DESC:TECHNICAL FIELD
[0001] The present disclosure relates generally to the field of wireless communication systems. In particular, the present disclosure pertains to a system and method for doppler shift eatimation of communication devices on high-speed trains (HST).

BACKGROUND
[0002] A fourth generation (4G) or Long-Term Evolution (LTE) cellular communication system is generally designed to provide an optimized performance to a user who moves at a low speed. Such a system may be designed to support a user who moves at a speed of 350 km/h, but the performance thereof is inferior to that of a user who moves at a low speed. If such a cellular communication system is applied to a high-speed train, link quality between a network and the high-speed train is deteriorated and sufficient link capacity cannot be ensured due to high mobility of 350 km/h. If the speed of the high-speed train increases further, performance deterioration will become more serious and quality of a wireless data service provided to a passenger will be significantly decreased. If a scenario in which a high-speed train uses some capacity of a macro base station (BS), data communication of other users in a cell may be deteriorated.
[0003] Conventional wireless systems mainly use Physical Uplink Channel (PUSCH) based Demodulation Reference Signal (DMRS) symbols for estimation of Doppler shift/spread. Due to lack of DMRS density in a typical 4G/LTE network, i.e., only 2 PUSCH DMRS symbols per Transmission Time Interval (TTI), the maximum Doppler speed that can be estimated is around +/-1KHz whereas the maximum Doppler requirement can be up to +/-1340Hz for a User Equipment (UE) with a velocity of up to 350km/hr for Band 1 of the LTE network. Further, though higher Doppler frequency may be estimated with Physical Uplink Control Channel (PUCCH) by virtue of higher DMRS density for up to 4 DMRS symbols, however the maximum frequency estimation is limited to +/-1.75 KHz. This may not satisfy the requirements for band 41 of the LTE network with UE speed up to 350km/hr. PUCCH is not guaranteed to be scheduled every 1 millisecond sub-frame of the TTI. For simultaneous PUSCH and PUCCH transmissions, if “simultaneous PUCCH-PUSCH” parameter is enabled during estimation of the Doppler frequency, then number of reference symbols will be lesser than separate transmissions which will effect range of the estimation. Frequency offset estimation in time domain may also be used for estimation of the Doppler frequency. This can be done using cyclic prefix based correlation for all DMRS symbol in TTI which can estimate maximum Doppler frequency up to 1/T_s, where T_s is sample duration.
[0004] Efforts have been made in the past to develop systems for 4G/LTE networks for handling multiple UEs moving at high-speeds and having high Doppler frequencies. Fig. 1 shown a conventional system, in which the UEs of multiple users within each moving carriage of a high-speed train is served by antennas mounted on each train carriage roof-top (TCAS/ETCS: Train Collision Avoidance System/European Train Control System), which provide Line-of-Sight (LOS) visibility to each of a plurality of base stations disposed along a track on which the high-speed train is running. The antennas are mounted on each carriage which also provides LOS visibility to each of the base stations along the track. The base station should serve directly handheld UE devices within the end-carriages of a driver and a railway guard of the train for facilitating various communication processes therebetween, for example, Mission-Critical Push-to-Talk (MCPTT). Internet-of-Things (IOT) based asset monitoring, onboard Vehicle Signal Specification (VSS), a passenger information system (PIS) and an onboard Wi-Fi facility is generally provided in trains for monitoring, communicating, and assessing various parameters related to the trains and the UEs of the users.FIG. 2 shows comparison of PUSCH DMRS symbols with time-spacing of 7 Orthogonal Frequency-Division Multiplexing (OFDM) symbols in the conventional system, depicted by green colour, with 3 OFDM symbols apart, closely time-spaced DMRS occurrence followed by Sounding Reference Signal (SRS) shown in blue colour. .
[0005] FIG. 3 illustrates a process flow diagram generally employed in conventional wireless systems for handling single UEs moving at high-speeds and having high Doppler frequencies. In the process flow diagram, sample of a given OFDM symbol with length as Fast Fourier Transform (FFT) size is added with length of cyclic prefix ( ), where is number of DMRS symbols per TTI and is the sample duration. Autocorrelation is calculated between cyclic prefix (CP) samples and corresponding original samples within one OFDM symbol. The estimates are averaged across DMRS symbols per TTI, from which corresponding frequency offset is computed. However, such techniques are erroneous when multiple UEs with various speed are required to be supported in each TTI. This approach is computationally complex compared to other methods as it involves computation of received signal in time domain.
[0006] There is therefore a need in the art to provide a reliable and robust wireless communication system and method capable of addressing the above shortcomings, while efficiently catering to single or multiple UEs moving at high-speeds with same or different velocities.

OBJECTS OF THE INVENTION
[0007] An object of the present disclosure is to overcome the problems associated with the conventional wireless communication systems associated with user equipments (UEs) moving at high-speeds.
[0008] Another object of the present disclosure is to provide a wireless communication system for effectively resolving performance gaps in terms of throughput degradation due to high-frequency offset for moving users, and method thereof.
[0009] Another object of the present disclosure is to reliably compensate effects of high Doppler shift affecting fourth generation/ Long Term Evolution (4G/LTE) and fifth (5G) systems where number of reference symbols per Transmission Time Interval (TTI) is limited.
[0010] Another object of the present disclosure is to provide a wireless communication system and method capable of efficiently handling multiple UEs with various speeds having high Doppler frequencies.
[0011] Another object of the present disclosure is to provide a cost-effective solution for deployment of reliable and robust 4G and 5G network for a high-speed trains (HST), while eliminating the need for additional base station deployment to support UE on roof of carriages of the train and within the train.

SUMMARY
[0012] An aspect of the present disclosure is to provide a system for estimation of doppler shifts. The system includes a processor communicatively coupled to a network node of a base station. A memory is operatively coupled with the processor, where said memory stores instructions which, when executed by the processor, cause the processor to determine one or more user equipments UEs connected to the base station and measure a relative movement between the one or more UEs and the base station. The processor determines a doppler shift associated with the one or more UEs based on the measured relative movement, where the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
[0013] In an embodiment, the processor may be configured to determine a frequency offset associated with the doppler shift by measuring a time difference between a second DMRS symbol associated with the DMRS signal and a SRS symbol associated with the SRS signal, wherein the SRS symbol is positioned three symbols away from the second DMRS symbol. The processor may be configured to augment the time difference with an Orthogonal Frequency Division Multiplexing (OFDM) symbol duration.
[0014] In an embodiment, the processor may be configured to determine a transition region that comprises a variation in the doppler shift due to the relative movement between the one or more UEs and the base station, where the doppler shift may switch between a threshold range, and where the threshold range may be based on one or more DMRS symbols transmitted by the UE for a Transmission Time Interval (TTI). In response to a determination that the threshold range is below a threshold value, determine the doppler shift through a plurality of DMRS symbols received from the UE. In response to a determination that the threshold range is above the threshold value, the processor may determine the doppler shift through the SRS symbol and the second DMRS symbol received from the UE.
[0015] In an embodiment, the processor may be configured to compute the OFDM symbol phase estimate based on the frequency offset and compensate the OFDM symbol phase estimate by performing phase rotation of the OFDM symbol to remove effects associated with the frequency offset.
[0016] In an embodiment, the processor may be configured to maintain a 98 percent throughput during the transition region through Hybrid Automatic Repeat Request (HARQ) retransmissions.
[0017] In an embodiment, the processor may be configured to determine the doppler shift with the threshold range of +/-1340 Hz required by a High-Speed Train (HST) moving with a velocity of 350 Km/hr using the second DMRS signal and the SRS signal. The processor may be configured to determine the doppler shift with the threshold range of +/-1340 Hz required by the HST moving with the velocity of 350 Km/hr for 15KHz sub-carrier-spacing fifth generation (5G) deployment bands using the second DMRS signal and the SRS signal. The processor may be configured to determine the doppler shift with the threshold range of +/-2334 Hz required by the HST moving with the velocity of 350 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal. The processor may be configured to determine the doppler shift with the threshold range of +/-1740 Hz required by the HST moving with the velocity of 500 Km/hr for 15 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal. The processor may be configured to determine the doppler shift with the threshold range of +/-3334 Hz required by the HST moving with the velocity of 500 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.
[0018] An aspect of the present disclosure is to provide a method for estimation of doppler shifts. The method includes determining, one or more user equipments UEs connected to a base station and measuring a relative movement between the one or more UEs and the base station. The method includes determining, a doppler shift associated with the one or more UEs based on the measured relative movement, wherein the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
[0019] In an embodiment, the method may inlcude determining, a frequency offset associated with the doppler shift by measuring a time difference between a second DMRS symbol associated with the DMRS signal and a SRS symbol associated with the SRS signal, wherein the SRS symbol is positioned three symbols away from the DMRS symbol. The method may inlcude determining, a frequency offset associated with the doppler shift by augmenting the time difference with an Orthogonal Frequency Division Multiplexing (OFDM) symbol duration.
[0020] In an embodiment, the method may inlcude determining, a transition region that comprises a variation in the doppler shift due to the relative movement between the one or more UEs and the base station, wherein the doppler shift switches between a threshold range, and wherein the threshold range is based on one or more DMRS symbols transmitted by the UE for a Transmission Time Interval (TTI). The method may include in response to a determination that the threshold range is below a threshold value, determining the doppler shift through a plurality of DMRS symbols received from the UE. The method may include in response to a determination that the threshold range is above the threshold value, determining the doppler shift through the SRS symbol and the second DMRS symbol received from the UE.
[0021] In an embodiment, the method may include computing, the OFDM symbol phase estimate based on the frequency offset and compensating the OFDM symbol phase estimate by performing phase rotation of the OFDM symbol to remove effects associated with the frequency offset.
[0022] In an embodiment, the method may inlcude maintaining, a 98 percent throughput during the transition region through Hybrid Automatic Repeat Request (HARQ) retransmissions.
[0023] In an embodiment, the method may inlcude determining, the doppler shift with the threshold range of +/-1340 Hz required by a High-Speed Train (HST) moving with a velocity of 350 Km/hr using the second DMRS signal and the SRS signal. The method may inlcude determining, the doppler shift with the threshold range of +/-1340 Hz required by the HST moving with the velocity of 350 Km/hr for 15KHz sub-carrier-spacing fifth generation (5G) deployment bands using the second DMRS signal and the SRS signal. The method may inlcude determining, the doppler shift with the threshold range of +/-2334 Hz required by the HST moving with the velocity of 350 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal. The method may inlcude determining, the doppler shift with the threshold range of +/-1740 Hz required by the HST moving with the velocity of 500 Km/hr for 15 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal. The method may inlcude determining, the doppler shift with the threshold range of +/-3334 Hz required by the HST moving with the velocity of 500 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.
[0024] In an aspect, a user equipment (UE) for sending requests includes one or more processors communicatively coupled to a processor associated with a system, where the one or more processors are coupled with a memory, and where said memory stores instructions which, when executed by the one or more processors, cause the one or more processors to transmit one or more reference signals to the processor. The processor is configured to determine a relative movement of the UE and a base station. The processor is configured to determine a doppler shift associated with the UE based on the measured relative movement, wherein the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal among the one or more reference signals, with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal among the one or more reference signals.
[0025] In an aspect, a non-transitory computer readable medium comprising a processor with executable instructions, causing the processor to determine one or more user equipments UEs connected to the base station and measure a relative movement between the one or more UEs and the base station. The processor determines a doppler shift associated with the one or more UEs based on the measured relative movement, where the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
[0026] An aspect of the present disclosure is to provide a system for estimation of doppler shifts. The system includes a processor communicatively coupled to a network node of a base station. A memory is operatively coupled with the processor, where said memory stores instructions which, when executed by the processor, cause the processor to determine a doppler shift associated with one or more UEs based on the measured relative movement of the one or more UEs with a base station, where the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE. The processor determines a transition region that comprises a variation in the doppler shift due to the relative movement, where the doppler shift switches between a threshold range of +/-1340 Hz required by a High-Speed Train (HST) moving with a velocity of 350 Km/hr using the second DMRS signal and the SRS signal for 4G/LTE bands. The processor determines, the doppler shift with the threshold range of +/-1340 Hz required by a high-speed train (HST) moving with the velocity of 350 Km/hr for 15KHz sub-carrier-spacing fifth generation (5G) deployment bands using the second DMRS signal and the SRS signal. The processor determines, the doppler shift with the threshold range of +/-2334 Hz required by the HST moving with the velocity of 350 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal. The processor determines, the doppler shift with the threshold range of +/-1740 Hz required by the HST moving with the velocity of 500 Km/hr for 15 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal. The processor determines, the doppler shift with the threshold range of +/-3334 Hz required by the HST moving with the velocity of 500 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.

BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0028] FIG. 1 illustrates a schematic representation of a conventional wireless system deployed in a high-speed train.
[0029] FIG. 2 illustrates a spectrum diagram showing variation in Orthogonal Frequency-Division Multiplexing (OFDM) symbols of time-spaced Physical Uplink Channel (PUSCH) based Demodulation Reference Signal (DMRS) and Sounding Reference Symbols (SRS) of the conventional wireless system.
[0030] FIG. 3 illustrates a process flow diagram employed in the conventional wireless system for handling User Equipments (UEs) moving at high-speeds and having high Doppler frequencies.
[0031] FIG. 4 illustrates a schematic representation of a wireless communication system, in accordance with an embodiment of the present disclosure.
[0032] FIG. 5 illustrates a flowchart showing a wireless communication method for handling multiple user equiments (UEs) moving at high-speeds, in accordance with an embodiment of the present disclosure.
[0033] FIGs. 6A and 6B illustrates plots depicting variation in throughput of a plurality of frames of the wireless communication system with signal-to-noise (SNR) ratio, when the wireless communication system is deployed in a high-speed train, in accordance with an embodiment of the present disclosure.
[0034] FIG. 7 illustrates a plot depicting tracking on Doppler shift variation at high speed, in accordance with an embodiment of the present disclosure.
[0035] FIG. 8 illustrates a plot depicting Cyclic Redundancy Check (CRC) error occurring during Doppler transition regions with respect to number of sub-frames, in accordance with an embodiment of the present disclosure.
[0036] FIG. 9 illustrates a plot depicting performance of Long-Term Evolution (LTE) channel models with sounding reference signal (SRS) based frequency-offset estimation, in accordance with an embodiment of the present disclosure.
[0037] FIG. 10 illustrates a plot depicting management of the Doppler shift variation by the wireless communication system, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0038] Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits, systems, networks, processes, and other components may be shown as components in block diagram form in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments.
[0039] Embodiments of the present invention include various steps, which will be described below. The steps may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor programmed with the instructions to perform the steps. Alternatively, steps may be performed by a combination of hardware, software, firmware and/or by human operators.
[0040] Embodiments of the present invention may be provided as a computer program product or mobile application, which may include a machine-readable storage medium tangibly embodying thereon instructions, which may be used to program a computer (or other electronic devices) or mobile devices, to perform a process. The machine-readable medium may include, but is not limited to, fixed (hard) drives, magnetic tape, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), and magneto-optical disks, semiconductor memories, such as ROMs, PROMs, random access memories (RAMs), programmable read-only memories (PROMs), erasable PROMs (EPROMs), electrically erasable PROMs (EEPROMs), flash memory, magnetic or optical cards, or other type of media/machine-readable medium suitable for storing electronic instructions (e.g., computer programming code, such as software or firmware).
[0041] Various methods described herein may be practiced by combining one or more machine-readable storage media containing the code according to the present invention with appropriate standard computer or mobile hardware, along with a computer application or Android or IOs application, to execute the code contained therein. An apparatus for practicing various embodiments of the present invention may involve one or more computers (or one or more processors within a single computer) and storage systems containing or having network access to computer program(s) coded in accordance with various methods described herein, and the method steps of the invention could be accomplished by modules, routines, subroutines, or subparts of a computer program product.
[0042] The word “exemplary” and/or “demonstrative” is used herein to mean serving as an example, instance, or illustration. For the avoidance of doubt, the subject matter disclosed herein is not limited by such examples. In addition, any aspect or design described herein as “exemplary” and/or “demonstrative” is not necessarily to be construed as preferred or advantageous over other aspects or designs, nor is it meant to preclude equivalent exemplary structures and techniques known to those of ordinary skill in the art.
[0043] Reference throughout this specification to “one embodiment” or “an embodiment” or “an instance” or “one instance” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0044] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
[0045] Also, it is noted that individual embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed but could have additional steps not included in a figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination can correspond to a return of the function to the calling function or the main function.
[0046] While describing a particular figure, certain features have been also referred which are shown in some other figure. For the sake of convenience, the figure. Number is given for such features for understanding. Further, at the beginning only the structure of the apparatus is explained with reference to each figure. Thereafter, the functioning is explained separately and for that purpose the figures are again referred to highlighting on the functional part.
[0047] Embodiments explained herein relate to a wireless communication base station system deployed for high-speed moving objects, for example, a high-speed train (HST), and a method thereof. The wireless communication system facilitates efficient estimation of Doppler shift/spread, and reliably caters to communication requirements of single or multiple User Equipments (UEs) moving at high-speeds with same or different velocities.
[0048] Various embodiments of the present disclosure are described from FIGs. 4-10.
[0049] Referring to FIG. 4, according to an aspect, the wireless base station 400/communication system 400 can facilitate one or more UEs located in a high-speed train (HST). A person skilled in the art may undetsand that the wireless base station 400 may be interchangeably referred as the communication system 400 or as a system 400 throughout the disclsoure. The system 400 is capable of handling multiple UE with various speeds having high Doppler frequency (high speed train scenarios) in a case when the UEs of users within each moving carriage of the HST could be served by antennas mounted on each train carriage roof-top (TCAS /ETCS: Train Collision Avoidance System/European Train Control System) which provide Line-of-Sight (LOS) visibility to each of the base stations located along the track. The system 400 is also applicable to UEs mounted on each carriage which also provides LOS visibility to each of the base stations along the track. The system 400 enables the base stations to serve directly handheld UE devices within the end carriages of a driver and a railway Guard for performing, e.g., Mission-Critical Push-to-Talk (MCPTT). Also, the system 400 may facilitate robust LTE communication for UEs of users of multiple trains travelling with difference velocities in parallel tracks. Here, UEs need not be necessarily at LOS of the base stations. The system 400 may be applied for all these requirements for example, HST speeds of +/-1340Hz for single UE as well as multi-UE scenarios. The system 400 can be used to estimate and compensate Doppler shift/spread for multiple UEs having various speeds.
[0050] As illustrated, the wireless communication system 400 can include one or more processor(s) 402. The one or more processor(s) 402 can be implemented as one or more microprocessors, microcomputers, microcontrollers, digital signal processors, central processing units, logic circuitries, and/or any devices that manipulate data based on operational instructions. Among other capabilities, the one or more processor(s) 402 may be configured to fetch and execute computer-readable instructions stored in a memory 404. The memory 404 can store one or more computer-readable instructions or routines, which may be fetched and executed to create or share the data units over a network service. The memory 404 can include any non-transitory storage device including, for example, volatile memory such as RAM, or non-volatile memory such as EPROM, flash memory, and the like.
[0051] In an embodiment, the wireless communication system 400 may also include an interface(s) 406. The interface(s) 406 can include a variety of interfaces, for example, interfaces for data input and output devices referred to as I/O devices, storage devices, and the like. The interface(s) 406 can facilitate communication of the wireless communication system 400 with various devices coupled to the HST or base stations disposed at the track on which the HST is running. The interface(s) 406 can also provide a communication pathway for one or more components of the wireless communication system 400. Examples of such components include, but are not limited to, communication unit 414 and database 416.
[0052] In an embodiment, one or more units of the wireless communication system 400 can be implemented as a combination of hardware and programming (for example, programmable instructions) to implement one or more functionalities of the wireless communication system 400. In the examples described herein, such combinations of hardware and programming may be implemented in several different ways. For example, the programming for the wireless communication system 400 can be processor-executable instructions stored on a non-transitory machine-readable storage medium and the hardware for the wireless communication system 400 can include a processing resource (for example, one or more processors), to execute such instructions. In the present examples, the machine-readable storage medium may store instructions that, when executed by the processing resource, implement the wireless communication system 400. In such examples, the wireless communication system 400 can include the machine-readable storage medium storing the instructions and the processing resource to execute the instructions, or the machine-readable storage medium may be separate but accessible to the wireless communication system 400 and the processing resource. In other examples, the wireless communication system 400 can be implemented by electronic circuitry. The database 416 can include data that is either stored or generated as a result of functionalities implemented by any of the components of the wireless communication system 400.
[0053] In an embodiment, the processor 402 may be communicatively coupled to a network node of a base station. The processor 402 may determine one or more user equipments UEs connected to the base station and measure a relative movement between the one or more UEs and the base station. The processor 402 may determine a doppler shift associated with the one or more UEs based on the measured relative movement, where the doppler shift may be measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
[0054] In an embodiment, the processor 402 may be configured to determine a frequency offset associated with the doppler shift by measuring a time difference between a second (2nd) DMRS symbol associated with the DMRS signal and a SRS symbol associated with the SRS signal, wherein the SRS symbol is positioned three symbols away from the 2nd DMRS symbol. The processor 402 may be configured to determine a frequency offset associated with the doppler shift by augmenting (multiplying) the time difference with an Orthogonal Frequency Division Multiplexing (OFDM) symbol duration.
[0055] In an embodiment, the processor 402 may be configured to compute the OFDM symbol phase estimate based on the frequency offset and compensate the OFDM symbol phase estimate by performing phase rotation of the OFDM symbol to remove effects associated with the frequency offset.
[0056] In an embodiment, the processor 402 may be configured to determine the doppler shift with the threshold range of +/-1340 Hz required by a High-Speed Train (HST) moving with a velocity of 350 Km/hr using the second DMRS signal and the SRS signal.
[0057] In an embodiment, the processor 402 may be configured to determine the doppler shift with the threshold range of +/-1340 Hz required by the HST moving with the velocity of 350 Km/hr for 15KHz sub-carrier-spacing fifth generation (5G) deployment bands using the second DMRS signal and the SRS signal. The processor 402 may be configured to determine the doppler shift with the threshold range of +/-2334 Hz required by the HST moving with the velocity of 350 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.
[0058] In an embodiment, the processor 402 may be configured to determine the doppler shift with the threshold range of +/-1740 Hz required by the HST moving with the velocity of 500 Km/hr for 15 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal. Further, the processor 402 may be configured to determine the doppler shift with the threshold range of +/-3334 Hz required by the HST moving with the velocity of 500 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.
[0059] In an embodiment, the system 400 includes a correlation unit 408 configured to perform cross-correlation of channel estimates between Sounding Reference Signal (SRS) and Demodulation Reference Signal (DMRS) symbols to be transmitted by a 4G/LTE/5G network. A Least-Squares (LS) Channel estimates of received Physical Uplink Channel (PUSCH) based 2nd Demodulation Reference Signal (DMRS) symbols, HLS(k,ldmrs), is differentially correlated with the Least-Squares (LS) Channel estimates of following SRS symbols that is 3 OFDM symbols away, HLS(k,lsrs) to estimate a phase-change (R) using the below equation (1), where k is the DMRS subcarriers and ldmrs is the DMRS symbol index, lsrs is SRS symbol index:
… (1)
[0060] The system 400 includes a frequency offset (FO) estimation unit 410 configured to estimate a frequency offset by converting the phase-change (R) into Frequency-offset (FO) by taking into consideration angular Radians change in the 3 symbol time difference of the 2nd DMRS Symbol and the SRS Symbol, lsrs – ldmrs, in multiples of 71.3 microseconds (one OFDM symbol duration with cyclic prefix). The FO is estimated using the below equation (2):
… (2)
[0061] The system 400 also includes an FO compensation unit 412 configured to compute and compensate per-OFDM symbol phase estimate, from the estimated FO, by phase rotation of each OFDM symbol received to remove the effect of the frequency-offset caused due to Doppler shift/spread by virtue of the HST, using the below equation (3):
(3)
[0062] The system 400 exploits closer time-spacing of sounding reference signal (SRS) symbol to 2nd PUSCH DMRS symbol, which is 3 symbols apart. With such a configuration, an offset of the maximum Doppler frequency can be estimated around +/-2.33 KHz for 15KHz subcarrier spacing and +/-3.334 KHz for 30 KhZ subcarrier spacing. The system 400 may be configured to repeat one or more of its processes using correlation between two DMRS symbols within each TTI, to track Doppler transition regions jointly with SRS based approach.
[0063] In an embodiment, the wireless communication system 400 can include one or more other units 418 for implementing functionalities that supplement applications or functions performed by the wireless communication system 400. In an embodiment, the communication unit 414 may be operatively coupled to the processor(s) 402. The communication unit 414 may be configured to communicatively couple the wireless communication system 400 of the UEs or devices associated with the users of the HST.
[0064] The SRS based approach of the system 400 works for high Doppler cases with single or multiple UEs with similar or different velocity (speed and direction) in parallel tracks. The HST scenarios for LTE networks from Third Generation Partnership Project (3GPP) TS 36.141 (version 15.14.0 Release 15) were considered for performance validation of the proposed approach at high Doppler frequency. The two scenarios specified for High speed conditions are scenario 3 (300km/hr with maximum Doppler as 1150Hz) and scenarios 1 (350km/hr with maximum Doppler as 1340Hz). The parameters for the results shown below are given in Table 1 for scenario 3. The processing as per the correlation unit 408, the FO estimation unit 410 and the FO compensation unit 412 was tested for both the scenarios with maximum Doppler up to 1340Hz with 1200 frames to cover the Doppler transitions for Hz.

Parameters Values
Channel Model LTE HST Channel model
User equipment mobility [km/h] 300
Maximum Doppler (Hz) 1150
Sub-carrier spacing [kHz] 15
Bandwidth [MHz / PRBs] 20
FFT size 2048
SRS time periodicity (ms) 40, 80, 160
SRS frequency domain allocation Full/wide band

Table 1: Test parameters for HST scenario 3

[0065] Referring to FIG. 2, where a spectrum diagram showing variation in Orthogonal Frequency-Division Multiplexing (OFDM) symbols of time-spaced Physical Uplink Channel (PUSCH) based Demodulation Reference Signal (DMRS) of the wireless communication system 400 is shown. Fig. 2 shown 3 symbol apart closely time-spaced SRS to PUSCH based DMRS symbol (indicated by green and blue colours respectively).
[0066] In an embodiment, FIG. 5 shows a wireless communication method 500 for handling multiple UEs moving at high-speeds, where k is DMRS subcarrier index, Y(k,l) is received grid in frequency domain, ldmrs is and lsrs is DMRS and SRS symbol index, and Ts is symbol duration. The method 500 involves a step 502 of performing cross-correlation of channel estimates between the SRS and the DMRS symbols. Thereafter, at step 504, a frequency offset (FO) is estimated based on pre-defined parameters, and at step 506, from the frequency-offset, per-OFDM symbol phase estimate is computed and compensated by phase rotation of each OFDM symbol received to remove the effect of the frequency-offset caused due to Doppler shift/spread by virtue of the HST. The steps of the process may be repeated using correlation between two DMRS symbols within each TTI, to track Doppler transition regions jointly with SRS based approach. The method 500 utilizes SRS and 2nd DMRS reference signals for meeting Doppler requirements of +/-1340Hz of HST at 350Km/hr for all Bands, sub-Ghz bands as well as sub-3GHz bands of LTE networks, which cover the gambit of LTE deployment bands.
[0067] In an embodiment, the method 500 may include determining, by the processor 402, one or more user equipments UEs connected to a base station and measuring a relative movement between the one or more UEs and the base station. The method 500 may include determining, by the processor 402, a doppler shift associated with the one or more UEs based on the measured relative movement, where the doppler shift may be measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
[0068] As shown in FIG. 6A, without compensation of the Frequency-Offset by the FO compensation unit 412, throughput of the PUSCH UE reported is zero in the presence of +/-1150 KHz Doppler. As shown in FIG. 6B ]with compensation of the Frequency-offset, the throughput reported was above 70% throughput at a target Signal-to-Noise ratio (SNR) specified as per the 3GPP 36.141 requirement (red dashed-line) under High Speed Train speeds of 300Km/hr at +/-1150Hz Doppler frequency. The proposed method exceeds the requirement by at least 1dB margin from the required SNR (Signal Noise Ratio) specified in the 3GPP 36.141 specifications as indicated by the blue line in Fig. 6B. In the time-duration wherein the UE in the HST approaches the base station installed near the tracks on which the HST is running, the Doppler shift has a trajectory which has transitions from +1150Hz to -1150Hz and vice-versa every 3.5 seconds in the whereabouts of sub-frame numbers 2000, 5500, 9000 etc. The Doppler shift estimated using the method 500 was observed to be able to track the Doppler trajectory of HST scenarios, as specified in 36.141 even with higher SRS periodicity as shown in FIG. 7.
[0069] As illustrated in FIG. 7, in an embodiment, the processor 402 may determine the transition region that include a variation in the doppler shift due to the relative movement between the one or more UEs and the base station. The doppler shift may switch between a threshold range, where the threshold range may be based on one or more DMRS symbols transmitted by the UE for a Transmission Time Interval (TTI). In response to a determination that the threshold range is below a threshold value, the processor 402 may determine the doppler shift through a plurality of DMRS symbols received from the UE. In response to a determination that the threshold range is above the threshold value, the processor 402 may determine the doppler shift through the SRS symbol and the second DMRS symbol received from the UE. The processor 402 may be configured to maintain a 98 percent throughput during the transition region through Hybrid Automatic Repeat Request (HARQ) retransmissions.
[0070] With lower SRS periodicity, during these discontinuity intervals, block Cyclic Redundancy Check (CRC) errors are experienced as indicated in FIG. 8. These CRC block errors (BLER) may be corrected through Hybrid automatic repeat request (HARQ) Retransmission, and hence the performance saturation to 98% throughput occurs in the throughput graph, as shown in Fig. 6B, since this has not been accounted for in the process 500. The method 500 obtained 98% throughput by virtue of HARQ retransmissions, which is in transition region when the HST passes a base station wherein the Doppler frequency switches from +1150 Hz to -1150Hz and vice versa as descried in the HST 3GPP specifications. Further, the method 500 improves tracking of the frequency-offset transition for high speed scenarios. Better tracking helps reduce the CRC block errors for higher MCS in the transition regions. The PUSCH DMRS based scheme can estimate the Doppler frequencies up to 800Hz. Thus, the SRS based estimate can be compared with this threshold (+/-800Hz) and the estimation process can be switched to DMRS based approach for better tracking for Doppler region between +800Hz to -800Hz for lower SRS periodicity.
[0071] The method 500 based on SRS works well for multi-UE scenarios, and passed all the 3GPP throughput requirements with a clear margin. The method 500 was observed not to impact performance of non-HST test cases as shown in FIG. 9 (with Extended Vehicular A (EVA), Extended Pedestrian A (EPA) channel models), and complies the 3GPP throughput requirements (TS 36.141, Table 8.2.1.5-6) as per Table 2. The ETU channel model with Doppler frequency was also tested with same test parameters (test requirement not present in specification 36.141) for FRC (Fixed Reference Channel) considered.

# of Rx. Ants Propagation conditions Achieved throughput SNR(dB) # of frames
2 EPA 5Hz Low 95% (target: 70%) 0.2 100
2 EVA 70Hz Low 90% (target: 70%) 0.8 100

Table 2: Test requirements for PUSCH, 20 MHz Channel Bandwidth
[0072] The utilisation of low periodicity SRS (up to 160ms) provides an effective solution to handle signalling requirements in HST deployments. By reducing the need for periodic PUCCH transmissions (every 5th sub-frame in 10ms frame), network throughput can be significantly improved. This enhancement enables faster and more reliable data transmission in high-speed train environments. Along with low periodicity in time, SRS is allocated only on the last symbol of a sub-frame, whereas to achieve similar performance PUCCH Format 2 is used which occupies all 14 symbols in a sub-frame. Thus, the proposed system 400 and method 500 are cost-effective and efficient in high-speed train deployments.
[0073] As illustrated in FIG. 10, in an embodiment, the system 400 may determine a transition region that includes a variation in the doppler shift due to the relative movement between the one or more UEs and the base station. The doppler shift may switch between a threshold range, where the threshold range may be based on one or more DMRS symbols transmitted by the UE for a Transmission Time Interval (TTI). The threshold range may be between +800 Hz to -800 Hz. In response to a determination that the threshold range is below a threshold value, the system 400 may determine the doppler shift through a plurality of DMRS symbols received from the UE. In response to a determination that the threshold range is above the threshold value, the system 400 may determine the doppler shift through the SRS symbol and the second DMRS symbol received from the UE.
[0074] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE INVENTION
[0075] The present invention overcomes the problems associated with conventional wireless communication systems for user equipments (UEs) moving at high-speeds.
[0076] The present invention provides a wireless communication system for effectively resolving performance gaps in terms of throughput degradation due to high-frequency offset for moving users, and method thereof.
[0077] The present invention provides a wireless communication system and method capable of reliably compensating effects of high Doppler shift affecting fourth generation/ Long term Evolution/ fifth generation (4G/LTE/5G) systems where number of reference symbols per Transmission Time Interval (TTI) is limited.
[0078] The present invention provides a wireless communication system and method capable of efficiently handling multiple UEs with various speeds having high Doppler frequencies.
[0079] The present invention provides a cost-effective solution for deployment of reliable and robust LTE network for a high-speed train, while eliminating the need for additional base station deployment to support UE on roof of carriages of the train and within the train.
[0080] The present disclosure uses a sounding reference signal (SRS) for Doppler estimation, hence an additional DMRS symbol required for Doppler estimation for 5G networks may be avoided, thereby resulting in a higher sector throughput.
,CLAIMS:1. A system (400) for estimation of doppler shifts, the system (400) comprising:
a processor (402) communicatively coupled to a network node of a base station;
a memory (404) operatively coupled with the processor (402), wherein said memory (404) stores instructions which, when executed by the processor (402), cause the processor (402) to:
determine one or more user equipments UEs connected to the base station and measure a relative movement between the one or more UEs and the base station; and
determine a doppler shift associated with the one or more UEs based on the measured relative movement, wherein the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
2. The system (400) as claimed in claim 1, wherein the processor (402) is configured to determine a frequency offset associated with the doppler shift by:
measuring a time difference between a second DMRS symbol associated with the DMRS signal and a SRS symbol associated with the SRS signal, wherein the SRS symbol is positioned three symbols away from the second DMRS symbol; and
augment the time difference with an Orthogonal Frequency Division Multiplexing (OFDM) symbol duration.
3. The system (400) as claimed in claim 2, wherein the processor (402) is configured to:
determine a transition region that comprises a variation in the doppler shift due to the relative movement between the one or more UEs and the base station, wherein the doppler shift switches between a threshold range, and wherein the threshold range is based on one or more DMRS symbols transmitted by the UE for a Transmission Time Interval (TTI);
in response to a determination that the threshold range is below a threshold value, determine the doppler shift through a plurality of DMRS symbols received from the UE; and
in response to a determination that the threshold range is above the threshold value, determine the doppler shift through the SRS symbol and the second DMRS symbol received from the UE.
4. The system (400) as claimed in claim 2, wherein the processor (402) is configured to compute the OFDM symbol phase estimate based on the frequency offset and compensate the OFDM symbol phase estimate by performing phase rotation of the OFDM symbol to remove effects associated with the frequency offset.
5. The system (400) as claimed in claim 3, wherein the processor (402) is configured to maintain a 98 percent throughput during the transition region through Hybrid Automatic Repeat Request (HARQ) retransmissions.
6. The system (400) as claimed in claim 2, wherein the processor (402) is configured to:
determine the doppler shift with the threshold range of +/-1340 Hz required by a High-Speed Train (HST) moving with a velocity of 350 Km/hr using the second DMRS signal and the SRS signal;
determine the doppler shift with the threshold range of +/-1340 Hz required by the HST moving with the velocity of 350 Km/hr for 15KHz sub-carrier-spacing fifth generation (5G) deployment bands using the second DMRS signal and the SRS signal;
determine the doppler shift with the threshold range of +/-2334 Hz required by the HST moving with the velocity of 350 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal;
determine the doppler shift with the threshold range of +/-1740 Hz required by the HST moving with the velocity of 500 Km/hr for 15 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal; and
determine the doppler shift with the threshold range of +/-3334 Hz required by the HST moving with the velocity of 500 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.
7. A method 500 for estimation of doppler shifts, the method 500 comprising:
determining, one or more user equipments UEs connected to a base station and measuring a relative movement between the one or more UEs and the base station; and
determining, a doppler shift associated with the one or more UEs based on the measured relative movement, wherein the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
8. The method 500 as claimed in claim 7 comprising determining, a frequency offset associated with the doppler shift by:
measuring a time difference between a second DMRS symbol associated with the DMRS signal and a SRS symbol associated with the SRS signal, wherein the SRS symbol is positioned three symbols away from the DMRS symbol; and
augmenting the time difference with an Orthogonal Frequency Division Multiplexing (OFDM) symbol duration.
9. The method 500 as claimed in claim 8 comprising:
determining, a transition region that comprises a variation in the doppler shift due to the relative movement between the one or more UEs and the base station, wherein the doppler shift switches between a threshold range, and wherein the threshold range is based on one or more DMRS symbols transmitted by the UE for a Transmission Time Interval (TTI);
in response to a determination that the threshold range is below a threshold value, determining the doppler shift through a plurality of DMRS symbols received from the UE; and
in response to a determination that the threshold range is above the threshold value, determining the doppler shift through the SRS symbol and the second DMRS symbol received from the UE.
10. The method 500 as claimed in claim 8, comprising computing, the OFDM symbol phase estimate based on the frequency offset and compensating the OFDM symbol phase estimate by performing phase rotation of the OFDM symbol to remove effects associated with the frequency offset.
11. The method 500 as claimed in claim 9, comprising maintaining, a 98 percent throughput during the transition region through Hybrid Automatic Repeat Request (HARQ) retransmissions.
12. The method 500 as claimed in claim 8 comprising:
determining, the doppler shift with the threshold range of +/-1340 Hz required by a High-Speed Train (HST) moving with a velocity of 350 Km/hr using the second DMRS signal and the SRS signal;
determining, the doppler shift with the threshold range of +/-1340 Hz required by the HST moving with the velocity of 350 Km/hr for 15KHz sub-carrier-spacing fifth generation (5G) deployment bands using the second DMRS signal and the SRS signal;
determining, the doppler shift with the threshold range of +/-2334 Hz required by the HST moving with the velocity of 350 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal;
determining, the doppler shift with the threshold range of +/-1740 Hz required by the HST moving with the velocity of 500 Km/hr for 15 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal; and
determining, the doppler shift with the threshold range of +/-3334 Hz required by the HST moving with the velocity of 500 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.
13. A user equipment (UE) for sending requests, the UE comprising:
one or more processors communicatively coupled to a processor (402) associated with a system (400), wherein the one or more processors are coupled with a memory, and wherein said memory stores instructions which, when executed by the one or more processors, cause the one or more processors to:
transmit one or more reference signals to the processor (402),
wherein the processor (402) is configured to:
determine a relative movement of the UE and a base station,; and
determine a doppler shift associated with the UE based on the measured relative movement, wherein the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal among the one or more reference signals, with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal among the one or more reference signals.
14. A non-transitory computer readable medium comprising a processor with executable instructions, causing the processor to:
determine one or more user equipments UEs connected to the base station and measure a relative movement between the one or more UEs and the base station; and
determine a doppler shift associated with the one or more UEs based on the measured relative movement, wherein the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE.
15. A system (400) for estimation of doppler shifts, the system (400) comprising:
a processor (402) communicatively coupled to a network node of a base station;
a memory (404) operatively coupled with the processor (402), wherein said memory (404) stores instructions which, when executed by the processor (402), cause the processor (402) to:
determine a doppler shift associated with one or more UEs based on the measured relative movement of the one or more UEs with a base station, wherein the doppler shift is measured by differentially combining Least-Squares (LS) channel estimates of a received Physical Uplink Shared Channel (PUSCH) second Demodulation Reference Signal (DMRS) signal with Least-Squares (LS) channel estimates of a sounding reference signal (SRS) signal transmitted by the UE;
determine a transition region that comprises a variation in the doppler shift due to the relative movement, wherein the doppler shift switches between a threshold range of +/-1340 Hz required by a High-Speed Train (HST) moving with a velocity of 350 Km/hr using the second DMRS signal and the SRS signal for 4G/LTE bands;
determine, the doppler shift with the threshold range of +/-1340 Hz required by a high-speed train (HST) moving with the velocity of 350 Km/hr for 15KHz sub-carrier-spacing fifth generation (5G) deployment bands using the second DMRS signal and the SRS signal;
determine, the doppler shift with the threshold range of +/-2334 Hz required by the HST moving with the velocity of 350 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal;
determine, the doppler shift with the threshold range of +/-1740 Hz required by the HST moving with the velocity of 500 Km/hr for 15 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal; and
determine, the doppler shift with the threshold range of +/-3334 Hz required by the HST moving with the velocity of 500 Km/hr for 30 KHz sub-carrier-spacing 5G deployment bands using the second DMRS signal and the SRS signal.