Hybrid DCF Supporting Smart Antennas in WLANs

Hybrid distributed coordination function (H-DCF), a modified MAC protocol of IEEE 802.11, is proposed in this paper to support hybrid antennas, i.e., smart adaptive array antennas and omni-directional antennas in one WLAN. Smart antennas follow directional DCF (D-DCF) and omni-diretional antennas follow DCF. In D-DCF, before sending any data-frames, the sender and receiver node transmit its pilot sequence respectively by means of omni-directional RTS/CTS handshake mechanism. Based on the pilot, the directional beam can be formed by the smart antenna. Then the sender can transmit its data-frame in the directional mode. Based on virtual carrier sense mechanism, the omni-directional transmission between the sender and receiver cannot interfere with the other nodes. When the sender and receiver communicate in the directional mode, the other nodes can contend the channel to send their data-frames. Hence, D-DCF supports space division multiplexing. Moreover, D-DCF supports service differentiation. Smart antennas have a higher priority to access channel than omni-directional antennas by differentiating the contention parameters, such as arbitrary interframe space, initial contention window size, the maximum backoff stage and retry limit. Simulation results show that H-DCF can support hybrid antenna system effectively and provide much higher network throughput, lower delay and jitter than DCF does

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