System Implications in Designing a 60 GHz WLAN RF Front-End Ali Behravan, Florent Munier, Tommy Svensson, Maxime Flament Thomas Eriksson, Arne Svensson, and Herbert Zirath Dept. of Signals and Systems (S2) and Microtechnology Centre at Chalmers (MC2) Chalmers University of Technology, S-41296 Gothenburg, Sweden http://www.s2.chalmers.se http://www.mc2.chalmers.se/ Abstract In this paper, we seek to evaluate the performance of a 60 GHz WLAN system taking into account RF cir- cuitry imperfections and hardware requirements for var- ious modulation techniques. A model of an RF front- end is developed, including physical imperfections of the circuitry such as power amplifier (PA) nonlinearity and voltage controlled oscillator VCO phase noise. Given the RF front-end model, several modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) and Continuous Phase Modulation (CPM) are consid- ered. The evaluation of the system performance in terms of bit error rate allows a better understanding of physical circuitry limitations, and optimal modulation parameters as well as circuit design recommendations can be derived. Keywords OFDM, CPM, 60 GHz, Power Amplifier, Phase noise. 1. Introduction The need for high data rates communication systems has been dramatically increasing in recent years [1]. The 60 GHz band (59–64 GHz), an unlicensed frequency band, has been investigated as a potential band for wireless high data rate transmission. One of its main properties is the existence of strong attenuation due to the oxygen absorp- tion and obstacles, resulting in a good frequency reuse factor, but also limiting the coverage of the cells in the cellular communication network. Radio frequency (RF) hardware with ideal character- istics is difficult to design for the 60 GHz band. Problems such as power amplifier (PA) non-linearity and oscillator phase noise are more prominent for these circuits than for circuits designed for lower frequencies. Therefore, we should take these effects into account in the overall communication channel. This report is a first effort in an ongoing work aiming at evaluating the performance of various communication techniques in presence of hardware imperfections. We study how the performance of a system is affected by the non-linearity of the amplifier, and the phase noise of the oscillators and phase-locked loops at the transmitter and receiver. Even for a noiseless channel, the degradation due to non-linearities and phase noise can become severe, leading to poor performance. We have chosen to study an orthogonal frequency division multiplex (OFDM) sys- tem, due to its good performance for multipath channels. We also study a CPM system, which is more resistant to the non-linearities of the power amplifier. In section 2, we give a general background on non- idealities arising from the RF front-end of a wireless tran- ceiver as well as a brief insight on modulation methods employed in the baseband back-end. Section 3 describes the models of the RF hardware, including phase noise and power amplifier non-linearity. In section 4, some exper- iments are performed in order to characterize the effects of the hardware imperfections for an OFDM and a CPM system. 2. Background 2.1. System description Figure 1 shows the block diagram of the communication system. In the baseband modulator block, input sym- bols are modulated (using CPM or OFDM modulation) and fed into the analog RF front end. The signal is up- converted to the desired frequency band, and then ampli- fied to an adequate power level, to be transmitted through the channel. The receiver amplifies the input signal, and down-converts it to baseband, where it can be demodu- lated and further processed. Due to nonlinearities in the power amplifier and phase noise of the VCO, the demod- ulated bits may contain errors even on an noiseless chan- nel. 2.2. Front-end imperfections Power Amplifier (PA) linearity and Voltage Controlled Oscillator (VCO) stability are critically important in practical systems. If not considered, they may severely degrade the performance of a digital communication sys- tem. These imperfections, depicted in Figure 1 as solid line boxes, are studied in this paper.