Demodulation apparatus and method in a communication system employing 16-ary QAM

A 16-ary QAM (Quadrature Amplitude Modulation) demodulation apparatus for receiving an input signal Rk(Xk, Yk) comprised of a kth quadrature-phase component Yk and a kth in-phase component Xk, and generating soft values Λ(sk,0), Λ(sk,1), Λ(sk,2) and Λ(sk,3) for the input signal Rk(Xk, Yk) by a soft decision means, comprising:

• a first calculator for determining a soft value Λ(sk,2) of a third demodulated symbol among 4 demodulated symbols by subtracting a distance 2a between two demodulated symbols on the same axis of a mapping table from a level |Yk| of the quadrature-phase component Yk;
• a second calculator for determining a soft value Λ(sk,3) of a fourth demodulated symbol by calculating Yk+α*Zk using a first variable α determined by the soft value of the third demodulated symbol and a sign bit of the quadrature-phase component Yk, where Zk is the soft value of the third demodulated symbol;
• a third calculator for deciding a soft value Λ(sk,0) of a first demodulated symbol by subtracting the distance 2a from a level |Xk| of the in-phase component Xk;
• and
• a fourth calculator for deciding a soft value Λ(sk,1) of a second demodulated symbol by calculating Xk+β*Z′k using a second variable β determined by the soft value of the first demodulated symbol and a sign bit of the in-phase component Xk, where Z′k is the soft value of the first demodulated symbol.

A 16-ary QAM (Quadrature Amplitude Modulation) demodulation method for receiving an input signal Rk(Xk, Yk) comprised of a kth quadrature-phase component Yk and a kth in-phase component Xk, and generating soft values Λ(sk,0), Λ(sk,1), Λ(sk,2) and Λ(sk,3) for the input signal Rk(Xk, Yk) by a soft decision means, comprising the steps of:

• calculating soft values Λ(sk,0) and Λ(sk,1) of first and second demodulated symbols among 4 demodulated symbols, using the following equations, Z k ′ = [LeftBracketingBar] X k [RightBracketingBar] - 2 ⁢ a Λ ⁡ ( s k , 1 ) = X k + β · Z k ′ , where ⁢ ⁢ β = { 0 ⁢ if ⁢ ⁢ MSB ⁡ ( Z k ′ ) = 1 - 1 if ⁢ ⁢ MSB ⁡ ( Z k ′ ) = 0 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( X k ) = 1 1 if ⁢ ⁢ MSB ⁡ ( Z k ′ ) = 0 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( X k ) = 0 Λ ⁡ ( s k , 0 ) = Z k ′
• where Λ(sk,0) denotes the soft value of the first demodulated symbol, Λ(sk,1) denotes the soft value of the second demodulated symbol, MSB is the most significant bit or sign bit, and “2a” denotes a distance between two demodulated symbols on the same axis of a mapping table;
• and
• calculating soft values Λ(sk,2) and Λ(sk,3) of third and fourth demodulated symbols among the 4 demodulated symbols, using the following equations, Z k = [LeftBracketingBar] Y k [RightBracketingBar] - 2 ⁢ a Λ ⁡ ( s k , 3 ) = Y k + α · Z k , where ⁢ ⁢ α = { 0 ⁢ if ⁢ ⁢ MSB ⁡ ( Z k ) = 1 - 1 if ⁢ ⁢ MSB ⁡ ( Z k ) = 0 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( Y k ) = 1 1 if ⁢ ⁢ MSB ⁡ ( Z k ) = 0 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( Y k ) = 0 Λ ⁡ ( s k , 2 ) = Z k
• where Λ(sk,2) denotes the soft value of the third demodulated symbol, Λ(sk,3) denotes the soft value of the fourth demodulated symbol, and “2a” denotes the distance between two demodulated symbols on the same axis of a mapping table.

A 16-ary QAM (Quadrature Amplitude Modulation) demodulation method for receiving an input signal Rk(Xk, Yk) comprised of a kth quadrature-phase component Yk and a kth in-phase component Xk, and generating soft values Λ(sk,0), Λ(sk,1), Λ(sk,2) and Λ(sk,3) for the input signal Rk(Xk, Yk) by a soft decision means, comprising the steps of:

• (a) calculating a soft value Λ(sk,2) of a third demodulated symbol among 4 demodulated symbols by subtracting a distance 2a between two demodulated symbols on the same axis of a mapping table from a level |Yk| of the quadrature-phase component Yk;
• (b) determining a soft value Λ(sk,3) of a fourth demodulated symbol by calculating Yk+α*Zk using a first variable α determined by the soft value of the third demodulated symbol and a sign bit of the quadrature-phase component Yk, where Zk is the soft value of the third demodulated symbol;
• (c) calculating a soft value Λ(sk,0) of a first demodulated symbol by subtracting the distance 2a from a level |Xk| of the in-phase component Xk;
• and
• (d) determining a soft value Λ(sk,1) of a second demodulated symbol by calculating Xk+β*Z′k using a second variable β determined by the soft value of the first demodulated symbol and a sign bit of the in-phase component Xk, where Z′k is the soft value of the first demodulated symbol.

A method for demodulating a received signal in a data communication system employing a modulating technique for dividing an output sequence of a channel encoder into 4 bits and mapping the bits to a specific one of 16 signal points having an in-phase component Xk and a quadrature-phase component Yk, comprising the steps of:

• calculating a soft value Zk of a third demodulated symbol by subtracting a distance 2a between two demodulated symbols on the same axis of a mapping table from a level |Yk| of the quadrature-phase component Yk;
• setting a first variable a to “0” if the soft value Zk has a negative value, setting the first variable α to “−1” if the Zk has a positive value and the quadrature-phase component Yk has a negative value, and setting the first variable α to “1” if the Zk has a positive value and the quadrature-phase component Yk has a positive value;
• determining a soft value of fourth demodulated symbol by calculating Yk+α*Zk using the quadrature-phase component Yk, the soft value Zk and the first variable α;
• calculating a soft value Z′k of a first demodulated symbol by subtracting the distance 2a between two demodulated symbols on the same axis of a mapping table from a level |Xk| of the in-phase component Xk;
• setting a second variable β to “0” if the soft value Z′k has a negative value, setting the second variable β to “−1” if the Z′k has a positive value and the in-phase component Xk has a negative value, and setting the second variable β to “1” if the Z′k has a positive value and the in-phase component Xk has a positive value;
• and
• determining a soft value of second demodulated symbol by calculating Xk+β*Z′k using the in-phase component Xk, the soft value Z′k and the second variable β.