A transmitter device (102, 200) comprising: circuitry configured to receive an N-bit sequence to be conveyed from the transmitter device to a receiver device (104, 300) over a communication channel; circuitry (222, 232, 246) configured to map the N-bit sequence to a point selected from a four-dimensional constellation consisting of 2 N 4D points which form a subset of a set of 4D points generated from a Cartesian product of a first two-dimensional constellation (602, 702, 802, 902) and a second 2D constellation (604, 704, 804, 904), the first 2D constellation consisting of M1 2D points partitioned into first 2D points, second 2D points, and third 2D points, and the second 2D constellation consisting of M2 2D points partitioned into fourth 2D points, fifth 2D points, and sixth 2D points, wherein M1 ≥ 5, wherein M2 ≥ 5, and wherein log 2 (M1×M2) > N, and wherein the subset (i) includes any 4D point of the set that is generated by combining any one of the M1 2D points and any one of the fourth 2D points; (ii) includes any 4D point of the set that is generated by combining any one of the first 2D points and any one of M2 2D points; and (iii) excludes any 4D point of the set that is generated by combining any one of the third 2D points and any one of the sixth 2D points; and circuitry (206, 210, 214, 218, 224) configured to transmit over the communication channel an optical signal representing the selected point. circuitry configured to receive an N-bit sequence to be conveyed from the transmitter device to a receiver device (104, 300) over a communication channel; circuitry (222, 232, 246) configured to map the N-bit sequence to a point selected from a four-dimensional constellation consisting of 2 N 4D points which form a subset of a set of 4D points generated from a Cartesian product of a first two-dimensional constellation (602, 702, 802, 902) and a second 2D constellation (604, 704, 804, 904), the first 2D constellation consisting of M1 2D points partitioned into first 2D points, second 2D points, and third 2D points, and the second 2D constellation consisting of M2 2D points partitioned into fourth 2D points, fifth 2D points, and sixth 2D points, wherein M1 ≥ 5, wherein M2 ≥ 5, and wherein log 2 (M1×M2) > N, and wherein the subset (i) includes any 4D point of the set that is generated by combining any one of the M1 2D points and any one of the fourth 2D points; (ii) includes any 4D point of the set that is generated by combining any one of the first 2D points and any one of M2 2D points; and (iii) excludes any 4D point of the set that is generated by combining any one of the third 2D points and any one of the sixth 2D points; and (i) includes any 4D point of the set that is generated by combining any one of the M1 2D points and any one of the fourth 2D points; (ii) includes any 4D point of the set that is generated by combining any one of the first 2D points and any one of M2 2D points; and (iii) excludes any 4D point of the set that is generated by combining any one of the third 2D points and any one of the sixth 2D points; and circuitry (206, 210, 214, 218, 224) configured to transmit over the communication channel an optical signal representing the selected point.