WO2005043791A2

WO2005043791A2 - Method for constructing downlink frame in wireless communication system using orthogonal frequency division multiple access

Info

Publication number: WO2005043791A2
Application number: PCT/KR2004/002445
Authority: WO
Inventors: Hyoung-Soo Lim; Dong-Seung Kwon; Seong-Rag Kim; In-Kyeong Choi; Choong-Il Yeh; Seong-Chul Cho; Yu-Ro Lee; Jong-Ee Oh; Seung-Ku Hwang; Soon-Young Yoon; Sang-Hoon Sung; Jae-Hee Cho; In-Seok Hwang; Hoon Huh
Original assignee: Electronics And Telecommunications Research Institute; Samsung Electronics Co., Ltd.; Kt Corporation; Sk Telecom Co., Ltd.; Ktfreetel Co., Ltd.; Hanaro Telecom, Inc.
Priority date: 2003-10-30
Filing date: 2004-09-23
Publication date: 2005-05-12
Also published as: KR20050041857A; KR100657506B1; WO2005043791A3

Abstract

Disclosed is a method for constructing a downlink frame in a mobile data communication system using an OFDMA method. To construct the downlink frame, a single preamble is arranged at the head of the downlink frame, and a plurality of data symbols are arranged following the preamble. Then, a plurality of pilot symbols are inserted between the data symbols over the downlink frame at a predetermined period. Accordingly, a channel variation can be rapidly tracked using the preamble and pilot symbols. Furthermore, interference between adjacent cells is minimized and averaged to make a frequency reuse rate be 1, such that arrangement of multiple cells can be easily designed.

Description

Description METHOD FOR CONSTRUCTING DOWNLINK FRAME IN WIRELESS COMMUNICATION SYSTEM USING ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS Technical Field

[1] The present invention relates to a method for constructing a downlink frame in a mobile data communication system. More specifically, the present invention relates to a method for constructing a downlink frame in a mobile data communication system using orthogonal frequency division multiple access (OFDMA). Background Art

[2] When a signal is transmitted through a multipath channel, inter-symbol interference due to the multipath channel is generated in a received signal. In case of high-speed data transmission, especially, inter-symbol interference becomes severe because a symbol period is smaller than channel delay spreading. Thus, a complicated receiving technique is required in order to compensate for a distortion due to the inter-symbol interference to restore a transmission signal.

[3] As a modulation technique capable of compensating for a distortion in a multipath channel, orthogonal frequency division multiplexing (OFDM) has been proposed. The basic concept of OFDM converts serially input data streams into N parallel data items, respectively loads the N parallel data items on independent sub-carriers and transmits them so as to increase a data rate. Here, the sub-carriers should be appropriately selected such that orthogonality can be maintained. Since the sub-carriers can be transmitted simultaneously owing to orthogonality, a symbol period in each sub-carrier can be increased by as much as the number of sub-carriers, compared to a technique of sequentially transmitting data using a single sub-carrier.

[4] The OFDM method has bandwidth efficiency and symbol period superior to those of a frequency division multiplexing (FDM) method because OFDM uses sub-carriers having mutual orthogonality. Thus, OFDM has strong endurance against inter-symbol interference over a single sub-carrier modulation method.

[5] In the meantime, in a broadband mobile communication system, a radio channel is frequency-selective and time-varying. This means that a channel in an OFDM system in a frequency base is not identical to the corresponding channel in a time base. Furthermore, when a multiplex transmission antenna is used, symbols transmitted from the multiplex transmission antenna are independently subjected to fading, and then overlapped and received. Accordingly, effective channel estimation is required before demodulation of OFDM symbols because a channel is frequency-selective and time- varying in a mobile OFDM communication system using a multiplex transmission antenna.

[6] In an OFDM system, channel estimation is carried out such that a pilot is inserted into a sub-carrier previously known in a lattice of time base and frequency base. A channel estimation technique can be divided into a pilot-symbol-aided channel estimation technique or a decision-directed channel estimation technique based on the kind of data used for channel estimation. The pilot-symbol-aided channel estimation technique is suitable for a high-speed fading channel. Here, pilots are arranged in consideration of a coherence bandwidth and coherence time of a channel and a decrease in bandwidth efficiency due to use of a pilot tone. The decision-directed channel estimation technique estimates the channel of the next symbol period using detected data. Thus, the decision-directed channel estimation technique is suitable for a fixed low-speed fading channel or a low-speed fading channel having large time correlation.

[7] When the OFDM method is used for cellular mobile communication, wireless LAN, or wireless mobile Internet not used for broadcast, a multiple access method for multiple users is needed. Thus, OFDM is combined with TDMA, FDMA, and CDMA, which are typical multiple access methods

[8] An OFDM/TDMA/FDMA(OFDMA) method allows a user to use part of entire sub-channels and a part of OFDM symbols. This method allocates different numbers of sub-carriers and different numbers of OFDM symbols to respective users based on transfer rates the users require such that resources can be effectively distributed. Particularly, the OFDMA method is advantageous when large- size FFT is used.

[9] IEEE 802.16, which is a typical wireless data communication system standard employing the OFDMA method, is for fixed data communication so that it cannot support terminal mobility. Thus, IEEE 802.16e that will become a mobile data communication system standard is being standardized.

[10] However, since a frame structure used in the OFDMA mode of IEEE 802.16e partially supports terminal mobility, channel estimation is carried out only using a preamble. Though decision-directed channel estimation can be performed for data signals following the preamble, reliability of the decision-directed channel estimation is considerably lower than the reliability of channel estimation using the preamble. Disclosure of Invention Technical Problem

[11] It is an advantage of the present invention to provide a method for constructing a downlink frame for an OFDMA communication system, which enables channel estimation with high reliability.

[12] It is another advantage of the present invention to provide a method for constructing a downlink frame for an OFDMA communication system, which supports the frequency reuse rate of 1 for mobile packet data communication in a mobile data communication system using OFDMA. Technical Solution

[13] In one aspect of the present invention, a method for constructing a downlink frame of a wireless communication system comprises a) arranging a preamble at the head of the downlink frame; b) arranging a plurality of data symbols following the preamble; and c) inserting a pilot symbol between the data symbols over the downlink frame.

[14] The pilot symbol can be arranged between the data symbols at a predetermined period.

[15] The preamble can include a cyclic prefix and a pattern repeated twice in the time domain. The pilot symbol can use the cyclic prefix and one of the two patterns of the preamble.

[16] In another aspect of the present invention, a method for constructing a downlink frame of a wireless communication system supporting mobility using OFDMA, comprises a) arranging a single preamble at the head of the downlink frame; b) arranging a plurality of data symbols following the preamble; and c) repeatedly inserting a plurality of pilot symbols between the data symbols at a predetermined period over the downlink frame.

[17] The method for constructing a downlink frame can further comprise defining a sub-channel for the downlink frame section other than the preamble to minimize and average interference between sub-channels of adjacent cells. Defining a sub-channel comprises i) defining a sub-channel using a basic permutation for the downlink frame section other than the preamble; and ii) varying the physical position of a sub-carrier for each symbol section for the defined sub-channel and a slot.

[18] In another aspect of the present invention, a method for operating a terminal using an OFDMA downlink frame, which includes a preamble arranged at the head of the downlink frame, a plurality of data symbols arranged following the preamble and a pilot symbol arranged between the data symbols, comprises receiving the downlink frame; obtaining frame timing synchronization, symbol timing synchronization and carrier frequency synchronization using the preamble of the received downlink frame, and carrying out automatic gain control using the received preamble; and performing channel estimation using the received preamble or pilot symbol.

[19] An OFDMA downlink frame structure according to the present invention comprises a preamble arranged at the head of the downlink frame; a plurality of data symbols arranged following the preamble; and a pilot symbol arranged between the data symbols. Brief Description of the Drawings

[20] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention;

[21] FIG. 1 illustrates a downlink frame structure of a wireless communication system using OFDMA according to an embodiment of the present invention;

[22] FIG. 2 illustrates the form of a time domain of a preamble of a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention;

[23] FIG. 3 illustrates allocation of sub-carriers to a single slot and a single sub-channel in a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention;

[24] FIG. 4 illustrates allocation of resources to two slots and four sub-channels in a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention;

[25] FIG. 5 illustrates allocation of resources in a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention;

[26] FIGS. 6(a) and 6(b) are graphs showing initial synchronization performances using the preamble of a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention; and

[27] FIG. 7 is a graph showing channel estimation performance using the preamble of a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention. Best Mode for Carrying Out the Invention [28] In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. To clarify the present invention, parts which are not described in the specification are omitted, and parts for which similar descriptions are provided have the same reference numerals.

[29] A downlink frame structure of a wireless communication system using OFDMA and various methods for using the downlink frame structure according to preferred embodiments of the present invention will now be explained.

[30] FIG. 1 illustrates a downlink frame structure of a wireless communication system using OFDMA according to an embodiment of the present invention. Referring to FIG. 1, the first OFDM symbol section of a downlink frame 7 is used for transmitting a preamble 1, and an integer number of data symbols 3 are allocated to the OFDM symbol section following the preamble 1. A plurality of pilot symbols 5 are inserted between the data symbols 3 at a predetermined period over the downlink frame. The size of each pilot symbol is approximately half of the size of the OFDM symbol.

[31] According to an embodiment of the present invention, at least one OFDM symbol can be used for transmitting a control signal such as a common control channel between the preamble 1 and data symbols allocated to data transmission if required.

[32] According to the embodiment of the present invention, the preamble 1 that is the head of the downlink 7 can be used by a receiver of a terminal that receives the downlink frame signal for the following purposes.

[33] (1) Initial synchronization including frame timing synchronization, symbol timing synchronization, and carrier frequency synchronization

[34] (2) Automatic gain control

[35] (3) Cell search and identification, which is carried out in an initial cell search process and a cell search process in case of handoff

[36] (4) Channel estimation

[37] In the meantime, the pilot symbol 5 according to the embodiment of the present invention is inserted between the data symbols 3 at a predetermined period to allow a receiving part to estimate a degree of distortion of the channel. The result of channel estimation carried out using the pilot symbols is combined with the result of channel estimation using the preamble to track a channel variation or obtain channel estimation with higher reliability. In the embodiment of the present invention, one pilot symbol is inserted for three OFDM data symbols. The pilot symbol is used for estimating a channel or tracking a channel variation. As the period of inserting the pilot symbol is decreased, high-speed terminal mobility is supported but the overhead due to the pilot symbol is increased. Thus, it is preferable to minimize the number of pilot symbols within a range in which the maximum available terminal moving speed is supported in the actual system design. [38] FIG. 2 illustrates the form of the time domain of the preamble of the downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention. As shown in FIG. 2, the preamble according to the embodiment of the present invention is constructed such that a pattern (Pc) 13 is repeated twice in the time domain. A cyclic prefix (CP) 11 is arranged before the preamble. The pilot symbol according to the embodiment of the present invention is identical to the signal covering the CP 11 and the first pattern (Pc) 13 among the preamble signal. According to the present invention, each of the patterns can have 1024 samples. A frequency domain pattern P [k] (k=-N /2,-N /2+l,...,N /2-1) IDcell,s FFT FFT FFT for generating the preamble signal and pilot signal is represented as follows.

[39] MathFigure 1 P_IDeeuΛk] = C(l - 2q_ID rtm]), k = 2m - ^ N, , _{w = 0}-l,...,- N. -

^ *1 = C(l -2 ,_s[/« -l]), k = 2m -^, _{m =} rt ^L _{+ Xy} L ^L ₊ 2,...,^ ^l-

PiD_CSnA^k = ^ otherwise [40] Here, C is a constant corresponding to relative power of the preamble signal to transmission power of the data signal following the preamble. C can be

, for example.

ID_cell e {0X...,N_cell -l} represents a number identifying a total of

Ncell cells, and S G {0,1,...,L_W - \} is the number of Walsh code having a length of L w . In addition, k e {-N^ I2 -N_FFT I2 + \,-,N_FFT /2 -1} represents a sub-carrier number. The number of effective sub-carriers used for signal transmission is used and the entire number of sub-carriers including null sub-carriers that are not used for signal transmission and the effective sub-carriers is N ^ι FFT

[41] When the frequency domain pattern P [k] defined by Math Figure 1 is inverse- IDcell,s fast-Fourier-transformed, the pattern is repeated in the time domain. [42] In Math Figure 1, qlD [m] represents a frequency domain preamble pattern given cell,s by Math Figure 2. [43] MathFigure 2

QlDcell,

^~

[44] Here,

Θ represents the exclusive-OR (XOR) bit operator, and m mod _|y represents the ( m mod L_w )th code symbol value of Walsh code, which has a value 0 or 1, a length of and a number S . In addition, Pk represents the kth code symbol value of PN code, which has a value 0 or 1 and a length of

. In the embodiment of the present invention, the PN code is generated by a pseudo- 11 9 random binary sequence (PRBS). An example of a PRBS polynomial is X +X +1. An example of the initial vector of PRBS for preamble modulation is [01010101010].

[45] As described above, the preamble according to the embodiment of the present invention is generated by uniformly arranging the frequency domain pattern, which is composed of a combination of a PN code and a Walsh code to identify different cells, at an interval of two sub-carriers on the basis of a sub-carrier corresponding to a center frequency and OFDM^nodulating the frequency domain pattern. The preamble signal is one of the signals of a pattern appointed for a single OFDM symbol section. A terminal that receives the preamble signal can obtain reliable information for the aforementioned purposes.

[46] For the rest of the downlink frame section other than the preamble, a sub-channel and a slot are defined and used for resource allocation. The sub-channel is a frequency domain allocation unit of a resource allocated to a common control signal or each data signal, and the slot is a time domain allocation unit. The sub-channel allocation should be designed such that an interference signal from an adjacent cell is uniformly distributed to multiple sub-channels. In an embodiment of the present invention, assume that the number of sub-channel definition patterns is 97, the number of effective sub-carriers is 1552, a slot corresponds to three OFDM symbol sections other than pilot symbols, and a sub-channel size corresponds to 16 sub-carriers G to G for 0 15 each OFDM symbol section. An example of the result of designing under these conditions is as follows. [47] MathFigure 3 Carrier (s, m;n) = 97 x (m - \6n) + [P_s (m + m_σ) + ID_cell] mod 97, 0 < s < 96

= 97 x (m - \6n) + ID_cell , s = 96

[48] Here, Carrier (s₇ m; ₇ n) means a sub-carrier number, specifically, the number of the ( G {0,l ..,47} )th sub-carrier of the ( s <≡ {0,1,... , 96} )th sub-channel for the ( n e {0,1,2} )th OFDM symbol section belonging to one slot of the downlink frame. That is, Carrier (s m , n) represents sub-carriers by values 0 through 4655 for a total of 1552x3=4656 sub- carriers belonging to one slot. In Math Figure 3, n is the maximum integer that is not higher than m/16, m ₀ is sub-carrier allocation offset having a value 0 when a slot number is an even number and having a value 48 when the slot number is an odd number. In addition, represents the ( e {0,1,...,96} )th element of the permutation, obtained by cyclic- shifting a basic permutation of Math Figure 4 to the left s times. [49] MathFigure 4 P₀ = [5, 25, 28, 43, 21 , 8, 40, 6, 30, 53, 71 , 64, 29, 48, 46, 36, 83, 27, 38, 93, 77, 94, 82, 22, 13, 65, 34, 73, 74, 79, 7, 35, 78, 2, 10, 50, 56, 86, 42, 16, 80, 12, 60, 9, 45, 31 , 58, 96, 92, 72, 69, 54, 76, 89, 57, 91 , 67, 44, 26, 33, 68, 49, 51 , 61 , 14, 70, 59, 4, 20, 3, 15 75, 84, 32, 63, 24, 23, 18, 90, 62, 19, 95, 87, 47, 41 , 1 1 , 55, 81 17, 85, 37, 88, 52, 66, 39, 1 ]

[50] The sub-carrier allocation pattern minimizes and averages interference with a sub- carrier allocation pattern for sub-channels of different cells. That is, when one sub- channel is selected from 97 sub-channels of each of two different cells, a sub-carrier set having a size of 48 belonging to the two selected sub-channels is repeated in only less than one sub-carrier for one slot. This local interference is averaged over the subchannels according to an error correction code and thus interference between cells is averaged and a frequency reuse rate becomes 1.

[51] FIG. 3 illustrates allocation of sub-carriers to a single slot and a single sub-channel in a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention, and FIG. 4 illustrates allocation of resources to two slots and four sub-channels in a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention.

[52] That is, FIG. 3 illustrates an example of sub-channel allocation for one slot in time and sub-carrier domains according to the definition of sub-channel, and FIG. 4 illustrates an example of allocation of time and frequency resources to a single terminal or terminal group in units of sub-channel and slot in the time and sub-channel domains. The example of FIG. 4 shows the allocation of a resource having two slots in the time domain and four sub-channels in the sub-channel domain.

[53] FIG. 5 illustrates allocation of resources in a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention. Specifically, FIG. 5 shows an example of resource allocation for transmitting data to each terminal or each terminal group over the downlink frame. In FIG. 5, the preamble and pilot symbols are not shown in order to avoid confusion and represent only a logical control signal and traffic allocation.

[54] Referring to FIG. 5, the initial part of the traffic section following the common control channel is used for transmitting frames such as DL-MAP and UL-MAP in case of IEEE 802.16a and IEEE 802.16e, for example, and resource allocation information, and other downlink frame sections are used for transmitting downlink data based on allocation information transmitted to each terminal according to DL-MAP.

[55] In the embodiment of the present invention, the pilot symbols can be selectively used by a base station. The base station can transmit information about whether a frame is constructed using the pilot symbols to the common control channel such that a terminal identifies the pilot symbols.

[56] A method of generating a preamble, a method of generating a pilot symbol, a method of defining a sub-channel, and a method of operating a terminal according to an embodiment of the present invention will now be explained.

[57] First of all, the method of generating a preamble according to an embodiment of the present invention is described.

[58] The frequency domain pattern represented by Math Figure 1 is modulated to generate a preamble constructed such that a pattern is repeated integer times in the time domain. A PN code capable of identifying multiple cells is generated and used for the frequency domain pattern of the preamble, and a Walsh code is periodically modulated to generate a preamble pattern group having strong orthogonality.

[59] A pilot symbol is generated as follows.

[60] The pilot symbol is inserted at an interval of a predetermined number of OFDM symbol sections. The pilot symbol is composed of the pattern CP of the preamble and one of the repeated patterns. Then, the result of channel estimation using the pilot symbol is combined with the result of channel estimation using the preamble to track a channel variation. In addition, the result of channel estimation using the pilot symbol is combined with the result of cell search or channel estimation using the preamble to improve reliability of the cell search or channel estimation result.

[61] The method of defining a sub-channel according to an embodiment of the present invention will now be explained.

[62] The sub-channel is defined using Math Figure 3 for downlink sections other than the preamble to minimize and average interference between adjacent cell sub-channels. Then, the physical position of the sub-carrier is varied for each symbol section for the same sub-channels and slots with respect to the sub-channel defined using the basic permutation of Math Figure 4.

[63] The method of operating a terminal using the preamble and pilot symbol of the downlink frame according to an embodiment of the present invention will now be explained.

[64] Each terminal obtains frame timing synchronization, symbol timing synchronization, and carrier frequency synchronization using the received preamble and then carries out automatic gain control using the preamble. In addition, each terminal performs channel estimation using the preamble or pilot symbol.

[65] A terminal that is newly connected to a wireless communication system or a terminal that is being handed off identifies and searches for an optimum cell using the preamble or pilot symbol. A terminal that is connected to the wireless communication system and normally operated, identifies and measures preamble signals of the cell to which the terminal belongs and adjacent cells, to decide whether handoff is executed.

[66] The results of simulations about initial synchronization and channel estimation using the preamble and pilot symbol according to an embodiment of the present invention will now be explained.

[67] FIGS. 6(a) and 6(b) are graphs showing initial synchronization performances using a preamble of a downlink frame of a wireless communication system using OFDMA according to an embodiment of the present invention.

[68] Referring to FIGS. 6(a) and 6(b), AWGN, ITU-Ped-B, and ITU-Veh-A models were used for the simulation. FIG. 6(a) shows correct preamble detection probability. Auto-correlation of time domain replica was used for preamble detection. The point of time of a frame or initial FFT window is decided by correct preamble detection. As shown in FIG. 6(a), high probability was detected in a very low SNR range. Since SINR at the cell/sector edge is very low under the condition of frequency reuse rate 1 (much lower than OdB), it can be known that the preamble according to the present invention can be used in the conventional mobile communication system with the frequency reuse rate 1.

[69] FIG. 6(b) shows initial frequency estimation after frame detection. The product of conjugate of time domain replica was used for frequency estimation. An estimation range was set to [-1, 1] for the sub-carrier space due to frequency domain allocation of the preamble to sub-carriers. When the estimation range is extended, complexity of cell confirmation in the frequency domain is reduced. Furthermore, stable estimation performance was obtained due to the length of time domain replica in spite of relatively long delay spread. The simulation result shows a very satisfactory estimation result in a very low SNR range. Since SINR at the cell/sector edge is very low under the condition of frequency reuse rate 1 (much lower than OdB), it can be known that the preamble according to the present invention can be used in the conventional mobile communication system with the frequency reuse rate 1.

[70] FIG. 7 is a graph showing channel estimation performance (packet error rate (PER)) using the preamble of the downlink frame of the wireless communication system using OFDMA according to an embodiment of the present invention. In this simulation, it was assumed that the pilot symbol was inserted periodically (to every fourth symbol). ITU-Veh-A model was used for the simulation. Furthermore, PN-Walsh despreading was used in the frequency domain in order to remove noises. In addition, time domain interpolation was used for symbols interposed between pilot symbols. To test cell edge performance under the condition of frequency reuse rate 1, a convolution turbo code (CTC) of a low ratio (1/2 through 1/12) and QPSK were used in the simulation. Here, SINR is frequently reduced lower than OdB at the cell/sector edge under the condition of frequency reuse rate 1. The simulation result shows that low-rate coding is carried out using the preamble structure of the present invention to secure cell/sector edge coverage. When sectors in a cell use a preamble having orthogonal Walsh different from PN, inter-sector interference is reduced and thus channel estimation performance is improved.

[71] As described above, according to the embodiments of the present invention, the pilot symbol with reliability higher than that of a pilot tone is inserted into the downlink frame at a predetermined period so that channel estimation performance and terminal mobility can be improved.

[72] Furthermore, the present invention designs sub-channels such that interference between sub-channels of adjacent cells is minimized and averaged to provide a structure in which the frequency reuse rate becomes 1, thereby maximizing efficiency of frequency resource utilization and supporting high-speed mobile data communication.

[73] While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[74] According to the present invention, channel estimation performance can be improved and frequency resource utilization efficiency can be maximized in a wireless data communication system using OFDMA.

[75]

[76]

Claims

[I] A method for constructing a downlink frame of a wireless communication system, comprising: a) arranging a preamble at the head of the downlink frame; b) arranging a plurality of data symbols following the preamble; and c) inserting a pilot symbol between the data symbols over the downlink frame.

[2] The method as claimed in claim 1, further comprising d) arranging at least one symbol for a common control channel between the preamble and data symbols allocated to data transmission.

[3] The method as claimed in claim 1, wherein the size of the pilot symbol is approximately half of the size of the data symbol.

[4] The method as claimed in claim 1, wherein the pilot symbol is arranged between the data symbols at a predetermined period.

[5] The method as claimed in claim 4, wherein the pilot symbol is inserted for every three data symbols.

[6] The method as claimed in claim 1, wherein the preamble includes a cyclic prefix and a pattern repeated twice in the time domain.

[7] The method as claimed in claim 6, wherein the pilot symbol uses the cyclic prefix and one of the two patterns of the preamble.

[8] The method as claimed in claim 6, wherein the preamble is generated in a manner that a frequency domain pattern composed of a combination of a PN code and a Walsh code is uniformly arranged at an interval of two sub-carriers on the basis of a sub-carrier corresponding to a center frequency and modulated.

[9] The method as claimed in claim 1, wherein the preamble is used for initial synchronization, cell identification, channel estimation and automatic gain control.

[10] The method as claimed in claim 1, wherein the pilot symbol is used for channel estimation and tracking of a channel variation.

[I I] The method as claimed in claim 1, wherein the preamble is generated through a process comprising: modulating a frequency domain pattern in order to generate a preamble having a pattern that is repeated integer times in the time domain; modulating a PN code capable of identifying multiple cells in the frequency domain pattern of the preamble; and additionally modulating a Walsh code in the frequency domain pattern to generate a preamble pattern group having storing orthogonality.

[12] The method as claimed in claim 11, wherein the frequency domain pattern, P [k], is represented as follows. IDcell,s N N P,_D„„,m = C(\ - 2q,_D„_πι{m]), k = 2m -, _m = o,l_;...,-Jϋ-l *] = C0 - 2 .[* -1D. k = rt- , _{m =} rt rt 2..^

Pi_DcenAk] = , otherwise (Here, C is a constant corresponding to relative power of the preamble signal to transmission power of the data signal following the preamble, ∞ceU represents a number identifying a total of ^ cell cells, s represents a number of Walsh code having a length of L _w , and k represents a sub-carrier number.)

[13] A method for constructing a downlink frame of a wireless communication system supporting mobility using OFDMA, comprising: a) arranging a single preamble at the head of the downlink frame; b) arranging a plurality of data symbols following the preamble; and c) repeatedly inserting a plurality of pilot symbols between the data symbols at a predetermined period over the downlink frame.

[14] The method as claimed in claim 13, further comprising d) arranging at least one symbol for a common control channel between the preamble and data symbols allocated to data transmission.

[15] The method as claimed in claim 14, further comprising defining a sub-channel for the downlink frame section other than the preamble to minimize and average interference between sub-channels of adjacent cells, wherein defining a sub-channel comprises: i) defining a sub-channel using a basic permutation for the downlink frame section other than the preamble; and ii) varying the physical position of a sub-carrier for each symbol section for the defined sub-channel and a slot.

[16] The method as claimed in claim 15, wherein the sub-channel is represented as follows. Carrier (s,m n) = 97 x (m -I6n) + [P_s(m + m₀ ) + ID_cell] moά97 ^', O ≤ s < 96 = 91 (m - \6n) + ID_cell, s = 96 (Here,

Carrier (s,m„' ή) means a sub-carrier number of the m-th sub-carrier of the s-th sub-channel for the n-th OFDM symbol section belonging to one slot of the downlink frame, m o is a sub-carrier allocation offset having a value 0 when a slot number is an even number and having a value 48 when the slot number is an odd number, and P (j) s represents the j-th element of the permutation, obtained by cyclic- shifting the basic permutation to the left s times.)

[17] The method as claimed in claim 15 or 16, wherein the basic permutation defining the sub-channel is as follows. P₀ = [5, 25, 28, 43, 21 , 8, 40, 6, 30, 53, 71 , 64, 29, 48, 46, 36, 83, 27, 38, 93, 77, 94, 82, 22, 13, 65, 34, 73, 74, 79, 7, 35, 78, 2, 10, 50, 56, 86, 42, 16, 80, 12, 60, 9, 45, 31 , 58, 96, 92, 72, 69, 54, 76, 89, 57, 91 , 67, 44, 26, 33, 68, 49, 51 , 61 , 14, 70, 59, 4, 20, 3, 15 75, 84, 32, 63, 24, 23, 18, 90, 62, 19, 95, 87, 47, 41 , 1 1 , 55, 81 17, 85, 37, 88, 52, 66, 39, 1 ]

[18] A method for operating a terminal using an OFDMA downlink frame including a preamble arranged at the head of the downlink frame, a plurality of data symbols arranged following the preamble and a pilot symbol arranged between the data symbols, the method comprising: receiving the downlink frame; obtaining frame timing synchronization, symbol timing synchronization, and carrier frequency synchronization using the preamble of the received downlink frame, and carrying out automatic gain control using the received preamble; and performing channel estimation using the received preamble or pilot symbol. [19] The method as claimed in claim 18, further comprising deciding whether handoff is executed using the preamble or pilot symbol received from cells adjacent to a cell to which the terminal belongs. [20] An OFDMA downlink frame structure comprising: a preamble arranged at the head of the downlink frame; a plurality of data symbols arranged following the preamble; and a pilot symbol arranged between the data symbols. [21] The OFDMA downlink frame structure as claimed in claim 20, wherein the size of the pilot symbol is approximately half of the size of the data symbol. [22] The OFDMA downlink frame structure as claimed in claim 20, wherein the pilot symbol is arranged between the data symbols at a predetermined period. [23] The OFDMA downlink frame structure as claimed in claim 20, wherein the preamble includes a cyclic prefix and a pattern repeated twice in the time domain. [24] The OFDMA downlink frame structure as claimed in claim 23, wherein the pilot symbol uses the cyclic prefix and one of the two patterns of the preamble. [25] The OFDMA downlink frame structure as claimed in claim 23, wherein the preamble is generated in a manner that a frequency domain pattern composed of a combination of a PN code and a Walsh code is uniformly arranged at an interval of two sub-carriers on the basis of a sub-carrier corresponding to a center frequency and modulated.