Method of QAM soft demapping

1. A method of demapping in a receiver wherein the input signal is a) demodulated into I real and Q imaginary data pairs which was mapped using a constellation having M data bits for the I and Q pairs, b) demapped, c) deinterleaved and d) decoded, the method of demapping comprising:

deriving M intermediate soft bit values y j for the I and Q data pairs as a function of the spacing in the constellation;

limiting the range of the M values y j ;

deriving a look-up table index for each of the limited M values y j ;

indexing a look-up table, having 2 N+1 entries for supporting up to N soft bits, for each M data bit using the derived indices; and

outputting K soft bits for each data bit of the I and Q data pairs where K<=N wherein the range is limited to ±1 and the indices are derived by 2 K −(2 K −1)*y j .

2. The method according to claim 1 , wherein the indices are the locations and the boundaries of the points of the constellation.

3. The method according to claim 1 , wherein the indices are the locations and the boundaries of the points of the constellation.

4. The method according to claim 1 , wherein the look-up table includes only soft bits.

5. The method according to claim 1 , wherein for M equal to 2 (QPSK), the intermediate soft bits (y 0,q′ , y 1,q′ ) of the input signal (Z i +iZ q ) equal to:

y 0,q′ =Z i y 1,q′ =Z q .

6. The method according to claim 1 , wherein for M equal to 4 (16-QAM), the intermediate soft bits (y 0,q′ , y 1,q′ , y 2,q′ , y 3,q′ ) of the input signal (Z i +iZ q ) equal to:

y 0,q′ =Z i ,y 1,q′ =Z q ,y 2,q′ =|Z i |−2 ,y 3,q′ =|Z q |−2.

7. The method according to claim 1 , wherein for M equal to 6 (64-QAM), the intermediate soft bits (y 0,q′ , y 1,q′ , y 2,q′ , y 3,q′ , y 4,q′ , y 5,q′ ) of the input signal (Z i +iZ q ) equal to:

y 0,q′ =Z i y 1,q′ =Z q y 2,q′ =|Z i |−4 y 3,q′ =|Z q |−4

if |Z i |<4, y 4,q′ =2−|Z i |, else y 4,q′ =|Z i |−6 and

if |Z q |<4, y 5,q′ =2−|Z q |, else y 5,q′ =|Z q |−6.

8. A receiver including a) demodulator for demodulating an input signal into I real and Q imaginary data pairs which was mapped using a constellation having M bits for the I and Q pairs, b) a demapper with a look-up table, c) deinterleaver and d) decoder; and the demapper:

derives M intermediate soft bit values y j for the I and Q data pairs as a function of the spacing in the constellation;

limits the range of the M values y j ;

derives a look-up table index for each of the limited M values y j ;

indexes the look-up table, having 2 N+1 entries for supporting up to N soft bits using the derived indices; and

outputs K soft bits per M data bit of the I and Q data pairs where K<=N, wherein the range is limited to ±1 and the indices are derived by 2 K −(2 K −1)*y j .

9. The method according to claim 8 , wherein the look-up table includes only soft bits.