The ability to reliably and autonomously identify unused frequency bands plays an extremely important role in cognitive radio networks . Relying on the spectrum sensing, ongoing licensed operation must not be compromised and the secondary spectrum usage efficiency should be maintained. Thus, it is critical to ensure that the confidence level of the estimated signal status satisfies the primary user's requirement, whilst keeping the delay and computational complexity to a minimum. This paper provides a comprehensive comparison in terms of performance, reliability and complexity of stand alone sensing schemes for various cognitive radio application areas. We first give some new results on reliability performance, and then evaluate the sensing time required to achieve the target performance. Finally, we compare the computational complexity of various sensing approaches by calculating the number of arithmetic operations required by each approach.