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The anticipated 1000 fold traffic growth in 2020 and the explosion of new services and applications including the massive Internet of Things, tactile internet, etc. pose tough challenges for the next generation mobile network. A soft end-toend solution from the core network to the radio access network, as well as the air interface is envisioned as the key to meet the extremely stringent demands. This paper presents our perspective of the 5G air interface and raise the concept of software defined air interface (SDAI) as a framework of 5G air interface. Instead of a global optimized air interface for the previous communication generations, SDAI is envisioned to bring agility and heighten the efficiency into the design and implementation of the building blocks of the air interface. The fundamental building blocks and the related parameters including spectrum, bandwidth, multiple access schemes, waveform, and so on, are all tunable such that the air interface can be optimized to each individual application scenario. To clarify the adaptation mechanism of SDAI, we provide a case study on the link adaptation between the sparse code multiple access (SCMA) scheme and the orthogonal frequency division multiplexing access (OFDMA) from energy efficiency and spectral efficiency co-design perspective.Index Terms-Software defined air interface, link adaptation, energy efficiency, spectral efficiency

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