WO2005050887A2

WO2005050887A2 - Method for configuring uplink frame in ofdma-based wireless communication system

Info

Publication number: WO2005050887A2
Application number: PCT/KR2004/001958
Authority: WO
Inventors: Yu-Ro Lee; Dong-Seung Kwon; Seong-Rag Kim; In-Kyeong Choi; Choong-Il Yeh; Hyoung-Soo Lim; Seong-Chul Cho; Jong-Ee Oh; Seung-Ku Hwang; Jee-Hwan Ahn; 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: 2003-11-21
Filing date: 2004-08-04
Publication date: 2005-06-02
Also published as: KR20050049140A; WO2005050887A3

Abstract

Disclosed is a method for configuring an uplink frame in an OFDMA based wireless communication system. The total band of the uplink frame is divided into a predetermined number of subcarrier groups. Subcarriers within the respective allocated subcarrier groups are allocated to the subchannel defined for a predetermined number of consecutive symbols defined by slots in the uplink frame. Pilot subcarriers are allocated by the unit of the predetermined number of symbols so as to allow channel estimation from among the allocated subcarriers. Accordingly, data subcarriers are allocated by minimizing collision with specific users of adjacent cells so as to enable channel estimation using preambles in a cellular system, and some positions of the data subcarriers are allocated for pilots, thereby allowing synchronous demodulation and improving frequency reuse rates.

Description

Description Method for Configuring Uplink Frame in OFDMA-Based Wireless Communication System Technical Field

[1] The present invention relates to a method for configuring an uplink frame in the OFDMA (orthogonal frequency division multiple access)-based wireless communication system. More specifically, the present invention relates to a system and method for configuring an uplink frame for performing synchronous demodulation and improving frequency reuse rates in the uplink frame of the OFDMA-based wireless ccmriinication system. Background Art

[2] The IEEE, which is the US standardization organization for the computer and electronics industry, has recently confirmed the IEEE 802.11a wireless LAN standard on the basis of the OFDM method with data rates of from 6 to 54 Mbps in the frequency band of 5GHz, and the ETSI, which is the European standardization organization, has also confirmed and used the HiperLAN/2 having similar physical layers as those of the US standard as the standard of the high-speed wireless LAN. The physical layers of the HiperLAN/2 and the IEEE 802.1 la adopt the same OFDM method in the frequency band of 5GHz of the ISM band, and provide variable data rates of from 6 to 54Mbps.

[3] Further, the wireless MAN (metropolitan area network) which processes the physical layers and MAC layers for the frequency bands of from 10 to 66GHz and of from 2 to 11GHz has completed the IEEE 802.16 standard in the frequency band of from 10 to 66GHz, and has been progressing the IEEE 802.16a standard with an extended frequency band of from 2 to 16GHz.

[4] ISI (inter-symbol interference) caused by multiple paths is generated to receive signals when the signals are transmitted through a multi-path channel. In particular, since the period of a symbol is less than the delay spread of the channel in the case of transmitting data at high data rates, ISI becomes greater, and hence, a complex receiving method is required in order to compensate for distortion caused by ISI and accurately restore the transmit signals. The period of a symbol is to be greater than the delay spread of the channel in order to reduce the distortion phenomenon in the multi- path channel, and the OFDM method is proposed as a modulation method for easily compensating for the distortion in the multi-path channel. [5] The OFDM method uses a plurality of subcarriers with orthogonality to transmit data, differing from the transmission method which uses single carriers. That is, the OFDM method performs serial and parallel conversion on the input data by as many as the number of the subcarriers used for modulation, modulates the converted data by using the corresponding subcarriers, and hence, the data rates are maintained and the period of the symbol for each subcarrier is increased to the number of the subcarriers. Since the OFDM method uses the subcarriers having orthogonality, the OFDM method provides better bandwidth efficiency and has a longer symbol period than those of the existing FDM (frequency division multiplex) method, and accordingly, the OFDM method has a stronger characteristic against ISI compared to the single carrier modulation method.

[6] The modulation process and the demodulation process at a transmitter and a receiver in the OFDM system respectively correspond to performance of IDFT (inverse discrete Fourier transform) and DFT (discrete Fourier transform), which can also be realized efficiently by using IFFT (inverse fast Fourier transform) and FFT (fast Fourier transform). Also, when a guard interval which is longer than the delay spread of the channel is inserted for each transmitted symbol period, the orthogonality between the subcarriers is maintained to generate no ICI (inter-carrier interference), and no OFDM symbols caused by the multi-path channel are superimposed to completely eliminate the ISI between the adjacent symbols.

[7] Further, the frequency hopping methods for averaging inter-cell interference are used in the above-noted OFDMA-based cellular system, the methods for averaging the inter-cell interference by using different frequency hopping patterns for the respective cells so that a channel of one cell may be maximally uniformly collided with various channels of another cell are used, one of which is the above-described OFDMA method of the IEEE802.16a. However, since the interference is averaged without distinguishing channels in the IEEE802.16a, the required SNR (signal to noise ratio) is averaged with other mixed channels, and it is difficult to control the inter-cell interference.

[8] The above-noted OFDM systems including the IEEE 802.11a, the HperLAN type 2, and the IEEE802.16a use preambles in the uplinks.

[9] FIG. 1 shows an uplink data burst structure of the HperLAN type 2 and the IEEE 802.1 la of the general OFDM/TDMA system, including a short preamble 11, a long preamble 13, and a pay load 15.

[10] FIG. 2 shows an uplink frame structure of IEEE 802.16a of the general OFDM/ TDMA system, including a preamble 21, a UL (uplink) burst#l 23, a UL (uplink) burst#2 25, a UL (uplink) burst#3 27, and a ranging subchannel 29.

[11] Referring to FIGs. 1 and 2, the systems may generate cases having difficulty in performing channel estimation by use of preambles because of collision of the inter- cell preambles 11, 13, and 21 according to position distribution of users within a cell, and hence, a cell plan for preventing collision of the inter-cell preambles is needed. Also, the channel estimation using the preamble is allowed in the stoppage state, or is available only in the low-speed case which satisfies the condition that the time for performing demodulation by using the preamble is less than the channel variation time caused by the speed of a terminal. If the terminal has mobility of greater than the middle speed, errors of the channel value estimated by the preamble and the channel value of a symbol after the preamble occur, and the performance is deteriorated.

[12] As to prior art, Korean Application No. 1998-27484 (filed on July 8, Relates to a system and method for configuring an uplink frame for performingcharacterized in that provided is a method and device for synchronizing an OFDM receiver for obtaining symbol/frame timing synchronization, carrier frequency synchronization, and sampling clock timing synchronization on the OFDM signals by using a reference symbol, which still needs improvement as to the frequency reuse rates. Disclosure of Invention Technical Problem

[13] It is an advantage of the present invention to provide a method for configuring an uplink frame for improving the frequency reuse rates in an OFDMA-based wireless ccrnmunication system.

[14] It is another advantage of the present invention to provide a method for configuring an uplink frame for allowing channel estimation when a terminal may move in an OFDMA-based wireless communication system. Technical Solution

[15] In one aspect of the present invention, a method for configuring an uplink frame in an OFDMA-based wireless communication system comprises: (a) dividing the bandwidth of the uplink frame into a predetermined number of subcarrier groups; (b) allocating subcarriers within the divided subcarrier groups to subchannels defined on a predetermined number of consecutive symbols defined as slots in the uplink frame; and (c) allocating pilot subcarriers by the predetermined number of symbols so as to allow channel estimation from among the allocated subcarriers. [16] The subcarrier group divides valid subcarriers corresponding to the bandwidth into eight groups in (a), and the subchannel is defined by a unit of three OFDMA symbols, and two subcarriers are selected from the respective eight subcarrier groups and allocated to the subchannel in (b).

[17] The subcarriers belonging to the respective groups and allocated to the subchannel are adjacent to each other.

[18] The pilot subcarrier is allocated by a unit of three symbols by using two adjacent subcarriers for each symbol in the uplink in (c).

[19] The pilot subcarrier is provided to the symbol provided at the center of the allocated symbols.

[20] The pilot subcarrier is allocated to one of the two adjacent subcarriers corresponding to the arranged symbols.

[21] The pilot subcarrier is provided to the subcarrier with an even number from among the two adjacent subcarriers when the summation of a cell number and a slot number is even, and the pilot subcarrier is provided to the subcarrier with an odd number when the summation of a cell number and a slot number is odd. Brief Description of the Drawings

[22] 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:

[23] FIG. 1 shows an uplink data burst structure of the HperLAN type 2 and the IEEE 802.1 la of a general OFDM/TDMA system;

[24] FIG. 2 shows an uplink frame structure of IEEE 802.16a of a general OFDM/ TDMA system;

[25] FIG. 3 shows an uplink frame structure in a wireless communication system using the OFDMA method according to a preferred embodiment of the present invention; and

[26] FIGs. 4 and 5 show a method for allocating uplink pilots according to a preferred embodiment of the present invention, FIG. 4 showing pilot allocation in the case that ID mod 2=0, and FIG. 5 showing pilot allocation in the case that ID mod 2=1. cell cell Best Mode for Carrying Out the Invention

[27] 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.

[28] To clarify the present invention, parts which are not described in the specification are omitted, and parts which have similar descriptions have the same reference numerals.

[29] Data subcarriers are allocated by two respective adjacent subcarriers, and a pilot is inserted into a predetermined position so that collision of specific users between the adjacent cells may be minimized and channel estimation may be allowed in the uplink frame structure of the OFDMA-based wireless communication system.

[30] FIG. 3 shows an uplink frame structure in a wireless communication system using the OFDMA method according to a preferred embodiment of the present invention.

[31] Referring to FIG. 3, the total bandwidth is divided into subcarrier groups, data subcarriers are allocated so that collision with the terminal of an adjacent cell may be minimized, and pilots are allocated at some positions so as to allow channel estimation from among the allocated data subcarriers.

[32] In the OFDM frame structure according to the preferred embodiment of the present invention, subchannels are defined for three consecutive OFDM symbols with n (n=0, 1, 2) respectively defined by slots Slot t and Slot t+1 (31 and 33, and data bursts are allocated thereto. In this instance, division of the subcarriers into subchannels is performed by slots Slot t and Slot t+1. 1552 valid subcarriers of the respective OFDM symbols 31 and 33 are divided into eight groups G to G including 194 consecutive 0 7 subcarriers. Also, as to a single subchannel, two consecutive subcarriers are selected from the respective eight groups G to G , and 16 subcarriers per symbol are 0 7 identically used during a slot. [33] In this instance, the value with a small subcarrier number from among the subcarriers for configuring respective blocks is assigned and used as the subcarrier for configuring the subchannel, and its position is not changed in the same frame interval. [34] Also, the subchannels for data transmission are allocated per one slot and one subchannel, and the ranging channels for synchronizing the receive signals of the uplink terminals are allocated per one slot and six subchannels in the same frame period. [35] The subcarriers for configuring the UL (uplink) subchannels are defined as follows for each symbol interval belonging to each slot. [36] MathFigure 1 ₍ s (l 94m + 2 x {pXm + m₀ ) + ID_ceU ]moά97l 0 ≤ s < 96 Lamer s, m \ - { {\ 94m + 2ID_cell , s = 96

[37] where Carrier(s,m) is the m-th subcarrier configuring the s-th subchannel and has values of from 0 to 1551, s is a subchannel number and has values of from 0 to 96, m is a subcarrier index for configuring the subchannel and has values of from 0 to 7, m is a subcarrier index offset given as m =8 x [(number of frames) mod 12], P (j) is the j- o s th element of the permutation obtained by recursively moving the basic substitution permutation P to the left s times, ID is a cell number assigned by the MAC (medium 0 cell access control) and has values of from 0 to 96, and a mod b is a remainder obtained by dividing a by b. [38] A case with the cell number ID = 3 and the subchannel number s = 2 in the first cell frame will be described so as to exemplify subchannel division. [39] (1) It is defined that P = {28, 43, 21, 8, 40, 6, 30, 53, 71, 64, 29, 48, 46, 36, 83, 27, 2 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, 11, 55, 81, 17, 85, 37, 88, 52, 66, 39, 1, 5, 25}.

[40] (2) The eighth to fifteenth elements are used since the subcarrier index offset m o becomes 8 in the first frame. That is, {71, 64, 29, 48, 46, 36, 83, 27} are used from P . 2

[41] (3 Since the cell number ID = 3, all the elements of the permutation are added cell with 3, and are then divided by 97 to find the remainders. Hence, they are found as {74, 67, 32, 51, 49, 39, 86, 30}. [42] (4) Sixteen subcarriers are totally selected from the symbols each by two adjacent subcarriers. Therefore, it is found that Carrier(2, m) = { 148, 328, 452, 684, 874, 1048, 1336, 1418}, and the actually used subcarriers are found as { 148, 149, 328, 329, 452, 453, 684, 685, 874, 875, 1048, 1049, 1336, 1337, 1418, 1419}. [43] The reference numeral A indicates the pilot P allocated to a predetermined position so as to allow channel estimation from among the allocated data subcarriers, and a detailed pilot allocating method will now be described with reference to FIGs. 4A and 4B. [44] FIGs. 4A and 4B show a method for allocating uplink pilots according to a preferred embodiment of the present invention, FIG. 4 showing pilot allocation in the case that ID mod 2=0, and FIG. 5 showing pilot allocation in the case that ID mod cell cell 2=1. [45] Referring to FIGs. 4A and 4B, the pilot subcarriers of the uplink are allocated by a unit of three symbols for each two consecutive subcarriers. The pilot subcarriers are arranged to the first symbol, which corresponds to the central symbol of each slot. The pilot subcarrier is provided to the subcarrier with an even number when the summation of the cell number and the slot number is even as shown in FIG. 4, and the pilot subcarrier is provided to the subcarrier with an odd number when the summation of the cell number and the slot number is odd as shown in FIG. 5 (refer to the shaded parts of FIGs. 4 and 5.)

[46] Therefore, the present invention minimizes collision with specific users of the adjacent cell in order to minimize the channel estimation problem using the preambles and allow channel estimation in the cellular system, and further, divides the total bandwidth into subcarrier groups, allocates data subcarriers in order to minimize collision between terminals of adjacent cells, and allocates some positions of the data subcarriers as pilots, thereby enabling synchronous demodulation and improving frequency reuse rates.

[47] 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

[1] A method for configuring an uplink frame in an OFDMA (orthogonal frequency division multiple access)-based wireless communication system, comprising: (a) dividing the bandwidth of the uplink frame into a predetermined number of subcarrier groups; (b) allocating subcarriers within the divided subcarrier groups to subchannels defined on a predetermined number of consecutive symbols defined as slots in the uplink frame; and (c) allocating pilot subcarriers by the predetermined number of symbols so as to allow channel estimation from among the allocated subcarriers.

[2] The method of claim 1, wherein the subcarrier group divides valid subcarriers corresponding to the bandwidth into eight groups in (a), and the subchannel is defined by a unit of three OFDMA symbols, and two subcarriers are selected from the respective eight subcarrier groups and allocated to the subchannel in (b).

[3] The method of claim 2, wherein the subcarriers belonging to the respective groups and allocated to the subchannel are adjacent to each other.

[4] The method of claim 1, wherein the subcarriers for configuring the subchannel satisfies the equation: + 2 ^χ {P_s (m + m ) + ID_cen ]mod9ll 0 ≤ s < 96 Carrier

+ 2ID_ceU , s = 96 where Carrier(s,m) is the m-th subcarrier configuring the s-th subchannel, s is a subchannel number, m is a subcarrier index for configuring the subchannel, m is a subcarrier index offset, P (j) is the j-th element of the permutation obtained by s recursively moving the basic substitution permutation P to the left s times, ID 0 cell is a cell number assigned by the MAC (medium access control), and a mod b is a remainder obtained by dividing a by b.

[5] The method of claim 4, wherein the Carrier(s,m) has values of from 0 to 1551, s has values of from 0 to 96, m has values of from 0 to 7, and ID has values of cell from 0 to 96.

[6] The method of claim 5, wherein the subcarrier index offset m is given as m =8 x o o [(number of frames) mod 12].

[7] The method of claim 1, wherein the pilot subcarrier is allocated by a unit of three symbols by using two adjacent subcarriers for each symbol in the uplink in (c).

[8] The method of claim 1, wherein the pilot subcarrier is provided to the symbol provided at the center of the allocated symbols.

[9] The method of claim 8, wherein the pilot subcarrier is allocated to one of the two adjacent subcarriers corresponding to the arranged symbols.

[10] The method of claim 9, wherein the pilot subcarrier is provided to the subcarrier with an even number from among the two adjacent subcarriers when the summation of a cell number and a slot number is even, and the pilot subcarrier is provided to the subcarrier with an odd number when the summation of a cell number and a slot number is odd.