Quantizer and optical transmitter
A quantizer includes: a clipper configured to clip a portion exceeding a quantization range of a sample value sampled at a predetermined rate; and a noise shaper configured to determine a plurality of candidates for a quantization level based on the clipped sample value, and outputs, as a quantization value, a value obtained by adding a minimum noise in which a noise in a low-frequency region is minimum among noises generated in each candidate to the sample value before clipping.
1. A quantizer comprising:
a clipper circuit configured to remove a value exceeding a quantization range of a sample value which is sampled from a transmission data at a predetermined rate which is higher than a frequency of the transmission data and obtain clipped sample values; and
a noise shaper circuit coupled to an output of the clipper circuit and configured to determine a plurality of candidates for a quantization level based on each of the clipped sample values, obtain a value by adding a minimum noise in which a noise in a specific frequency region is minimum among noises generated in each candidate to the sample value and output an obtained value as a quantization value.
2. The quantizer according to claim 1 , wherein the noises include a clipping noise caused by an operation to remove the value and a quantization noise caused by a quantization.
3. The quantizer according to claim 1 , further comprising a filter configured to extract a specific frequency component of the noise.
4. The quantizer according to claim 3 , wherein the filter includes an adder configured to add the minimum noise before one symbol to the noise generated in each candidate, a delayer configured to delay the minimum noise in a current symbol by one symbol cycle, and a multiplier configured to multiply the minimum noise delayed by the delayer by a coefficient, and an output of the multiplier is coupled to an input of the adder.
5. The quantizer according to claim 4 , wherein the coefficient is set to a value to set the noise to the minimum noise, in accordance with a spectrum shape of a main signal or a transmission characteristic of a transmission line.
6. The quantizer according to claim 4 , further comprising a selection circuit configured to select the minimum noise from outputs of the adder.
7. The quantizer according to claim 1 , further comprising a second adder configured to add the minimum noise to the sample value.
8. An optical transmitter comprising:
a signal processor that outputs a sample value;
a quantizer;
a digital-to-analog converter that performs analog conversion on a quantization value; and
an optical modulator that is driven based on an output of the digital-to-analog converter,
the quantizer includes:
a clipper circuit configured to remove a value exceeding a quantization range of the sample value which is sampled from a transmission data at a predetermined rate which is higher than a frequency of the transmission data and obtain clipped sample values; and
a noise shaper circuit coupled to an output of the clipper circuit and configured to determine a plurality of candidates for a quantization level based on each of the clipped sample values, obtain a value by adding a minimum noise in which a noise in a specific frequency region is minimum among noises generated in each candidate to the sample value and output an obtained value as a quantization value.
9. The optical transmitter according to claim 8 , wherein the quantizer receives, from the signal processor, the sample value which is expanded in an amplitude direction to exceed a maximum input voltage range of the digital-to-analog converter.