RF Transmitter Architecture Investigation for Power Efficient Mobile WiMAX Applications Liang Rong, Fredrik Jonsson, Lirong Zheng Dept. of ECS, School of ICT KTH Royal Institute of Technology Stockholm, Sweden {liangr, fjon, lirong}@kth.se Mats Carlsson, Charlotta Hedenäs Catena Wireless Electronics AB Kronoborgsgränd 19 SE-16446 Kista Stockholm, Sweden {mcarlsson, chedenas}@catena.se Abstract—Wireless broadband digital communication systems with high spectral efficiency suffer from severe power efficiency problem. Peak-to-Average Power Ratio is reported up to 12dB for WiMAX 802.16e systems implementing OFDM IFFT-1024 and 64-QAM modulation. In this work, outphasing (LINC) and polar transmitter architectures are investigated and compared with direct conversion (DC) architecture. Complete system solution targeting 23dBm output power is evaluated. System level simulation result shows that, with linear power combiner, LINC consumes more power than DC if non-clipping modulation scheme used. And polar system has stringent 3 degree phase matching and 0.5dB gain matching requirements to meet EVM and spectrum mask specifications. I. INTRODUCTION Prevailing wireless digital communication systems are evolving towards highly efficient spectrum usage in both mobile and fixed access. In WiMAX 802.16e-2005 system, OFDM (Orthogonal Frequency Division Multiplexing) and 64-QAM are implemented. However, with Direct Conversion (DC) system (Figure 1), the tradeoff for spectral efficiency is the demanding linearity of the system especially the power amplifier, it is required to linearly output 23dBm average power according to Power Class 2 for mobile WiMAX. With reported PAPR up to 12dB for IFFT- 1024/64-QAM system [1], class-A amplifiers will dissipate high percentage power and cause thermal problems, and this situation is even worse for base stations applications. Figure 1. Direct Conversion Architecture Block Diagram LINC (Linear amplifier with Nonlinear Components) modulation (Figure 2) can avoid power amplifier efficiency obstacle by processing signal into 2 correlated equal envelop, half amplitude signals and combine the amplified signals after PAs, thus nonlinear high efficiency PAs can be used. However, linear power combiner’s low efficiency prohibits the direct use of LINC principle, simulation result shows a maximum 7dB loss during the combination of 2 paths. So it is possible that combiner will counteract all the gain achieved by nonlinear amplifier pair. Figure 2. LINC (MLINC) Architecture Block Diagram Besides these 2 systems, Polar modulation (Figure 3) is gaining more and more attention due to its concise modulation architecture. The separation of phase and envelop signal raise the efficiency of PA without too much increase in system blocks number. Previous work achieving high efficiency of 34% by implementing polar modulation in EDGE system is reported [2]. So it is also a promising candidate to be investigated for broadband system in this work. Envelop tracking on power modulation is often used in polar power amplifier design and simple multiplier model is used here to represent the last stage amplifier. Figure 3. Polar system Architecture Block Diagram This investigation is supported by cooperation project between iPack Center, KTH Royal Institute of Technology, Sweden AND Catena Wireless Electronics AB, Sweden. 1-4244-2542-6/08/$20.00 ©2008 IEEE