  Frequency spectrum is one of the valuable resources in wireless
communications. Using cognitive radio, spectrum efficiency will increase by
making use of the spectrum holes. Dynamic Spectrum Access techniques allows
secondary users to transmit on an empty channel not used by a primary user for
a given time. In this paper, a Distributed Dynamic Spectrum Access based TDMA
protocol (DDSAT) is designed and implemented on USRP. The proposed protocol
performs two main functions: Spectrum Sensing, and Spectrum Management.
Spectrum Sensing is performed to find spectrum holes in a co-operative manner
using the contributing secondary users. Spectrum Management works
distributively on the secondary users to allocate the spectrum holes in a
fairly and efficient utilization. The DDSAT protocol is implemented using
Software Defined Radio (SDR) and Universal Software Radio Peripheral (USRP).
Evaluation and performance tests are conducted to show throughput and fairness
of the system.
