Method and apparatus for prioritizing information protection in high order modulation symbol mapping

[0002] This application claims priority under 35 USC § 119 (e)(1) of provisional application Serial No. 60/232,357, filed Sep. 14, 2000.

[0003] The present invention relates generally to wireless communication and, more particularly, to a system and method for selectively protecting encoded information bits in M-ary quadrature amplitude modulation (QAM) transmissions.

[0004] As a function in the modulation and transmission process, the output of turbo encoder consists of systematic bits and punctured parity bits as shown in FIG. 1. A different code rate is realized by puncturing the parity bits at a certain rate.

[0005] In the prior art [1], and as shown in FIG. 2, the output of a turbo, or any other suitable encoder is interleaved without any consideration of importance of each code bit, or without consideration of the protection level afforded in the modulation of the encoded bits. In another prior art reference [2], the punctured parity bits are mapped to the most well-protected modulation bits without any justification.

[0006] In a third prior art reference [3], a turbo trellis modulation scheme is employed by jointly designing high order modulation and turbo code. FIG. 3 shows the turbo trellis modulation scheme where the modulation symbol boundary in time is kept by interleaving at the modulation symbol level rather than interleaving, and scattering in time the code bits. Here the systematic bits are mapped to the most well-protected modulation bits. However, this turbo trellis modulation scheme is not applicable to conventional wireless communication systems which require an interleaver between the channel encoder and the modulation symbol mapper. Since a fading channel often causes burst errors, wireless communications systems use a code bit interleaver to protect against burst errors which cause the decoder to lose both systematic and parity code bits for the same information bit during a fading period. Losing both systematic and parity code bits for a single information bit is worse for turbo decoder performance than losing a systematic bit of one information bit and a parity bit of another information bit. So, the output of the channel encoder should be interleaved before the encoder output is mapped to modulation symbols.

[0007] Thus, although it is well known that the mapping of information into modulation symbols provides different levels of noise immunity or protection, this principle has yet to be meaningfully applied to conventional wireless communication systems.

[0008] [1] TIA 1XTREME Ad hoc group, IS-2000 1XTREME Delta Specification, V2.0d

[0009] [2] S. Le Goff, A. Glavieux and C. Berrou, “Turbo-codes and high spectral efficiency modulation,” IEEE ICC 94 Proceedings, pp. 645-649, 1994.

[0010] [3] P. Robertson and T. Wortz, “Extensions of turbo trellis coded modulation to high bandwidth efficiencies,” IEEE ICC 97 Proceedings, pp. 1251-1255, 1997

[0011] The invention achieves better channel decoder performance than the prior art by providing a greater level of protection to the more important encoded bits, such as the systematic bits, with little additional hardware.

[0012] Modern communication systems provide higher rates of data transmission through the use of multiple level modulation such as M-ary quadrature amplitude modulation (QAM). Each bit in the QAM constellation is protected against channel impairment at a different level. On the other hand, each code bit of channel encoder output has different importance level in terms of decoding. So, a method and apparatus are described herein for efficiently mapping important code bits to well-protected input bits for modulation symbol mapper, and for a corresponding receiving method and apparatus. The present invention takes advantage of the inherent structure of such a priority conversion to provide improved decoder performance and less hardware complexity.

[0013] In most wireless communication systems the output of the channel encoder is typically interleaved before the encoder output is mapped to modulation symbols, since in wireless communication systems a fading channel often causes burst errors. The present invention method and apparatus provides a means of interleaving, while matching important code bits to well-protected modulation bits.

[0014] The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

[0015] FIG. 1 shows a block diagram of a turbo encoder.

[0016] FIG. 2 shows a block diagram of a prior art communication system.

[0017] FIG. 3 shows a block diagram of a prior art turbo encoder providing a trellis modulation scheme.

[0018] FIG. 4 shows a diagram of a QAM constellation used in one example of the present invention.

[0019] FIG. 5 shows a block diagram of a communication system in accordance with the invention.

[0020] FIGS. 6A-6C shows distribution ratios for different modulation schemes in accordance with the invention.

[0021] FIG. 4 illustrates an example of a specific Gray-coded 64-QAM constellation, and shows how code bits are mapped into modulation bits. The principle of mapping important information bits to well protected areas of the modulation symbol applies to other high order modulation schemes as well, 64-QAM is merely used as an example of such a high order modulation scheme. Likewise, although the example presented below uses turbo codes, the principle applies to other channel coding schemes as well.

[0022] Each point of the 64-QAM constellation is represented by two real numbers (c_li, c_Qi) i=1, . . . ,64 and corresponds to b₀b₁. . . b₅. The first two modulation bits b₀b₁determine one of four quadrants in the constellation. The protection level against channel impairment is related to the distance between the center of each quadrant and the decision boundary B0 or B1, with that distance being equal to D₁=0.617, as illustrated in FIG. 4. The second pair of bits b₂b₃separate the subsets of signal points in a certain quadrant. The protection level is similarly given by the distance between the center of each subset and the decision boundary B2 or B3, such that the distance is equal to D₂=0.308. The protection level of the last pair of bits b₄b₅is similarly given by D₃=0.154. The first two bits are the most well-protected, while protection decreases for each subsequent bit pair.

[0023] Some of channel encoder output bits are more important than others. For example, since the systematic bit is used for both constituent decoders in the receiver, the systematic bits should be better protected than parity bits. The systematic bit output of turbo encoder is identical with the information bit. Thus, the invention proposes to map the systematic bits to well-protected bits while mapping the parity bits to the least-protected bits whenever possible. Note that other encoding schemes may require alternate algorithms to determine bit importance.

[0024] FIG. 5 is a block diagram of the invention in the case of 64-QAM modulation with three code bit interleavers. The number of code bit interleavers is equal to the number of protection classification sets. The number of the classification sets is no larger than log2(M) in the case of M-QAM. The first code bit interleaver (INT1) output is mapped to the most well-protected pair of bits b₀b₁, the second INT2 to b₂b₃, and the third INT3 to b₄b₅.

[0025] The demultiplexer (DEMUX) distributes the systematic bits and the parity bits according to the number of code bit interleavers and the code rate. For instance, Table 1 shows an example of the distribution ratio for 64-QAM with three code bit interleavers. For code ¼, the first interleaver (INT 1) takes three systematic bits and any one parity bit, the second interleaver takes four other parity bits, and the third interleaver takes the remaining four parity bits, repetitively. Table 2 shows the distribution pattern for 64-QAM with two code bit interleavers, while Table 3 shows the pattern for 16-QAM.

[0026] Before being interleaved, each DEMUX output arm goes through a cyclic shift. The cyclic shift guarantees that the systematic bits and parity bits corresponding to the same information bits are well separated in time. A completely independent and unique shifting system would be optimal for system performance, but such a scheme is not practical. Therefore, independent, or at least a pseudo-random, cyclic shift patterns are introduced in each interleaver path. This permits the same circuits and software to be used for each of the demultiplexed interleaver paths. The cyclic shift of k/m is given by $Y\left(n\right)=X\left(\mathrm{mod}\left(n+\mathrm{kN}/m,N\right)\right)k=1,2,\dots ,m-1.$

[0027] Mod(n,N) is the modulo function, N is the size of the code bit interleaver, m is the number of code bit interleavers. The output of the cyclic shift and interleavers is used for modulation symbol mapping.

[0028] In general, the number of interleaver paths (n) is equal to the number of protection levels offered by the modulation symbol mapping. For example, 16 QAM and 8 PSK offer two levels of protection, therefore, there are two interleaver paths. 32 and 64 QAM offer three levels of protection, so there are three interleaver paths. 128 and 256 QAM offer four levels of protection, so there four interleaver paths are optimal. Since BPSK offers only one level, there need be only one interleaver path, as is conventional. With QAM, the level of protection is determined as follows:

[0029] 2²ⁿQAM offer n levels of protection.

[0030] The above operations are performed in the reverse order in the receiver.