Over on GitHub we've recently seen the release of some interesting code called "Nexmon Software Defined Radio" which demonstrates a discovery that allows a Broadcom 802.11ac WiFi chip to be used as a transmit capable software defined radio. This means that it can be used to transmit (within the 2.4 GHz and 5 GHz WiFi bands) any arbitrary signal from IQ data. The specific WiFi chip used in their experiments is the US$10 BCM4339 which has been found in smartphones such as the Nexus 5. It's not clear if other Broadcom 802.11ac WiFi chips could also work.
To begin with Nexmon SDR you'll need a development environment set up on a Nexus 5 smartphone. Then it's a matter of downloading the dependencies, installing the Android NDK, and compiling Nexmon. IQ data can then be transmitted in code using from special system commands.
The Nexmon team have indicated on Twitter that they plan to present a paper with more information on Nexmon SDR at the MobiSys 2018 conference which will be held in June.