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We study capacity improvements in WLAN technology that uses orthogonal frequency division multiplexing (OFDM) and, conventionally, time division multiple access (TDMA) to deliver high quality of service (QoS) traffic. We show that improvements are attainable via the adaptation of the cyclic prefix to the channel conditions w.r.t. the baseline system that uses a fixed value of CP. Furthermore, we compare TDMA with orthogonal frequency division multiple access (OFDMA) and we show that the latter scheme provides large capacity gains w.r.t. TDMA both with and without CP adaptation. To simplify complexity and lower the amount of feedback, we propose to adapt over a finite set of pre-determined CP values. These values are computed using a method that is based on the analysis of the distribution of the capacity-optimal CP. For the numerical results, we have used the widely adopted channel model of the IEEE 802.11 standard, and we have found that a single value of CP for each channel class of this model is sufficient to achieve nearly optimal capacity for a wide range of distances.