A joint design of MIMO-OFDM transceiver and power-saving MAC in WLANs

The combination of multiple input multiple output (MIMO) and orthogonal frequency division multiplexing (OFDM) is regarded as one of the most promising solutions for improving spectrum efficiency and enhancing data rate for next-generation wireless communication systems. However, the applications of these advanced physical-layer techniques also impose great challenges on upper layer protocol designs. In this paper, we propose a cross-layer approach to exploring the utility of the joint design of multiuser MIMO-OFDM technique and power-saving MAC protocol. Specifically, a novel MIMO-OFDM transceiver architecture and an improved power-saving MAC protocol (namely, PSM-MIMO/OFDM) are proposed. In the proposed downlink transceiver architecture, by processing the data according to the channel state, the access point (AP) makes the data for one user appear as zero at the other user on each subcarrier such that it can send distinct packets to two users simultaneously. In the dual uplink, two users can concurrently transmit data to the AP without co-channel interference by uplink-downlink duality. The key ideas of PSM-MIMO/OFDM can be summarized as follows. We employ a similar timing structure to IEEE 802.11 power-saving mechanism (PSM). By utilizing ATIM (Ad-hoc Traffic Indication Message) window, negotiations are conducted between the AP and multiple candidate users via ATIM messages to achieve multiuser diversity gains. At the end of ATIM window, the AP sends an additional beacon to indicate the estimated time duration for data exchange with the measurement result of the number of active links which have been successfully negotiated during ATIM window. The length of a beacon interval is adjustable to satisfy the requirements of active links to the maximum extent. During the data exchange, the AP always communicates with two compatible users simultaneously both in the downlink and uplink, which effectively creates extra dimensions in the spatial domain. Extensive simulations have been conducted and the results demonstrate that the new protocol significantly outperforms other power-saving MAC protocols with only minimum additional overhead and minor modifications to the IEEE 802.11 PSM.