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Wireless backhaul systems have been considered as a promising candidate of beyond 3G wireless broadband system for mobile communications. The achievable transmission performance over radio relay channel depends on antenna directivity and radiation patterns of each antenna element. To improve the transmission performance and keep radio relay channel in an acceptable condition, it is effective to control the antenna directivity by employing multi-antenna signal transmission and reception. For this purpose, we have proposed a simple method to extend the existing single antenna relay node based on IEEE802.11a to multi-antenna system, where a cross configuration multi-antenna is applied to the existing relay nodes as external equipment. The multi-antenna system is controlled by detecting the antenna control information assigned to the lengths of the packets transmitted from the existing node. In this paper, to clarify the effectiveness of multi-antenna system in wireless multi-hop networks, we show an indoor radio propagation model based on two dimensional ray-tracing that provides realistic indoor channel characteristics and evaluate throughput performance by system level simulations. Simulation results ensure that application of directional antenna to multi-hop networks with highly efficient packet forwarding protocol, called Intermittent Periodic Transmit (IPT), improves throughput performance of the radio relay transmission in wireless backhaul.