  Despite exhibiting very high theoretical data rates, in practice, the
performance of LTE-U/LAA and WiFi networks is severely limited under
cross-technology coexistence scenarios in the unlicensed 5GHz band. As a
remedy, recent research shows the need for collaboration and coordination among
co-located networks. However, enabling such collaboration requires an
information exchange that is hard to realize due to completely incompatible
network protocol stacks. We propose OfdmFi, the first cross-technology
communication scheme that enables direct bidirectional over-the-air
communication between LTE-U/LAA and WiFi with minimal overhead to their legacy
transmissions. Requiring neither hardware nor firmware changes in commodity
technologies, OfdmFi leverages the standard-compliant possibility of generating
message-bearing power patterns, similar to punched cards from the early days of
computers, in the time-frequency resource grid of an OFDM transmitter which can
be cross-observed and decoded by a heterogeneous OFDM receiver. As a
proof-of-concept, we have designed and implemented a prototype using commodity
devices and SDR platforms. Our comprehensive evaluation reveals that OfdmFi
achieves robust bidirectional CTC between both systems with a data rate of up
to 84kbps, which is more than 125x faster than state-of-the-art.
