KR100911307B1 - Method of transmiting reference signal - Google Patents

Abstract

A method of transmitting a reference signal is provided to improve the efficiency of transmission power of a transmitter and increase the performance of the signal detection of a receiver by providing high cross correlation and a PAPR(Peat-to-Average Power Ratio) having a low reference signal sequence. A reference signal sequence defined by a mathematical formula is generated(S510), and the some or entire f the reference signal is mapped to at least one resource block(S520). The reference signal is transmitted to at least one of resource block(S530).

Description

Reference signal transmission method {METHOD OF TRANSMITING REFERENCE SIGNAL}

The present invention relates to wireless communications, and more particularly, to the generation and application of sequences in a wireless communications system.

Wireless communication systems are widely deployed to provide various kinds of communication services such as voice and data. In general, a wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available system resources (bandwidth, transmission power, etc.). Examples of multiple access systems include code division multiple access (CDMA) systems, frequency division multiple access (FDMA) systems, time division multiple access (TDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, and single carrier frequency (SC-FDMA). division multiple access) system.

In a wireless communication system, a sequence is generally used for various purposes such as a reference signal, a scrambling code, and the like. The characteristics that a sequence used in a wireless communication system should generally satisfy are as follows.

(1) Good correlation characteristics to provide high detection performance.

(2) Low peak-to-average power ratio (PAPR) to increase the efficiency of the power amplifier.

(3) Generating a large number of sequences for ease of transmitting large amounts of information or cell planning.

Although CAZAC (Constant Amplitude and Zero Auto Correlation) sequences having good PAPR characteristics of the sequence have been proposed, this is limited in the number of available sequences. Therefore, many wireless communication systems use sequences generated in a pseudo-random manner. Pseudorandom sequences are advantageous in that the number of available sequences is high, but it is necessary to consider the high PAPR problem that occurs in a particular pattern.

Various binary or non-binary pseudo-random sequences have been used in wireless communication systems. The pseudo random sequence can be easily generated using an m-stage m-stage linear feedback shift register (LFSR) and has a very good random characteristic. Since m-sequences are much simpler than non-binary pseudorandom sequences, they are used as scrambling codes in wideband CDMA (WCDMA) systems.

Gold sequences generate pseudo-random number sequences using two different binary m-sequences. Gold sequences are easily implemented by two m-stage LFSRs. Gold sequences have the advantage of being able to generate different pseudorandom sequences for that period, while varying the initial state of each m-step LFSR.

There is a need for a technique capable of generating sequences with improved PAPR and correlation characteristics.

An object of the present invention is to provide a method and apparatus for transmitting a reference signal in a wireless communication system. Also provided is a receiver for receiving the transmitted reference signal.

An object of the present invention is to provide a sequence transmission method and apparatus in a wireless communication system. Also provided is a receiver for receiving the transmitted sequence.

In one aspect, a method is provided by a transmitter in a wireless communication system for transmitting a reference signal. The method includes generating a reference signal sequence, mapping part or all of the reference signal sequence to at least one resource block, and transmitting the reference signal to the at least one resource block.

The reference signal sequence is defined as follows.

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, N _RB ^{max , DL} is the maximum number of resource blocks, and the pseudo random number sequence c (i) is (2N _ID ^cell +1). N _ID ^cell is a cell identifier (ID).

The pseudo random number sequence c (i) is

It can be defined as. x (i) and y (i) are m-sequences and Nc is a constant. The Nc may have a value between 1500 and 1800. The m-sequence x (i) is x (0) = 1, x (i) = 0, i = 1,2,... The initial value of 30 may be initialized, and the m-sequence y (i) may be initialized to the initial values.

The initial values may change as the OFDM symbol number changes. The initial values may be obtained from l (2N _ID ^cell +1).

One resource block may include 12 subcarriers in the frequency domain. In this case, two modulation symbols of the reference signal sequence may be mapped to two subcarriers in one resource block.

The reference signal may be a cell common reference signal or a terminal specific reference signal.

In another aspect, a transmitter includes a reference signal generator for generating a reference signal, and a transmission circuit for transmitting the reference signal. The reference signal generator generates a reference signal sequence defined as follows:

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, N _RB ^{max , DL} is the maximum number of RBs, and the pseudo random number sequence c (i) is (2N _ID ^cell +1). Generated from a gold sequence generator initialized with initial values obtained from the N _ID ^cell , wherein the N _ID ^cell is a cell ID. The reference signal is generated by mapping a part or all of the reference signal sequence to at least one resource block. .

In another aspect, a receiver includes a receiving circuit for receiving a reference signal and a received signal, a channel estimator for estimating a channel using the reference signal, and a data processor for processing the received signal using the estimated channel. do. The reference signal is generated based on a reference signal sequence defined as follows.

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, N _RB ^{max , DL} is the maximum number of RBs, and the pseudo random number sequence c (i) is (2N _ID ^cell +1). N _ID ^cell is a cell identifier (ID), which is generated from a gold sequence generator initialized with initial values obtained from.

The generated sequence provides low PAPR and high cross correlation. Thus, it is possible to provide efficient transmit power at the transmitter and to improve signal detection performance at the receiver. The generated sequence can be applied to a reference signal requiring high reliability, and can be applied to other scrambling codes.

The following techniques include code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), and the like. It can be used in various wireless communication systems. CDMA may be implemented with a radio technology such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented with wireless technologies such as Global System for Mobile communications (GSM) / General Packet Radio Service (GPRS) / Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may be implemented in a wireless technology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, Evolved UTRA (E-UTRA). UTRA is part of the Universal Mobile Telecommunications System (UMTS). 3rd Generation Partnership Project (3GPP) long term evolution (LTE) is part of Evolved UMTS (E-UMTS) using Evolved-UMTS Terrestrial Radio Access (E-UTRA), which employs OFDMA in downlink and SC in uplink -FDMA is adopted. LTE-A (Advanced) is the evolution of 3GPP LTE.

For clarity, the following description focuses on 3GPP LTE / LTE-A, but the technical spirit of the present invention is not limited thereto.

1 shows a wireless communication system.

Referring to FIG. 1, the wireless communication system 10 includes at least one base station 11 (BS). Each base station 11 provides a communication service for a particular geographic area (generally called a cell) 15a, 15b, 15c. The cell can in turn be divided into a number of regions (called sectors). The user equipment (UE) 12 may be fixed or mobile, and may include a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), It may be called other terms such as a wireless modem and a handheld device. The base station 11 generally refers to a fixed station communicating with the terminal 12, and may be referred to as other terms such as an evolved-NodeB (eNB), a base transceiver system (BTS), an access point, and the like. have.

Hereinafter, downlink means communication from the base station to the terminal, and uplink means communication from the terminal to the base station. In downlink, a transmitter may be part of a base station, and a receiver may be part of a terminal. In uplink, a transmitter may be part of a terminal, and a receiver may be part of a base station.

2 shows a structure of a radio frame in 3GPP LTE.

Referring to FIG. 2, a radio frame consists of 10 subframes, and one subframe consists of two slots. The time it takes for one subframe to be transmitted is called a transmission time interval (TTI). For example, one subframe may have a length of 1 ms and one slot may have a length of 0.5 ms.

One slot includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols in the time domain. The OFDM symbol is used to represent one symbol period since 3GPP LTE uses OFDMA in downlink, and may be referred to as an SC-FDMA symbol or a symbol period according to a system. A resource block (RB) includes a plurality of consecutive subcarriers in one slot in resource allocation units.

The structure of the radio frame is only an example, and the number of subframes included in the radio frame or the number of slots included in the subframe and the number of OFDM symbols included in the slot may be variously changed.

3 is an exemplary diagram illustrating a resource grid for one downlink slot.

Referring to FIG. 3, the downlink slot includes a plurality of OFDM symbols in a time domain. Here, one downlink slot includes 7 OFDM symbols and one resource block (RB) includes 12 subcarriers (subcarriers) in the frequency domain, but is not limited thereto.

Each element on the resource grid is called a resource element, and one RB includes 12 × 7 resource elements. The number N ^DL of RBs included in the downlink slot depends on the downlink transmission bandwidth set in the cell.

4 shows an example of a structure of a downlink subframe.

Referring to FIG. 4, the subframe includes two slots. Up to three OFDM symbols in the first slot of the subframe are control regions to which control channels are allocated, and the remaining OFDM symbols are data regions to which a Physical Downlink Shared Channel (PDSCH) is allocated.

Downlink control channels used in 3GPP LTE include a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH), a Physical Hybrid-ARQ Indicator Channel (PHICH), and the like. The PCFICH transmitted in the first OFDM symbol of a subframe carries information about the number of OFDM symbols used for transmission of control channels in the subframe. Control information transmitted through the PDCCH is called downlink control information (DCI). DCI includes uplink or downlink scheduling information or an uplink transmission power control command for certain UE groups.

Now, the reference signal will be described.

When transmitting data in a wireless communication system, the transmitted data may be distorted on the wireless channel. In order for the receiver to correctly restore the distorted data to the original data, it is necessary to find out the channel state and correct distortion in the received signal by the channel state. In order to find out the channel state, a signal known to both the transmitter and the receiver is used, which is called a reference signal or a pilot. Since the reference signal is an important signal for determining the channel state, the transmitter transmits the reference signal at a larger transmission power than other signals. In addition, in order to distinguish the reference signals transmitted between cells in a multi-cell environment, the peak-to-average power ratio (PAPR) and correlation characteristics of the reference signals should be good.

The reference signal may be divided into a cell common reference signal and a UE specific reference signal. The cell common reference signal is a reference signal used by all terminals in a cell, and the terminal specific reference signal is a reference signal used by a specific terminal or a specific terminal group in a cell. The cell common reference signal can be used for channel estimation by all terminals in a cell, whereas the terminal specific reference signal can be used only for channel estimation by a specific terminal.

5 shows an example of a downlink common reference signal structure when the base station uses one antenna. 6 shows an example of a downlink common reference signal structure when the base station uses two antennas. 7 shows an example of a downlink common reference signal structure when the base station uses four antennas. This may be referred to Section 6.10.1 of 3GPP TS 36.211 V8.0.0 (2007-09) "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8)". Rp represents a reference signal for antenna p (p ∈ {0, 1, 2, 3}). R0 to R3 do not overlap with each other. Each Rp in one OFDM symbol is located at 6 subcarrier intervals. Therefore, when one RB includes 12 subcarriers, one RB requires a sequence of length 2 (or two modulation symbols) as a reference signal. The number of R0 and the number of R1 in the subframe is the same, the number of R2 and the number of R3 is the same. The number of R2 and R3 in the subframe is less than the number of R0 and R1. Rp is not used for any transmission through any antenna other than antenna p. This is to avoid interference between antennas.

Now, the generation of the sequence for the reference signal will be described.

Consider a reference signal using a gold sequence generator. The gold sequence can be implemented by two 31-step linear feedback shift registers (LFSRs). 1st LFSR 'x (30) x (29) x (28) ... of 2 LFSR'. It is assumed that x (2) x (1) x (0) 'is initialized to' 0000000000000000000000000000001 ', and the initial value of the second LFSR is determined by a cell identifier, a subframe number, and an OFDM symbol number. The cell ID refers to a cell unique ID. The subframe number is the index of the subframe within the radio frame, and the OFDM symbol number is the index of the OFDM symbol within the subframe (or slot).

8 shows an example of a gold sequence generator. Use the sequence generation polynomial D ³¹ + D ³ +1 for the first m-sequence x (i), and the sequence generation polynomial D ³¹ + D ³ + D ² + D for the second m-sequence y (i). Use +1. Using these two m-sequences, a pseudo-random sequence c (i) is generated. The generation formula of the pseudo random number sequence c (i) can be expressed as follows.

Where i = 0, 1, ..., M _max -1, where M _max is the length of the binary pseudo-random number sequence generated using the gold sequence. Only part of the pseudorandom sequence of length M _max may be used. If M is the length of a sequence that takes only part of a pseudorandom sequence of length M _max and uses M, then M ≦ M _max . M may vary depending on the number of RBs used to transmit data. Since the number of usable RBs varies according to the frequency bands available in the 3GPP LTE system, the value of M may also vary according to the number of allocated RBs.

In the case of the first LFSR, as described above, the initial value is fixed to '0000000000000000000000000000001', and the initial value of the second LFSR is determined by a cell ID, a subframe number, and an OFDM symbol number.

9 illustrates initial value setting of the second LFSR. Of the initial values composed of 31 bits, 17 bits of LSB (least significant bits) are initialized with 9 bits of cell ID, 4 bits of subframe number, and 4 bits of OFDM symbol number. Since 3GPP LTE supports 504 unique cell IDs, cell IDs range from 0 to 503. Since one radio frame includes 10 subframes, the subframe number ranges from 0 to 9. Since up to 14 OFDM symbols may be included in one subframe, the OFDM symbol number has a range of 0 to 13. The remaining 14 most significant bit (MSB) bits are initialized to '0'. The initial value of the second LFSR can be represented as the following table.

Here, the range of the cell ID, the subframe number, the OFDM symbol number, and the corresponding number of bits are only examples and may be changed. For example, the subframe number may be a slot number. In this case, since 20 slots may be included in one radio frame, the slot number may have a range of 0 to 19.

As described above, after the initial value of the first LFSR and the initial value of the second LFSR are determined, some or all of the pseudo random sequences generated by the gold sequence generator are used as reference signals. The generated sequence is modulated into modulation symbols through Quadrature Phase Shift Keying (QPSK) modulation and mapped to each resource element. At this time, only some of the predetermined length of gold sequence can be used. For example, a reference signal may be mapped to two subcarriers at six subcarrier intervals as shown in FIG. 5 among 12 subcarriers constituting 1 RB.

However, when generating a pseudo random number sequence as described above, '0' and '1' are not included in a similar ratio in the generated binary pseudo random number sequence, and the number of '0' is greater than the number of '1'. The number of '1's may be greater than the number of' 0's. In this case, even with QPSK modulation, the DC component is present due to the biased pseudorandom sequence, which may deteriorate the PAPR characteristics while undergoing an inverse fast fourier transform (IFFT). Typically, if the cell ID, the subframe number, and the OFDM symbol number are all '0', all 31 bits of the initial value of the second LFSR become '0'.

FIG. 10 is a graph comparing sizes between a reference signal and arbitrary data when the initial values of the second LFSR are all '0'. When all of the initial 32 bits of the gold sequence generator are initialized to '0', a reference signal much larger than any other data is generated in some time samples, which indicates a deterioration of the PAPR characteristic.

11 illustrates a problem due to an initial value of a gold sequence in a multi-cell environment. In a multicell environment, each cell has a unique cell ID, but by default only 9 bits out of 31 bits are generated, so if the remaining 22 bits are the same, nearly identical pseudorandom sequence can be generated for each cell. . In particular, if a cell ID is consecutive for each cell, up to 30 bits of the initial value 31 bits may overlap. Therefore, correlation characteristics may deteriorate when the generated pseudo random number sequence is used as a reference signal.

Hereinafter, a sequence generation and an application of the generated sequence to solve the above problems will be described.

First, a method of generating a sequence by changing a Most Significant Bit (MSB) among initial values of a gold sequence will be described.

In generating the pseudo random number sequence using the gold sequence, the MSB 14 bits are changed to an appropriate value so that the ratio of '0' and '1' included in the initial value of the second LFSR is uniform. Since the cell ID, subframe number, and OFDM symbol number are all '0', the remaining 14 bits are changed to an appropriate value to define a gold sequence having good PAPR characteristics. Since the gold sequence has a pseudo random number sequence generated according to the initial value, it is important to set the initial value in order to generate a good PAPR sequence.

In one embodiment, all 14 bits of the MSB may be set to '1'. Since all initial values of the gold sequence can be avoided as '0', the deterioration of the PAPR characteristic can be prevented. Table 2 shows PAPR when all MSB 14 bits are set to '0', and Table 3 shows PAPR when all MSB 14 bits are set to '1'. Tables 2 and 3 show basic sequences generated by setting LSB 17 bits of the gold sequence generator differently according to cell ID, subframe number, and OFDM symbol number when the number of RBs is 6, 12, 25, 50, and 100, respectively. It represents the largest peak value when used as a reference signal.

As can be seen from Tables 2 and 3, it can be seen that the PAPR characteristic is better when all MSB 14 bits are set to '1' than when all are set to '0'.

In another embodiment, the MSB 14 bits can be set to a bit sequence that can be cyclically mapped on the QPSK constellation. Since the sequence values initially output from the gold sequence generator are the same as the initial values, the initial values are evenly placed at the four symbol positions in the QPSK constellation, thereby ensuring that the modulation symbols of the generated pseudorandom sequence are concentrated in a specific QPSK modulation symbol. You can prevent it.

 12 shows an example of setting a cyclically mapped bit sequence to an initial value when using QPSK modulation. Suppose that the bit sequences '00', '01', '11', and '10' in the QPSK constellation correspond to modulation symbols ①, ②, ③, and ④, respectively. The bit sequence is set so that four modulation symbols appear evenly with 14 bits of MSB. First, the first bit sequence '00011110000111' is defined such that modulation symbols appear in the order of ①, ②, ③, ④, ①, ②, ③. In fact, since the output of the gold sequence generator starts from the LSB, the first bit sequence is inverted to define a second bit sequence '11100001111000' in reverse order. In addition, since the LSB 17 bits are given according to the cell ID, subframe number, and OFDM symbol number, and one QPSK modulation symbol corresponds to 2 bits, the second bit sequence is cyclically shifted by 1 bit to the left. To generate a third sequence '11000011110001'. The bit closest to the LSB 17 bit among the 14 bits of the MSB is set to an arbitrary bit, and is mapped to one modulation symbol by 2 bits from the bit after the nearest bit (ie, the 19th bit from the LSB). As a result, when outputting from the LSB, the modulation symbols are output in the order of (1), (2), (3), (4), (1) and (2) according to the third sequence.

When the MSB 14 bit is set to '11000011110001', the PAPR characteristics according to the number of RBs are shown in Table 4 below.

As can be seen from Table 4, the MSB 14-bit is set to the proposed value, and the PAPR characteristic is improved.

In another embodiment, a combination of various MSB 14 bits is proposed to improve PAPR characteristics. While changing the value of MSB 14 bit from '00000000000000' to '11111111111111', a value having an optimum PAPR characteristic can be found in all cases, but this is too complicated. Here, it is assumed that the number of RBs is 6, 12, 25, 50, 100, and a sequence having a length corresponding to the number of RBs is used as a reference signal. LSB 17 bits are set differently according to the cell ID, subframe number, and OFDM symbol number for each number of RBs. IFFT is performed on the reference signal for OFDM modulation, and if a peak value of an OFDM symbol as a time domain signal exceeds a specific threshold, the candidate is excluded.

Table 5 shows MSB 14 bits having the best PAPR characteristics when the number of RBs is 6, 12, 25, 50, and 100, respectively.

If the optimal value shown in Table 5 is used for MSB 14 bits according to the number of each RB, the PAPR increase due to bias can be prevented.

Table 6 shows peak values and PAPR when the MSB 14-bit '00010001110001' of Table 5 is applied to the number of all RBs. This shows that applying the best value to the number of specific RBs to the number of other RBs may not be optimal.

In order to be selected as one optimal value, it is important to have a good PAPR characteristic evenly across several RBs that is not optimal for the number of one RB. To this end, based on the algorithm described above, in case of having various number of RBs, the sum of the peak values in the various RBs generated when the value is used is the smallest based on the values that do not exceed a certain threshold. I found a value to create, and that value is '00111101101100'. Table 7 shows peak values and PAPR when the MSB 14-bit '00111101101100' is applied to the number of all RBs.

Although the PAPR characteristics deteriorate more than the results of Table 5, which can be considered optimal, the results show better results than the MSB 14-bit '00010001110001' used in Table 6, and the peak or PAPR characteristics are uniform throughout. This has the advantage of lowering complexity and reducing memory size compared to using different MSB 14 bits depending on the number of RBs.

The above describes a method of improving the PAPR characteristic of the sequence by setting an initial value of the second LFSR of the gold sequence generator. Hereinafter, a method of improving PAPR characteristics of a sequence by setting an initial value of the first LFSR will be described.

In an embodiment, the initial value of the first LFSR may be designated as a specific value. For example, an initial value is set for a bit sequence in which modulation symbols can be uniformly mapped in a QPSK constellation. For example, if you sort the sequence of bits '00', '01', '11', and '10' in reverse order (since the gold sequence is output from the LSB) and map only up to 31 iterations, The value becomes '1111000011110000111100001111000'. Table 8 shows peak values and PAPR according to the number of RBs and the initial value of the lower LFSR when the initial value of the first LFSR is '1111000011110000111100001111000'. This can be seen that when compared with the results of Table 2, the PAPR is much reduced.

In another embodiment, the initial value of the first LFSR may be set to 1's complement of the initial value of the second LFSR. 13 shows an example in which the initial value of the first LFSR is set to one's complement of the initial value of the second LFSR. Although the initial value of the second LFSR of the gold sequence generator is set to '0', the initial value of the first LFSR is all set to '1', which is one's complement of the initial value of the second LFSR. This may produce a sequence of more random characteristics, thus preventing deterioration of the PAPR characteristics. Table 9 shows the result when the initial value of the first LFSR is set to one's complement of the initial value of the second LFSR.

Meanwhile, in order to distinguish between reference signals between cells or between terminals, the correlation characteristics of the reference signals should be good. As described with reference to FIG. 11, when the initial value of the gold sequence generator differs only in the cell ID, and the remaining values (subframe number and OFDM symbol number) are the same, the generated pseudo random number sequence may overlap in some intervals. Can be. This problem occurs because only the value of the cell ID 9 bits is different among the initial values of 31 bits. To solve this problem, only part of the sequence generated as the reference signal is used. This is because even if a pseudo-random sequence of length M _max (which is called a basic sequence) is generated, a sequence of length M (which is called a used sequence) is used according to the number of RBs. Therefore, if the use sequence is selected at different offsets from the base sequence generated based on the cell ID, the problem of overlapping the sequence in some intervals due to the almost identical initial value can be solved.

Now, we describe how to set the offset of the sequence based on the cell ID.

After generating the length M _max basic sequence c (i) of the (i = 0,1, ..., M max -1) by the gold sequence generator, suppose use of a sequence of length M. At this time, M≤M _max . The offset of the use sequence, that is, the start point of the use sequence is set differently according to the cell ID.

14 illustrates that the offset of the available sequence is changed according to the cell ID. This offsets the interval N times according to the cell ID from the base sequence of length M _max to select the use sequence of length M. If the used sequence goes beyond the scope of the base sequence, it is cyclically shifted. The use sequence cu (i) (i = 0,1, ..., M-1) can be expressed from the basic sequence c (i) (i = 0,1, ..., M _max -1) by have.

Here, 'mod' represents a modulo operation, N is an offset interval, and N _ID ^cell is a cell ID. Here, although the same offset is defined for each cell ID, this is merely an example and different offsets may be defined for each cell ID.

 Since the starting point of the use sequence varies according to the cell ID, the use sequence may be different even though the initial value is similar. Therefore, it is possible to ensure random characteristics and to prevent deterioration of PAPR characteristics.

Equation 2 is expressed as a format of a reference signal for a 3GPP LTE system in which resources are allocated in units of RB.

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, r _{l , ns} is a reference signal sequence, N _RB ^{max , DL} is a maximum number of RBs, m is an index of a reference signal sequence, m 'is an index to take a part from the reference signal sequence, N _RB ^DL is the number of RBs used, α _{k, l} (p) is the modulation symbols used as reference symbols for the pth antenna port in the n _s slot, k is a subcarrier index used for transmission of a reference signal, and N _RS ^interval is an interval of a start point according to cell ID N _cell ^ID . r _{l , ns} (m) is the basic sequence, and r _{l , ns} (m ') is the use sequence.

15 cyclically shifts and uses a basic sequence according to a cell ID. After generating the basic sequence c (i) (i = 0,1, ..., M _max -1) of length M _max by the gold sequence generator, the cyclic shift amount N based on the cell ID Is determined, and the base sequence is cyclically shifted by the cyclic shift amount N. In this case, the starting point of the use sequence can always be in the same position. The use sequence cu (i) (i = 0,1, ..., M-1) can be expressed from the basic sequence c (i) (i = 0,1, ..., M _max -1) by have.

Here, c _shift (i) is a sequence in which the basic sequence is cyclically shifted by the cyclic shift amount N.

Equation 4 is expressed as a format of a reference signal for a 3GPP LTE system in which resources are allocated in RB units.

As another example, when generating a pseudo random number sequence from the gold sequence generator, some of the initial sequences may be excluded. The sequence of length Nc among the initially generated gold sequences may be discarded, and the subsequent sequence may be used as the reference signal sequence. The initial value affects the sequence generated initially, thus preventing the similar initial value from deteriorating the PAPR characteristic. This is expressed as the following equation.

Representation of Equation 6 in the form of Equation 1 is as follows.

The Nc value may be set to a length that is generated randomly enough so that the generated pseudo random number sequence is not affected by the initial value. For example, the Nc value may have a value between 1500 and 1800.

A pseudo random number sequence c (i) of Equation 7 is used to express a format of a reference signal for a 3GPP LTE system in which resources are allocated in units of RB.

We now describe the cross-correlation properties between the generated pseudorandom number sequences.

The pseudorandom sequence G (D) generated using two m-sequences X (D) and Y (D) is expressed as a polynomial expression.

Here, the first m-sequence X (D) = I ₁ (D) / g ₁ (D) and the second m-sequence Y (D) = I ₂ (D) / g ₂ (D). Here, g ₁ (D) and g ₂ (D) are primitive polynomials for generating X (D) and Y (D) as follows.

I ₁ (D) and I ₂ (D) are initial values for generating X (D) and Y (D) and are defined as follows.

Here, I (CELLID) represents an initial value according to the cell ID CELLID, and I (N _sf ) D ⁹ represents an initial value according to the slot number and the OFDM symbol number.

If the timing between multiple cells is a synchronous environment, the slot number and OFDM symbol number between neighboring cells will be the same. It is assumed that the slot number and the OFDM symbol number are identical, and the cross correlation between pseudo random number sequences generated in two adjacent cells having different cell IDs CELLID1 and CELLID2 is obtained as follows.

According to the above equation, it can be seen that the cross correlation property is determined only by the cell ID. Since there is no change in the cross-correlation property between cells according to the change of the slot number and the OFDM symbol number, it may be difficult to obtain a sequence having good cross-correlation property in this manner.

When a modulated sequence composed of modulation symbols by QPSK modulation of the generated pseudo random number sequence is referred to as R1 [n] and R1 [n] for two cells, they may be defined as follows.

Here, S [n] is a cell common sequence depending on the subframe number and the OFDM symbol number, and X1 [n] and X2 [n] are cell specific sequences obtained from cell IDs, respectively. The cross correlation of the sequences R1 [n] and R1 [n] can be obtained as follows.

Here, () * means a complex conjugate. The cross-correlation result of the two modulated sequences R1 [n] and R1 [n] indicates that the common axis component of the Q axis is changed by the subframe number and the OFDM symbol number, but the common axis component of the I axis is removed. Because of this, it is difficult to exhibit good cross-correlation between cells.

Therefore, a method for improving the cross correlation property between generated pseudo random number sequences is proposed.

In one embodiment, the starting point of the usage sequence may be changed based on the subframe number and / or the OFDM symbol number. 16 illustrates changing the starting point of the usage sequence based on the subframe number and / or the OFDM symbol number. Generates a long pseudo random sequence based on each cell ID. From the long pseudo-random sequence, a plurality of basic sequences of length Mmax capable of supporting the maximum number of RBs are obtained based on the subframe number and the OFDM symbol number. The sequence of length M used for the transmission of the actual reference signal is obtained from the base sequence. Accordingly, the cross correlation characteristic of the reference signal between cells can be improved.

The following is the format of a reference signal for a 3GPP LTE system in which resources are allocated in RB units.

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, r _{l , ns} is a reference signal sequence, N _RB ^{max , DL} is a maximum number of RBs, m is an index of a reference signal sequence, N _symb ^DL is the number of OFDM symbols included in the slot. The gold sequence generator of the base sequence c (i) is initialized to N _ID ^cell +1 at the beginning of each OFDM symbol.

In another embodiment, the initial value used to generate the base sequence may be changed to improve the cross correlation characteristic. If the subframe number and / or OFDM symbol number coincide in a synchronous environment where timing between multiple cells is consistent, the initial value depending on the subframe number and / or OFDM symbol number is almost similar between cells, resulting in poor correlation. Can get properties In addition, even in an asynchronous environment, in consideration of the transmission time difference between adjacent cells, it is necessary to ensure that the initial values are not the same continuously.

A change in an initial value according to a change in a subframe number and / or an OFDM symbol number between cells may be different. For example, the initial value of the first cell increases as the OFDM symbol number increases, but the second cell may be configured to increase or decrease the initial value as the OFDM symbol number increases. For example, a cell having a cell ID of CELLID1 causes the initial value to increase by n as the OFDM symbol number increases by one. In addition, the cell having the cell ID CELLID2 causes the initial value to increase by n + 1 as the OFDM symbol number increases by one.

In order to set the change of initialization differently according to the change of the OFDM symbol number, the OFDM symbol number may be extended in units of radio frames rather than in a subframe or a slot. When there are N _sym OFDM symbols per subframe, the q th OFDM symbol number of the k th subframe of the radio frame may be represented by k * N _sym + q.

Alternatively, in a system in which the number of OFDM symbols included per subframe is variable, the maximum number of OFDM symbols N _{sym and max} per subframe may be defined. In this case, the q th OFDM symbol number of the k th subframe of the radio frame may be represented by k * N _{sym , max} + q. This is to ensure that each OFDM symbol has a unique OFDM symbol number in one radio frame.

The gold sequence generator increases or decreases the initial value of the m-sequence by a predetermined interval as the OFDM symbol number increases. For example, a cell having a cell ID of CELLID1 causes an initial value to increase by a value defined by CELLID1 such as CELLID1 or CELLID1 + 1 as the OFDM symbol number increases by one. In addition, a cell having a cell ID of CELLID2 causes the initial value to increase by a value defined by CELLID2 such as CCELLID2 or CELLID2 + 1 as the OFDM symbol number increases by one. However, this may be a problem if the cell ID between the cells is about twice as different. For example, if CELLID1 = 5, CELLID2 = 11 and cell ID + 1 is increased (that is, CELLID1 + 1 and CELLID2 + 1), the initial values increase as the OFDM symbol number increases to 6 and 12. Doubled. This results in one bit shifted when expressed in binary. 6 is binary '0110' and 12 is binary '1100'. When a 1-bit shift occurs, cross-correlation characteristics deteriorate due to overlap between the I-axis component of the reference signal of the first cell and the Q-axis component of the reference signal of the second cell when QPSK modulation is used.

Therefore, the initial value needs to be set not to be twice the increase of other cells as the OFDM symbol number and / or subframe number increases. To facilitate this, the initial value is increased or decreased by an odd multiple as the DM symbol number and / or subframe number is increased. For example, the initial value of the gold sequence generator having the cell ID of n is increased or decreased by (2n + 1) times as the OFDM symbol number is increased or decreased.

If this is expressed as a format of a reference signal for a 3GPP LTE system in which resources are allocated in units of RB, the following equation is used.

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, r _{l , ns} is a reference signal sequence, N _RB ^{max , and DL} is the maximum number of RBs. In this case, the sequence generator may be initialized as in the following equation.

Here, l '= 8 n _s + l is an OFDM symbol number in a radio frame.

On the other hand, the cross correlation between pseudo random number sequences is dependent on the binary addition result of initial values used for generating two pseudo random number sequences as shown in the following equation.

(c_init(n₂,l))와 (c_init(n₁,l))

(2c_init(n₂,l)) 역시 OFDM 심벌 번호 l에 따라 변동되어야 좋은 교차 상관 특성을 가질 수 있다.Therefore, when generating a pseudo random number sequence with different initial values for each OFDM symbol number, it can be said that the binary addition result of the initial values of each cell must be changed as the OFDM symbol number is changed to have a good cross-correlation property. This means that the previous addition result of the initial value c _init (n ₁ , l ) of the first cell and the initial value c _init (n ₂ , l ) of the second cell changes as the OFDM symbol number l changes. n ₁ is a cell ID of the first cell and n ₂ is a cell ID of the second cell. Also, consider QPSK modulation (2c _init (n ₁ , l ))

(c _init (n ₂ , l )) and (c _init (n ₁ , l ))

(2c _init (n ₂ , l )) may also have good cross-correlation characteristics when it is changed according to OFDM symbol number l .

c_init(n₂,l)가 OFDM 심벌 번호 l에 따라 변동되도록 한다.17 shows the initial value setting of the gold sequence generator. The 31-bit initial value of the second LFSR is divided into two regions (region # 1 and region # 2). Each region has 14 bits, and region # 2 is on the LSB side. The remaining 4 bits of the MSB can give any value. Region # 1 and region # 2 each contain a binary sequence of cell IDs. In this case, region # 1 cyclically shifts the binary sequence of the cell ID by the first cyclic shift m ₁ according to the OFDM symbol number l , and region # 2 performs the binary shift of the cell ID by the second cyclic shift m ₂ according to the OFDM symbol number l . Cyclic shift the sequence. For example, the region # 1 may cyclically shift the binary sequence of the cell ID by the cyclic shift l m ₁ , and the region # 2 may cyclically shift the binary sequence of the cell ID by the cyclic shift l m ₂ . C _init (n ₁ , l ) by dividing the initial value into two regions and including a binary sequence of cell IDs with different cyclic shifts in each region.

Let c _init (n ₂ , l ) vary with OFDM symbol number l .

Assuming that the size of the region # 1 is b ₁ and the size of the region # 2 is b ₂ , here b ₁ = b ₂ = 14. The size of the regions # 1 and # 2 can be arbitrarily defined within the range of the initial value. In order to increase the generation period of the gold sequence, b ₁ and b ₂ may be set to be relatively prime with each other. In addition, m ₁ and b ₁ , and m ₂ and b ₂ may also be set to be relatively prime with each other.

A format of a reference signal for a 3GPP LTE system in which resources are allocated on a RB basis is expressed as follows.

는 프루어(flooring) 연산으로 x 보다 작은 가장 큰 정수이다 (단, x>0).Where l 'is an OFDM symbol number in a radio frame, CS _b (M, a) is a cyclic shift function,

Is the largest integer smaller than x in the flooring operation (where x> 0).

The proposed sequence describes the application of the 3GPP LTE / LTE-A to the downlink reference signal, but can also be applied to the uplink reference signal. In addition, although PAPR and cross-correlation characteristics are described for reference signals between cells, the same may be applied to reference signals between terminals and / or antennas.

The reference signal to which the proposed sequence is applied may be a cell common reference signal or a terminal specific reference signal.

18 is a flowchart illustrating a method of transmitting a reference signal according to an embodiment of the present invention. This method may be performed by a transmitter for transmitting a reference signal. The transmitter may be part of the base station if the transmission of the downlink reference signal, and may be part of the terminal if the transmission of the uplink reference signal. In step S510, a reference signal sequence is generated. The reference signal sequence may be defined as follows.

Here, n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, r _{l , ns} is a reference signal sequence, N _RB ^{max , and DL} is the maximum number of RBs. The pseudo random sequence c (i) may be defined as shown in Equation 7. Where m-sequence x (i) is x (0) = 1, x (i) = 0, i = 1,2,... It may be initialized to an initial value of 30, and the m-sequence y (i) may be initialized to an initial value obtained from (2N _ID ^cell +1). N _ID ^cell is a cell ID. Initial values of the m-sequence y (i) change as the OFDM symbol number l changes. Thus, the initial value of the m-sequence y (i) can be obtained from l (2N _ID ^cell +1).

In step S520, all or part of the reference signal sequence is mapped to at least one RB. One RB may include 12 subcarriers. If the cell common reference signal, two modulation symbols of the reference signal sequence may be mapped to two subcarriers in one RB. If it is a UE specific reference signal, three modulation symbols of the reference signal sequence may be mapped to three subcarriers in one RB.

In step S530, a reference signal is transmitted through the RB. The proposed reference signal sequence provides improved PAPR and cross correlation. Therefore, the transmission power efficiency of the transmitter can be improved, and high detection performance can be provided to the receiver.

19 is a block diagram illustrating a transmitter and a receiver to which an embodiment of the present invention is applied. The transmitter 800 includes a data processor 810, a reference signal generator 820, and a transmission circuit 830. The data processor 810 processes the information bits to generate a transmission signal. The reference signal generator 820 generates a reference signal. The reference signal generation of FIG. 18 may be performed by the reference signal generator 820. The transmission circuit 830 transmits a transmission signal and / or a reference signal.

The receiver 810 includes a data processor 910, a channel estimator 920, and a receiving circuit 930. The reception circuit 930 receives a reference signal and a reception signal. The channel estimator 920 estimates a channel using the received reference signal. The data processor 910 processes the received signal using the estimated channel.

The above embodiment mainly describes an example of applying the proposed sequence to the reference signal, but the proposed sequence may be applied to various other signals. For example, it may be applied to a scrambling code, a synchronization signal, a preamble, a masking code, and the like. This generates a basic sequence as shown in Equation 20 based on the pseudorandom sequence c (i) of Equation 7. The initial value of the m-sequence y (i) used for the pseudo random number sequence c (i) can be obtained from l (2N _ID ^cell +1). Apply this basic sequence to the desired target signal and / or target code. While applying the base sequence to the target signal and / or target code, only a part of the base sequence may be applied according to the allocated resource or the length (or size) of the target signal and / or the target code. Send the applied sequence. The applied sequence may be used by the receiver of FIG. 19 for various purposes according to the target signal and / or the target code.

The invention can be implemented in hardware, software or a combination thereof. In hardware implementation, an application specific integrated circuit (ASIC), a digital signal processing (DSP), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, and a microprocessor are designed to perform the above functions. , Other electronic units, or a combination thereof. In the software implementation, the module may be implemented as a module that performs the above-described function. The software may be stored in a memory unit and executed by a processor. The memory unit or processor may employ various means well known to those skilled in the art.

Although the present invention has been described above with reference to the embodiments, it will be apparent to those skilled in the art that the present invention may be modified and changed in various ways without departing from the spirit and scope of the present invention. I can understand. Therefore, the present invention is not limited to the above-described embodiment, and the present invention will include all embodiments within the scope of the following claims.

1 shows a wireless communication system.

2 shows a structure of a radio frame in 3GPP LTE.

3 is an exemplary diagram illustrating a resource grid for one downlink slot.

4 shows an example of a structure of a downlink subframe.

5 shows an example of a downlink common reference signal structure when the base station uses one antenna.

6 shows an example of a downlink common reference signal structure when the base station uses two antennas.

7 shows an example of a downlink common reference signal structure when the base station uses four antennas.

8 shows an example of a gold sequence generator.

9 illustrates initial value setting of the second LFSR.

FIG. 10 is a graph comparing sizes between a reference signal and arbitrary data when the initial values of the second LFSR are all '0'.

11 illustrates a problem due to an initial value of a gold sequence in a multi-cell environment.

12 shows an example of setting a cyclically mapped bit sequence to an initial value when using QPSK modulation.

13 shows an example in which the initial value of the first LFSR is set to one's complement of the initial value of the second LFSR.

14 illustrates that the offset of the available sequence is changed according to the cell ID.

15 cyclically shifts and uses a basic sequence according to a cell ID.

16 illustrates changing the starting point of the usage sequence based on the subframe number and / or the OFDM symbol number.

17 shows the initial value setting of the gold sequence generator.

18 is a flowchart illustrating a method of transmitting a reference signal according to an embodiment of the present invention.

19 is a block diagram illustrating a transmitter and a receiver to which an embodiment of the present invention is applied.

Claims (14)

In a method for transmitting a reference signal by a transmitter in a wireless communication system, Generating a reference signal sequence defined as follows;

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, N _RB ^{max, DL} is the maximum number of resource blocks, and c (2m) and c (2m + 1) are pseudo random number sequences. defined by c (i), i = 0, 1, ..., M _max -1, M _max is the length of the pseudo-random sequence c (i), The pseudo random number sequence c (i) is generated from a gold sequence generator initialized with initial values based on (2N _ID ^cell +1), and N _ID ^cell is a cell identifier (identifier). Mapping part or all of the reference signal sequence to at least one resource block; And Transmitting the reference signal to the at least one resource block. The method of claim 1, The pseudo random sequence c (i) is

Wherein x (i) and y (i) are m-sequences and Nc is a constant. The method of claim 2, The m-sequence x (i) is x (0) = 1, x (i) = 0, i = 1,2,... Initialized to initial values of 30, and wherein the m-sequence y (i) is initialized to the initial values. The method of claim 2, Wherein Nc has a value between 1500 and 1800. The method of claim 1, Wherein the initial values change as the OFDM symbol number changes. The method of claim 5, wherein The initial values are obtained from l (2N _ID ^cell +1). The method of claim 1, The size of the initial values is 31 bits. The method of claim 1, One resource block includes 12 subcarriers in the frequency domain. The method of claim 8, Two modulation symbols of the reference signal periodic sequence are mapped to two subcarriers in one resource block. The method of claim 1, The reference signal is a cell common reference signal or a terminal specific reference signal. A reference signal generator for generating a reference signal; And Including a transmission circuit for transmitting the reference signal, The reference signal generator generates a reference signal sequence defined as follows:

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, N _RB ^{max, DL} is the maximum number of resource blocks, and c (2m) and c (2m + 1) are pseudo random number sequences. defined by c (i), i = 0, 1, ..., M _max -1, M _max is the length of the pseudo-random sequence c (i), The pseudo random number sequence c (i) is generated from a gold sequence generator initialized with initial values based on (2N _ID ^cell +1), and N _ID ^cell is a cell identifier (identifier). And transmitting the reference signal by mapping part or all of the reference signal sequence to at least one resource block. The method of claim 11, The pseudo random sequence c (i) is

Wherein x (i) and y (i) are m-sequences and Nc is a constant. A receiving circuit for receiving a reference signal and a received signal; A channel estimator for estimating a channel using the reference signal; And A data processor for processing the received signal using the estimated channel, The reference signal is generated based on a reference signal sequence defined as follows.

Where n _s is a slot number in a radio frame, l is an OFDM symbol number in a slot, N _RB ^{max, DL} is the maximum number of resource blocks, and c (2m) and c (2m + 1) are pseudo random number sequences. defined by c (i), i = 0, 1, ..., M _max -1, M _max is the length of the pseudo-random sequence c (i), The pseudo random number sequence c (i) is generated from a gold sequence generator initialized with initial values based on (2N _ID ^cell +1), and the N _ID ^cell is a cell identifier. The method of claim 13, The pseudo random sequence c (i) is

Where x (i) and y (i) are m-sequences and Nc is a constant.

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