Envelope tracking (ET) is an important technology for enabling LTE devices to work across many bands and now it has its own standard. The MIPI Alliance has published the Analog Reference Interface for Envelope Tracking, a standardized specification that supports interoperability between 3G/4G modem chipsets and ET power supplies.
Nujira, the UK company which has pioneered, and to some extent standardized, ET technology for devices and base stations (through MIPI and its own OpenET initiative), welcomed the new release, which should accelerate the creation of an ecosystem around ET by allowing vendors to mix and match components.
ET allows dynamic control of the supply voltage of RF power amplifiers, which makes them operate more efficiently, so devices can deliver more transmit power. Battery life is extended and the designs can support more flexible multiband RF front ends.
The main specialists in the mobile world have been Nujira and Quantance, while Qualcomm moved the goalposts considerably when it announced its RF360 product (though it is not known who supplies the ET element).
The MIPI eTrak specification has already been incorporated by various modem and ETPS vendors, including Nujira, whose NCT-L1300 ET power modulator is already compatible with the eTrak interface. CEO Tim Haynes said: ‘Open standards like eTrak are integral to the future success of envelope tracking. The highest system performance can only be achieved with the right interface, and we welcome the introduction of products that unlock the full performance potential of envelope tracking. We fully expect ET to achieve a 100% attach rate in smartphones and other categories of wireless devices in the next year, which would simply not be possible without the work done by the MIPI Alliance in producing the final specification.’
The eTrak specification is available for members to download from the MIPI Alliance website and is available under MIPI Alliance IP rules.
ET will be an increasingly common feature of RF components for mobile devices as LTE-Advanced, with its wide or aggregated frequency bands, rolls out. Qualcomm and Intel have both implemented the technology, the latter via its acquisition of Fujitsu Semiconductor Wireless Products. Earlier this year, Nujira unveiled Woodstock, a reference design for smartphone RF front ends, which can support 16 frequencies while keeping power consumption low. This should turn up in commercial smartphones next year.
LTE is today deployed in 20 frequencies between 700MHz and 2.7GHz, and currently, each one requires its own power amplifier (PA) and RF filter. Nujira can apply its ET supply modulator to PAs from different chip suppliers, and in Woodstock three of these are supported at once ‘ a multimode, multiband PA; a 2.3-2.7GHz PA; and a region or operator specific LTE PA. Also included in the reference design are antenna switches and filters. The firm says all the major PA vendors have now developed ET-capable products.
‘Adding envelope tracking to a couple of specific LTE bands is relatively straightforward, but doesn’t by itself solve the cost, footprint and power consumption challenges the industry is facing,’ Haynes said. ‘What the mobile industry really needs is multimode, multiband reference designs that can be quickly incorporated into handsets.’
The energy saving comes because the ET modulator chip can drive up to four PA devices in parallel even on 20MHz LTE channels. And it enables the handset to deliver full power to the antenna port, maximizing coverage and boosting data rates, even for LTE signals with high PAPR (peak to average power ratios).