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700 MHz shift needs GaAs linearity

The issues surrounding the impending deployment of the LTE standard in the analog TV spectrum will demand high-performance GaAs PAs, say both Avago and Strategy Analytics.

On February 17 2009 RF chip technology is set to enter uncharted territory, as US analog TV signals are switched off.

Now the dust has settled from the US auction for the 700 MHz spectrum, the winning wireless communications companies say these airwaves will be used for Long-Term Evolution (LTE) cellular services. The efforts to bring the 4G standard online are being led by Verizon, which is expected to deploy the first LTE networks, and AT&T.

Although this frequency is lower than those currently used by most 3G communications, LTE will help ensure that GaAs remains dominant in 700 MHz RF power amplifiers (PAs).

“Where the focus is on data and LTE, then linearity requirements will ensure that GaAs remains the primary technology for handset front-ends,” said Asif Anwar, director of market analyst Strategy Analytics GaAs and compound semiconductor program.

“The fact that 700 MHz looks like it will be used for LTE argues for the use of GaAs rather than silicon,” agreed William Mueller, manager for RF chip manufacturer Avago Technologies wireless semiconductor division.

Mueller and Anwar also agree that products look set to be physically released for use in the analog TV spectrum by 2010. However, Mueller pointed out that in the handset market there is normally an 18-month lag between product release and volume manufacturing.

As well as the tough linearity requirements, Mueller said that design and performance expectations for LTE will call on PAs to be especially small, efficient and operate within tight current and temperature limits.

He highlighted that, although it is reputed for its excellent radio signal propagation properties, the 700 MHz spectrum s bandwidth constraints mean that some services will still need to operate at higher frequencies. This will limit overall demand for 700 MHz products.

Furthermore, inconsistency in the frequencies freed by turning off analog TV signals in different countries and interference experienced at these frequencies will mean PA manufacturers have to think hard about how they tackle the transition.

Despite these challenges Mueller said that Avago is well positioned to take advantage of the move to LTE, thanks in part to expertise developed producing PAs for the 3G CDMA standard.

Anwar says that Avago and all the major GaAs PA suppliers are readily able to address the 700 MHz spectrum. “We may also see a challenge from Mitsubishi Electric which is looking to expand beyond its Asian market base into North America and Europe,” he said.

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