Method of communications between MIMO stations

A link adaption method for multi input multi output (MIMO) system having a multi-antenna structure and a communicating method of MIMO stations in a basic service set (BSS), which constitutes a wireless communication network. The communicating method includes receiving from a predetermined MIMO station, the number of MIMO antennas and transmission rates supported by the respective MIMO stations. It further includes, storing one or more channel estimations indicators obtained while receiving various framed form the predetermined MIMO station for each MIMO antenna and for each transmission rate supported by the MIMO antenna, determining the threshold levels of the channel estimation for each MIMO antenna, and comparing the threshold levels with an average of channels estimation indicators for each MIMO antenna. The transmission rate is determined based on the comparison result and the data is then transmitted to the predetermined MIMO station ad the determined transmission rate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority of Korean Patent Application No. 10-2004-0002969 filed on Jan. 15, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to multiple input multiple output (MIMO) technology, and more particularly, to a link adaptation method suitable for a MIMO system having a multi-antenna structure.

2. Description of the Related Art

In accordance with the proliferation and development of digital devices, digital technology has demanded a high-speed wireless local area network (LAN) system that will operate at data rates of 100 Mbits/sec or higher. To meet such demand, multiple input multiple output (MIMO) technology has been introduced as a candidate for one of the most promising technologies for speeding up next generation wireless LAN systems.

The MIMO technology is classified into a spatial multiplexing technique, which enables higher-speed data transmission by simultaneously transmitting different types of data using multiple transmitting and receiving antennas without the necessity of increasing the bandwidth of an entire system, and a spatial diversity technique, which enables transmission diversity by transmitting one kind of data using multiple transmitting antennas.

Specifically, the spatial multiplexing technique is an adaptive array antenna technique which electrically controls directionality using multiple antennas, in which a plurality of independent transmission paths are established by decreasing the directionality in a narrow-beam pattern, thereby increasing the transmission speed according to the number of antennas. In this case, the same frequency and transmission timing are utilized by the respective antennas.

In a conventional single input single output (SISO)-based wireless LAN system (IEEE 802.11 or 802.11a), a link adaptation method, which varies data transmission methods adaptively to the communication network environment between stations, employs a state of a wireless channel as a factor that can be used in data transmission by a current transmitter, thereby achieving efficient data transmission between the stations.

FIG. 1illustrates the relationship between a media access control (MAC) layer20and a physical layer10according to the IEEE 802.11 standard. Referring toFIG. 1, the MAC layer20performs data communication with higher layers via a MAC service access point (SAP)30and with the physical layer10via a physical SAP40. The physical layer10comprises two sublayers, including a physical layer convergence procedure (PLCP) sublayer11and a physical medium dependent (PMD) sublayer12. The PLCP sublayer11and the PMD sublayer12perform data communication via a PMD SAP50.

The PLCP sublayer11is a layer defined to allow the MAC layer20to be minimally associated with the PMD sublayer12. In other words, the PLCP sublayer11converts a service occurring in the MAC layer20into a service compatible with an orthogonal frequency division multiplexing (OFDM) physical layer or converts a signal obtained from the OFDM physical layer into a signal compatible with the service occurring in the MAC layer20so that the MAC layer20can operate independently of the OFDM physical layer.

The PMD sublayer12provides the OFDM physical layer with a predetermined signal transmission/reception method. In other words, the PMD sublayer12, which is closely related to the OFDM physical layer, converts the service occurring in the MAC layer20into a service compatible with the OFDM physical layer.

The physical layer10of a receiving station, specifically the PLCP sublayer11, transmits RXVECTOR60to the MAC layer20via the physical SAP40. Here, RXVECTOR60includes many parameters, including a received signal strength indicator (RSSI). The MAC layer20of a transmitting station transmits TXVECTOR70to the PLCP sublayer11via the physical SAP40. Here, TXVECTOR70includes parameters, such as data transmission rate, power and the like.

FIG. 2Aillustrates a function, to which TXVECTOR70is applied, and parameters of the function are also shown inFIG. 2A.FIG. 2Billustrates a function, to which RXVECTOR60is applied.FIG. 2Balso displays parameters of the function. Referring toFIGS. 2A and 2B, TXVECTOR70is used as a factor of a function PHY-TXSTART.request, and RXVECTOR60is used as a factor of a function PHY-RXSTART.indicate.

More specifically, as shown inFIG. 2A, the TXVECTOR70includes parameters LENGTH, DATARATE, SERVICE, and TXPWR LEVEL. The parameter LENGTH indicates the number of data octets to be transmitted from a MAC layer of a transmitting station to a receiving station via a physical layer of the transmitting station and has a value between 1 and 4095. The parameter DATARATE indicates a transmission rate of signals transmitted over a wireless LAN, which can be selected among transmission rates supported by the IEEE 802.11a standard, i.e., 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Among the transmission rates, 6, 12, and 24 Mbps are essentially supported. The parameter SERVICE includes 7 null bits reserved for initialization of a scrambler and 9 null bits reserved for later use. The parameter TXPWR_LEVEL is used for determining the power of signals to be transmitted and has a value between 1 and 8.

In such a wireless LAN environment, a transmitting station transmits data to a receiving station at a transmission rate. Alternatively, a transmitting station transmits data to a receiving station based on the power of the signal selected by a transmission rate switching mechanism. In this case, the transmitting station performs rate switching through various indicators of states of channels, such as the transmission success proportion of previous frames.

There is another conventional link adaptation method that increases, decreases, or maintains a transmission rate based on a result obtained by comparing an RSSI value measured at an antenna of a conventional SISO system with a predetermined threshold value.

There is a still another conventional link adaptation method that checks packet error rate (PER), which is another parameter used in a link adaptation process, i.e., that checks the transmission success proportion of an acknowledgement (ACK) frame transmitted from a receiving station in response to the transmission of data to the receiving station.

The conventional link adaptation methods described above are inappropriate for MIMO systems using a multi-antenna structure even though they are still effective for SISO systems using a single antenna structure. Therefore, there exists a need for development of a link adaptation method for MIMO systems.

SUMMARY OF THE INVENTION

The present invention provides an improved wireless LAN system for a MIMO system having multiple antennas, in which the most efficient transmission rate is selected for data transmission according to states of communication network channels and the number of receiver antennas.

In accordance with an aspect of the present invention, there is provided a communicating method of MIMO stations in a basic service set (BSS), which constitutes a wireless communication network, the communicating method comprising, receiving the number of MIMO antennas and transmission rates supported by the respective MIMO antennas from a predetermined MIMO station, storing one or more channel estimation indicators obtained while receiving various frames from the predetermined MIMO station for the respective MIMO antennas and for the respective transmission rates supported by the MIMO antennas, determining threshold levels of the channel estimation indicators for the respective MIMO antennas, comparing the threshold levels of the channel estimation indicators for the respective MIMO antennas with averages of the channel estimation indicators for the respective MIMO antennas, and determining a transmission rate based on comparison results to then transmit data to the predetermined MIMO station at the determined transmission rate.

The method of determining a transmission rate may comprise, determining, the highest transmission rate for each MIMO antenna for which the average of the channel estimation indicators stored for each MIMO antenna respectively exceeds the threshold level of the channel estimation indicators, summing up the highest transmission rate determined for the respective MIMO antennas, and selecting a lower transmission rate among the entire transmission rates obtained as the summing result and the entire transmission rates determined for the predetermined MIMO stations, and transmitting data to the predetermined MIMO station at the selected transmission rate.

The channel estimation table may include a received signal strength indicator (RSSI) defined by the IEEE 802.11 standard.

Also, the channel estimation table may include an average packet error rate (PER) as one of the channel estimation indicators.

The channel estimation table may further include an average number of retries of data transmission until an acknowledgement (ACK) frame is received as one of the channel estimation indicators.

The RSSI value measured for each antenna is preferably converted into vectors to be transmitted to a MAC layer and is stored in the channel estimation table.

In the present invention, MIMO communications can be realized by using conventional spatial multiplexing MIMO chipsets, a detailed dcscription of which, however, will not be presented here. For example, MIMO communications can be realized using an AGN 100 Wi-Fi chip set manufactured by Airgo Networks. Thc AGN 100 Wi-Fi chip set, which has a much higher transmission rate of 108 Mbps per channel, compared with a conventional Wi-Fi chip set, is perfectly compliant with all of the Wi-Fi standards and supports the IEEE 802.11a, 802.11b, and 802.11g standards.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3is a diagram illustrating the format of an IEEE 802.11 management frame100. The management frame100may be a beacon frame, an association request frame, a dissociation frame, an association response frame, a probe request frame, or a probe response frame.

The management frame100includes a media access control (MAC) header110, a frame body120, and a frame check sequence (FCS) field130. The MAC header110includes a frame control field, a duration field, a destination address (DA) field, a source address (SA) field, a basic service set identification (BSSID) field, and a sequence control field.

The frame body120of the management frame100is filled with one or more information elements (IEs)200, as shown inFIG. 4. Each of the IEs200includes an element ID field210in which the type of IE200is recorded, an information field230in which data to be actually transmitted through the IE200is recorded, and a length field220in which the size of the information field230is recorded.

FIG. 5is a tabulated diagram illustrating information elements and their respective element IDs recorded in their respective element ID fields. The respective element ID determines the type of information element. In the prior art, information elements having element IDs of 32-255 are reserved. In the present invention, the information element having an element ID of 32, called a MIMO capability information212, is further added to the conventional information elements.

In the conventional SISO technology, the transmission rate is determined by an information element having an element ID of 1, which is a supported rate information element211.

The link adaptation method for a wireless communication network which uses at least one antenna configured to transmit and receive, comprises generating information on a management frame. The information on the management frame comprises single input single output (SISO) information that indicates at least one transmission rate of a first plurality of transmission rates supported by a single SISO antenna configured to transmit and receive, and multi input multi output (MIMO) information that indicates at least one transmission rate of a second plurality of transmission rates supported by a plurality of MIMO antennas configured to transmit and receive. The information on the management frame is transmitted using either the single SISO antenna or at least one of the plurality of MIMO antennas.

FIG. 6illustrates the format of a supported rates information element300according to a preferred embodiment of the present invention. The supported rates information element300includes a 1-byte element ID field310, a 1-byte length field320, and a 1 to 8 byte supported rates field330. At least one of the eight transmission rates supported by the IEEE 802.11 standard, that is, 6, 9, 12, 18, 24, 36, 48, and 54 Mbps correspond to one of the bytes recorded in the supported rates information element300.

FIG. 7illustrates the format of a MIMO capability information element400according to a preferred embodiment of the present invention, which is an improvement of the supported rates information element300shown inFIG. 6. The MIMO capability information element400includes a 1-byte element ID field410, a 1-byte length field420, a 1-byte MIMO antenna field430, and a MIMO supported rates set field440, which has as many bits as a total number of transmission rates supported by each MIMO antenna.

The element ID of the MIMO capability information element400, that is, 32, is recorded in the element ID field410, and the sum of sizes of the MIMO antenna field430and the MIMO supported rates set field440is recorded in the length field420. Supposing that there are n MIMO antennas, the size of the MIMO supported rates set field440can be obtained by the following equation:

N=Qi=1n⁡[number⁢⁢of⁢⁢transmission⁢⁢rates⁢⁢supported⁢⁢by⁢⁢i⁢-⁢th⁢⁢MIMO⁢⁢ant⁢⁢enna].
Therefore, a value of N+1 is recorded in the length field420.

The MIMO antenna field430includes subfields, such as ‘The number of MIMO antennas’ field431comprising 3 bits in which the number n of MIMO antennas is recorded, and a 5-bit Reserved field432.

As described above, the MIMO supported rates set field440has a size of N bytes, and N is determined depending on the number n of MIMO antennas and the number of transmission rates supported by each MIMO antenna. Data rates supported by each MIMO antenna are recorded in the 1-byte subfields441through443. In other words, at least one of the eight transmission rates supported by the IEEE 802.11 standard, i.e., 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, may be recorded in each of the subfields441through443.

FIG. 8illustrates the relationship between an MAC layer20and a physical layer10in a MIMO system to which the present invention is applied. The MAC layer20and the physical layer10are the same as their respective counterparts illustrated inFIG. 1in terms of their structures and the way they transmit/receive data to/from their respective upper and lower layers. However, the MAC layer20and the physical layer10shown inFIG. 8are different in that RSSI as a parameter of the RXVECTOR60shown inFIG. 1is replaced by ‘RSSI of each antenna’ (or RSSI Per Antenna) of the RXVECTOR60shown inFIG. 8, and ‘transmission rate’ and ‘power’ as parameters of the TXVECTOR70shown inFIG. 1are replaced by ‘transmission rate of each antenna’ (or Transmission Rate Per Antenna) and ‘power of each antenna’ (or Power Per Antenna) shown inFIG. 8.

FIG. 9illustrates a history-based channel estimation table500according to the present invention.FIG. 9shows m stations existing in a basic service set (BSS), each of the m stations manages channel estimation indicators for the rest of the m stations using the history-based channel estimation table500.

Referring toFIG. 9, the history-based channel estimation table500records channel estimation indicators530together with a serial number510of each station and an antenna index520of each antenna of each station. In the present embodiment, the channel estimation indicators530are an average RSSI, an average packet error rate (PER), and an average number of retries of data transmission until an acknowledgement (ACK) frame is received. However, exemplary embodiments of the present invention may contain additional channel estimation indicators530.

The average RSSI is determined by measuring RSSI values of a unicast frame, a broadcast frame, a multicast frame, and an ACK frame, received by a station in a MIMO system, at each MIMO antenna. The average PER and the average number of retries of data transmission until an ACK frame is received are determined in almost the same manner as the average RSSI. The channel estimation indicators530are updated whenever new data is transmitted.

A user may set a threshold value for each of the channel estimation indicators530, based on experience or by trial and error, with reference to, for example, the average of each of the channel estimation indicators530. For example, as shown inFIG. 9, when an antenna index of a station1is 0, transmission rates at which an average RSSI exceeds a RSSI threshold, a transmission rate at which an average PER exceeds a PER threshold, and transmission rates at which an average number of retries of data transmission until an ACK frame is received exceeds a threshold value of retries of data transmission until an ACK frame is received are marked by hatched lines.

A critical transmission rate at which the average RSSI exceeds the RSSI threshold, a critical transmission rate at which the average PER exceeds the PER threshold, and a critical transmission rate at which the average number of retries of data transmission until an ACK frame is received exceeds the threshold value of retries of data transmission until an ACK frame is received are transmission rates 6, 3, and 4, respectively. Therefore, a critical transmission rate at which the averages of the channel estimation indicators530exceed the threshold of the channel estimation indicators530is the transmission rate 3. Then, a current transmission rate is switched to the transmission rate 3, thereby actually transmitting/receiving data to/from stations at the transmission rate 3.

While it has been described that threshold values are independently set for the respective channel estimation indicators530, an overall threshold value may be set for all of the channel estimation indicators530. In other words, the user may determine transmission rate depending on whether a value obtained by adding a predetermined weight on the averages of the channel estimation indicators530and summing up the resultant averages exceeds a threshold obtained by adding the predetermined weight on the thresholds of the channel estimation indicators530and summing up the resultant thresholds.

FIG. 10is a flowchart illustrating the overall operation of the present invention.

First, referring toFIG. 10, a first station receives a MIMO capability information element of a management frame transmitted from another station in its BSS in step S10. In step S20, the first station generates a history-based channel estimation table using information obtained from the MIMO capability information element, such as the number of MIMO antennas and transmission rates supported by each MIMO antenna.

While the first station transmits/receives various frames to/from another station, an RSSI value is measured for each MIMO antenna in step S30. As illustrated inFIG. 8, the first station transmits the RSSI value from a PLCP sublayer to an MAC layer.

In step S40, an average RSSI value in the history-based channel estimation table is renewed in the MAC layer for each MIMO antenna of each station by using the RSSI value transmitted from the PLCP sublayer. In step S50, an average PER and an average number of retries of data transmission until an ACK frame is received in the history-based channel estimation table is renewed based on a PER and the number of retries of data transmission until an ACK frame is received, which are measured for each MIMO antenna during transmission of various frames.

In step S60, an RSSI threshold, a PER threshold, and a retry threshold are determined based on a comprehensive evaluation of the RSSI, PER and retry channel estimation indicators530. The thresholds are determined, for example, based on experience, by trial and error, or in consideration of the purpose for using the stations.

When the first station attempts to transmit data, a highest transmission rate, at which the average RSSI, the average PER, and the average number of retries of data transmission until an ACK frame is received respectively exceed the RSSI threshold, the PER threshold, and the threshold number of retries of data transmission until an ACK frame is received, is set for each MIMO antenna in step S70. In step S80, transmission rates, at which the first station can transmit data to the second station, are determined by summing up the highest transmission rate set for each MIMO antenna.

In step S90, steps S10through S80are performed for a second station, which is to receive data from the first station, so that a highest transmission rate, at which an average RSSI, an average PER, and an average number of retries of data transmission until an ACK frame is received respectively exceed an RSSI threshold, a PER threshold, and a threshold number of retries of data transmission until an ACK frame is received, can be set for each MIMO antenna and so that transmission rates, at which the second station can receive data from the first station, can be determined by summing up the highest transmission rate set for each MIMO antenna.

In step S91, the lowest transmission rate among the total transmission rates corresponding to the first station and the total transmission rates corresponding to the second station is selected as a final transmission rate. In step S92, the first station transmits data to the second station at the final transmission rate determined in step S91.

Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood and included within the scope of the present invention as defined by the appended claims unless they depart therefrom. Therefore, the described embodiments are to be considered in all respects only as illustrative and not restrictive of the scope of the invention.

According to the present invention, it is possible to dynamically change transmission rates according to communication environments of various antennas in a MIMO system. In addition, it is possible to make the MIMO system compliant with a conventional wireless LAN environment by defining the format of frames to be suitable for the MIMO system according to the IEEE 802.11 standard.