Vector signaling code with improved noise margin
Methods are described allowing a vector signaling code to encode multi-level data without the significant alphabet size increase known to cause symbol dynamic range compression and thus increased noise susceptibility. By intentionally restricting the number of codewords used, good pin efficiency may be maintained along with improved system signal-to-noise ratio.
1. A method comprising:
generating a codebook using an offline process prior to system initialization, the codebook comprising a plurality of constrained-alphabet codewords of a constrained-alphabet code, the plurality of constrained-alphabet codewords being a subset of a maximal set of vectors having an associated unconstrained alphabet;
obtaining a set of input bits;
mapping the set of input bits to a set of symbols corresponding to a constrained-alphabet codeword having symbol values selected from a constrained alphabet having at least three values, the constrained alphabet having a fewer number of symbol values than the unconstrained alphabet, wherein mapping the set of input bits comprises selecting a corresponding constrained-alphabet codeword from the codebook; and
transmitting each symbol of the set of symbols over a respective wire of a multi-wire bus.
2. The method of claim 1 , wherein the constrained-alphabet code is generated by selecting, from the maximal set of vectors, codewords having symbol values within the constrained alphabet, the maximal set of vectors representing a transformation of a plurality of PAM-M codewords with a transformation matrix of size N, wherein M is an integer greater than 1 and N is an integer power of 2 greater than 1.
3. The method of claim 2 , wherein each of the plurality of PAM-M codewords has at least k zero elements, wherein k is an integer greater than or equal to 1.
4. The method of claim 3 , wherein the constrained alphabet is less than or equal to (N−k−1)·(M−1)+1.
5. The method of claim 2 , wherein M=2, and wherein each PAM-M codeword has symbol values selected from the group (+1, −1).
6. The method of claim 2 , wherein M=3, and wherein each PAM-M codeword has symbol values selected from the group (+1, 0, −1).
7. The method of claim 2 , wherein the transformation matrix is a Hadamard matrix.
8. The method of claim 1 , wherein the codebook comprises at least two constrained-alphabet codes.
9. The method of claim 8 , wherein a constrained-alphabet code is selected at system initialization.
10. An apparatus comprising:
a codebook generator configured to generate a codebook using an offline process prior to system initialization, the codebook comprising a plurality of constrained-alphabet codewords of a constrained-alphabet code, the plurality of constrained-alphabet codewords being a subset of a maximal set of vectors having an associated unconstrained alphabet;
a codebook encoder configured to obtain a set of input bits, and to responsively map the set of input bits to a set of symbols corresponding to a constrained-alphabet codeword having symbol values selected from a constrained alphabet having at least three values, the constrained alphabet having a fewer number of symbol values than the unconstrained alphabet, wherein the codebook encoder is configured to select, for the set of input bits, a corresponding constrained-alphabet codeword from the codebook; and
a plurality of drivers configured to transmit each symbol of the set of symbols over a respective wire of a multi-wire bus.
11. The apparatus of claim 10 , wherein the codebook generator is configured to generate the constrained-alphabet code by selecting, from the maximal set of vectors, codewords having symbol values within the constrained alphabet, the maximal set of vectors representing a transformation of a plurality of PAM-M codewords with a transformation matrix of size N, wherein M is an integer greater than 1 and N is an integer power of 2 greater than 1.
12. The apparatus of claim 11 , wherein each of the plurality of PAM-M codewords has at least k zero elements, wherein k is an integer greater than or equal to 1.
13. The apparatus of claim 12 , wherein the constrained alphabet is less than or equal to (N−k−1)·(M−1)+1.
14. The apparatus of claim 11 , wherein M=2, and wherein each PAM-M codeword has symbol values selected from the group (+1, −1).
15. The apparatus of claim 11 , wherein M=3, and wherein each PAM-M codeword has symbol values selected from the group (+1, 0, −1).
16. The apparatus of claim 11 , wherein the transformation matrix is a Hadamard matrix.
17. The apparatus of claim 10 , wherein the codebook comprises at least two constrained-alphabet codes.
18. The apparatus of claim 17 , wherein the codebook encoder is further configured to select a constrained-alphabet code at system initialization.