KR20050012551A - Data Transmission Method for OFDM-Wireless LAN System - Google Patents

Abstract

PURPOSE: A method for transmitting data in a wireless LAN system using an OFDM method is provided to group data symbols according to a high-speed mobile environment, and to insert pilot symbols used for a channel characteristic estimation between each data symbol group, thereby transmitting/demodulating data without an error and a fading influence even though a mobile moves at high speed. CONSTITUTION: OFDM frames modulated by an OFDM method consist of data fields including plural training symbols and plural data symbols. The data fields include plural data symbol groups and plural pilot symbols for estimating and compensating for channel changes by being disposed between the plural data symbol groups, respectively. The plural data symbol groups are composed of plural sets of guard intervals and data symbol pairs, and transmit data to a wireless terminal through an access point.

Description

Data transmission method in ODF system wireless LAN system {Data Transmission Method for OFDM-Wireless LAN System}

The present invention relates to a wireless LAN system, and more particularly, to a data transmission method in a wireless LAN system that can maintain the quality of wireless LAN service in an environment in which a user terminal moves at a high speed.

Wireless LAN connects an access point to the end of high-speed wired Internet to ensure high transmission speeds and limited mobility at low speeds, making it convenient for hot-spot areas such as homes, schools, hotels and conference halls. Internet service can be provided. Wireless LAN-based Internet service has the disadvantage that service coverage is relatively narrow and mobility is limited than mobile network-based Internet service, but it is more economical because it is faster than mobile communication network-based internet service and the usage fee is very low.

1 is a view for explaining a general wireless LAN system.

As shown, a user may wirelessly connect to an access point 20 located in a communication area using user wireless terminals 10 and 12 having a wireless communication module (wireless LAN card). A plurality of access points 20 installed in each area are collected through the Internet 30, and a user accesses a desired content provider server (CP server) 40 through the access point 20 and the Internet 30 to provide a desired service. It is available. The access point 20 serves as a hub of a wired LAN and becomes a basic configuration that can use a wireless LAN card.

The current wireless LAN is based on IEEE 802.11b and the frequency band used is 2.400 ~ 2.4835GHz ISM (Industrial, Scientific and Medical) band. In Korea, the number of channels is set to 13, but it is independent without interference in actual cell configuration. The number of available channels is only three, which makes it difficult to transmit broadband data and accommodate a large number of subscribers. In addition, since it is fundamentally an ISM (Industrial, Scientific, Medical Bands) band, frequency interference with Bluetooth, home RF, microwave ovens, and medical devices is inevitable. This is getting worse. For the deterioration of service quality and the efficient use of frequency, there are discussions on ways to improve service quality and joint use of access points around the high-speed wireless LAN forum.

On the other hand, research has been actively conducted on a wireless LAN using a 5GHz frequency to provide a wireless LAN service in a mobile environment. As a standard for wireless LAN using 5GHz frequency, the IEEE 802.11a standard centered on the US and the HyperLAN / 2 standard centered on ETSI in Europe were completed in 1999. 5GHz wireless LAN basically has a frequency bandwidth of 20MHz per channel, the number of channels is 12 channels for IEEE 802.11a, 18 channels for Hyper LAN / 2, and is capable of high-speed data transmission due to strong fading characteristics according to multipath of radio waves. Frequency Division Multiplex) is used and the modulation method that can be up to 64QAM is adopted, which enables broadband, multi-channel and high quality compared to the existing wireless LAN. In addition, the wireless LAN using 5GHz can provide a transmission rate of up to 54Mbps at a relatively low rate compared to a mobile network-based wireless Internet service, as well as support IP, ATM applications and QoS.

In a high-speed data transmission with a short symbol period in a wireless communication channel having multipath fading, the inter-symbol interference becomes more severe when a single carrier method is used, whereas the complexity of the receiver is greatly increased. Since the symbol period in each subcarrier can be extended by the number of subcarriers while maintaining the same, a simple equalizer having a single tap can cope with a severe frequency selective fading channel by multipath. In the OFDM scheme, since a plurality of carriers having mutual orthogonality are used, frequency utilization efficiency is increased, and a process of modulating and demodulating the plurality of carriers in a transceiver may be performed at high speed.

The transmission signal of OFDM is a sum of a plurality of digital modulation waves. QPSK, QAM, etc. are used as a modulation method of each carrier. Data transmission by OFDM is in units of transmission symbols, and each transmission symbol is composed of a valid symbol interval and a guard interval. The guard interval is a signal interval to reduce the effects of multipath.

FIG. 2 is a diagram illustrating a frame structure applied to an OFDM WLAN system. FIG. 3 is a view illustrating a frame structure applied to an OFDM WLAN system in time and frequency domains.

As shown in FIG. 2, the data frame in the OFDM scheme consists of training symbols and data, and the training symbol field includes, for example, first and second training symbols having a length of 8 ms (TRAINING SYMBOL 1, TRAINING SYMBOL 2). And a data field includes a plurality of data symbols DATA SYMBOL #X and a plurality of guard intervals (GIs) disposed between the data symbols, and includes N guard intervals and data symbols. Consists of a set. Here, N is the number of OFDM symbols transmitted in one frame, and when the data rate is 36 Mbps, data is transmitted by 18 octets per OFDM symbol. The length of the guard section is set to 0.8 ms, for example, and the length of the data symbol is set to 3.2 ms, for example.

The transmitter transmits two long training symbols having a length of 8 으로 to the receiver to estimate the information of the radio channel, and then uses the estimated values to restore the data values of the data field transmitted to the end of the frame. Do it. As shown in FIG. 3, pilots are included in the data symbols, but are mainly used for correcting phase errors, and a complex algorithm is required because the intervals between pilots are large for estimating the information of the radio channel. That is, when data is transmitted in this manner, if the moving object moves at a high speed, the environmental change of the wireless channel is severe in the environment, and thus there is a problem in that the data cannot be normally restored only by the information of the wireless channel estimated by the training symbol.

As such, in the current OFDM frame structure, since the change in the radio channel during transmission of one frame is sufficiently small to effectively compensate for the influence of the channel, the performance deterioration is small in a fixed environment such as an office and a low speed mobile environment. In the high-speed mobile environment in which the channel environment changes rapidly during data field transmission, the degradation of performance becomes serious.

Therefore, in order to apply the WLAN system in a high speed mobile environment, the frame structure must be changed to be robust to fast fading. To this end, a method of transmitting the frame length by reducing the length of the channel environment to a small length has been proposed, but in this case, the average network rate is lowered and thus it is not a fundamental solution. That is, since the length of an OFDM frame can be variably transmitted from 1 to 4095 bytes, when transmitting the length of the frame divided by the length of the channel environment with little change, overhead is required for every packet and multiple packets must be transmitted. There is a problem that the transmission characteristics are degraded.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and by changing the frame structure in an OFDM type WLAN system, data in a WLAN system that can minimize the effects of fading due to a fast channel environment change in a high speed mobile environment. There is a technical problem in providing a transmission method.

1 is a view for explaining a general wireless LAN system,

2 is a view for explaining a frame structure applied to a wireless LAN system of the OFDM method,

3 is a diagram illustrating a frame structure applied to an OFDM wireless WLAN system in time and frequency domains;

4 is a view for explaining a frame structure applied to a data transmission method in a wireless LAN system according to the present invention;

5 is a view for explaining a PPDU frame structure applied to a data transmission method in a wireless LAN system according to the present invention.

<Description of the symbols for the main parts of the drawings>

10, 12: user wireless terminal 20: access point

30: Internet 40: CP Server

The present invention is a data transmission method in a wireless LAN system providing a wireless LAN service by the OFDM scheme, wherein an OFDM frame modulated by the OFDM scheme is composed of a data field including a plurality of training symbols and a plurality of data symbols, The data field is configured to include a plurality of data symbol groups and a plurality of pilot symbols respectively disposed between the plurality of data symbol groups, and the plurality of data symbol groups are configured by accessing a plurality of sets of guard intervals and data symbol pairs. Send to the wireless terminal through the point.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention.

4 is a view for explaining a frame structure applied to a wireless LAN service method according to the present invention. In the present invention, by dividing a data field into groups of a certain length and inserting a short length pilot symbol between the groups, the length of a data set consisting of one pilot symbol and one data symbol group is limited to less than the coherence time. Effectively compensates for radio channel changes in fading. Coherence time refers to a time when it is estimated that there is little power difference between data symbols. Therefore, in the present invention, the symbols are grouped with little power difference.

As shown, the data frame according to the present invention includes first and second training symbols (TRAINING SYMBOL 1, TRAINING SYMBOL 2), a plurality of data symbol groups (DATA SYMBOL GROUP # 1, ..., DATA SYMBOL GROUP #M) and A plurality of pilot symbols (PILOT SYMBOL) are added between each group of data symbols. In addition, each data symbol group DATA SYMBOL GROUP # 1, ..., DATA SYMBOL GROUP #M may be arranged between a plurality of data symbols DATA SYMBOL # 1, ..., DATA SYMBOL #L and a plurality of data symbols. It consists of L sets of guard intervals and data symbols, including a guard interval (GI).

As described above, in order to configure a frame, the OFDM data symbols transmitted in one frame must be divided into a coherent data symbol group. For this purpose, the number of data symbols included in one data symbol group must be determined. The number of data symbols included in one data symbol group is the number of data symbols included in the coherence time.

To this end, first, when the total number of OFDM data symbols transmitted in one frame is N, N OFDM symbols are divided by L to form an OFDM symbol group. Here, the number M of OFDM symbol groups is extracted by Equation 1.

M = Ceiling {N / L}

In the above formula, Ceiling {x} means the minimum integer greater than or equal to x. In addition, the number L of OFDM symbols included in one OFDM symbol group is derived by [Equation 2].

L = Ceiling {Q}

In Equation 2, T _C is the coherence time, for example, Equation 3, T _S represents the length of an OFDM symbol, and Δ represents other performances such as modulation scheme and channel coding rate. As a margin considering the part that affects, it can be defined as shown in [Table 1] according to the modulation method, but is not limited thereto and may be determined differently considering the overall condition.

Where f _c [GHz] is the center frequency and v [km / h] is the speed of the moving object.

Modulation method Δ BPSK 0 QPSK 0 ~ 1 16-QAM 1 ~ 2 64-QAM 2 ~ 3

For example, in a mobile environment moving at a speed of 80 km / h, using a frequency band of 5 GHz as in IEEE 80.211a, and applying all modulation schemes of BPSK to 64QAM, and T _S is 4㎲, Δ is 2 Therefore, the number L of data symbols is seven. Since the number of data symbols is extracted as described above and the total number of OFDM symbols is already known, the number of data symbol groups can be obtained by Equation 1.

After dividing the OFDM data symbols into M symbol groups consisting of L data symbols by this method, pilot symbols for estimating and compensating for channel changes are inserted before each OFDM symbol group. In this frame structure, the actual data rate R_new due to the insertion of pilot symbols can be obtained by using Equation 4 when the data rate of the conventional OFDM WLAN system is R_old.

R_new = R_old * (1-1 / (L + 1))

When the OFDM frame structure is changed by the above method, the current access point and the user terminal cannot accommodate the proposed fast mobile frame structure, so it is necessary to confirm whether the new frame structure is accepted between the access point and the user terminal. There is a need for information bits for a set of rates for travel (data rates per second depending on the modulation scheme).

In FIG. 5, one packet protocol data unit (PPDU) frame structure is shown in IEEE 802.11a, the OFDM scheme. 5 is a view for explaining a PPDU frame structure applied to a wireless LAN service method according to the present invention.

As shown, the PPDU frame includes a physical layer convergence procedure (PLCP) header portion, a physical layer convergence protocol service data unit (PSDU) field, a tail bit field, and a pad field, and a physical layer (PLCP). The Convergence Procedure header includes a rate field (RATE), a reserved field (reserved), a length information field (LENGTH), a parity bit field (Parity), a tail bit field, and a service field (SERVICE).

More specifically, after 4 bits of the rate field, 1 bit of the reserved field and the lower 9 bits of the 16 bits of the service field are prepared for future use, and the information transmitted in 4 bits of the rate field is 8 Specified as. Therefore, it can be seen that 8 additional information can be transmitted using the rate field.

In addition, although the lower 9 bits of the service field are not used, the modulation scheme and the coding rate change depending on the rate, and thus are not suitable for the transmission of an information signal requiring reliability. Therefore, the information of the improved pilot symbol should be transmitted using only the rate field and the spare field.

That is, whether or not the access point and the user terminal support the fast mobile frame structure can be represented by one bit, so that transmission is performed using a reserved field. That is, 1 (or 0) is transmitted when the frame structure for fast movement is supported, and 0 (or 1) is transmitted when it is not supported.

In addition, the data rate per second according to the modulation scheme is transmitted using the rate field 4 bits. As described above, in the current OFDM-based WLAN standard, since there are a total of eight rate sets, eight of the total 16 sets available in the rate field 4 bits remain. Therefore, eight rate fields for a fast moving object can be added. The rate field value for the fast moving object is used by using the complement of the current rate field value of 8, as shown in [Table 2].

Rate (Mbps) R1-R4 Remarks 6 1101 Existing Standard 6 0010 For high speed moving objects 9 1111 Existing Standard 9 0000 For high speed moving objects 12 0101 Existing Standard 12 1010 For high speed moving objects 18 0111 Existing Standard 18 1000 For high speed moving objects 24 1001 Existing Standard 24 0110 For high speed moving objects 36 1011 Existing Standard 36 0100 For high speed moving objects 48 0001 Existing Standard 48 1110 For high speed moving objects 54 0011 Existing Standard 54 1100 For high speed moving objects

In this manner, the new rate field value can also have the reliability of the existing field value.

In summary, the present invention is to propose a data transmission scheme suitable for a high-speed mobile environment, and in transmitting a plurality of OFDM symbols, a plurality of OFDM symbols (N) including a plurality of data symbols (L) Divide into M groups of data symbols. In determining L, it is necessary to consider factors (Δ) affecting transmission performance such as coherence time (T _C ), modulation scheme, and channel coding rate in consideration of transmission characteristics between successive data symbols. desirable.

After determining L, accordingly, a plurality of OFDM symbols are grouped into a plurality of data symbol groups including a plurality of data symbols, and then communication equipment (access point and wireless terminal) has such a high-speed mobile frame structure in order to successfully transmit them. It is necessary to distinguish whether or not the device supports the device. In the case of the device that supports the frame structure for the high-speed movement, the data rate according to the modulation method is inserted into the data packet format and transmitted.

As such, those skilled in the art will appreciate that the present invention can be implemented in other specific forms without changing the technical spirit or essential features thereof. Therefore, the above-described embodiments are to be understood as illustrative in all respects and not as restrictive. The scope of the present invention is shown by the following claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention. do.

As described above, according to the present invention, when a OFDM symbol is transmitted in a WLAN system, data symbols are grouped according to a high-speed mobile environment, and a pilot symbol used for channel characteristic estimation is inserted between groups of data symbols, thereby moving objects in the WLAN system. Even in the case of moving at high speed, data transmission and demodulation can be performed without errors without fading.

In addition, whether or not the communication device that will receive the frame in the data packet format supports the frame structure for high-speed movement, and in the case of communication equipment that supports the frame structure for high-speed movement, the data transmission rate according to the modulation scheme is transmitted by transmitting the frame. In a wireless LAN system in which a moving object moves at a high speed at a receiving end, it is possible to improve the deity of data transmission.

Claims (6)

A plurality of wireless terminals equipped with a wireless LAN card, a plurality of access points located in an area communicatable with the wireless terminal for performing wireless communication with the wireless terminal, and an internet for collecting the plurality of access points. A data transmission method in a WLAN system for providing a WLAN service by using an OFDM frame, the OFDM frame modulated by the OFDM scheme includes a data field including a plurality of training symbols and a plurality of data symbols, The data field is arranged between a plurality of data symbol groups and the plurality of data symbol groups, respectively, and includes a plurality of pilot symbols for estimating and compensating for a change in a channel. The plurality of data symbol groups are composed of a plurality of sets of guard intervals and data symbol pairs, and are transmitted to the wireless terminal through the access point. The method of claim 1, The number M of data symbol groups is determined as a minimum integer greater than or equal to a value obtained by dividing the total number N of the data symbols by the number L of specified data symbols, The number of specified data symbols L is determined by Q ² = T _C / [(1 + Δ) * T _S ] and L = [least integer greater than or equal to Q], where T _C is the coherence time. , T _S is the length of the OFDM symbol, Δ represents the margin in consideration of the part affecting the data transmission performance according to the modulation scheme and the channel coding rate. The method of claim 2, The margin Δ considering the part affecting the data transmission performance according to the modulation scheme and the channel coding rate is 0 for the BPSK modulation scheme, 0 to 1 for the QPSK modulation scheme, and 1 to 16 for the 16-QAM modulation scheme. 2, the case of the 64-QAM modulation method data transmission method in an OFDM method wireless LAN system characterized in that 2 to 3. The method of claim 1, The OFDM frame is included in a packet protocol data unit (PPDU) frame and transmitted. The OFDM frame includes information indicating a data rate of the OFDM frame in a rate field constituting the PPDU frame. How to transfer data in the system. The method of claim 4, wherein The rate field value for indicating the data rate of the OFDM frame is 0010 when the data rate is 6 (Mbps), 0000 when the data rate is 9 (Mbps), 1010 when the data rate is 12 (Mbps), and 1010 when the data rate is 1000 for 18 (Mbps), 0110 for data rate of 24 (Mbps), 0100 for data rate of 36 (Mbps), 1110 for data rate of 48 (Mbps), 1110 for data rate of 54 (Mbps) A data transmission method in an OFDM type WLAN system, characterized in that set to 1100. The method of claim 1, The OFDM frame is transmitted by a packet protocol data unit (PPDU) frame, and information indicating whether an access point receiving the OFDM frame and a wireless terminal support the OFDM frame structure in a preliminary field constituting the PPDU frame. Data transmission method in a wireless LAN system of the OFDM method, including transmitting.