Hybrid DCF Supporting Hybrid Antennas in a WLAN

Hybrid distributed coordination function (H-DCF), a modified medium access control protocol of IEEE 802.11, is proposed in this paper to support hybrid antennas, i.e., smart adaptive array antennas and normal omni-directional antennas in one WLAN. Nodes equipped with smart antennas follow directional DCF (D-DCF) and nodes equipped with omnidirectional 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/omni-directional CTS (ORTS/OCTS) 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. D-DCF is compatibility with DCF by means of ORTS/CTS or RTS/OCTS mechanism. Moreover, D-DCF supports service differentiation. Smart antennas have a higher priority to access channel than normal antennas by differentiating the contention parameters, such as initial contention window size, the maximum backoff stage and retry limit. A simple analytical model is presented to compute saturation throughput and bandwidth. 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, and that the analytical model is valid