Communication unit for soft-decision demodulation and method therefor
A communication unit for performing soft-decision demodulation includes a receiver that receives a transmitted signal conveying a first set of bits including k bits selected from a set of 2 k possible signals. A demodulator includes a bank of 2 k correlators that detects a transmission of each possible transmitted signal, and outputs 2 k magnitudes of correlator outputs, based on the detected possible transmitted signals, as a first set of inputs. A de-mapper circuit receives the first set of inputs and determines derived from a plurality of aggregated correlator output magnitude distributions of the first set of inputs, wherein the plurality of aggregated correlator output magnitude distributions is fewer than 2 2k ; and calculates therefrom a first set of aposteriori soft bits including k soft bits. In this manner, high quality soft-decisions can be obtained in a robust and practical manner.
1. A communication unit for performing soft-decision demodulation comprising a receiver, wherein the receiver is arranged to receive a transmitted signal conveying a first set of bits comprising k bits that has been selected from a set of 2 k possible signals, wherein the receiver comprises:
a demodulator comprising a bank of 2k correlators and configured to:
detect a transmission of each possible transmitted signal, and
output 2k magnitudes of correlator outputs, based on the detected possible transmitted signals, by the bank of 2k correlators as a first set of inputs; and
a de-mapper circuit coupled to the demodulator and configured to receive the first set of inputs and:
determine statistics derived from a plurality of aggregated correlator output magnitude distributions of the first set of inputs, wherein the plurality of aggregated correlator output magnitude distributions is fewer than 2 2k ; and
calculate a first set of aposteriori soft bits comprising k soft bits from the determined statistics and output the first set of aposteriori soft bits comprising k soft bits.
2. The communication unit of claim 1 , wherein the plurality of aggregated correlator output magnitude distributions is two, and wherein a first aggregated correlator output magnitude distribution approximates an aggregation of the 2 k distributions of the output magnitudes of the 2 k correlators when a corresponding one of the 2 k possible signals was selected as the transmission signal, and wherein a second aggregated correlator output magnitude distribution approximates an aggregation of a remaining 2 2k -2 k distributions of the output magnitudes.
3. The communication unit of claim 2 , wherein the first and second aggregated correlator output magnitude distributions are each represented by a first set of distribution parameters.
4. The communication unit of claim 3 , wherein the first set of distribution parameters comprises a non-centrality parameter(s) and a scale parameter, sigma.
5. The communication unit of claim 3 further comprising a look-up-table, wherein the de-mapper circuit is further configured to perform one of the following:
estimate the first set of distribution parameters of the first aggregated correlator output magnitude distribution by fitting a first probability distribution to the magnitudes of correlator outputs obtained by correlating received synchronization signals with correlators corresponding to known synchronization signals;
estimate the first set of distribution parameters of the second aggregated correlator output magnitude distribution by fitting a second probability distribution to the magnitudes of correlator outputs obtained by correlating received synchronization signals with the 2 k −1 correlators corresponding to signals other than the known synchronization signal;
estimate the first set of distribution parameters of the first aggregated correlator output magnitude distribution by fitting a third probability distribution to the magnitudes of correlator outputs having the greatest magnitude among sets of 2 k correlator outputs obtained by the bank of 2 k correlators;
estimate the first set of distribution parameters of the second aggregated correlator output magnitude distribution by fitting a fourth probability distribution to the magnitudes of the 2 k −1 correlator outputs that do not have the greatest magnitude among sets of 2 k correlator outputs obtained by the bank of 2 k correlators; or
select the first set of distribution parameters of the first and second aggregated correlator output magnitude distributions from the look-up-table coupled to the de-mapper circuit.
6. The communication unit of claim 5 , wherein the first set of distribution parameters comprises a non-centrality parameter(s) and a scale parameter, sigma, and wherein at least one of the first, second, third or fourth probability distributions is a Rician distribution.
7. The communication unit of claim 3 , wherein the de-mapper circuit is further configured to combine output magnitudes of the 2 k correlators with the first set of distribution parameters of the first and second aggregated correlator output magnitude distributions to obtain a first set of apriori soft signals comprising 2 k soft magnitudes.
8. The communication unit of claim 7 , wherein the de-mapper circuit is further configured to combine all the apriori soft signals to generate a set of 2 k aposteriori soft signals and wherein the set of 2 k aposteriori soft signals are combined to obtain the first set of aposteriori soft bits.
9. The communication unit of claim 1 , wherein the first set of aposteriori soft bits are combined with the soft bits of the second set of apriori soft signals, to obtain a first set of extrinsic soft bits comprising k soft bits.
10. The communication unit of claim 9 , wherein the de-mapper circuit is further configured to combine the first set of aposteriori soft bits with the soft bits of the second set of apriori soft signals, to obtain a first set of extrinsic soft bits comprising k soft bits.
11. The communication unit of claim 1 , wherein the transmitted signal also conveys a second set of bits comprising Qm bits where a phase of the received transmitted signal has been rotated using a rotation selected from a set of 2 Qm possible rotations, and the demodulator is configured to output an additional 2 k phases of the correlator outputs as a second set of inputs, to the de-mapper circuit and wherein the de-mapper circuit is further configured to:
determine statistics derived from a number of aggregated correlator output phase distributions of the second set of inputs; and
calculate a second set of aposteriori soft bits comprising Qm soft bits based on the derived statistics.
12. The communication unit of claim 11 , wherein the number of aggregated correlator output phase distributions is one, and the aggregated correlator output phase distribution approximates an aggregation of the 2 k distributions of the additional 2 k phases of the correlator outputs when a corresponding one of the 2 k possible signals was selected as the transmission signal.
13. The communication unit of claim 12 , wherein the aggregated correlator output phase distribution is represented by a second set of distribution parameters.
14. The communication unit of claim 13 , wherein the second set of distribution parameters comprises a spreading parameter, sigma phase .
15. The communication unit of claim 13 , further comprising a look-up-table, wherein the de-mapper circuit is further configured to perform one of the following:
estimate the second set of distribution parameters of the aggregated correlator output phase distribution by fitting a fifth probability distribution to phase errors of correlator outputs obtained by correlating received synchronization signals with correlators corresponding to known synchronization signals;
estimate the second set of distribution parameters of the aggregated correlator output phase distribution by fitting a sixth probability distribution to phase errors of correlator outputs having the greatest magnitude among sets of 2 k correlator outputs obtained by the bank of 2 k correlators; or
select the second set of distribution parameters of the aggregated correlator output phase distribution from the look-up-table coupled to the de-mapper circuit.
16. The communication unit of claim 15 , wherein at least one of the fifth or sixth probability distributions is a Gaussian distribution.
17. The communication unit of claim 15 , wherein the de-mapper circuit is further configured to combine output phases of the 2 k correlators with the second set of distribution parameters of the aggregated correlator output phase distribution to obtain a third set of apriori soft signals comprising 2 k+Qm soft phases.
18. The communication unit of claim 11 , wherein the de-mapper circuit is further configured to accept a fourth set of apriori soft signals comprising Qm soft bits as a fourth set of inputs and wherein the second set of aposteriori soft bits is combined with soft bits of the fourth set of apriori soft signals to obtain a second set of extrinsic soft bits comprising Qm soft bits.
19. The communication unit of claim 18 , wherein the de-mapper circuit is further configured to combine all the apriori soft signals to generate a set of 2 k+Qm aposteriori soft signals and wherein the set of 2 k+Qm aposteriori soft signals is combined to obtain the first set of aposteriori soft bits and the second set of aposteriori soft bits.
20. A method for performing soft-decision demodulation comprising a communication unit having a receiver, the receiver having a de-mapper circuit coupled to a demodulator that comprises a bank of 2 k correlators, the method at the receiver comprising:
receiving a transmitted signal that conveys a first set of bits comprising k bits that has been selected from a set of 2 k possible signals:
detecting a transmission of each possible transmitted signal;
outputting 2 k magnitudes of correlator outputs, based on the detected possible transmitted signals, by the bank of 2 k correlators as a first set of inputs;
determining statistics derived from a plurality of aggregated correlator output magnitude distributions of the first set of inputs, wherein the plurality of aggregated correlator output magnitude distributions is fewer than 2 2k ; and
calculating a first set of aposteriori soft bits comprising k soft bits from the determined statistics and outputting the first set of aposteriori soft bits comprising k soft bits.