  This paper describes a radio architecture for distributed spectrum sharing of
multiple channels among secondary users (SUs) in a wide band of frequencies and
a localized area. A novel Multichannel Immediate Multiple Access (MIMA)
physical layer is developed such that each SU can monitor all the channels
simultaneously for incoming signals and achieve fast rendezvous within the
multiple channels. The spectrum utilized by an SU pair can be changed
dynamically based upon spectrum sensing at the transmitter and tracking
synchronization and control messages at the receiver. Although information
about the number of active SUs can be used to improve the spectrum sharing
efficiency, the improvement is small relative to the cost of obtaining such
information. Therefore, the architecture adopts Multichannel Carrier Sense
Multiple Access (CSMA) for medium access control regardless of the number of
active SUs. A prototype implementation of the architecture has been developed
using an advanced software defined radio (SDR) platform. System tests
demonstrate that the spectrum sharing efficiency of the prototype is close to
an upper bound if the signal-to-noise ratio (SNR) is sufficiently high. Among
other practical issues, imaged interference caused by hardware IQ imbalance
limits system performance. In the prototype, the MIMA is based on an LTE
waveform. Therefore, the spectrum sharing radio can be potentially applied to
the 3.5 GHz radar band for Citizens Broadband Radio Service (CBRS).
