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Wireless LANs are an attractive networking technology for industrial applications. A major obstacle toward the fulfillment of hard real-time requirements is the error-prone behavior of wireless channels. A common approach to increase the probability of a message being transmitted successfully before a prescribed deadline is to use feedback from the receiver and subsequent retransmissions (automatic repeat request-ARQ-protocols). In this paper, three modifications to an ARQ protocol are investigated. As one of these modifications a specific transmit diversity scheme, called antenna redundancy, is introduced. The other modifications are error-correcting codes and the transmission of multiple copies of the same packet. In antenna redundancy the base station/access point has several antennas. The base station transmits on one antenna at a time, but whenever a retransmission is needed, the base station switches to another antenna. The relative benefits of using FEC versus adding antennas versus sending multiple copies are investigated under different error conditions. One important result is that for independent Gilbert-Elliot channels between the base station antennas and the wireless station the antenna redundancy scheme effectively decreases the probability of missing a deadline, in a numerical example approximately an order of magnitude per additional antenna can be observed. As a second benefit, antenna redundancy decreases the number of transmission trials needed to transmit a message successfully, thus saving bandwidth.
Date of Publication: Aug. 2005