Measurement Based Throughput Evaluation of Residual Frequency Offset Compensation in WiMAX Qi Wang, Sebastian Caban, Christian Mehlf¨ uhrer and Markus Rupp Institute of Communications and Radio-Frequency Engineering Vienna University of Technology, Gusshausstrasse 25/389, A-1040 Vienna, Austria Email: {qwang, scaban, chmehl, mrupp}@nt.tuwien.ac.at Abstract - WiMAX utilizes a physical-layer based on OFDM that is very sensitive to carrier frequency offset. Even though most of this offset can be compensated using the initial training sequence, there still remains a residual fre- quency offset due to estimation errors. The methods proposed to correct for this remaining offset are mostly tested by means of pure simulation. In this work, we present outdoor-to-indoor WiMAX measurements in an alpine scenario in which four residual fre- quency offset compensation schemes are investigated. We evaluate the performance of these schemes in terms of measured throughput rather than only frequency offset estimation error. If a-priori knowledge of the previous receive frame is exploited in a symbol-wise frequency offset estimator, the measurement results show worse performance than simulations predict. Consistent with simulations, the data-aided method effectively compensates the throughput loss due to the residual frequency offset. Keywords - Residual Frequency Offset Compensation, WiMAX, Measurement 1. INTRODUCTION Carrier frequency synchronization is a crucial issue for OFDM based WiMAX since the Carrier Frequency Off- set (CFO) introduces inter-carrier interference. Numer- ous papers dealing with carrier frequency synchroniza- tion in OFDM can be found (e.g. [1–3]). The basic idea is to split the CFO into the Fractional Frequency Off- set (FFO), the Integer Frequency Offset (IFO) and the Residual Frequency Offset (RFO). For WiMAX, the FFO and the IFO are estimated and corrected using the preamble at the beginning of each frame. To estimate the RFO during the data transmission, pilot-based and decision directed methods have been developed [2, 3] and improved [4]. To the authors’ knowledge, how- ever, all evaluations presented in literature are based on simulations. In this work, we measure a WiMAX transmission with CFO in a realistic alpine scenario. Four RFO compensation schemes are evaluated. The results are based on outdoor-to-indoor measurements utilizing the Vienna MIMO testbed [5, 6]. 2. RESIDUAL FREQUENCY OFFSET COMPEN- SATION IN WIMAX In this section, we first introduce the basic idea of RFO estimation. Then we review the four RFO compen- sation schemes utilized in the measurement, namely the pilot-based frame-wise approach, the pilot-based symbol-wise approach, the symbol-wise approach with pre-knowledge and the data-aided approach [4]. 2.1. System Model In an OFDM system, the CFO Δf CFO is normalized to the subcarrier spacing f s and denoted by ε CFO = ΔfCFO fs . After the FFO and the IFO are corrected, the RFO is typically in the order of 10 -3 . In the following, we denote the OFDM symbol in- dex within one frame by l, the receive antenna index by m, the subcarrier index by k, the FFT size by N and the Cyclic Prefix (CP) length by N g . The received symbol in the frequency domain is referred to as R (m) l,k , the transmitted symbol as X (m) l,k , the channel frequency response as H (m) l,k and the additive Gaussian noise as V (m) l,k . According to [2, 4], the RFO can be derived from the phase variation j 2π ˜ ε RFO in two consecutive OFDM symbols using W (m) l,k = R (m) l-1,k R (m)* l,k (X (m) l-1,k X (m)* l,k ) * =(H (m) l-1,k X (m) l-1,k + V (m) l-1,k ) (1) · (H (m) l-1,k X (m) l,k e j2π ˜ ε + V (m) l,k ) * (X (m) l-1,k X (m)* l,k ) * =|H (m) l-1,k | 2 |X (m) l-1,k | 2 |X (m) l,k | 2 e -j2π ˜ εRFO + ˜ V (m) l,k which leads to the estimated RFO ε RFO =˜ ε RFO ·N/(N + N g ). The additional noise terms are contained in ˜ V (m) l,k . 2.2. Pilot-based Frame-wise Approach Assuming N R receive antennas, we denote the subset of pilot subcarrier indices by N p and the total number of OFDM symbols in one frame by N f . The estimated RFO can be derived by averaging over N f OFDM sym- bols in the current frame. This yields the estimated RFO ˆ ε RFO, Frame = - 1 2π N N + Ng arg    N f  l=2 N R  m=1  k∈Np W (m) l,k    . (2) 51st International Symposium ELMAR-2009, 28-30 September 2009, Zadar, Croatia 233