What is wrong/right with IEEE 802.11n spatial multiplexing power save feature?

The IEEE 802.11n standard has proposed a new Spatial Multiplexing Power Save (SMPS) feature, which allows for a station to retain one active receive chain, to mitigate MIMO circuitry power consumption. But does it work in all cases? Our experiments reveal that SMPS may not always save power compared with multiple active chains at the receiver. Even when it does, it may be proven more energy hungry. In this work, we seek to uncover the "good", the "bad" and the "ugly" of SMPS using real experiments. We further devise a MIMO Receiver Energy Save (MRES) algorithm, which seeks to identify and set the most energy-efficient receive chain setting, by using a novel, low-overhead sampling scheme. Our prototype experiments show that, MRES outperforms SMPS with energy savings up to 37%.