  Software-defined radios (SDRs) are often used in the experimental evaluation
of next-generation wireless technologies. While crowd-sourced spectrum
monitoring is an important component of future spectrum-agile technologies,
there is no clear way to test it in the real world, i.e., with hundreds of
users each with an SDR in their pocket participating in RF experiments
controlled by, and data uploaded to, the cloud. Current fully functional SDRs
are bulky, with components connected via wires, and last at most hours on a
single battery charge. To address the needs of such experiments, we design and
develop a compact, portable, untethered, and inexpensive SDR we call Sitara.
Our SDR interfaces with a mobile device over Bluetooth 5 and can function
standalone or as a client to a central command and control server. The Sitara
offers true portability: it operates up to one week on battery power, requires
no external wired connections and occupies a footprint smaller than a credit
card. It transmits and receives common waveforms, uploads IQ samples or
processed receiver data through a mobile device to a server for remote
processing and performs spectrum sensing functions. Multiple Sitaras form a
distributed system capable of conducting experiments in wireless networking and
communication in addition to RF monitoring and sensing activities. In this
paper, we describe our design, evaluate our solution, present experimental
results from multi-sensor deployments and discuss the value of this system in
future experimentation.
