Linearly approximated log map algorithm for turbo decoding

Abstract: A method and apparatus for implementing a linearly approximated Log MAP algorithm, said implementation involves calculating MAX*(a(n),b(n)) function, said method comprising the steps of: (A) Receiving a(n), b(n) and DE; (B) calculating (a(n)+b(n)+DE)/2 and generating at least one intermediate result, said at least one intermediate result reflecting at least one relationship between at least two elements out of a(n), b(n) and DE; and (C) providing an MAX*(a(n),b(n)) result selected from a group comprising of a(n), b(n) or (a(n)+b(n)+DE)/2, said selection dependent upon the at least one intermediate result.

Description: FIELD OF THE INVENTION Apparatus and method for implementing a linearly approximated Log MAP algorithm and especially an apparatus and method for implementing a linearly approximated Log MAP algorithm for turbo decoding and turbo equalization. BACKGROUND OF THE INVENTION Turbo Coding (i.e.- TC) is used for error control coding in digital communications and signal processing. The following references give some examples of various implementations of the TC: "Near Shannon limit error correcting coding and decoding: turbo-codes", by Berrou, Glavieux, Thitimajshima, IEEE International Conference of Communication. Geneva Switzerland, pp. 1064-1070, May 1993; "Implementation and Performance of a Turbo/MAP Decoder", Pietrobon, International Journal of Satellite Communication; "Turbo Coding", Heegard and Wicker, Kluwer Academic Publishers 1999. MAP algorithm and soft output Viterbi algorithm (SOVA) are Soft Input Soft Output (i.e.- SISO) decoding algorithms that have gained wide acceptance in the area of communications. Both algorithms are mentioned in U.S patent 5,933,462 of Viterbi et al. The TC has gained wide acceptance in the area of communications, such as in cellular networks, modems, and satellite communications. Some turbo encoders consists of two parallel-concatenated systematic convolutional encoders separated by a random interleaver. A turbo decoder has two soft-in soft-out (SISO) decoders. The output of the first SISO is coupled to the input of the second SISO via a first interleaver, while the output of the second SISO is coupled to an input of the first SISO via a feedback loop that includes a deinterleaver. A common SISO decoder uses either a maximum a posteriori (i.e.- MAP) decoding algorithm or a Log MAP decoding algorithm. The latter algorithm is analogues to the former algorithm but is performed in the logarithmic domain. Another common decoding algorithm is the max log MAP algorithm. The max log MAP is analogues to the log MAP but the implementation of the former involves an addition of correction factor. Briefly, the MAP finds the most likely information bit to have been transmitted in a coded sequence. The output signals of a convolutional encoder are transmitted via a channel and are received by a receiver that has a turbo decoder. The channel usually adds noise to the transmitted signal. During the decoding process a trellis of the possible states of the coding is defined. The trellis includes a plurality of nodes (states), organized in T stages, each stage has N=2sup(K-1) nodes, whereas T being the number of received samples taken into account for evaluating which bit was transmitted from a transmitter having the convolutional encoder and K is the constraint length of the code used for encoding. Each stage is comprised of states that represent a given time. Each state is characterized by a forward state metric, commonly referred to as alpha (α or a) and by a backward state metric, commonly referred to as beta (β or b) . Each transition from a state to another state is characterized by a branch metric, commonly referred to as gamma (γ). Alphas, betas and gammas are used to evaluate a probability factor that indicates which signal was transmitted. This probability factor is commonly known as lambda (Λ). A transition from a stage to an adjacent stage is represented by a single lambda. A function MAX*(a(n),b(n)) is frequently used when alphas, betas and gammas are calculated. Conveniently, said calculation involves in a comparison of a(n) and b(n). Said elements a(n) and b(n) usually have real values. MAX*(a(n), b(n)) equals MAX(a(n), b(n)) + Log(1+EXP{-|a(n)-b(n)|}). The first portion of said equation is usually calculated. The calculation of the second portion is relatively complicated and time consuming. Usually an approximation of said second portion is calculated. Said approximation is either a linear approximation, a step approximation or a multi step approximation. A linear approximation method is described in "Linearity Approximated for Log-MAP Algorithms for Turbo Decoding", by Jung-Fu Cheng and Tony Ottoson, 51 st IEEE Vehicular Technology Conference Proceedings, 13-15 May 2000, pages 2252-2256. Said method suggests to approximate the second portion by a the following functions: MAX{0, (C-|a(n)-b(n)|/4} and by : MAX{0, (C-|a(n)-b(n)|/8}. Both estimations provide relatively poor performances when |a(n)-b(n)| is relatively small, and their implementation is relatively time consuming. Another simplified linear approximation method is described in "Simplified Log-MAP Algorithm", Research Disclosure, No. 421, XP888685, May 1999, page 612. There is a need for providing a fast and high performance apparatus and method for implementing a linearly approximated Log MAP algorithm. BRIEF DESCRIPTION OF THE DRAWINGS While the invention is pointed out with particularity in the appended claims, other features of the invention are disclosed by the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1. is a schematic description of a portion of a MAX*(a(n),b(n)) function and a linear approximation of said function, according to a preferred embodiment of the invention; FIGS. 2-3 are schematic diagrams illustrating two apparatuses for implementing a linearly approximated Log MAP algorithm, according to a preferred embodiment of the invention; and FIG. 4 is a flow chart illustrating a method for implementing a linearly approximated Log MAP algorithm, according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS It should be noted that the particular terms and expressions employed and the particular structural and operational details disclosed in the detailed description and accompanying drawings are for illustrative purposes only and are not intended to in any way limit the scope of the invention as described in the appended claims. The invention provides a fast and high performance apparatus and method for implementing a linearly approximated Log MAP algorithm. Said approximation is a linear approximation having a slope of -0.5, said slope providing an improved approximation when a(n) is substantially equal to b(n). Conveniently, method is executed during a single clock cycle. The invention provides a fast and high performance apparatus and method for implementing a linearly approximated Log MAP algorithm. The invention provides a method for implementing a linearly approximated Log MAP algorithm, said implementation involves calculating MAX*(a(n),b(n)) function, said method comprising the steps of: (A) Receiving a(n), b(n) and DE; (B) Calculating (a(n)+b(n)+DE)/2 and generating at least one intermediate result, said at least one intermediate result reflecting at least one relationship between at least two elements out of a(n), b(n) and DE; and (C) Providing an MAX*(a(n),b(n)) result selected from a group comprising of a(n), b(n) or (a(n)+b(n)+DE)/2, said selection dependent upon the at least one intermediate result. Conveniently, the linearly approximated Log MAP algorithm is implemented in either an iterative decoding process or in an iterative equalization process. The invention provides a method for implementing a linearly approximated Log MAP algorithm wherein a first intermediate result indicates whether an absolute value of a difference between a(n) and b(n) is greater than DE. A second intermediate result indicates whether a(n) is greater than b(n). Conveniently, the at least one intermediate result comprising of either a sign(a(n)-b(n)-DE) signal and a sign(b(n)-a(n)-DE) signal or of a sign(a(n)-b(n)) signal and a sign(|a(n)-b(n)|-DE) signal. The invention provides a method for implementing a linearly approximated Log MAP algorithm wherein the MAX*(a(n),b(n)) result equals (I) MAX(a(n),b(n)) when |a(n)-b(n)| is not smaller than DE, and (II) (a(n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. |a(n)-b(n)| is smaller than DE when (a(n)-b(n)-DE) < 0 and (-a(n)+b(n)-DE) < 0. The invention provides a method for implementing a linearly approximated Log MAP algorithm wherein the MAX*(a(n),b(n)) result equals (I) a(n) when (a(n)-b(n)-DE) > 0 and (b(n)-a(n) -DE) < 0; (II) b(n) when (a(n)-b(n)-DE) < 0 and (b(n)-a(n) -DE) > 0; and (III) (a(n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. The invention provides an apparatus for implementing a linearly approximated Log MAP algorithm, said implementation involves calculating MAX*(a(n),b(n)) function, said apparatus comprising of: (A) Calculating means for receiving a(n), b(n) and DE; for Calculating (a(n)+b(n)+DE)/2 and for generating at least one intermediate result, said at least one intermediate result reflecting at least one relationship between at least two elements out of a(n), b(n) and DE; and (B) Selecting means for providing an MAX*(a(n),b(n)) result selected from a group comprising of a(n), b(n) or (a(n)+b(n)+DE)/2, said selection dependent upon the at least one intermediate result. Conveniently, the linearly approximated Log MAP algorithm is implemented in an iterative decoding process or in an iterative equalization process. The invention provides an apparatus for implementing a linearly approximated Log MAP algorithm wherein a first intermediate result indicates whether an absolute value of a difference between a(n) and b(n) is greater than DE and a second intermediate result indicates whether a(n) is greater than b(n). Conveniently, the at least one intermediate result comprising of a sign(a(n)-b(n)-DE) signal and a sign(b(n)-a(n)) signal. The at least one intermediate result comprising of a sign(a(n)-b(n)) signal and a sign(|a(n)-b(n)|-DE) signal. The invention provides an apparatus for implementing a linearly approximated Log MAP algorithm that is adapted to calculate the MAX*(a(n),b(n)) function during a single clock cycle. The invention provides an apparatus for implementing a linearly approximated Log MAP algorithm wherein the MAX*(a(n),b(n)) result equals MAX(a(n),b(n)) when |a(n)-b(n) | is not smaller than DE, and equals (a (n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. The invention provides an apparatus for implementing a linearly approximated Log MAP algorithm wherein the MAX*(a(n), b(n)) result equals a(n) when (a(n)-b(n)-DE) > 0 and (b(n)-a(n) -DE) < 0; wherein the MAX*(a(n), b(n)) result equals b(n) when (a(n)-b(n)-DE) < 0 and (b(n)-a(n) -DE) > 0; and wherein the MAX*(a(n), b(n)) result equals (a(n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. For convenience of explanation, the following description relates to turbo decoding. The invention is not limited to said decoding method. For example said invention is also applied in various detection processes such as equalization processes. |a(n)-b(n)| is an absolute value of a difference between a(n) and b(n). EXP{-|a(n)-b(n)|} equals e by the power of -|a(n)-b(n)|. FIG. 1 is a graph illustrating a portion of a MAX*(a(n), b(n)) versus an absolute value of a difference between a(n) and b(n). Said graph further illustrates a linear approximation AP(n) of said portion. Said portion equals Log(1+EXP{-|a(n)-b(n)|}). Curved line 4 illustrates portion Log(1+EXP{-|a(n)-b(n)|}), while line 5 illustrates a linear approximation of said portion. The slope of said linear approximation substantially equals - 0.5. AP(n) = MAX{0,(DE-(a(n)-b(n))/2)}. Preferably DE=2*LOG(2). Said linear approximation provides superior performances when the difference between a(n) and b(n) is very small. Furthermore, such an approximation allows to speed the calculation of the MAX* function, as further illustrated in Figs. 2-3. Referring to Fig. 2 illustrating an apparatus 10 for implementing a linearly approximated Log MAP algorithm, according to a preferred embodiment of the invention. Apparatus 10 implements the following equations : [1] MAX*(a(n), b(n)) = MAX(a(n),b(n)) when |a(n)-b(n)| is not smaller than DE. [2] MAX*(a(n), b(n)) = (a(n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. Apparatus 10 allows to calculate in parallel the following values: MAX(A(n), b(n)); (a(n)+b(n)+DE)/2 and the sign of (|a(n)-b(n)| -DE) and then to determine, according to said sign) which of the first two values to provide as a result of the calculation. Accordingly, apparatus 10 comprises of first means 11 for calculating the following values: MAX(A(n), b(n)); (a(n)+b(n)+DE)/2 and the sign of (|a(n)-b(n)| -DE) and of a second means 19 to determine which of the first two elements to provide as a result, according to said sign. First means 11 comprising of first to third subtracting units 12-14, adder 15 and two multiplexers 16 and 17. First and second subtracting units 12 - 13 and adder 15 receive a(n), b(n) and DE. Third subtracting unit 14 receives a(n) and b(n) and generates sign(a(n)-b(n)). Sign(a(n)-b(n)) is provided a a control signal to first and second multiplexers 16 and 17. Second multiplexer 17 receives a(n) and b(n) and selects the greater element out of them, in response to sign(a(n)-b(n)). Therefore, second multiplexer 17 generates MAX(a(n), b(n)) and provide it to second means 19. Adder 15 receives a(n), b(n) and DE and provides to second means 19 (a(n)+b(n)+DE)/2. The division by two is a shift of the bit position by one. Second means receives a control signal sign(|a(n)-b(n)|-DE) and accordingly selects whether to provide MAX(a(n),b(n)) provided by second multiplexer 17 or (a(n)+b(n)+DE)/2, provided by first multiplexer 16. First subtracting unit 12 receives a(n), b(n) and DE and generates sign(b(n)-a(n)-DE). Second subtracting unit 13 receives a(n), b(n) and DE and generates sign(a(n)-b(n)-DE). Both sign signals are provided to first multiplexer 16, that selects the appropriate sign signal according to sign(a(n)-b(n)) provided by third subtracting unit 14. Conveniently, apparatus 10 performs said calculation in a single clock cycle. Referring to Fig. 3 illustrating an apparatus 20 for implementing a linearly approximated Log MAP algorithm, according to a preferred embodiment of the invention. Apparatus 20 implements the following equations: [3] MAX*(a(n), b(n)) = a(n) when (a(n)-b(n)-DE) > 0 and (b(n)-a(n) -DE) < 0. [4] MAX*(a(n), b(n)) = b(n) when (a(n)-b(n)-DE) < 0 and (b(n)-a(n) -DE) > 0. [5] MAX*(a(n), b(n)) = (a(n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. Apparatus 20 allows to calculate in parallel the following values: (a(n)+b(n)+DE)/2, the sign of (a(n)-b(n) -DE) and the sign of (b(n)-a(n)-DE) and to determine, according to said values which of following values to provide as a result of the calculation : a(n), b(n) and (a(n)+b(n)+DE)/2. Accordingly, apparatus 20 comprises of first means 21 for calculating the following values: (a(n)+b(n)+DE)/2, the sign of (a(n)-b(n) -DE) and sign of (b(n)-a(n)-DE) and a second means 29 to determine which of the following elements to provide (a(n), b(n) or (a(n)+b(n)+DE)/2) as a result, according to said sign values. First means 21 comprising of first to second subtracting units 22-23, adder 24. First and second subtracting units 22 - 23 and adder 24 receive a(n), b(n) and DE. Adder 24 receives a(n), b(n) and DE and provides to second means 29 (a(n)+b(n)+DE)/2. The division by two is a shift of the bit position by one. First subtracting unit 22 generates sign(b(n)-a(n)-DE). Second subtracting unit 23 generates sign(a(n)-b(n)-DE). Both sign signals are provided to control inputs of second means 29. Second means 29 is conveniently a multiplexer that receives a(n), b(n) and (a(n)+b(n)+DE)/2 and selects which signal to provide according to said control signals. Conveniently, apparatus 20 performs said calculation in a single clock cycle. FIG. 4 is a flow chart illustrating a method 30 for implementing a linearly approximated Log MAP algorithm. Method 30 starts at step 31 of receiving a(n) and b(n). Step 31 is followed by step 32 of calculating (a(n)+b(n)+DE)/2 and determining at least one intermediate result reflecting at least one relationship between a(n), b(n) and DE. Step 32 is followed by step 33 of providing a one of the values of a(n), b(n) or (a(n)+b(n)+DE)/2, dependent upon the at least one intermediate result. It should be noted that the particular terms and expressions employed and the particular structural and operational details disclosed in the detailed description and accompanying drawings are for illustrative purposes only and are not intended to in any way limit the scope of the invention as described in the appended claims. Thus, there has been described herein an embodiment including at least one preferred embodiment of an apparatus and method for implementing a fast Log-MAP (Maximum-a- Posteriori) algorithm for turbo decoding and turbo equalization. It will be apparent to those skilled in the art that the disclosed subject matter may be modified in numerous ways and may assume many embodiments other than the preferred form specifically set out and described above.

Claims: A method for implementing a linearly approximated Log MAP algorithm involving the calculation of a MAX* (a(n),b(n)) function, said method comprising the steps of: receiving a(n), b(n) and a value DE; calculating (a(n)+b(n)+DE)/2 and generating at least one intermediate result, said at least one intermediate result reflecting at least one relationship between at least two elements out of a(n), b(n) and DE; and providing an MAX*(a(n),b(n)) result selected from a group comprising of a(n), b(n) or (a(n)+b(n)+DE)/2, said selection dependent upon the at least one intermediate result. An iterative decoding method including the linearly approximated Log MAP algorithm implementing method of claim 1. An iterative equalization method including the linearly approximated Log MAP algorithm implementing method of claim 1. The method of claim 1 wherein a first intermediate result indicates whether an absolute value of a difference between a(n) and b(n) is greater than DE. The method of claim 1 a second intermediate result indicates whether a(n) is greater than b(n). The method of claim 5 wherein a first intermediate result indicates whether an absolute value of a difference between a(n) and b(n) is greater than DE. The method of claim 1 wherein the at least one intermediate result comprising of a sign(a(n)-b(n)-DE) signal and a sign(b(n)-a(n)-DE) signal. The method of claim 1 wherein the at least one intermediate result comprising of a sign(a(n)-b(n)) signal and a sign(|a(n)-b(n)|-DE) signal. The method of claim 1 wherein said method is executed during 2 or 3 clock cycles. The method of claim 1 wherein the MAX*(a(n),b(n)) result equals MAX(a(n),b(n)) when |a(n)-b(n)| is not smaller than DE, and equals (a(n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. The method of claim 1 wherein the MAX*(a(n), b(n))result equals a(n) when (a(n)-b(n)-DE) > 0 and (b(n)-a(n) -DE) < 0; wherein the MAX*(a(n), b(n)) result equals b(n) when (a(n)-b(n)-DE) < 0 and (b(n)-a(n) -DE) > 0; and wherein the MAX*(a(n), b(n)) result equals (a(n) +b (n) +DE)/2 when (a(n)-b(n)-DE) < 0 and (b(n)-a(n)-DE) < 0. An apparatus for implementing a linearly approximated Log MAP algorithm involving the calculation of a MAX* (a(n),b(n)) function, said apparatus comprising: calculating means for receiving a(n), b(n) and a value DE; means for calculating (a(n)+b(n)+DE)/2 and for generating at least one intermediate result, said at least one intermediate result reflecting at least one relationship between at least two elements out of a(n), b(n) and DE; and selecting means for providing an MAX*(a(n),b(n)) result selected from a group comprising of a(n), b(n) or (a(n)+b(n)+DE)/2, said selection dependent upon the at least one intermediate result. An iterative decoder comprising linearly approximated Log MAP algorithm implementing apparatus of claim 12. An iterative equalizer comprising the linearly approximated Log MAP algorithm implementing apparatus of claim 12. The apparatus of claim 12 wherein a first intermediate result indicates whether an absolute value of a difference between a(n) and b(n) is greater than DE. The apparatus of claim 12 a second intermediate result indicates whether a(n) is greater than b(n). The apparatus of claim 16 wherein a first intermediate result indicates whether an absolute value of a difference between a(n) and b(n) is greater than DE. The apparatus of claim 12 wherein the at least one intermediate result comprising of a sign(a(n)-b(n)-DE) signal and a sign(b(n)-a(n)) signal. The apparatus of claim 12 wherein the at least one intermediate result comprising of a sign(a(n)-b(n)) signal and a sign(|a(n)-b(n)|-DE) signal. The apparatus of claim 12 wherein said apparatus calculates the MAX*(a(n),b(n)) function during a single clock cycle. The apparatus of claim 12 wherein the MAX*(a(n),b(n)) result equals MAX(a(n),b(n)) when |a(n)-b(n)| is not smaller than DE, and equals (a(n)+b(n)+DE)/2 when |a(n)-b(n)| is smaller than DE. The apparatus of claim 12 wherein the MAX*(a(n), b(n))result equals a(n) when (a(n)-b(n)-DE) > 0 and (b(n)-a(n) -DE) < 0; wherein the MAX*(a(n), b(n)) result equals b(n) when (a(n)-b(n)-DE) < 0 and (b(n)-a(n) -DE) > 0; and wherein the MAX*(a(n), b(n)) result equals (a(n)+b(n)+DE)/2 when (a(n)-b(n)-DE) < 0 and (b(n)-a(n)-DE) < 0.

Inventors: AIDAN FABRICE, LESHIN GREGORY, SIVAN NOAM, AIDAN, FABRICE, LESHIN, GREGORY, SIVAN, NOAM
Applicants: MOTOROLA INC, MOTOROLA INC.
Application year: 2000
Main classification: H03M13
IPC codes: H03M13