WO2020095101A1

WO2020095101A1 - Multiple access method in wireless telecommunications system

Info

Publication number: WO2020095101A1
Application number: PCT/IB2018/060490
Authority: WO
Inventors: Saif Khan Mohammed; Venkatesh KHAMMAMMETTI
Original assignee: Indian Institute Of Technology, Delhi
Priority date: 2018-11-06
Filing date: 2018-12-21
Publication date: 2020-05-14

Abstract

The present invention provides a multiple-access method for OTFS based communication systems, which is free from multiuser interference (MUI) and inter-carrier interference (ICI). The present invention resolves the problem of the prior art by being able to avoid interference between the information transmitted by different terminals on adjacent DDRBs, without the need for Guard bands. In the present invention, DDRBs allocated to each user terminal are spaced apart at equal intervals both in the Delay as well as in the Doppler domain. Due to this type of regular spacing in both the Delay and the Doppler domain, each terminal's information signal (i.e., after OTFS modulation) is limited to a subset of the entire time-frequency domain and does not occupy the entire time-frequency region. The time-frequency signal of each mobile terminal is allocated to a different subset of the entire time-frequency region so that they do not interfere with each other.

Description

MULTIPLE ACCESS METHOD IN WIRELESS

TELECOMMUNICATIONS SYSTEM

TECHNICAL FIELD [001] The present subject matter described herein, in general, relates to a wireless telecommunications system and particularly, to a wireless telecommunications system for multiplexing access from different user terminals (UTs) on the same physical resource. More particularly, the invention relates to a wireless telecommunications system that can mitigate impairments to the transmitted radio signal caused by the transmission medium and also due to mobility of transmitter and/or receiver.

BACKGROUND

[002] Wireless communication channels under high mobility (terminal speed up to 500 Km/hr) and high delay spread are known to increase multi user interference (MUI) and thereby degrade system throughput in current communication systems.

[003] Achieving high data rates at high mobility i.e., when mobile terminal has high speed (e.g., high speed train running at 500 Km/hr) and/or high delay spread (when the channel is such that the received information is received after several micro seconds, for example either in very large cells or in dense urban environments) is very challenging and is one of the goals of IMT-2020 for Fifth generation (5G) wireless communication systems [IMT2020 - Framework and the overall objective of Future

Deployment of 1MT for 2020 and Beyond, Recommendation ITU-R M-2083-0,

Sept 2015 (www.itu.inf)] . When several mobile terminals share the same wireless channel, a "Multiple Access" method is required to ensure that their communication does not interfere with each other. In 4G communication systems, the multiple access method is Orthogonal Frequency Division Multiple Access (OFDMA) where mobile terminals are allocated separate frequency channels. However at very high mobile speed, the high Doppler spread shifts the centre frequency of the mobile terminal's channel when the signal is received at the base station. As this shift depends on the speed of the mobile, it is different for the different frequency channels due to which adjacent frequency channels overlap with each other because of which the information transmitted from one mobile terminal interferes strongly with the information transmitted by another mobile terminal on an adjacent frequency channel. This is known as inter-carrier interference (ICI) and it degrades channel capacity at high mobility [OFDMICI - T -Wang, J.-G.-Proakis,

E.~Masry and J.~R.~Zeidler, Performance Degradation of OFDM Systems due to Doppler Spreading," IEEE Trans on Wireless Commun., vol. 5, no. 6, June 2006 ].

Similarly, very high delay spread requires a long cyclic prefix to avoid inter-symbol interference, which also reduces the channel capacity as cyclic prefix is an overhead.

[004] Recently, a new modulation technique "Orthogonal Time Frequency Space Modulation " (OTFS - R. Hadani, S. Rakib, M. Tsatsanis, A. Monk, A. J. Goldsmith, A.

F. Molisch, Orthogonal Time Frequency Space Modulation, "IEEE Wireless Communications and Networking Conference (WCNC 2017), March 2017) has been developed, which has been shown to be more robust to Doppler spread as compared to OFDM [OTFS,CohereWhitePaper - R.Hadani and A. Monk, "OTFS: A New Generation of Modulation Addressing the Challenges of 5G," arXiv: 1802.02623[cs.IT], www.arxiv. org, Feb. 2018] In OTFS, the information symbols are transmitted in the Delay Doppler (DD) domain which is T seconds by Af Hz (i.e., the Delay domain is T seconds wide, and the Doppler domain is D/ Hz wide). The Delay domain is divided into I sub divisions (each sub-division is T/I seconds) and the Doppler domain is divided into J sub divisions (each sub-division is Af/J Hz). This way, the entire Delay Doppler domain is divided into / x / sub-divisions, where each sub-division is called a Delay Doppler Resource Block (DDRB). Each information symbol is transmitted on a single DDRB. At the transmitter, the OTFS modulation then spreads each information symbol (localized on a DDRB) in the Delay Doppler domain to the entire time-frequency domain (i.e., JT seconds in the time-domain and I Af Hz in the frequency domain) due to which each information symbol sees the same effective channel gain irrespective of the DDRB on which the information symbol was transmitted. On the other hand, in an OFDM based system, the information symbols are localized to a small region of the time-frequency region due to which the effective channel gain of each information symbol is different and can be small at times due to fading. In contrast, in an OTFS system, since each information symbol is spread over the entire time -frequency region (and not localized to a small region) the channel gain does not suffer from deep fades. If the Delay spread and the Doppler spread of the wireless channel is not high (i.e., Delay spread is less than T/I and Doppler spread is less than Af/J) then the information symbols transmitted on adjacent delay Doppler Resource Blocks (DDRBs) do not interfere with each other, which makes the OTFS based system more robust to Doppler and Delay spread. [005] However, when Doppler spread is higher than D// / Hz and/or the Delay spread is higher than T/I seconds, information symbols transmitted in adjacent DDRBs start to interfere with each other, which can degrade performance. This problem becomes severe when multiple terminals communicate at the same time, i.e., multiple access. [006] To alleviate this problem, in the prior art patent US9722741, it is proposed to multiplex the mobile terminals in the Delay Doppler domain, by allocating DDRBs in such a manner that no two DDRBs allocated to two different mobile terminals he adjacent to each other. In [USPatent] it is proposed to use Guard bands between the DDRBs allocated to different mobile terminals. No information is communicated on the DDRBs located in the Guard band, and therefore it is an overhead which reduces the total system capacity (see Figure 1). In Figure 1, the Delay-Doppler domain is divided into 64 DDRBs (I = J = 8) and there are 4 mobile terminals (UT1, UT2, UT3 and UT4). In the left sub-figure, mobile terminals are allocated DDRBs along the Delay domain and they are separated from DDRBs allocated to other mobile terminals by Guard bands in the Doppler domain. In the right sub-figure another approach is shown where the mobile terminals are allocated DDRBs along the Doppler domain and are separated from the DDRBs allocated to other mobile terminals by Guard bands in the Delay domain. Note that the Guard bands occupy 32 DDRBs which is half of the total resource of 64 DDRBs. [007] Reference is made to the prior art figure 1, where allocation of DDRBs to terminals

UT1, UT2, UT3 and UT4 in the Delay Doppler domain is shown. Guard bands separate the DDRBs allocated to different terminals so as to reduce interference between them. The main problem is that Guard band is an overhead as no information is communicated through it. It is used to avoid interference between the information symbols communicated by different terminals in the Delay Doppler domain. [008] Hence, at high Doppler spread (which happens at high mobile speed) and at high

Delay spread, the system capacity achieved by the state-of-art methods (e.g., OTFS technology proposed in US9722741) degrades severely with increasing Doppler and Delay spread as the required Guard band size needs to be increased. [009] Reference is further made to a non-patent literature, A Survey on High Mobility

Wireless Communications: Challenges, Opportunities and Solutions”, https://ieeexplore.ieee.org/stamp/stamp.isp7amumbeF7383229. providing reliable broadband wireless communications in high mobility environments, such as high-speed railway systems, remains one of the main challenges faced by the development of the next generation wireless systems. This paper provides a systematic review of high mobility communications. It first summarizes a list of key challenges and opportunities in high mobility communication systems, and then provides comprehensive review of techniques that can address these challenges and utilize the unique opportunities. The review covers a wide spectrum of communication operations, including the accurate modeling of high mobility channels, the transceiver structures that can exploit the properties of high mobility environments, the signal processing techniques that can harvest the benefits (e.g., Doppler diversity) and mitigate the impairments (e.g., carrier frequency offset, inter-carrier interference, channel estimation errors) in high mobility systems, and the mobility management and network architectures that are designed specifically for high mobility systems. The survey focuses primarily on physical layer operations, which are affected the most by the mobile environment, with some additional discussions on higher layer operations, such as handover management and control-plane/user-plane decoupling, which are essential to high mobility operations. [0010] Still further, reference is made to a non-patent literature, “ Position-Based

Compressed Channel Estimation and Pilot Design for High-Mobility OFDM Systems”, httDs://ieeexnlore. ieee.org/stamD/stamD.isn7amumbeF68620l3. With the development of high-speed trains (HSTs) in many countries, providing broadband wireless services in HSTs is crucial. Orthogonal frequency-division multiplexing (OFDM) has been widely adopted for broadband wireless communications due to its high spectral efficiency. However, OFDM is sensitive to the time selectivity caused by high-mobility channels, which costs much spectrum or time resources to obtain the accurate channel state information (CSI). Therefore, the channel estimation in high-mobility OFDM systems has been a long-standing challenge. In this paper, a new position-based high-mobility channel model is considered, in which the HST’s position information and Doppler shift are utilized to determine the positions of the dominant channel coefficients. Then, a joint pilot placement and pilot symbol design algorithm is proposed for channel estimation which improves channel estimation accuracy. Simulation results demonstrate that the proposed method performs better than existing channel estimation methods over high-mobility channels. Thus, this prior art literature, solves the problem of accurate channel estimation in high-mobility wireless channels.

[0011] Reference is then made to a non-patent literature,“ Orthogonal Time Frequency Space Modulation” , https ://ieeexpl ore. ieee .org/document/7925924/. discloses a new two- dimensional modulation technique called Orthogonal Time Frequency Space (OTFS) modulation. Through this design, which exploits full diversity over time and frequency, OTFS coupled with equalization converts the fading, time-varying wireless channel experienced by modulated signals such as OFDM into a time-independent channel with a complex channel gain that is roughly constant for all symbols. Thus, transmitter adaptation is not needed. This extraction of the full channel diversity allows OTFS to greatly simplify system operation and significantly improves performance, particularly in systems with high Doppler. Simulation results indicate at least several dB of block error rate performance improvement for OTFS over OFDM in these settings. In addition these results show that even at very high Doppler shift (e.g., mobile speed of 500 Km/h), OTFS achieves linear scaling of throughput with increasing MIMO order, whereas the performance of OFDM under high Doppler shift breaks down completely. Thus, this non-patent literature proposes Orthogonal Time Frequency Space Modulation as a new modulation scheme, which is robust to very high Doppler and delay spread. This paper however does not discuss any multiple-access method when using OTFS waveforms for transmission. [0012] Reference is then made to a non-patent literature, “ OFDM high speed train communication systems in 5G cellular networks”, https://ieeexplore.ieee.org/document/83 l9l72/. which discloses a reliable high-speed wireless communication system, which is essential for high speed trains (HSTs) in upcoming 5G cellular communication networks. Orthogonal frequency division multiplexing (OFDM) has been used for wideband communication standards because of its efficiency and its robustness to multipath propagations and is a good candidate for 5G communication systems as well. However, the expected high carrier frequency of 5G, along with the high speed of HSTs, causes high Doppler shifts that results in severe inter carrier interferences (ICIs) in OFDM systems. To mitigate ICI, channel state information (CSI) is needed at the receiver. In this paper, an earlier proposed method for HST channel estimation in conventional communication standards is enhanced for 5G communication systems. In the enhanced method, an accurate estimate of the Doppler shift is applied to calculate the channel tap gains. Besides this, by exploiting the estimated CSI, an adaptive channel coding scheme is proposed which employs different coding rates based on the channel condition of the subcarriers. Thus, the objective of this non-patent literature is to address the problem of inter-carrier interference in multi-carrier communication systems due to very high Doppler spread in high mobility scenarios. The solution to this problem as proposed in this literature document is to estimate the Doppler shift and to use different coding rates based on the channel condition on each subcarrier. The literature document however does not discuss or propose multiple-access methods which are robust to multi-user interference (MUI) under high mobility conditions. [0013] Accordingly, in view of existing wireless communication systems for high mobility and high delay spread wireless channels and in view of the drawbacks of the prior arts as mentioned herein above, there is a dire need to provide an improved multiple access method that can avoid multi-user interference between the information transmitted by different user terminals on adjacent DDRBs, without the need for Guard bands. SUMMARY OF THE PRESENT INVENTION

[0014] The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later. [0015] An objective of the present invention is to provide multiple -access method for

OTFS based communication systems, which is free from multiuser interference (MUI) and inter-carrier interference (ICI).

[0016] Another objective of the present invention is to provide a multiple access method for OTFS based systems, which avoids the use of Guard bands and whose throughput does not degrade as severely as other prior art methods with increasing Doppler and delay spread.

[0017] Yet another objective of the present invention is provide multiple access method that allow several users to communicate simultaneously using Orthogonal Time Frequency Space (OTFS) waveforms in such a way that they achieve higher data throughput in a high Doppler and delay spread wireless channel, when compared to existing multiple access methods which use guard bands in the delay-Doppler domain to reduce multi-user interference.

[0018] Another objective of the present invention is to provide a multi-user interference (MUI) and ICI free multiple-access method without explicitly estimating the Doppler shift.

[0019] Yet another objective of the present invention is to provide multiple-access method wherein DDRBs allocated to each user terminal are spaced apart at equal intervals both in the Delay as well as in the Doppler domain. [0020] Still another objective of the present invention is to provide multiple-access method with smart allocation of non-overlapping DDRBs and TFRBs to each UT in such a manner that there is no multi-user interference and no guard bands are used in either the delay Doppler or the time frequency domain.

[0021] Accordingly to one aspect, in one implementation, the present invention provides a multiple access method for wireless communication system for transmitting information from plurality of user terminals (UTs), wherein said method at each of the said plurality of UTs comprises of:

· mapping, at each UT, plurality of information symbols of the said UT to Delay

Doppler Resource Blocks (DDRBs) allocated to said UT, to obtain a plurality of Delay Doppler signals, wherein said DDRBs are spaced apart at regular intervals along delay and Doppler domain;

• transforming said Delay Doppler signals of the said UT to a plurality of time frequency signals which are mapped to the Time Frequency Resource Blocks

(TFRBs) allocated to said UT;

• transforming said plurality of time -frequency signals at each UT to a plurality of continuous time domain signals, which are then up-converted and transmitted to a base station (BS). [0022] In one aspect, in one implementation, at transmitter of each UT, the plurality of information signals are mapped to the DDRBs allocated to that UT.

[0023] In one aspect, in one implementation, wherein each of said UTs from said plurality of UTs are allocated with non-overlapping said TFRBs so that said plurality of time frequency signals of all the UTs are separable from each other.

[0024] In one aspect, in one implementation, BS converts the received time frequency signals of each UT back to the corresponding delay Doppler signals. [0025] In one aspect, in one implementation, wherein the base station performs separate channel equalization on said delay Doppler signals of each UT to obtain an estimate of said information symbols transmitted by each UT of the plurality of UTs to the base station. [0026] In one aspect, in one implementation, each of the said plurality of continuous-time domain signals of a UT are transmitted from different antenna units of the said UT.

[0027] In one aspect, in second implementation, the present invention provides a multiple access method for wireless communication system for receiving information from plurality of user terminals (UTs), wherein said method comprising:

• receiving, at a base station, plurality of continuous time signals from each of the said plurality of UTs;

• transforming said received plurality of continuous time signal to plurality of time frequency signals and thereby separating said plurality of time frequency signals for each UT based on Time Frequency Resource Block (TFRB) allocation;

• conversion, by said base station, of the time frequency signals received from each UT to the corresponding Delay Doppler signals for the said UT;

• followed by, channel equalization of the delay Doppler signals for each UT to obtain an estimate of plurality of information signals transmitted by each of said UTs.

[0028] In one aspect, in third implementation, the present invention provides a wireless communication system comprising plurality of user terminals (UTs) and a base station, wherein said plurality of UTs transmit information to said base station on same time- frequency resource:

Wherein, each UT of the said plurality of UTs is adapted to:

having plurality of information symbols, and perform mapping of said plurality of information symbols to Delay Doppler Resource Blocks (DDRBs) to obtain a plurality of Delay Doppler signals, wherein said DDRBs are spaced apart at regular intervals along delay and Doppler domain; transform said plurality of Delay Doppler signals to a plurality of time frequency signal and mapping them to Time Frequency Resource Blocks (TFRB) allocated to said UT ;

transform said plurality of time -frequency signals to a plurality of continuous time signals which are then up-converted and transmitted;. said base station adapted to:

receive said plurality of continuous time signals from said plurality of UTs;

transform said received plurality of continuous time signals to plurality of time frequency signals and thereby separate said plurality of time frequency signals for each UT based on Time Frequency Resource Blocks (TFRBs) allocated to said UT; convert plurality of time frequency signals for each UT to plurality of Delay Doppler signals;

wherein, channel equalization is performed on said plurality of delay Doppler signals for each UT to obtain an estimate of plurality information symbols transmitted by each of said UTs.

[0029] According to another aspect of the present invention there is provided a multiuser downlink communication method for wireless communication system for transmitting information from a base station (BS) to a plurality of user terminals (UTs), wherein said method comprising:

mapping, at the base station, plurality of information symbols intended for each UT from said plurality of UTs to Delay Doppler Resource Blocks (DDRBs) to obtain plurality of Delay Doppler signals for each UT, wherein for each said UT, the said DDRBs for that UT are spaced apart at regular intervals along delay and Doppler domain;

transforming said Delay Doppler signals of each of the said plurality of UTs to a plurality of time frequency signals which are mapped to the Time Frequency Resource Blocks (TFRBs) allocated to said UT;

jointly transforming together the said plurality of time-frequency signals from all the said plurality of UTs to a plurality of continuous time domain signals, which are then up-converted and transmitted from the antenna/antennas of said BS. [0030] According to another aspect of the present invention there is provided a downlink communication method for wireless communication system for receiving information at each UT from a plurality of UTs, wherein said method at each UT comprising: receiving, at said UT, plurality of continuous time signals from a base station (BS); transforming said received plurality of continuous time signals to plurality of time frequency signals and separating the time frequency signals intended for said UT based on Time Frequency Resource Block (TFRB) allocation;

converting, by said UT, said plurality of time frequency signals intended for said UT to plurality of Delay Doppler signals;

followed by channel equalization of said plurality of Delay Doppler signals to obtain an estimate of the plurality information symbols intended for said UT.

[0031] According to another aspect of the present invention there is provided a downlink communication method comprising plurality of user terminals (UTs) and a base station (BS), wherein said plurality of UTs receive information from said base station on the same time-frequency resource:

wherein each UT from said plurality of UTs is adapted to:

receive plurality of continuous time signals from the said BS;

transform said received plurality of continuous time signal to plurality of time frequency signals and thereby separate plurality of time frequency signals intended for said UT based on Time Frequency Resource Block (TFRB) allocation;

convert said plurality of time frequency signals intended for said UT to plurality of Delay Doppler signals;

perform channel equalization on said plurality of delay Doppler signals to obtain an estimate of plurality information signals intended for said UT.

Wherein, the said base station is adapted to:

having plurality of information symbols intended for each UT, and perform mapping of said plurality of information symbols for each UT to Delay Doppler Resource Blocks (DDRBs) allocated to said UT to obtain plurality of Delay Doppler signals for said UT, wherein for each UT the said allocated DDRBs are spaced apart at regular intervals along delay and Doppler domain; transform said Delay Doppler signals for each UT to a plurality of time frequency signals intended for said UT which are mapped to the Time Frequency Resource Blocks (TFRBs) allocated to said UT; jointly transform together said plurality of time-frequency signals for all the UTs to a plurality of continuous time domain signals, which are then up-converted and transmitted from the antenna/antennas of said BS.

[0032] According to the various implementation of present invention, several user terminals communicate simultaneously with the base station using Orthogonal Time Frequency Space (OTFS) waveforms in such a way that they achieve higher data throughput in a high Doppler and delay spread wireless channel, when compared to other multiple access methods which use guard bands in the delay-Doppler domain to reduce multi-user interference. Thus, the inventive multiple-access method is free from multi-user interference (MUI), and inter-carrier interference (ICI) and also achieves diversity by embedding information symbols in the Delay-Doppler domain followed by OTFS modulation, which spreads the information in the entire time-frequency domain. Thus, the multiplexing method achieves high throughput even in high mobility and high delay spread channels as it guarantees MUI free communication. [0033] Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0034] The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which: [0035] Figure 1: illustrates the allocation of DDRBs by using guard bands to terminals UT1, UT2, UT3 and UT4 in the Delay Doppler domain according to prior art.

[0036] Figure 2: illustrates the allocation of DDRBs to the terminals in the Delay Doppler domain according to an exemplary implementation of the present invention.

[0037] Figure 3: illustrates the allocation of TFRBs to the terminals in the time-frequency domain, according to one implementation of the present invention.

[0038] Figure 4: illustrates that (a) processed Delay Doppler signal at the base station for UT1 being localized only to the DDRBs allocated to UT1, (b) processed Delay Doppler signal at the base station for UT2 is localized only to the DDRBs allocated to UT2, (c) processed Delay Doppler signal at the base station for UT3 is localized only to the DDRBs allocated to UT3, (d) processed Delay Doppler signal at the base station for UT4 is localized only to the DDRBs allocated to UT4, according to various exemplary implementations of the present invention.

[0039] Figure 5: illustrates the block diagram of the Transmitter signal processing at each User Terminal (UT), according to one implementation of the present invention.

[0040] Figure 6: illustrates the block diagram of the signal processing at the base station (BS) receiver, according to one implementation of the present invention.

[0041] Figure 7: illustrates process flow diagram for transmitting signal processing at each user terminal (UT), according to one implementation of the present invention.

[0042] Figure 8: illustrates process flow diagram for signal processing at the base station (BS) receiver, according to one implementation of the present invention.

[0043] Figure 9: illustrates an uplink communication system with K single antenna UTs communicating with a single antenna BS on the same time frequency resource, according to one implementation of the present invention. [0044] Figure 10: illustrates the graphic plot between spectral efficiency per User terminal versus received signal to noise ratio p, according to one implementation of the present invention.

[0045] Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE PRESENT INVENTION [0046] The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. [0047] Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness. [0048] The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. [0049] It is to be understood that the singular forms“a,”“an,” and“the” include plural referents unless the context clearly dictates otherwise.

[0050] By the term“substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. [0051] Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.

[0052] It should be emphasized that the term“comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0053] The present invention solves the problems of the prior art by being able to avoid interference between the information transmitted by different terminals on adjacent DDRBs, without the need for Guard bands.

[0054] According to the present invention, the DDRBs allocated to each terminal are spaced apart at equal intervals both in the Delay as well as in the Doppler domain.

[0055] In an exemplary implementation, reference is made to figure 2, where the resource allocation is shown for the four mobile terminals. Note that each mobile terminal is allocated 16 DDRBs. For each mobile terminal there is a constant spacing of one DDRB in both the Delay and the Doppler domain. No two mobile terminals are allocated the same DDRB. DDRBs allocated to UT1 are denoted by parallel // lines, DDRBs allocated to UT2 are denoted by parallel horizontal lines, DDRBs allocated to UT3 are denoted by parallel vertical lines and those allocated to UT4 are denoted by both parallel horizontal and vertical lines. Each terminal is allocated 16 DDRBs as opposed to only 8 DDRBs in the prior methods as shown in figure 1.

[0056] Due to this type of regular spacing in both the Delay and the Doppler domain as shown in figure 2, each terminal's information signal (i.e., after OTFS modulation) gets limited to a subset of the entire time-frequency domain and does not occupy the entire time- frequency region. Using this fact, the time-frequency signal of each mobile terminal is allocated a different subset of the entire time-frequency region so that they do not interfere with each other. This allows the receiver (base station) to separate the information signals received from different terminals in the time-frequency domain.

[0057] In an exemplary implementation, reference is made to figure 3, which shows UT1 is allocated the lower left 16 TFRBs (Time Frequency Resource Block), UT2 is allocated the lower right 16 TFRBs, UT3 is allocated the upper left 16 TFRBs and UT4 is allocated the upper right 16 TFRBs. Due to regular spacing between the information symbols in the Delay Doppler domain, the corresponding time-frequency signal for each terminal (which is obtained after OTFS modulation on the Delay Doppler information symbols at the terminal's transmitter) is limited to a subset of the entire time-frequency region. The time- frequency signal of each terminal is allocated a different subset of the time-frequency domain so that they do not interfere with each other and can be separated easily at the base station receiver.

[0058] The separation of the time-frequency signals from different terminals at the base station receiver is followed by conversion of the separated time-frequency signal of each terminal to the corresponding Delay Doppler signal using the inverse OTFS transform. The resulting Delay Doppler signals for each terminal are restricted to only those DDRBs, which are allocated to that terminal.

[0059] In one exemplary implementation, reference is made to figure 4(a)-(d), which illustrates the occupancy of the Delay Doppler signal (i.e., output of the inverse OTFS transform applied to the time -frequency signal) of each terminal. Figure 4(a) illustrates that the processed Delay Doppler signal at the base station for UT1 is localized only to the DDRBs allocated to UT1, and therefore the information symbols transmitted by other UTs do not interfere with it. Figure 4(b) illustrates that the processed Delay Doppler signal at the base station for UT2 is localized only to the DDRBs allocated to UT2, and therefore the information symbols transmitted by other UTs do not interfere with it. Figure 5(c) illustrates that the processed Delay Doppler signal at the base station for UT3 is localized only to the DDRBs allocated to UT3, and therefore the information symbols transmitted by other UTs do not interfere with it. Figure 5(d) illustrates that the processed Delay Doppler signal at the base station for UT4 is localized only to the DDRBs allocated to UT4, and therefore the information symbols transmitted by other UTs do not interfere with it.

[0060] Therefore, in the present invention, without the use of Guard bands, interference between the terminals is avoided by spacing their information symbols at equal intervals in both the Delay and Doppler domain, followed by allocation of different non-overlapping time-frequency regions to the time-frequency signals of different UTs. In the above exemplary implementation, each UT is allocated 16 DDRBs as compared to the prior art i.e., Guard band based multiple-access method as shown in the figure 1 where each terminal is only allocated 8 DDRBs, i.e., the present invention doubles the overall system capacity in high Doppler (i.e., high mobile speed) and Delay spread scenarios. [0061] Reference is made to figure 5, which illustrates the block diagram of Transmitter signal processing at each User Terminal (UT), according to one implementation of the present invention. Further, reference is made to figure 6, which illustrates the block diagram of the signal processing at the base station (BS) receiver. [0062] Reference is made to figure 7 which illustrates process flow diagram for transmitter signal processing at each user terminal (UT), according to one implementation of the present invention. Further, reference is made to figure 8 which illustrates process flow diagram for signal processing at the base station (BS) receiver. [0063] According to figures 5-8, the present invention is described in the following steps: [0064] STEP1: At the transmitter of each UT, the information symbols are mapped to the DDRBs allocated to that UT. The allocation is such that the DDRBs are spaced apart at regular intervals both along the delay as well as the Doppler domain (block 101; step 701).

[0065] STEP 2 : The delay Doppler symbols are then transformed to the time frequency domain. Due to regular spacing of the information symbols in the delay Doppler domain, the time frequency signal of each UT occupies only a portion of the entire time frequency domain. The TFRBs are therefore allocated to each UT in a manner that TFRBs of two different UTs do not overlap (block 102; step 702).

[0066] STEP 3: The time-frequency signal is then transformed to a continuous time signal which is up-converted and transmitted (block 103 and block 104; step 703 and step 704).

[0067] STEP4: The continuous time signal received at the base station (BS) from all the UTs is firstly transformed back to the time frequency domain (block 200; step 801). As the UTs are allocated with non-overlapping TFRBs, the time frequency signals of the UTs are separable from each other (step 802).

[0068] STEP5: For each UT, the BS converts the UT's time frequency signal back to the delay Doppler domain (see block 201; step 803).

[0069] STEP6: Separate channel equalization is performed on the delay Doppler signal of each UT to get an estimate of the delay Doppler information symbols transmitted by each UT (see block 202; step 804).

[0070] Reference is made to figure 9 which illustrates an uplink communication system with K single antenna UTs communicating with a single antenna BS on the same time frequency resource, according to one implementation of the present invention. It depicts the overall communication system where K different UTs (10) transmit their information to a base station (20) on the same time-frequency resource. WORKING EXAMPLES:

[0071] Let the Delay Doppler domain be Af Hz wide along the Doppler domain and T seconds along the delay domain. With I subdivisions along the delay domain and J subdivisions along the Doppler domain, each DDRB is therefore Af/J Hz X T/l seconds. The (a, b)- th DDRB occupies the a-th subdivision along the Doppler domain and the b-th subdivision along the Delay domain (a = 0,1, · ·· ,]— 1, b = 0,1, · ·· , /— 1). The corresponding time frequency domain is JT seconds along the time domain and / D/ Hz along the frequency domain. Each TFRB is Af Hz X T seconds.

[0072] In the present invention, UT's are allocated distinct DDRBs in the Delay Doppler domain as well as distinct TFRBs in the time frequency domain. In one embodiment of the invention, each UT is allocated DDRBs spaced apart regularly by d₁ DDRBs along the Doppler domain and d₂ DDRBs along the delay domain. For example, Figure 2 shows the multiplexing of K = 4 UTs with d₁ = 2 and d₂ = 2. Note that c^and d₂ need not be the same. Further, the spacing for all UTs need not be the same. In the embodiment where all UTs use the same spacing, the number of UTs is K = d₁ d₂. The /c-th UT is allocated DDRBs in the set

where [ ] denotes the floor operation and (/c)_d denotes the value of k modulo d_x . Fet z_k[d, b] , 0 < a < J/d₂ , 0 < b < I/d₁ denote the information symbols of the k- th UT. Based on the DDRBs allocated to k- th UT in the set C_k, the /c-th UT transmits z_k[d, b] on the

Fet z_k[a, b] denote the signal transmitted by the /c-th UT on the (a, b)-th DDRB 0 £ a < J , 0 £ b < I. From the above discussion it is clear that z_k [a, b] = z_k[a, b\ for (a = [/c/c/ + d₂ a , b = (/c)_di + d₁ b), and z_k[a, b] = 0 for (a, b) g C_k . The k-th UT firstly transforms these delay Doppler information symbols into the discrete time frequency domain signal F_k [l, w] through the equation

w = 0,1, - , (/ - l) .

[0073] Due to the regular spacing of the allocated DDRBs, the discrete time frequency signal F_k[l, w] is invariant to shifts by integral multiples of J/d₂ along the time domain and to shifts by integral multiples of I /d along the frequency domain. Hence the time frequency signal can be localized to a portion of the entire time frequency domain which is JT / d₂ seconds wide along the time domain and IAf / d₁ Hz wide along the frequency domain. In other words, the present invented method allocates

d, x - TFRBs to the k-th

UT in the time-frequency domain. These TFRBs could be spread apart across the entire time-frequency domain or they could be placed together. Also, the allocation of TFRBs is made in such a way that no two terminals share a common TFRB. This ensures that there is no multi user interference as the UTs occupy non-overlapping TFRBs. One such allocation of TFRBs is depicted in Fig. 3 for if = 4 UTs, with J = I = 8 and d_t = d₂ = 2. One possible non-overlapping allocation of contiguous TFRBs is where the k-th UT is allocated the interval \JT (k)_d2/d₂ , JT (k)_d2/d₂ + ]T/d₂] seconds along the time domain and the interval [I Af [k/d₂\/d₁ , I Af [k/d₂\/d₁ + I A//d₁] Hz along the frequency domain. In general, let the TFRBs allocated to the k-th UT be denoted by the set D_k. For example, for the contiguous TFRB allocation the set D_k is given by

[0074] The time frequency signal F_k [l, w] is then converted to the continuous time domain signal s_k (t) . For example, for the contiguous TFRB allocation given above, s_k (t) is given by

EQN4:

where h_Lx(t) is the transmit pulse which is bandwidth limited to D/ Hz and approximately time limited to T seconds.

[0075] The receiver firstly transforms the received continuous time domain signal r(t) to the discrete time frequency domain signal R[l,w] as follows.

where h_rx(t) is the receive pulse and * denotes the complex conjugation operator. Due to the non-overlapping allocation of TFRBs to different UTs in the time frequency domain, there is no multi user interference and R [l, w] contains the time frequency signal of only that UT which has been allocated the (/, w)-th TFRB. For the k-th UT, the time frequency signal received on the TFRBs allocated to the k-th UT i.e., R [l, w] , (/, w) e D_k is given by (for the contiguous TFRB allocation)

J/d₂ , 0 < h < I/d₁ .

[0076] This received time frequency signal for the k-th UT is then transformed back to the delay Doppler domain. The received delay Doppler signal of the k-th UT is given by

where the received delay Doppler signals z_k\d, b] are related to the delay Doppler information symbols transmitted by the k-th UT, through a 2-D convolution in the delay Doppler domain. [0077] To be precise, let the transmitted information symbols be represented by

Then received delay Doppler signal ¾[. , . ] and the transmitted information symbols ¾[. , . ] are related by

N[ d, b] where H[ ·,·] is the effective 2-D delay Doppler channel and N[a, b] is the additive noise. Although the information symbols transmitted from different UTs do not interfere with each other at the receiver (i.e., there is no multi-user interference), there still exists inter symbol interference (ISI) due to the dispersive nature of the effective channel in equation EQN-8 above. The ISI can however be removed by using channel equalization techniques known in prior art.

[0078] In an exemplary implementation, reference is made to figure 10, which illustrates the spectral efficiency achieved by each mobile terminal for the standardized 3GPP Extended Typical Urban (ETU) channel model where the maximum delay spread is 5 ps and the maximum Doppler spread is 300 Hz, which corresponds to a mobile speed of about 80 Km/hr at a carrier frequency of 4 GHz. In IMT-2020, 5G systems have been expected to operate even at a mobile speed of 350 Km/hr (e.g. high speed train). Therefore to test the robustness of the present invention towards high delay and Doppler spread, in comparison with the current state of art method (i.e., Guard band based multiple-access), the maximum delay spread is doubled to 10 microseconds, and the maximum Doppler is 1300 Hz which correspond to mobile speeds of 350 Km/hr respectively. The tapped delay line (TDL) type delay profile is 2 x

[0 , 50 , 120 , 200 , 230 , 500 , 1600 , 2300 , 5000] ns and the corresponding power profile is [-1 , -1 , -1 , 0 , 0 , 0 , -3 , -5 , -7] dB. We also have Af = 15 KHz (T = 1/D/) and J = I = 45. There are K = 15 mobile terminals and the total bandwidth is I Af = 675 KHz. Spectral efficiency is defined to be the data rate (in bits per second) divided by the bandwidth used (in Hz). In Fig. 10 we plot the per-user terminal spectral efficiency as a function of increasing p which is a quantity proportional to the ratio of the total energy transmitted by each mobile terminal to the noise power spectral density at the receiver.

[0079] In the exemplary implementation for the present invention. d₁ = 5, d₂ = 3. From figure 10, it is observed that the prior art i.e., the guard band based method is unable to achieve high spectral efficiency due to multi user interference and saturates at high values of p. On the other hand the data rate achieved by the present invention increases steadily with increasing p. For example at p = 27 dB, even at a mobile speed of 350 Km/hr the present invention achieves a per-user spectral efficiency of 0.55 bits/sec/Hz which corresponds to a data rate of 371 kbits/sec, whereas the guard band based method achieves a per-user data rate of only 23 kbits/sec when guard bands are in the Doppler domain (as in the left sub-figure of Fig. 1) and a per-user data rate of 104 kbits/sec when the guard bands are in the Delay domain (as in the right sub-figure of Fig. 1). The total system throughput achieved by the present invention is therefore 371 Kbps/terminal X 15 terminals = 5.57 Mbps using only a bandwidth of 675 KHz whereas the guard band based method achieves a total system throughput of only 1.56 Mbps which is more than three times less than that achieved by the present invention.

[0080] Accordingly, some of the non-limiting advantages of the present invention are as follows:

• The multiplexing method achieves high throughput even in high mobility and high delay spread channels as it guarantees MUI free communication.

• The multiple-access method is free from multi-user interference (MUI), and inter carrier interference (ICI) and also achieves diversity by embedding information symbols in the Delay-Doppler domain followed by OTFS modulation, which spreads the information in the entire time-frequency domain.

• The present invention allows several users to communicate simultaneously using Orthogonal Time Frequency Waveforms (OTFS) in such a way that they achieve higher data throughput in a high Doppler and delay spread wireless channel, when compared to other multiple access methods which use guard bands in the delay- Doppler domain to reduce multi-user interference. • The multiple access method avoids the use of Guard bands and its throughput does not degrade as severely with increasing Doppler and delay spread.

[0081] Although a multiple access method in wireless telecommunications system thereof been described in language specific to structural features and/or methods, it is to be understood that the embodiments disclosed in the above section are not necessarily limited to the specific features or methods or devices described. Rather, the specific features are disclosed as examples of implementations of the multiple access method in wireless telecommunications system. Another implementation example may be when both the base station and the UT have more than one transmit and receive antenna, in which case each UT could transmit multiple delay Doppler signals, one delay Doppler signal from each transmit antenna. Yet another implementation example is when the base station transmits multiple delay Doppler signals, one each to a user in the downlink.

Claims

1. A multiple access method for wireless communication system for transmitting information from plurality of user terminals (UTs), wherein said method comprising: mapping, at each of said plurality of UTs, plurality of information symbols to Delay

Doppler Resource Blocks (DDRBs) to obtain plurality of Delay Doppler signals, wherein said DDRBs are spaced apart at regular intervals along delay and Doppler domain; transforming said Delay Doppler signals of the said UT to a plurality of time frequency signals which are mapped to the Time Frequency Resource Blocks

(TFRBs) allocated to said UT; transforming said plurality of time -frequency signals at each UT to a plurality of continuous time domain signals, which are then up-converted and transmitted to a base station (BS).

2. The multiple access method as claimed in claim 1, wherein at transmitter of each said

UT, said plurality of information signals are mapped to said DDRBs allocated to that UT.

3. The multiple access method as claimed in claim 1, wherein each of said UT from said plurality of UTs are allocated with non-overlapping said TFRBs so that said plurality of time frequency signals of each UT is separable from that of other UTs.

4. The multiple access method as claimed in any of claims 1 to 3, wherein for each said UT from said plurality of UTs, the said BS recovers the said plurality of time frequency signals for the said UT from the received time-domain signals and converts them back to said delay Doppler signals.

5. The multiple access method as claimed in claim 4, wherein for each said UT from said plurality of UTs, the said base station performs separate channel equalization on said delay Doppler signals of said UT to obtain an estimate of said information symbols transmitted by said UT of said plurality of UTs to said base station.

6. The multiple access method as claimed in claim 5, wherein either each UT from said plurality of UTs or the said BS or both have multiple antennas.

7. The multiple access method as claimed in claim 6, wherein each of the said UTs from the said plurality of UTs transmits different Delay Doppler signals from each of its antennas.

8. A multiple access method for wireless communication system for receiving information from plurality of user terminals (UTs), wherein said method comprising: receiving, at a base station, plurality of continuous time signal from said plurality of UTs;

transforming said received plurality of continuous time signal to plurality of time frequency signals and thereby separating said plurality of time frequency signals for each UT based on Time Frequency Resource Block (TFRB) allocation;

converting, by said base station, plurality of time frequency signals for each UT to plurality of Delay Doppler signals;

followed by separate channel equalization on said plurality of delay Doppler signals for each UT to obtain an estimate of plurality information signals transmitted by each of said UTs.

9. A wireless communication system comprising plurality of user terminals (UTs) and a base station, wherein said plurality of UTs transmit information to said base station on same time-frequency resource:

Wherein, each UT of said plurality of UTs is adapted to:

have plurality of information symbols and perform mapping of said plurality of information symbols to Delay Doppler Resource Blocks (DDRBs) to obtain plurality of Delay Doppler signals, wherein said DDRBs are spaced apart at regular intervals along delay and Doppler domain; transform said Delay Doppler signals of the said UT to a plurality of time frequency signals which are mapped to the Time Frequency Resource Blocks (TFRBs) allocated to said UT; transform said plurality of time -frequency signals at said UT to a plurality of continuous time domain signals, which are then up-converted and transmitted to a base station (BS). said base station adapted to:

receive said plurality of continuous time signal from said plurality of UTs;

transform said received plurality of continuous time signal to plurality of time frequency signals and thereby separate said plurality of time frequency signals for each UT based on Time Frequency Resource Block (TFRB) allocation;

convert said plurality of time frequency signals for each UT to plurality of Delay Doppler signals;

perform separate channel equalization on said plurality of delay Doppler signals for each UT to obtain an estimate of plurality information signals transmitted by each of said UTs.

10. A multiuser downlink communication method for wireless communication system for transmitting information from a base station (BS) to a plurality of user terminals (UTs), wherein said method comprising: mapping, at the base station, plurality of information symbols intended for each UT from said plurality of UTs to Delay Doppler Resource Blocks (DDRBs) to obtain plurality of Delay Doppler signals for each UT, wherein for each said UT, the said

DDRBs for that UT are spaced apart at regular intervals along delay and Doppler domain; transforming said Delay Doppler signals of each of the said plurality of UTs to a plurality of time frequency signals which are mapped to the Time Frequency Resource Blocks (TFRBs) allocated to said UT; jointly transforming together the said plurality of time-frequency signals from all the said plurality of UTs to a plurality of continuous time domain signals, which are then up-converted and transmitted from the antenna/antennas of said BS.

11. The downlink method as claimed in claim 10, wherein at the transmitter end of said BS, for each UT from said plurality of UTs, said plurality of information signals of said UT are mapped to DDRBs allocated to said UT.

12. The downlink method as claimed in claim 10, wherein said UTs from said plurality of UTs are allocated with non-overlapping said TFRBs so that said plurality of time frequency signals of each UT is separable from that of other UTs from said plurality of UTs.

13. The downlink method as claimed in any of claims 10 to 12, wherein each said UT from said plurality of UTs recovers its intended time frequency signals from the time-domain signal received at the antenna/antennas of said UT and converts them back to delay Doppler signals.

14. The downlink method as claimed in claim 13, wherein each said UT from said plurality of UTs performs channel equalization on said delay Doppler signals of said UT to obtain an estimate of said information symbols intended for said UT as transmitted by said BS.

15. The downlink method as claimed in claim 14, where either the said UTs from said plurality of UTs or the said BS or both have multiple antennas.

16. The downlink method as claimed in claim 14 or claim 15, where for each UT from said plurality of UTs, the BS precodes information symbols intended for said UT to information signals which are then mapped to DDRBs allocated to said UT to obtain a plurality of Delay Doppler signals for said UT.

17. A downlink communication method for wireless communication system for receiving information at each UT from a plurality of UTs, wherein said method at each UT comprising: receiving, at said UT, plurality of continuous time signals from a base station (BS);

transforming said received plurality of continuous time signals to plurality of time frequency signals and separating the time frequency signals intended for said UT based on Time Frequency Resource Block (TFRB) allocation; converting, by said UT, said plurality of time frequency signals intended for said UT to plurality of Delay Doppler signals;

18. A downlink communication method comprising plurality of user terminals (UTs) and a base station (BS), wherein said plurality of UTs receive information from said base station on the same time -frequency resource:

wherein each UT from said plurality of UTs is adapted to:

receive plurality of continuous time signals from the said BS;

Wherein, the said base station is adapted to:

having plurality of information symbols intended for each UT, and perform mapping of said plurality of information symbols for each UT to Delay Doppler Resource Blocks (DDRBs) allocated to said UT to obtain plurality of Delay Doppler signals for said UT, wherein for each UT the said allocated DDRBs are spaced apart at regular intervals along delay and Doppler domain; transform said Delay Doppler signals for each UT to a plurality of time frequency signals intended for said UT which are mapped to the Time Frequency Resource Blocks (TFRBs) allocated to said UT; jointly transform together said plurality of time -frequency signals for all the UTs to a plurality of continuous time domain signals, which are then up-converted and transmitted from the antenna/antennas of said BS.