When receiving a signal, a communication system first performs packet detection to make sure whether the receiving signal is a target signal matching the system specification.  When the result of the packet detection reveals that the receiving signal is the target signal matching the system specification, the system continues to receive the signal.  Otherwise, when the result of the packet detection reveals that the receiving signal is not the target signal matching the system specification; the system terminates receiving the subsequent signal. 
If the packet detection encounters an error, such that the signal supposed to be received is misjudged as a non-target signal, which causes the system to lose the signal, which causes the signal unstable or unclear.  However, if the signal is not supposed be received and is misjudged as a target signal, but the system receives and processes unrelated signal; such that the system resources are wasted, which further expels those signals really needing received and processed.  Therefore, the correctness of the packet detection matters for the correctness and the stability of the system. 
When the receiving signal is conducted periodically, a detection mechanism called delay correlation is often used on the packet detection.  FIG.  1 is a diagram of the receiving signal after sampled for both a conventional practice and the current invention.  The arrow represents the order to feed in the signal processing circuit (not shown).  r(t) represents the sample data at time t. The period of the sample data is N, i.e.,r(t+N)=r(t)
The delay correlation of the signal from the prior art is obtained by the following equation:
            ∑              k        =        0                    N        -        1              ⁢                  r        ⁡                  (                      t            +            k                    )                    ·                        r          *                ⁡                  (                      t            +            N            +            k                    )                                ∑              k        =        0                    N        -        1              ⁢                  r        ⁡                  (                      t            +            k                    )                    ·                        r          *                ⁡                  (                      t            +            k                    )                    
When the result of the above equation is larger than a threshold, the input signal can be determined to be the target signal matching the system specification.  If not, the receiving signal may be not the target signal, and the system should stop receiving the present signal. 
From the above equation, one can find out that the denominator only relates to the N sample data, i.e. r(t) to r(t+N−1), and the influence of the other sample data at the numerator r(t+N) to r(t+2N−1) is ignored.  That is, calculating the denominator need N sample data, which means the reference data are a half of calculating the numerator.  Since the sample value needed for calculating the denominator is a half of calculating the numerator, the existing information is wasted.  Furthermore, since the number of the sampling points of the numerator and the denominator on the above equation are different, the energy considered in the denominator is related to only a half of the sample data at the numerator, i.e. the denominator cannot reflect the energy of all sample data.  Consequently, the peak interval of delay correlation is calculated more unstably such that the misjudging probability is increased.  Therefore, it is urgently needed a target signal determination method and associated apparatus using delay correlation for determining a target signal more correctly.