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A method of encoding a stream of data elements is provided which involves splitting the stream of data elements into a first stream and a second stream; encoding the first stream to produce a first encoded stream; performing a constellation mapping using a combination of the first encoded stream and

A method of encoding a stream of data elements is provided which involves splitting the stream of data elements into a first stream and a second stream; encoding the first stream to produce a first encoded stream; performing a constellation mapping using a combination of the first encoded stream and a third stream which is based on the second stream. This may involve defining a signal constellation; defining a plurality of co-sets within the constellation such that a minimum distance between constellation points within each co-set is larger than a minimum distance between any constellation points within the signal constellation; performing said constellation mapping by using the first encoded stream to identify a sequence of co-sets of said plurality of co-sets, and by using the third stream to identify a sequence of constellation points within respective co-sets of the sequence of co-sets identified by said first encoded stream.

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1. A method of encoding a stream of data elements comprising: defining a signal constellation comprising a plurality of constellation points; defining a plurality of co-sets within the plurality of constellation points such that a minimum distance between constellation points within each co-set is l

1. A method of encoding a stream of data elements comprising: defining a signal constellation comprising a plurality of constellation points; defining a plurality of co-sets within the plurality of constellation points such that a minimum distance between constellation points within each co-set is larger than a minimum distance between any constellation points within the signal constellation; splitting the stream of data elements into a first stream and a second stream; encoding the first stream to produce a first encoded stream; performing a constellation mapping using the first encoded stream to identify a sequence of co-sets of said plurality of co-sets, and using a third stream which is based on the second stream to identify a sequence of constellation points within respective co-sets of the sequence of co-sets identified by said first encoded stream. 2. The method according to claim 1 wherein labels of constellation points within each co-set are Gray mapped. 3. The method according to claim 2 wherein the third stream is identical to the second stream. 4. The method according to claim 2 wherein: the constellation mapping uses the data elements of the first encoded stream for least significant bits of the constellation mapping and the constellation mapping uses the data elements of the third stream for most significant bits of the constellation mapping. 5. The method according to claim 1 wherein the constellation mapping uses the data elements of the first encoded stream for least significant bits of the constellation mapping and the constellation mapping uses the data elements of the third stream for most significant bits of the constellation mapping. 6. The method according to claim 1 wherein the plurality of constellation points comprises a regular array, and wherein each co-set comprises a respective set of equally spaced points within the regular array. 7. The method according to claim 1 further comprising: encoding the second stream to produce the third stream using relatively weak encoding compared to that used in encoding the first stream to produce the third stream. 8. The method according to claim 1 further comprising: performing shaping on the second stream to produce the third stream. 9. The method according to claim 8 wherein the shaping is performed using a Huffman tree based addressing scheme to get a substantially Gaussian amplitude distribution. 10. The method according to claim 8 wherein performing constellation shaping for a given signal constellation comprising a plurality of constellation points comprises: associating a cost with each of the plurality of constellation points; defining a hierarchy of blocks, the hierarchy having a plurality of layers comprising at least a first layer and a last layer, each layer having fewer blocks than each previous layer; wherein the first layer is formed by ordering all of the constellation points according to cost, and then assigning a first lowest cost group of constellation points to a first shaping partition, a second lowest cost group of constellation points to a second shaping partition dividing comprises a plurality of shaping partitions, and so on until a highest cost group of constellation points assigned to a last shaping partition, each shaping partition being assigned a cost based on the costs of the constellation points in the shaping partition, each shaping partition being a first layer block; wherein an element in each other layer is formed by combining two blocks of a previous layer and is assigned a cost based on the costs of the two blocks of the previous layer, a block of each layer being comprised of one of the elements according to cost, or a group of the elements according to cost; the last layer having a single block comprising a plurality of elements; shaping gain being achieved by only mapping to a subset of the elements of the last layer. 11. The method according to claim 1 further comprising: performing shaping and channel coding on the second stream to produce the third stream with relatively weak encoding compared to that used in encoding the first stream. 12. The method according to claim 11 wherein the shaping is performed using a Huffman tree based addressing scheme to get a substantially Gaussian amplitude distribution. 13. The method according to claim 1 wherein the encoding performed on the first stream is turbo encoding. 14. The method according to claim 13 wherein the Turbo encoding is symbol-based Turbo encoding. 15. A transmitter comprising: a de-multiplexer operable to split an input stream into a first stream and a second stream; a first encoder operable to encode the first stream to produce a first encoded stream; a constellation mapper operable: to map to a signal constellation comprising a plurality of constellation points within which is defined a plurality of co-sets such that a minimum distance between constellation points within each co-set is larger than a minimum distance between any constellation points within the signal constellation; to use the first encoded stream to identify a sequence of co-sets of said plurality of co-sets; and to use a third stream which is based on the second stream to identify a sequence of constellation points within respective co-sets of the sequence of co-sets identified by said first encoded stream. 16. The transmitter according to claim 15 wherein the plurality of constellation points comprises a rectangular array, and wherein each co-set comprises a respective set of equally spaced points within the rectangular array. 17. The transmitter according to claim 15 wherein the constellation mapper performs Gray mapping in mapping the third stream. 18. The transmitter according to claim 15 wherein the third stream is identical to the second stream. 19. The transmitter according to claim 15 further comprising: a second encoder adapted to encode the second stream to produce the third stream using relatively weak encoding compared to that used in the first encoder. 20. The transmitter according to claim 15 wherein the first encoder implements a turbo code. 21. The transmitter according to claim 15 further comprising: a shaper and a second encoder adapted to perform shaping and encoding on the second stream to produce the third stream. 22. The transmitter according to claim 21 wherein the shaper is adapted to perform constellation shaping for a given signal constellation comprising a plurality of constellation points by: associating a cost with each of the plurality of constellation points; defining a hierarchy of blocks, the hierarchy having a plurality of layers comprising at least a first layer and a last layer, each layer having fewer blocks than each previous layer; wherein the first layer is formed by ordering all of the constellation points according to cost, and then assigning a first lowest cost group of constellation points to a first shaping partition, a second lowest cost group of constellation points to a second shaping partition dividing comprises a plurality of shaping partitions, and so on until a highest cost group of constellation points assigned to a last shaping partition, each shaping partition being assigned a cost based on the costs of the constellation points in the shaping partition, each shaping partition being a first layer block; wherein an element in each other layer is formed by combining two blocks of a previous layer and is assigned a cost based on the costs of the two blocks of the previous layer, a block of each layer being comprised of one of the elements according to cost, or a group of the elements according to cost; the last layer having a single block comprising a plurality of elements; shaping gain being achieved by only mapping to a subset of the elements of the last layer. 23. The transmitter according to claim 15 wherein the constellation mapper uses the data elements of the first encoded stream for least significant bits of the constellation mapping and the constellation mapper uses the data elements of the third stream for most significant bits of the constellation mapping.