  In this paper, we study cooperative cognitive radio networks consisting of a
primary user and multiple secondary users. Secondary users transmit only when
primary user is sensed as silent and may interfere with primary transmission
due to imperfect sensing. When primary activity is sensed correctly, secondary
users cooperate with primary user by assisting retransmission of failed packets
of primary user. We analyze packet throughput of primary and secondary users
for three variations of proposed cooperation method. Signal flow graph (SFG)
based approach is employed to obtain closed form expressions of packet
throughput. The analysis is done for two cases; individual sensing and
cooperative sensing. Further, we characterize optimal transmission probability
of secondary users that maximizes individual secondary packet throughput
keeping all queues in the system stable. Results present a comparison of
throughput performance of proposed cooperation methods under different
scenarios and show their benefits for both primary as well as secondary user
throughput.
