TKN Technical Reports Series Editor: Prof. Dr.-Ing. Adam Wolisz Redundancy Concepts to Increase Transmission Reliability in Wireless Industrial LANs

Wireless LAN are potentially an attractive networking technology for industrial applications. A major obstacle towards the fulfillment of hard real-time requirements is the error-prone behavior of wireless channels. A common approach to combat channel errors and to increase the probability of a message being transmitted successfully before a prescribed deadline is to employ redundancy and diversity techniques. In this paper we introduce a specific transmit diversity scheme, called antenna redundancy, and integrate this with two other redundancy / diversity schemes, namely error-correcting codes and multicopy-ARQ, into a common framework allowing to investigate the tradeoffs between these methods. In antenna redundancy the wireless stations are equipped with a single antenna, but the base station / access point has several of them. For each trial to transmit a packet from the base station to the wireless station another antenna is used. The relative benefits of using FEC versus adding antennas are investigated. One important result obtained analytically and by simulation is that for independent Gilbert-Elliot channels between the base station antennas and the wireless station the antenna redundancy scheme decreases the probability of missing a deadline by approximately an order of magnitude per additional antenna. As a second benefit, antenna redundancy decreases the number of transmission trials needed to transmit a message successfully, thus saving bandwidth