Method and apparatus for transmitting a reference signal in a wireless communication system

Abstract

An uplink transmission method in a wireless communication system, and a user equipment (UE) therefore, are discussed. The method includes, for example, generating uplink control information (UCI) which includes at least one of a hybrid automatic repeat request (HARQ) acknowledgement (ACK)/negative-acknowledgement (NACK) signal, a channel quality indicator (CQI) and a scheduling request (SR); generating a reference signal (RS) based on one or more indices selected from among a set {0, 3, 6, 8, 10}; transmitting the UCI on a physical uplink control channel (PUCCH); and transmitting the generated RS for the PUCCH based on the selected one or more indices.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is

In Equation 1, u denotes a root index, and n denotes a component index in the range of 0≤n≤N−1, where N is a length of the base sequence. b(n) is defined in the section 5.5 of 3GPP TS 36.211 V8.7.0.

A length of a sequence is equal to the number of elements included in the sequence. u can be determined by a cell identifier (ID), a slot number in a radio frame, etc. When it is assumed that the base sequence is mapped to one RB in a frequency domain, the length N of the base sequence is 12 since one RB includes 12 subcarriers. A different base sequence is defined according to a different root index.

The base sequence r(n) can be cyclically shifted by Equation 2 below to generate a cyclically shifted sequence r(n, Ics).

$\begin{array}{cc}r\left(n,I_{\mathrm{cs}}\right)=r\left(n\right)·\exp \left(\frac{j2\pi I_{\mathrm{cs}}n}{N}\right),0\le I_{\mathrm{cs}}\le N-1& \left[\mathrm{Equation}2\right]\end{array}$

In Equation 2, Ics denotes a CS index indicating a CS amount (0≤Ics≤N−1).

Hereinafter, the available CS of the base sequence denotes a CS index that can be derived from the base sequence according to a CS interval. For example, if the base sequence has a length of 12 and the CS interval is 1, the total number of available CS indices of the base sequence is 12. Alternatively, if the base sequence has a length of 12 and the CS interval is 2, the total number of available CS indices of the base sequence is 6.

Now, transmission of an HARQ ACK/NACK signal in PUCCH formats 1a/1b will be described.

FIG. 3 shows a PUCCH format 1b in a normal CP in 3GPP LTE. One slot includes 7 OFDM symbols. Three OFDM symbols are used as reference signal (RS) OFDM symbols for a reference signal. Four OFM symbols are used as data OFDM symbols for an ACK/NACK signal.

In the PUCCH format 1b, a modulation symbol d(0) is generated by modulating a 2-bit ACK/NACK signal based on quadrature phase shift keying (QPSK).

A CS index Ics may vary depending on a slot number ns in a radio frame and/or a symbol index 1 in a slot.

In the normal CP, there are four data symbols for transmission of an ACK/NACK signal in one slot. It is assumed that CS indices mapped to the respective data OFDM symbols are denoted by Ics0, Ics1, Ics2, and Ics3.

The modulation symbol d(0) is spread to a cyclically shifted sequence r(n,Ics). When a one-dimensionally spread sequence mapped to an (i+1)th OFDM symbol in a subframe is denoted by m(i), it can be expressed as follows.
{m(0),m(1),m(2),m(3)}={d(0)r(n,Ics0),d(0)r(n,Ics1),d(0)r(n,Ics2),d(0)r(n,Ics3)}

In order to increase UE capacity, the one-dimensionally spread sequence can be spread by using an orthogonal sequence. An orthogonal sequence wi(k) (where i is a sequence index, 0≤k≤K−1) having a spread factor K=4 uses the following sequence.

TABLE 2

Index (i) [wi(0), wi(1), wi(2), wi(3)]

0         [+1, +1, +1, +1]
1         [+1, −1, +1, −1]
2         [+1, −1, −1, +1]

An orthogonal sequence wi(k) (where i is a sequence index, 0≤k≤K−1) having a spread factor K=3 uses the following sequence.

TABLE 3

Index (i) [wi(0), wi(1), wi(2)]

0         [+1, +1, +1]
1         [+1, ej2π/3, ej4π/3]
2         [+1, ej4π/3, ej2π/3]

A different spread factor can be used for each slot.

Therefore, when any orthogonal sequence index i is given, a two-dimensionally spread sequences {s(0), s(1), s(2), s(3)} can be expressed as follows.
{s(0),s(1),s(2),s(3)}={wi(0)m(0),wi(1)m(1),wi(2)m(2),wi(3)m(3)}

The two-dimensionally spread sequences {s(0), s(1), s(2), s(3)} are subjected to inverse fast Fourier transform (IFFT) and thereafter are transmitted in corresponding OFDM symbols. Accordingly, an ACK/NACK signal is transmitted on a PUCCH.

A reference signal for the PUCCH format 1b is also transmitted by cyclically shifting the base sequence r(n) and then by spreading it by the use of an orthogonal sequence. When CS indices mapped to three RS OFDM symbols are denoted by Ics4, Ics5, and Ics6, three cyclically shifted sequences r(n,Ics4), r(n,Ics5), and r(n,Ics6) can be obtained. The three cyclically shifted sequences are spread by the use of an orthogonal sequence wRSi(k) having a spreading factor K=3.

An orthogonal sequence index i, a CS index Ics, and a resource block index m are parameters required to configure the PUCCH, and are also resources used to identify the PUCCH (or UE). If the number of available cyclic shifts is 12 and the number of available orthogonal sequence indices is 3, PUCCHs for 36 UEs in total can be multiplexed to one resource block.

In the 3GPP LTE, a resource index n(1)PUUCH is defined in order for the UE to obtain the three parameters for configuring the PUCCH. The resource index n(1)PUUCH is defined to nCCE+N(1)PUUCH, where nCCE is an index of a first CCE used for transmission of corresponding DCI (i.e., DL resource allocation used to receive DL data mapped to an ACK/NACK signal), and N(1)PUUCH is a parameter reported by a BS to the UE by using a higher-layer message.

Time, frequency, and code resources used for transmission of the ACK/NACK signal are referred to as ACK/NACK resources or PUCCH resources. As described above, an index of the ACK/NACK resource required to transmit the ACK/NACK signal on the PUCCH (referred to as an ACK/NACK resource index or a PUCCH index) can be expressed with at least any one of an orthogonal sequence index i, a CS index Ics, a resource block index m, and an index for obtaining the three indices. The ACK/NACK resource may include at least one of an orthogonal sequence, a cyclic shift, a resource block, and a combination thereof.

FIG. 4 shows an example of performing HARQ.

By monitoring a PDCCH, a UE receives a DL resource allocation on a PDCCH 501 in an nth DL subframe. The UE receives a DL transport block through a PDSCH 502 indicated by the DL resource allocation.

The UE transmits an ACK/NACK signal for the DL transport block on a PUCCH 511 in an (n+4)th UL subframe. The ACK/NACK signal can be regarded as a reception acknowledgement for a DL transport block.

The ACK/NACK signal corresponds to an ACK signal when the DL transport block is successfully decoded, and corresponds to a NACK signal when the DL transport block fails in decoding. Upon receiving the NACK signal, a BS may retransmit the DL transport block until the ACK signal is received or until the number of retransmission attempts reaches its maximum number.

In the 3GPP LTE, to configure a resource index of the PUCCH 511, the UE uses a resource allocation of the PDCCH 501. That is, a lowest CCE index (or an index of a first CCE) used for transmission of the PDCCH 501 is nCCE, and the resource index is determined as n(1) PUUCH=nCCE+N(1)PUUCH.

Now, the proposed PUCCH structure and the method of performing HARQ by using the PUCCH structure will be described.

FIG. 5 shows an example of a PUCCH structure according to an embodiment of the present invention.

One slot includes 7 OFDM symbols. 1 denotes an OFDM symbol number, and has a value in the range of 0 to 6. Two OFDM symbols with 1=1, 5 are used as RS OFDM symbols for a reference signal, and the remaining OFDM symbols are used as data OFDM symbols for an ACK/NACK signal.

A symbol sequence {d(0), d(1), d(2), d(3), d(3), d(4)} is generated by performing QPSK modulation on a 10-bit encoded ACK/NACK signal. d(n)(n=0, 1, 2, 3, 4) is a complex-valued modulation symbol. The symbol sequence can be regarded as a set of modulation symbols. The number of bits of the ACK/NACK signal or a modulation scheme is for exemplary purposes only, and thus the present invention is not limited thereto.

The symbol sequence is spread with an orthogonal sequence wi. Symbol sequences are mapped to respective data OFDM symbols. An orthogonal sequence is used to identify a PUCCH (or UE) by spreading the symbol sequence across the data OFDM symbols.

The orthogonal sequence has a spreading factor K=5, and includes five elements. As the orthogonal sequence, one of four orthogonal sequences of Table 4 below can be selected according to an orthogonal sequence index i.

TABLE 4

Index (i) [wi(0), wi(1), wi(2), wi(3), wi(4)]

0         [+1, +1, +1, +1, +1]
1         [+1, ej2π/5, ej4π/5, ej6π/5, ej8π/5]
2         [+1, ej4π/5, ej8π/5, ej2π/5, ej6π/5]
3         [+1, ej6π/5, ej2π/5, ej8π/5, ej4π/5]
4         [+1, ej8π/5, ej6π/5, ej4π/5, ej2π/5]

Two slots in the subframe can use different orthogonal sequence indices.

Each spread symbol sequence is cyclically shifted by a cell-specific CS value ncellcs(ns,1). Each cyclically shifted symbol sequence is transmitted by being mapped to a corresponding data OFDM symbol.

ncellcs(ns,1) is a cell-specific parameter determined by a pseudo-random sequence which is initialized on the basis of a physical cell identity (PCI). ncellcs(ns,1) varies depending on a slot number ns in a radio frame and an OFDM symbol number 1 in a slot.

Two RS OFDM symbols are transmitted by mapping an RS sequence used for demodulation of an ACK/NACK signal.

The RS sequence is acquired by cyclically shifting the base sequence of Equation 1. Since the number of subcarriers per RB is 12, a length of the base sequence N is 12.

As described above, since the ACK/NACK signal is spread with an orthogonal sequence having a spreading factor K=5, up to five UEs can be identified by changing an orthogonal sequence index. This implies that up to five PUCCHs can be multiplexed in the same RB.

Only one RB is used in the PUCCH, and thus the maximum number of available RS sequences is determined by the number of available cyclic shifts and the number of available orthogonal sequences. Since the number of subcarriers per RB is 12, the maximum number of available cyclic shifts is 12. Since the number of RS OFDM symbols is 2, the number of available orthogonal sequences is 2. Therefore, the maximum number of available RS sequences is 24.

It is not necessary to use all of the 24 RS sequences. This is because only 5 UEs can be multiplexed in the PUCCH.

The proposed invention relates to how to select five RS sequences from the 24 RS sequences.

First, it is assumed that two orthogonal sequences are used, and an orthogonal sequence index thereof is iRS. Further, CS values are identified by CS indices 0 to 11.

FIG. 6 shows reference signal allocation according to an embodiment of the present invention.

A reference signal is allocated according to the following rule.

(1) Three CS indices and two CS indices have different orthogonal sequence indices. Herein, three CS indices are allocated to iRS=0, and two CS indices are allocated to iRS=1.

(2) A difference between respective CS indices is maximized in the same orthogonal sequence index.

A sub-figure (A) shows an example in which a difference between CS indices is set to at least 4 with respect to CS indices in iRS=0, and a difference between CS indices is set to at least 6 with respect to CS indices in iRS=1.

In the example of the sub-figure (A), an offset can be given to the CS indices. A sub-figure (B) shows an example in which, while the difference between CS indices is maintained to at least 6 with respect to CS indices in iRS=1, a start point thereof is changed.

FIG. 7 shows reference signal allocation according to another embodiment of the present invention.

A reference signal is allocated according to the following rule.

(1) Three CS indices and two CS indices have different orthogonal sequence indices. Herein, three CS indices are allocated to iRS=0, and two CS indices are allocated to iRS=1.

(2) A difference between respective CS indices is 4 in the same orthogonal sequence index. A sub-figure (A) shows an example in which a difference between CS indices is set to 4 with respect to CS indices in iRS=0 and iRS=1.

A sub-figure (B) shows an example in which an offset is given to CS indices in the example of the sub-figure (A).

FIG. 8 shows reference signal allocation according to another embodiment of the present invention.

A reference signal is allocated according to the following rule.

(1) Five CS indices have the same orthogonal sequence index. That is, only one orthogonal sequence index can be used. Herein, five CS indices are allocated to iRS=0. For example, the orthogonal sequence may be [1 1].

(2) A difference between respective CS indices is at least 2.

A sub-figure (A) shows a selected CS index set {0, 3, 5, 8, 10}. A sub-figure (B) shows a selected CS index set {0, 3, 6, 8, 10}. A sub-figure (C) shows a selected CS index set {0, 2, 4, 6, 8}.

Considering that 5 CS indices capable of overcoming a path loss or fading can be selected from 12 CS indices, it is proposed to use one orthogonal sequence.

Further, considering that a low CS index is preferably used in general, it is better to have a great difference between the low CS indices.

Therefore, it is proposed to determine a CS index from a CS index set {0, 3, 6, 8, 10} in the proposed embodiments.

It is assumed hereinafter that a reference signal is spread with one orthogonal sequence, and a CS index is determined from the CS index set {0, 3, 6, 8, 10}.

Returning to FIG. 5, assume that Ics denotes a determined CS index. The Ics is selected from the CS index set {0, 3, 6, 8, 10}.

A cyclically shifted sequence is generated by cyclically shifting a base sequence on the basis of the Ics. The cyclically shifted sequence is transmitted by being mapped to each RS OFDM symbol.

The Ics to be applied may differ for each RS OFDM symbol. For example, a UE may determine a first CS index Ics(1)={ncellcs(ns,1)+Ics} mod N with respect to an RS OFDM symbol with 1=1, and may determine a second CS index Ics(5)={ncellcs(ns,1)+Ics} mod N with respect to an RS OFDM symbol with 1=5.

FIG. 9 is a flowchart showing a method of performing HARQ according to an embodiment of the present invention. This is a process of performing HARQ on the basis of the PUCCH structure according to the embodiment of FIG. 5.

A BS transmits a resource configuration to a UE (step S910). The resource configuration can be transmitted by using a radio resource control (RRC) message for configuring/modification/reconfiguration of a radio bearer.

The resource configuration includes information regarding a plurality of resource index candidates. The plurality of resource index candidates may be a set of resource indices that can be configured to the UE. The resource configuration may include information regarding four resource index candidates.

The BS transmits a DL grant to the UE through a PDCCH (step S920). The DL grant includes a DL resource allocation and a resource index field. The DL resource allocation includes resource allocation information indicating a PDSCH. The resource index field indicates a resource index used to configure a PUCCH among the plurality of resource index candidates. If there are four resource index candidates, the resource index field may have two bits.

The UE receives a DL transport block through a PDSCH on the basis of the DL resource allocation (step S930). The UE generates an HARQ ACK/NACK signal for the DL transport block.

The UE configures the PUCCH on the basis of a resource index (step S940). In the structure of FIG. 5, a PUCCH resource includes an orthogonal sequence index used to spread the ACK/NACK signal and a CS index for a reference signal.

The orthogonal sequence index used to spread the ACK/NACK signal can be obtained as follows.
i₁=n_PUCCH mod N_SF,i₂=3i₁ mod N_SF   [Equation 3]

Herein, i1 is an orthogonal sequence index used in a first slot, i2 is an orthogonal sequence index used in a second slot, NSF is a spreading factor of an orthogonal sequence, and nPUCCH is a resource index.

Since the PUCCH is transmitted in one subframe, that is, in two slots, two orthogonal sequence indices are determined. Since one slot includes five data OFDM symbols, NSF is 5.

A CS index Ics for a reference signal is selected from a CS index set {0, 3, 6, 8, 10}. More specifically, a relationship between the orthogonal sequence index and the CS index Ics can be defined by Table 5 below.

TABLE 5

i1 or i2 Ics


0        0
1        3
2        6
3        8
4        10

That is, the orthogonal sequence index and the CS index can be 1:1 mapped.

A cyclic shift for two RS OFDM symbols is obtained on the basis of the CS index. For example, the UE may determine a first CS index Ics(1)={ncellcs(ns,1)+Ics} mod N with respect to an RS OFDM symbol with 1=1, and may determine a second CS index Ics(5)={ncellcs(ns,1)+Ics} mod N with respect to an RS OFDM symbol with 1=5.

The UE determines a PUCCH resource on the basis of a resource index nPUCCH, and configures a PUCCH having the same structure of FIG. 5.

The UE transmits an ACK/NACK signal through the PUCCH (step S950).

FIG. 10 is a flowchart showing a method of transmitting a reference signal according to an embodiment of the present invention. This is a process of transmitting the reference signal on the basis of a PUCCH structure according to the embodiment of FIG. 5.

A UE generates a base sequence (step S1010). According to Equation 1, the UE generates a base sequence with a length N=12.

The UE determines a CS index on the basis of a resource index (step S1020). The resource index can be transmitted by a BS by being included in a DL grant. As shown in step S940 of FIG. 9 described above, the UE can select one CS index Ics from a CS index set {0, 3, 6, 8, 10} on the basis of the resource index.

The UE cyclically shifts a base sequence on the basis of the selected CS index (step S1030).

The UE transmits the cyclically shifted sequence (step S1040).

FIG. 11 is a block diagram showing a wireless communication system for implementing an embodiment of the present invention.

A BS 50 includes a processor 51, a memory 52, and a radio frequency (RF) unit 53. The memory 52 is coupled to the processor 51, and stores a variety of information for driving the processor 51. The RF unit 53 is coupled to the processor 51, and transmits and/or receives a radio signal. The processor 51 implements the proposed functions, processes and/or methods. The processor 51 can implement the operation of the BS 50 according to the embodiments of FIG. 9 and FIG. 10.

A UE 60 includes a processor 61, a memory 62, and an RF unit 63. The memory 62 is coupled to the processor 61, and stores a variety of information for driving the processor 61. The RF unit 63 is coupled to the processor 61, and transmits and/or receives a radio signal. The processor 61 implements the proposed functions, processes and/or methods. The processor 61 can implement the operation of the UE 60 according to the embodiments of FIG. 9 and FIG. 10.

The processor may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits, and/or data processors. The memory may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media and/or other storage devices. The RF unit may include a baseband circuit for processing a radio signal. When the above-described embodiment is implemented in software, the above-described scheme may be implemented using a module (process or function) which performs the above function. The module may be stored in the memory and executed by the processor. The memory may be disposed to the processor internally or externally and connected to the processor using a variety of well-known means.

In the above exemplary systems, although the methods have been described on the basis of the flowcharts using a series of the steps or blocks, the present invention is not limited to the sequence of the steps, and some of the steps may be performed at different sequences from the remaining steps or may be performed simultaneously with the remaining steps. Furthermore, those skilled in the art will understand that the steps shown in the flowcharts are not exclusive and may include other steps or one or more steps of the flowcharts may be deleted without affecting the scope of the present invention.

Claims

1. An uplink transmission method in a wireless communication system, the method comprising:

generating, by a user equipment (UE), uplink control information (UCI) which includes at least one of comprises a hybrid automatic repeat request (HARQ) acknowledgement (ACK)/negative-acknowledgement (NACK) signal, a channel quality indicator (CQI) and a scheduling request (SR);

generating, by the UE, a reference signal (RS) based on one or more indices selected from among a set {0, 3, 6, 8, 10};

transmitting, by the UE and to a base station (BS), the UCI on a physical uplink control channel (PUCCH); and

transmitting, by the UE and to the BS, the generated RS for the PUCCH based on the selected one or more indices.

2. The method of claim 1, wherein the RS is transmitted to the BS on second and sixth orthogonal frequency division multiplexing (OFDM) symbols among 7 OFDM symbols in one slot.

3. The method of claim 1, further comprising:

generating, by the UE, a base sequence for the RS; and

cyclically shifting, by the UE, the base sequence based on the one or more indices selected from the set {0, 3, 6, 8, 10}.

4. The method of claim 1, further comprising:

receiving, by the UE from the BS, a resource configuration including information regarding a plurality of resource candidates.

5. The method of claim 4, further comprising:

receiving, by the UE from the BS, information indicating a resource index among the plurality of resource candidates; and

determining, by the UE, a cyclic shift index according to the resource index.

6. The method of claim 5, wherein the HARQ ACK/NACK signal is spread to an orthogonal sequence, and

wherein an orthogonal sequence index for identifying the orthogonal sequence is determined on the basis of the resource index.

7. A user equipment for performing an uplink transmission in a wireless communication system, the user equipment comprising:

a transceiver configured to transmit and receive a radio signal; and

a processor operatively coupled to the transceiver and configured to:

generate uplink control information (UCI) which includes at least one of comprises a hybrid automatic repeat request (HARQ) acknowledgement (ACK)/negative-acknowledgement (HACK) (NACK) signal, a channel quality indicator (CQI) and a scheduling request (SR),

generate a reference signal (RS) based on one or more indices selected from among a set {0, 3, 6, 8, 10},

control the transceiver to transmit, to a base station (BS), the UCI on a physical uplink control channel (PUCCH), and

control the transceiver to transmit, to the BS, the generated RS for the PUCCH based on the selected one or more indices.

8. The user equipment of claim 7, wherein the RS is transmitted on second and sixth orthogonal frequency division multiplexing (OFDM) symbols among 7 OFDM symbols in one slot.

9. The user equipment of claim 7, wherein the processor is further configured to:

generate a base sequence for the RS; and

cyclically shift the base sequence based on the one or more indices selected from the set {0, 3, 6, 8, 10}.

10. The user equipment of claim 7, wherein the processor is further configured to:

control the transceiver to receive, from the BS, a resource configuration including information regarding a plurality of resource candidates.

11. The user equipment of claim 10, wherein the processor is further configured to:

control the transceiver to receive, from the BS, information indicating a resource index among the plurality of resource candidates; and

determine a cyclic shift index according to the resource index.

12. The user equipment of claim 11, wherein the HARQ ACK/NACK signal is spread to an orthogonal sequence, and

wherein an orthogonal sequence index for identifying the orthogonal sequence is determined on the basis of the resource index.

13. The method of claim 1, wherein the UCI further comprises a channel quality indicator (CQI).

14. The method of claim 1, wherein the UCI further comprises a scheduling request (SR).

15. The user equipment of claim 7, wherein the UCI further comprises a channel quality indicator (CQI).

16. The user equipment of claim 7, wherein the UCI further comprises a scheduling request (SR).