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A parallel concatenated coder (20) outputs a sequence of data sets in which earlier data sets contain data bits (d) and non-interleaved parity bits (p) without interleaved parity bits (q), while later data sets include the interleaved parity bits (q). Each data set is modulated as one symbol. The ou

A parallel concatenated coder (20) outputs a sequence of data sets in which earlier data sets contain data bits (d) and non-interleaved parity bits (p) without interleaved parity bits (q), while later data sets include the interleaved parity bits (q). Each data set is modulated as one symbol. The output format partially overcomes the delay incurred by interleaving the parity bits (q), with a substantially even distribution of data and parity bits in the sequence of data sets. The delay may be reduced further by interleaving with an index constraint.

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A parallel concatenated coder (20) outputs a sequence of data sets in which earlier data sets contain data bits (d) and non-interleaved parity bits (p) without interleaved parity bits (q), while later data sets include the interleaved parity bits (q). Each data set is modulated as one symbol. The ou

A parallel concatenated coder (20) outputs a sequence of data sets in which earlier data sets contain data bits (d) and non-interleaved parity bits (p) without interleaved parity bits (q), while later data sets include the interleaved parity bits (q). Each data set is modulated as one symbol. The output format partially overcomes the delay incurred by interleaving the parity bits (q), with a substantially even distribution of data and parity bits in the sequence of data sets. The delay may be reduced further by interleaving with an index constraint. beams having beam widths equal to the beam width of the first laser beam and advancing in mutually different directions; a condensing means for condensing each second laser beam to mutually advance in near the same direction; and a shaping means for converting the intensity distribution of a cross section perpendicular to a system optical path of each second laser beam in a laser optical path of each of the mutually separated second laser beams, wherein the dividing means is an element which divides a first laser beam by diffraction to produce a second laser beam; and wherein the condensing means is an element having a focal point on or near the dividing means, and arranged in the system optical path in a position where the second laser beams overlap; and which satisfies the relationship f·{1-m·λ·f/(p·W)}≤Zd≤f·{1+m·λ·f/(p·W)} where λ represents the wavelength of the first and second laser beams, f represents the focal length of the condensing means, m represents the minimum value of the absolute value of the difference of the degree of diffraction of the second laser beams in the same diffraction direction, W represents the beam width of the first laser beam in this diffraction direction, p represents the sequence period of the grating unit of the dividing means in this diffraction direction, and Zd represents the distance from the condensing means to the shaping means with the advancing direction of the second laser beam designated positive. 5. The laser irradiation optical system claimed in claim 1, wherein the condensing means is an element having a focal point on the dividing means, and arranged in the system optical path in a position where the second laser beams do not overlap. 6. A laser irradiation optical system comprising: a dividing means for dividing a first entering laser beam once, and producing a plurality of second laser beams having beam widths equal to the beam width of the first laser beam and advancing in mutually different directions; a condensing means for condensing each second laser beam to mutually advance in near the same direction; and a shaping means for converting an intensity distribution of a cross section perpendicular to a system optical path of each second laser beam in a laser optical path of each of the mutually separated second laser beams, wherein the dividing means is an element which divides a first laser beam by diffraction to produce a second laser beam; and wherein the condensing means is an element having a focal point on or near the dividing means, and arranged in the system optical path in a position where the second laser beams do not overlap; and which satisfies the relationship f·{p·W/(m·λ·f)-1}≤Zd≤f·{1+m·λ·f/(p·W)} where λ represents the wavelength of the first and second laser beams, f represents the focal length of the condensing means, m represents the minimum value of the absolute value of the difference of the degree of diffraction of the second laser beams in the same diffraction direction, W represents the beam width of the first laser beam in this diffraction direction, p represents the sequence period of the grating unit of the dividing means in this diffraction direction, and Zd represents the distance from the condensing means to the shaping means with the advancing direction of the second laser beam designated positive. 7. The laser irradiation optical system claimed in claim 1, wherein the condensing means and the shaping means are a single integrated element. 8. The laser irradiation optical system claimed in claim 1, wherein the shaping means is a single element having a plurality of parts for converting the intensity distribution of the second laser beam. 9. The laser irradiation optical system claimed in claim 1, wherein the shaping means is an element for converting the intensity distribution of the second laser beam by diffraction. 10. A method of converting a laser beam comprising the steps of: dividing the las