SYSTEM AND METHOD OF GENERATING SOFT BITS

• US 20100142649A1
• Filed: 12/04/2008
• Published: 06/10/2010
• Est. Priority Date: 12/04/2008
• Status: Active Grant

• Current Assignee: Altobridge Limited (Singapore Technologies Engineering Limited)
• Original Assignee: ADC Telecommunications Incorporated (TE Connectivity Limited)
• Inventors: Xie, Kai, Elyasi, Sattar
• Granted Patent: US 8,184,745 B2
• Time in Patent Office: Days
• US Class Current: 375/320
• CPC Class Codes: H04L 25/067 providing soft decisions, i.e. decisions together with an estimate of reliability H04L25/068 and H04L25/069 take precedence; sequence estimation techniques H04L25/03178
• CPC Class Codes: H04L 27/38 Demodulator circuits; Receiver circuits

Abstract

A receiver receives a received signal from a communication link, the received signal comprising a stream of symbols modulated onto a carrier, each of the stream symbols selected from a constellation of symbols to represent a plurality of encoded data bits. The receiver comprises a demodulator to produce a stream of received symbols derived from the received signal; a channel decoder to produce a received stream of data bits derived from the stream of received symbols; and a memory device to store information related to the plurality of symbols in the constellation. For each of the symbols in the stream of received symbols, the demodulator: determines which of the plurality of symbols in the constellation is a respective closest symbol to that received symbol; identifies a respective nearest neighbor symbol for each data bit represented by the closest symbol for that received symbol based on the information stored in the memory device; and for each data bit represented by the respective closest symbol for that received symbol, subtracts the distance between the respective closest symbol and that received symbol from the distance between the respective nearest neighbor symbol and that received symbol in order to produce respective soft bits for that received symbol.

Claims

1. A receiver that receives a received signal from a communication link, the received signal comprising a stream of symbols modulated onto a carrier, each of the stream symbols selected from a constellation of symbols to represent a plurality of encoded data bits, the receiver comprising:

a demodulator to produce a stream of received symbols derived from the received signal;

a channel decoder to produce a received stream of data bits derived from the stream of received symbols; and

a memory device to store information related to the plurality of symbols in the constellation;

wherein, for each of the symbols in the stream of received symbols, the demodulator;

determines which of the plurality of symbols in the constellation is a respective closest symbol to that received symbol;

identifies a respective nearest neighbor symbol for each data bit represented by the closest symbol for that received symbol based on the information stored in the memory device; and

for each data bit represented by the respective closest symbol for that received symbol, subtracts the distance between the respective closest symbol and that received symbol from the distance between the respective nearest neighbor symbol and that received symbol in order to produce respective soft bits for that received symbol.

2. The receiver of claim 1, wherein the demodulator is operable to produce a stream of received symbols by using one of a phase-shift keying (PSK) modulation scheme and a Quadrature Amplitude Modulation (QAM) modulation scheme.

3. The receiver of claim 2, wherein the PSK modulation scheme is a Global System for Mobile communications (GSM) 8-PSK modulation scheme.

4. The receiver of claim 2, wherein the QAM modulation scheme is a modulation scheme based on the IEEE 802.16e standard.

5. The receiver of claim 1, wherein the demodulator is further operable, for each bit represented by the closest symbol for a given received symbol, to retrieve, from the memory device, polarity data related to the soft bits for that received symbol.

6. The receiver of claim 1, wherein the received signal comprises a radio frequency signal and wherein the receiver further comprises a radio frequency module that down-converts the received signal.

7. A communication system comprising:

a transmitter operable to transmit a modulated signal, the transmitter comprising;

a channel encoder to encode a stream of data bits; and

a modulator to map the encoded steam of data bits to a plurality of symbols from a constellation of symbols such that each symbol in the modulated signal represents a plurality of encoded data bits; and

a receiver operable to receive the modulated signal, the receiver comprising;

a demodulator to produce a stream of received symbols derived from the received signal;

a channel decoder to produce a received stream of data bits derived from the stream of received symbols; and

a memory device to store information related to the plurality of symbols in the constellation;

wherein, for each of the symbols in the stream of received symbols, the demodulator;

determines which of the plurality of symbols in the constellation is a respective closest symbol to that received symbol;

identifies a respective nearest neighbor symbol for each data bit represented by the closest symbol for that received symbol based on the information stored in the memory device; and

for each data bit represented by the respective closest symbol for that received symbol, subtracts the distance between the respective closest symbol and that received symbol from the distance between the respective nearest neighbor symbol and that received symbol in order to produce respective soft bits for that received symbol.

8. The communication system of claim 7, wherein the demodulator is operable to produce a stream of received symbols by using one of a phase-shift keying (PSK) modulation scheme and a Quadrature Amplitude Modulation (QAM) modulation scheme.

9. The communication system of claim 8, wherein the PSK modulation scheme is a Global System for Mobile communications (GSM) 8-PSK modulation scheme.

10. The communication system of claim 8, wherein the QAM modulation scheme is a modulation scheme based on the IEEE 802.16e standard.

11. The communication system of claim 7, wherein the demodulator is further operable, for each bit represented by the closest symbol for a given received symbol, to retrieve, from the memory device, polarity data related to the soft bits for that received symbol.

12. The communication system of claim 7, wherein the transmitter further comprises a radio frequency module that up-converts the modulated signal and the receiver further comprises a radio frequency module that down-converts the received signal.

13. A method of demodulating a received signal comprised of a stream of symbols, the method comprising:

receiving the signal comprised of a stream of symbols, each of the stream symbols selected from a constellation of symbols to represent a plurality of encoded data bits;

for each received symbol, determining which symbol in the constellation has the minimum distance to that received symbol;

for each bit represented by the symbol with the minimum distance to that received symbol, identifying a nearest neighbor symbol that reverses the value of the respective bit based on information stored in a memory device; and

for each bit represented by the symbol with the minimum distance to that received symbol, subtracting the minimum distance from the distance between each respective bit'"'"'s nearest neighbor symbol and that received symbol to obtain soft bits for that received symbol.

14. The method of claim 13, wherein the constellation is a constellation of one of a phase-shift keying (PSK) modulation scheme and a Quadrature Amplitude Modulation scheme.

15. The method of claim 14, wherein the modulation scheme is a Global System for Mobile communications (GSM) 8-PSK modulation scheme.

16. The method of claim 14, wherein the QAM modulation scheme is a modulation scheme based on the IEEE 802.16e standard.

17. The method of claim 13, further comprising, for each bit represented by the symbol with the minimum distance, retrieving, from the memory device, polarity data related to the reliability information for that received symbol.

18. The method of claim 13, wherein the received signal is a radio frequency signal and receiving the signal comprises down-converting the received signal to a baseband signal.

19. A program product comprising a processor-readable medium on which program instructions are embodied, wherein the program instructions are operable, when executed by at least one programmable processor included in a device that demodulates received communication signals, to cause the device to:

calculate the distance between a received symbol representing a plurality of bits and a symbol in a constellation which is closest to the received symbol;

for each bit represented by the closest symbol, identify a nearest neighbor symbol that reverses the value of the respective bit based on information retrieved from a memory device;

for each bit represented by the closest symbol, subtract the calculated distance between the closest symbol and the received symbol from the distance between each respective bit'"'"'s nearest neighbor symbol and the received symbol to obtain soft bits for the received symbol; and

output the soft bits for use in decoding the bits represented by the closest symbol.

20. The program product of claim 19, wherein the program instructions are further operable to cause the device to:

retrieve polarity information related to the soft bits from the memory device for each bit represented by the closest symbol; and

apply the polarity information to the corresponding soft bits prior to outputting the soft bits.