Demodulation apparatus and method in a communication system employing 8-ary PSK modulation

An 8-ary PSK (Phase Shift Keying) 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 for generating soft decision values Λ(sk,0), Λ(sk,1), and Λ(sk,2) for the input signal Rk(Xk, Yk) by a soft decision means, comprising the steps of:

• calculating a soft value Zk of the input signal Rk(Xk, Yk) according to equation Zk=|Xk|−|Yk|, and determining a first parameter α and a second parameter β by the input signal;
• and
• calculating the soft decision values for the input signal Rk(Xk, Yk), using the first parameter α, the second parameter β, and the received signal Rk(Xk, Yk), according to
• 10 Λ ⁡ ( s k , 2 ) = 2 ⁢ Y k + α · Z k , w ⁢ ⁢ h ⁢ ⁢ e ⁢ ⁢ r ⁢ ⁢ e ⁢ ⁢ α = { ⁢ 0 ⁢ i ⁢ ⁢ f ⁢ ⁢ MSB ⁡ ( Z k ) = 0 ⁢ - 1 ⁢ i ⁢ ⁢ f ⁢ ⁢ MSB ⁡ ( Z k ) = 1 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( Y k ) = 0 ⁢ ⁢ 1 ⁢ i ⁢ ⁢ f ⁢ ⁢ MSB ⁡ ( Z k ) = 1 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( Y k ) = 1 ⁢ ⁢ Λ ⁡ ( s k , 1 ) = 2 ⁢ X k + β · Z k , w ⁢ ⁢ h ⁢ ⁢ e ⁢ ⁢ r ⁢ ⁢ e ⁢ ⁢ β = { ⁢ 0 ⁢ i ⁢ ⁢ f ⁢ ⁢ MSB ⁡ ( Z k ) = 1 ⁢ - 1 ⁢ i ⁢ ⁢ f ⁢ ⁢ MSB ⁡ ( Z k ) = 0 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( X k ) = 1 ⁢ ⁢ 1 ⁢ i ⁢ ⁢ f ⁢ ⁢ MSB ⁡ ( Z k ) = 0 ⁢ ⁢ and ⁢ ⁢ MSB ⁡ ( X k ) = 0
• Λ(sk,0)=Zk
• where Λ(sk,i) indicates a soft decision value corresponding to sk,i(i=0,1,2), and sk,i indicates an ith bit in a coded signal sequence mapped to a kth symbol.

An 8-ary PSK (Phase Shift Keying) 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 for generating soft decision values Λ(sk,0), Λ(sk,1), and Λ(sk,2) for the input signal Rk(Xk, Yk) by a soft decision means, comprising:

• a calculator for calculating a soft value Zk by subtracting a level |Yk| of the quadrature-phase signal component Yk from a level |Xk| of the in-phase signal component Xk of the received signal Rk(Xk, Yk), and outputting the Zk as a first soft decision value;
• a first selector for receiving the Zk from the calculator and an inverted value −Zk of the Zk, and selecting the Zk or the −Zk according to a most significant bit (MSB) of the quadrature-phase signal component Yk;
• a second selector for receiving the Zk from the calculator and the −Zk, and selecting the Zk or the −Zk according to an MSB of the in-phase signal component Xk;
• a third selector for receiving an output of the second selector and a value “0”, and selecting the output of the second selector or the value “0” according to an MSB of the Zk;
• a first adder for adding a value calculated by multiplying the quadrature-phase signal component Yk by {square root}{square root over (2)} to an output of the third selector, and outputting a result value as a third soft decision value;
• a fourth selector for receiving an output of the second selector and a value “0”, and selecting the output of the second selector or the value “0” according to the MSB of the Zk;
• and
• a second adder for adding a value calculated by multiplying the in-phase signal component Xk by {square root}{square root over (2)} to an output of the fourth selector, and outputting a result value as a second soft decision value.

An 8-ary PSK (Phase Shift Keying) 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 for generating soft decision values Λ(sk,0), Λ(sk,1), and Λ(sk,2) for the input signal Rk(Xk, Yk) by a soft decision means, comprising the steps of:

• (a) calculating a soft value Zk of a first demodulated symbol by subtracting a level |Yk| of the quadrature-phase signal component Yk from a level |Xk| of the in-phase signal component Xk of the received signal Rk(Xk, Yk);
• (b) setting a first variable α to “0” if the soft value Zk has a positive value, setting the first variable α to “−1” if the Zk has a negative value and the quadrature-phase component Yk has a positive value, and setting the first variable α to “1” if the Zk has a negative value and the quadrature-phase component Yk has a negative value;
• (c) determining a soft value of a third demodulated symbol by calculating {square root}{square root over (2)}Yk+α*Zk using the quadrature-phase component Yk, the soft value Zk and the first variable α;
• (d) setting a second variable β to “0” if the soft value Zk has a negative value, setting the second variable β to “−1” if the Zk has a positive value and the in-phase component Xk has a negative value, and setting the second variable β to “1” if the Zk has a positive value and the in-phase component Xk has a positive value;
• and
• (e) determining a soft value of a second demodulated symbol by calculating {square root}{square root over (2)}Xk+β*Zk using the in-phase component Xk, the soft value Zk and the second variable β.