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In cognitive radio, secondary user (SU) performs spectrum sensing with a certain sensing range. This sensing range has a similar definition as the carrier sensing range of IEEE 802.11 MAC. It is widely considered that a SU is permitted to utilize the primary channel if no primary user (PU) transmits data inside its sensing range. However, it is observed that a busy PU outside the sensing range still can be detected by SU. As a result, the SU misinterprets that this busy PU is inside its sensing range, and hereby loses opportunity to utilize the primary channel. This new sensing issue is termed as spatial false alarm (SFA) problem. In this paper, we show the SFA can obviously reduce the SU's sensing performance, and cause the probabilities of false alarm and detection evaluated in current research have a great deviation with the actual results. To solve this SFA issue, the principle of SFA is studied thoroughly: i) this paper reveals the cause of this sensing problem, and presents the related approaches to ameliorate its negative impact; ii) this paper provides a reliable performance evaluation method to allow the SU to be aware of the real sensing results rather than the current works. Furthermore, with regarding the SFA, this paper derives the general upper and lower bounds of SU's sensing performance to have an insight into various sensing applications.