WO2005053198A2

WO2005053198A2 - Method for allocating pilot subcarriers, method and device for transmitting and receiving pilot subcarriers in orthogonal frequency division multiplex system

Info

Publication number: WO2005053198A2
Application number: PCT/KR2004/001836
Authority: WO
Inventors: Yu-Ro Lee; Choong-Il Yeh; Hyoung-Soo Lim; Jong-Ee Oh; Dong-Seung Kwon
Original assignee: Electronics And Telecommunications Research Institute; Samsung Electronics Co., Ltd.; Sk Telecom Co., Ltd; Kt Corporation; Ktfreetel Co., Ltd.; Hanaro Telecom, Inc.
Priority date: 2003-11-28
Filing date: 2004-07-22
Publication date: 2005-06-09
Also published as: KR20050051865A; KR100600672B1; WO2005053198A3

Abstract

Disclosed is a method for allocating pilot subcarriers, and a method and device for transmitting and receiving pilot subcarriers in an OFDMA system. The method for allocating pilot subcarriers partitions the total bandwidth of the downlink channel into subcarrier groups having a predetermined number of subcarriers, allocates pilots to the subcarriers of the subcarrier group according to specific pilot patterns, and cycles positions of the inter-cell pilots within a specific group by a specific cycle to exchange the determined pilots according to a predetermined reference. Accordingly, pilot collision is reduced to improve accuracy of channel estimation, a number of distinguishable pilots is increased to easily perform cell planning, and a system is deployed without special cell planning.

Description

METHOD FOR ALLOCATING PILOT SUBCARRIERS, METHOD AND DEVICE FOR TRANSMITTING AND RECEIVING PILOT SUBCARRIERS IN ORTHOGONAL FREQUENCY DIVISION MULTIPLEX SYSTEM

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korea Patent Application No. 2003-85530 filed on November 28, 2003 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention The present invention relates to a method for allocating pilot subcarriers, and a method and device for transmitting and receiving pilot subcarriers in an OFDMA (orthogonal frequency division multiple access) system. More specifically, the present invention relates to a method for allocating pilot subcarriers of a base station for improving frequency reuse rates in an OFDM (orthogonal frequency division multiplex) system.

(b) Description of the Related Art In order to realize a BMWS (broadband multimedia wireless service)

system which enables reliability of high speed and large-capacity services,

OFDM transmission methods for transmitting signals with high data rates in the

millimeter wave bandwidths of from several to several tens GHz have been

used. The OFDM method represents a frequency multiplex system for

perform an IFFT (inverse fast Fourier transform) on the data to be transmitted,

dividing available bandwidths into a plurality of subcarriers, transmitting them, allowing an OFDM receiver to perform a FFT (fast Fourier transform) on the transmitted subcarriers, and converting them into original data, and it also represents a multiplex communication system for providing a specific orthogonal condition between subcarrier frequencies, and separating respective subcarriers from the receiver irrespective of spectral superposition. FIG. 1 shows a block diagram of a conventional OFDM system, and configuration and operation of a transmitter and a receiver of the OFDM system will be described with reference to FIG. 1. An OFDM transmitter 10 comprises a serial/parallel converter 2, a modulator 4, an IFFT (inverse fast Fourier transform) unit 6, a parallel/serial converter 8, and a D/A (digital/analog) converter and filter 12. The serial/parallel converter 2 converts high-speed transmit data received in series into low-speed parallel data. The modulator 4 modulates the data parallel-converted by the serial/parallel converter 2 through a predetermined modulation method. The IFFT unit 6 transforms the data modulated by the modulator 4 into signals on the time axis, and outputs results. The parallel/serial converter 8 converts the parallel data output by the

IFFT unit 6 into serial signals. The D/A converter and filter 12 converts the serial signals output by the parallel/serial converter 8 into analog signals, filters the analog signals, and outputs filtered results to the receiver through an RF (radio frequency) terminal. That is, the data symbols output by the serial/parallel converter 2 are modulated by corresponding carriers, OFDM symbols are configured through the IFFT unit 6, and finally input to the RF terminal and transmitted to channels. Also, the OFDM symbols are transmitted per symbol unity, but they are influenced by previous symbols while transmitted through a multipath channel.

In order to prevent OFDM inter-symbol interference, a CP (cyclic prefix) is

provided to the parallel/serial converter 8 so that the CP may be additionally

inserted between the adjacent OFDM symbols by establishing a length of the

CP to be greater than the maximum delay spreading of a channel.

Next, the OFDM receiver 20 comprises an A/D (analog/digital)

converter and filter 29, a serial/parallel converter 28, an FFT (fast Fourier

transform) unit 26, a channel estimator 23, a demodulator 24, and a

parallel/serial converter 22. The A/D converter and filter 29 receives the analog signals from the

transmitter 10 through the RF terminal, filters the received signals, and

converts them into digital signals. The serial/parallel converter 28 eliminates the CP inserted into the

digital data converted by the A/D converter and filter 29, and converts them into

parallel signals. The FFT unit 26 performs an FFT on the time-axis data of the parallel

signals converted by the serial/parallel converter 28, and generates frequency-

axis data signals. The channel estimator 23 estimates channel estimates of the

frequency-axis data signals transformed by the FFT unit 26.

The demodulator 24 uses the channel estimates found by the channel

estimator 23 and demodulates the data.

The parallel/serial converter 22 converts the parallel signals

demodulated by the demodulator 24 into serial signals. Since the above-configured OFDM system parallels a predetermined

data sequence by the number of subcarriers used for modulation, and modulates the corresponding subcarriers by using the parallel data, the total data rates maintain the original high speed, and the symbol period of the subchannels including the respective subcarriers is increased by the number of subcarriers. Therefore, the frequency-selective multipath fading channel is approximated as a frequency-nonselective channel with respect to each subchannel, and corresponding distortions can be easily compensated by using a simple receiver. As described above, the OFDM method has an advantage of reducing complexity of the receiver in the broadband transmission with severe frequency selective fading, and in order to reduce the complexity, the OFDM method uses the CP and eliminates influences caused by delay spreading. Also, when modifying a channel for synchronization detection, the OFDM method inserts a pilot as shown in FIG. 2. FIG. 2 shows an exemplified case of inserting a pilot following more than a ratio of satisfying the Nyquist sampling theorem in the conventional OFDM system. In order to accurately estimate the channel, an insertion period of the pilot is determined in consideration of terminal mobility on the time axis, and is determined in consideration of delay spreading on the frequency axis. The insertion period is given in Equations 1 and 2. Equation 1

' 2τ„άf Equation 2 D < ¹ ' 2d_fT_s where τ_m is maximum delay spreading, Δ is a subcarrier interval, d_f

is a Doppler frequency, and T_s is and OFDM symbol length. FIG. 3 shows a conventional pilot inserting method in the OFDM system, showing a pilot structure of the IEEE 802.16a. As shown, the pilots are inserted with respect to the time axis and the frequency axis. This method allows channel estimation for a single cell, but the terminal provided on the border of a cell generates errors of channel estimation because of inter-cell pilot collision since a pilot is provided as the same position as that of a pilot of an adjacent cell.

SUMMARY OF THE INVENTION It is an advantage of the present invention to provide a method for allocating pilot subcarriers, and a method and device for transmitting and receiving pilot subcarriers in an OFDMA system for allowing a terminal on the border of a cell to estimate channels when a base station is provided on an adjacent cell of the cell in which the terminal belongs, by maintaining intervals of the pilots and allocating specific pilot patterns to the respective base stations so as to allow channel estimation. To achieve the advantage, a method for allocating pilot subcarriers to corresponding downlink channels in an OFDMA system is provided. In one aspect of the present invention, a method for allocating pilot subcarriers to a downlink channel in an OFDMA system comprises: i) partitioning a total bandwidth of the downlink channel into a plurality of

subcarrier groups having a predetermined number of subcarriers; ii) allocating

a pilot according to a specific pilot pattern to each subcarrier of the subcarrier

groups; and iii) cycling positions of pilots between cells within a specific group

from among the subcarrier groups, by a predetermined period.

The predetermined period in iii) is determined by an environment of

the downlink channel caused by mobility and delay spreading and complexity of the terminal.

The step of ii) comprises allocating the pilots so that the pilots may

have a rule according to the subsequent equation, and generating pilot

patterns specific to the subcarrier groups:

N_p(gn, en, sgn) = N^*sgn+(cn+sgn*G(gn))modN where N is a prime number (e.g., N=3, 5, 7, 11 , 13, 17, ...), N_p is a

subcarrier number for pilot allocation, sgn is a subcarrier group number (sgn=0,

1 , 2 K/N-1 , and K is a number of subcarriers of the total bandwidth), G of

G(gn) is a group, gn of G(gn) is a group number, and en is a cell number within a subcarrier group.

The pilot patterns are formed to have a partitioning feature with respect

to the frequency axis, and a continuity feature with respect to the time axis. The specific pilot patterns formed per unity of subcarriers are

respectively combined, and the number of pilot patterns allocable per unity of

subcarriers is increased.

The step of iii) follows the subsequent equation:

N_p(gn, en, sn, sgn) = (cn+sgn*G(gn)+(sn mod pls)*ss_(Sn mo_{d P}is₎)modN where sn is a number of symbols, pis is a number of cycled cells, and

ss is a number of subcarriers according to the cycled pilot positions. The positions of the cycled pilots of iii) are uniformly provi according to the subsequent equation: sn mod pis =0 ss₀=0 sn mod pis =1 ssι=|^~N/ / ] sn mod pis =2 ss₂= [^~(N - ss_l ) /(pis - l ] pls-2 sn mod pis =pls-1 ss_pι_s-ι= (N- ss_n)/2 n=l where sn is a number of symbols, pis is a number of cycled cells, ss is a number of subcarriers according to the cycled pilot positions. The method further comprises: iv) exchanging the position of the ir symbol pilots within a specific cell cycled in iii) according to a specific refere The step of iv) comprises exchanging the pilots so that the | provided in the center of the specific subcarrier group may be provided nea a symbol on which frequency domain interpolation will be performed. In another aspect of the present invention, a method for a transm to transmit transmit data into which a pilot is inserted to a receiver throuc downlink channel in an OFDMA system, comprises: a) determining position pilots which correspond to respective cells belonging to a specific subca group by a specific reference, wherein the positions of pilots are cy between a predetermined number of adjacent cells belonging to the spe subcarrier group by a specific period; b) transmitting information accordin the determined positions of pilots, and inserting a pilot into the transmit ι based on the determined positions of pilots; and c) transmitting the tran data into which the pilot is inserted to the receiver. The positions of the pilots determined in a) are exchanged so th specific pilot provided in the center of the specific subcarrier group provided nearest the symbol to be interpolated. The step of b) comprises: d) respectively converting in parallel data and pilots according to the data and the number of pilot subcarriers; e) modulating the data and the pilots converted in parallel in d); and f) inserting the pilots modulated in e) into the positions of pilots determined in a), inserting the data into residual positions, performing an IFFT on them, and converting them into time domain signals. The step of c) comprises: adding a cyclic prefix to the time domain signals converted in f), and converting them into serial signals; and converting the serial signals into analog signals, filtering the analog signals, and transmitting the filtered signals to the receiver. In still another aspect of the present invention, a method for receiving data with an inserted pilot from a transmitter through a downlink channel in an OFDMA system comprises: a) receiving information according to a position of a pilot from the transmitter, and detecting the position of the pilot, wherein the position of the pilot is cycled between a predetermined number of adjacent cells belonging to a specific subcarrier group by a predetermined period, and is determined by exchanging the cycled pilots by a specific reference; b) eliminating the pilot from the transmit data based on the detected position of the pilot; and c) demodulating the pilot-eliminated transmit data, and receiving them. The step of b) comprises: d) filtering the data transmitted by the transmitter, and converting them into digital signals; e) eliminating a CP (of the converted digital signals, and converting them into parallel signals; f) performing an FFT on the parallel signals, and converting them into frequency domain signals; and g) separating pilots and data from the frequency domain signals converted in f) according to the position of the pilot detected in a). The step of c) comprises: h) using the pilot separated in g), and estimating a channel; i) using a channel estimate estimated in h), and demodulating data; and j) converting the demodulated parallel data into serial data. The step of i) comprises: shifting the pilot to the time domain, inserting the pilot into the frequency domain, and estimating a channel. In further another aspect of the present invention, a transmitter for transmitting pilot-inserted transmit data to a receiver through a downlink channel in an OFDMA system comprises: a serial/parallel converter for converting data and pilots into parallel data according to a number of pilots and data subcarriers; a modulator for modulating the data and the pilots parallel- converted by the serial/parallel converter; a pilot pattern controller for determining the position of the pilot according to a specific reference, and transmitting information according to the determined position of the pilot to the receiver, wherein the position of the pilot is cycled between a predetermined number of adjacent cells belonging to a specific subcarrier group by a predetermined period, and is determined by exchanging specific pilots within the cell by the specific reference; a multiplexer for inserting a pilot into the position of the pilot, inserting data into residual positions, and multiplexing them; an IFFT unit for transforming the multiplexed data and pilots into time domain signals, and outputting results; a parallel/serial converter for adding a cyclic prefix to the signals output by the IFFT unit, and converting them into serial signals; and a digital/analog converter and filter for converting the serial signals output by the parallel/serial converter into analog signals, filtering the analog signals, and transmitting filtered results to the receiver through an RF terminal.

In still yet another aspect of the present invention, a receiver for

receiving pilot-inserted transmit data from a transmitter through a downlink

channel in an OFDMA system comprises: an A/D converter and filter for

converting the data transmitted by the transmitter into digital signals; a

serial/parallel converter for eliminating a cyclic prefix from the digital signals,

and converting them into parallel signals; an FFT unit for performing an FFT on

the parallel signals, and outputting frequency domain signals; a pilot pattern

controller for receiving information according to the pilot position transmitted by

the transmitter, and detecting the pilot position wherein the position of the pilot is cycled between a predetermined number of adjacent cells belonging to a

specific subcarrier group by a predetermined period, and is determined by

exchanging specific pilots within the cell by the specific reference; a

demultiplexer for separating pilots and data from the detected position of the pilot; a channel estimator for using the separated pilots and estimating a

channel of the separated data; a demodulator for using the estimated channel

estimate and demodulating the separated data; and a parallel/serial converter

for converting the demodulated parallel data into serial data.

BRIEF DESCRIPTION OF THE DRAWINGS

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: FIG. 1 shows a block diagram of a conventional OFDM system; FIG. 2 shows an exemplified case of inserting a pilot in the conventional OFDM system;

FIG. 3 shows a conventional pilot inserting method in the OFDM

system;

FIG. 4 shows a subcarrier partitioning structure according to a first

preferred embodiment; FIG. 5 shows a pilot pattern according to a second preferred

embodiment of the present invention;

FIG. 6 shows a pilot pattern according to a third preferred embodiment

of the present invention; FIG. 7 shows a pilot pattern according to a fourth preferred embodiment of the present invention;

FIG. 8 shows a pilot pattern according to a fifth preferred embodiment

of the present invention; FIG. 9 shows a pilot pattern according to a sixth preferred embodiment

of the present invention; and FIG. 10 shows a block diagram of a transmitter and a receiver of an

OFDM system according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 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. FIG. 4 shows a subcarrier partitioning structure according to a first

preferred embodiment.

An OFDM system divides a downlink (a usable bandwidth) into a

plurality of subcarriers, and transmits them. As shown in FIG. 4, the OFDM

system separates a total usable bandwidth having K subcarriers into K N

subcarrier groups, each group having N subcarriers, and the number of

subcarrier groups being referred to as "sgn" hereinafter. Each subcarrier group

includes a plurality of adjacent cells.

Each base station allocates pilots as given in Equation 3 Equation 3

N_p(gn, en, sgn) = N^*sgn+(cn+sgn^*G(gn))modN where N is a prime number (N=3, 5, 7, 11 , 13, 17, ...), N_p is a

1 , 2 K N-1 , and K is a number of subcarriers of the total bandwidth), G of

N*N pilot patterns are formed by combining the N pilot patterns

generated by Equation 3. That is, accuracy of channel estimation can be

enhanced since collision of pilots can be reduced by differentiating the pilot

patterns of the adjacent cells and increasing the number of distinguishable

pilots.

FIG. 5 shows pilot patterns allocated by the base station according to a

second preferred embodiment of the present invention when G(gn) =4 and

cn=2 according to Equation 3. The base station generates pilot patterns of Table 1 when G(gn) =4

and cn=2. Table 1

Since the pilot patterns are the same in the symbols within a frame, fixed pilot subcarriers can be used. However, in order to perform channel delay spreading required by the system, the pilots are inserted at intervals of a short time, and since increase of a pilot ratio per symbol functions as strong interference boosted in the data region of the adjacent cell, the pilot ratio per symbol is to be reduced. In order to reduce the pilot ratio and accommodate the channel delay spreading, inter-cell pilot position within a group is cycled as given in Equation 4. The cycling period is determined by channel environments caused by the delay spreading and the complexity of the terminal. Equation 4 N_p(gn, en, sn, sgn) = (cn+sgn*G(gn)+(sn mod pls)^*sS(_Sn m_od pi_s))modN where sn is a number of symbols, pis is a number of cycled cells, and ss is a number of subcarriers according to the cycled pilot positions. Since the pilots are to be uniformly arranged during the ss symbols regarding the relation between ss and pis, ss is given as Equation 5. Equation 5 sn mod pis =0 ss₀=0

sn mod pis =2 ss^N-^ ^-l)]

sn mod pis =pls-1 ss. pls-1 _n)/2

FIG. 6 shows a pilot pattern according to a third preferred embodiment of the present invention when cycling between two cells is performed in the case of Ν=7 from Equations 3 to 5. In order to generate an equal interval of the pilot between the two symbols, it is given that pls=2 and ssι=4 since

when the cycling between the two cells is performed in the case of N=7. Positions of the pilots in this instance are given in Table 2. Table 2

FIG. 7 shows a pilot pattern according to a fourth preferred embodiment of the present invention when cycling among three cells is performed in the case of N=7 from Equations 3 to 5. When the cycling among three cells is performed in the case of N=7, it is given that pls=3, ssι=3 from sn=1 , and ss₂=2 from sn=2. Positions of the pilots in this instance are given in Table 3. Table 3

In a like manner, when a cycling among four cells is performed in the case of N=7 based on Equations 3 to 5, it is given that pls=4, ss-ι=2 from sn=1 , ss₂=2 from sn=2, and ss₃=2 from sn=3. Arrangement of the pilots is not restricted to the above description. For example, the pilots can be reversely arranged so that pls=2, ss-ι=2 from sn=1 , and ss₂=3 from sn=2 in the cycling among three cells. Table 4 shows relations between en and sn in the cycling according to pilot positions given that N=7. Table 4

The cycling according to the pilot positions exchanges the pilot positions and improves performance of channel estimation of the receiver. FIG. 8 shows a pilot pattern according to a fifth preferred embodiment of the present invention. The positions of the pilots are determined as shown in FIG. 8 when the cycling according to the pilot positions among four cells given that N=11 is performed. The relations between sn and cn are given as Table 5 when N=1 and the cycling according to the pilot positions of a four symbol unit is performed. Table 5

The pilots determined as shown in FIG. 8 reduce complexity of channel estimation of the receiver by modifying the positions of the pilots. FIG. 9 shows a pilot pattern according to a sixth preferred embodiment of the present invention when cycling caused by the pilot position among four cells is performed and the pilot position is exchanged between the symbols given that N=11. In detail, a pilot arrangement of (sn mod pis = 2) is used for the symbol of (sn mod pls=1) in the case of (sn mod pls=1), and a pilot arrangement of (sn mod pis = 1) is used for the symbol of (sn mod pls=2) in the case of (sn mod pls=2). In a like manner, a pilot arrangement of (sn mod pis = 4) is used for the symbol of (sn mod pls=3) in the case of (sn mod pls=3), and a pilot arrangement of (sn mod pis = 3) is used for the symbol of (sn mod pls=4) in the case of (sn mod pls=4). Referring to FIGs. 8 and 9, exchanges of the pilot positions of the odd symbols and the even symbols within pis of FIG. 8 produce FIG. 9. Table 6 shows relations between cn and sn when the pilots between the symbols are exchanged so as to increase performance of channel estimation. Table 6

As shown in FIG. 9, the pilots after the exchanges of pilots are positioned nearest the symbols to be interpolated by the pilot positioned within the subcarrier group when frequency domain interpolation is performed, and accordingly, the performance of channel estimation is better than that of FIG. 8. FIG. 10 shows a block diagram of a transmitter and a receiver of an OFDM system according to a preferred embodiment of the present invention. As shown in FIG. 10, the OFDM transmitter 100 comprises a serial/parallel converter 110, a modulator 120, a pilot pattern controller 130, a multiplexer 140, an IFFT unit 150, a parallel/serial converter 160, and a D/A converter and filter 170. The serial/parallel converter 110 converts high-speed transmit data received in series into low-speed parallel data, and converts pilots received in series into parallel data. The modulator 120 modulates the parallel-converted and input data

and pilots according to a predefined modulation method.

The QAM method is used for the data by a QAM modulator 124, and

the BPSK or QPSK modulation method is used for the pilots by a BPSK or

QPSK modulator 122 in FIG. 10, but without being restricted to this, one of 1-

bit BPSK, 2-bit QPSK, 4-bit 16QAM, 6-bit 64QAM, and 8-bit 256QAM can be

used for the modulation method used by the subcarriers according to an

amount of data transmittable by a single subcarrier in the IEEE802.1 1a.

The pilot pattern controller 130 maintains intervals of the pilots,

allocates specific pilot patterns to the respective base stations, and generates

positions of the pilots.

The multiplexer 140 inserts modulated pilots according to positions of the pilots determined by the pilot pattern controller 130, inserts modulated data

into residual positions, and outputs them as a single signal. The IFFT unit 150 performs an IFFT on the signals output by the

multiplexer 140 into temporal signals to thus perform an OFDM conversion

output. The data output by the IFFT unit 150 are defined to be OFDM symbols,

and in order to prevent OFDM inter-symbol interference output by the IFFT unit

150, a CP is provided to the parallel/serial converter 160 so that the CP may be

additionally inserted between the adjacent OFDM symbols by establishing a

length of the CP to be greater than the maximum delay spreading of a channel.

The parallel/serial converter 160 converts the OFDM symbols of the

parallel signals to which the CP is added into serial signals, and outputs the

serial signals. The D/A converter and filter 170 converts the digital signals converted

as serial signals into analog signals, filters the analog signals, and outputs filtered results to the receiver 200 through the RF terminal. Next, the OFDM receiver 200 comprises an A/D converter and filter 210, a serial/parallel converter 220, an FFT unit 230, a pilot pattern controller 240, a demultiplexer 250, a channel estimator 260, a demodulator 270, and a parallel/serial converter 280. The A/D converter and filter 210 receives the analog signals with the inserted CP from the transmitter 10 through the RF terminal, filters the received signals, and converts them into digital signals. The serial/parallel converter 220 eliminates the CP from the OFDM symbols, and converts them into parallel signals. The FFT unit 230 performs an FFT on the parallel signals converted by the serial/parallel converter 220, and converts time domain signals into frequency domain OFDM symbols. The pilot pattern controller 240 maintains the intervals of the pilots, allocates specific pilot patterns to the respective base stations, and generates position of the pilots so as to demultiplex the received FFT-performed signals into pilots and data. The demultiplexer 250 receives frequency domain OFDM symbols output by the FFT unit 230, separates the OFDM symbols into data and pilots according to positions of the generated pilots, and outputs results. The channel estimator 260 receives the pilots output by the demultiplexer 250, and estimates channels of the received signals. In this instance, the channel estimator 260 shifts the pilots in the time domain and interpolates them in the frequency domain so as to estimate the channels of the received signals. The method for estimating the channel of the received signals is not restricted to this. For example, general channel estimation methods such as a one-dimensional (a frequency domain) interpolation method, a one-dimensional (a time domain) + a one-dimensional (a frequency domain) interpolation method, and a two-dimensional (time and frequency domains) interpolation method can also be used. The demodulator 270 uses the channel estimates, uses the QAM method which corresponds to the data modulation method of the modulator 120 of the transmitter 100, and demodulates the data. The parallel/serial converter 280 converts the demodulated parallel signals into serial signals. According to the preferred embodiments of the present invention, the distinguishable cells have their own specific pilot patterns, and by exchanging the positions of the inter-symbol pilots, collision between the pilots is reduced to thereby minimize interference between adjacent cells of the cell in which the terminal belongs, allow the terminal provided on the border of the cell to estimate a channel, and enhance the accuracy of channel estimation. Also, the adjacent cells are distinguishable in the broadband, cell planning is easily performed by increasing the number of distinguishable pilots, and deploy of the system is possible without special cell planning. 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.

Claims

WHAT IS CLAIMED IS:

1. A method for allocating pilot subcarriers to a downlink channel in an OFDMA (orthogonal frequency division multiple access) system, comprising: i) partitioning a total bandwidth of the downlink channel into a plurality of subcarrier groups having a predetermined number of subcarriers; ii) allocating a pilot according to a specific pilot pattern to each subcarrier of the subcarrier groups; and iii) cycling positions of pilots between cells within a specific group from among the subcarrier groups, by a predetermined period. 2. The method of claim 1 , wherein the step of ii) comprises allocating the pilots so that the pilots may have a rule according to the subsequent equation, and generating pilot patterns specific to the subcarrier groups: N_p(gn, cn, sgn) = N^*sgn+(cn+sgn^*G(gn))modN where N is a prime number (e.g., N=3, 5, 7, 11 , 13, 17, ...), N_p is a subcarrier number for pilot allocation, sgn is a subcarrier group number (sgn=0,

1 ,

2, ..., K/N-1 , and K is a number of subcarriers of the total bandwidth), G of

G(gn) is a group, gn of G(gn) is a group number, and cn is a cell number within a subcarrier group.

3. The method of claim 1 , wherein the pilot patterns are formed to have a partitioning feature with respect to the frequency axis, and a continuity feature with respect to the time axis.

4. The method of claim 1 , wherein the specific pilot patterns formed per

unity of subcarriers are respectively combined, and the number of pilot patterns

allocable per unity of subcarriers is increased.

5. The method of claim 1 , wherein the step of iii) follows the

subsequent equation: N_p(gn, cn, sn, sgn) = (cn+sgn^*G(gn)+(sn mod pls)^*sS(_Sn mod pis))modN where sn is a number of symbols, pis is a number of cycled cells, and ss is a number of subcarriers according to the cycled pilot positions.

6. The method of claim 1 , wherein the positions of the cycled pilots of iii) are uniformly provided according to the subsequent equation: sn mod pis =0 sso=0

sn mod pis =2 ss₂= |^~(N - ss_x ) /(pis - 1)] p -2 sn mod pis =pls-1 ss_pι_s-1= (N- ss_n)/2 H=l where sn is a number of symbols, pis is a number of cycled cells, and ss is a number of subcarriers according to the cycled pilot positions.

7. The method of one of claims 1 to 6, wherein the predetermined period in iii) is determined by an environment of the downlink channel caused by mobility and delay spreading and complexity of the terminal.

8. The method of claim 1 , further comprising: iv) exchanging the position of the inter-symbol pilots within a specific cell cycled in iii) according to a specific reference.

9. The method of claim 8, wherein the step of iv) comprises exchanging the pilots so that the pilot provided in the center of the specific subcarrier group may be provided nearest a symbol on which frequency domain interpolation will be performed.

10. A method for a transmitter to transmit transmit data into which a pilot is inserted to a receiver through a downlink channel in an OFDMA (orthogonal frequency division multiple access) system, comprising: a) determining positions of pilots which correspond to respective cells belonging to a specific subcarrier group by a specific reference, wherein the positions of pilots are cycled between a predetermined number of adjacent cells belonging to the specific subcarrier group by a specific period; b) transmitting information according to the determined positions of pilots, and inserting a pilot into the transmit data based on the determined positions of pilots; and c) transmitting the transmit data into which the pilot is inserted to the receiver.

11. The method of claim 10, wherein the positions of the pilots determined in a) are exchanged so that a specific pilot provided in the center of the specific subcarrier group may be provided nearest the symbol to be interpolated.

12. The method of claim 10, wherein the step of b) comprises: d) respectively converting in parallel data and pilots according to the data and the number of pilot subcarriers; e) modulating the data and the pilots converted in parallel in d); and f) inserting the pilots modulated in e) into the positions of pilots determined in a), inserting the data into residual positions, performing an IFFT (inverse fast Fourier transform) on them, and converting them into time domain signals.

13. The method of claim 12, wherein the step of c) comprises: adding a cyclic prefix to the time domain signals converted in f), and converting them into serial signals; and converting the serial signals into analog signals, filtering the analog signals, and transmitting the filtered signals to the receiver.

14. A method for receiving data with an inserted pilot from a transmitter through a downlink channel in an OFDMA (orthogonal frequency division multiple access) system, comprising: a) receiving information according to a position of a pilot from the transmitter, and detecting the position of the pilot, wherein the position of the pilot is cycled between a predetermined number of adjacent cells belonging to a specific subcarrier group by a predetermined period, and is determined by exchanging the cycled pilots by a specific reference; b) eliminating the pilot from the transmit data based on the detected position of the pilot; and c) demodulating the pilot-eliminated transmit data, and receiving them.

15. The method of claim 14, wherein the step of b) comprises: d) filtering the data transmitted by the transmitter, and converting them into digital signals; e) eliminating a CP (cyclic prefix) of the converted digital signals, and converting them into parallel signals; f) performing an FFT (fast Fourier transform) on the parallel signals, and converting them into frequency domain signals; and g) separating pilots and data from the frequency domain signals

converted in f) according to the position of the pilot detected in a).

16. The method of claim 15, wherein the step of c) comprises: h) using the pilot separated in g), and estimating a channel; i) using a channel estimate estimated in h), and demodulating data;

and j) converting the demodulated parallel data into serial data.

17. The method of claim 16, wherein the step of i) comprises: shifting

the pilot to the time domain, inserting the pilot into the frequency domain, and estimating a channel.

18. A transmitter for transmitting pilot-inserted transmit data to a

receiver through a downlink channel in an OFDMA (orthogonal frequency

division multiple access) system, comprising: a serial/parallel converter for converting data and pilots into parallel

data according to a number of pilots and data subcarriers; a modulator for modulating the data and the pilots parallel-converted

by the serial/parallel converter; a pilot pattern controller for determining the position of the pilot

according to a specific reference, and transmitting information according to the

determined position of the pilot to the receiver, wherein the position of the pilot

is cycled between a predetermined number of adjacent cells belonging to a specific subcarrier group by a predetermined period, and is determined by

exchanging specific pilots within the cell by the specific reference; a multiplexer for inserting a pilot into the position of the pilot, inserting data into residual positions, and multiplexing them; an IFFT (inverse fast Fourier transform) unit for transforming the

multiplexed data and pilots into time domain signals, and outputting results; a parallel/serial converter for adding a cyclic prefix to the signals output

by the IFFT unit, and converting them into serial signals; and a digital/analog converter and filter for converting the serial signals

output by the parallel/serial converter into analog signals, filtering the analog

signals, and transmitting filtered results to the receiver through an RF (radio

frequency) terminal.

19. The transmitter of claim 18, wherein the downlink channel is

partitioned into a plurality of subcarrier groups having a predetermined number of subcarriers.

20. The transmitter of claim 19, wherein the adjacent cells within the subcarrier group have specific pilot patterns.

21. The transmitter of claim 20, wherein the position of the pilot allocated based on the pilot pattern is cycled in the specific subcarrier group according to the subsequent equation: N_P(gn, cn, sn, sgn) = (cn+sgn^*G(gn)+(sn mod pls)*ss_(sn m_od pi_s))modN where sn is a number of symbols, pis is a number of cycled cells, and ss is a number of subcarriers according to the cycled pilot positions.

22. The transmitter of claim 18, wherein the position of the cycled and determined pilot is exchanged according to a specific reference so that the pilot provided in the center of the specific subcarrier group may be provided nearest the symbol on which frequency domain interpolation will be performed.

23. A receiver for receiving pilot-inserted transmit data from a transmitter through a downlink channel in an OFDMA (orthogonal frequency division multiple access) system, comprising: an A/D (analog/digital) converter and filter for converting the data transmitted by the transmitter into digital signals; a serial/parallel converter for eliminating a cyclic prefix from the digital signals, and converting them into parallel signals; an FFT (fast Fourier transform) unit for performing an FFT on the parallel signals, and outputting frequency domain signals; a pilot pattern controller for receiving information according to the pilot position transmitted by the transmitter, and detecting the pilot position wherein the position of the pilot is cycled between a predetermined number of adjacent cells belonging to a specific subcarrier group by a predetermined period, and is determined by exchanging specific pilots within the cell by the specific reference; a demultiplexer for separating pilots and data from the detected position of the pilot; a channel estimator for using the separated pilots and estimating a channel of the separated data; a demodulator for using the estimated channel estimate and demodulating the separated data; and a parallel/serial converter for converting the demodulated parallel data into serial data.

24. The receiver of claim 23, wherein the channel estimator shifts the pilot separated by the demultiplexer to the time domain, inserts the pilot into the frequency domain, and estimates a channel.