FSK/MSK decoder
View Patent ↗A decoder for a modulation scheme is configured to operate close to the radio noise floor. A correlation value may be constantly updated, in an effort to match to a signature to a preamble of a packet. A low clamp value may act as a floor to which a calculated correlation value is set, if it is less than the low clamp value. If a correlation threshold is exceeded, then the correlation value is examined to determine it is a peak value. If the peak is found, power of the preamble is compared to a power threshold that is relative to the radio noise floor. If the power threshold is exceeded, positive correlation is detected. A channel optimizer is used to remove the frequency misalignment. This enables the use of a filter that is approximately equal to the occupied bandwidth of the incoming signal, further rejecting noise and interference.
1. A decoder, comprising:
a correlator to calculate a correlation value between incoming samples and a signature of a preamble of a packet, wherein correlation is based in part on correlation techniques comprising confining the calculated correlation value to a region above a low clamp value;
a bit constructor to construct bits from a payload of the packet having the preamble; and
a channel optimizer configured to:
estimate frequency misalignment based at least in part on a point of correlation to the preamble of the packet;
tune to remove the estimated frequency misalignment; and
filter at a location indicated by the tuning.
2. The decoder as recited in claim 1 , wherein confining the calculated correlation value to the region above the low clamp value comprises:
setting the low clamp value at a level such that if a valid packet arrives a low value of the calculated correlation value will not prevent correlation.
3. The decoder as recited in claim 1 , wherein the correlation techniques additionally comprise:
resetting the calculated correlation value if a preamble power value is less than a power threshold.
4. The decoder as recited in claim 1 , wherein the correlation techniques additionally comprise:
basing correlation on both a correlation value threshold and a power threshold, wherein the power threshold is based in part on a radio noise floor level.
5. The decoder as recited in claim 1 , wherein the correlation techniques additionally comprise:
a correlation value threshold set to indicate correlation between the calculated correlation value of the samples and the signature of the preamble; and
a power threshold to compare to a preamble power value, wherein:
the preamble power value is based at least in part on bits in samples used in the calculated correlation value; and
the power threshold is a relative value, and based in part on a radio noise floor.
6. The decoder as recited in claim 5 , additionally comprising:
tracking a noise floor to determine a level of the noise floor; and
using the noise floor to set the power threshold.
7. The decoder as recited in claim 1 , wherein:
the filter is set to a width of approximately an occupied band width (OBW) of an incoming signal.
8. The decoder as recited in claim 1 , wherein the correlation techniques additionally comprise:
detecting a peak in the calculated correlation value; and
using the detected peak to synchronize to a beginning of the payload of the packet.
9. A decoder, comprising:
a correlator to calculate a correlation value between incoming samples and a signature of a preamble of a packet, wherein the correlation value is based at least in part on confining the calculated correlation value to a region above a low clamp value;
a bit constructor to construct bits from a payload of the packet having the preamble; and
a channel optimizer configured to:
estimate frequency misalignment;
tune to remove the estimated frequency misalignment using a complex mixer; and
filter in the time domain at a location indicated by the tuning, wherein the filter is set to approximately an occupied bandwidth (OBW) of an incoming signal.
10. A decoder to decode a modulation scheme, comprising:
a correlator to calculate a correlation value between incoming samples and a signature;
a channel optimizer configured to:
estimate frequency misalignment based at least in part on a point of correlation to a preamble of a packet;
tune according to the estimated misalignment; and
filter at a location indicated by the tuning; and
a bit constructor to construct bits from a payload of the packet having the preamble indicated by the signature.
11. The decoder as recited in claim 10 , wherein the correlator comprises:
a correlation value threshold set to indicate correlation between the calculated correlation value of the samples and the signature; and
a power threshold to compare to a preamble power value, wherein:
the preamble power value is based at least in part on the incoming samples; and
the power threshold is based in part on a noise floor.
12. The decoder as recited in claim 10 , wherein the correlator is configured to:
restrict the calculated correlation value to a region above a low clamp value.
13. The decoder as recited in claim 10 , wherein the correlator is configured to:
track the noise floor to determine a level of the noise floor; and
use the noise floor to set a power threshold.
14. The decoder as recited in claim 10 , wherein:
the filtering is at a width of approximately an occupied band width (OBW) of an incoming signal.
15. The decoder as recited in claim 10 , wherein the channel optimizer is additionally configured to:
estimate the frequency misalignment based in part on the samples associated with the preamble; and
remove bias intrinsic with the preamble signature.
16. A method of decoding a modulation scheme, comprising:
identifying a packet to decode by correlating to a preamble of the packet;
estimating frequency misalignment, between a transmitter and a receiver, based at least in part on a point of correlation to the preamble of the packet;
tuning according to the estimated frequency misalignment; and
filtering at a location indicated by the tuning.
17. The method as recited in claim 16 , additionally comprising:
calculating a correlation value of sample bits to a signature of a preamble;
requiring the correlation value to be greater than or equal to a low clamp value; and
finding correlation when:
the correlation value exceeds a correlation value threshold; and
a correlation power value exceeds a power threshold.
18. The method as recited in claim 16 , additionally comprising:
detecting a peak in the calculated correlation value; and
using the detected peak to synchronize to a beginning of a payload of the packet.
19. The method as recited in claim 16 , wherein:
estimating frequency misalignment is performed using adders and shifters;
tuning according to the estimated misalignment is performed using a complex mixer; and
filtering at a location indicated by the tuning is performed by use of a filter set to approximately an occupied bandwidth of an incoming signal.
20. The method as recited in claim 16 , wherein the tuning uses a complex mixer.