Rach procedure for non-terrestrial networks

Abstract

A system and method for a random access procedure in a non-terrestrial network. In some embodiments, the method includes calculating, by a user equipment (UE), a timing advance based on location information of the UE and ephemeris of a non-terrestrial network node. The method can also include randomly selecting, from a plurality of values, a random value, and transmitting, by the UE, a random access (RA) message to the non-terrestrial network node at a transmission time, the random access (RA) message can include a signature. The transmission time can differ from a nominal transmission time by an amount based on the random value, the nominal transmission time is based on the timing advance, or the signature can differ from a nominal signature by a cyclic shift based on the random value.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to and the benefit of U.S. Provisional Application No. 63 / 056,352, entitled “METHODS FOR RACH PROCEDURES FOR NON-TERRESTRIAL NETWORKS” and filed on July 24, 2020, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] One or more aspects of embodiments according to the present disclosure relate to mobile communications, and more particularly to mobile communications involving non-terrestrial networks. BACKGROUND

[0004] In mobile communications systems, it can sometimes be advantageous for a user equipment to communicate with a non-terrestrial node, e.g., a communication satellite. In such cases, the round-trip delay can be significantly larger than the round-trip delay between the user equipment (UE) and a terrestrial network node. This can pose challenges, e.g., a reliable random access procedure between the UE and the terrestrial network node can not be reliable when there is a large round-trip delay.

[0005] Accordingly, there is a need for systems and methods for random access procedures in non-terrestrial networks. SUMMARY

[0006] According to one embodiment of the present disclosure, a method is provided, comprising: calculating, by a user equipment (UE), a timing advance based on: location information of the UE; and ephemeris of a non-terrestrial network node; randomly selecting, from a plurality of values, a random value; and transmitting, by the UE, a random access (RA) message to the non-terrestrial network node at a transmission time, the RA message comprising a signature, wherein: the transmission time differs from a nominal transmission time by an amount based on the random value, the nominal transmission time being based on the timing advance, or the signature differs from a nominal signature by a cyclic shift based on the random value.

[0007] In some embodiments, the method further comprises receiving, by the UE, the ephemeris information.

[0008] In some embodiments, the transmission time differs from the nominal transmission time by the amount based on the random value.

[0009] In some embodiments, the RA message comprises a preamble sequence.

[0010] In some embodiments, the RA message is sent via a PRACH occasion (RO), and the amount based on the random value is less than a cyclic prefix (CP) of the RO.

[0011] In some embodiments, the RA message includes a preamble sequence, and the signature differs from a nominal signature by a cyclic shift based on the random value.

[0012] In some embodiments, each of the plurality of cyclic shifts respectively corresponds to one of a plurality of values; a first cyclic shift of the plurality of cyclic shifts is a largest one of the plurality of cyclic shifts that is smaller than a second cyclic shift of the plurality of cyclic shifts; and the preamble sequence is shifted: the second cyclic shift, minus a maximum supported channel delay spread of the UE and the non-terrestrial network node, minus an uncertainty of the computed time advance, orthogonal to: the preamble sequence shifted the first cyclic shift.

[0013] In some embodiments, the method further comprises: randomly selecting a delay value from a set of delay values; and transmitting, by the UE, a demodulation reference signal (DMRS) via a physical uplink shared channel (PUSCH) occasion (PO), the transmission of the DMRS starting a delay value after a start of the PO.

[0014] In some embodiments, the set of delay values includes integer multiples of a DMRS duration.

[0015] In some embodiments, the RA message includes a preamble sequence, the RA message is transmitted via a PRACH occasion (RO); and a length of the preamble sequence is at least equal to a maximum supported channel delay spread.

[0016] In some embodiments, the length of the preamble sequence is a smallest prime number that is at least equal to the maximum supported channel delay spread.

[0017] In some embodiments, the RA message does not include a preamble.

[0018] According to one embodiment of the present invention, there is provided a system comprising: a user equipment (UE), the UE comprising: a radio; and processing circuitry configured to: compute a timing advance based on: location information of the UE; and ephemeris of a non-terrestrial network node; randomly select a random value from a plurality of values; and transmit, by the UE to the non-terrestrial network node at a transmission time, a random access (RA) message, the RA message comprising a signature, wherein: the transmission time differs from a nominal transmission time by an amount based on the random value, the nominal transmission time being based on the timing advance, or the signature differs from a nominal signature by a cyclic shift based on the random value.

[0019] In some embodiments, the processing circuitry is further configured to receive the ephemeris information.

[0020] In some embodiments, the transmission time differs from the nominal transmission time by the amount based on the random value.

[0021] In some embodiments, the RA message includes a preamble sequence.

[0022] In some embodiments, the RA message is sent via a PRACH occasion (RO), and the amount based on the random value is less than a cyclic prefix (CP) of the RO.

[0023] In some embodiments, the RA message includes a preamble sequence, and the signature differs from a nominal signature by a cyclic shift based on the random value.

[0024] In some embodiments, each of the plurality of cyclic shifts respectively corresponds to one of a plurality of values; a first cyclic shift of the plurality of cyclic shifts is a largest one of the plurality of cyclic shifts that is less than a second cyclic shift of the plurality of cyclic shifts; and the preamble sequence is shifted: the second cyclic shift, minus a maximum supported channel delay spread of the UE and the non-terrestrial network node, minus an uncertainty of the computed time advance, orthogonally to: the preamble sequence shifted the first cyclic shift.

[0025] According to one embodiment of the present invention, there is provided a system comprising: a user equipment (UE), the UE comprising: a radio; and means for processing configured to: compute a timing advance based on: location information of the UE; and ephemeris of a non-terrestrial network node; randomly select a random value from a plurality of values; and transmit, by the UE, a random access (RA) message to the non-terrestrial network node at a transmission time, the RA message comprising a signature, wherein: the transmission time differs from a nominal transmission time by an amount based on the random value, the nominal transmission time being based on the timing advance, or the signature differs from a nominal signature by a cyclic shift based on the random value. BRIEF DESCRIPTION OF DRAWINGS

[0026] These and other features and advantages of the present invention will be understood and appreciated by those skilled in the art upon studying the following specification, claims, and appended drawings in which:

[0027] Figure 1A is a sequence diagram of a random access procedure according to embodiments of the present disclosure;

[0028] Figure 1B is a sequence diagram of a random access procedure according to embodiments of the present disclosure;

[0029] Figure 2A is a correlator output diagram according to embodiments of the present disclosure;

[0030] Figure 2B is a correlator output diagram according to embodiments of the present disclosure;

[0031] Figure 2C is a structure for a medium access control (MAC) random access response according to embodiments of the present disclosure;

[0032] Figure 3A is a table of cyclic shift values according to embodiments of the present disclosure;

[0033] Figure 3B is a correlator output diagram according to embodiments of the disclosure;

[0034] Figure 4 is a timing diagram of transmissions from two UEs according to embodiments of the disclosure;

[0035] Figure 5A is a sequence diagram of a random access procedure according to embodiments of the disclosure;

[0036] Figure 5B is a structure of an uplink common control channel message for a radio resource control connection request according to embodiments of the disclosure;

[0037] Figure 5C is a structure of a MAC payload with timing advance fine tuning according to embodiments of the disclosure;

[0038] Figure 5D is a structure of a MAC payload without timing advance fine tuning according to embodiments of the disclosure;

[0039] Figure 6 is a flow diagram of a method according to embodiments of the disclosure; and

[0040] Figure 7 is a block diagram of a portion of a mobile communication system according to embodiments of the disclosure. DETAILED DESCRIPTION

[0041] The detailed description set forth below, in connection with the appended drawings, is intended as a description of exemplary embodiments of systems and methods for random access procedures in non-terrestrial networks provided in accordance with the present disclosure and is not intended to represent the only forms in which the present disclosure can be constructed or utilized. The description sets forth the features of the present application in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures can be accomplished by different embodiments and that they are intended to be included in the scope of the application. As shown elsewhere herein, like element numbers are intended to indicate like or similar elements or features.

[0042] Satellite communication systems are expected to provide a solution to complement ground networks in order to extend services to remote areas with insufficient and no service. Recently, commercialization and standardization efforts have started to incorporate satellite communications into existing cellular networks. In particular, 3GPP has initiated a study item on 5G New Radio (NR), i.e., Non-Terrestrial Networks (NTN), to deploy satellite systems as standalone networks or as integration with 5G ground networks in scenarios such as mobile broadband and machine-type communications. However, typical satellite channel impairments such as large propagation delay and high Doppler shift pose challenges to the implementation of some components of NTN such as random access (RA) and timing advance (TA).

[0043] The random access procedure serves multiple purposes, for example, (i) allowing a UE to establish a connection with a gNB, (ii) synchronizing uplink timing with the network node (gNB), and (iii) beam management. One role of random access is to enable a UE in radio resource control (RRC) idle mode to establish a connection with a gNB during initial access. In this case, multiple UEs can perform the procedure. Therefore, a contention-based solution is specified. The specified random access procedure is an iterative procedure in which multiple transmissions occur between a UE and a gNB. As used herein, the phrase “user equipment” is used as a count noun, even though the noun it contains (“equipment”) can not be countable in ordinary English. Similarly, the phrase downlink control information (DCI) is also used as a count noun.

[0044] One distinguishing factor between NTN and terrestrial scenarios is that the round-trip delay is longer in the NTN case, and its impact can be amplified in the random access channel (RACH) procedure, which has multiple iterations. However, a UE capable of communicating with an NTN can be able to compute and apply a timing advance (TA) to pre-compensate for the round-trip delay, such that some of the problems that can arise from the long round-trip delay can be addressed.

[0045] During the RACH procedure, a UE transmits a preamble to a gNB over the PRACH channel to obtain uplink (UL) synchronization. In 5G NR, 64 preambles are defined in each time-frequency PRACH occasion. A preamble consists of two parts, a cyclic prefix (CP) and a preamble sequence (or simply “preamble”). Typically, the preamble is followed by a guard time (GT) to absorb propagation delays. The length of the GT is referred to as the guard period (GP). In 5G NR, 13 preamble formats are supported. The 13 preamble formats can be divided into two categories: long preambles and short preambles. The time-domain differences between the different preamble formats include different (i) CP lengths, (ii) sequence lengths, (iii) GP lengths, and (iv) repetition numbers.

[0046] Long preambles are based on Zadoff-Chu (ZC) sequences of length 839. The subcarrier spacing (SCS) of long preambles can be 1.25 kHz or 5 kHz. The numerology used for long preambles is different from any other NR transmission. Long preambles can only be used for FR1 bands, which are below 6 GHz. There are four different formats for long preambles, namely Format 0, Format 1, Format 2, and Format 3. The preamble format is part of the cell random access configuration, and each cell is limited to a single preamble format. Long preambles with 1.25 kHz numerology occupy six resource blocks in the frequency domain, while preambles with 5 kHz numerology occupy 24 resource blocks.

[0047] Short preambles are based on length-139 sequences. The subcarrier spacing of short preambles is aligned with the normal NR subcarrier spacing of different numerologies, i.e., 15 kHz, 30 kHz, 60 kHz, and 120 kHz. This allows the gNB receiver to use the same Fast Fourier Transform (FFT) engine for data and random access preamble detection. Short preambles use a subcarrier spacing of 15 kHz or 30 kHz in case of sub-6 GHz (FR1) operation, and 60 kHz or 120 kHz in case of higher NR bands (FR2) operation. Short preambles occupy 12 resource blocks in the frequency domain, regardless of the preamble’s numerology. In general, short preambles are shorter than long preambles and can span only a few OFDM symbols. The short preamble format is designed such that the last part of each OFDM symbol acts as a CP for the next OFDM symbol, and the length of a preamble OFDM symbol is equal to the length of a data OFDM symbol. Therefore, in most cases, multiple preamble transmissions can be time-multiplexed within a single PRACH occasion (RO). Furthermore, for short preambles, there can be multiple PRACH occasions in the frequency domain within a single RACH slot, as well as in the time domain. Short preambles can be targeted mainly for small or “normal” cell and indoor deployment scenarios. Short preambles support analog beam sweeping during PRACH reception, enabling the same preamble to be received at the gNB with different beams.

[0048] Cyclic shifts can be applied to ZC sequences to obtain more preamble resources. Sequences obtained from cyclic shifts of different ZC sequences are not perfectly orthogonal. Therefore, orthogonal sequences obtained by cyclic shifting a single root sequence are preferred over non-orthogonal sequences; in some embodiments, additional ZC root sequences are used only when the required number of sequences (e.g., 64) cannot be generated by cyclic shifts of a single root sequence. Thus, the cyclic shift size is very important in RACH design.

[0049] Cyclic shift compensates for N CS The size is determined such that the zero correlation zone (ZCZ) of the sequence guarantees the orthogonality of the PRACH sequences, regardless of the delay spread and time uncertainty of the UEs. Therefore, the minimum value of N CS may be the smallest integer greater than the sequence sampling period of the maximum delay spread and time uncertainty of the uplink unsynchronized UEs, plus some additional guard samples provided for the overlap of the impulse shaping filter envelopes present in the PRACH receiver.

[0050] The lower bound of the resulting cyclic shift N CS can be written as:

[0051]

[0052] where r is the cell size (in km), τ ds is the maximum delay spread, N ZC and T SEQ are the PRACH sequence length and duration (measured in μβ) respectively, and n f is the number of additional guard samples due to the receiver pulse-shaping filter.

[0053] For a given environment, the delay spread can be assumed constant. However, the larger the cell, the larger the cyclic shift needed to generate an orthogonal sequence, and therefore, the larger the number of ZC root sequences necessary to provide the required 64 preambles.

[0054] The relationship between the cell size and the number of ZC root sequences needed allows for some system optimization. Typically, the gNB can configure N CS Since the expected inter-cell interference and load (user density) increase as the cell size decreases, smaller cells need more protection than larger cells to avoid co-preamble interference.

[0055] In Rel. 16, two types of random access procedures are specified, namely the 4-step random access procedure and the 2-step random access procedure. The 2-step is an alternative to the 4-step RACH, which reduces the number of consecutive steps or iterative transmissions between the UE and the gNB. In other words, the 2-step RACH reduces the duration of the initial access by reducing the number of consecutive downlink and uplink transmissions. This characteristic of the 2-step RACH makes it suitable for NTN scenarios, where the round-trip delay between the gNB and the UE can be much larger than in typical terrestrial cases.

[0056] The following paragraphs illustrate the comparison between the 2-step RACH and the 4-step RACH in terms of steps or transmissions.

[0057] In the 4-step RACH, as Figure 1AThe described specifies the following four steps, called "Step 1" to "Step 4". In Step 1, at 105, the UE randomly selects a ZC preamble from the available pool of preambles and transmits Msgl or preamble in the specified RO. The gNB uses the preamble to estimate the TA and to coordinate the beam management. In Step 2, at 110, the gNB transmits one random access response (RAR) or Msg2 for each received preamble in the RO regardless of the number of UEs that selected the received preamble. This response includes the estimated TA, the UE's random access preamble identification (RAPID), an UL grant to the UE, a temporary cell radio network temporary identifier (TC-RNTI), and a fallback indicator for UEs that received the RAR but do not have a RAPID that matches the RAPID in the RAR. In Step 3, at 115, the UE whose RAPID matches the RAR RAPID will transmit Msg3 containing a contention resolution ID (CRID) according to the UL grant included in the RAR message. This means that only when the UE's TA matches the TA indicated by the RAR, the gNB will receive Msg3 at the expected time. This helps the UE with the best estimated TA to have a higher chance of correct decoding at the gNB side. In Step 4, at 120, if the gNB correctly decodes Msg3, it will transmit Msg4 including the CRID of the successful UE. For the UE, if the content of Msg4 matches its CRID, the contention is completed. In this case, the TC-RNTI becomes the assigned C-RNTI and the UE transmits a HARQ acknowledgement to Msg4 using the common resource set. As mentioned above, the 4-step RACH requires at least two round-trip delays to complete.

[0058] In 2-step RACH, the number of sequential transmissions is reduced. To achieve this, typically, in 2-step RACH, the UE transmits PUSCH containing CRID before receiving RAR from the gNB. As Figure 1BAs shown, the following steps are referred to as "Step 1" and "Step 2" and are designated for 2-step RACH. In Step 1, at 125, two messages are transmitted in two different slots. These two messages are combined together to form MsgA. The first message is MsgA PRACH, which is a preamble randomly selected from a pool of preambles. The second message is referred to as MsgA PUSCH, which contains the UE's CRID. The MsgA PRACH transmission occurs in an RO, and the subsequent MsgA PUSCH occurs in a PUSCH occasion (PO), both of which depend on the selected preamble. The mapping between preambles and ROs and POs is configured by the gNB prior to random access. In Step 2, at 130, if MsgA is decoded correctly at the gNB, the gNB sends a RAR message referred to as MsgB to the UE. This message contains the UE's TA, TC-RNTI, and CRID. In a special case where MsgA PRACH is received but MsgA PUSCH is not, MsgB contains an UL grant for the UE to send its PUSCH as a fallback. If the UE's CRID matches the CRID contained in the decoded RAR, the random access is completed, and the UE sends a HARQ acknowledgement at 135, and its TC-RNTI is promoted to C-RNTI. As mentioned above, 2-step RACH requires completion of at least one round trip compared to two round trip delays (RTD) in 4-step RACH.

[0059] Due to Doppler shifts in NTN scenarios, the current preamble design of Rel-16 can not support large RTD and frequency offsets. Therefore, in the latest work item for NTN, a UE with the capability to compute and pre-compensate for TA is proposed. Such a UE can be a Global Navigation Satellite System (GNSS)-based UE that has access to ephemeris data containing information about the orbital trajectory of a man-made satellite (e.g., the UE can receive ephemeris data from the network). Such a UE is able to compute the distance to the satellite and, thereby, estimate the round trip delay. In the case where the gNB is located on the ground and the satellite is a network node separate from the gNB (and the satellite retransmits transmissions received from the gNB to the UE and retransmits transmissions received from the UE to the gNB), the UE can also receive information about a timing advance corresponding to the delay between the gNB and the satellite (which can be referred to as "common timing advance"), or it can compute the common timing advance based on (i) the location of the gNB and (ii) the ephemeris data. Given these assumptions, the RACH procedure can be redesigned to support NTN UEs. Thus, in some embodiments, the RACH design can be modified in several aspects.

[0060] In some embodiments, an intentional random delay is applied to the MsgA preamble or Msgl preamble. In 2-step RACH, as mentioned above, MsgA consists of a PRACH preamble and a PUSCH transmission, referred to as MsgA PRACH and MsgA PUSCH, respectively. The 2-step MsgA PRACH preamble is separate from the 4-step PRACH preamble, however, they can be transmitted in the same PRACH occasion (RO) as the 4-step PRACH-RO or in a separate RO. The PUSCH transmission is organized into PUSCH occasions (POs), which can span multiple symbols and physical resource blocks (PRBs) with optional guard periods and guard bands between consecutive POs. Each PO consists of multiple demodulation reference signal (DMRS) ports and DMRS sequences. Each DMRS port or DMRS sequence is referred to as a PUSCH resource unit (PRU). The 2-step RACH supports at least one-to-one and many-to-one mapping between preambles and PRUs.

[0061] For NTN operation, it is assumed that the UE has GNSS capability so that it knows its own geographical location information. The network will provide the UE with sufficient information so that it can accurately calculate the timing advance value. For example, the following all or part of information can be sent to the UE to calculate the timing advance value: ephemeris data of the satellite, geographical location information of the gateway, geographical location information of any reference point in space, and cell-specific common timing advance value.

[0062] This information can be broadcast in the cell and sent to all UEs of the cell via, for example, system information block (SIB). The ephemeris data of the satellite can also be broadcast in the cell and sent to all UEs of the cell via, for example, SIB. Alternatively, the information can be indicated to the UE by dedicated signaling. For example, when the UE is in RRC connected mode and the satellite switches the gateway, the satellite can update the reference point geographical location information in the broadcast channel, and also convey the updated information about the new reference point geographical location to the connected UEs via dedicated signaling, such as in DCI (downlink control information) or MAC-CE (MAC control element). Thus, the UE can accurately calculate the timing advance value for all or part of the timing advance compensation.

[0063] It is also assumed that the UE has obtained the downlink timing information before initiating the RACH procedure. In this way, the UE has information about both 2-step and 4-step ROs. With the above assumption, the UE can transmit the preamble with precise timing so that the preamble arrives at the gNB precisely at the beginning of the PRACH occasion.

[0064] If two or more UEs select the same signature (same preamble sequence with the same cyclic shift), then since they use the precisely calculated propagation delay and timing advance to transmit their signature, their signatures arrive together at the gNB, i.e., at the same time. As a result, a contention occurs and the gNB cannot distinguish the different UEs that have initiated the RACH procedure. Eventually, in further steps of the RACH procedure, after the contention resolution step, in the best case scenario, only one UE will succeed and the other UEs will be discarded. Figure 2A An example is shown of how the power delay profile (PDP) of two UEs using the same signature in the same RO would collide. As shown, the PDPs 205, 210 in the two signatures received by the gNB within the observation interval overlap with each other. In this case, the gNB can not be able to distinguish the two UEs.

[0065] In some embodiments, a UE initiating a random access procedure will select a signature from the available PRACH signatures by transmitting a MsgA preamble in a 2-step RACH or by transmitting a Msgl preamble in a 4-step RACH. The UE then calculates the accurate propagation delay and timing advance and then adds a randomly selected delay value and transmits the signature by applying the accurate timing advance and the randomly selected delay. The UE is configured by the network with a set of available random delay values. The random delay values can be cell-specific, i.e., the set of available random delay values is broadcast to the cell and all UEs belonging to the cell use the same set. The consecutive delay values of the set of available random delay values can differ (i.e., can be separated) by the maximum assumed delay spread the network is operating plus an additional guard time provided for errors in the UE’s location-based timing advance estimation. On the other hand, the delay values can not be larger than the cyclic prefix (CP) of the PRACH. In one example, the set of delay values τ k may be:

[0066]

[0067] where K is the number of available delays, which is determined as:

[0068]

[0069] where τ ds is the assumed maximum delay spread, and e TA is the guard time provided for errors in the UE’s timing advance estimation.

[0070] For example, if the CP duration is 103.13 μ8, the assumed delay spread is 5.2 μ8, and if e TA is 0.5 μ8, then K = 18, and the delay values can be selected to be equally spaced (i.e., separated) by 5.73 μ8, that is, the set of delay values τk Will be:

[0071] τ k = 5.73 kμs, k = 0, 1,..., 17

[0072] In some embodiments, the delay values are chosen to be further apart from each other (e.g., such that every two adjacent signatures are separated by a larger delay), to provide a larger marginal distance between them. The network determines the available delay values for all PRACH formats according to the above criteria, and configures the UEs with the available delay values table. Figure 2B An example is shown in which two UEs transmit the same signature using two different intentional random delay values. The gNB can distinguish the two signatures and knows that there are two different UEs trying to initiate random access in the same RO. The gNB then takes appropriate actions to continue the RACH procedure for two or more UEs, respectively, depending on whether a 2-step or 4-step RACH procedure is employed.

[0073] To distinguish the UEs that applied different random delays, a new identification field, Random Access Delay Identification (RADID), can be included in the random access response. For example, for 4-step RACH, in the Msg2 random access response, the gNB can address the UE not only by its Random Access Preamble Identification (RAPID) but also by its RADID. Figure 2C An exemplary structure of the MAC random access response is illustrated. As shown, in addition to the RAPID, a field called RADID is added to indicate that the addressed UE is the one that used the specific random delay with the RADID identification. The field size (in bits) in Figure 2C may be chosen as needed. Figure 2C The sizes shown in are for illustration only and are not scaled.

[0074] For 2-step RACH, the RADID field can similarly be included in the MsgB PDSCH so that the addressed UE recognizes that the gNB is addressing the UE with the indicated RADID.

[0075] In some embodiments, an intentional cyclic shift can be applied to the MsgA preamble and the Msg1 preamble. In one such embodiment, a UE selects a signature from the available PRACH signatures and initiates a random access procedure by transmitting a MsgA preamble in 2-step RACH or by transmitting a Msg1 preamble in 4-step RACH. The UE computes the accurate propagation delay and the corresponding timing advance, and then transmits the signature preamble using the computed timing advance.

[0076] The cyclic shift compensates for N CSThe size can be determined in such a way that the zero-correlation zone (ZCZ) of the sequence guarantees the orthogonality of the preamble sequences irrespective of the delay spread and time uncertainty of the timing advance computed by the UE. Thus, N CS The minimum value can be the smallest integer greater than the sequence sample period of the maximum delay spread plus some additional guard samples provided for the overflow of the pulse-shaping filter envelope present in the PRACH receiver plus additional samples to account for the time uncertainty of the timing advance computed by the UE.

[0077] The lower bound of the resulting cyclic shift N CS can be written as:

[0078]

[0079] where τ ds is the maximum delay spread in μs, N ZC and T SEQ are the preamble sequence length and duration (measured in μs) respectively, n f is the number of additional guard samples due to the receiver pulse-shaping filter, and n TA is the number of additional samples to compensate for the time uncertainty of the timing advance computed by the UE. Using cyclic shifts that satisfy this lower bound ensures that two shifted preamble sequences, one of which is shifted by N CS , greater than the other), will be orthogonal at the gNB even if both the delay spread and uncertainty in the computed timing advance are reduced at the gNB by the effective cyclic shift difference.

[0080] For a given environment, the delay spread can be assumed to be constant. However, the larger the cell, the larger the cyclic shift required to generate orthogonal sequences and, therefore, the larger the number of ZC root sequences necessary to provide the required number of preambles.

[0081] By choosing a larger value for n TA , the cyclic shift values can be chosen such that two adjacent signatures are further apart from each other (i.e., have a larger difference in their respective cyclic shift offsets) to provide a larger marginal distance between them. The network determines the available cyclic shift values for all PRACH formats according to the above criteria and configures the UE with the available cyclic shift table.

[0082] Unlike equation (1), in equation (2), N CSIt is independent of cell radius. Therefore, it does not require the 16 different zero-correlation zone configurations specified in 3GPP TS38.211v16.0.0, "Physical channels and modulation (Release 16)" (referred to herein as the "Physical Channels and Modulation Document"). The Physical Channels and Modulation Document specifies 16 different zero-correlation zone configurations to support different cell radius ranges. Theoretically, only one zero-correlation zone structure would be sufficient. However, because different UEs may have different timing advance estimation capabilities in practice, in order to support n TA Multiple values ​​of can define several zero-correlation region configurations. In one example, using equation (2), only four zero-correlation region configurations are defined to support n. TA Four different values. Figure 3A The table provides, for example, an example of N for four different zero-correlation regions. CS The values ​​are respectively related to n TA The values ​​0, 2, 4, and 6 correspond to each other. In Figure 3A In the table, it is assumed that for the preamble subcarrier spacing Δf RA =1.25kHz, N ZC =L RA =839、τ ds =5.2μs, n f =2 and T SEQ It was selected as 800μs.

[0083] Figure 3A The table can replace Table 6.3.3.1-5 in the Physical Channel and Modulation File. Other tables, such as Tables 6.3.3.1-6 and 6.3.3.1-6 in the Physical Channel and Modulation File, can be replaced using the same method.

[0084] Figure 3B An example is shown where two UEs send the same signature using two different randomly selected cyclic shift values. The gNB is able to distinguish between the two signatures and knows that two different UEs are attempting to initiate random access in the same RO. The gNB then takes appropriate action, depending on whether a 2-step or 4-step RACHUE procedure is used, to continue the RACH procedure for the two or more UEs respectively.

[0085] In some embodiments, a very short sequence is used for the PRACH preamble. One factor in determining the appropriate sequence length for the PRACH preamble is the required size of the supported cell. For example, the length of the cyclic prefix can be greater than the maximum supported round-trip time plus the maximum channel delay spread (see, for example, [link to relevant documentation]). Figure 2B). The preamble sequence length can be larger than the cyclic prefix. Thus, the preamble sequence length can also be larger than the maximum supported round trip delay plus the maximum channel delay spread. For GNSS based UEs capable of pre-compensating the round trip delay, the cyclic prefix does not have to account for the round trip delay and can be as short as the maximum channel delay spread only. Thus, in this case, the length of the PRACH preamble sequence can also be as short as the maximum supported channel delay spread in theory.

[0086] In one embodiment, the length of the PRACH preamble sequence is chosen to be at least larger than the maximum supported channel delay spread. On the other hand, if the preamble sequence is selected from the ZC sequence family, the length of the sequence can be a prime number in order to guarantee the best cross-correlation properties of the sequence. Thus, in one example, the length of the PRACH preamble sequence is chosen to be the smallest prime number larger than the maximum supported channel delay spread.

[0087] The principle of using a cyclic shift on the sequence to increase the number of preamble resources can also be applied to this embodiment. In particular, if the length of the preamble sequence is chosen to be moderately larger than the channel delay spread for any reason, a cyclic shift can be applied to the sequence to provide multiple sequences. The cyclic shift length can be larger than the supported channel delay spread.

[0088] In some embodiments, an intentional random delay is applied to the MsgA PUSCH. In some embodiments, if the UE is using a 2-step RACH procedure, the UE selects the same delay applied to the preamble (selected according to the method described above for applying an intentional random delay to the MsgA preamble or Msgl preamble) and also applies it to the MsgA PUSCH transmission. Alternatively, if an intentional cyclic shift is applied to the MsgA preamble or Msgl preamble as described above, the UE can select a delay corresponding to the selected cyclic shift. The mapping between delay values and cyclic shift values is predetermined and known to both the UE and the network.

[0089] In case of one-to-one mapping between preambles and PRUs, if two or more UEs use the same preamble in the same RO, they will end up using the same PRU for MsgA-PUSCH transmission. In case of many-to-one mapping between preambles and PRUs, even if some UEs use different preambles in different ROs, they can still use the same PRU for MsgA-PUSCH transmission. In both cases, the PRU DMRS ports from multiple UEs will interfere with each other. This leads to channel estimation degradation and thus the performance of MsgA PUSCH detection will be affected. As a result of the UE applying a random delay, the probability of DMRS from different UEs interfering with each other can be reduced as shown in Figure 4 Figure 6.Figure 4 Two transmissions 405, 410 from two respective UEs are shown, whose DMRSs do not overlap in time. Therefore, the PUSCH channel estimation performance will be improved. The random delay applied to the MsgA PUSCH can be an arbitrary value, e.g. the same value as the deliberate random delay applied to the MsgA preamble (in the manner explained above). However, to avoid DMRSs from different UEs interfering with each other, the DMRS duration, i.e. integer multiples of the OFDM symbol duration, can be chosen as the random delay value.

[0090] In one embodiment, there can be a mapping between the deliberate random delay applied to the MsgA preamble (in the manner explained above) and the deliberate random delay applied to the MsgA PUSCH. The mapping can be one-to-one, one-to-many or many-to-one, depending on the number of delays available in both mechanisms. In a simple example, the same deliberate delay can be applied to both the MsgA preamble and the MsgA PUSCH, i.e. a simple one-to-one mapping can be used.

[0091] In another embodiment, there can be a mapping between the deliberate cyclic shift applied to the preamble (in the manner explained above) and the deliberate random delay applied to the MsgA PUSCH. The mapping can be one-to-one, one-to-many or many-to-one, depending on the number of cyclic shifts and delays available in both mechanisms, respectively.

[0092] In such an embodiment, the gNB will be able to detect the presence of the preamble in the preceding step and will also extract the timing information of the preamble. Therefore, the gNB will have the timing information of the MsgA PUSCH and DMRS at the slot level and symbol level. In other words, the gNB will not have to extract the MsgA timing information by blind detection.

[0093] In some embodiments, the UEs of a cell are configured with a set of MsgA PUSCH POs that partially overlap in the time domain. This configuration provides a set of starting points for the MsgA PUSCH in the resources allocated for the POs. In one example, the UE can randomly select the configuration and transmit the MsgA PUSCH accordingly. There can be a mapping between the above configuration and (i) a random delay applied to the MsgA preamble (as explained above for the embodiment of applying a random delay to the preamble intentionally), or between the above configuration and (ii) a random cyclic shift applied (as explained above for the embodiment of applying a random cyclic shift to the preamble intentionally), or between the above configuration and (iii) a short preamble sequence (as explained above for the embodiment of using a short preamble sequence). In the case of a preamble-less random access procedure, where the MsgA preamble is not present (as described below), there is no such mapping between the MsgA PUSCH configuration and the preamble (as the preamble is not present), and the UE can randomly select the MsgA PUSCH configuration.

[0094] In some embodiments, a contention-based preamble-less random access procedure is employed. In 2-step RACH, the MsgA consists of a PRACH preamble and a PUSCH transmission, referred to as MsgA-PRACH and MsgA-PUSCH, respectively. The UE initiates the random access procedure by transmitting a preamble in a pre-configured RO. By receiving the preamble in the RO, the gNB will be informed of the presence of a UE that is trying to access the network. The gNB will also extract the timing information of the UE’s uplink transmission from the received preamble and will send the UE’s timing advance value in MsgB. However, in NTN, if the UE is GNSS capable, it is able to accurately compute the value of the timing advance and does not need the gNB to transmit the timing advance information in MsgB.

[0095] In one embodiment, the UE initiates the random access procedure by first transmitting a MsgA PUSCH. Prior to the random access procedure, the network has configured a pool of available POs and corresponding PRUs. The UE randomly selects a PO and an available PRU and transmits an RRC connection request in the MsgA PUSCH using an accurately computed timing advance. Figure 5A A procedure for a 2-step contention-based preamble-less random access procedure is shown.

[0096] For example, the RRC connection request can be transmitted in a common control channel (CCCH) consisting of six bytes, as Figure 5BAs shown. An example of an uplink CCCH message used for an RRC connection request CCCH message can include a UE Contention Resolution Identifier (CRID) and an RRC connection request type, and is applicable to UEs transitioning from RRC idle or RRC deactivation to an RRC connection. In other words, the CCCH message can be either an RRC Setup Request or an RRC Resume Request. It can also be used for UE re-establishing an RRC connection and for UE requests for on-demand system information.

[0097] Upon receiving the MsgA PUSCH, the gNB monitoring the PO will detect its presence and decode it. The gNB first performs channel estimation using the DMRS in the MsgA PUSCH. When two or more UEs use the same PO, to avoid DMRS interference, techniques similar to the deliberate cyclic shift described above can be applied to MsgA PUSCH transmission to reduce the level of DMRS interference. Then, the gNB calculates the precise timing advance value for the UE. Even if the UE has already calculated the timing advance before the MsgA PUSCH transmission, final fine-tuning of the timing advance may still be necessary.

[0098] During the random access process where the gNB calculates the complete TA, the gNB then sends a timed advance command in the next step, T. A It contains the complete TA value (as specified in 3GPP TS 38.321v16.0.0, “Medium Access Control (MAC) protocol specification (Release 16)”). In this case, the timing advance command, T A Instructions to press T A The index values ​​= 0, 1, 2, ..., 3846 list N TA Values, where 2 μ • The time alignment amount for SCS at 15kHz is:

[0099] N TA =T A .16.64 / 2 μ

[0100] N TA Defined in the physical channel and modulation document, and associated with the SCS transmitted from the UE on the first uplink after receiving the random access response.

[0101] However, in some embodiments (where the addressed UE has already calculated the timing advance value), the gNB can send a timing advance fine-tuning command in the next step. The timing advance fine-tuning command is indicated via, for example, T... A= 0, 1, 2,..., 63, where for 2 μ • SCS of 15 kHz, the UE currently calculates the value N TA_UE adjusts to the new N TA value N TA_new .

[0102] N TA_new = N TA_UE +(T A -31)·16·64 / 2 μ (3)

[0103] In the next step, the gNB forms the MAC payload for the random access response to be transmitted as MsgB-PDSCH. This MAC payload includes the first 48 bits belonging to the uplink CCCH service data unit (SDU) within the MsgA PUSCH (which serves as the UE contention resolution ID, CRID), the HARQ feedback timing indicator, the PUCCH resource indicator, the TPC command, the timing advance fine tuning, and the temporary C-RNTI. Figure 5C An example of the content of the MsgB PDSCH MAC payload and the amount of information required is shown. In this example, the MAC payload as shown is octet aligned. The bit field shown as R is a reserved bit for future use and is set to “0”. The description of the UE contention resolution ID, the HARQ feedback timing indicator, the PUCCH resource indicator, and the TPC command can be found in 3GPP TS 38.213 v16.0.0, “Physical layer procedures for control (Release 16)” and 3GPP TS 38.321 v16.0.0, “Medium Access Control (MAC) protocol specification (Release 16)”. The timing advance fine tuning is a 6-bit field used in equation (3) for T A .

[0104] In another embodiment, the timing advance fine tuning is not sent in the MAC payload random access response. For cases where the timing advance measured by the GNSS based UE is accurate enough, the timing advance fine tuning does not need to be sent in the random access response. Any timing drift can be handled periodically later by the timing advance adjustment command. In this case, the MAC payload random access response can include the 48-bit CRID, the HARQ feedback timing indicator, the PUCCH resource indicator, the TPC command, and the temporary C-RNTI. Figure 5D An example of the content of the MsgB-PDSCH MAC payload and the amount of information required in this case is shown.

[0105] MsgB PDSCH is scheduled using PDCCH containing DCI format 1_0 and scrambled using MsgB RNTI. When the UE receives and successfully decodes the MsgB PDSCH, it compares the CRID in MsgB with the CRID the UE transmitted in MsgA PUSCH. If it matches, the UE will assume that the gNB has identified the MsgA. Timing advance fine tuning is then applied and using the temporary C-RNTI provided by the gNB, the UE transmits HARQ ACK feedback to the gNB. This will complete the random access procedure.

[0106] If the CRID in MsgB does not match the CRID in MsgA, it means contention has occurred and a MAC control element (CE) is used for a different UE. In this case, the UE goes back to transmission of MsgA PUSCH (assuming the maximum number of MsgA PUSCH has not been reached).

[0107] In step 1, after transmitting MsgA PUSCH, the UE will start a timer, e.g., msgB-ResponseWindow. If the UE does not receive MsgB PDCCH (and PDSCH) before the timer expires, the UE will assume that the gNB did not receive the MsgA PUSCH. The UE will then re-initiate a new random access procedure by selecting a PO and transmitting a new MsgA PUSCH.

[0108] Figure 6 A method for random access involving NTN is shown in accordance with some embodiments. The method includes calculating, by a user equipment (UE), a timing advance based on location information of the UE and location information of a non-terrestrial network node (gNB) at 605; randomly selecting, from a plurality of values, a random value at 610; and transmitting, by the UE, a random access (RA) message to the gNB at a transmission time at 615. In this method, the RA message can include a signature. Further, the transmission time can differ from a nominal transmission time (i.e., the nominal transmission time is the transmission time corresponding to the TA without any intentionally added random delay) by an amount based on the random value, or the signature can differ from a nominal signature (the nominal signature lacking a cyclic shift) by a cyclic shift based on the random value. Figure 7 A system including a UE 705 and a gNB 710 in communication is shown. The UE can include a radio 715 and processing circuitry (or means for processing) 720, which can perform various methods disclosed herein, e.g., the method shown. Figure 6 The method shown. For example, the processing circuitry 720 can receive, via the radio 715, a transmission from the gNB 710, and the processing circuitry 720 can transmit, via the radio 715, a signal to the gNB 710.

[0109] As used herein, “a part of something” means “at least a part of something” and thus can mean less than the entirety of the thing. Thus, “a part of” a thing includes as a particular example the entire thing, i.e., the entire thing is an example of a part of the thing. As used herein, the term “or” is to be interpreted as “and / or”, such as “A or B” means any one of A, B, or A and B.

[0110] The term “processing circuitry” and “means for processing” are each used herein to refer to any combination of hardware, firmware, and software for processing data or digital signals. The processing circuitry hardware can include, for example, special-purpose application specific integrated circuits (ASICs), general-purpose or special-purpose central processing units (CPUs), digital signal processors (DSPs), graphics processing units (GPUs), and programmable logic devices such as field programmable gate arrays (FPGAs). In processing circuitry as used herein, each function is performed by either hardware configured to perform that function (i.e., hardwired) or by more general hardware such as a CPU that is configured to execute instructions stored in a non-transitory storage medium. The processing circuitry can be fabricated on a single printed circuit board (PCB) or distributed among multiple interconnected PCBs. The processing circuitry can contain other processing circuitry; for example, the processing circuitry can include both an FPGA and a CPU interconnected on a PCB. The processing circuitry or means for processing in a UE can perform the methods described herein, for example, by sending messages (through a radio of the UE) or by receiving messages (through a radio of the UE), and in some cases, by performing further processing.

[0111] As used herein, when a method (e.g., adjustment) or a first quantity (e.g., a first variable) is referred to as being “based on” a second quantity (e.g., a second variable), it means that the second quantity is input to the method or affects the first quantity, e.g., the second quantity can be an input (e.g., the only input, or one of multiple inputs) to a function that computes the first quantity, or the first quantity can be equal to the second quantity, or the first quantity can be the same as the second quantity (e.g., stored in one or more locations of memory that are the same as the second quantity).

[0112] It should be understood that although the terms “first,” “second,” “third,” etc. can be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed herein could be termed a second element, component, region, layer or section without departing from the spirit and scope of the inventive concept.

[0113] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive concept. As used herein, the terms "substantially," "approximately," and similar terms are used as terms of approximation and not as terms of degree, unless otherwise indicated herein and are intended to account for the inherent deviations in measured or calculated values that would be recognized by those skilled in the art.

[0114] 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. Expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. Also, use of "can" in describing embodiments of the inventive concept means "one or more embodiments of the invention." Further, the term "exemplary" is intended to mean an example or an illustration. As used herein, the terms "using," "used," and "has been used" can be considered synonymous with the term "utilizing."

[0115] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed in the recited range. For example, a range of "1.0 to 10.0" or "1.0 to 10.0" is intended to include all sub-ranges, for example, 2.4 to 7.6, between and including the recited minimum and maximum values, and, in the same manner, expressing a preference for a range of values over a range of values. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein.

[0116] While exemplary embodiments of systems and methods for random access procedures in non-terrestrial networks have been particularly shown and described herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that systems and methods for random access procedures in non-terrestrial networks constructed in accordance with the principles of the present disclosure can be embodied within a form that is not specifically described herein but nevertheless falls within the scope of the present disclosure. The present disclosure is also defined in the following claims and their equivalents.

Claims

1. A method for a random access procedure in a non-terrestrial network, the method comprising: calculating, by a user equipment (UE), a timing advance based on: location information of the UE; and ephemeris of a non-terrestrial network node; randomly selecting, from a plurality of values, a random value; and transmitting, by the UE, a random access (RA) message to the non-terrestrial network node at a transmission time, the RA message comprising a signature, wherein: the transmission time differs from a nominal transmission time by an amount based on the random value, the nominal transmission time based on the timing advance, or the signature differs from a nominal signature by a cyclic shift based on the random value, wherein the RA message is sent via a physical random access channel (PRACH) occasion (RO), and the amount based on the random value is less than a cyclic prefix (CP) of the RO.

2. The method of claim 1, further comprising receiving, by the UE, the ephemeris information. the transmission time differs from the nominal transmission time by the amount based on the random value, and 3. The method of claim 1, wherein, the RA message comprises a preamble sequence.

4. The method of claim 1, wherein: the signature differs from the nominal signature by the cyclic shift based on the random value, and the RA message comprises a preamble sequence.

5. The method of claim 4, wherein: each of a plurality of cyclic shifts respectively corresponds to one of a plurality of values; a first cyclic shift of the plurality of cyclic shifts is a largest one of the plurality of cyclic shifts that is less than a second cyclic shift of the plurality of cyclic shifts; and the preamble sequence is shifted according to: the second cyclic shift, minus a maximum supported channel delay spread of the UE and the non-terrestrial network node, minus an uncertainty of the calculated timing advance, the shifted preamble sequence is orthogonal to: a preamble sequence shifted by the first cyclic shift.

6. The method of claim 1, further comprising: randomly selecting, from a set of delay values, a delay value; and transmitting, by the UE, a demodulation reference signal (DMRS) via a physical uplink shared channel (PUSCH) occasion (PO), the transmission of the DMRS beginning a delay value after a beginning of the PO. the set of delay values comprises integer multiples of a DMRS duration.

8. The method of claim 1, wherein: the RA message comprises a preamble sequence, 7. The method of claim 6, wherein, the RA message is sent via a PRACH occasion (RO); and a length of the preamble sequence is at least equal to a maximum supported channel delay spread. the length of the preamble sequence is a smallest prime number that is at least equal to the maximum supported channel delay spread. the RA message does not contain a preamble.

11. A system for a random access procedure in a non-terrestrial network, the system comprising:

9. The method of claim 8, wherein, a user equipment (UE), the UE comprising:

10. The method of claim 1, wherein, a radio; and processing circuitry, the processing circuitry configured to: calculate a timing advance based on: location information of the UE; and ephemeris of a non-terrestrial network node; randomly select, from a plurality of values, a random value; and transmit, by the UE, a random access (RA) message to the non-terrestrial network node at a transmission time, the RA message comprising a signature, wherein: the transmission time differs from a nominal transmission time by an amount based on the random value, the nominal transmission time based on the timing advance, or the signature differs from a nominal signature by a cyclic shift based on the random value, ​ ​ ​ wherein the RA message is transmitted via a PRACH occasion (RO), and the amount based on the random value is less than a cyclic prefix (CP) of the RO.

12. The system of claim 11, wherein, The processing circuitry is further configured to receive ephemeris information.

13. The system of claim 11, wherein, The transmission time differs from a nominal transmission time by an amount based on the random value, and The RA message includes a preamble sequence.

14. The system of claim 11, wherein: The signature differs from a nominal signature by a cyclic shift based on the random value, and The RA message includes a preamble sequence.

15. The system of claim 14, wherein: each of the plurality of cyclic shifts respectively corresponds to one of a plurality of values; a first cyclic shift of the plurality of cyclic shifts is a largest one of the plurality of cyclic shifts that is less than a second cyclic shift of the plurality of cyclic shifts; and The preamble sequence is shifted according to: the second cyclic shift, subtracting a maximum supported channel delay spread of the UE and the non-terrestrial network node, subtracting an uncertainty of the computed timing advance, the shifted preamble sequence is orthogonal to: the preamble sequence shifted by the first cyclic shift.

16. A system for a random access procedure in a non-terrestrial network, the system comprising: a user equipment (UE), the UE comprising: a radio; and means for processing, the means for processing is configured to: compute a timing advance based on: location information of the UE; and ephemeris of a non-terrestrial network node; randomly select a random value from a plurality of values; and transmit, by the UE to the non-terrestrial network node at a transmission time, a random access (RA) message, the RA message includes a signature, wherein: the transmission time differs from a nominal transmission time by an amount based on the random value, the nominal transmission time based on the timing advance, or the signature differs from a nominal signature by a cyclic shift based on the random value, wherein the RA message is transmitted via a PRACH occasion (RO), and the amount based on the random value is less than a cyclic prefix (CP) of the RO.

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