FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
1. Title of the invention: DIGITAL SIGNAL DEMODULATION
2. Applicant(s)
NAME NATIONALITY ADDRESS
TATA CONSULTANCY Indian Nirmal Building, 9th Floor, Nariman Point,
SERVICES LIMITED Mumbai, Maharashtra 400021, India
3. Preamble to the description
COMPLETE SPECIFICATION
The following specification particularly describes the invention and the manner in which it
is to be performed.

TECHNICAL FIELD
[0001] The present subject matter relates, in general, to communication systems and, in particular, to demodulation of digital signals.
BACKGROUND
[0002] In a communication system, signals are transmitted between a transmission unit of a fixed base station and a communication device, such as cellular phones, personal digital assistants, and portable computers. The signals travel over a physical medium, usually referred to as a channel, and are received by a receiver of the communication device, or the fixed base station. The transmitted signal and the received signal are usually not identical as the transmitted signal is subjected to various impairments during its propagation in the physical medium before being received as the received signal. For example, the transmitted signal may be subjected to noise, interferences, multipath propagation, and frequency variations (e.g. due to Doppler spread). A conventional method of recovering, from a received signal, an estimated signal that is substantially similar to the originally transmitted signal involves down-converting the received signal to obtain data symbols. The data symbols thus obtained, typically estimate the data symbols transmitted in the originally transmitted signal. Down-converting the received signal typically involves mixing the received signal with a reference signal corresponding to the carrier frequency of the transmitted signal to generate a baseband or intermediate frequency (IF) signal for further processing by the receiver.
[0003] Processing the baseband signal typically involves computation of various channel coefficients, also referred to as channel impulse response (CIR), equalizing the received signal, and demodulating the equalized symbol to obtain demodulated data symbols, also known as demodulated symbols. Further, in digital communication, the baseband signal is converted to a digital signal prior to computation of the CIR and the equalization signals such that the equalized symbols thus produced are digitally modulated and are subsequently demodulated to obtain demodulated symbols. However, due to the various impairments due to the channel and various receiver components, such as filters and oscillators, the demodulated symbols may not be similar to the data symbols originally transmitted, thus resulting in degradation of the performance of the receiver.

SUMMARY
[0004] This summary is provided to introduce concepts related to a method and a system for demodulation of a digital signal. The concepts are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.
[0005] Method(s) and a system(s) for demodulation of digital signals are described herein. In one implementation, a constellation diagram having a predetermined number of axes sets and one or more constellation points corresponding to one or more equalized symbols is obtained. Additionally, a quantization region corresponding to each of the axes sets is determined. Further, a first axis of an axes set with respect to a second axis of the axes set is folded to obtain a folded region such that the folded region includes a sub-quantization region corresponding to the quantization region associated with the axes set. Based at least on the folded region and the sub-quantization region, soft bit corresponding to a constellation point selected from among the one or more constellation points is ascertained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The detailed description is provided with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components. For simplicity and clarity of illustration, elements in the figures are not necessarily to scale. Some embodiments of devices and/or methods in accordance with embodiments of the present subject matter are now described, by way of example only, and with reference to the accompanying figures, in which:
[0007] Fig. 1 illustrates an exemplary communication network environment implementing communication devices for digital signal demodulation, according to an embodiment of the present subject matter.
[0008] Fig. 2(a) illustrates a communication device implementing a system for digital signal demodulation, according to an embodiment of the present subject matter.

[0009] Fig. 2(b), 2(c), 2(d), and 2(e) illustrate various stages of folding technique used by the communication device for digital signal demodulation, according to an embodiment of the present subject matter.
[0010] Fig. 3 illustrates a method of digital signal demodulation in a communication device, according to an embodiment of the present subject matter.
DETAILED DESCRIPTION
[0011] Systems and methods for demodulating digital signals in a communication device are described herein. The systems and methods can be implemented in a variety of communication devices. The communication devices that can implement the described method(s) include, but are not limited to, fixed telephones, hand-held devices, laptops or other portable computers, mobile phones, personal digital assistants (PDAs), global positioning system (GPS) tracking devices, and the like. Additionally, the method can be implemented in any communication network, such as Global System for Mobile Communication (GSM) network, Enhanced Data rates for GSM Evolution (EDGE) network, Universal Mobile Telecommunications System (UMTS) network, Personal Communications Service (PCS) network, Time Division Multiple Access (TDMA) network, Code Division Multiple Access (CDMA) network, Next Generation Network (NGN), and IP-based network, Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), Long Term Evolution (LTE) network, and satellite communication network. In general, the systems and methods may be implemented in any network where frequency offset may be experienced and/or in any communication device. Although the description herein is with reference to certain communication networks, the systems and methods may be implemented in other communication networks and devices, albeit with a few variations, as will be understood by a person skilled in the art.
[0012] Typically, in a digital communication network, the data, decoded as data symbols, is transmitted in the form of transmitted signals by one or more transmitters of the communication network. The transmitted signals traverse through a channel and are received, as received signals, by a receiver unit of a communication device. However, the transmitted signal, while traversing in the channel, is adversely affected by various channel impairments, such as noise, interferences, multipath propagation, and frequency variations, for example, due to

Doppler spread. As would be well known to those skilled in the art, noise in the channel is typically caused due to distortions while interferences are typically caused, for example, by operation of other communication devices. Multipath propagation occurs when the transmitted signals travel through multiple paths of propagation to reach the communication device while Doppler spread is typically caused due to movement of the communication devices in relation to each other and the surroundings.
[0013] Further, the received signals are also adversely affected by frequency variations caused due to operation of various components, such as filters and oscillators implemented for signal processing at the receiver unit. Hence the received signals are seldom identical to the transmitted signals. Usually, the receiver unit is configured to recover, from a received signal, an estimated signal that is substantially similar to the originally transmitted signal. However, various impairments introduced by the channel and various components of the receiver unit result in a signal distortion, thus affecting the estimation of the originally transmitted signal. For instance, complex values representing the data obtained by decoding the received signal may not be similar to complex values representing the data originally transmitted. Thus, the receiver unit typically uses corrective processes, such as data symbol equalization and mapping to obtain complex values similar to the originally transmitted data prior to demodulation of the received signal.
[0014] One approach of symbol mapping involves obtaining imaginary and real values corresponding to decoded data symbols and representing the imaginary and real values on imaginary and real planes on a data symbol map respectively. Further, the real values and the imaginary values are combined in a single plane by a process referred to as folding. Further, a mean value of location, on the data symbol map, of all the decoded symbols is calculated based on which error margins in location of each of the data symbols is computed to obtain equalized symbols. However, using mean values for estimating the error margins may affect a computation of the equalized symbols as the mean values are themselves obtained based on the defective data symbols.
[0015] The present subject matter discloses a system and a method of demodulating digital signals in a communication device. Data transmitted in the form of a transmitted signal, by one or more transmitters, such as a transmission unit of a fixed base station or a

communication device, such as a cellular phone, a personal digital assistant, or a portable computer is received, as a received signal, by a receiver of the communication device, or the fixed base station, respectively. Although the description herein is with reference to a downlink of a transmission system, the systems and methods may be implemented in an uplink of the transmission system, albeit with a few variations, as will be understood by a person skilled in the art.
[0016] The transmitted signal propagates over a physical medium, usually referred to as a channel, to reach the receiving communication device. The received signal is subsequently processed to estimate a channel impulse response (CIR) of the channel. The CIR may be understood as channel coefficients that help in estimating the characteristics of the channel, when the channel is considered as a filter. Estimating the characteristics of the channel enables to undo the distortions caused by the channel and recover an estimated signal which is substantially similar to the originally transmitted signal. The CIR, as will be known to a person skilled in the art, may be estimated using various methods that have not been explained here for the sake of brevity. The CIR thus estimated is used for determining equalized symbols corresponding to the received signal. The equalized symbols may be understood as the symbols or known information obtained by equalizing data derived from the received signal. The equalized symbols may be further demodulated to obtain demodulated symbols having the data similar to the data originally transmitted.
[0017] In one embodiment, to obtain the demodulated symbols a constellation diagram representing the data symbols received in the received signal is obtained based on the equalized symbols. The constellation diagram, as will be understood by a person skilled in the art, is a representation of a signal modulated using a digital modulation scheme, such as quadrature amplitude modulation (QAM), phase-shift keying (PSK), quadrature phase-shift keying (QPSK) 16QAM, and 64QAM. The constellation diagram thus represents possible data symbols that may be selected by the modulation scheme as points in a complex plane along an inphase (I) and a quadrature phase (Q) axis. In one implementation, the constellation diagram includes one or more constellation points corresponding to the equalized symbols, such that for each equalized symbol a corresponding constellation point is represented in the complex plane.

[0018] The constellation diagram may be subsequently analyzed to determine soft bits corresponding to each of the constellation points and in turn the equalized symbols such that the data symbols obtained using the soft bits are substantially similar to the originally transmitted data symbols. In one implementation, the soft bits may be determined using a method of shifting and folding of a predetermined number of axes sets of the constellation diagram. For instance, the constellation diagram may include a pair of I1, Q1, I2, Q2, I3, Q3, and I4, Q4 axes. In order to determine the number of axes sets, system parameters, such as power levels used during transmission and modulation schemes used for modulating the transmitted signal are ascertained. Subsequently, the number of sets of axes to be used is determined, for instance, based on the modulation schemes used.
[0019] Further, one or more quantization regions corresponding to the axes sets are determined such that for each axes set at least one quantization region is determined. Quantization may be defined as the process of mapping a large set of input values to a smaller set, while the quantization region may be defined as that region on the constellation diagram in which the smaller set of values lie. The quantization regions may thus be used to map the constellation points to soft bits substantially similar to the originally transmitted data symbols. In one implementation, the quantization region may be determined based on an out-bit width (w) parameter representing the number of bits required to represent a soft bit to be determined.
[0020] Based on the quantization regions and the axes sets, soft bits corresponding to each of the constellation points may be computed. In order to obtain the soft bits, a folding is performed for each axes set to minimize the sets of quantization regions to be analyzed for ascertaining the soft bits corresponding to the constellation points lying near the quantization regions. In one implementation, number of times the folding may be performed, in order to determine soft bits corresponding to all the complex points present in the digital signal, is equal to twice the number of axes sets. For instance, in a case of 2 axes sets, initially two foldings may be performed for a first axes set to obtain a first intermediate soft bit and a second intermediate soft bit. Subsequently, a second axes set may be folded in two steps to obtain a third intermediate soft bit and a fourth intermediate soft bit. Intermediate soft bits, such as the first intermediate soft bit, the second intermediate soft bit, the third intermediate soft bit, and the fourth intermediate

soft bit may be combined to obtain a soft bit representation corresponding to the constellation points. The soft bits thus obtained may be utilized as the demodulated symbols.
[0021] The system and the method of the present subject matter thus facilitate the demodulation of a digital signal in a communication device using a technique of folding of axes for soft bit mapping of equalized symbols. Using the technique of folding of axes allows the system and the method to be efficiently used in wireless communicating devices as the technique significantly reduces the time and resources required as instead of analyzing all quantization regions only about half the quantization regions are analyzed to efficiently compute soft bits corresponding to the equalized symbols for demodulation.
[0022] These and other advantages of the present subject matter would be described in greater detail in conjunction with the following figures. While aspects of described systems and methods for demodulation of digital signals can be implemented in any number of different computing systems, environments, and/or configurations, the embodiments are described in the context of the following exemplary system(s).
[0023] Fig. 1 illustrates a network environment 100 implementing one or more digital signal demodulation systems 102 configured to demodulate digital signals, according to an embodiment of the present subject matter. In one implementation, the network environment 100 includes one or more digital signal demodulation systems 102-1, 102-2, 102-3, …, 102-N (collectively referred to as digital signal demodulation systems 102) communicating with each other through a network 104. The digital signal demodulation systems 102 may include, without limitation, communication devices, such as hand-held devices, laptops, tablets or other portable computers, smart phones, mobile phones, global positioning system (GPS) receivers, personal digital assistants (PDAs), and the like. Each of the digital signal demodulation systems 102, hereinafter referred to as system 102, works on a communication protocol as defined by the network 104 to which the system 102 is coupled.
[0024] The network 104 may be a wireless or a wired network, or a combination thereof. The network 104 can be a collection of individual networks, interconnected with each other and functioning as a single large network. Examples of such individual networks include, but are not limited to, GSM network, Enhanced Data rates for GSM Evolution (EDGE) network, UMTS network, PCS network, TDMA network, CDMA network, NGN, IP-based network, PSTN,

ISDN, LTE, and satellite communication. Depending on the technology, the network 104 includes various network entities, such as base stations, mobile switching centres, transmission towers, gateways, routers; as would be apparent to a person skilled in the art, and such details have been omitted for brevity. Although the description herein is with reference to certain communication networks, the systems and methods may be implemented in other communication networks and devices, albeit with a few variations, as will be understood by a person skilled in the art.
[0025] In operation, originally transmitted signals, i.e., signals transmitted by a base transceiver station (BTS) 106 of the network 104 over a medium, usually referred to as a channel, such as air are received by the system 102 as received signals 108-1, 108-2, 108-3, …, 108-N, collectively referred to as the received signals 108. The transmitted signal, as will be understood, is an analog signal obtained by modulating data to be transmitted. For instance, in digital communications, the data to be transmitted is initially digitally modulated to obtain originally transmitted data symbols that are further converted to analog signals and up-converted before being transmitted as the originally transmitted signals. While traversing through the channel, the originally transmitted signals are subjected to various impairments leading to changes in its properties, such as phase, frequency, and amplitude. Impairments in the originally transmitted signals may be caused due to various factors, such as noise, interferences, multipath propagation, and frequency variations, for example, due to Doppler shift.
[0026] Noise in the channel may be due to various factors, such as distortions due to, for example, faulty electrical equipments. Interferences are typically caused, for example, by operation of other communication devices in the path of signal propagation. Multipath propagation occurs when the originally transmitted signals travel through multiple paths of propagation to reach the system 102. For example, a first path may be the line of sight; a second path may be due to reflection at an obstacle such as a building; and so on. Doppler shift, as would be well known to those skilled in the art, is caused due to movement of the system 102 or other objects in the network environment 100 leading to a change in path length between the BTS 106 and the system 102 or between different communication devices.
[0027] Due the above stated impairments, the received signals 108 are seldom identical to the originally transmitted signals. The channel may thus be considered to be behaving as a

filter causing the impairments in signal properties of the originally transmitted signals with distortion of the originally transmitted signals being regarded as a characteristic property of the channel. Further, the received signals 108 are also adversely affected by the noise and frequency variations caused due to operation of the various components, such as filters and oscillators, implemented for signal processing in the system 102. The systems 102 are thus configured to estimate and correct the distortions in the received signal 108.
[0028] The system 102 are configured to down-convert the received signal 108 in order to recover an estimated signal that is substantially similar to the original transmitted signal. For the purpose, the system 102 initially mix the received signal 108 with a reference signal, also known as a local carrier signal to generate a baseband or intermediate frequency (IF) signal that may be demodulated to obtain demodulated symbols. Further, in case of digital communication, the system 102 convert the analog signal into digital signals using various intermediate processes, such as channel estimation and signal equalization to obtain complex digital symbols, also referred to as equalized symbols, prior to the demodulation. However, the various impairments, as discussed above, may result in an incorrect estimation of the equalized symbols thus affecting the estimation of the received signal and in turn the data symbols received in the received signal.
[0029] The system 102 implements a process of soft bit mapping to obtain demodulated symbols having data similar to originally transmitted data. In one implementation, the channel estimates obtained using channel estimation of the received signal are provided to a demodulation module 110 configured to obtain the demodulated symbols. The demodulation module 110 processes the equalized symbols in order to obtain the demodulated symbols. In one implementation, the demodulation module 110 initially represents the received signal on a constellation diagram.
[0030] As previously discussed, a constellation diagram may be defined as a representation of possible data symbols, present in a signal, where the data symbols may be selected by a digital modulation scheme as points in a complex plane along I and Q axis while modulating data to obtain a modulated signal. Further, the demodulation module 110 obtains the constellation diagram such that the constellation diagram includes one or more constellation

points. In one implementation, the demodulation module 110 represents each equalized symbol using a corresponding constellation point in the complex plane.
[0031] A constellation diagram representing the data symbols received in the received signal is further obtained based on the equalized symbols. The constellation diagram, as will be understood by a person skilled in the art, is a representation of a signal modulated using a digital modulation scheme, such as quadrature amplitude modulation (QAM), phase-shift keying (PSK), quadrature phase-shift keying (QPSK) 16QAM, and 64QAM. The constellation diagram thus represents possible data symbols that may be selected by the modulation scheme as points in a complex plane along I and Q axis. In one implementation, the constellation diagram includes one or more constellation points corresponding to the equalized symbols, such that for each equalized symbol a corresponding constellation point is represented in the complex plane.
[0032] The demodulation module 110 further analyzes the constellation diagram to determine soft bits corresponding to each of the constellation points. In one implementation, the demodulation module 110 determines the soft bits using a method of shifting and folding of a predetermined number of axes sets of the constellation diagram. The axes sets may be understood as different sets of I and Q axes defined in the complex plane in which the constellation diagram is defined, such that each set of axes encompasses a predefined region on the constellation diagram and constellation points present in the predefined region. The demodulation module 110 initially determines the number of axes sets, using one or more system parameters, such as modulation schemes used for obtaining the modulated originally transmitted signal.
[0033] The demodulation module 110 subsequently determines one or more quantization regions corresponding to the axes sets such that for each axes set at least one quantization region is determined. A quantization region may be defined as a high power region to which a large set of constellation points can be mapped to obtain a smaller and more accurate set of soft bits corresponding to the constellation points. In one implementation, the demodulation module 110 may determine the quantization region based on an out-bit width (w) parameter representing the number of bits required to represent a soft bit to be determined.
[0034] Further, based at least on the quantization regions and the axes sets, the demodulation module 110 computes soft bits corresponding to each of the constellation points.

In one implementation, the demodulation module 110 performs a folding for each axes set in order to minimize corresponding quantization regions for ascertaining the soft bits corresponding to the constellation points lying near the quantization regions. The working of the demodulation module 110 will be explained in greater detail with the relation to figures 2(a), 2(b), 2(c), 2(d), and 2(e). The soft bits thus obtained from the demodulation module 110 can be converted into hard bits by specific decoders used in receivers.
[0035] Although, the demodulation system 102 has been explained with reference to the communication devices, it would be understood that such an explanation is only for the purposes of illustration and should not be construed as a limitation. The concepts relating to the demodulation system 102 implementing the process of folding of the axes, explained herein with reference to the communication devices, can be extended to various other devices and systems. The demodulation system 102 may be implemented in any transceiver using digital modulations, for example, the demodulation system 102 may be implemented as the BTS 106 for demodulating data symbols received from the communication devices.
[0036] Fig. 2(a) illustrates components of the system 102, in accordance with an embodiment of the present subject matter. The system 102 includes interface(s) 202, one or more processor(s) 204, and a memory 206 coupled to the processor(s) 204. The interfaces 202 may include a variety of software and hardware interfaces, for example, interfaces for peripheral device(s), such as a keyboard, a mouse, an external memory, a camera device, and a printer. Further, the interfaces 202 may enable the system 102 to communicate with other devices, such as web servers and external databases. The interfaces 202 can facilitate multiple communications within a wide variety of networks and protocol types, including wired networks, for example, local area network (LAN), cable, etc., and wireless networks, such as Wireless LAN (WLAN), cellular, or satellite. For the purpose, the interfaces 202 may include one or more ports for connecting a number of computing systems with one another or to another server computer.
[0037] The processor(s) 204 can be a single processing unit or a number of units, all of which could include multiple computing units. The processor 204 may be implemented as one or more microprocessors, microcomputers, microcontrollers, digital signal processors, central processing units, state machines, logic circuitries, and/or any devices that manipulate signals

based on operational instructions. Among other capabilities, the processor 204 is configured to fetch and execute computer-readable instructions and data stored in the memory 206.
[0038] The memory 206 may include any computer-readable medium known in the art including, for example, volatile memory, such as static random access memory (SRAM) and dynamic random access memory (DRAM), and/or non-volatile memory, such as read only memory (ROM), erasable programmable ROM, flash memories, hard disks, optical disks, and magnetic tapes. The memory 206 also includes module(s) 208 and data 210.
[0039] The modules 208, amongst other things, include routines, programs, objects, components, data structures, etc., which perform particular tasks or implement particular abstract data types. The modules 208 further include a receiver front-end module 212, a channel estimation module 214, an equalization module 216, the demodulation module 110, and other module(s) 218. The other modules 218 may include programs that supplement applications on the system 102, for example, programs in the operating system. Further, the other modules 218 may include decoders (not shown in the figure), such as turbo decoders and Viterbi decoders for processing the soft bits obtained by the demodulation module 110. On the other hand, the data 210 serves, amongst other things, as a repository for storing data processed, received, and generated by one or more of the modules 208. The data 210 includes signal data 220, channel impulse response data 222, equalization data 224, modulation data 226, and other data 228. The other data 228 includes data generated as a result of the execution of one or more modules in the other modules 218.
[0040] In operation, the originally transmitted signal, transmitted over the channel, is received as the received signal 108 by the system 102. As mentioned earlier, the received signal 108 and the originally transmitted signal are seldom identical. Hence, the system 102 is configured to recover an estimated signal, which is substantially identical to the originally transmitted signals, from the received signal 108. The received signal 108 is initially received by an antenna (not shown in this figure) and saves the received signal in the signal data 220. Further the signal data 220 is accessed by the receiver front-end module 212 to obtain the received signal. The receiver front-end module 212 is configured to down-convert the received signal 108 to generate the baseband signal. In one implementation, the receiver front-end module 212 mixes the received signal 108 with a reference signal, for example, a local carrier signal generated by a

local oscillator (not shown in the figure) connected to the receiver front-end module 212. A down-converted signal thus received is sampled using an Analog to Digital converter (not shown in the figure) provided in the receiver front-end module 212 to obtain a digital received baseband signal r(n), hereinafter referred to as the received signal r(n). The received signal r(n) is subsequently saved in the signal data 220 and fed to the channel estimation module 214 and the equalization module 216.
[0041] The channel estimation module 214 is configured to estimate the CIR. The CIR may be understood as channel coefficients that help in determining characteristics of the channel when the channel is considered as a filter. As explained previously, the CIR of the channel is thus used to undo the distortions caused by the channel and recover an estimated signal, which is substantially similar to the originally transmitted signals. The CIR, as will be known to a person skilled in the art, may be estimated using various methods that have not been explained here for the sake of brevity. In one implementation, the CIR may be estimated using a training sequence present in the received signal 108. The training sequence, as will be apparent to a person skilled in the art, may be provided in the transmitted signal corresponding to the received signal 108 and may be used as a timing reference and for equalization of the received signal 108.
[0042] The channel estimation module 214 then saves the CIR in the channel impulse response data 222. Subsequently, the channel estimation module 214 provides the CIR to the equalization module 216. In one implementation, the equalization module 216 is configured to equalize the received signal 108 using the CIR and the received signal r(n) to obtain equalized symbols. The equalized symbols may be further provided to the demodulation module 110 to obtain demodulated symbols. However, as discussed previously, various impairments subjected to the transmitted signal as well as the received signal r(n) result in an incorrect estimation of the equalized symbols, thus affecting demodulation. The demodulation module 110 may thus use corrective techniques to obtain the demodulated symbols using the equalized symbols that may be impaired. In one implementation, the demodulation module 110 uses a process of soft bit mapping to obtain soft bits corresponding to the equalized symbols to obtain the demodulated symbols.
[0043] Further as described in fig. 1, the demodulation module 110 maps the received signal on the constellation diagram having the one or more constellation points such that for each

of the equalized symbol a corresponding constellation point is represented in the complex plane on which the constellation diagram is defined. For example, the demodulation module 110 may map the received signal on a constellation diagram 230 as illustrated in fig. 2(b). The constellation diagram 230 represents, along I1 and Q1 axis, a signal modulated using 16 QAM technique and includes 16 distinct constellation points having symbol energy levels varying in the range of [-3p, -p, p, 3p]. Although, the constellation diagram 230 is illustrated for a signal modulated using 16 QAM technique, it will be understood that a similar constellation diagram may be generated for signals modulated using other modulation techniques, for example, QAM, QPSK, and the like albeit with a few variations, as will be understood by a person skilled in the art.
[0044] The demodulation module 110 subsequently analyzes the constellation diagram to determine soft bits corresponding to each of the constellation points and in turn the equalized signals. In one implementation, the demodulation module 110 determines the soft bits using a method of shifting and folding of a predetermined number of the axes sets of the constellation diagram. For instance, the constellation diagram may include a pair of I1, Q1, I2, Q2, I3, Q3, and I4, Q4 axes. In order to determine the number of axes sets, the demodulation module 110 determines various system parameters, such as symbol energy levels used during transmission and modulation schemes used for modulating the transmitted signal. Subsequently, the demodulation module 110 determines the number of sets of axes to be used based on the modulation schemes used at the BTS 106. In one implementation, the demodulation module 110 determines the number of axes sets using the following equation:
Number of axes sets = N/2………………..(1)
where, N = number of bits required to uniquely represent constellation points on a constellation diagram. For instance, in the previous example of the constellation diagram 230, N = 4 for the 16 QAM modulation technique. The demodulation module 110 thus determines the number of axes sets as 2. The number of sets of axes thus obtained may be stored in the equalization data 224.
[0045] Further, the demodulation module 110 determines the one or more quantization regions corresponding to the axes sets. For the purpose, the demodulation module 110 may initially determine the out-bit width (w) parameter based on, for instance, the precision required to estimate the soft bits. Further, based on the out-bit width parameter, the demodulation module

110 determines the one or more quantization regions such that each of the quantization regions includes a predetermined number of quantization levels. The quantization levels may be defined as a range of symbol energy levels of the equalized signal within which values of the soft bits fall. In one implementation, the demodulation module 110 computes the predetermined number of quantization levels using the following equation:
Number of Quantization levels = 2W – 1…………………(2)
[0046] Further, demodulation module 110 computes size of each of the quantization levels using the following equation:
Size of each Quantization level = 2p/ (2W – 1)…………………(3)
where 2p represents distance between 2 adjacent constellation points on the constellation diagram.
[0047] For example, for an out-bit width (w) parameter = 3, the demodulation module 110 may determine the number of quantization levels as 7. Based on the quantization levels, the quantization region may be determined as illustrated in table 1.
Table 1

111 110 101 000 001 010 011
(-3) (-2) (-1) 100 (0) (1) (2) (3)

[0048] Further, in the above example, the quantization levels thus obtained act as sampling ranges to which the soft bits will be sampled. The soft bits thus will have a value in a range of -3q and 3q. The quantization levels thus obtained may be stored in the equalization data 224.
[0049] Based on the determined quantization regions and the number of axes sets the demodulation module 110 logically synthesizes the constellation diagram as a constellation diagram having the predetermined number of axes and the quantization regions corresponding to each of the axes sets. For instance, in the previous example of the constellation diagram 230, the demodulation module 110 may synthesize the constellation diagram 230 to include two axes sets I1, Q1, and I2, Q2 as illustrated in fig. 2(c). The fig. 2(c) further illustrates five quantization

regions 232-1, 232-2, 232-3, 232-4, and 232-5 corresponding to I1 and I2 axis and having quantization levels between -3q and 3q as determined in table 1. Further, as illustrated in the fig. 2(c), the quantization regions for each of the axes sets are determined such that a quantization region is ascertained for each intersection of the I and Q axis of axes set. Although the fig. 2(c) illustrates quantization regions corresponding to I1 and I2 axis, similar quantization regions can be obtained for Q1 and Q2 axis.
[0050] Further, the demodulation module 110 performs a folding for each axes set in the constellation diagram in order to compute soft bits corresponding to each of the constellation points. In one implementation, the demodulation module 110 performs the folding such that for each axes set, say I1, Q1 the demodulation module 110 performs two steps of folding with a first folding along vertical axis (Q1) and a second folding along horizontal axis (I1) or vice versa. Thus the number of times the demodulation module 110 performs the folding is equal to twice the number of axes sets. Further, for each axes set the demodulation module 110 determines corresponding intermediate soft bits that may be combined to obtain a soft bit representation corresponding to the constellation points. For the purpose of explanation and not as a limitation, the folding process is further explained with respect to one of the constellation points illustrated in the previous example of the constellation diagram 230. In order to determine the soft bits corresponding to the constellation point the demodulation module 110 initially performs a vertical fold for the axis I1 with respect to the origin to provide a first folded region 234 as illustrated in the fig. 2(d).
[0051] As illustrated in the fig. 2(d), the quantization regions 232-1, 232-2, 232-3, 232-4, and 232-5 are folded such that a first-half portion 236-a of the quantization region 232-3 is folded while a second-half portion 236-b of the quantization region 232-3 lies in the first folded region 234. Further, the demodulation module 110 folds the axis I1 with respect to the origin such that a first mirror image of the constellation point under consideration is obtained in the first folded region 234. The demodulation module 110 subsequently determines the soft bit value corresponding to the constellation point based on a co-ordinate location of the constellation point in the first folded region 234 along the I1 axis. In one implementation, the demodulation module 110 determines the location of the constellation point with respect to a sub-quantization region, for instance, the second half portion of the quantization region 232-3 in the folded region 234.

The demodulation module 110 subsequently maps the location of the constellation point to a suitable quantization level of the sub-quantization region. For instance the demodulation module 110 may map the location of the constellation point to the equalization level ‘3q’. Mapping the constellation point to the quantization level in the sub-quantization region facilitates in estimating the soft bit value corresponding to the constellation point. Further, estimating the soft-bit value with respect to only the sub-quantization region having three quantization levels reduces the time required to analyze and estimate the soft bit value based on mapping to the quantization region having seven quantization levels.
[0052] In one implementation, the demodulation module 110 determines one or more secondary bits of the soft bit value corresponding to the constellation point based on the value of the quantization level of the sub-quantization region to which the constellation point is mapped. The secondary bits may be defined as all sequence bits apart from the most significant bit of the soft bit value. For instance, if the quantization level of the sub-quantization region corresponding to the location of the constellation point is 3q, the equalization module may determine values of the secondary bits of the soft bit value as ‘11’ based on the corresponding sequence bit values of the quantization level as given in the table 1. Additionally, in case the location of the constellation point falls beyond the sub-quantization region, the demodulation module 110 may not map the constellation point and assume a predefined value, say ‘011’ or ‘111’ as the soft bit value corresponding to the constellation point. Further, in order to determine the most significant bit of the soft bit value, the demodulation module 110 determines whether, the constellation point in the first folded region 234 is an actual image of the constellation point under consideration or a mirror image of the constellation point under consideration. In case the constellation point in the first folded region 234 is the mirror image of the constellation point under consideration, the demodulation module 110 may determine a negative value, say ‘1’ as the most significant bit of the soft bit value. On the other hand, if the constellation point in the first folded region 234 is the actual image of the constellation point under consideration, the demodulation module 110 may determine a positive value, say ‘0’ as the most significant bit of the soft bit value. In the present case, thus the demodulation module 110 determines the most significant bit as 1, as the constellation point in the folded region is a mirror image. The soft bit value thus obtained may be stored as a first intermediate soft bit value in the equalization data 224. Similarly, the

demodulation module 110 may fold the Q1 axis along the origin to obtain a second intermediate soft bit value using the process defined for folding the I1 axis.
[0053] Further, the demodulation module 110 performs a similar vertical folding for the second axes set I2, Q2 to determine a third intermediate soft bit value and a fourth intermediate soft bit value corresponding to the constellation point. In order to determine the soft bits corresponding to the constellation point under consideration the demodulation module 110 folds the axis I2 with respect to the Q2 axis, such that the first folded region 234 is folded to provide a second folded region 238 as illustrated in the fig. 2(e).
[0054] The fig. 2(e) illustrates the quantization regions 232-1, 232-2, 232-3, 232-4, and 232-5 folded such that first-half portions 240-a and 242-a of the quantization regions 232-2 and 232-5, respectively, with respect to the Q2 axis. While second-half portions 240-b and 242-b of the quantization regions 232-2 and 232-5, respectively, lie in the folded region 238 and may be utilized as sub-quantization regions for the constellation point under consideration. Further, a second mirror image corresponding to the first mirror image and in turn the constellation point under consideration is obtained in the second folded region 238. The demodulation module 110 subsequently determines the location of the constellation point with respect to the sub-quantization region, for instance, the second half portion of the quantization region 232-2 in the second folded region 238. The demodulation module 110 subsequently maps the location of the constellation point to a quantization level of the sub-quantization region such that co-ordinates of the constellation point along the I2 axis lie within sampling region covered by the equalization level.
[0055] Further, as previously described, the demodulation module 110 determines one or more secondary bits of the soft bit value corresponding to the constellation point based on the value of the quantization level. As illustrated in the fig. 2(e), since the constellation point falls beyond the sub-quantization region, the demodulation module 110 assumes a predefined value '111’ as the soft bit value corresponding to the constellation point. The soft bit value thus obtained is stored as the third intermediate soft bit value in the equalization data 224. Similarly, the demodulation module 110 folds the Q2 axis along the origin to obtain the fourth intermediate soft bit value using the process defined for folding the I1 axis.

[0056] The intermediate soft bits, i.e., the first intermediate soft bit value, the second intermediate soft bit value, the third intermediate soft bit value, and the fourth intermediate soft bit value are combined by the demodulation module 110 to obtain a soft bit representation corresponding to the constellation point under consideration. Soft bits thus obtained may be stored as the demodulated symbols in the modulation data 226. Although the soft bit value determination is explained with reference to only a single constellation point, it will be understood that the same process can be used for estimating soft bits corresponding to other constellation points also.
[0057] In one implementation, the soft bits may be further converted into hard bits by a decoder (not shown in the figure) of the system 102. For instance, the turbo decoders and the Viterbi decoders provided in the other modules 218 may process the soft bits obtained by the demodulation module 110. The hard bits may be understood as a representation of the data corresponding to the demodulated symbols in the form of only a value representing the symbol. Using the soft bits for decoding facilitates efficient decoding process as the decoders may need to require weighted algorithms used by conventional decoders for the decoding process. The system 102 thus facilitates in minimizing the decoder processing requirements by providing the soft bit representation of data required for decoding.
[0058] The present system thus facilitate an efficient and accurate demodulation of the received signal using a technique of folding of axes sets for soft bit mapping of the equalized symbols. Further, using the technique of folding of axes sets reduces the time required to analyze and map the soft bits as the system 102 analyzes about half the quantization regions to estimate the soft bits. Further, mapping the soft bits to the equalization levels ensures an accurate estimation of the originally transmitted data as a probability of the soft bits being closer to the originally transmitted data is higher as compared to the conventional systems.
[0059] Fig. 3 illustrates a method 300 of demodulating digital signals in a communication device, such as the system 102, in accordance with an embodiment of the present subject matter. The method 300 is implemented in computing device, such as the system 102. The method may be described in the general context of computer executable instructions. Generally, computer executable instructions can include routines, programs, objects, components, data structures, procedures, modules, functions, etc., that perform particular

functions or implement particular abstract data types. The method may also be practiced in a distributed computing environment where functions are performed by remote processing devices that are linked through a communications network. Herein, some embodiments are also intended to cover program storage devices, for example, digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of the described method. The program storage devices may be, for example, digital memories, magnetic storage media such as magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media.
[0060] The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method, or an alternative method. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein. Furthermore, the method can be implemented in any suitable hardware, software, firmware, or combination thereof.
[0061] At block 302, an originally transmitted signal is received as a received signal, such as the received signal 108 by a communication device, such as the system 102. The received signal 108 is seldom identical to the originally transmitted signal, as the originally transmitted signal suffers from various impairments, such as frequency variations due to Doppler spread. Additionally, the received signal 108 is also subjected to impairments caused due to the operation of various components, such as clock circuits, and oscillators of the receiving device, i.e., the system 102. The impairments result in the received signal being different from the originally transmitted signal thus affecting decoding of the originally transmitted signal.
[0062] At block 304, a channel impulse response (CIR) of a channel used to transmit the originally transmitted signal is determined based on the received signal. In one implementation, the CIR may be estimated, for example, by the channel estimation module 214, using a training sequence present in the received signal 108. The training sequence, as will be apparent to a person skilled in the art, is provided in each of the received signal and is used as a timing reference and for equalization of the received signal.

[0063] At block 306, the received signal is processed to obtain equalized symbols corresponding to the received signal, for example, by the equalization module 110. In one embodiment, the received signal 108 is initially equalized based on the CIR and the received signal to obtain the equalized symbols. The equalized symbols, however, may be distorted due to the various impairments subjected to the received symbol thus affecting the demodulation of the received signal to obtain originally transmitted data symbols.
[0064] At block 308, a constellation diagram having one or more constellation points corresponding to the equalized symbols is obtained, for example, by the demodulation module 110. The constellation diagram, as will be known, represents possible data symbols that may be selected by a modulation scheme as points in a complex plane along I and Q axis. In one implementation, the constellation diagram includes one or more constellation points corresponding to the equalized symbols such that for each equalized symbol a corresponding constellation point is represented in the complex plane. Further, a predetermined number of axes sets are defined for the constellation diagram, such that each set of axes encompasses a predefined region on the constellation diagram and constellation points present in the predefined region. In one implementation, the number of axes sets may be determined, for example, by the demodulation module 110 using one or more system parameters, such as modulation schemes used for obtaining the modulated originally transmitted signal.
[0065] At block 310, one or more quantization regions corresponding to each of the predetermined number of axes are determined. In one implementation, the quantization regions are determined such that for each axes set at least one quantization region is determined. In order to estimate the quantization region, an out-bit width parameter representing the number of bits required to represent a soft bit may be determined based on, for instance, the precision required to estimate the soft bit. Further, based on the out-bit width parameter the quantization regions may be determined such that each of the quantization regions includes a predetermined number of quantization levels. The quantization levels may be defined as a range of symbol power of the signal within which values of the soft bits fall. The quantization regions thus obtained may be used to map the constellation points to soft bits substantially similar to the originally transmitted data symbols.

[0066] At block 312, soft bits corresponding to each of the one or more constellation points are computed based on the quantization regions and the axes sets. In one implementation, the soft bits are computed using a technique of folding of the axes set such that for each constellation point the number of times the folding is performed is equal to the number of axes, i.e., twice the number of axes sets determined in the constellation diagram. For instance, if two axes sets are determined by the demodulation module 110, the folding may be performed four number times, thus minimizing the sets of quantization regions to be analyzed for ascertaining the soft bits corresponding to the constellation points lying near the quantization regions. In one implementation, for each first step of folding an intermediate soft bit value may be determined, such that for each constellation point a corresponding soft bit value may be computed by combining the intermediate soft bits obtained after each folding of axes. The soft bits thus obtained may be further decoded to hard bits to obtain data similar to originally transmitted data. Using the soft bits for decoding process helps in eliminating the requirement of complex decoding algorithms, thus facilitating in fast and efficient decoding of the data.
[0067] Although implementations for demodulation of digital signals in a communication device have been described in language specific to structural features and/or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as specimen implementations for demodulation of the digital signals.

I/We claim:
1. A method of demodulating a digital signal in a communication device, the method
comprising:
obtaining a constellation diagram having a predetermined number of axes sets and one or more constellation points corresponding to one or more equalized symbols;
determining a quantization region corresponding to each of the predetermined number of axes sets;
folding a first axis of an axes set with respect to a second axis of the axes set to obtain a folded region, wherein the folded region includes an sub-quantization region corresponding to the quantization region associated with the axes set; and
ascertaining, for a constellation point selected from among the one or more constellation points, a soft bit corresponding the constellation point based at least on the folded region and the sub-quantization region.
2. The method as claimed in claim 1, wherein the ascertaining comprises:
determining a location of the constellation point in the folded region with respect to a quantization level of the sub-quantization region of the folded region;
calculating one or more secondary bits of the soft bit value corresponding to the constellation point based at least on the determination;
determining whether the location of the constellation point in the folded region corresponds to an actual image of the constellation point or a mirror image of the constellation point to obtain a most significant bit of the soft bit value corresponding to the constellation point; and
computing an intermediate soft bit value corresponding to the constellation point based at least on the one or more secondary bits and the most significant bit.
3. The method as claimed in claim 1, wherein the ascertaining comprises:
obtaining one or more intermediate soft bit values for each of the predetermined number of axes sets; and

combining the one or more intermediate soft bit values to obtain the soft bit value corresponding to the constellation point.
4. The method as claimed in claim 2, wherein the calculating comprises assigning a predetermined value as the one or more secondary bits for the constellation point having the location outside the sub-quantization region.
5. The method as claimed in claim 2, wherein the calculating comprises assigning, as the one or more secondary bits, a value associated with the quantization level corresponding to the location of the constellation point.
6. A digital signal demodulation system (102) comprising:
a processor (204); and
a memory (206) coupled to the processor (204), the memory (206) comprising: an equalization module (216) configured to generate one or more equalized
symbols based at least on a received signal; and a demodulation module (110) configured to:
obtain a constellation diagram having a predetermined number of axes sets and one or more constellation points corresponding to the one or more equalized symbols obtained by equalizing the received signal;
determine a quantization region corresponding to each of the predetermined number of axes sets to obtain one or more quantization regions;
folding a first axis of an axes set with respect to a second axis of the axes set to obtain a folded region, wherein the folded region includes a sub-quantization region corresponding to the quantization region associated with the axes set; and
ascertaining, for a constellation point selected from among the one or more constellation points, a soft bit corresponding the constellation point based at least on the folded region and the sub-quantization region.
7. The digital signal demodulation system (102) as claimed in claim 6, wherein the
demodulation module (110) is further configured to:

obtain one or more intermediate soft bit values for each of the predetermined number of axes sets; and
combine the one or more intermediate soft bit values to obtain the soft bit value corresponding to the constellation point.
8. The digital signal demodulation system (102) as claimed in claim 6, wherein the
demodulation module (110) is further configured to:
determine a location of the constellation point in the folded region with respect to a quantization level of the sub-quantization region of the folded region;
calculate one or more secondary bits of the soft bit value corresponding to the constellation point based at least on the determination;
determine whether the location of the constellation point in the folded region corresponds to an actual image of the constellation point or a mirror image of the constellation point to obtain a most significant bit of the soft bit value corresponding to the constellation point; and
compute an intermediate soft bit value corresponding to the constellation point based at least on the one or more secondary bits and the most significant bit.
9. The digital signal demodulation system (102) as claimed in claim 8, wherein the demodulation module (110) is further configured to assign a predetermined value as the one or more secondary bits for the constellation point having the location outside the sub-quantization region.
10. The digital signal demodulation system (102) as claimed in claim 8, wherein the demodulation module (110) is further configured to assign as the secondary bits a value associated with the quantization level corresponding to the location of the constellation point.
11. The digital signal demodulation system (102) as claimed in claim 6, wherein the digital signal demodulation system (102) further includes a channel estimation module (214) configured to compute channel impulse response (CIR) for the received signal.
12. The digital signal demodulation system (102) as claimed in claim 6, wherein the digital signal demodulation system (102) further includes a decoder configured to decode the soft bit to obtain a corresponding hard bit, wherein the hard bit includes data similar to data originally transmitted to the digital signal demodulation system (102).

13. A computer-readable medium having embodied thereon a computer program for executing a method comprising:
obtaining a constellation diagram having a predetermined number of axes sets and one or more constellation points corresponding to one or more equalized symbols;
determining a quantization region corresponding to each of the predetermined number of axes sets;
folding a first axis of an axes set with respect to a second axis of the axes set to obtain a folded region, wherein the folded region includes an sub-quantization region corresponding to the quantization region associated with the axes set; and
ascertaining, for a constellation point selected from among the one or more constellation points, a soft bit corresponding the constellation point based at least on the folded region and the sub-quantization region.