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Abstract-Increasingly wireless networks use multi-antenna nodes as in IEEE 802.11n and 802.16. The Physical layer (PHY) in such systems may use the antennas to provide multiple streams of data (spatial multiplexing) or to increase the robustness of fewer streams. These physical layers also provide support for sending packets at different rates by changing the modulation and coding of transmissions. Rate adaptation is the problem of choosing the best transmission mode for the current channel and in these systems requires choosing both the level of spatial multiplexing and the modulation and coding.Hydra is an experimental wireless network node prototype in which both the MAC and PHY are highly programmable. Hydra's PHY is essentially the 802.11n PHY, and currently supports two antennas and the same modulations and codings as 802.11n. Because of limitations of our hardware platform, the actual rates are a factor of 10 smaller than 802.11n. The MAC is essentially the 802.11 MAC with extensions, including the ability to feedback channel state or rate information from the receiver. Hydra was designed to allow experimentation with real radios, PHYs, and network stacks over real-world channels and it is well suited to studying rate adaptation in multi-antenna systems. To allow controlled experimentation, we also have the ability to perform experiments over emulated channels using exactly the same MAC and PHY used for RF transmissions.We present rate control experiments based on transmission over both real and emulated channels. Our experiments include measurements for single antenna systems and two antenna systems using a single or multiple spatial streams. We study rate adaptation algorithms using both explicit and implicit feedback from the receiver. A novel aspect of our results is the first experimental study of adaptation between single and multiple spatial streams for 802.11n style systems.Increasingly wireless networking technologies, including IEEE 802.11n and IEEE 802.16, support radios with multiple antennas. These antennas can be used to support multiple data streams (spatial multiplexing) or to increase robustness by taking advantage of channel diversity [1], [2]. Choosing between

The performance of a wireless network protocol is inseparably linked to the physical layer algorithms on which it is built, the hardware used to implement the radio, and the wireless environment in which it operates. This paper identifies three features of wireless networking protocols impacted by these lower-level characteristics that are often overlooked or misunderstood by many researchers developing wireless protocols or using testbed-based evaluation methods. These features are temporal scaling, measurement reciprocity, and cross-layer adaptation. Temporal scaling refers to the time resolution with which events, such as broadcast or feedback, occur in the wireless network. This feature is tightly coupled with processing time at the physical layer and time selectivity in the wireless channel. Measurement reciprocity is an assumption used to estimate parameters of the forward link of a bidirectional communication channel, based on observations from the reverse link. This assumption directly depends on the interference properties and hardware symmetry of nodes in a wireless network. System adaptation, based on reciprocity or feedback, inevitably requires careful scrutiny of power and rate control applied to physical wireless devices. This paper also provides recommendations to guide researchers in setting up interesting and useful wireless experiments. Three concerns for wireless experimentation are addressed, namely: ambient interference, RF hardware profiling, and fading properties of the wireless channel. The motivation for this paper stems from experience prototyping and experimenting with Hydra, a wireless cross-layer testbed developed at the University of Texas at Austin.