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We address the problem of delivering streaming video over changing wireless conditions in the context of HTTP Adaptive Streaming (HAS) systems, such as those based on the Dynamic Adaptive Streaming over HTTP (DASH) standard [1, 2]. In HAS, the video client plays the primary role in rate adaptation unlike server controlled video delivery systems. Performance of adaptive streaming over rapidly changing wireless conditions degrades due to the slower time-scale of response at the application layer compared to the time scale of changing radio link conditions. We propose a novel physical layer aware video rate adaptation strategy for HAS systems in which the HAS client uses physical layer modem statistics as a complement to application layer throughput estimate to enable quick identification and response to changing radio link conditions. This novel HAS rate adaptation algorithm includes a "Replace Request" feature wherein the HAS client can replace a current video segment request at the HAS server under suitable conditions. We demonstrate improved video streaming performance of our approach through simulations on a 3GPP LTE (Long Term Evolution) system level simulator.