WO2016004591A1 - Apparatus and methods for non-linear symbol detection in td-scdma - Google Patents

Abstract

Apparatus, methods, and computer program product for wireless communication, including receiving, in a time division synchronous code division multiple access (TD-SCDMA) network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols; determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions; determining,based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol; determining a target posterior probability for the target symbol based on the posterior probabilities; and detecting the target symbol based on the target posterior probability.

Description

APPARATUS AND METHODS FOR NON-LINEAR SYMBOL DETECTION

IN TD-SCDMA

REFERENCE TO CO-PENDING APPLICATIONS FOR PATENT

[0001] The present Application for Patent is related to the following co-pending U.S. Patent Application:

"APPARATUS AND METHODS FOR JOINT CHANNEL ESTIMATION AND NON-LINEAR SYMBOL DETECTION IN TD-SCDMA," having Attorney Docket No. 141787, filed concurrently herewith, assigned to the assignee hereof, and expressly incorporated by reference herein.

BACKGROUND

[0002] Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to apparatus and methods for nonlinear symbol detection in Time Division-Synchronous Code Division Multiple Access (TD-SCDMA).

[0003] Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the Universal Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division - Code Division Multiple Access (TD-CDMA), and Time Division - Synchronous Code Division Multiple Access (TD-SCDMA). For example, in some countries like China, TD-SCDMA is being considered as the underlying air interface in the UTRAN architecture with existing GSM infrastructure as the core network. The UMTS also supports enhanced 3G data communications protocols, such as High Speed Downlink Packet Data (HSDPA), which provides higher data transfer speeds and capacity to associated UMTS networks.

[0004] Conventionally, in TD-SCDMA, linear multi-user detection is performed at a receiver. However, linear receivers may not perform well under some channel conditions such as severe channel conditions where the channel impulse response has nulls in the frequency domain. Therefore, there is a need for improved receivers in TD-SCDMA.

SUMMARY

[0005] The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

[0006] In one aspect, a method for wireless communication is provided that includes receiving, in a time division synchronous code division multiple access (TD- SCDMA) network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols; determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions; determining, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol; determining a target posterior probability for the target symbol based on the posterior probabilities; and detecting the target symbol based on the target posterior probability.

[0007] In another aspect, an apparatus for wireless communication is provided that includes a processing system configured to receive, in a TD-SCDMA network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols; determine forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions; determine, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol; determine a target posterior probability for the target symbol based on the posterior probabilities; and detect the target symbol based on the target posterior probability.

[0008] In a further aspect, an apparatus for wireless communication is provided that includes means for receiving, in a TD-SCDMA network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols; means for determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions; means for determining, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol; means for determining a target posterior probability for the target symbol based on the posterior probabilities; and means for detecting the target symbol based on the target posterior probability.

[0009] In yet another aspect, a computer program product for wireless communication in provided that includes a computer-readable medium including code for receiving, in a TD-SCDMA network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols; code for determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions; code for determining, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol; code for determining a target posterior probability for the target symbol based on the posterior probabilities; and code for detecting the target symbol based on the target posterior probability.

[0010] To the accomplishment of the foregoing and related ends, the one or more aspects comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative features of the one or more aspects. These features are indicative, however, of but a few of the various ways in which the principles of various aspects may be employed, and this description is intended to include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The disclosed aspects will hereinafter be described in conjunction with the appended drawings, provided to illustrate and not to limit the disclosed aspects, wherein like designations denote like elements, and in which:

[0012] FIG. 1 is a diagram illustrating an example of a wireless communications system according to some present aspects;

[0013] FIG. 2 is a block diagram illustrating an example posterior recursion performed at a receiver in some present aspects;

[0014] FIG. 3 is a diagram illustrating an example computational flow of a receiver in some present aspects;

[0015] FIGs. 4-10 are flow charts of example methods of wireless communication in aspects of the wireless communications system of FIG. 1;

[0016] FIG. 11 is a diagram of a hardware implementation for an apparatus employing a processing system, including aspects of the wireless communications system of FIG. 1;

[0017] FIG. 12 is a diagram illustrating an example of a telecommunications system, including aspects of the wireless communications system of FIG. 1;

[0018] FIG. 13 is a diagram illustrating an example of a frame structure in a telecommunications system, in aspects of the wireless communications system of FIG. 1; and

[0019] FIG. 14 is a diagram illustrating an example of a Node B in communication with a UE in a telecommunications system, including aspects of the wireless communications system of FIG. 1.

DETAILED DESCRIPTION

[0020] The detailed description set forth below, in connection with the appended drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

[0021] Some present aspects provide non-linear symbol detection in Time Division - Synchronous Code Division Multiple Access (TD-SCDMA). In some aspects, a recursive Bayesian symbol detector is provided that performs online symbol detection in a time slot without needing to buffer all symbols of that time slot. In these aspects, in order to detect a target symbol and upon receiving K subsequent symbols, a non-linear receiver recursively computes forward and backward probabilities of the K symbols using corresponding transition probabilities. Then, the non-linear receiver determines a target posterior probability based on the forward and backward probabilities and uses the target posterior probability for MAP detection of the target symbol after a fixed lag of K symbols, thereby providing symbol detection by fixed-lag posterior recursions.

[0022] The present aspects may be implemented similar to a turbo decoder and may be easily integrated in dual subscriber identity module (SIM) dual active (DSDA) applications. Also, the present aspects may provide performance improvement compared to conventional linear multi-user receivers. For example, the non-linear receiver in the present aspects may result in performance gain in single cell scenarios with a spreading factor of 1.

[0023] The performance gain of the non-linear receiver in the present aspects may depend on the puncturing level used for transmitting the symbols. For example, in some aspects, at a certain block error rate (BLER), a higher performance gain may be achieved at a higher puncturing level.

[0024] Referring to FIG. 1, a wireless communications system 100 is illustrated with aspects, including non-linear symbol detector component 110, configured to improve symbol detection in TD-SCDMA network 112. Wireless communications system 100 includes user equipment (UE) 102 that is receiving downlink signals 108 from base station 104 and transmitting uplink signals 106 to base station 104 in TD- SCDMA network 112.

[0025] Conventionally, in TD-SCDMA network 112, the chip rate is 1.28 megachips per second (Mcps) and the downlink time slot is 675 microseconds (μβ) or 874 chips. Table 1 shows an example configuration of chips in a TD-SCDMA downlink time slot.

Table 1

ample configuration of chips in a TD-SCDMA downlink time slot

[0026] As shown in Table 1, there are 144 chips in the midamble of a TD- SCDMA downlink time slot. The midambles are training sequences for channel estimation and power measurements at UE 102. Each midamble can potentially have its own beamforming weights. Also, there is no offset between the power of the midamble and the total power of the associated channelization codes. The TD- SCDMA downlink time slot further includes 704 data chips and 16 guard period (GP) chips.

[0027] Conventionally, in TD-SCDMA, linear multi-user detection such as minimum mean-square error (MMSE) detection is performed on downlink signals 108 received at a receiver 114 of UE 102. However, linear receivers may not perform well under some channel conditions. For example, linear receivers may result in high symbol error rate (SER) under severe channel conditions, e.g., when the channel impulse response has nulls in the frequency domain (such as the three tap [1 1 1] channel).

[0028] As used herein, for a channel memory of L, a shift register or shift, d^shlft(m), refers to the vector of transmitted symbols d(m) to d(m - L), in which the last symbol d(m - L) is referred to as the tail, and the vector of the first L symbols d(m) to d(m - L +1) is referred to as the tunnel of symbols, d¹™¹¹⁶^):

(m} ~ \ (m) d(m~i) *** (m~L*l) (m-L l {tx{L+1))

d*^ (^j«)™[<f (i») {m-l) — «f (.ffi ~L l)] (IxL)

Further, as used herein, M is the transmitted symbol constellation cardinality (e.g., M=4 for quadrature phase shift keying (QPSK) and M=16 for 16 QPSK). The received symbol y(m) in a single cell with a spreading factor of 1 may be modeled as: h{i s{m -i od l6)d{m - i) ÷ v(m) u{m }h + v{m)

where s(m) is the scrambling sequence with period 16, u(m) is s(m mod 16)*d(m), and h(m) is the channel impulse response (CIR).

[0029] In some present aspects, receiver 114 of UE 102 includes non-linear symbol detector component 110 that operates to address one or more deficiencies of conventional receivers in TD-SCDMA via performing online symbol detection in a time slot without needing to buffer all symbols of that time slot. Although non-linear symbol detector component 110 is illustrated as a part of receiver 114, it should be understood that non-linear symbol detector component 110 may be separate from, but in communication with, receiver 114. For instance, non-linear symbol detector component 110 may be implemented as one or more processor modules in a processor of UE 102, as computer-readable instructions stored in a memory of UE 102 and executed by a processor of UE 102, or some combination of both.

[0030] In an aspect, for example, receiver 114 or non-linear symbol detector component 110 include an MAP symbol detector component 126 that performs fixed- lag detection of received symbols. For example, in some aspects, during a current time slot, MAP symbol detector component 126 detects a symbol d(m) upon receiving y(m) and K subsequent symbols (e.g., y(m+l) up to y(m+K)) but without waiting to receive all symbols in that time slot. In these aspects, a K-lag MAP detection, d^" (m), of symbol d(m) may be modeled as:

where the function "Pr(A | B)" denotes "probability of event A given event B".

[0031] In some present aspects, in order to perform this K-lag MAP detection recursively, a forward probability, a (m), of symbol d(m) may be modeled as: m ) = Pr| d¹""'¹" rn ) , y j

= ∑ P (m - 1) , ^> _w_, )Pr(^ (m).y{m) (m

{m-l }

= y ( m - 1 } γ ( m - 1, m ) where γ (m - 1, m) is the transition probability of symbol d(m):

and _m is the vector of the received symbols 1 to m:

v = >'(!) y(2) — y m)

[0032] Further, in some present aspects, backward probability, fi(m), of symbol d(m) may be modeled as:

where y>_m is the vector of the received symbols m+1 to N+L:

V = y(m +l) y(m+2) — y(N+ L) ] [0033] Accordingly, in these aspects, receiver 114 and/or non-linear symbol detector component 110 include transition probability determiner component 120 that determines transition probabilities of the symbols, and also include forward probability determiner component 116 and backward probability determiner component 118 that, respectively, recursively determine forward and backward probabilities of symbol d(m) according to: a m )= ( m -l) γ (m

β m -l}= y γ {m - L m ) β( m }

[0034] In some aspects, the initial conditions for the recursions of forward and backward probabilities are set, respectively, as: a(0) ₌ -L. β (m + K)= a (m + K)

[0035] In some aspects, receiver 114 and/or non-linear symbol detector component 110 include tunnel posterior probability determiner component 122 that determines a tunnel posterior probability,

¾_ase(j _on the product of forward and backward probabilities of the most recent symbol of the tunnel of symbols, where:

Prf ^A¾t* { m + L- 1) v )

= Vr(d*^aai*i(m+L-l).v )

i

— m +L-l ) β{ m + 1.-1} where λ is a constant and the tunnel of symbols, J^unnel, is: rf (iH + £ -l) = [^"rf(jif + Z - l) d(m + L - 2) ^■■■ d(m f]

[0036] In some present aspects, receiver 114 and/or non-linear symbol detector component 110 include target posterior probability determiner component 124 that determines a target posterior probability,

for a target symbol d(m) by marginalizing over the symbols of the tunnel of symbols d^timnel(m + L - 1) except for the target symbol d(m), which is equivalent to sum/multiplication over the forward and backward probabilities of the symbols of the tunnel of symbols d^timnel(m + L - 1) except for the target symbol d(m), where:

= i {m + L - l)J3 (m + L -l)

[0037] Accordingly, in these aspects, MAP symbol detector component 126 performs MAP detection of the target symbol d(m) according to:

^(m) = ar ma 2 {m +L—\) fi[m +L-i)

[0038] Thus, UE 102 having receiver 114 and/or non-linear symbol detector component 110 configured with the above-noted components executing the above- noted functions may perform online symbol detection in a time slot without needing to buffer all symbols of that time slot. For example, receiver 114 and/or non-linear symbol detector component 110 may perform MAP detection of the ^th symbol through recursions of forward and backward probabilities of symbols up to m + K, that is, by fixed-lag posterior recursion with a fixed lag of K symbols. Also, UE 102 having receiver 114 and/or non-linear symbol detector component 110 configured with the above-noted components executing the above-noted functions may perform online symbol detection at symbol rate which is lower than chip rate.

[0039] Fig. 2 shows an example block diagram 200 of a process executed by receiver 114 or non-linear symbol detector component 110 of UE 102 for determining a target posterior probability of a received symbol d(m) by recursively deriving forward and backward probabilities of symbols in a single input single output (SISO) first order auto regressive (AR1) model. At each iteration, at block 202, forward probability determiner component 116 and backward probability determiner component 118 perform a recursion of a respective one of the forward and backward probabilities, by combining (e.g. multiplying or equivalently adding in the logarithmic domain) the transition probability (determined by transition probability determiner component 120) with the forward and backward probabilities as follows: a {m = y a{m - 1 ) -/{m - i_s m) β^Μ - l)= y* Y ( m - m ) β(ιη )

[0040] Then, at block 204, tunnel posterior probability determiner component 122 performs Markov grouping on the forward and backward probabilities to group those probabilities that correspond to the tunnel of symbols that has symbol d(m) as its tail.

[0041] Then, at block 206, target posterior probability determiner component 124 performs marginalization on the forward and backward probabilities over the tunnel of symbols except for symbol d(m) to derive the target posterior for symbol d(m): if (m = ar max V. {m +L -l) 0(m +L -l)

[0042] Finally, at block 208, receiver 114 and/or non-linear symbol detector component 110 send delayed forward probabilities and advanced backward probabilities to the next iteration.

[0043] Fig. 3 shows an example computational flow 300 with reference to received symbols 308, executed by receiver 114 and/or non-linear symbol detector component 110 of UE 102 for determining a target posterior probability of a target symbol d(m) by recursively deriving forward and backward probabilities of symbols, according to some present aspects.

[0044] Computational flow 300 includes portions 302, 304, 306 corresponding to the flow of computations at receiver 114 and/or non-linear symbol detector component 110, with reference to symbol indices m + L - 2 (index of received symbol y(m + L - 2)), m + L - 1 (index of received symbol y(m + L - 1)), and m + K (index of received symbol y(m + K)), respectively.

[0045] In portion 302, index m + L - 2 represents an initial state in which forward probability determiner component 116 determines an initial forward probability, for example, according to a uniform distribution. Also, in portion 302, receiver 114 and/or non-linear symbol detector component 110 may determine an initial detected tunnel of symbols for this state which may include, for example, null or random entries.

[0046] Portion 304 follows portion 302 and corresponds to the computations performed with reference to subsequent index m + L - 1. According to the system model in the present aspects, in portion 304, based on the respective symbol, d( m + L - 1), and the tunnel of symbols of the previous portion, d¹™¹¹⁶^ + L - 2), receiver 114 and/or non-linear symbol detector component 110 determine the new shift, d^shlft(m + L - 1). Also, based on the new shift, receiver 114 and/or non-linear symbol detector component 110 determine the new tunnel of symbols d¹¹™⁶^ + L - 1). Then, based on the new shift and the received symbol y(m + L - 1), transition probability determiner component 120 determines the transition probability y(m + L - 2,m + L - 1). Subsequently, forward probability determiner component 116 determines the forward probability a(m + L - 1) based on the transition probability y(m + L - 2,m + L - 1) and the forward probability of the previous portion a(m + L - 2).

[0047] Computational flow 300 may include other successive portions (not shown), where in each portion, similar computations are performed by forward probability determiner component 116 based on a corresponding received symbol and a corresponding predecessor portion.

[0048] The last portion 306 corresponds to the computations performed with reference to index m + K. In portion 306, backward probability determiner component 118 uses the last forward probability, a(m + K), as an initial state for the backward probability β(ηι + K). Then, backward probability determiner component 118 performs backward iterations of backward probabilities by determining the backward probability of each portion based on the backward and transition probabilities of the successor portion, until reaching back to portion 304.

[0049] In portion 304, after backward probability determiner component 118 determines the backward probability β(ηι + L - 1), target posterior probability determiner component 124 determines a target posterior probability, Pr(d(m) | (m + K)), for symbol d(m) based on marginalization over forward and backward probabilities a(m + L - 1) and β(ηι + L - 1).

[0050] In some alternative aspects, various components of receiver 114 and/or non-linear symbol detector component 110 may perform computations in the logarithmic domain. For example, in some aspects, transition probability determiner component 120, forward probability determiner component 116, backward probability determiner component 118, and target posterior probability determiner component 124 may, respectively, determine the transition probability y(m - 1 , m), the forward probability a(m), the backward probability fi(m), and the target posterior probability Pr(d(m) I y(m + K)) of symbol d(m), according to: i I

γ { m - 1 m ) -— - >^?{ m )— u m ) h\ a { n = ma a [m + y(^{m ~ m})) β {m— l) - max γ (m - 1 , m ) 4- β [m ) Pr I d ( m ) v | ------ max ( { m - L ~ I ) + β ( m + L ~~ I } )

[0051] Accordingly, in the present aspects, MAP detection of the ^th symbol is performed by recursions of forward and backward probabilities of symbols up to m + K, that is, by fixed-lag posterior recursion with a fixed lag of K symbols.

[0052] FIGs. 4-10 describe methods 400, 500, 600, 700, 800, 900, and 1000, respectively, in aspects of the wireless communications system of FIG. 1. For example, methods 400, 500, 600, 700, 800, 900, and 1000 may be performed by UE 102 executing receiver 114 and/or non-linear symbol detector component 110 (FIG. 1) as described herein, where method 400 relates to an aspect of detecting a target symbol, method 500 relates to an aspect of determining posterior probabilities, method 600 relates to an aspect of determining forward and backward probabilities, method 700 relates to a further aspect of determining forward probabilities, method 800 relates to a further aspect of recursively determining backward probabilities, method 900 relates to an aspect of determining a target posterior probability, and method 1000 relates to a further aspect of detecting a target symbol.

[0053] Referring now to FIG. 4, in an aspect of a method of wireless communication in which receiver 114, non-linear symbol detector component 110, and/or UE 102 perform online symbol detection in a time slot at symbol rate and without needing to buffer all symbols of that time slot, at block 402, method 400 includes receiving, in a TD-SCDMA network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols. For example, in some aspects, receiver 114 and/or non-linear symbol detector component 110 of UE 102 may receive, in TD- SCDMA network 112, a target symbol (e.g., d(m)), a tunnel of symbols whose least recent symbol is the target symbol (e.g., d¹™¹¹⁶^ + L - 1)), and a first number of symbols subsequent to the tunnel of symbols (e.g., d(m + L), d(m + L + 1), ... d(m + K)). In some aspects, for example, TD-SCDMA network 112 is a single cell network with a spreading factor of one.

[0054] At block 404, method 400 includes determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, where each of the forward and backward probabilities is recursively determined by performing a second number of recursions. For example, in some aspects, receiver 114 and/or non-linear symbol detector component 110 and/or a respective one of forward probability determiner component 116 and backward probability determiner 118 may determine forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol (e.g., for d(m + L - 1), d(m + L - 2), ..., d(m + 1)), where each of the forward and backward probabilities is recursively determined by performing a second number of recursions. For example, for symbol d(m + L - 1), forward probability a(m + L - 1) and backward probability β(ηι + L - 1) may be determined by recursions corresponding to indices between m + L - 1 and m + K, equivalent to K - L + 2 recursions.

[0055] At block 406, method 400 includes determining, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol. For example, in some aspects, receiver 114 and/or non-linear symbol detector component 110 and/or tunnel posterior probability determiner component 122 may determine, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol. For example, tunnel posterior probability determiner component 122 may determine a posterior probability for symbol d(m + L - 1) based on forward probability a(m + L - 1) and backward probability β(ηι + L - 1).

[0056] At block 408, method 400 includes determining a target posterior probability for the target symbol based on the posterior probabilities. For example, in some aspects, receiver 114 and/or non-linear symbol detector component 110 and/or target posterior probability determiner component 124 may determine a target posterior probability for target symbol d(m) based on the posterior probabilities of the symbols of tunnel of symbols d¹™¹¹⁶^ + L - 1) excluding the target symbol d(m) itself.

[0057] At block 410, method 400 includes detecting the target symbol based on the target posterior probability. For example, in some aspects, receiver 114 and/or non-linear symbol detector component 110 and/or MAP symbol detector component 126 may detect target symbol d(m) based on the target posterior probability for the target symbol d(m) which was determined based on the posterior probabilities of the symbols of tunnel of symbols d¹™¹¹⁶^ + L - 1) excluding the target symbol d(m) itself.

[0058] Referring to FIG. 5, method 500 includes further, and optional, aspects related to block 406 of method 400 of FIG. 4 for determining the posterior probabilities.

[0059] At optional block 502, method 500 includes determining a posterior probability of a symbol of the tunnel of symbols to be proportional to a product of a forward probability and a backward probability of the symbol. For example, receiver 114 and/or non-linear symbol detector component 110 and/or tunnel posterior probability determiner component 122 may determine a posterior probability for symbol d(m + L - 1) of tunnel of symbols d¹™¹¹⁶^ + L - 1) to be proportional to a product of forward probability a(m + L - 1) and backward probability β(ηι + L - 1).

[0060] Referring to FIG. 6, method 600 includes further, and optional, aspects related to block 404 of method 400 of FIG. 4 for determining the forward and backward probabilities.

[0061] At optional block 602, method 600 includes recursively determining forward probabilities for a middle symbol up to a last symbol, where the middle symbol is a most recent symbol of the tunnel of symbols and the last symbol is a most recent symbol of the first number of symbols. For example, in one aspect, receiver 114 and/or non-linear symbol detector component 110 and/or forward probability determiner component 116 may recursively determine forward probabilities for middle symbol d(m + L - 1) up to last symbol d(m + K), where middle symbol d(m + L - 1) is a most recent symbol of tunnel of symbols d^timnel(m + L - 1) and last symbol d(m + K) is a most recent symbol of the first number of symbols d(m), ..., d(m + K).

[0062] At optional block 604, method 600 includes recursively determining backward probabilities for a preceding symbol of the last symbol back to the middle symbol. For example, in one aspect, receiver 114 and/or non-linear symbol detector component 110 and/or backward probability determiner component 118 of receiver 114 may recursively determine backward probabilities for symbol d (m + K - 1) (which is the preceding symbol of last symbol d(m + K)) back to middle symbol d(m + L - 1).

[0063] Referring to FIG. 7, method 700 includes further, and optional, aspects related to block 602 of method 600 of FIG. 6 for recursively determining the forward probabilities.

[0064] At optional block 702, method 700 includes assigning an initial forward probability to a symbol preceding the middle symbol. For example, in one aspect, receiver 114 and/or non-linear symbol detector component 110 and/or forward probability determiner component 116 may assign an initial forward probability to symbol d(m + L - 2) that precedes middle symbol d(m + L - 1). In some aspects, such initial forward probability may be determined according to a uniform probability distribution function.

[0065] At optional block 704, method 700 includes, starting with the middle symbol and up to the last symbol, determining each forward probability as a function of a respective transition probability and a preceding forward probability. For example, in one aspect, starting with middle symbol d(m + L - 1) and up to last symbol d(m + K), receiver 114 and/or non-linear symbol detector component 110 and/or forward probability determiner component 116 may determine forward probability a(m + L + i) of a symbol d(m + L + i) as a function of transition probability y(m + L + i) and preceding forward probability a(m + L + i - 1). [0066] Referring to FIG. 8, method 800 includes further, and optional, aspects related to block 604 of method 600 of FIG. 6 for recursively determining the backward probabilities.

[0067] At optional block 802, method 800 includes assigning a backward probability of the last symbol to be equal to a forward probability of the last symbol. For example, in one aspect, receiver 114 and/or non-linear symbol detector component 110 and/or backward probability determiner component 118 may assign backward probability of last symbol d(m + K) to be equal to forward probability of last symbol d(m + K).

[0068] At optional block 804, method 800 includes, starting with the preceding symbol of the last symbol and back to the middle symbol, determining each backward probability as a function of transition and backward probabilities of a respective subsequent symbol. For example, in one aspect, starting with d(m + K - 1) (which is the preceding symbol of last symbol d(m + K)) and back to middle symbol d(m + L - 1), receiver 114 and/or non-linear symbol detector component 110 and/or backward probability determiner component 118 may determine backward probability β(ηι + L + i) of a symbol d(m + L + i) as a function of transition probability y(m + L + i +1) and backward probability β(ηι + L + i +1).

[0069] Referring to FIG. 9, method 900 includes further, and optional, aspects related to block 408 of method 400 of FIG. 4 for determining the target posterior probability.

[0070] At block 902, method 900 includes determining the target posterior probability to be a sum of the posterior probabilities. For example, in one aspect, receiver 114 and/or non-linear symbol detector component 110 and/or target posterior probability determiner component 124 may determine a target posterior probability for target symbol d(m) to be the sum of posterior probabilities of the symbols of tunnel of symbols d^timnel(m + L - 1) excluding the target symbol d(m) itself.

[0071] Referring to FIG. 10, method 1000 includes further, and optional, aspects related to block 410 of method 400 of FIG. 4 for detecting the target symbol.

[0072] At optional block 1002, method 1000 includes performing MAP detection on the target posterior probability. For example, in one aspect, receiver 114 and/or non-linear symbol detector component 110 and/or MAP symbol detector component 126 may detect target symbol d(m) by performing MAP detection on the target posterior probability determined by target posterior probability determiner component 124.

[0073] Referring to FIG. 11, an example of a hardware implementation for an apparatus 1100 including non-linear symbol detector component 110 and employing a processing system 1114 is shown. In an aspect, apparatus 1100 may be UE 102 of FIG. 1, including receiver 114, and may be configured to perform any functions described herein with reference to UE 102 and/or receiver 114 and/or non- linear symbol detector component 110. In this aspect, non-linear symbol detector component 110 is illustrated as being optionally implemented separate from, but in communication with, receiver 114. Further, in this aspect, non-linear symbol detector component 110 may be implemented as one or more processor modules in a processor 1104 of UE 102, as computer-readable instructions stored in a computer-readable medium 1106 in a memory 1107 of UE 102 and executed by processor 1104 of UE 102, or some combination of both.

[0074] In this example, the processing system 1114 may be implemented with a bus architecture, represented generally by the bus 1102. The bus 1102 may include any number of interconnecting buses and bridges depending on the specific application of the processing system 1114 and the overall design constraints. The bus 1102 links together various circuits including one or more processors, represented generally by the processor 1104, one or more communications components, such as, for example, non-linear symbol detector component 110 of FIG. 1, and computer- readable media, represented generally by the computer-readable medium 1106. The bus 1102 may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. A bus interface 1108 provides an interface between the bus 1102 and receiver 114, which may be part of a transceiver (not shown). The receiver 114 and/or transceiver (not shown) provide a means for communicating with various other apparatus over a transmission medium. Depending upon the nature of the apparatus, a user interface 1112 (e.g., keypad, display, speaker, microphone, joystick) may also be provided.

[0075] The processor 1104 is responsible for managing the bus 1102 and general processing, including the execution of software stored on the computer-readable medium 1106. For example, in some aspects, non-linear symbol detector component 110 may be software stored on the computer-readable medium 1106 and may be executed by processor 1104. The software, when executed by the processor 1104, causes the processing system 1114 to perform the various functions described herein for any particular apparatus.

[0076] The computer-readable medium 1106 may also be used for storing data that is manipulated by the processor 1104 when executing software, such as, for example, software modules represented by non-linear symbol detector component 110. In one example, the software modules (e.g., any algorithms or functions that may be executed by processor 1104 to perform the described functionality) and/or data used therewith (e.g., inputs, parameters, variables, and/or the like) may be retrieved from computer-readable medium 1106. The modules may be software modules running in the processor 1104, resident and/or stored in the computer-readable medium 1106, one or more hardware modules coupled to the processor 1104, or some combination thereof.

[0077] Turning now to FIG. 12, a block diagram is shown illustrating an example of a telecommunications system 1200. Telecommunications system 1200 includes UEs 1210 which may be examples of UE 102 of FIG. 1 and which may include and execute non-linear symbol detector component 110 to perform any functions described herein. The various concepts presented throughout this disclosure may be implemented across a broad variety of telecommunication systems, network architectures, and communication standards. By way of example and without limitation, the aspects of the present disclosure illustrated in FIG. 12 are presented with reference to a UMTS system employing a TD-SCDMA standard. In this example, the UMTS system includes a (radio access network) RAN 1202 (e.g., UTRAN) that provides various wireless services including telephony, video, data, messaging, broadcasts, and/or other services. The RAN 1202 may be divided into a number of Radio Network Subsystems (RNSs) such as an RNS 1207, each controlled by a Radio Network Controller (RNC) such as an RNC 1206. For clarity, only the RNC 1206 and the RNS 1207 are shown; however, the RAN 1202 may include any number of RNCs and RNSs in addition to the RNC 1206 and RNS 1207. The RNC 1206 is an apparatus responsible for, among other things, assigning, reconfiguring and releasing radio resources within the RNS 1207. The RNC 1206 may be interconnected to other RNCs (not shown) in the RAN 1202 through various types of interfaces such as a direct physical connection, a virtual network, or the like, using any suitable transport network.

[0078] The geographic region covered by the RNS 1207 may be divided into a number of cells, with a radio transceiver apparatus serving each cell. A radio transceiver apparatus is commonly referred to as a Node B in UMTS applications, but may also be referred to by those skilled in the art as a base station (BS), a base transceiver station (BTS), a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), an access point (AP), or some other suitable terminology. For clarity, two Node Bs 1208 are shown; however, the RNS 1207 may include any number of wireless Node Bs. The Node Bs 1208 provide wireless access points to a core network 1204 for any number of mobile apparatuses. Examples of a mobile apparatus include a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a notebook, a netbook, a smartbook, a personal digital assistant (PDA), a satellite radio, a global positioning system (GPS) device, a multimedia device, a video device, a digital audio player (e.g., MP3 player), a camera, a game console, or any other similar functioning device. The mobile apparatus is commonly referred to as user equipment (UE) in UMTS applications, but may also be referred to by those skilled in the art as a mobile station (MS), a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communications device, a remote device, a mobile subscriber station, an access terminal (AT), a mobile terminal, a wireless terminal, a remote terminal, a handset, a terminal, a user agent, a mobile client, a client, or some other suitable terminology. For illustrative purposes, three UEs 1210, which may be the same as or similar to UE 102 of FIG. 1, are shown in communication with the Node Bs 1208, which may be the same as or similar to base station 104 of FIG. 1. The downlink (DL), also called the forward link, refers to the communication link from a Node B to a UE, and the uplink (UL), also called the reverse link, refers to the communication link from a UE to a Node B.

[0079] The core network 1204, as shown, includes a GSM core network. However, as those skilled in the art will recognize, the various concepts presented throughout this disclosure may be implemented in a RAN, or other suitable access network, to provide UEs with access to types of core networks other than GSM networks. [0080] In this example, the core network 1204 supports circuit-switched services with a mobile switching center (MSC) 1212 and a gateway MSC (GMSC) 1214. One or more R Cs, such as the RNC 1206, may be connected to the MSC 1212. The MSC 1212 is an apparatus that controls call setup, call routing, and UE mobility functions. The MSC 1212 also includes a visitor location register (VLR) (not shown) that contains subscriber-related information for the duration that a UE is in the coverage area of the MSC 1212. The GMSC 1214 provides a gateway through the MSC 1212 for the UE to access a circuit-switched network 1216. The GMSC 1214 includes a home location register (HLR) (not shown) containing subscriber data, such as the data reflecting the details of the services to which a particular user has subscribed. The HLR is also associated with an authentication center (AuC) that contains subscriber-specific authentication data. When a call is received for a particular UE, the GMSC 1214 queries the HLR to determine the UE's location and forwards the call to the particular MSC serving that location.

[0081] The core network 1204 also supports packet-data services with a serving GPRS support node (SGSN) 1218 and a gateway GPRS support node (GGSN) 1220. GPRS, which stands for General Packet Radio Service, is designed to provide packet- data services at speeds higher than those available with standard GSM circuit- switched data services. The GGSN 1220 provides a connection for the RAN 1202 to a packet-based network 1222. The packet-based network 1222 may be the Internet, a private data network, or some other suitable packet-based network. The primary function of the GGSN 1220 is to provide the UEs 1210 with packet-based network connectivity. Data packets are transferred between the GGSN 1220 and the UEs 1210 through the SGSN 1218, which performs primarily the same functions in the packet- based domain as the MSC 1212 performs in the circuit-switched domain.

[0082] The UMTS air interface is a spread spectrum Direct-Sequence Code Division Multiple Access (DS-CDMA) system. The spread spectrum DS-CDMA spreads user data over a much wider bandwidth through multiplication by a sequence of pseudorandom bits called chips. The TD-SCDMA standard is based on such direct sequence spread spectrum technology and additionally calls for a time division duplexing (TDD), rather than a frequency division duplexing (FDD) as used in many FDD mode UMTS/W-CDMA systems. TDD uses the same carrier frequency for both the uplink (UL) and downlink (DL) between a Node B 1208 and a UE 1210, but divides uplink and downlink transmissions into different time slots in the carrier.

[0083] FIG. 13 shows a frame structure 1300 for a TD-SCDMA carrier, which may be used for communications between base station 104 of FIG. 1, and UE 102, also of FIG. 1. The TD-SCDMA carrier, as illustrated, has a frame 1302 that is 10 milliseconds (ms) in duration. The frame 1302 has two 5 ms subframes 1304, and each of the subframes 1304 includes seven time slots, TS0 through TS6. The first time slot, TS0, is usually allocated for downlink communication, while the second time slot, TS1, is usually allocated for uplink communication. The remaining time slots, TS2 through TS6, may be used for either uplink or downlink, which allows for greater flexibility during times of higher data transmission times in either the uplink or downlink directions. A downlink pilot time slot (DwPTS) 1306, a guard period (GP) 1308, and an uplink pilot time slot (UpPTS) 1310 (also known as the uplink pilot channel (UpPCH)) are located between TS0 and TS1. Each time slot, TS0-TS6, may allow data transmission multiplexed on a maximum of 16 code channels. Data transmission on a code channel includes two data portions 1312 separated by a midamble 1314 and followed by a guard period (GP) 1316. The midamble 1314 may be used for features, such as channel estimation, while the GP 1316 may be used to avoid inter-burst interference.

[0084] FIG. 14 is a block diagram of a Node B 1410 in communication with a UE 1450 in a RAN 1400. In an aspect, Node B 1410 may be an example of base station 104 of FIG. 1, and UE 1450 may be an example of UE 102 of FIG. 1 and may include and execute non-linear symbol detector component 110 of FIG. 1, either in receiver 1454 (which may be the same as or equivalent to receiver 114 of FIG. 1) or optionally separate from receiver 1454, for example, in memory 1492 and/or controller/processor 1490, to perform any functions described herein.

[0085] In the downlink communication, a transmit processor 1420 may receive data from a data source 1412 and control signals from a controller/processor 1440. The transmit processor 1420 provides various signal processing functions for the data and control signals, as well as reference signals (e.g., pilot signals). For example, the transmit processor 1420 may provide cyclic redundancy check (CRC) codes for error detection, coding and interleaving to facilitate forward error correction (FEC), mapping to signal constellations based on various modulation schemes (e.g., binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), M-phase-shift keying (M-PSK), M-quadrature amplitude modulation (M-QAM), and the like), spreading with orthogonal variable spreading factors (OVSF), and multiplying with scrambling codes to produce a series of symbols. Channel estimates from a channel processor 1444 may be used by a controller/processor 1440 to determine the coding, modulation, spreading, and/or scrambling schemes for the transmit processor 1420. These channel estimates may be derived from a reference signal transmitted by the UE 1450 or from feedback contained in the midamble 1314 (FIG. 13) from the UE 1450. The symbols generated by the transmit processor 1420 are provided to a transmit frame processor 1430 to create a frame structure. The transmit frame processor 1430 creates this frame structure by multiplexing the symbols with a midamble 1314 (FIG. 13) from the controller/processor 1440, resulting in a series of frames. The frames are then provided to a transmitter 1432, which provides various signal conditioning functions including amplifying, filtering, and modulating the frames onto a carrier for downlink transmission over the wireless medium through smart antennas 1434. The smart antennas 1434 may be implemented with beam steering bidirectional adaptive antenna arrays or other similar beam technologies.

[0086] At the UE 1450, a receiver 1454 receives the downlink transmission through an antenna 1452 and processes the transmission to recover the information modulated onto the carrier. The information recovered by the receiver 1454 is provided to a receive frame processor 1460, which parses each frame, and provides the midamble 1314 (FIG. 13) to a channel processor 1494 and the data, control, and reference signals to a receive processor 1470. The receive processor 1470 then performs the inverse of the processing performed by the transmit processor 1420 in the Node B 1410. More specifically, the receive processor 1470 descrambles and despreads the symbols, and then determines the most likely signal constellation points transmitted by the Node B 1410 based on the modulation scheme. These soft decisions may be based on channel estimates computed by the channel processor 1494. The soft decisions are then decoded and deinterleaved to recover the data, control, and reference signals. The CRC codes are then checked to determine whether the frames were successfully decoded. The data carried by the successfully decoded frames will then be provided to a data sink 1472, which represents applications running in the UE 1450 and/or various user interfaces (e.g., display). Control signals carried by successfully decoded frames will be provided to a controller/processor 1490. When frames are unsuccessfully decoded by the receiver processor 1470, the controller/processor 1490 may also use an acknowledgement (ACK) and/or negative acknowledgement (NACK) protocol to support retransmission requests for those frames.

[0087] In the uplink, data from a data source 1478 and control signals from the controller/processor 1490 are provided to a transmit processor 1480. The data source 1478 may represent applications running in the UE 1450 and various user interfaces (e.g., keyboard). Similar to the functionality described in connection with the downlink transmission by the Node B 1410, the transmit processor 1480 provides various signal processing functions including CRC codes, coding and interleaving to facilitate FEC, mapping to signal constellations, spreading with OVSFs, and scrambling to produce a series of symbols. Channel estimates, derived by the channel processor 1494 from a reference signal transmitted by the Node B 1410 or from feedback contained in the midamble transmitted by the Node B 1410, may be used to select the appropriate coding, modulation, spreading, and/or scrambling schemes. The symbols produced by the transmit processor 1480 will be provided to a transmit frame processor 1482 to create a frame structure. The transmit frame processor 1482 creates this frame structure by multiplexing the symbols with a midamble 1314 (FIG. 13) from the controller/processor 1490, resulting in a series of frames. The frames are then provided to a transmitter 1456, which provides various signal conditioning functions including amplification, filtering, and modulating the frames onto a carrier for uplink transmission over the wireless medium through the antenna 1452.

[0088] The uplink transmission is processed at the Node B 1410 in a manner similar to that described in connection with the receiver function at the UE 1450. A receiver 1435 receives the uplink transmission through the antenna 1434 and processes the transmission to recover the information modulated onto the carrier. The information recovered by the receiver 1435 is provided to a receive frame processor 1436, which parses each frame, and provides the midamble 1314 (FIG. 13) to the channel processor 1444 and the data, control, and reference signals to a receive processor 1438. The receive processor 1438 performs the inverse of the processing performed by the transmit processor 1480 in the UE 1450. The data and control signals carried by the successfully decoded frames may then be provided to a data sink 1439 and the controller/processor, respectively. If some of the frames were unsuccessfully decoded by the receive processor, the controller/processor 1440 may also use an acknowledgement (ACK) and/or negative acknowledgement (NACK) protocol to support retransmission requests for those frames.

[0089] The controller/processors 1440 and 1490 may be used to direct the operation at the Node B 1410 and the UE 1450, respectively. For example, the controller/processors 1440 and 1490 may provide various functions including timing, peripheral interfaces, voltage regulation, power management, and other control functions. The computer readable media of memories 1442 and 1492 may store data and software for the Node B 1410 and the UE 1450, respectively. A scheduler/processor 1446 at the Node B 1410 may be used to allocate resources to the UEs and schedule downlink and/or uplink transmissions for the UEs.

[0090] Several aspects of a telecommunications system has been presented with reference to a TD-SCDMA system. As those skilled in the art will readily appreciate, various aspects described throughout this disclosure may be extended to other telecommunication systems, network architectures and communication standards. By way of example, various aspects may be extended to other UMTS systems such as W- CDMA, High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), High Speed Packet Access Plus (HSPA+) and TD-CDMA. Various aspects may also be extended to systems employing Long Term Evolution (LTE) (in FDD, TDD, or both modes), LTE-Advanced (LTE-A) (in FDD, TDD, or both modes), CDMA2000, Evolution-Data Optimized (EV-DO), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Ultra- Wideband (UWB), Bluetooth, and/or other suitable systems. The actual telecommunication standard, network architecture, and/or communication standard employed will depend on the specific application and the overall design constraints imposed on the system.

[0091] Several processors have been described in connection with various apparatuses and methods. These processors may be implemented using electronic hardware, computer software, or any combination thereof. Whether such processors are implemented as hardware or software will depend upon the particular application and overall design constraints imposed on the system. By way of example, a processor, any portion of a processor, or any combination of processors presented in this disclosure may be implemented with a microprocessor, microcontroller, digital signal processor (DSP), a field-programmable gate array (FPGA), a programmable logic device (PLD), a state machine, gated logic, discrete hardware circuits, and other suitable processing components configured to perform the various functions described throughout this disclosure. The functionality of a processor, any portion of a processor, or any combination of processors presented in this disclosure may be implemented with software being executed by a microprocessor, microcontroller, DSP, or other suitable platform.

[0092] Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside on a computer-readable medium. A computer- readable medium may include, by way of example, memory such as a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., compact disc (CD), digital versatile disc (DVD)), a smart card, a flash memory device (e.g., card, stick, key drive), random access memory (RAM), read only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), a register, or a removable disk. Although memory is shown separate from the processors in the various aspects presented throughout this disclosure, the memory may be internal to the processors (e.g., cache or register).

[0093] Computer-readable media may be embodied in a computer-program product. By way of example, a computer-program product may include a computer- readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system.

[0094] It is to be understood that the specific order or hierarchy of steps in the methods disclosed is an illustration of exemplary processes. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the methods may be rearranged. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented unless specifically recited therein.

[0095] The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language of the claims, wherein reference to an element in the singular is not intended to mean "one and only one" unless specifically so stated, but rather "one or more." Unless specifically stated otherwise, the term "some" refers to one or more. A phrase referring to "at least one of a list of items refers to any combination of those items, including single members. As an example, "at least one of: a, b, or c" is intended to cover: a; b; c; a and b; a and c; b and c; and a, b and c. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, or 35 U.S.C. § 112(f), whichever is appropriate, unless the element is expressly recited using the phrase "means for" or, in the case of a method claim, the element is recited using the phrase "step for."

Claims

CLAIMS WHAT IS CLAIMED IS:

1. A method of wireless communication, comprising:

receiving, in a time division synchronous code division multiple access (TD- SCDMA) network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols; determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions; determining, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol; determining a target posterior probability for the target symbol based on the posterior probabilities; and

detecting the target symbol based on the target posterior probability.

2. The method of claim 1, wherein determining the posterior probabilities comprises:

determining a posterior probability of a symbol of the tunnel of symbols to be proportional to a product of a forward probability and a backward probability of the symbol.

3. The method of claim 1, wherein determining the forward and backward probabilities comprises:

recursively determining forward probabilities for a middle symbol up to a last symbol, wherein the middle symbol is a most recent symbol of the tunnel of symbols and the last symbol is a most recent symbol of the first number of symbols; and

recursively determining backward probabilities for a preceding symbol of the last symbol back to the middle symbol.

4. The method of claim 3, wherein recursively determining the forward probabilities comprises: assigning an initial forward probability to a symbol preceding the middle symbol; and

starting with the middle symbol and up to the last symbol, determining each forward probability as a function of a respective transition probability and a preceding forward probability.

5. The method of claim 4, wherein recursively determining the backward probabilities comprises:

assigning a backward probability of the last symbol to be equal to a forward probability of the last symbol; and

starting with the preceding symbol of the last symbol and back to the middle symbol, determining each backward probability as a function of transition and backward probabilities of a respective subsequent symbol.

6. The method of claim 1, wherein determining the target posterior probability comprises:

determining the target posterior probability to be a sum of the posterior probabilities.

7. The method of claim 1, wherein detecting the target symbol comprises: performing maximum a posteriori (MAP) detection on the target posterior probability.

8. The method of claim 1, wherein the TD-SCDMA network is a single cell network with a spreading factor of one.

9. An apparatus for wireless communication, comprising:

a processing system configured to:

receive, in a time division synchronous code division multiple access (TD-SCDMA) network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols; determine forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions;

determine, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol;

determine a target posterior probability for the target symbol based on the posterior probabilities; and

detect the target symbol based on the target posterior probability.

10. The apparatus of claim 9, wherein to determine the posterior probabilities the processing system is configured to:

determine a posterior probability of a symbol of the tunnel of symbols to be proportional to a product of a forward probability and a backward probability of the symbol.

11. The apparatus of claim 9, wherein to determine the forward and backward probabilities the processing system is configured to:

recursively determine forward probabilities for a middle symbol up to a last symbol, wherein the middle symbol is a most recent symbol of the tunnel of symbols and the last symbol is a most recent symbol of the first number of symbols; and

recursively determine backward probabilities for a preceding symbol of the last symbol back to the middle symbol.

12. The apparatus of claim 11, wherein to recursively determine the forward probabilities the processing system is configured to:

assign an initial forward probability to a symbol preceding the middle symbol; and

starting with the middle symbol and up to the last symbol, determine each forward probability as a function of a respective transition probability and a preceding forward probability.

13. The apparatus of claim 12, wherein to recursively determine the backward probabilities the processing system is configured to:

assign a backward probability of the last symbol to be equal to a forward probability of the last symbol; and

starting with the preceding symbol of the last symbol and back to the middle symbol, determine each backward probability as a function of transition and backward probabilities of a respective subsequent symbol.

14. The apparatus of claim 9, wherein to determine the target posterior probability the processing system is configured to:

determine the target posterior probability to be a sum of the posterior probabilities.

15. The apparatus of claim 9, wherein to detect the target symbol the processing system is configured to:

perform maximum a posteriori (MAP) detection on the target posterior probability.

16. The apparatus of claim 9, wherein the TD-SCDMA network is a single cell network with a spreading factor of one

17. An apparatus for wireless communication, comprising:

means for receiving, in a time division synchronous code division multiple access (TD-SCDMA) network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols;

means for determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions;

means for determining, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol; means for determining a target posterior probability for the target symbol based on the posterior probabilities; and

means for detecting the target symbol based on the target posterior probability.

18. The apparatus of claim 17, wherein the means for determining the posterior probabilities comprises:

means for determining a posterior probability of a symbol of the tunnel of symbols to be proportional to a product of a forward probability and a backward probability of the symbol.

19. The apparatus of claim 17, wherein the means for determining the forward and backward probabilities comprises:

means for recursively determining forward probabilities for a middle symbol up to a last symbol, wherein the middle symbol is a most recent symbol of the tunnel of symbols and the last symbol is a most recent symbol of the first number of symbols; and

means for recursively determining backward probabilities for a preceding symbol of the last symbol back to the middle symbol.

20. The apparatus of claim 19, wherein the means for recursively determining the forward probabilities comprises:

means for assigning an initial forward probability to a symbol preceding the middle symbol; and

means for, starting with the middle symbol and up to the last symbol, determining each forward probability as a function of a respective transition probability and a preceding forward probability.

21. The apparatus of claim 20, wherein the means for recursively determining the backward probabilities comprises:

means for assigning a backward probability of the last symbol to be equal to a forward probability of the last symbol; and means for, starting with the preceding symbol of the last symbol and back to the middle symbol, determining each backward probability as a function of transition and backward probabilities of a respective subsequent symbol.

22. The apparatus of claim 17, wherein the means for determining the target posterior probability comprises:

means for determining the target posterior probability to be a sum of the posterior probabilities.

23. The apparatus of claim 17, wherein the means for detecting the target symbol comprises:

means for performing maximum a posteriori (MAP) detection on the target posterior probability.

24. The apparatus of claim 17, wherein the TD-SCDMA network is a single cell network with a spreading factor of one.

25. A computer program product for wireless communication, comprising: a computer-readable medium comprising:

code for receiving, in a time division synchronous code division multiple access (TD-SCDMA) network, a target symbol, a tunnel of symbols whose least recent symbol is the target symbol, and a first number of symbols subsequent to the tunnel of symbols;

code for determining forward and backward probabilities for each symbol in the tunnel of symbols excluding the target symbol, wherein each of the forward and backward probabilities is recursively determined by performing a second number of recursions;

code for determining, based on the forward and backward probabilities, posterior probabilities for each symbol in the tunnel of symbols excluding the target symbol;

code for determining a target posterior probability for the target symbol based on the posterior probabilities; and code for detecting the target symbol based on the target posterior probability.

26. The computer program product of claim 25, wherein the code for determining the posterior probabilities comprises:

code for determining a posterior probability of a symbol of the tunnel of symbols to be proportional to a product of a forward probability and a backward probability of the symbol.

27. The computer program product of claim 25, wherein the code for determining the forward and backward probabilities comprises:

code for recursively determining forward probabilities for a middle symbol up to a last symbol, wherein the middle symbol is a most recent symbol of the tunnel of symbols and the last symbol is a most recent symbol of the first number of symbols; and

code for recursively determining backward probabilities for a preceding symbol of the last symbol back to the middle symbol.

28. The computer program product of claim 27, wherein the code for recursively determining the forward probabilities comprises:

code for assigning an initial forward probability to a symbol preceding the middle symbol; and

code for, starting with the middle symbol and up to the last symbol, determining each forward probability as a function of a respective transition probability and a preceding forward probability.

29. The computer program product of claim 28, wherein the code for recursively determining the backward probabilities comprises:

code for assigning a backward probability of the last symbol to be equal to a forward probability of the last symbol; and

code for, starting with the preceding symbol of the last symbol and back to the middle symbol, determining each backward probability as a function of transition and backward probabilities of a respective subsequent symbol.

30. The computer program product of claim 25, wherein the code for determining the target posterior probability comprises:

code for determining the target posterior probability to be a sum of the posterior probabilities.