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In this paper we provide a baseband simulation model which is used to study the performance of the physical layer of IEEE 802.11a OFDM based wireless LANs. In our study we provide two different system alternatives called system 1 and system 2. System 1 represents a low complexity implementation that meets the minimum requirements of the standards. We use system 1, which employs a zero forcing channel estimator, hard decision Viterbi decoder and a non-overlapped windowing spectral shaping, as a reference model. In system 2 we provide a more efficient system that utilizes channel estimation using least square error (LSE) estimation and soft decision Viterbi decoding. We then evaluate the performance of both systems. The simulation results demonstrate an increase in the performance of system 2 over system 1 that coincides with the classical results for LSE estimation and soft decision decoding, which confirms the accuracy of the simulation model. Our results also show that channel estimation degrades the system performance when the channel is mainly AWGN with very low fading effects. Finally in order to reduce the out of band emissions of OFDM WLANs, we design a new spectral shaping filter. We show that the filter's effect on the overall system performance is negligible while it significantly reduces the out of band spectral leakage.