WO2006107135A1

WO2006107135A1 - Method for inserting postfix into ofdma symbol and method for constructing frame of portable internet using the same

Info

Publication number: WO2006107135A1
Application number: PCT/KR2005/004518
Authority: WO
Inventors: Hae Chul Lee; Kyoo-Tae Ryoo; Seong-Choon Lee; Jeong-Hwi Kim; Sang-Ho Yi
Original assignee: Kt Corporation; Solid Technologies, Inc.
Priority date: 2005-04-08
Filing date: 2005-12-24
Publication date: 2006-10-12

Abstract

A method for inserting postfix into an Orthogonal Frequency Division Multiple Access (OFDMA) symbol in a portable Internet for use of a repeater includes removing an Inter Carrier Interference (ISI) caused by a postfix by inserting a copied data into an end of a useful symbol duration (Tb) among an entire symbol duration (T5) of an OFDMA symbol, where the copied data correspond to T5 for securing an orthogonality in symbol, transmission at a front portion of a useful symbol duration (Tb); and setting a postfix length (time) and a cyclic prefix (CP) length (time) to be equal to each other, thereby making downlink cell coverage and uplink cell coverage equal to each other.

Description

METHOD FOR INSERTING POSTFIX INTO OFDMA SYMBOL AND METHOD FOR CONSTRUCTING FRAME OF PORTABLE INTERNET USING THE

SAME

Description

Technical Field

The present invention relates to a method for inserting postfix into Orthogonal Frequency Division Multiple Access (OFDMA) symbol of a portable Internet and a method for constructing a frame structure of the portable Internet by using the postfix-inserted OFDMA symbol to use a repeater in the portable Internet.

Background Art

Referring to Fig. 1, a structure of an OFDMA symbol used in a portable Internet will be described. As shown in Fig. 1, the OFDMA symbol uses a cyclic prefix (CP) to remove signal distortion caused by multi-path signal having delay time of less than a cyclic prefix (CP) time. The CP removes Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI) by adding a copy data corresponding to an end portion T_g of a useful symbol duration T_b to a beginning of the OFDMA symbol.

That is, when a total length of the OFDMA symbol is assumed to be T₃, T₃ is given by the T_b duration of the original OFDM symbol and the T₉ duration corresponding to an end portion of the T_b duration. T_g is provided for securing the orthogonality in transmitting the OFDMA symbol and for decoding the original data without loss even if the sample is extracted with T_b time during the T₅ duration. Because the data of the beginning potion is identical to the data of the end portion, the T₉ duration is stored as a sample and then an end portion is searched through an auto correlation within the symbol.

To maintain the orthogonal characteristic, the OFDMA symbol has the same data at the beginning and end portions. This is called a cyclic prefix (CP). Symbol synchronization can be detected using the condition that data before and after the symbol are identical to each other .

Fig. 2 illustrates a construction of a portable Internet system having a base station and a mobile station.

Referring to Fig. 2, the mobile station receives a downlink signal from the base station through multipaths.

When a maximum delay difference of the multipaths is less than CP time, the signal can be decoded without influence of ISI due to the multipaths.

Meanwhile, the uplink signals transmitted from the mobile station have to arrive at the base station at the same time. If the uplink signals do not arrive at the base station at the same time, the base station cannot decode the uplink signals due to the ISI and ICI between the uplink signals. Therefore, a "method for uplink time synchronization using a ranging signal" is used in the portable Internet. In this method, the mobile station transmits the uplink signals as early as a Round Trip

Delay (RTD) from the base station and the mobile station.

Fig. 3 illustrates a timing diagram of the uplink signals that the base station receives from the mobile stations when the method for uplink time synchronization using the ranging signal is used in the portable Internet system.

In Fig. 3, "on-time" is an absolute time at which the base station wants to receive the uplink signals of the mobile stations. For example, an error allowable time of on-time in the portable Internet is less than ±1/4 of the CP time, that is, ±3.2 us.

Like the case of the downlink, since the base station receives the uplink signals of the mobile station through the multipaths, the maximum delay time difference of the multipaths of the uplink signals has to be less than the CP time in order to decode the signals without influence of ISI and ICI due to the multipaths.

Referring to Fig. 4, when the portable Internet system uses the repeater, the mobile station receives a downlink signal transmitted from the base station and a downlink signal transmitted from the base station through the repeater, that is, multipath signals. A time difference t_D of the multipath signals received by the mobile station can be expressed as Eq. (1) below.

In Eq. (1), t_A, t_R, to, and t_s represent a propagation delay time between the base station and the mobile station, a propagation delay time between the repeater and the mobile station, an optical cable or air propagation delay time between the base station and the repeater, and a system delay time of the repeater. Therefore, "t_A" means the downlink propagation time taken from the base station to the mobile station, and "t_R+t_s+t₀" means the downlink propagation time taken from the base station through the repeater to the mobile station.

In order for the mobile station to receive the multipath signals without ISI, the time difference between the multipaths has to be less than the CP time T₉. Using this condition, downlink cell coverage t_RD of the repeater is limited to Eq. (2) below.

^RD — tg ^~*^~ M ^~~ h ^~~ ^O Eq. ( 2 ) Due to the uplink time synchronization method using the ranging signal, the mobile station transmits the uplink signal faster than on-time by the time RTD of the base station and the mobile station. In Fig. 4, the RTD time of the base station and the mobile station is divided into a first case of not passing through the repeater and a second case of passing through the repeater. First, the case where the propagation delay time t_A between the base station and the mobile station is recognized as RTD will be described below.

In this case, the mobile station transmits the uplink signal faster than on-time by the "propagation delay time t_A between the base station and the mobile station. The timing diagram of the uplink received by the base station is illustrated in Fig. 5. Fig. 5 illustrates the multipath signals arriving at a receiver of the base station when RTD is recognized as t_A in Fig. 4.

As illustrated in Fig. 5, the signals directly propagated to the base station among the uplink signals transmitted from the mobile station arrive at the receiver of the base station at on-time. The signals transmitted through the repeater to the base station are delayed longer than on-time and then arrive at the receiver of the base station.

Therefore, in order to decode the multipath signals without ISI, the time difference t_R+t_s+t₀-t_A of the multipaths has to be equal to or less than the CP time T₉. Since uplink cell coverage t_RU of the repeater is limited to Eq. (3) below, the maximum uplink cell coverage and the maximum downlink cell coverage of the repeater are equal to each other. t_RD ^ t_g + t_A - t_s - t, Eq . ( 3 )

Next, the case where the propagation delay time t_R+t_s+t₀ among the base station, the repeater and the mobile station is recognized as RTD will be described below.

In this case, the mobile station transmits the uplink signal faster than on-time by the "propagation delay time t_R+t_s+t₀ among the base station, the repeater and the mobile station. The timing diagram of the uplink received by the base station is illustrated in Fig. 6. Fig. 6 illustrates the multipath signals arriving at the receiver of the base station when RTD is recognized as t_R+t_s+t₀ in Fig. 4. As illustrated in Fig. 6, the signals transmitted to through the repeater to the base station among the uplink signals transmitted from the mobile station arrive at the receiver of the base station at on-time. The signals directly propagated to the base station arrives at the receiver of the base station faster than on-time by the time t_R+t_s+to-t_A.

An allowable timing error of on-time in the portable Internet is less than ±1/4 of the CP time, that is, ±3.2 us. When the time difference t_R+t_s+t₀-t_A of the multipaths is less than 3.2 us, the base station can decode the uplink signal. In this case, uplink cell coverage t_RU of the repeater is limited to Eq. (4) below. Since T_g=12.8μs>3.2μs , the maximum coverage of the repeater is limited by the maximum uplink cell coverage.

Ϊ_RD — 3.2/JS + t_A — t_s — t_{0 Eq # ( 4 )}

However, the repeater cannot be used to expand the base station coverage under this condition. Also, the use of the repeater is very restricted even when it is used to cover a shadow area within the base station coverage, which is equal to that of an in-building repeater. Since ISI occurs when the uplink signal is inputted faster than on-time, data error rate increases in decoding the OFDMA symbol .

Disclosure Technical Problem

It is, therefore, an object of the present invention to provide a method for inserting postfix into OFDMA symbol and a method for constructing a frame structure of a portable Internet using the same, in which the repeater can be used to expand the base station coverage, the base station can decode uplink signal without ISI, and the frame structure of the portable Internet can be constructed using the OFDMA symbol into which the postfix is inserted.

Other objects and advantages of the present invention can be understood more fully through the embodiments of the present invention. Also, the objects and advantages of the present invention can be easily implemented by means of the following claims and combination thereof.

Technical Solution

In accordance with one aspect of the present invention, there is provided a method for inserting postfix into an Orthogonal Frequency Division Multiple Access (OFDMA) symbol in a portable Internet for use of a repeater, the method including the steps of: removing an Inter Carrier Interference (ISI) caused by a postfix by inserting a copied data into an end of a useful symbol duration (T_b) among an entire symbol duration (T_s) of an OFDMA symbol, the copied data corresponding to T₉ for securing an orthogonality in symbol transmission at a front portion of a useful symbol duration (Tb); and setting a postfix length (time) and a cyclic prefix (CP) length (time) to be equal to each other, thereby making downlink cell coverage and uplink cell coverage equal to each other.

In accordance with another aspect of the present invention, there is provided a method for constructing a frame structure of a portable Internet using an OFDMA symbol into which a postfix is inserted, the method including the steps of: defining system parameters of the OFDMA symbol and frame structure parameters by using the OFDMA symbol into which the postfix is inserted, wherein the system parameters of the OFDMA symbol includes a channel bandwidth of 10 MHz, a sampling frequency ( F_s ) of 10MHz, a sampling interval (1/F_S) of 100 nsec, an FFT size (N_FFT) of 1024, number of used subcarrier of 864, number of data subcarriers of 768, number of pilot subcarriers of 96, subcarrier frequency interval of 9.765625 KHz, a useful symbol duration (T_b=l/Δf) of 102.4 μs, a CP time (T_g=T_b/8) of 12.8 μs , a postfix time (T_P=T_g) of 12.8 μs, an OFDMA symbol duration (T_s=T_b+T_g+T_P) of 128 μs , and a TDD frame length of 5 ms; and the frame structure parameters includes number of symbols per frame of 38, a TTG time of 102 μs , an RTG time of 34 μs .

In accordance with a further aspect of the present invention, there is provided a method for constructing a frame structure of a portable Internet using an OFDMA symbol into which a postfix is inserted, the method including the steps of: defining system parameters of the OFDMA symbol and frame structure parameters by using the OFDMA symbol only in an uplink, wherein the system parameters of the OFDMA symbol includes a channel bandwidth of 10 MHz, a sampling frequency (F₅) of 10MHz, a sampling interval (1/F_S) of 100 nsec, an FFT size (N_FFT) of 1024, number of used subcarrier of 864, number of data subcarriers of 768, number of pilot subcarriers of 96, subcarrier frequency interval of 9.765625 KHz, a useful symbol duration (T_b=l/Δf) of 102.4 μs , a CP time (T_g=T_b/8) of 12.8 μs, a postfix time (T_P=T_g) of 12.8 μs , a downlink OFDMA symbol duration (T_s=T_b+T_g) of 115.2 μs , an uplink OFDMA symbol duration ( T_s=T_b+T_g+T_P ) of 128 μs , and a TDD frame length of 5 ms ; and the frame structure parameters includes number of downlink symbols per frame of 28, number of uplink symbols of 12, a TTG time of 178.8 μs , an RTG time of 59.6 μs .

Advantageous Effects

In accordance with the present invention, in the frame structure of the portable Internet using OFDMA symbol into which the postfix is inserted, it is possible to prevent Inter Symbol Interference (ISI) from occurring in the base station (AP) reception signal and the mobile terminal reception signal during the use of the repeater due to the uplink time synchronization scheme by the ranging signal, thereby providing high quality of communication services and expanding cell coverage of the repeater.

Description of Drawings

The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 illustrates a structure of an OFDMA symbol in a portable Internet; Fig. 2 illustrates a construction of a portable Internet system having a base station and a mobile station;

Fig. 3 is a signal timing diagram of the base station and the mobile station in the portable Internet system;

Fig. 4 illustrates a construction of a portable Internet system when a repeater is used;

Fig. 5 is a timing diagram of multipath signals arriving at the receiver of the base station when RTD is recognized as t_A in Fig. 4;

Fig. 6 is a timing diagram of multipath signals arriving at the receiver of the base station when RTD is recognized as t_R+t_s+to in Fig. 4;

Fig. 7 illustrates a method for inserting postfix into an OFDMA symbol of a portable Internet according to an embodiment of the present invention;

Fig. 8 illustrates a method for inserting postfix into an OFDMA symbol of a portable Internet according to another embodiment of the present invention; Fig. 9 is a timing diagram of multipath signals arriving at the receiver of the base station when RTD is recognized as t_R+t_s+t₀ by using OFDMA symbol into which postfix is inserted in accordance with the present invention; Fig. 10 illustrates a frame structure of a portable Internet using OFDMA symbol into which postfix is inserted in accordance with an embodiment of the present invention; and

Fig. 11 illustrates a frame structure of a portable Internet using OFDMA symbol into which postfix is inserted in accordance with another embodiment of the present invention.

Best Mode for the Invention Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. Figs. 7 and 8 illustrate a method for inserting postfix into OFDMA symbol of a portable Internet in accordance with the present invention.

Referring to Figs. 7 and 8, if postfix is inserted into OFDMA symbol of a portable Internet, a repeater can be used to expand base station coverage in the portable Internet system and an uplink signal can be decoded at the base station without ISI. Also, a frame structure of the portable Internet can be constructed using the OFDMA into which the postfix is inserted. In Figs. 7 and 8, an Inter Carrier Interference

(ISI) caused by the postfix is removed by inserting a copied data corresponding to a front portion T₅ of a useful symbol duration T_b into an end of the OFDMA symbol.

In the portable Internet system using the repeater shown in Fig. 4, even when the propagation delay time t_R+t_s+to among the base station, the repeater and the mobile station is recognized as RTD due to the ranging and a postfix length T_P is larger than the time difference t_R+ts+t₀-t_A of the multipaths (see Eq. (5)), the base station can decode the uplink signal without ISI by inserting the postfix into the OFDMA symbol as illustrated in Figs. 7 and 8.

t_p ≥ t_R + t_s + t_Q — t_{A Eq} . _{( 6 )}

Fig. 9 is a timing diagram of multipath signals arriving at the receiver of the base station when RTD is recognized as t_R+t_s+t₀ by using OFDMA symbol into which postfix is inserted in accordance with the present invention. Referring to Fig. 9A, when the postfix length T_P is greater than 3.2 us, uplink cell coverage t_RU of the repeater is limited to Eq. 6 below, so that the uplink cell coverage of the repeater is expanded.

^RU — tp ⁺ ^A ^~ *s ^~~ ^O Eq . ( 6 )

Accordingly, the cell coverage t_RC of the repeater is limited by the less time of the CP time T₉ and the postfix time T_P from the relational expression of the uplink cell coverage t_RD and the relational express of the downlink cell coverage t_RU, and is defined as Eq. (7) below.

t_RC ≤MIN{τ ,T_P}+ t_A -t_s -t_o

Eq. (7)

Thus, the cell coverage of the repeater can be expanded by the postfix. In other words, the repeater can be used to expand the cell coverage of the base station. If the postfix time T_P is equal to the CP time Tg, the repeater can be used by setting the uplink cell coverage and the downlink cell coverage of the repeater to be equal to each other. Fig. 10 illustrates a frame structure of a portable Internet using OFDMA symbol into which postfix is inserted in accordance with an embodiment of the present invention. The frame structure of the portable Internet is constructed using the OFDMA symbol of Fig. 7 into which the postfix is inserted. Table 1 below shows system parameters of the OFDMA symbol and frame structure parameters .

[Table 1]

Fig. 11 illustrates a frame structure of a portable Internet using OFDMA symbol into which postfix is inserted in accordance with another embodiment of the present invention. The frame structure of the portable Internet is constructed by applying the OFDMA of Fig. 7 only to the uplink signal. Table 2 below shows system parameters of the OFDMA symbol and frame structure parameters .

[Table 2]

While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims

What is claimed is;

1. A method for inserting postfix into an Orthogonal Frequency Division Multiple Access (OFDMA) symbol in a portable Internet to use a repeater, the method comprising the steps of: removing an Inter Carrier Interference (ISI) caused by a postfix by inserting a copied data into an end of a useful symbol duration (T_b) among an entire symbol duration (T₃) of an OFDMA symbol, the copied data corresponding to T₉ for securing an orthogonality in symbol transmission at a front portion of a useful symbol duration (T_b); and setting a postfix length (time) and a cyclic prefix (CP) length (time) to be equal to each other, thereby making downlink cell coverage and uplink cell coverage equal to each other.

2. The method as recited in claim 1, wherein when a propagation delay time (t_R+t_s+t₀) among the base station, the repeater and the mobile station is recognized a Round Trip Delay (RTD) due to the ranging in the portable Internet system using the repeater and the postfix length (T_P) is longer than the time difference ( t_R+t_s+to-t_A) of multipaths, a base station decodes the uplink signal without ISI by inserting the postfix into the OFDMA symbol.

3. The method as recited in claim 1, wherein when the postfix length (time) (T_P) is longer than an error allowable time (3.2 us) of an absolute time (on- time) at which the base station wants to receive the uplink signals of the mobile stations, the uplink cell coverage of the repeater is expandable and the repeater is used to expand the cell coverage of the base station.

4. The method as recited in claim 3, wherein when the postfix length (time) (T_P) is set to be equal to the CP length (time) (T₉), the uplink cell coverage and the downlink cell coverage of the repeater are equal to each other, whereby the repeater is usable.

5. A method for constructing a frame structure of a portable Internet by using an OFDMA symbol with a postfix, the method comprising the step of: defining system parameters of the OFDMA symbol and frame structure parameters by using the OFDMA symbol with the postfix inserted thereto, wherein the system parameters of the OFDMA symbol includes a channel bandwidth of 10 MHz, a sampling frequency (F_s) of 10MHz, a sampling interval (1/F_S) of

100 nsec, an FFT size (N_FFT) of 1024, 864 used subcarriers,

768 data subcarriers, 96 pilot subcarriers, subcarrier frequency interval of 9.765625 KHz, a useful symbol duration (T_b=l/Δf) of 102.4 μs , a CP time (T_g=T_b/8) of

12.8 μs, a postfix time (T_P=T_g) of 12.8 μs, an OFDMA symbol duration (T_s=T_b+T_g+T_P) of 128 μs , and a TDD frame length of 5 ms; and the frame structure parameters includes 38 symbols per frame, a TTG time of 102 μs, and an RTG time of 34 μs .

6. A method for constructing a frame structure of a portable Internet by using an OFDMA symbol with a postfix inserted thereto, the method comprising the step of: defining system parameters of the OFDMA symbol and frame structure parameters by using the OFDMA symbol only in an uplink, wherein the system parameters of the OFDMA symbol includes a channel bandwidth of 10 MHz, a sampling frequency (F₃) of 10MHz, a sampling interval (1/F_S) of 100 nsec, an FFT size (N_FFT) of 1024, 864 used subcarriers, 768 data subcarriers, 96 pilot subcarriers, subcarrier frequency interval of 9.765625 KHz, a useful symbol duration (T_b=l/Δf) of 102.4 μs , a CP time (T_g=T_b/8) of 12.8 μs, a postfix time (T_P=T_g) of 12.8 μs, a downlink OFDMA symbol duration (T_s=T_b+T_g) of 115.2 μs , an uplink OFDMA symbol duration (T_s=T_b+T_g+Tp) of 128 μs , and a TDD frame length of 5 ms ; and the frame structure parameters includes 28 downlink symbols per frame, 12 uplink symbols, a TTG time of 178.8 μs, and an RTG time of 59.6 μs .