  Dual-function radar communication (DFRC) systems implement both sensing and
communication using the same hardware. Such schemes are often more efficient in
terms of size, power, and cost, over using distinct radar and communication
systems. Since these functionalities share resources such as spectrum, power,
and antennas, DFRC methods typically entail some degradation in both radar and
communication performance. In this work we propose a DFRC scheme based on the
carrier agile phased array radar (CAESAR), which combines frequency and spatial
agility. The proposed DFRC system, referred to as multi-carrier agile joint
radar communication (MAJoRCom), exploits the inherent spatial and spectral
randomness of CAESAR to convey digital messages in the form of index
modulation. The resulting communication scheme naturally coexists with the
radar functionality, and thus does not come at the cost of reduced radar
performance. We analyze the performance of MAJoRCom, quantifying its achievable
bit rate. In addition, we develop a low complexity decoder and a codebook
design approach, which simplify the recovery of the communicated bits. Our
numerical results demonstrate that MAJoRCom is capable of achieving a bit rate
which is comparable to utilizing independent communication modules without
affecting the radar performance, and that our proposed low-complexity decoder
allows the receiver to reliably recover the transmitted symbols with an
affordable computational burden.
