  Collaborative spectrum sensing can significantly improve the detection
performance of secondary unlicensed users (SUs). However, the performance of
collaborative sensing is vulnerable to sensing data falsification attacks,
where malicious SUs (attackers) submit manipulated sensing reports to mislead
the fusion center's decision on spectrum occupancy. Moreover, attackers may not
follow the fusion center's decision regarding their spectrum access. This paper
considers a challenging attack scenario where multiple rational attackers
overhear all honest SUs' sensing reports and cooperatively maximize attackers'
aggregate spectrum utilization. We show that, without attack-prevention
mechanisms, honest SUs are unable to transmit over the licensed spectrum, and
they may further be penalized by the primary user for collisions due to
attackers' aggressive transmissions. To prevent such attacks, we propose two
novel attack-prevention mechanisms with direct and indirect punishments. The
key idea is to identify collisions to the primary user that should not happen
if all SUs follow the fusion center's decision. Unlike prior work, the proposed
simple mechanisms do not require the fusion center to identify and exclude
attackers. The direct punishment can effectively prevent all attackers from
behaving maliciously. The indirect punishment is easier to implement and can
prevent attacks when the attackers care enough about their long-term reward.
