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We evaluate the performance of a W-CDMA reverse link receiver system with closed loop fast transmit power control (TPC). For fast power control, signal-to-interference power ratio (SIR) must be measured precisely. We propose a new SIR measurement algorithm having a simple structure. The proposed algorithm uses unused orthogonal variable spreading factor (OVSF) code for interference power evaluation. The proposed SIR measurement algorithm is compared to the conventional SIR measurement algorithm (see Higuchi, K. et al., IEEE. J. Sel. Areas Commun., vol.18, no,8, p.1526-35, 2000; Seo, S. et al., IEICE Trans. Commun., vol.E81-B, no.7, p.1508-16, 1998; Andoh, H. et al., IEICE Trans. Commun., vol.E81-B, no.7, p.1517-26, 1998) under closed loop fast TPC. We adopted a weighted multi-slot averaging (WMSA) channel estimation filter with K=2 for mobile radio channel estimation and considered one slot TPC delay. Extensive computer simulation results show that the proposed algorithm using unused OVSF code reduces the required Eb/I0 at the BER of 10-3 up to 0.9 dB and has an improved TPC error performance compared to the conventional algorithm.