard associated with WiMAX, there are still a

myriad of system variables that will affect the implementation of the RF solution.Currently, WiMAX is being deployed in the 3.5 GHz and 5.6 GHz bands, and the

unlicensed 2.5 GHz band. Additionally, there are emerging bands at 4.9 GHz and 700

MHz. In the realm of interoperability and global compatibility, companies must look for

creative and innovative ways to gain a performance advantage and differentiatethemselves from their competitors and still maintain compliance to the WiMAX standard.

The RF chipset used to implement WiMAX radio must be flexible enough to handle a

variety of implementations, and have enough sufficient performance to meet thespecifications. The design challenge is to maintain the basic functional requirements and

to understand the architecture ramifications on the more subtle performance parameters

that are still required for compliance. Ensuring these parameters meet WiMAXspecifications is imperative to maintaining a robust, manufacturable design.


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WiMAX Transmitter

The key performance parameter of the transmitter is its error vector magnitude (EVM) ata given power. The EVM indicates the integrity of the digital constellation after it has

passed through the transmitter. The primary contributors to the transmitter EVM

degradation are the phase noise of the local oscillator (LO) sources and the final poweramplifier. Because the power amplifier is a key contributor, it makes sense to discuss the

transmitters EVM performance related to a given power out. The specification for EVM

is 2.7%. In contrast to cellular systems or 802.11 specs, this EVM requirement is much

stricter and the EVM is often expressed in decibels to better resolve the finer scale. Thestandard of 2.7% equates to -31.4 dB. The transmitter performance is characterized by the

maximum rated modulated power while still maintaining an EVM of -31.4 or better.

Customer premise equipment (CPE) outdoor systems, which have primarily clear line of

sight to the base station, are generally rated to +20 dBm output power. Indoor systems

that are employed inside buildings have to deal with a severe multipath environment

which increases the rated power in the range of +24 dBm to +27 dBm. More stringentrated power is required from the base stations that need to transmit at 4 W to 20 W,

depending on the desired range and implementation of the cell.

As the designer progresses up the rated power output of the system, the primary device

that must be altered is the power amplifier. Since the power amplifier is one of the key

contributors to EVM, using a larger, beefier device will allow higher output powers whilemaintaining the EVM spec of -31.4 dB. That, however, is not sufficient to ensure

complete compliance to the standard. The standard specifies an absolute spurious output

spec of -40 dBm. No spurs are allowed higher than this limit regardless of output power.

As the rated power increases, the gain of the transmitter must also be increased

accordingly, assuming the input power supplied from the base band processor remainsfixed. This increase in transmitter gain will apply not only to the desired signal but to any

undesired spurious signals as well. Since the spurious output spec is fixed, this increase in

gain reduces any output spurious performance margin to spec. Systems that may have

been compliant at a +20 dBm output power turn out to be non-compliant at +24 dBmoutput powers or higher due to the increased gain. For the RF chipset to be sufficiently

flexible to meet the wide range of output powers for a variety of situations, it must

maintain a good EVM performance of roughly -37 dB leading up to the PA and maintain7 dB to10 dB or more margin to the spurious output spec. This affords the designer

flexibility to choose the proper power amplifier device suitable for the system. At the

same time he can be assured of not only meeting the EVM requirements, but also thespurious output limitations.


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WiMAX Receiver

The key performance parameter of the receiver is its sensitivity. The specificationsdiscuss a minimum BER of 1E-6 to maintain compliance. When testing with RF and

analog circuitry only, it is difficult to take an actual BER measurement. It is more

common to relate the BER to an equivalent EVM number. The correlation defines thespec of -21.5 dB EVM at sensitivity level for 64-QAM signals. Commonly, 2 dB margin

is added into the parameter to bring the target to -23.5 dB.

The receiver also employs an AGC function to keep the output power fed into an ADCconstant. The dynamic range of the ADC is fixed and it is desirable to use its full

dynamic range. The AGC varies the gain as the input signal changes to keep the output

power constant. Achieving the required sensitivity has generally required a suitablesystem noise figure performance at the desired input power setting with the AGC

function engaged. Designers can modify the system noise figure by adding additional

LNAs or by reducing the noise figure of the existing stages. Either technique will lower

the overall system noise figure and improve the system sensitivity.

The designer must also be concerned with WiMAX specifications related to adjacent

channel blocking performance. This parameter indicates how immune the receiver is tounwanted signals and blockers that are located one or two channels away. This is a very

stringent receiver specification and dictates the linearity performance requirements of the

active stages in the receiver, as well as the location and selectivity of filtering in thelineup. Arbitrarily adding more LNAs to reduce the system noise figure and improve

sensitivity can have detrimental consequences toward blocking performance.

Receiver blocking specifications are outlined for two different situations: operating near

sensitivity level, and operating near maximum input power. The two extremes represent

cases in which the receiver is in its maximum and minimum gain settings. For themaximum gain scenario, specifications call for operating the receiver at its sensitivity

level, which is dictated by the input power at which an EVM of -23.5 dB is measured,

plus 3 dB. This results in an EVM performance that is better than the -23.5 dB. The

blocker signal at its adjacent or alternate channel is injected at the same power level asthe desired signal, and raised until the system EVM degrades back down to -23.5 dB. The

blocker performance is denoted by the delta between the desired signal and the unwanted

signal. Similarly, for the minimum gain case the input power is injected at a relativelyhigh power of -30 dBm. The blocker signal is injected at this same level and the desired

signal is reduced, while adjusting the AGC for required set-point, until the system EVM

degrades down to the -23.5 dB level. The blocker performance in this case is also denotedby the delta between the desired and unwanted signal. The specification requires the delta

between the desired and blocker be 4 dB for the adjacent channels and 11 dB for alternate

channels.


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Measurement Performance

WiMAX system parameters for the transmitter and receiver have been measured using aTexas Instruments WiMAX chipset on the TSW5003 reference board. The block diagram

of the five chipset solution using a superheterodyne architecture is shown in Fig. 1.

Fig. 1: TSW5003 Reference Design Block Diagram

This solution employs a 10-MHz wide surface acoustic wave (SAW) filter to facilitateuse with a variety of common signal bandwidths: 3.5 MHz, 7 MHz, 5 MHz and 10 MHz.

The TRF1223 PA is a 1-W Class-A amplifier. It is capable of delivering a modulated

output power of +20 dBm over temperature and frequency. The EVM performance curveis shown in Fig. 2.

Fig. 2: TSW5003 Transceiver EVM Performance Vs Power Out


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The curve clearly indicates the rated output performance of the radio with respect to

EVM performance, but this still does not tell the entire story. Though parameters such as

the transceiver spectral mask will likely fall into line given a satisfactory EVMperformance, the spurious output performance is still not accounted for. The expected

spurs related to the carrier feed-through of the mixer, the DAC images, and the second

harmonics can be attenuated sufficiently by judicious use of baseband, IF, and RF filters.The more unpredictable spurs related to the harmonics and intermodulation mixing

products of the clock signals and LO signals can be a serious hindrance, especially as the

chipsets move toward higher levels of integration where isolation is more difficult to

maintain.

The TSW5003 design at the rated +20 dBm has spurious output no greater than -50 dBm

at rated power which provides 10 dB of margin. Though the design margin is nice tohave, the margin allows more flexibility with this solution to move to higher rated power.

Say, for example, the rated power needed to be raised to +24 dBm to +27 dBm for an

indoor application. The existing PA could be replaced with a suitable power amplifier

and minimal other changes. The additional 4 to 7 dB of gain needed to get to the higherpowers may eat into the spurious output margin, but still leave at least 3 to 6 dB to spare.

Fig. 3: TSW5003 Receiver EVM Curve Vs Input Power

The receiver EVM curve (Fig. 3) has a constant output power to the ADC of -20 dBm.

Though sensitivity can be extracted from this information, compliance related to blockermeasurements is not accounted for. In contrast to the transmitter, the receiver requires

specific bandwidth limiting filters to satisfy the adjacent channel blocking requirements.

The architecture allows for a high-IF SAW filter and for two switchable on-the-fly low IFfilters. The high IF filter has a 10-MHz bandwidth to pass all the desired signal

bandwidths of 10 MHz or below. The low-frequency IF SAWs allow for two separate

signal bandwidths to be supported without any hardware change. This filtering coupled


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with the high dynamic range of the AGC amplifiers in the TRF1212 facilitates meeting

the stringent blocking specs at the sensitivity level. The higher power blocking tests are

aided by the above and by the integrated switchable attenuator in the TRF1216 LNA thatimproves the linearity at higher powers. The blocking performance of the TSW5003 is

shown in Fig. 4.

Fig. 4: TSW5003 Receiver Blocking Performance At Low And High Power

The high dynamic range of the receiver coupled with the switchable filters provide for a

flexible implementation with a variety of system conditions. Though not initially evident

in the EVM sweeps, these other receiver characteristics are critical to maintainingcompliance and flexibility in the system implementation.

Conclusion

When working with an RF chipset for WiMAX, it is important to look closer at criteria

other than just the standard EVM performance. Sufficient margin with specifications suchas spurious output and receiver blocking facilitate the system designer's ability to offer

creative solutions to satisfy market demand while maintaining a robust, manufacturable

product. As the fixed standard is being finalized, the 802.16e mobile WiMAX standard(not completely solidified) is beginning to get more attention and the wide influx of new

baseband providers working on mobile products is growing. Using a high-performance,

flexible RF solution to meet the design challenges is important for market success.

The Texas Instruments chipset is shown as one example of a solution that is flexible

enough to work in the three major WiMAX frequency bands, with a variety of rated

output power, and with a variety of base band processors that employ either a low IF orI/Q (quadrature) interface.


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About The Author

Russell Hoppenstein is an RF Applications Engineer working in the WiMAX productgroup of the Wireless Infrastructure Business Unit at Texas Instruments in Dallas, TX.

He has over 14 years experience in product development and applications related to

cellular infrastructure and WiMAX systems. Hoppenstein holds a Masters degree inelectrical engineering from University of Texas at Arlington and his undergraduate

degree from University of Texas at Austin. He can be reached at [email protected]

Challenges of Wimax

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