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The time to frequency protocol is the first frequency-domain contention protocol in wireless LANs, and has attracted a great deal of attention, since it can improve channel efficiency significantly. However, this protocol mainly provides a uniform service by letting each user conduct channel contention over the same frequency-domain range. In this paper, we first propose a novel weighted frequency-domain contention (WFC) scheme for service differentiation. In WFC, different frequency-domain ranges are assigned to different users to achieve weighted channel access opportunity, and a signature-assisted method is employed to completely exclude collisions. We then develop a theoretical framework to study the performance of WFC. With this framework, we can evaluate system throughput, optimize system parameter settings, and achieve proportional fairness. Extensive simulations verify that our theoretical model is very accurate.