Microwave Engineering Europe - November 2008 - (Page 28) 28 MMICs — RF FRONT END existing amplifier technologies adapted for a wideband supply feed to the RF device. Optimising average power output Envelope tracking also provides maximum efficiency throughout a probability distribution set by average output power requirements as the terminal moves across a mobile network cell. The result is a huge improvement in the PA’s power dissipation for a given output power. Calculations show that the HAT-enabled PA uses half the battery power of those using conventional MMICs. See Table 1. Conclusion RF subsystem design for the next generation of network terminals faces efficiency challenges on two fronts: the new standards are inherently less efficient to transmit owing to their complex, amplitude based modulation schemes, and spectrum availability restrictions worldwide have led to the need to support a very wide range of frequency bands. HAT technology provides a practical, and easy to implement solution to enhance the efficiency of the subsystem, more than compensating for the efficiency lost through compromises like the use of wide-band PA devices. It is emerging as one of the key enabling technologies supporting the development of compact, attractive 4G terminals at a price point that the market will accept. HAT described Powering an RF subsystem transmitting at variable power levels from a fixed supply voltage, leaves a great deal of excess power over the required output waveform and this is dissipated as heat. Tracking the supply voltage closely with the signal envelope makes for a much lower differential, dramatically lowering the excess power. The supply voltage is varied by the HAT power modulator device, which replaces the normal DC-DC converter delivering power to the output transistor. HAT is based on envelope tracking, a principle that has been known for half a century, but has rarely been implemented in a technically or commercially viable product. Critical performance issues include modulator power conversion efficiency, and compliance with spurious-signal and noise specifications, together with sufficient bandwidth to accurately track a 5, 10 or even 20 MHz modulated signal. Nujira is the first vendor to successfully meet these challenges, developing a groundbreaking power modulator to enable High Accuracy Tracking™. Already adopted in cellular base stations and DVB transmitters, HAT based solutions for handsets offer an impressive improvement in efficiency, going from typically a low 20 percent for standard a class AB amplifier to mid 40 percent with HAT. High-accuracy tracking can be applied to all major RF devices and process technologies, providing greater efficiency at broader bandwidth to GaAs FETs and HBTs as well as silicon LDMOS and bipolar devices. Table 1: The HAT-enabled PA uses half the battery power of conventional MMICs. Figure 6: HAT reduces the voltage difference between the supply voltage and the signal envelope, dramatically reducing the energy wasted as heat. Ericsson blowing in the wind for green basestations Quantenna Communications has joined the ranks of companies crafting high-end variations of 802.11n silicon. The startup is using 4x4 MIMO (multiple input, multiple output) antennas along with beam forming and mesh technologies to deliver an aggregate throughput of up to a Gbit/second. The company uses two chips, one each for 2.4 and 5 GHz links. Two other startups recently announced 5 GHz chips that aim to offer new levels of Wi-Fi performance based on variations of the .11n standard. Celano said its switched MIMO approach uses up to eight antennas and beam forming to carry up to four high definition video streams across 50 meters and penetrate multiple walls. Amimon uses a novel encoding technique and a 4x5 MIMO configuration to hit up to 1.5 Gbits/s, and has gathered an ad hoc group of consumer electronics giants to refine its specification. All three companies compete with an expanding set of wireless options for the digital home including some using 60 GHz frequencies or ultrawideband approaches. For its part, Quantenna claims it delivers twice the 300 Mbits/s physical layer throughput of existing Wi-Fi chips from established vendors including Atheros and Broadcom. Founder and chief executive Behrooz Rezvani said one of those companies may move to 4x4 MIMO by the end of next year, but they may not also adopt the beam forming approach. The chips use a version of mesh networking, based on concepts under discussion in the 802.11s group, where each node can act as an access or relay point — claimed to be the best way to ensure whole house coverage across the broad range of homes. The Quantenna chips can be configured for 2x2 or 4x4 operation using only 5 GHz, 2.4 GHz or both. www.mwee.com/211200283 Microwave Engineering Europe ● November 2008 ● www.mwee.com http://www.mwee.com/211200283 http://www.mwee.com
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