for illustrative purposes , the following description is directed to a cellular radio communication system , but it will be understood that this invention is not so limited and applies to other types of communication systems . one example of the cellular system using different modulation types is the proposed egprs ( enhanced gprs ) system in which gmsk ( the modulation type used in gsm systems today ) will be used as the robust low data rate modulation and offset 8psk ( epsk ) will be used as the modulation for higher data rates . please refer to fig1 . fig1 is a functional block diagram of the modulation scheme detecting apparatus 10 according to a preferred embodiment of the present invention . the modulation scheme detecting apparatus 10 is applied in an egprs system , and comprises a gmsk power estimating module 40 , an 8 - psk power estimating module 60 , and a decision module 80 . r represents the received signal with the training sequence code . to demodulate the transmitted data by the gmsk modulation , the elements of received data are rotated by −( nπ )/ 2 radians , which n is the sample index . here , the gmsk power estimating module 40 then evaluates a nominal power η 2 gmsk of the received signal r rotated by −( nπ )/ 2 radians . to demodulate the transmitted data by the 8 - psk modulation , the elements of received data are rotated by −( 3nπ )/ 2 radians , which n is the sample index . here , the 8 - psk power estimating module 60 then evaluates a nominal power η 2 epsk of the received signal rotated by −( 3nπ )/ 8 radians . next , the decision module 80 determines the modulation scheme determined by a transmitter according to the nominal powers η 2 gmsk and η 2 epsk . the operations of generating the nominal powers η 2 gmsk and η 2 epsk are detailed in the following paragraphs . please refer to fig2 . fig2 is a functional block diagram of the gmsk power estimating module 40 and the epsk power estimating module 60 according to the preferred embodiment of the present invention . the gmsk power estimating module 40 comprises a derotator 42 , a decision logic 44 , an accumulating unit 46 , and a computing unit 48 . in the egprs system , each data block transmitted via the received signal r comprises a plurality of received data r ( n ), where “ n ” denotes the input timing of the receiver . the derotator 42 rotates the received data r ( n ) by −( nπ )/ 2 radians in a constellation diagram to generate a plurality of data r gmsk ( n ). the decision logic 44 divides the plurality of data r gmsk ( n ) into several groups corresponding to states s ( n ). each state s ( n ) can be expressed as the following equation : s ( n )=[ tsc m tsc m + 1 tsc m + 2 ]= 4 * tsc m + 2 * tsc m + 1 + tsc m + 2 equation ( 1 ) in equation ( 1 ), tsc m is the m th bit of known training sequence code provided by the radio link . for simplify , in the present embodiment , the received signals from a 3 - tap fir model channel are assumed , meaning that each state corresponds to three bits . however , it should be noted that the method and system of the present invention could be applied for channels with any taps . as a result , eight kinds of states and eight kinds of groups are utilized in the present embodiment . in some embodiment , the eight groups can be further reduced into four groups by reducing the states s ( n ) into s ′( n ) for better implements . the operation of generating the states s ′( n ) are represented in the following equation : in equation ( 2 ), the operator ^ is an operation with exclusive - or . the sign of the received data r ( n ) will then be changed after phase derotation according to s ( n ). for example , a plurality of data r gmsk ( n ) having the same absolute values are classified into the same group , in order to decrease the number of groups and increase the number of the elements of each group . in the present embodiment , if the bits correspond to two states are complementary to each other , such as binary bits “ 100 ” and binary bits “ 011 ”, the two states correspond to the same group . for example , a data r gmsk ( p ) corresponding to a state expressed as bits “ 100 ” and a data r gmsk ( q ) corresponding to a state expressed as bits “ 011 ” are classified in the same group . the operation of the decision logic 44 according to the present embodiment is represented by the following equation : r gmsk ′( n )=− sgn [ s ( i ) xor 2 n − 1 ]· r gmsk ( n ) equation ( 3 ) then , the decision logic 44 further divides the data r gmsk ′( n ) into 4 groups as mentioned in the previous embodiment . the accumulating unit 46 generates a plurality of accumulated values t gmsk ( i ) corresponding to each state by accumulating the data r gmsk ( n ) in the same group , wherein “ i ” denotes the index of the group . the operation of the accumulating unit 46 is represented by the following equation : t gmsk ⁡ ( i ) = ∑ n ⁢ r gmsk i ⁡ ( n ) equation ⁢ ⁢ ( 4 ) in equation ( 4 ), r gmsk i ( n ) denotes the data r gmsk ( n ) or r gmsk ′( n ) divided into the i th group . finally , the computing unit 48 generates a weighted average of the accumulated values t gmsk ( i ). as more data r gmsk ( n ) corresponds to a specific group ( i . e ., i = m ), the weight of the accumulated value t gmsk ( m ) increases . in the present embodiment , the weighted average is the nominal power η gmsk 2 . the operation of the computing unit 48 is represented by the following equation : η gmsk 2 = 1 n ⁢ ∑ i ⁢ n i ⁢  t gmsk i n i  2 = 1 n ⁢ ∑ i ⁢  ( n i ) - 0 . 5 · t gmsk i  2 equation ⁢ ⁢ ( 5 ) in equation ( 5 ), n denotes the total number of the data r gmsk ( n ), and n i denotes the number of a plurality of data r gmsk i ( n ) divided into the i th group . ( n i ) − 0 . 5 can be pre - calculated for each training sequence code as static rom table . the 8 - psk power estimating module 60 comprises a derotator 62 , a decision logic 64 , an accumulating unit 66 , and a computing unit 68 . the derotator 62 rotates the received data r ( n ) by −( 3nπ )/ 8 radians in a constellation diagram to generate a plurality of data r epsk ( n ). the decision logic 64 divides the data r epsk ( n ) into several groups as the decision logic 44 . the accumulating unit 66 accumulates the data r epsk ( n ) in the same group to generate a plurality of accumulated values t epsk ( i ) corresponding to each state , wherein “ i ” denotes the group index . finally , the computing unit 68 generates a weighted average of the accumulated values t epsk ( i ). in the present embodiment , the weighted average is the nominal power η epsk 2 . since the operations of the decision logic 64 , the accumulating unit 66 , and the computing unit 68 are the same as the components of the same names in the gmsk power estimating module 40 , detailed descriptions are omitted for brevity . in an embodiment of the present invention , if the nominal power η gmsk 2 is greater than the nominal power η epsk 2 , the target modulation scheme is determined to be the gmsk modulation scheme ; otherwise , the target modulation scheme is determined to be the 8 - psk modulation scheme . in another embodiment of the present invention , a cost function c k is further calculated according to the nominal power η gmsk 2 and η epsk 2 based on the following equation : c k = η gmsk 2 ( k )− η epsk 2 ( k ) equation ( 6 ) in equation ( 6 ), k represents the index of each burst . here , the target modulation scheme is determined by comparing the cost function c k with a predetermined threshold . for example , if the cost function c k is greater than the threshold , the modulation scheme of the burst is gmsk ; otherwise , the modulation scheme of the burst is epsk . it should be understood that the detection method of the present invention is not limited to the above embodiment . any alternative way to detection the modulation scheme of each burst according to the nominal power η gmsk 2 and η epsk 2 should be covered by the present invention . however , it is possible that the received signal r might be seriously disturbed by noise , and thus the calculated nominal powers η epsk 2 and η gmsk 2 may be incorrect . since the modulation scheme for a data block is kept fixed , the same modulation scheme will be used over four successive radio bursts . in order to correct error detection , the decision module 80 observes the nominal powers η epsk 2 and η gmsk 2 of each burst and combines the information of previous bursts to generate a more precise decision of the target modulation scheme . please refer to fig3 . fig3 is a schematic diagram of the decision module 80 according to a preferred embodiment of the present invention . the decision module 80 comprises a diversity combining unit 82 and a criterion unit 84 . the diversity combining unit 82 performs a sequential detection by generating a decision value d k according to the following equation : d k = d k - 1 + c k σ k 2 , ⁢ where ⁢ ⁢ d - 1 = 0 ⁢ ⁢ and ⁢ ⁢ σ ⁢ k ⁢ 2 = e ⁡ [  r ⁢ ( n )  2 ] - η ⁢ x ⁢ 2 ⁡ ( k ) , equation ⁢ ⁢ ( 7 ) in equation ( 7 ), x may be gmsk or epsk . when c k & gt ; 0 , gmsk modulation is adopted and the noise power is estimated as σ k 2 = e [| r ( n )|]− η gmsk 2 ( k ), and vise versa . the criterion unit 84 determines the target modulation scheme according to the decision value d k . as shown in fig3 , the diversity combining unit 82 further comprises a plurality of adders 92 , 96 , a scaling unit 94 , and a multiplexer 98 . firstly , the adder 92 calculates the cost function c k , which is the difference between the nominal powers η gmsk 2 and η epsk 2 of the k th data burst in a data block . secondly , the scaling unit 94 further calculates the value by dividing the cost function c k by a noise variance value σ k 2 of the k th data burst . thirdly , the adder 96 and the multiplexer 98 add the value with the decision value d k − 1 of the k − 1 th burst to generate the decision value d k . the operations of the adder 96 and the multiplexer 98 are based on equation ( 7 ) mentioned above . it should be noted that equation ( 7 ) is performed in each data block individually , meaning that the decision value d k of one data block will not be utilized in other block . that is , k is between 0 - 3 in this embodiment . for example , in one data block , if k = 0 , the multiplexer 98 outputs an initial value d − 1 , then the adder 96 adds the value to d − 1 wherein d − 1 is equal to zero according to the preferred embodiment . if k & gt ; 0 , the multiplexer 98 outputs the previous decision value d k − 1 , which is the decision value of the k − 1 th burst , then the adder 96 adds the value to the previous decision value d k − 1 . in this invention , the decision value d k is related to both the value and the previous decision value d k − 1 . in other words , not just current burst data in this data block are considered for modulation scheme detection , the previous information of all previous bursts in this data block with the same modulation scheme will also be used to further correct the current decision result . therefore , with further consideration of previous burst information in the present data block , the reliability of the decision value d k increases accordingly . finally , the criterion unit 84 determines the target modulation scheme according to the decision value d k and a threshold value t . in the present embodiment , t is equal to zero . if the decision value d k is greater than the threshold value t , the target modulation scheme is determined to be the gmsk modulation scheme . on the contrary , if the decision value d k is not greater than the threshold value t , the target modulation scheme is determined to be the 8 - psk modulation scheme . compared with the related art , the modulation scheme detecting apparatus utilizes the nominal powers η gmsk 2 and η epsk 2 to determine the target modulation scheme according to the present invention . the operation of generating the nominal power is obviously simpler than that in the related art and therefore both the computation power and memory requirement are reduced . in addition , the decision module of the modulation scheme detecting apparatus observes the nominal powers η epsk 2 and η gmsk 2 of all bursts in one data block transmitted via the same modulation scheme to determine the target modulation scheme . therefore the reliability of the modulation scheme detecting apparatus is significantly improved . those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention . accordingly , the above disclosure should be construed as limited only by the metes and bounds of the appended claims .