Microwave Engineering Europe - November 2008 - (Page 25) MMICs — RF FRONT END 25 around 3.5 dB. OFDM systems, such as those used in 3GPP long-term evolution (LTE) standards and WiMAX, depend on even higher crest factors – up to 9.0 or 9.5 dB – resulting in even lower PA efficiencies. Best in class RF amplifiers for WiMAX typically dissipate -80 percent of the power they draw as heat. In Figure 1, the solid blue curve represents drain efficiency versus power output and the dashed curve is the probability distribution of instantaneous output power value for a typical W-CDMA transmission. As can be seen, for much of the time the signal power lies well below the peak power and hence the device is operating at low efficiency. The wideband challenge A marketable handset will need to operate globally. So far, ten different FDD frequency bands and four different TDD frequency bands have been defined in 3GPP that can be used for LTE, and it is likely that more bands will be added to this list such as 700 MHz in the US. To address this, current 3G terminals use multiple PAs. Figure 2 shows a conventional RF sub system with single mode PAs. Five are used to cover the W-CDMA bands and a further two for the GPRS/EDGE bands. Infrastructure solutions Essentially the same issues exist at the network end, and the solutions adopted here include crest factor reduction, digital pre-distortion (DPD), linearization, Doherty, and a new technique called High Accuracy Tracking (HAT™). Base station RF transmitters operate at an order of magnitude higher power than at the terminal end – say 40 W typical rather than 0.25 W for a W-CDMA handset. They also essentially transmit at a constant power level, whereas handset RF circuits must control the power over a 74 dB range. Whilst the above techniques can deliver savings in the high power network end, some draw too much power for use in the terminal. Crest factor reduction, DPD and linearization all rely on intensive signal processing techniques, which need to be implemented in the baseband. Running a 2 W DSP to double the efficiency of a 40 W transmitter delivers a worthwhile benefit. At the handset end, this might translate to a 500 mW DSP linearizing the power of a transmitter running at 250 mW full power which gives a limited advantage at best, and might easily reduce efficiency when the handset is close to a base station and transmitting minimum power. Doherty is essentially narrow-band, so would need to be implemented seven times in the RF subsystem shown in Figure 1. It also requires linearization to cancel out the two nonlinear transfer characteristics of the amplifiers. High accuracy tracking High Accuracy Tracking (HAT™) uses a completely different technique to enhance efficiency – and it does so over a wide Microwave Engineering ● November 2008 ● www.mwee.com http://www.microsemi.com http://www.microsemi.com http://www.mwee.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.