GB2557930A

GB2557930A - A method for detecting a number of active users in a communication system

Info

Publication number: GB2557930A
Application number: GB1621509.7A
Authority: GB
Inventors: Al-Mari Mohammed; Nekovee Maziar
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2016-12-16
Filing date: 2016-12-16
Publication date: 2018-07-04
Also published as: GB201621509D0

Abstract

The number of user equipments accessing a base station in a grant-free multiple access GFMA or RACH situation is determined based on signals received from the user equipments. The radio resource defined for a user equipment to access the base station comprises an activity area 210 with a first modulation scheme and a data area 220 with a second modulation scheme, wherein the first scheme uses a subset of available subcarriers. The two schemes may be the same, and may be FQAM or QAM. The determination may comprise the step of estimating the number of active subcarriers based on the activity area, and comparing this with the expected number for a given number of users. The base station may transmit an uplink resource allocation specifying activity area and data area information. The system is directed for use with mMTC or URLLC in 5G.

Description

(71) Applicant(s):

Samsung Electronics Co., Ltd.

(Incorporated in the Democratic People's Republic of Korea)

129 Samsung-ro, Yeongtong-gu Suwon-Si, Gyeonggi-do 443-742,

Democratic People's Republic of Korea (72) Inventor(s):

Mohammed Al-Mari Maziar Nekovee (74) Agent and/or Address for Service:

Appleyard Lees IP LLP

Clare Road, HALIFAX, West Yorkshire, HX1 2HY, United Kingdom (51) INT CL:

H04W 74/08 (2009.01) (56) Documents Cited:

US 20120272118 A1 US 20100014434 A1

KR 1020160074076

XP051189264; Performance of mMTC and URLLC channel coding candidates; 3GPP Draft; R1-1612288; 2016-11-04 (58) Field of Search:

INT CL H04L, H04W

Other: EPODOC, WPI, Patent fulltext (54) Title of the Invention: A method for detecting a number of active users in a communication system

Abstract Title: Estimating the number of active users in a RACH or GFMA system based on signals received from the user equipments (57) The number of user equipments accessing a base station in a grant-free multiple access GFMA or RACH situation is determined based on signals received from the user equipments. The radio resource defined for a user equipment to access the base station comprises an activity area 210 with a first modulation scheme and a data area 220 with a second modulation scheme, wherein the first scheme uses a subset of available subcarriers. The two schemes may be the same, and may be FQAM or QAM. The determination may comprise the step of estimating the number of active subcarriers based on the activity area, and comparing this with the expected number for a given number of users. The base station may transmit an uplink resource allocation specifying activity area and data area information. The system is directed for use with mMTC or URLLC in 5G.

Grant-Free Access Area 200

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OFDM Modulation 40

130 120 110

80 70

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Fig. 3

Fig. 4

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DL-SCH RA Response Temporary C-RNTI

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UE2: UL-SCH RRC Connection Request

UE identity

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Fig. 5

Uplink resource allocation

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Payload

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Fig. 8

A method for detecting a number of active users in a communication system

The present invention relates to a communication system where multiple users can initiate communication with a base station. Such a situation can result in an overload of information at the base station which can make it difficult or impossible for some or all of the users to communicate. Embodiments of the present invention aim to address shortcomings in the prior art.

In wireless communication systems, there are many scenarios when the basestation is not aware of how many users are currently transmitting. Such scenarios include grant-free multiple access (GFMA), where the users can transmit data to the base-station without prior scheduling permission/grant.

According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

Figure 1 shows a representation of available radio resources, including grant free access areas;

Figure 2a shows 2 UEs and a BS in communication;

Figure 2b shows a schematic of a communication chain according to an embodiment of the present invention;

Figure 3 shows an example of FQAM modulation with 4 subcarriers;

Figure 4 shows how the grant free access area is subdivided according to an embodiment of the present invention;

Figure 5 shows a RACH procedure according to an embodiment of the present invention;

Figure 6 shows how uplink resources are allocated in a specific RAR according to an embodiment of the present invention;

Figure 7 shows a 2-step RACH procedure according to an embodiment of the present invention; and

Figure 8 shows a base station according to an embodiment of the present invention.

It has been agreed in the 3GPP standard that grant-free multiple access (GFMA) should be studied for mMTC (massive Machine-Type-Communications) and URLLC (Ultra-Reliable and Low Latency Communications) use cases in the New Radio NR (5G - Fifth Generation) study item. In GFMA, the base-station will allocate part of the available radio resources for the users to transmit in grant-free multiple access approach. This is shown in Figure 1 which demonstrates schematically the available radio resources. Time is represented on the horizontal axis and frequency on the vertical axis. Three grant free access areas are shown at designated time/frequency pairs.

In the designated grant free access areas, the users are able to transmit data to the base-station without prior scheduling permission/grant from the base-station. As such, the base-station will not be aware of how many users are transmitting to it at a given time, since there is no pre-arrangement and any users in the service area of the base station could, in principle, transmit without warning.

Figure 2a shows a typical configuration of 2 User Equipments (UE) 1a and 1b in communication with a Base Station (BS) 2.

In order to detect/decode the individual users’ signals that transmit in any grant free access area, the base station is required to detect/estimate the number of users. Figure 2b shows an example of the system with a total of K users 10 transmitting to the base-station. Although Figure 2b illustrates an OFDM system, multicarrier modulation methods, other than OFDM, may also be used. The system shown in

Figure 2b is as known from the prior art, with the exception of feature 100 which in concerned with detecting the value of K- the number of users.

Since the other features of Figure 2b are known, detailed explanations will not be required, beyond identifying them here. There are multiple (K) users 10. Each user 10 transmits data which is channel encoded in an a channel encoder 20. The output of channel encoder 20 is modulated in a modulator 30. The output of modulator 30 is then OFDM modulated 40 and transmitted over radio channel 50. At the Base station, the signals are received along with additive Gaussian white noise AWGN 60. The received signal is OFDM demodulated (removing Cyclic Prefix (70), serial to parallel conversion (80) and passed through an FFT stage (90)).

The output of FFT 90 feeds into a Detector 100 for detecting the number of users K. The output of FFT 90 also feeds into A Multiuser detection stage 110, which also received information from the Detector 100, which is used to adjust the behaviour of the Multiuser detection stage as will be described later.

The output(s) of the Multiuser detection stage will relate to one or more users’ data, each of which is channel decoded 120 to give the K individual users’ data 130.

Embodiments of the present invention are related to a method and associated apparatus for detecting the number of users (K) at the base-station, which is represented by the detection stage 100, shown in Figure 2.

Another scenario, aside from GFMA already mentioned, where the base-station is not aware of the number active users is when more than one user chose the same preamble sequence in PRACH (Physical Random Access Channel). As part of the PRACH procedure, each user equipment (UE) selects one of a predefined list of preamble sequences. There is a finite possibility that two or more UEs will select the same preamble sequences, meaning that the base station will not be able to distinguish them.

In this scenario, the base-station will think it is being accessed by only one user and will send an uplink grant as a response. In this case, all UEs who selected that same preamble will transmit data based on the uplink grant information, while the base-station is expecting transmission from one user. This will result in data collision and a failed access attempt for one or more of the users.

Frequency quadrature amplitude modulation (FQAM) is a combination of frequency shift keying (FSK) and quadrature amplitude modulation (QAM). FQAM carries Q = log₂(/W_Q) information bits by selecting one subcarrier among M_Fsubcarriers (the selected subcarrier is depicted as shaded heavily in Figure 3) and modulating the selected subcarrier with an /W_Q-ary QAM constellation. The rest of the subcarriers (M/=-1) will be kept inactive/unused in the current time unit, as shown in Figure 3. This feature of transmitting by activating one out of subset of subcarriers will be utilized by embodiments of the present invention to detect the number of active users. FQAM modulation is used on a subset of the resources of the grant-free access area to enable the detection of the number of active users.

Embodiments of the present invention relate to a method for a detecting the number of active users in an uplink direction by use of FQAM modulation. This method can be utilized in different scenarios including grant-free multiple access and RACH schemes. In embodiments of the method, users use FQAM modulation when transmitting in a grant-free access period. This enables the detection of the number of active users by estimating the number of active subcarriers within the grant-free access area. By comparing the estimated number of active subcarriers to the expected number of active subcarriers, the number of active users can be determined.

Embodiments of the method are applicable for any modulation scheme that utilizes a subset of the available subcarriers, such as subcarrier-index modulation where more than one subcarrier will be active. FQAM is given as an example but it will be appreciated that other modulation techniques may be utilized.

Consider an uplink transmission scenario where a plurality of users transmit on the same radio resources using FQAM. The base-station is not aware of how many users are active at a given time instant. The actual number of active users is defined as K.

In a Grant-free access scenario, such as that shown in Figure 1, to estimate the number of active users, the base-station performs the following steps:

A) Information to be predefined by the base-station:

1) The base-station will specify the grant-free access areas 200: the resource elements 202 that the users are allowed to access the systems without prior scheduling permission/grant;

2) Each grant-free access area 200 can consist of two parts, as shown in Figure 4. in the first part, which will be referred to as the Activity Area 210, FQAM modulation is used to enable the detection of user activity. The modulation in the second part of the grant-free access area, which will be referred to as Data Area 220, can be any suitable scheme, such as FQAM, QAM or any other. In one embodiment, the grant-free access area can consist of the Activity Area only.

3) The modulation and coding parameters (i.e. M_F, M_Q, code rate, etc.) that the users should use in the Activity Area 210 in each grant-free access area 200 is predefined by the base-station and known to both UEs and base-station.

4) The base-station will convey to the users (via control broadcast for example):

i) the locations and sizes of the grant-free access areas 200, ii) the size of the Activity Area 210 in each grant-free access area 200, iii) the modulation and coding parameters to be used on each Activity Area 210, and iv) the parameters to be for the open loop power control (e.g. pathloss compensation factor, expected received power at the base-station).

B) Procedures for the users to access the system in grant-free access area:

1) Each user will select one of the grant-free access areas 200 to transmit in.

2) The user will use the modulation and coding parameters of the Activity Area 210 in the selected grant-free access area 200.

3) The user will use a modulation and coding parameters in the Data Area 220 based on the channel conditions and required QoS (e.g. BLER), These parameters may also be set by the base station.

4) In the data transmitted in the Activity Area 210, the user will include;

i) modulation and coding information of the Data Area 220 (if not know to the base station), ii) its user ID (and other identification information if needed), iii) small amount of data (depending of the Activity Area 210 size and the modulation and coding scheme).

5) In the data transmitted in the Data Area 220, the user can include;

i) its user ID (if not included in the Activity Area 210), ii) any other data.

C) Procedures for the base-station to detect the users’ activity and data:

1) For each grant-free access area 200, the base-station will detect the number of active users (i.e. users transmitted on this grant free access area) as follows:

2) The base-station will calculate the expected average number of active subcarriers a(K|N,M_F) on the Activity Area 210 for a given number of users. For example, let N be the number of resource elements 202 in the Activity Area 210, if:

3)

a) one user is active, the total number of active subcarriers must be

b) two users are active, the average total number of active subcarriers should be '

c) three users are active, the average total number of active subcarriers should be

d) etc.

In addition to the number of active users, the average number of active subcarriers is function of the number of available resource elements 202 (N) and M_F. Thus, this step can be done once and there is no need to perform it with each transmission time.

4) The base-station estimates the number of active subcarriers (β) based on the received signal in the Activity Area 210. If there is no signal transmitted on a given subcarrier, then the base-station should receive noise only (and inter-cell interference in some scenarios) on that subcarrier. Thus, the probability that the subcarrier is inactive pin) “ exp (——ZniP)

Based on this probability, (or using energy detection methods), the base-station can estimate the number of active subcarriers (β) in the Activity Area 210.

5) The base-station will compare the estimated number of active subcarriers (β) with the expected number of active subcarriers a(K|N,M_F), and estimates the number of active users accordingly.

6) The estimated number of users (K) will be passed to the Multiuser detection stage 110 and decoding blocks (as shown in Figure 2b) to retrieve the users’ data 130 in the Activity Area 210. For example, if the base-station 2 estimated that only one user is active, it will perform single-user detection and channel decoding for one user. However, if the base-station 2 detected two active users, it will perform multiuser detection 110 then channel decoding 120 for two users.

7) The base-station 2 uses the retrieved information from the Activity Area 210 to detect and decode the users’ data in the Data Area 220, noting that the modulation scheme used in the Data Area 220 may be different (e.g. QAM) to that used in the Activity Area 210 (e.g. FQAM).

D) Procedures for the base-station after retrieving the users’ data:

1) If the base-station 2 successfully retrieved all the users’ IDs from the Activity Area, 210 it will be able to transmit:

i) ACK+ID for each user that its data was successfully decoded, ii) NACK+ID for each user that its data was not successfully decoded,

The RACH procedure is different to the grant free access procedure described, and details of the RACH procedure now follow.

As mentioned previously, there is a potential problem if more than one UE selects the same preamble sequence for simultaneous transmission.

To estimate the number of users that selected the same preamble sequence and time-frequency resource, the base-station 2 and UEs 1a, 1b follow these steps as shown in Figure 5 ( noting that steps 1-3 are based on the LTE RACH procedure):

1) Two UEs (UE1 1a and UE2 1b) select the same preamble ‘Preamble T sequence and time-frequency resource. Shown as Msg 1 in Figure 5.

2) The base-station 2 detects the preamble transmission (which appears to the basestation 2 as a single user). The base-station 2 derives the RA-RNTI (Random Access-Radio Network Temporary Identifier) from the time-frequency location in which the preamble is received. A Temporary C-RNTI (Cell RNTI) is assigned to the seemingly single UE to be used to address the UE in subsequent messages. Shown as Msg 2 in Figure 5.

3) In the second step, the UEs receive Random Access Response (RAR) to ‘Preamble 1’ on DL-SCH resources that are assigned on PDCCH using the RARNTI. The RAR message carries the uplink resource allocation (in addition to other information) and the Temporary C-RNTI.

4) Unlike the prior art LTE system, the allocated uplink resource 300 (see Figure 6) consists of Activity Area 310 and Data Area 320. The modulation and coding schemes to be used on these areas is pre-configured or transmitted to the UEs by the base-station.

5) The base-station assigned uplink resources will be used to transmit the message (e.g. RRC Connection Request) to the base-station by both UEs (UE1 1a and UE2 1b). Each UE will include its identity in the Msg3 sent to the base-station. Each user assumes the assigned uplink resources are exclusively for it.

6) In a similar to the steps set out above in “C) Procedures for the base-station to detect the users’ activity and data”, the base-station 2 uses the number of active subcarriers in the Activity Area 310 to determine the number of users that are transmitting on the resources allocated to a specific RAR 300.

7) The base-station responds to the UEs in Msg4 with acknowledgement along with the user-ID (and PUSCH grant or DL-SCH assignment).

The description above is based on the conventional 4-step RACH procedure. However, embodiments of the invention can be based on other RACH procedures, such as the simplified 2-step RACH 400 (as illustrated in Figure 7). In 2-step RACH there is a possibility that two users will use the same preamble, and hence, the basestation may not be able to detect how many users used the same preamble 410. To this end, the payload area 420 may be divided into an activity area and data area (in a similar manner to that shown in Figures 4 and 6).

As can be seen from the foregoing, by adopting either the Grant Free Access or RACH scheme used, it is possible to determine with suitable accuracy the number of users attempting access at any given time. This allows the base station to implement a suitable detection and/or decoding approach to offer the best opportunity to properly serve the users who are seeking access.

Figure 8 shows a base station 500 according to an embodiment of the invention. The major changes required to a prior art base station are of a software, rather than a hardware nature. The base station controller function 510 requires a Number of users detector 520, which performs the function of stage 100 illustrated in Figure 2b and described previously. Also shown is transceiver 530. Other parts of the base station are omitted for clarity and brevity.

The Grant-free access scheme described is an important aspect of 5G systems for mMTC (massive Machine-Type-Communications) and URLLC (Ultra-Reliable and Low Latency Communications). In grant-free access, the base-station is required to estimate the number of active users in order to perform the appropriate detection and decoding procedures.

Embodiments of the invention enable the base-station to detect the number of active users with high accuracy. Also, embodiments can be used for detecting the number of users when more than one user chooses the same preamble sequence in PRACH (Physical Random Access Channel).

Throughout this application, reference has been made specifically to Grant Free Multiple Access and/or RACH, but embodiments of the invention may be beneficial in other scenarios where the base-station is not aware of the number of users transmitting on the same resources.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method of determining a number of user equipments accessing a base station in a grant-free multiple access or RACH situation, comprising the steps of:

defining a radio resource for a user equipment to access the base station; sub-diving the radio resource in a plurality of resource elements, including an activity area and a data area, wherein a first modulation scheme is used in the activity area and a second modulation scheme is used in the data area; wherein the first modulation scheme uses a subset of available subcarriers and a determination of the number of user equipments based on signals received from the user equipments.

2. The method of claim 1 wherein the first and second modulation schemes are the same.

3. The method of claim 1 wherein the first modulation schemes are FQAM.

4. The method of claim 3 wherein the second modulation scheme is QAM.

5. The method of any preceding claims wherein the determination of the number of user equipments comprises the steps of calculating the expected average number of active sub carriers in the Activity area for a given number of users; estimating the number of active sub carriers based on a received signal in the Activity Area; comparing the estimated number with the expected average number and estimating the number of users accordingly.

6. The method of any of claims 1 to 4 wherein if first and second user equipments select an identical preamble for RACH access on the same time/frequency resource, the base station detects the preamble and derives a temporary identifier from the time/frequency resource; and the base station transmits an uplink resource allocation, comprising Activity Area and Data Area information; and each user equipment transmitting its identity to the base station.

7. Apparatus arranged to perform the method of any preceding claim.

8. Apparatus as claimed in claim 7 wherein the apparatus is a base station.

9. Apparatus as claimed in claim 7 wherein the apparatus is a user equipment.

5

10. A system comprising the base station of claim 8 and the user equipment of claim 9.

Intellectual

Property

Office

Application No: GB1621509.7 Examiner: Dr Stephen Bevan