JP2008512024A - Method and system for link adaptation in a wireless network - Google Patents

Abstract

  A mobile station transmission rate from among a plurality of transmission rates between a plurality of stations in a wireless local area network (WLAN) using a function for determining whether a transmission error is caused by an increased path loss. Methods and systems for adapting are provided. Depending on the transmission time-out situation of the transmitted frame, the determination whether the transmission error associated with the time-out situation was caused by an increased path loss is from a plurality of input frames received by the mobile station. This is done using the received signal strength (RSS) value. If the determination is true, the RSS threshold is increased (eg, link adaptation rate threshold) and the frame is retransmitted.

Description

  The present invention relates to a communication system. More particularly, the present invention adapts the transmission rate using the discrimination between errors due to increased path loss and errors due to interference and collisions, such as a wireless LAN such as an IEEE 802.11 wireless local area network (WLAN). The present invention relates to a system and method for more accurately adjusting the transmission rate of each station in a network.

  The IEEE 802.11 standard specifies medium access control (MAC) and physical characteristics for wireless local area networks (WLANs) to support physical layer units. The IEEE 802.11 standard is defined in the international standard ISO / IEC 8802-11 “Information Technology-Telecommunications and information exchange area networks” (1999 edition) (which is incorporated herein by reference in its entirety).

  The IEEE 802.11 physical layer (PHY) defines multiple transmission rates based on various modulation and channel coding schemes, so that the sender of a frame can specify one of the multiple rates as a specific one. The selection can be made based on the radio channel condition between the receiving side and the transmitting side itself at the time. In general, the lower the transmission rate, the higher the transmission reliability.

  In such wireless systems, the propagation and interference environment varies with time and location due to factors such as station mobility, time-varying interference and collisions. As a result, there is no single PHY mode (modulation and coding scheme) that can be optimal under all circumstances. In order to alleviate this problem, link adaptation (LA) algorithms have been proposed. In the algorithm, the efficiency of the system is improved by adapting the modulation scheme optimal for the current link situation.

  In particular, a specific example of the LA algorithm allows a transmission station to adapt the transmission rate by utilizing the received signal strength (RSS). The RSS is measured from a frame received from an access point (AP) as a measure of link quality. Without taking into account rapid variations in SNR / SIR due to interference or multipath fading, it is assumed that RSS averages have a linear relationship with SNR. Therefore, the received power as a function of distance and path loss situation is very useful for estimating the most suitable matching PHY mode for the next transmission. Such a link adaptation algorithm is described in co-pending US Patent Application No. XXXXXXXXX, hereby incorporated by reference.

  A change in RSS indicates that the situation on the radio link between the station and the AP is changing, and because of the large distance or high path loss between the station and the AP, the transmission rate is adapted accordingly. There may be a need. However, since the prior art LA adaptation algorithm always assumes that the error is due to low RSS, the adaptation can be performed improperly.

  For example, other sources of errors that appear in a radio radio channel are interference noise at the receiving station, collisions at the receiving station, and multipath fading. The behavior of interference by other stations and other transmission devices in the same channel, such as microwaves, is almost pulsed and has a short lifetime. The LA algorithm should not adapt the rate to transmission errors caused by interference or collision, and in this case should not change the RSS threshold. Therefore, for LA algorithms that can provide dynamic rate adaptation based on RSS measured from received frames, it is necessary to distinguish between errors due to low RSS (long distance) and errors due to interference. There is.

  The present invention provides increased path loss, shading and high path (due to increased distance) in order to better adjust the transmission rate in a wireless network such as a wireless local area network (WLAN). It is directed to a system and method for adapting transmission rates with the ability to distinguish between errors due to loss and errors due to interference and collisions.

  One aspect of the present invention is a method for determining a transmission rate of a mobile station from a plurality of transmission rates, the step of measuring an average received signal strength value from a plurality of input frames received by the mobile station; Comparing the average received signal strength value to a predetermined reference table having a received signal strength that is minimally required to transmit a specific frame length at one of the plurality of transmission rates; and the comparison Based on the result, a step of selecting a new transmission rate of the wireless device for subsequent transmission of a new frame, and a transmission error related to the timeout situation according to a transmission timeout situation of the transmitted frame Using the average received signal strength value to determine whether or not it was caused by increased path loss; For transmission of a new frame of, to a method and a step of selecting a new transmission rate of the mobile station.

  Was another aspect of the invention caused by increased path loss due to increased transmission errors for use in determining a mobile station's transmission rate among a plurality of transmission rates, eg, a link adaptation algorithm? It relates to a method for determining whether or not. In accordance with a transmission timeout situation of a transmitted frame, the method determines whether a transmission error associated with the timeout situation is caused by an increased path loss, and a plurality of inputs received by the wireless device. Determining by using the received signal strength value from the frame, and if so, increasing the RSS threshold, eg, a link adaptation rate threshold, and retransmitting the frame; if not, retransmitting the frame And a step of performing.

  Another aspect of the present invention is a system for adapting a link adaptive transmission rate of a mobile station among a plurality of transmission rates, a receiving circuit for demodulating an input frame, and a received signal of the received input frame A power measurement circuit for measuring an intensity, and a processor coupled to the power measurement circuit, for determining a transmission timeout situation of a transmitted frame, and causing a transmission error associated with the timeout situation to increase due to a path loss. Is determined by using received signal strength values from a plurality of input frames received by the mobile station. If true, the received signal strength threshold is increased and the frame is retransmitted. Or a processor having a processor for transmitting a new frame.

  Still another aspect of the present invention is a system for determining a transmission rate from a plurality of transmission rates, a receiving circuit for demodulating an input frame, and measuring a received signal strength of the received input frame A power measurement circuit and a processor coupled to the power measurement circuit, which calculates an average received signal strength and sets a new transmission rate for transmission of a new frame based on a comparison result with a predetermined reference table. The predetermined reference table includes a received signal strength that is minimally required to transmit at one of the plurality of transmission rates for a particular frame length, the processor comprising: Depending on the transmission timeout situation of the transmitted frame, the transmission error associated with the timeout situation may increase the path loss. If true, the new transmission rate of the mobile station for subsequent transmission of the new frame or retransmission of the frame is determined. The system further comprises a memory coupled to the processor, the system comprising a memory for storing the predetermined reference table and the average received signal strength for subsequent acquisition.

  These and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment, which is illustrated in the accompanying drawings. Throughout the various drawings, reference symbols refer to the same parts.

  Although the present invention will now be described with particular reference to the system block diagram of FIG. 1, the apparatus and method according to the present invention may be used with other infrastructures in which stations communicate with other stations via a wireless medium. This should be understood at the beginning of the following description.

FIG. 1 shows a typical network to which an embodiment of the present invention is applied. As shown in FIG. 1, an access point (AP) 2 is coupled to a plurality of mobile stations (STA _i ) 10. The plurality of mobile stations communicate with each other and with the AP through a wireless link. An important principle of the present invention is that the frame error probability depends on the signal-to-noise ratio (SNR), transmission rate and length at the receiver, regardless of receiver performance and channel behavior. On average, the transmitting STA can relatively estimate the path loss and channel behavior by tracking the RSS measured from the frames transmitted by the receiving STA. It should be noted that in a typical IEEE 802.11 STA implementation, RSS can be used for the medium access control (MAC) protocol. Therefore, as long as the receiving STA uses a constant transmission power level for all transmissions, it generates a transmission rate reference for the transmission of subsequent frames so that the frame is transmitted at the appropriate transmission rate. And changes in RSS can be utilized to provide a mechanism for updating.

  Referring to FIG. 2, the AP and each STA in the WLAN of FIG. 1 may include a system having the architecture shown in the block diagram of FIG. Both the AP and the STA may include a receiver 12, a demodulator 14, a power measurement circuit 16, a memory 18, a control processor 20, a timer 22, a modulator 24, and a transmitter 26. Although this description refers to terms commonly used in describing a particular mobile station, the description and concepts may be applied to other processing systems, including systems having an architecture similar to that shown in FIG. Is true as well. The processor 20 may represent a microprocessor, a central processing unit, a computer, a circuit card, an application specific integrated circuit (ASIC). The memory 18 may represent a part or combination of disk-based optical or magnetic storage units, electronic memory, and other and other memory devices. However, in other embodiments, hardware circuitry may be utilized instead of or in combination with software instructions to implement the present invention.

  In operation, the receiver 12 and transmitter 26 are connected to an antenna (not shown) to convert the received signal through the demodulator 14 and modulator 24, respectively, and transmit the desired data to the corresponding digital data. Not). The power measurement circuit 16 operates under the control of the processor 20 to detect the RSS of frames received by the circuit. RSS for other stations is estimated and stored in memory 18. Memory 18 is coupled to processor 20 for subsequent acquisition. The estimated RSS for other stations in the same BSS is updated and used to generate a reference table that is later used to select an appropriate transmission rate. The timer 22 is used to delete old RSS estimates stored in the memory 18. In an embodiment, RSS is updated because it tends to change due to the time-varying nature of the radio channel and the potential mobility of WLAN STAs.

  FIG. 3 shows a transmission reference table for selecting an appropriate transmission rate as an example. Each time a transmitting STA transmits a frame with a specific length and receives a corresponding response signal, the transmitting STA is measured in a reference table to be used in the transmission of subsequent frames. A threshold boundary is generated or updated based on RSS. Once the RSS threshold boundary was established for each different frame interval (eg, 0-100 bytes, 100-1000 bytes and 1000-2400 bytes), the sending STA was measured from the frames received from the receiving STA. The transmission rate is adapted depending on the RSS. Note that a change in RSS indicates that the situation on the radio link between the transmitting STA and the receiving STA is changing. As shown in FIG. 3, each threshold boundary indicates the minimum RSS value required for a particular transmission PHY rate. For example, when an STA that is monitoring RSS from a frame transmitted by a receiving STA detects that the RSS is lower than one of the threshold values (ie, between the receiving STA and the transmitting STA). Due to the increased distance, subsequent transmission attempts may be made at a lower rate to ensure normal reception of the frame.

The following is a list of variables used in FIGS.

  Briefly, the frame length interval j = {1, 2, 3} represents various frame sizes 0-100 bytes, 100-1000 bytes, and 1000-2400 bytes, respectively. Data rate i = {1, 2, 3, 4} represents one of the four data rates 1, 2, 5.5 and 11 Mbps available in this example. An RSS threshold is defined for each of the intervals. The threshold LA_th [i, j] represents the minimum RSS_avg or RSS threshold value for transmitting frames at the data rate i and the length interval j.

  FIG. 4 shows (1) errors due to increased path loss, shading and high path loss (due to increased distance) and (2) interference and collisions to adjust the transmission rate in the wireless network. The entire operation for identifying an error due to is shown.

  In general, the mobile unit is configured to operate in two modes: (1) receive mode and (2) transmit mode (step 100). The STA sends a request signal to send data, and then selects a transmission rate based on the RSS average (RSS_avg) threshold, the frame size, and the number of retransmission attempts. Here, rate adaptation occurs when the average RSS measured from the received frame exceeds some threshold in the reference table that includes the minimum RSS value required for a particular transmission rate. Thereafter, the STA transmits a frame at the selected transmission rate. Depending on whether the transmission was successful and whether the frame error was caused by interference and collision or due to path loss, the STA updates the corresponding “threshold” in the reference table. Thus, the STA selects a transmission rate according to RSS_avg, frame length, and retransmission attempt. Note that in FIG. 4, the block diagram is shown for an 802.11 STA operating in a basic service set where all frames are always transmitted / received to / from the AP. Therefore, the receiving STA mentioned here is always an AP.

  Each time a transmitting STA transmits a frame having a specific length, the STA receives a corresponding response signal (ie, a response (ACK) frame). If a response signal is received, this means that the transmission rate was appropriate. If a response signal is not received within a predetermined time, a response signal timeout situation has occurred (step 102). In a timeout situation, it is determined in step 104 whether the error was caused by receiver performance (eg, RSS that is too low). If no, the cause of the error is determined to be, for example, interference, collision, multipath, etc. For example, after a timeout situation (step 102) has occurred, a determination is made in step 104 as to whether the RSS has changed more than a particular RSS threshold within a predetermined period.

The change of RSS with time, that is, ΔRSS / Δt can be calculated by the following equation.
ΔRSS / Δt = RSS_avg−actual RSS

  In the determination at step 104, as the change in RSS during a particular time increases, so does the probability that an error will be caused because the RSS at the receiver is too low. This further indicates that errors are caused by receiver performance (PER) and are caused by low power at the receiver. Therefore, in step 108, the RSS threshold / link adaptation rate threshold (LA_th [i] [j]) is increased, and then in step 106, after calculating the optimum data rate as shown in FIG. Sent. If the determination in step 104 indicates that there is no change in RSS for a particular time or only a slight change (eg when compared to a predetermined threshold), an error due to collision or interference at the receiver. There is a high possibility that it has occurred and the channel status has not deteriorated. Accordingly, in step 106, the frame is transmitted without a change in the link adaptation rate threshold (LA_th [i] [j]). It should be noted that the probability of collision with each retransmission is reduced, so that those skilled in the art may determine different retransmission threshold rates for different conditions.

  In FIG. 5, at step 120, a transmission rate is selected. In step 122, the highest possible transmission rate is checked in the LA_th table. For example, if RSSI_avg> = LA_th [i] [j] is false, a new (higher) rate is selected. Otherwise, the selected rate is utilized in step 124.

  As is apparent from the above, the present invention is advantageous in that, unlike the prior art, the RSS threshold is increased only after a channel-related error (eg, there is no update after a collision or interference). . The transmission rate is only adapted after an associated change in RSS or long (after a certain number of packets) interference. Furthermore, the present invention allows the selection of an appropriate transmission rate without making any changes to the current IEEE 802.11 WLAN medium access control specification.

  While the preferred embodiment of the invention has been illustrated and described, various changes and modifications can be made without departing from the true scope of the invention, and equivalents may be substituted for elements of the invention. It will be appreciated by those skilled in the art. In addition, many modifications may be made to adapt to a particular situation and teachings of the invention without departing from the central scope. Therefore, the present invention is not intended to be limited to the embodiments disclosed as the best way to carry out the invention, and all embodiments that fall within the scope of the appended claims. It is intended to include.

FIG. 2 is a simplified block diagram illustrating the architecture of a wireless communication system to which embodiments of the present invention are applied. FIG. 2 shows a simplified circuit diagram of access points and stations within a particular basic service set (BSS) according to an embodiment of the present invention. FIG. 4 is a graphical illustration of a transmission reference used to adjust a transmission rate according to an embodiment of the present invention. FIG. 6 is a flow diagram illustrating operational steps for identifying errors due to increased path loss and errors due to interference and collisions for adjusting transmission rates according to an embodiment of the present invention. Fig. 4 illustrates the calculation of an optimal data rate according to an embodiment of the present invention.

Claims (16)

A method for adapting a link adaptive transmission rate of a wireless device from a plurality of transmission rates,
Depending on the transmission timeout situation of the transmitted frame, whether the transmission error associated with the timeout situation was caused by an increased path loss, received from a plurality of input frames received by the wireless device Determining by utilizing signal strength values;
If true, increasing the received signal strength threshold and retransmitting the frame or transmitting a new frame;
Having a method.   The method of claim 1, further comprising the step of resending the frame or transmitting a new frame if transmission errors associated with the timeout situation are not caused by increased path loss.   Measuring an average received signal strength value from a plurality of input frames received by the wireless device and determining whether the received signal strength has changed more than a predetermined value during a predetermined time; The method of claim 1.   The method of claim 1, wherein the received signal strength threshold is a link adaptation rate threshold.   If the received signal strength change over time is greater than the predetermined value, the transmission error associated with the timeout situation is caused by increased path loss and the received signal strength threshold is increased. The method according to claim 3.   The method of claim 1, wherein the input frame is transmitted by an access point.   The method of claim 1, wherein the input frame is transmitted by another wireless device. A method for determining a transmission rate of a wireless device from a plurality of transmission rates,
Measuring an average received signal strength value from a plurality of input frames received by the wireless device;
Comparing the average received signal strength value to a predetermined reference table having a received signal strength that is minimally required to transmit a specific frame length at one of the plurality of transmission rates;
Selecting a new transmission rate of the wireless device for subsequent transmission of a new frame based on the comparison result; and
Using the average received signal strength value to determine whether a transmission error associated with the timeout situation is caused by an increased path loss in response to a transmission timeout situation of the transmitted frame;
If true, selecting a new transmission rate of the wireless device for subsequent transmission of a new frame or retransmission of the frame;
Having a method. A wireless device adapting a link adaptive transmission rate from among a plurality of transmission rates,
A receiving circuit for demodulating the input frame;
A power measurement circuit for measuring the received signal strength of the received input frame;
A processor coupled to the power measurement circuit for determining a transmission timeout situation of a transmitted frame and determining whether a transmission error associated with the timeout situation is caused by an increased path loss; Determined by using received signal strength values from multiple input frames received by the wireless device, and if true, increase the received signal strength threshold and retransmit the frame or transmit a new frame A processor;
A wireless device.   10. The wireless device of claim 9, wherein the processor further comprises retransmission of the frame or transmission of a new frame if transmission errors associated with the timeout situation are not caused by increased path loss. .   The processor further measures an average received signal strength value from a plurality of input frames received by the wireless device and determines whether the received signal strength has changed more than a predetermined value during a predetermined time. The wireless device according to claim 9, comprising:   The radio apparatus according to claim 9, wherein the received signal strength threshold is a link adaptive rate threshold.   If the received signal strength change over time is greater than the predetermined value, the transmission error associated with the timeout situation is caused by increased path loss and the received signal strength threshold is increased. The wireless device according to claim 10.   The wireless device according to claim 9, wherein the input frame is transmitted by an access point.   The wireless device according to claim 9, wherein the input frame is transmitted by another wireless device. A wireless device for determining a transmission rate from a plurality of transmission rates,
A receiving circuit for demodulating the input frame;
A power measurement circuit for measuring the received signal strength of the received input frame;
A processor coupled to the power measurement circuit for calculating an average received signal strength and selecting a new transmission rate for transmission of a new frame based on a comparison result with a predetermined reference table;
The predetermined reference table includes a received signal strength that is minimally required to transmit at one of the plurality of transmission rates for a particular frame length, and the processor Depending on the transmission timeout situation, it is determined whether the transmission error related to the timeout situation is caused by an increased path loss, using the average received signal strength. Selecting a new transmission rate for the wireless device for transmitting a new frame or for retransmitting the frame,
A wireless device having a memory coupled to the processor for storing the predetermined reference table and the average received signal strength for subsequent acquisition.

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