Method of hybrid automatic repeat request implementation for data transmission with multilevel coding

Abstract

A method of Hybrid Automatic Repeat Request implementation which efficiently combines received signals from multiple H-ARQ block transmission attempts encoded by the Multi-Level Coding approach with an uncoded subset of information bits, is presented. The method provides full error correction gains of the H-ARQ scheme and decoder computational complexity reduction due to transmission of uncoded bits that does not cause significant demodulator and signal processing complexity growths. The advantages are achieved via calculation of likelihood ratio metrics and the combination of at least two different data block transmission attempts for both encoded and uncoded bits of a data block. Additionally, the calculation of likelihood ratio metrics for uncoded bits is performed in consideration of the results of the decoding of the encoded bits. Receiver decisions are then determined on values of uncoded bits based on values of the combined likelihood ratio metrics for uncoded bits.

Claims

1. A method of Hybrid Automatic Repeat Request implementation for data transmission with Multi-Level Coding in which Forward Error Correction (FEC) encoded and uncoded subsets of bits are selected in accordance with different levels of noise immunity in a multi-level Quadrature Amplitude Modulation (QAM) symbol, such that the encoded bits are mapped onto less noise immune bits of a QAM symbol and the uncoded bits are mapped onto more noise immune bits of the QAM symbol, the method comprising: a. performing a first transmission of a data block containing N signal samples with a portion of bits of the data block encoded with a FEC code due to having lower noise immunity and another portion of bits of the data block remaining uncoded due to having higher noise immunity; b. receiving the first transmission of the data block and performing demodulation, FEC decoding of the encoded bit portion, and performing hard decisions on the uncoded bit portion of the received data block; c. checking the received data block for errors and sending a retransmission request to the transmitter if errors are detected; d. performing a second transmission of the same data block upon receipt of a retransmission request; and e. receiving the second transmission of the data block and performing demodulation, FEC decoding of the encoded bit portion, and performing hard decisions on the uncoded bit portion of the received data block accompanied by combining the information obtained after reception of the first and the second transmissions of the data block; wherein receiving the first transmission of a data block comprises: b.1. a first demodulation of signal samples of the first transmission and calculation of likelihood ratio metrics for the encoded bits; b.2. FEC decoding of the encoded bits using the calculated likelihood ratio metrics; b.3. a second demodulation of signal samples of the first transmission and calculation of likelihood ratio metrics for the uncoded bits using the results of the FEC decoding of the encoded bits as a priori information; and b.4. performing the hard decisions on the transmitted uncoded bits using the calculated likelihood ratio metrics for the uncoded bits; wherein combining the information obtained after reception of the first and the second transmissions of the data block comprises: d.1. a first demodulation of signal samples of the second transmission and calculation of likelihood ratio metrics for the encoded bits; d.2. combining the likelihood ratio metrics calculated for the encoded bits of the first and the second transmissions of the data block; d.3. FEC decoding of the encoded bits using the combined likelihood ratio metrics for the encoded bits; d.4. the second demodulation of signal samples of the second transmission and calculation of likelihood ratio metrics for the uncoded bits using the results of the FEC decoding of the encoded bits as a priori information; d.5. combining the likelihood ratio metrics calculated for the uncoded bits of the first and the second transmissions of the data block; and d.6. performing the hard decisions on the transmitted uncoded bits using the combined likelihood ratio metrics for the uncoded bits.

2. The method of claim 1, wherein the first and the second transmissions of a data block are two consecutive transmissions in a sequence of two or more transmissions of the same data block.

3. The method of claim 1, wherein likelihood ratio metrics for encoded and uncoded bits are calculated in the logarithmic scale.

4. The method of claim 3, wherein a piecewise linear approximation is used to calculate likelihood ratio metrics in the logarithmic scale as a function of a received signal sample.

5. The method of claim 3, wherein a combination of likelihood ratio metrics in the logarithmic scale consists in their algebraic addition.

6. The method of claim 3, wherein the performing hard decisions on the transmitted uncoded bits consists of determining a sign of a likelihood ratio metric in the logarithmic scale.

7. The method of claim 1, wherein encoded and uncoded bits are modulated using the Ungerboeck modulation.

8. The method of claim 1, wherein a block code is used to encode bits and encoded bits of a data block are divided into equal groups which are encoded and decoded independently.

9. The method of claim 1, wherein a Low-Density Parity Check code is used to encode bits.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present invention will become apparent from the following description of the disclosed embodiments with reference to accompanying drawings.

(2) FIG. 1 (Prior Art): depicts a conventional scheme of multi-level decoding of a received signal;

(3) FIG. 2 (Prior Art): depicts a structure of the apparatus disclosed in US application 2010/0091909;

(4) FIG. 3: depicts data transmission scheme 100 with H-ARQ, in accordance with various embodiments of the present disclosure;

(5) FIG. 4: depicts a functional diagram of transmitter 200, in accordance with various embodiments of the present disclosure;

(6) FIG. 5: depicts a functional diagram of receiver 300, in accordance with various embodiments of the present disclosure;

(7) The following reference numerals denote various features of the figures: 101: a source of information; 102: a data block generation module; 103: a transmitter; 104: a data transmission channel; 105: a receiver; 106: a data block error checking module; 107: a feedback control channel; 108: a recipient of information; 201: a transmitted data block; 202: a module of data separation into the encoded and uncoded parts; 203: an encoder; 204: a modulator of encoded bits; 205: a modulator of uncoded bits; 206: a resulting modulated signal; 301: a received signal; 302: a demodulator of encoded bits; 303: a module for combining LLR values of encoded bits from multiple transmissions; 304: a decoder; 305: a read and write controller for the combined LLR values of encoded bits; 306: memory; 307: a demodulator of uncoded bits; 308: a module for combining LLR values of uncoded bits from multiple transmissions; 309: a threshold device; 310: a read and write controller for the combined LLR values of uncoded bits; 311: memory; 312: a data combining module; and 313: a resulting bit sequence of a data block.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

(8) The instant disclosure is directed to address the noted deficiencies of the current state of the art. Accordingly, a description of the embodiments of the proposed method of Hybrid Automatic Retransmission Request (H-ARQ) implementation for data transmission with Multi-Level Coding (MLC) is presented. The presented H-ARQ with MLC method is configured to allow for coded and uncoded subsets of bits to be selected in accordance with different levels of noise immunity in a multi-level QAM symbol, such that the coded bits are mapped onto less immune bits of a symbol and the uncoded bits are mapped onto more immune bits of a symbol.

(9) The presented method includes: (a) performing the first transmission of a data block with a part of bits encoded with a forward error correction (FEC) code and with the other part of bits uncoded; (b) receiving the first transmission of a data block, performing demodulation and decoding of the encoded part of bits of the received data block; (c) checking the received data block for errors and sending a retransmission request to the transmitter if errors are detected; (d) performing the second transmission of the same data block upon receipt of a retransmission request; and (e) receiving the second transmission of the data block, performing demodulation and decoding of the encoded part of bits of the received data block accompanied by combining the information obtained after reception of the first and the second transmissions of the data block.

(10) The transmitted data block b={b.sub.0, b.sub.1, . . . b.sub.N−1} with the size of N bits is divided into two bit sequences b.sub.c and u representing the encoded and uncoded subsets of the block bits. A size of the encoded sequence is N.sub.c=N.sub.symb.Math.m.sub.c.Math.R, a size of the uncoded sequence is N.sub.u=N.sub.symb.Math.m.sub.u provided that N=N.sub.c+N.sub.u, where N.sub.symb is the number of signal samples (symbols), m.sub.c and m.sub.u are the numbers of encoded and uncoded bits of a single signal sample respectively, R is a code rate for the encoded bits. The bit sequence b.sub.c is converted by a FEC code with a rate of R into an encoded bit sequence c={c.sub.0, c.sub.1, . . . c.sub.Ncod−1} with the length of N.sub.cod=N.sub.symb.Math.m.sub.c. The encoded sequence c is divided into N.sub.symb blocks c(k), k=0, . . . , N.sub.symb−1, each of them has m.sub.c bits provided that each block is used to modulate a separate signal sample. The uncoded sequence u is divided into N.sub.symb blocks u(k), k=0, . . . , N.sub.symb−1, each of them has m.sub.u bits provided that each block is used to modulate a separate signal sample.

(11) The modulation of each signal sample s(k), k=0, . . . , N.sub.symb−1 is performed as follows: a block of the encoded sequence c(k) is mapped onto a intermediate signal sample s.sub.g(k) using the Gray code; a block of the uncoded sequence u(k) is mapped onto an offset signal s.sub.u(k) using the Ungerboeck code; and the resulting signal sample s(k) is calculated by adding the offset s.sub.u(k) to the signal sample s.sub.g(k), as represented below:
s(k)=s.sub.g(k)+s.sub.u(k).

(12) After modulation of all N.sub.symb signal samples of the block, the signal s={s(0), s(1), . . . , s(N.sub.symb−1)} is transmitted through a communication channel. The transmitted signal s is the same for both the first and the second transmissions of the data block.

(13) The demodulation and decoding of the received signal r={r(0), r(1), . . . r(N.sub.symb−1)} corresponding to the transmitted signal s for both the first and the second transmissions include demodulation of signal samples and calculation of likelihood ratio metrics for the encoded bits; decoding the encoded bits using the calculated metrics, demodulation of signal samples and calculation of likelihood ratio metrics for the uncoded bits using the results of the decoding of the encoded bits; and making decisions on values of the uncoded bits using the calculated likelihood ratio metrics for the uncoded bits.

(14) As such, the demodulation of the encoded bits c(k) mapped onto less immune bits of multi-level QAM symbols is independently performed for each received signal sample r(k), k=0, . . . , N.sub.symb−1. In a representative embodiment, a likelihood ratio metric in the logarithmic scale (LLR) is calculated for each bit c(k).sub.i from c(k), i=0, . . . , m.sub.c−1, as noted below:

(15) $\mathrm{LLR}\left({c\left(k\right)}_i\right)=\log \frac{\underset{\alpha \in S_i^{\left(1\right)}}{.Math.}p\left(r\left(k\right)|s\left(k\right)=\alpha \right)}{\underset{a\in S_i^{\left(0\right)}}{.Math.}p\left(r\left(k\right)|s\left(k\right)=\alpha \right)},$

(16) where S.sub.i.sup.(1) and S.sub.i.sup.(0) denote subsets of a set of all possible values of a transmitted signal sample s with the i-th encoded bit defined as c.sub.i=1 and c.sub.i=0 respectively, p( ) denotes a conditional (a posteriori) probability density function.

(17) In another embodiment, a piecewise linear approximation is applied to calculate likelihood ratio metrics in the logarithmic scale as a function of a received signal sample which can be expressed as:

(18) $LLR\left({c\left(k\right)}_i\right)=\frac{1}{2{\sigma }^2}\left(\underset{\alpha \in S_i^{\left(0\right)}}{\min }{.Math.r\left(k\right)-\alpha .Math.}^2-\underset{\alpha \in S_i^{\left(1\right)}}{\min }{.Math.r\left(k\right)-\alpha .Math.}^2\right),$

(19) where σ.sup.2 additionally denotes the noise variance in a received signal sample r(k).

(20) The set of N.sub.symb.Math.m.sub.c calculated metrics for all bits of the encoded sequence c is used for decoding. Decisions of the receiver on values of the bits in the b.sub.c and c sequences that are denoted by b.sub.c.sup.(est) and c.sup.(est) respectively are expressed in the results of the decoding.

(21) The demodulation of the uncoded bits u(k) mapped onto high immune bits of multi-level QAM symbols is also independently performed for each received signal sample r(k), k=0, . . . , N.sub.symb−1. In a representative embodiment, an LLR is calculated for each bit u(k).sub.i from u(k), i=0, . . . , m.sub.u−1, as follows:

(22) $\mathrm{LLR}\left({u\left(k\right)}_j\right)=\log \frac{\underset{\alpha \in S_j^{\left(1\right)}\left({c\left(k\right)}^{\left(\mathrm{est}\right)}\right)}{.Math.}p\left(r\left(k\right)|s\left(k\right)=\alpha \right)}{\underset{a\in S_j^{\left(0\right)}\left({c\left(k\right)}^{\left(\mathrm{est}\right)}\right)}{.Math.}p\left(r\left(k\right)|s\left(k\right)=\alpha \right)},$

(23) where S.sub.j.sup.(1)(c(k).sup.(est)) and S.sub.j.sup.(0)(c(k).sup.(est)) denote subsets of a set of all possible values of a transmitted signal sample s with the j-th uncoded bit defined as u.sub.j=1 and u.sub.j=0 respectively and m.sub.c values of the encoded bits are equal to the values of bits in c(k).sup.(est) for the current k-th signal sample.

(24) In one embodiment, a piecewise linear approximation is applied to calculate likelihood ratio metric in the logarithmic scale as a function of a received signal sample which can be expressed as:

(25) $LLR\left({u\left(k\right)}_j\right)=\frac{1}{2{\sigma }^2}\left(\underset{\alpha \in S_j^{\left(0\right)}\left({c\left(k\right)}^{\left(\mathrm{est}\right)}\right)}{\min }{.Math.r\left(k\right)-\alpha .Math.}^2-\underset{\alpha \in S_j^{\left(1\right)}\left({c\left(k\right)}^{\left(\mathrm{est}\right)}\right)}{\min }{.Math.r\left(k\right)-\alpha .Math.}^2\right).$

(26) In another representative embodiment, making decisions on values of the uncoded bits is achieved by determining a sign of an LLR for the uncoded bits. For the above notations the positive sign of an LLR(u(k).sub.j) corresponds to the bit value of 1 and the negative sign corresponds to the bit value of 0.

(27) The sequence of decisions u.sup.(est) for all the N.sub.symb.Math.m.sub.u uncoded bits and the sequence of decisions b.sub.c.sup.(est) for the encoded bits are combined into a sequence of decisions for the entire data block b.sup.u(est).

(28) A combination of LLR metrics for the encoded bits LLR(c(k).sub.i) and the uncoded bits LLR(u(k).sub.j) calculated for the first and the second data block transmissions after the second data block transmission is performed independently for each bit via their algebraic addition. The combined metrics for the encoded bits are then used for decoding of the encoded bits and the combined metrics for the uncoded bits are then used for making decisions on values of the uncoded bits.

(29) The first and the second transmissions of a data block in the above description are two consecutive transmissions in a sequence of two or more transmissions of the same data block.

(30) A general scheme of data transmission using MLC and H-ARQ 100 is provided by FIG. 3, in accordance with the embodiments of the present disclosure. The scheme 100 provides the following operational steps: data is transmitted from a source of information 101 to a data block generation module 102 where a checksum for integrity verification is added. The generated data block is then provided to transmitter 103 to generate a signal sequence. The signal sequence is transmitted through a data transmission channel 104 to a receiver 105 performing demodulation of the sequence and data decoding.

(31) A data block error checking module 106 then performs error checks in the data block using the checksum. If data block error checking module 106 detects no errors, the received block is transmitted to a recipient of information 108 and an acknowledgment (ACK) is sent over a feedback control channel 107. If errors are detected, a negative acknowledgment (NACK) is sent over the feedback channel 107.

(32) Upon receiving a NACK, the transmitter 103 then retransmits the same signal sequence through the data transmission channel 104 to receiver 105. The receiver 105 performs a combination of the information of the first and the second transmissions of the data block and performs demodulation and decoding of the data again. Then, the data block error checking module 106 performs error checks in the decoded data block again and the procedure is repeated until the block is correctly received or the maximum number of retransmissions of the data block is reached.

(33) A functional diagram of the transmitter unit 200 configuration is provided by FIG. 4, in accordance with the embodiments of the present disclosure. As shown, the transmitter unit configuration 200 comprises a data separation module 202 that separates the transmitted data block 201 into encoded and uncoded portions, an encoder 203 configured to encode the encoded portion of data block bits, a modulator 204 configured to modulate encoded bits 204 to intermediate Gray coded signal samples, and a modulator 205 configured to convert the intermediate Gray coded signal samples of the encoded bits into a resulting transmitted modulated signal by adding an offset signal of the uncoded portion of data bits 206.

(34) A functional diagram of the receiver unit 300 configuration is provided by FIG. 5, in accordance with the embodiments of the present disclosure. As shown, the receiver unit configuration 300 accepts a received signal 301 that has been transmitted through a communication channel The received signal 301 is processed by demodulator 302 configured to calculate likelihood ratio (LLR) metrics of the transmitted encoded bits. The LLR metric values of transmitted encoded bits for multiple transmissions are then combined by a first aggregating unit 303 and supplied to a first read/write controller 305 for storage into a first memory 306 associated with LLR metric values of the encoded portion of transmitted data bits.

(35) The combined LLR metric values for encoded bits provided by first aggregating unit 303 are also supplied to decoder 304 configured to decode a sequence of LLR metric values of encoded bits, in which the decoded values of encoded bits are forwarded to demodulator 307 configured to demodulate uncoded bits and calculate the LLR metric values of uncoded bits. The LLR metric values of the uncoded bits for multiple transmissions are then combined by a second aggregating unit 308 and supplied to a second read/write controller 310 for storage into memory 311, associated with LLR metric values of uncoded portion of transmitted data bits.

(36) The combined LLR metric values for uncoded bits provided by second aggregating unit 308 are also supplied to threshold device 309 configured to perform decisions on values of uncoded bits based on the values of the combined LLR metrics and data combining module 312 configured to restore the resulting bit sequence into data block 313 according to the separation of bits performed by transmitter unit configuration 200.

(37) In this embodiment, the combined LLR metric values of encoded bits from multiple transmissions provided by first aggregating unit 303 and the combined LLR values metric values of uncoded bits from multiple transmissions provided by second aggregating unit 308 are performed during the second and subsequent transmissions of a data block. For the first transmission, the corresponding LLR metric values that are calculated by the respective demodulators are passed to decoder 304 and threshold device 309 and subsequently written into the memory without changes.

(38) In a representative embodiment, a block code is used to encode bits and the bits of a data block to be encoded are divided into equal groups encoded and decoded by decoder 304 independently.

(39) In another representative embodiment, a Low-Density Parity Check (LDPC) code applied in many communication systems may also be used to encode bits that makes the claimed method applicable to modern multi-Gigabit data transmission networks.

(40) In the embodiments presented above, data processing by receiving units do not require additional computational operations with signal samples. The information combining for uncoded bits of the first and the second transmissions of a data block is performed by operations of calculation and addition of LLR metrics for uncoded bits which have complexity not exceeding the complexity of a standard demodulator for encoded bits. In addition, in specific embodiments, the same receiver computational resources may be used for the calculation of LLR metrics for the encoded and uncoded bits. At the same time, gains in error correction due to H-ARQ usage and gains in the decoder computational resources due to transmission of uncoded bits are fully realized in the described method.

(41) Therefore, the described method provides a reduction of receiver computational complexity for joint implementation of H-ARQ and MLC with an uncoded subset of information bits while keeping all advantages of the said schemes.

(42) Although the present invention has been described with reference to specific features and embodiments thereof, it is evident that various modifications and combinations can be made thereto without departing from the invention. The specification and drawings are, accordingly, to be regarded simply as an illustration of the invention as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present invention.