System and method for PAM-4 transmitter bit equalization for improved channel performance
A method includes receiving a serial data stream at a transmitter of a serial channel, encoding the serial data stream using a quaternary pulse amplitude modulation (PAM-4) scheme into a stream of 2-bit symbols, wherein a particular symbol is represented as a signal at one of four signal levels provided for a unit interval of time, determining that a first symbol of the encoded serial data stream is represented as a highest state of the PAM-4 scheme, and, in response, providing a first output signal on an output of the transmitter, wherein the first output signal includes a first portion at a first voltage level associated with the highest state for a first half of a first unit interval of time associated with the first symbol, followed by a second portion at a second voltage level associated with a de-emphasized highest state of the PAM-4 scheme for a second half of the first unit interval of time.
1. A method, comprising:
receiving, at a transmitter of a serial channel, a serial data stream;
encoding the serial data stream using a quaternary pulse amplitude modulation (PAM-4) scheme into a stream of 2-bit symbols, wherein a particular symbol is represented as a signal at one of four signal levels provided for a unit interval of time;
determining that a first symbol of the encoded serial data stream is represented as a highest state of the PAM-4 scheme; and
in response to determining that the first symbol is represented as the highest state, providing a first output signal on an output of the transmitter, wherein the first output signal includes a first portion at a first voltage level associated with the highest state for a first half of a first unit interval of time associated with the first symbol, followed by a second portion at a second voltage level associated with a de-emphasized highest state of the PAM-4 scheme for a second half of the first unit interval of time.
2. The method of claim 1 , further comprising:
determining that a second symbol of the encoded serial data stream is represented as a lowest state of the PAM-4 scheme; and
in response to determining that the second symbol is represented as the lowest state, providing a second output signal on the output, wherein the second output signal comprises a third portion at a third voltage level associated with the lowest state for a first half of a second unit interval of time associated with the second symbol, followed by a fourth portion at a fourth voltage level associated with a de-emphasized lowest state of the PAM-4 scheme for a second half of the second unit interval of time.
3. The method of claim 1 , wherein:
a first symbol represents a bit sequence comprising a digital 0 followed by a digital 0, and is represented in the PAM-4 scheme by a lowest state;
a second symbol represents a bit sequence comprising a digital 0 followed by a digital 1, and is represented in the PAM-4 scheme by a first higher state, wherein the first higher state is higher than the lowest state;
a third symbol represents a bit sequence comprising a digital 1 followed by a digital 0, and is represented in the PAM-4 scheme by a second higher state, wherein the second higher state is higher than the first higher state; and
a fourth symbol represents a bit sequence comprising a digital 1 followed by a digital 1, and is represented in the PAM-4 scheme by a highest state, wherein the highest state is higher than the second higher state.
4. The method of claim 1 , further comprising:
sending the output signal to a receiver via a transmission channel.
5. The method of claim 4 , further comprising:
equalizing, at the receiver, the output signal; and
decoding the output signal to recover the serial data stream.
6. The method of claim 1 , wherein the de-emphasized highest state comprises one of 20 de-emphasis levels, each de-emphasis level prescribing a different amount of de-emphasis.
7. The method of claim 6 , wherein the different amounts of de-emphasis comprise de-emphasis levels 0.5 dB to 10 dB, in 0.5 dB steps.
8. The method of claim 1 , wherein the transmitter comprises a transmitter of a PCI-e link.
9. A serial data channel, comprising:
a transmitter; and
a receiver configured to receive an output signal from the transmitter;
the transmitter configured to:
receive a serial data stream;
encode the serial data stream using a quaternary pulse amplitude modulation (PAM-4) scheme into a stream of 2-bit symbols, wherein a particular symbol is represented as a signal at one of four signal levels provided for a unit interval of time;
determine that a first symbol of the encoded serial data stream is represented as a highest state of the PAM-4 scheme; and
in response to determining that the first symbol is represented as the highest state, provide a first output signal on an output of the transmitter, wherein the first output signal includes a first portion at a first voltage level associated with the highest state for a first half of a first unit interval of time associated with the first symbol, followed by a second portion at a second voltage level associated with a de-emphasized highest state of the PAM-4 scheme for a second half of the first unit interval of time.
10. The serial data channel of claim 9 , the transmitter further configured to:
determine that a second symbol of the encoded serial data stream is represented as a lowest state of the PAM-4 scheme; and
in response to determining that the second symbol is represented as the lowest state, provide a second output signal on the output, wherein the second output signal comprises a third portion at a third voltage level associated with the lowest state for a first half of a second unit interval of time associated with the second symbol, followed by a fourth portion at a fourth voltage level associated with a de-emphasized lowest state of the PAM-4 scheme for a second half of the second unit interval of time.
11. The serial data channel of claim 9 , wherein:
a first symbol represents a bit sequence comprising a digital 0 followed by a digital 0, and is represented in the PAM-4 scheme by a lowest state;
a second symbol represents a bit sequence comprising a digital 0 followed by a digital 1, and is represented in the PAM-4 scheme by a first higher state, wherein the first higher state is higher than the lowest state;
a third symbol represents a bit sequence comprising a digital 1 followed by a digital 0, and is represented in the PAM-4 scheme by a second higher state, wherein the second higher state is higher than the first higher state; and
a fourth symbol represents a bit sequence comprising a digital 1 followed by a digital 1, and is represented in the PAM-4 scheme by a highest state, wherein the highest state is higher than the second higher state.
12. The serial data channel of claim 9 , the transmitter further configured to:
send the output signal to the receiver via a transmission channel.
13. The serial data channel of claim 12 , the receiver configured to:
equalize the output signal; and
decode the output signal to recover the serial data stream.
14. The serial data channel of claim 9 , wherein the de-emphasized highest state comprises one of 20 de-emphasis levels, each de-emphasis level prescribing a different amount of de-emphasis.
15. The serial data channel of claim 14 , wherein the different amounts of de-emphasis comprise de-emphasis levels 0.5 dB to 10 dB, in 0.5 dB steps.
16. The serial data channel of claim 9 , wherein the serial data channel comprises a PCI-e link.
17. A non-transitory computer-readable medium including code for performing a method, the method comprising:
receiving, at a transmitter of a serial channel, a serial data stream;
encoding the serial data stream using a quaternary pulse amplitude modulation (PAM-4) scheme into a stream of 2-bit symbols, wherein a particular symbol is represented as a signal at one of four signal levels provided for a unit interval of time;
determining that a first symbol of the encoded serial data stream is represented as a highest state of the PAM-4 scheme; and
in response to determining that the first symbol is represented as the highest state, providing a first output signal on an output of the transmitter, wherein the first output signal includes a first portion at a first voltage level associated with the highest state for a first half of a first unit interval of time associated with the first symbol, followed by a second portion at a second voltage level associated with a de-emphasized highest state of the PAM-4 scheme for a second half of the first unit interval of time.
18. The computer-readable medium of claim 17 , the method further comprising:
determining that a second symbol of the encoded serial data stream is represented as a lowest state of the PAM-4 scheme; and
in response to determining that the second symbol is represented as the lowest state, providing a second output signal on the output, wherein the second output signal comprises a third portion at a third voltage level associated with the lowest state for a first half of a second unit interval of time associated with the second symbol, followed by a fourth portion at a fourth voltage level associated with a de-emphasized lowest state of the PAM-4 scheme for a second half of the second unit interval of time.
19. The computer-readable medium of claim 17 , wherein:
a first symbol represents a bit sequence comprising a digital 0 followed by a digital 0, and is represented in the PAM-4 scheme by a lowest state;
a second symbol represents a bit sequence comprising a digital 0 followed by a digital 1, and is represented in the PAM-4 scheme by a first higher state, wherein the first higher state is higher than the lowest state;
a third symbol represents a bit sequence comprising a digital 1 followed by a digital 0, and is represented in the PAM-4 scheme by a second higher state, wherein the second higher state is higher than the first higher state; and
a fourth symbol represents a bit sequence comprising a digital 1 followed by a digital 1, and is represented in the PAM-4 scheme by a highest state, wherein the highest state is higher than the second higher state.
20. The computer-readable medium of claim 17 , the method further comprising:
sending the output signal to a receiver via a transmission channel.