WO2009091143A2

WO2009091143A2 - Method of pilot subcarrier transmitting

Info

Publication number: WO2009091143A2
Application number: PCT/KR2008/007792
Authority: WO
Inventors: Min Seok Noh; Yeong Hyeon Kwon; Jin Sam Kwak; Dong Cheol Kim; Sung Ho Moon; Seung Hee Han; Hyun Woo Lee; Jin Soo Choi
Original assignee: LG Electronics Inc
Current assignee: LG Electronics Inc
Priority date: 2008-01-18
Filing date: 2008-12-30
Publication date: 2009-07-23
Anticipated expiration: 2010-07-18
Also published as: WO2009091143A3

Abstract

A pilot symbol structure and a pilot transmission method for improving channel estimation performance in a wireless access system using multiple carriers are provided. In the method for transmitting subcarriers using a pilot structure, pilot subcarriers are repeatedly allocated at predetermined subcarrier spacings and at predetermined symbol spacings in a basic resource block, and the basic resource block is periodically and repeatedly allocated in a time domain and a frequency domain within a subframe, and then the pilot subcarriers and data subcarriers included in the subframe are transmitted. When one or more antennas are used, it is possible to transmit pilot symbols while maintaining predetermined spacings in the time domain and the frequency domain, thereby improving channel estimation performance.

Description

Description METHOD OF PILOT SUBCARRIER TRANSMITTING

Technical Field

[1] The present invention relates to a pilot symbol structure and a pilot transmission method for improving channel estimation performance in a wireless access system using multiple carriers. Background Art

[2] The following is a brief description of a channel estimation method and pilot signals.

[3] To detect a synchronous signal, a receiver should have information regarding wireless channels such as attenuation, phase shift, or time delay. Here, the term channel estimation refers to estimation of the reference phase and the size of each carrier. Wireless channel environment has fading characteristics such that the condition of a channel irregularly changes in the time and frequency domains as time passes. Channel estimation serves to estimate the amplitude and phase of such a channel. Namely, channel estimation serves to estimate a frequency response of a wireless link or a wireless channel.

[4] In one channel estimation method, a reference value is estimated based on pilot symbols of several base stations using a two-dimensional channel estimator. Here, the term pilot symbols refers to symbols that do not contain actual data but instead have high power to support carrier phase synchronization and acquisition of base station information. The transmitting and receiving ends can perform channel estimation using such pilot symbols. Specifically, the transmitting and receiving ends estimate a channel using pilot symbols known to both the transmitting and receiving ends and reconstruct data using the estimated value.

[5] FIG. 1 illustrates an example of a general pilot structure used in a single- transmit- antenna structure.

[6] The pilot structure of FIG. 1 is applied when one transmit antenna is used. When one antenna is used, two pilot subcarriers are used for each even symbol and two pilot sub- carriers are used for each odd symbol. In this case, an overhead of about 14.28% may occur due to pilot subcarriers.

[7] FIG. 2 illustrates an example of a general pilot structure used in a two- transmit- antenna structure.

[8] In downlink, Spacing-Time Coding (STC) is used to provide high-order transmit diversity. Here, two or more transmit antennas are needed to support STC.

[9] As shown in FIG. 2, two transmit antennas (first and second antennas) can simultaneously transmit different data symbols. Here, data symbols are repeatedly transmitted in the time domain (space-time) and the frequency domain (space-frequency). Accordingly, the pilot structure of FIG. 2 can exhibit higher performance when transmitting data although receiver complexity is increased.

[10] The method of allocating data in the example of FIG. 2 can be changed in order to use two antennas having the same channel estimation capabilities. A respective pilot symbol is transmitted twice in each antenna. The position of the pilot symbol is changed over four symbol durations. Symbols are counted starting from the beginning of the current region, and the first symbol number is even.

[11] In the example of FIG. 2, pilot subcarriers are used for channel estimation. Here, an overhead of about 14.28% may occur due to pilot subcarriers.

[12] FIG. 3 illustrates an example of a general pilot structure used in a four- transmit- antenna structure.

[13] When four antennas (first, second, third, and fourth antennas) are used, transmit diversity can be improved, compared to when two antennas are used. The pilot structure of FIG. 3 is characterized in that it does not change the channel estimation procedure of the user even though four antennas are used.

[14] As shown in FIG. 3, respective pilot channels of the antennas are allocated to each symbol. For example, when one symbol includes 14 subchannels, respective pilots of the four antennas are allocated to subcarriers of each symbol. Thus, an overhead of about 28.57% may occur due to pilot subcarriers.

[15] As described above, an overhead of about 14.28% may occur due to pilot subcarriers when one transmit antenna is used and when two transmit antennas are used. In addition, an overhead of about 28.57% may occur due to pilot subcarriers when four transmit antennas are used.

[16] From FIGs. 1 to 3, it can be seen that significant overhead occurs due to pilot subcarriers in the general Orthogonal Frequency Division Multiplexing (OFDM) system. Such overhead may reduce link throughput, thereby causing a reduction in system performance. The conventional pilot structure also has a problem in that it does not maintain commonality of multiple antennas of the multiple-antenna system. Disclosure of Invention Technical Problem

[17] An object of the present invention devised to solve the problem lies on providing a pilot structure that can guarantee excellent channel estimation performance in an OFDM system.

[18] Another object of the present invention devised to solve the problem lies on providing a pilot structure that maintains a uniform time spacing and a uniform frequency spacing to improve channel estimation performance while reducing pilot overhead. Technical Solution

[19] To achieve the above objects, the present invention provides a pilot symbol structure and a pilot transmission method for improving channel estimation performance in a wireless access system using multiple carriers.

[20] In one aspect of the present invention, a method for transmitting subcarriers using a pilot structure may include repeatedly allocating pilot subcarriers at a predetermined subcarrier spacing and at a predetermined symbol spacing in a basic resource block, periodically and repeatedly allocating the basic resource block in a time domain and a frequency domain within a subframe, and transmitting the pilot subcarriers and data subcarriers included in the subframe.

[21] The basic resource block may include 18 subcarriers in a frequency axis and 6

OFDM symbols in a time axis.

[22] In the basic resource block, the predetermined subcarrier spacing may be one of a

6-subcarrier spacing or a 9-subcarrier spacing. Here, the predetermined symbol spacing may be one of a 2-OFDM-symbol spacing, a 3 -OFDM- symbol spacing, or a 4-OFDM-symbol spacing.

[23] When the pilot subcarriers are transmitted using two antennas, the predetermined subcarrier spacing may be a 9-subcarrier spacing and the predetermined symbol spacing may be a 2-OFDM-symbol spacing in the first and second antennas.

[24] When the pilot subcarriers are transmitted using four antennas, the predetermined subcarrier spacing may be a 9-subcarrier spacing and the predetermined symbol spacing may be a 2-OFDM-symbol spacing in the first and second antennas, and the predetermined subcarrier spacing may be a 9-subcarrier spacing and the predetermined symbol spacing may be a 3-OFDM-symbol spacing in the third and fourth antennas.

[25] When the pilot subcarriers are transmitted using four antennas, the predetermined subcarrier spacing may be a 9-subcarrier spacing and the predetermined symbol spacing may be a 2-OFDM-symbol spacing in the first and second antennas, and the predetermined subcarrier spacing may be a 9-subcarrier spacing and the predetermined symbol spacing may be a 4-OFDM-symbol spacing in the third and fourth antennas.

[26] In another aspect of the present invention, a method for receiving data using a pilot structure for channel estimation may include receiving pilot subcarriers that are allocated at a predetermined subcarrier spacing and at a predetermined symbol spacing in a basic resource block, and performing channel estimation using the pilot subcarriers.

Advantageous Effects

[27] The present invention has the following advantages. [28] First, when one or more antennas are used, pilot symbols can be transmitted while maintaining predetermined spacings in the time domain and the frequency domain, thereby improving channel estimation performance.

[29] Second, transmission of data using the pilot structures suggested in the present invention can reduce pilot overhead of the OFDM system while improving system throughput. Brief Description of Drawings

[30] The accompanying drawings, which are included to provide a further understanding of the invention, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.

[31] In the drawings :

[32] FIG. 1 illustrates an example of a general pilot structure used in a single- transmit- antenna structure.

[33] FIG. 2 illustrates an example of a general pilot structure used in a two- transmit- antenna structure.

[34] FIG. 3 illustrates an example of a general pilot structure used in a four- transmit- antenna structure.

[35] FIG. 4 illustrates an example pilot structure in the case where a single antenna is used according to the present invention.

[36] FIG. 5 illustrates an example pilot structure in the case where two antennas are used according to the present invention.

[37] FIG. 6 illustrates an example pilot structure in the case where four antennas are used according to the present invention.

[38] FIG. 7 illustrates another example pilot structure in the case where four antennas are used according to the present invention.

[39] FIG. 8 illustrates another example pilot structure in the case where four antennas are used according to the present invention.

[40] FIG. 9 illustrates another example pilot structure in the case where four antennas are used according to the present invention. Mode for the Invention

[41] To accomplish the objects described above, the present invention provides a pilot symbol structure and a pilot transmission method for improving channel estimation performance in a wireless access system using multiple carriers.

[42] The embodiments described below are provided by combining components and features of the present invention in specific forms. The components or features of the present invention can be considered optional if not explicitly stated otherwise. The components or features may be implemented without being combined with other components or features. The embodiments of the present invention may also be provided by combining some of the components and/or features. The order of the operations described below in the embodiments of the present invention may be changed. Some components or features of one embodiment may be included in another embodiment or may be replaced with corresponding components or features of another embodiment.

[43] The embodiments of the present invention have been described focusing mainly on the data communication relationship between a terminal and a Base Station (BS). The BS is a terminal node in a network which performs communication directly with the terminal. Specific operations which have been described as being performed by the BS may also be performed by an upper node as needed.

[44] That is, it will be apparent to those skilled in the art that the BS or any other network node may perform various operations for communication with terminals in a network including a number of network nodes including BSs. The term "base station (BS)" may be replaced with another term such as "fixed station", "Node B", "eNode B (eNB)", or "access point". The term "terminal" may also be replaced with another term such as "user equipment (UE)", "mobile station (MS)", or "mobile subscriber station (MSS)".

[45] The embodiments of the present invention can be implemented by a variety of means. For example, the embodiments of the present invention can be implemented by hardware, firmware, software, or any combination thereof.

[46] In the case where the present invention is implemented by hardware, methods according to the embodiments of the present invention may be implemented by one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, or the like.

[47] In the case where the present invention is implemented by firmware or software, methods according to the embodiments of the present invention may be implemented in the form of modules, processes, functions, or the like which perform the features or operations described below. Software code can be stored in a memory unit so as to be executed by a processor. The memory unit may be located inside or outside the processor and can communicate data with the processor through a variety of known means.

[48] Specific terms used in the following description are provided for better understanding of the present invention and can be replaced with other terms without departing from the spirit of the present invention.

[49] The present invention suggests pilot structures that can guarantee excellent performance of channel estimation in the OFDM system. The present invention will now be described in detail with reference to the accompanying drawings.

[50] FIG. 4 illustrates an example pilot structure in the case where a single antenna is used according to the present invention.

[51] As shown in FIGs. 4(a) and 4(b), a basic resource block may include 6 OFDM symbols along the horizontal axis and 18 subcarriers along the vertical axis. In the embodiments of the present invention described below, it is assumed that the same basic resource block as that of FIGs. 4(a) and 4(b) is used.

[52] Here, a pilot structure included in the basic resource block can be designed such that pilot subcarriers are allocated at 9-subcarrier spacings along the frequency axis and at 2- OFDM- symbol spacings along the time axis. That is, the pilot structure in the case where a single antenna is used can be designed so as to have the following characteristics.

[53] (1) The pilot structure is designed such that pilot subcarriers are allocated at predetermined time intervals and at predetermined frequency intervals in one basic resource block.

[54] (2) Pilot subcarriers can be repeatedly allocated in the same manner in the time and frequency domains in a frame or subframe. Here, when basic resource blocks are repeated, the pilot structure is designed such that the time interval and the frequency interval of pilots in each basic resource block are kept constant.

[55] (3) When preamble OFDM symbols are transmitted at intervals of a predetermined period at the head of each subframe, OFDM symbols, starting from the second OFDM symbol, can be used as pilot subcarriers.

[56] The following Math Figure 1 represents the method for allocating pilot subcarriers as shown in FIGs. 4(a) and 4(b).

[57] MathFigure 1

[Math.l] k_{m s} = (9m + subcarrier offset + 3sfloor(s / 2) + symbol offset) mod 18

[58] In Math Figure 1, k_{m s} represents the position of a subcarrier corresponding to an mth pilot in an sth OFDM symbol, 'm' represents an index of the number of pilots occupied by each antenna in one OFDM symbol. For example, 'm' may be set to 0 if the number of pilot subcarriers in one symbol unit is 1 and may be set to 1 if the number of pilot subcarriers is 2. V represents the index of an OFDM symbol to which a pilot is allocated in a basic resource block. In addition, the subcarrier offset represents the index of a pilot subcarrier initially allocated in the frequency axis and the symbol offset represents the index of a pilot subcarrier initially allocated in the time axis.

[59] The following is a description of FIGs. 4(a) and 4(b) with reference to Math Figure 1.

[60] In the pilot structure of FIG. 4(a), the subcarrier offset is 1 and the OFDM symbol offset is 0. Here, 'm' = 0 and 1, and V = 0, 2, and 4 in Math Figure 1.

[61] In the pilot structure of FIG. 4(b), the subcarrier offset is 1 and the OFDM symbol offset is 1. Here, 'm' = 0 and 1 and V = 1, 3, and 5 in Math Figure 1. It can be seen from FIGs. 4(a) and 4(b) that overhead due to pilot subcarriers is about 5.56%.

[62] While FIGs. 4(a) and 4(b) illustrate an example pilot structure where pilots are at frequency and time intervals of 9X2, any other type of pilot structure having such a 9X2 format can be used according to the present invention.

[63] FIG. 5 illustrates an example pilot structure in the case where two antennas are used according to the present invention.

[64] As shown in FIGs. 5 (a) to 5(f), a basic resource block may include 6 OFDM symbols along the horizontal axis and 18 subcarriers along the vertical axis. Here, a pilot structure included in the basic resource block can be designed such that pilot symbols are allocated at 9-subcarrier spacings along the frequency axis and at 2-OFDM-symbol spacings along the time axis. That is, the pilot structure in the case where one antenna is used can be basically designed so as to have the characteristics described above with reference to FIGs. 4(a) and 4(b).

[65] The following is a description of FIGs. 5(a) to 5(f) with reference to the description of FIGs. 4(a) and 4(b) and Math Figure 1. In FIGs. 5 (a) to 5(f), each pilot subcarrier of the first antenna can be represented by Rl and each pilot subcarrier of the second antenna can be represented by R2.

[66] In the pilot structure of FIG. 5(a), the subcarrier offset of the first antenna is 1 and the

OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = 0, 2, and 4 in Math Figure 1.

[67] In the pilot structure of FIG. 5(a), the subcarrier offset of the second antenna is 1 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = 1, 3, and 5 in Math Figure 1.

[68] In the pilot structure of FIG. 5(b), pilot subcarriers of the first and second antennas are repeated alternately in reverse order. That is, pilot subcarriers of each of the first and second antennas may have two offset values.

[69] In the pilot structure of FIG. 5(b), one of the two subcarrier offsets of the first antenna is 1 and one of the two OFDM symbol offsets thereof is 0. Here, 'm' = 0 and V = 0, 2, and 4 in Math Figure 1. The other subcarrier offset of the first antenna is 10 and the other OFDM symbol offset is 1. Here, 'm' = 0 and V = 1, 3, and 5 in Math Figure 1.

[70] In addition, in the pilot structure of FIG. 5(b), one of the two subcarrier offsets of the second antenna is 10 and one of the two OFDM symbol offsets thereof is 0. Here, 'm' = 0 and V = 0, 2, and 4 in Math Figure 1. The other subcarrier offset of the second antenna is 1 and the other OFDM symbol offset is 1. Here, 'm' = 0 and V = 1, 3, and 5 in Math Figure 1. [71] In the pilot structure of FIG. 5(c), the subcarrier offset of the first antenna is 1 and the

OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = 0, 2, and 4 in Math

Figure 1. [72] In addition, in the pilot structure of FIG. 5(c), the subcarrier offset of the second antenna is 7 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V =

1, 3, and 5 in Math Figure 1. [73] In the pilot structure of FIG. 5(d), the subcarrier offset of the first antenna is 1 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O, 2, and 4 in

Math Figure 1. [74] In addition, in the pilot structure of FIG. 5(d), the subcarrier offset of the second antenna is 5 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V =

0, 2, and 4 in Math Figure 1. [75] In the pilot structure of FIG. 5(e), the subcarrier offset of the first antenna is 1 and the

Figure 1. [76] In addition, in the pilot structure of FIG. 5(e), the subcarrier offset of the second antenna is 6 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V =

0, 2, and 4 in Math Figure 1. [77] In the pilot structures of FIGs. 5(d) and 5(e), pilot subcarriers are presented only in the same OFDM symbols. [78] In the pilot structure of FIG. 5(f), the subcarrier offset of the first antenna is 1 and the

OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = I, 3, and 5 in Math

Figure 1. [79] In addition, in the pilot structure of FIG. 5(f), the subcarrier offset of the second antenna is 2 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V =

0, 2, and 4 in Math Figure 1. [80] From the above description, it can be seen that the pilot structures of FIGs. 5 (a) to

5(f) are designed so as to have a pilot overhead of about 11.11% when two transmit antennas are used. Here, reversed versions of the pilot structures of the first and second antennas illustrated in FIGs. 5 (a) to 5(f) may also be applied. [81] FIG. 6 illustrates an example pilot structure in the case where four antennas are used according to the present invention. [82] As shown in FIGs. 6(a) to 6(f), a basic resource block may include 6 OFDM symbols

(0, 1, ..., 5) along the horizontal axis and 18 subcarriers (0, 1, ..., 17) along the vertical axis. Here, a pilot structure included in the basic resource block can be designed such that pilot symbols are allocated at 9-subcarrier spacings along the frequency axis and at

2-OFDM-symbol spacings along the time axis. That is, the pilot structure can be basically designed so as to have the characteristics described above with reference to FIG. 4.

[83] In FIG. 6, each pilot subcarrier of the first antenna can be represented by Rl, each pilot subcarrier of the second antenna can be represented by R2, each pilot subcarrier of the third antenna can be represented by R3, and each pilot subcarrier of the fourth antenna can be represented by R4.

[84] The pilot structures of the first and second antennas of FIGs. 6(a) to 6(f) are identical to the pilot structure of FIG. 5(a). Thus, refer to FIG. 5(a) for the pilot allocation positions of the first and second antennas in FIGs. 6(a) to 6(f). The following is a description of the allocation positions of the third and fourth antennas of FIG. 6.

[85] The pilot structures of FIGs. 6(a) to 6(e) are designed such that the spacings of pilots of each of the third and fourth antennas are maintained at 9 subcarriers along the frequency axis and are maintained at 3 OFDM symbols along the time axis. However, the pilot structure of FIG. 6(f) is designed such that the spacings of pilots of the third and fourth antennas are maintained at 4 OFDM symbols along the time axis.

[86] That is, while pilots of the first, second, third, and fourth antennas are allocated at the same subcarrier spacings along the frequency axis, pilots thereof are allocated at different OFDM symbol spacings. The purpose of linearly increasing the OFDM symbol spacing is to reduce pilot overhead that linearly increases.

[87] The following Math Figure 2 represents a method for allocating pilot subcarriers while maintaining the 9-subcarrier spacing and the 3-OFDM-symbol spacing.

[88] MathFigure 2

[Math.2] k_{m s} - (9m + subcarrier offset + Asβoor(s / 3) + symbol offset) mod 18

[89] Descriptions of 'm', V, the subcarrier offset, and the symbol offset in Math Figure 2 are identical to those of Math Figure 1.

[90] However, the coefficient A of a floor function "floor()" may be changed according to the positional relationship between pilot subcarriers of the third antenna or the fourth antenna. For example, "A" represents an absolute subcarrier spacing between the first pilot subcarrier and the last subcarrier of the third antenna in the basic resource block. In this case, the floor function is changed to "floor(s/3)" to set the OFDM symbol spacing to 3 OFDM symbols as can be seen from Math Figure 2.

[91] The following Math Figure 3 represents a method for allocating pilot subcarriers while maintaining the 9-subcarrier spacing and the 4-OFDM-symbol spacing.

[92] MathFigure 3

[Math.3] k_{m s} = (9m + subcarrier offset + Asfloor(s 14) + symbol offsei)moά\% [93] Math Figure 3 is basically identical to Math Figure 2. However, the floor function of

Math Figure 3 is different from that of Math Figure 2. That is, the floor function is set to "floor(s/4)" to set the OFDM symbol spacing to 4 OFDM symbols. [94] The following is a description of FIG. 6 with reference to Math Figures 2 and 3.

[95] In the pilot structure of FIG. 6(a), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math

Figure 2. In this case, the floor coefficient A is 3. [96] In addition, in the pilot structure of FIG. 6(a), the subcarrier offset of the fourth antenna is 7 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 4 in Math Figure 2. In this case, the floor coefficient A is 3. [97] In the pilot structure of FIG. 6(b), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math

Figure 2. In this case, the floor coefficient A is 7. [98] In addition, in the pilot structure of FIG. 6(b), the subcarrier offset of the fourth antenna is 5 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 5. [99] In the pilot structure of FIG. 6(c), the subcarrier offset of the third antenna is 4 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math

Figure 2. In this case, the floor coefficient A is 6. [100] In addition, in the pilot structure of FIG. 6(c), the subcarrier offset of the fourth antenna is 1 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 3. [101] In the pilot structure of FIG. 6(d), the subcarrier offset of the third antenna is 6 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math

Figure 2. In this case, the floor coefficient A is 3. [102] In addition, in the pilot structure of FIG. 6(d), the subcarrier offset of the fourth antenna is 6 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 4 in Math Figure 2. In this case, the floor coefficient A is 3. [103] In the pilot structure of FIG. 6(e), the subcarrier offset of the third antenna is 8 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math

Figure 2. In this case, the floor coefficient A is 3. [104] In addition, in the pilot structure of FIG. 6(e), the subcarrier offset of the fourth antenna is 8 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 4 in Math Figure 2. In this case, the floor coefficient A is 3. [105] In the pilot structure of FIG. 6(f), the subcarrier offset of the third antenna is 2 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math

Figure 3. In this case, the floor coefficient A is 4. [106] In addition, in the pilot structure of FIG. 6(f), the subcarrier offset of the fourth antenna is 1 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 3. In this case, the floor coefficient A is 4.

[107] FIG. 7 illustrates another example pilot structure in the case where four antennas are used according to the present invention.

[108] The method for forming pilot structures illustrated in FIG. 7 is basically similar to that of FIG. 6. However, the method of FIG. 7 is different from that of FIG. 6 in the allocation positions of pilot subcarriers. The pilot subcarrier allocation structures of the first and second antennas Rl and R2 illustrated in FIG. 7 are identical to that of FIG. 5 (a). Thus, reference will now be made to a method for allocating pilot subcarriers of the third and fourth antennas R3 and R4.

[109] In the pilot structure of FIG. 7(a), the subcarrier offset of the third antenna is 5 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 0.

[110] In addition, in the pilot structure of FIG. 7(a), the subcarrier offset of the fourth antenna is 5 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 0.

[I l l] In the pilot structure of FIG. 7(b), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 3.

[112] In addition, in the pilot structure of FIG. 7(b), the subcarrier offset of the fourth antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 3.

[113] In the pilot structure of FIG. 7(c), the subcarrier offset of the third antenna is 5 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 4.

[114] In addition, in the pilot structure of FIG. 7(c), the subcarrier offset of the fourth antenna is 8 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 5.

[115] In the pilot structure of FIG. 7(d), the subcarrier offset of the third antenna is 5 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 2.

[116] In addition, in the pilot structure of FIG. 7(d), the subcarrier offset of the fourth antenna is 1 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 3.

[117] In the pilot structure of FIG. 7(e), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 4 in Math Figure 3. In this case, the floor coefficient A is 3.

[118] In addition, in the pilot structure of FIG. 7(e), the subcarrier offset of the fourth antenna is 7 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = 1 and 5 in Math Figure 3. In this case, the floor coefficient A is 3.

[119] In the pilot structure of FIG. 7(f), the subcarrier offset of the third antenna is 6 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 4 in Math Figure 3. In this case, the floor coefficient A is 5.

[120] In addition, in the pilot structure of FIG. 7(f), the subcarrier offset of the fourth antenna is 6 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 5 in Math Figure 3. In this case, the floor coefficient A is 5.

[121] FIGs. 8 illustrates still another example pilot structure in the case where four antennas are used according to the the present invention.

[122] The method for forming pilot structures illustrated in FIG. 8 is basically similar to that of FIG. 7. However, the method of FIG. 8 is different from that of FIG. 7 in the allocation positions of pilot subcarriers. The pilot subcarrier allocation structures of the first and second antennas Rl and R2 illustrated in FIG. 8 are identical to that of FIG. 5(b). Thus, reference will now be made to a method for allocating pilot subcarriers of the third and fourth antennas R3 and R4.

[123] In the pilot structure of FIG. 8(a), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 3.

[124] In addition, in the pilot structure of FIG. 8(a), the subcarrier offset of the fourth antenna is 7 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 4 in Math Figure 2. In this case, the floor coefficient A is 3.

[125] In the pilot structure of FIG. 8(b), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 7.

[126] In addition, in the pilot structure of FIG. 8(b), the subcarrier offset of the fourth antenna is 5 and the OFDM symbol offset thereof is 3. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 5.

[127] In the pilot structure of FIG. 8(c), the subcarrier offset of the third antenna is 4 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 6.

[128] In addition, in the pilot structure of FIG. 8(c), the subcarrier offset of the fourth antenna is 5 and the OFDM symbol offset thereof is 3. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 3.

[129] In the pilot structure of FIG. 8(d), the subcarrier offset of the third antenna is 6 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 3.

[130] In addition, in the pilot structure of FIG. 8(d), the subcarrier offset of the fourth antenna is 6 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = 1 and 4 in Math Figure 2. In this case, the floor coefficient A is 3.

[131] In the pilot structure of FIG. 8(e), the subcarrier offset of the third antenna is 8 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 3.

[132] In addition, in the pilot structure of FIG. 8(e), the subcarrier offset of the fourth antenna is 8 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 4 in Math Figure 2. In this case, the floor coefficient A is 3.

[133] In the pilot structure of FIG. 8(f), the subcarrier offset of the third antenna is 2 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 4 in Math Figure 3. In this case, the floor coefficient A is 4.

[134] In addition, in the pilot structure of FIG. 8(f), the subcarrier offset of the fourth antenna is 2 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 5 in Math Figure 3. In this case, the floor coefficient A is 3.

[135] FIG. 9 illustrates still another example pilot structure in the case where four antennas are used according to the present invention.

[136] The method for forming pilot structures illustrated in FIG. 9 is basically similar to that of FIG. 8. However, the method of FIG. 9 is different from that of FIG. 8 in the allocation positions of pilot subcarriers. The pilot subcarrier allocation structures of the first and second antennas Rl and R2 illustrated in FIG. 9 are identical to that of FIG. 5(b). Thus, reference will now be made to an allocation structure of pilot subcarriers of the third and fourth antennas R3 and R4.

[137] In the pilot structure of FIG. 9(a), the subcarrier offset of the third antenna is 5 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 0.

[138] In addition, in the pilot structure of FIG. 9(a), the subcarrier offset of the fourth antenna is 5 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 0.

[139] In the pilot structure of FIG. 9(b), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 5.

[140] In addition, in the pilot structure of FIG. 9(b), the subcarrier offset of the fourth antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 3.

[141] In the pilot structure of FIG. 9(c), the subcarrier offset of the third antenna is 5 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 4.

[142] In addition, in the pilot structure of FIG. 9(c), the subcarrier offset of the fourth antenna is 8 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 5.

[143] In the pilot structure of FIG. 9(d), the subcarrier offset of the third antenna is 5 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 3 in Math Figure 2. In this case, the floor coefficient A is 3.

[144] In addition, in the pilot structure of FIG. 9(d), the subcarrier offset of the fourth antenna is 1 and the OFDM symbol offset thereof is 2. Here, 'm' = 0 and 1, and V = 2 and 5 in Math Figure 2. In this case, the floor coefficient A is 3.

[145] In the pilot structure of FIG. 9(e), the subcarrier offset of the third antenna is 7 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 4 in Math Figure 3. In this case, the floor coefficient A is 3.

[146] In addition, in the pilot structure of FIG. 9(e), the subcarrier offset of the fourth antenna is 7 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 5 in Math Figure 3. In this case, the floor coefficient A is 3.

[147] In the pilot structure of FIG. 9(f), the subcarrier offset of the third antenna is 6 and the OFDM symbol offset thereof is 0. Here, 'm' = 0 and 1, and V = O and 4 in Math Figure 3. In this case, the floor coefficient A is 5.

[148] In addition, in the pilot structure of FIG. 9(f), the subcarrier offset of the fourth antenna is 6 and the OFDM symbol offset thereof is 1. Here, 'm' = 0 and 1, and V = I and 5 in Math Figure 3. In this case, the floor coefficient A is 5.

[149] The allocation structures of pilot subcarriers when four antennas are used have been described above with reference to FIGs. 6 to 9. It can be seen from FIGs. 6 to 9 that overhead due to pilot subcarriers in a basic resource block is about 14.3%. This overhead is far lower than overhead caused by a general pilot structure employing four antennas. Using the pilot structures described above in the embodiments of the present invention, it is possible to maintain commonality of transmit antennas when multiple antennas are used.

[150] Those skilled in the art will appreciate that the present invention may be embodied in other specific forms than those set forth herein without departing from the spirit and essential characteristics of the present invention. The above description is therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by reasonable interpretation of the appended claims and all changes coming within the equivalency range of the invention are intended to be embraced in the scope of the invention. It will also be apparent that claims which are not explicitly dependent on each other can be combined to provide an embodiment or new claims can be added through amendment after this application is filed. Industrial Applicability The embodiments of the present invention can be applied to a variety of wireless access systems. Examples of the variety of wireless access systems include 3rd Generation Partnership Project (3GPP), 3GPP2, and/or Institute of Electrical and Electronic Engineers 802 (IEEE 8O2.xx) systems. The embodiments of the present invention can be applied not only to the variety of wireless access systems but also to all technical fields which employ the variety of wireless access systems.

Claims

[1] A method for transmitting subcarriers using a pilot structure, the method comprising: repeatedly allocating pilot subcarriers at a predetermined subcarrier spacing and at a predetermined symbol spacing in a basic resource block; periodically and repeatedly allocating the basic resource block in a time domain and a frequency domain within a subframe; and transmitting the pilot subcarriers and data subcarriers included in the subframe.

[2] The method according to claim 1, wherein the basic resource block includes 18 subcarriers in a frequency axis and 6 OFDM symbols in a time axis.

[3] The method according to claim 2, wherein the predetermined subcarrier spacing is one of a 6-subcarrier spacing or a 9-subcarrier spacing.

[4] The method according to claim 3, wherein the predetermined symbol spacing is a

2-OFDM-symbol spacing.

[5] The method according to claim 3, wherein the predetermined symbol spacing is a

3-OFDM-symbol spacing.

[6] The method according to claim 3, wherein the predetermined symbol spacing is a

4-OFDM-symbol spacing.

[7] The method according to claim 2, wherein, when the pilot subcarriers are transmitted using two antennas, the predetermined subcarrier spacing is a 9-subcarrier spacing and the predetermined symbol spacing is a 2-OFDM-symbol spacing in the first and second antennas.

[8] The method according to claim 2, wherein, when the pilot subcarriers are transmitted using four antennas, the predetermined subcarrier spacing is a 9-subcarrier spacing and the predetermined symbol spacing is a 2-OFDM-symbol spacing in the first and second antennas, and the predetermined subcarrier spacing is a 9-subcarrier spacing and the predetermined symbol spacing is a 3-OFDM-symbol spacing in the third and fourth antennas.

[9] The method according to claim 2, wherein, when the pilot subcarriers are transmitted using four antennas, the predetermined subcarrier spacing is a 9-subcarrier spacing and the predetermined symbol spacing is a 2-OFDM-symbol spacing in the first and second antennas, and the predetermined subcarrier spacing is a 9-subcarrier spacing and the predetermined symbol spacing is a 4-OFDM-symbol spacing in the third and fourth antennas.

[10] A method for receiving data using a pilot structure for channel estimation, the method comprising: receiving pilot subcarriers that are allocated at a predetermined subcarrier spacing and at a predetermined symbol spacing in a basic resource block; and performing channel estimation using the pilot subcarriers.