Analog Angle Article

“The envelope, please” takes on new meaning with Nujira’s RF PA efficiency approach for 4G handsets

Cambridge, UK—In the wireless-communication world, it's all about power and efficiency, whether at the base station infrastructure or handset levels. Coolteq-l from Nujira Limited ( adapts their proprietary High Accuracy Tracking (HAT) technology to 4G systems (LTE and WiMax) to maximize efficiency of a wideband RF front end, yielding what the vendor claims is three to five times the bandwidth and with at double the efficiency of the more conventional approaches.

According to Julian Hildersley, VP for Handset Products and Strategy, the problem with such handsets is that they must cover a wide set of different bands (up to 14) as defined by the 3GPP for Long Term Evolution (LTE) and its operating modes (GSM, EDGE, WCDMA, HSUPA, and LTE), which in turn may dictate as many as seven distinct power amplifiers (PAs). In contrast, he noted that the Nujira approach would need only two PAs, for an obviously significant saving in board space, BOM and parts cost, and circuit-design intricacies, while also reducing power consumption by 1.4 to 4×.

The Coolteq-l HAT technique works by not providing a constant dc voltage to the final RF-stage transistor, which is normally operating in Class A/B mode for acceptable linearity despite its high dissipation and inefficiency. Instead, the supply voltage dynamically tracks and changes in synchronization with the envelope of the RF input to that stage. By modulating the supply, far less power is wasted given the wide peak-to-average-power ratio (PAPR) of these signals.

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While the power-modulation principle has been known for many years, it is difficult to achieve in practice. Providing this dynamic modulation, he added, required development of wideband, fast-response, yet stable closed-loop power control at relatively high power levels. In contrast, one technique in widespread use in base stations, known as the Doherty amplifier, needs to be tuned to each frequency band and PAPR value, using digital predistortion and a complex DSP circuit and algorithm to achieve required linearity specifications, which requires additional components and power consumption compared to the HAT method.

Since low parts count, minimum PCB space, and low cost are driving factors in handset design, Nurija's Coolteq-l is not available as a stand-alone IC for this market. Instead, the company is offering it as licensed IP to be incorporated into handset ICs. This is in contrast to the Coolteq-h module which is available for base stations, and the Coolteq-u module for broadcast transmitter, both already on the market. An evaluation system for the Coolteq-l is available now for $50,000, with a test IC scheduled for 3Q-2009.—by Bill Schweber

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