System and method of generating soft bits

• US 8,184,745 B2
• Filed: 12/04/2008
• Issued: 05/22/2012
• Est. Priority Date: 12/04/2008
• Status: Expired due to Fees

• Current Assignee: Altobridge Limited (Singapore Technologies Engineering Limited)
• Original Assignee: Altobridge Limited (Singapore Technologies Engineering Limited)
• Inventors: Xie, Kai, Elyasi, Sattar
• Primary Examiner(s): LEE, SIU M
• Application Number:
  US12/328,359
• Publication Number:
  US 20100142649A1
• Time in Patent Office: 1,265 Days
• Field of Search: 375/320, 375/348, 375/262, 375/316, 375/329, 375/340, 375/341, 375/342
• 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 radio frequency (RF) signal from a communication link, the RF signal including a stream of transmitted symbols modulated onto a carrier, each of the transmitted symbols being selected from a constellation of base symbols to represent a plurality of encoded data bits, the receiver comprising:

a radio frequency (RF) module operable to produce a stream of received symbols derived from the stream of transmitted symbols included within the RF signal;

a demodulator operable to receive the stream of receive symbols and to produce a stream of soft bits derived from the stream of received symbols;

a memory device operable to store information related to a plurality of base symbols in the constellation, the information including a nearest-neighbor look-up table and a polarity look-up table for each data bit represented by a base symbol in the constellation; and

a channel decoder operable to receive the stream of soft bits from the demodulator and to produce a stream of decoded data bits derived from the stream of soft bits,

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

determines which of the plurality of base symbols in the constellation is a closest symbol to the received symbol, the closest symbol including a plurality of first data bits;

calculates a first distance between the received symbol and the closest symbol; and

for each first data bit included within the closest symbol, the demodulator;

identifies a nearest neighbor symbol from the nearest-neighbor look-up table stored in the memory device;

calculates a second distance between the received symbol and the nearest-neighbor symbol;

subtracts the first distance from the second distance to produce a first soft bit associated with the first data bit; and

applies a polarity value from the polarity look-up table stored in the memory device to the first soft bit, prior to including the first soft bit into the stream of soft bits to be delivered to the channel decoder.

2. The receiver of claim 1, wherein the demodulator is operable to produce the stream of soft bits 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 RF module is operable to down-convert the RF signal to a baseband signal.

6. The receiver of claim 5, wherein the RF module is operable to separate the baseband signal into an in-phase (I) component and a quadrature (Q) component.

7. The receiver of claim 1, wherein the demodulator determines an absolute value of each first soft bit prior to applying the polarity value from the polarity look-up table.

8. A communication system comprising:

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

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

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

a receiver operable to receive the modulated signal and to derive a stream of received symbols from the stream of transmitted symbols included within the modulated signal, the receiver including;

a demodulator operable to produce a stream of soft bits derived from the stream of received symbols;

a channel decoder operable to produce a stream of decoded data bits derived from the stream of soft bits; and

a memory device operable to store information related to a plurality of base symbols in the constellation, the information including a nearest-neighbor look-up table and a polarity look-up table for each data bit represented by a base symbol in the constellation,

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

determines which of the plurality of base symbols in the constellation is a closest symbol to the received symbol, the closest symbol including a plurality of first data bits;

calculates a first distance between the received symbol and the closest symbol; and

for each first data bit included within the closest symbol, the demodulator;

identifies a nearest neighbor symbol from the nearest-neighbor look-up table stored in the memory device;

calculates a second distance between the received symbol and the nearest-neighbor symbol;

subtracts the first distance from the second distance to produce a first soft bit associated with the first data bit, and

applies a polarity value from the polarity look-up table stored in the memory device to the first soft bit, prior to including the first soft bit into the stream of soft bits to be delivered to the channel decoder.

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

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

11. The communication system of claim 9, wherein the QAM modulation scheme is a modulation scheme based on the IEEE 802.16e standard.

12. The communication system of claim 8, wherein the transmitter further comprises a transmitting radio frequency (RF) module that up-converts the modulated signal and the receiver further comprises a receiving radio frequency (RF) module that down-converts converts the received modulated signal to a baseband signal including the stream of received symbols.

13. A method of decoding a communications signal having of a stream of transmitted symbols modulated onto a carrier, each of the transmitted symbols being selected from a constellation of base symbols to represent a plurality of encoded data bits, the method comprising:

receiving the communications signal;

deriving a stream of received symbols from the stream of transmitted symbols included within the communications signal;

for each received symbol in the stream of received symbols;

determining which base symbol in the constellation of base symbols has the minimum distance to that received symbol, the closest symbol including a plurality of first data bits;

measuring a first distance between the received symbol and the closest symbol; and

for each of the plurality of first data bits represented by the closest symbol;

identifying a nearest neighbor symbol that reverses the value of the first data bit from a nearest-neighbor look-up table; and

measuring a second distance between the received symbol and the nearest-neighbor symbol;

subtracting the first distance from the second distance to obtain a soft bit associated with the first data bit;

applying a polarity value from a polarity look-up table to the first soft bit; and

incorporating the first soft bit into a stream of soft bits; and

deriving a stream of decoded data bits from the stream of soft bits.

14. The method of claim 13, wherein the constellation of base symbols incorporates one of a phase-shift keying (PSK) modulation scheme and a Quadrature Amplitude Modulation (QAM) 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, wherein the communications signal is a radio frequency (RF) signal and receiving the communications signal comprises down-converting the RF signal to a baseband signal.

18. The method of claim 17, wherein down-converting the RF signal to a baseband signal further comprises separating the baseband signal into an in-phase (I) component and a quadrature (Q) component.

19. The method of claim 13, further comprising obtaining an absolute value of each first soft bit prior to applying the polarity value from the polarity look-up table.

20. A program product comprising a non-transitory 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 a received communication signals having a plurality of received symbols containing encoded data bits, to cause the device to:

calculate a first distance between each of the plurality of received symbols and a base symbol in a constellation which is closest to the received symbol, the closest symbol representing a plurality of first data bits;

for each first data bit represented by the closest symbol,

identify a nearest neighbor symbol that reverses the value of the first data bit based on a nearest-neighbor look-up table retrieved from a memory device;

calculate a second distance between the received symbol and the nearest-neighbor symbol;

subtract the first distance from the second distance obtain a soft bit associated with the first data bit;

apply a polarity value from a polarity look-up table stored in the memory device to the first soft bit; and

incorporate the first soft bit into a plurality of soft bits represented by the received symbol; and

output the plurality of soft bits for use in decoding the plurality of received symbols.