CN101010900A - Method and system for link adaptation in wireless network - Google Patents

Abstract

A method and system are provided for modifying a transmission rate of a mobile station of a plurality of transmission rates between a plurality of stations in a Wireless Local Area Network (WLAN) with the ability to determine whether a transmission error is caused by increased path loss. In response to a transmission timeout condition for a transmitted frame, a Received Signal Strength (RSS) value from a plurality of incoming frames received by a mobile station is used to determine whether a transmission error associated with the timeout condition is caused by an increased path loss. If the determination is true, the RSS is increased, e.g., the link adaptation rate threshold, and the frame is retransmitted.

Description

Method and system for link adaptation in wireless network

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

The IEEE 802.11 standard specifies medium access control (MAC) and physical characteristics for wireless local area networks (WLANs) to support physical layer elements. The IEEE 802.11 standard is defined in the International Standard ISO/IEC 8802-11 "Information Technology-Telecommunications and Information Exchange Area Networks" published in 1999, which is hereby incorporated by reference in its entirety.

IEEE 802.11 Physical Layer (Physical Layers PHY) defines multiple transmission rates according to different modulation and channel coding schemes, so that the sender of the frame can select multiple transmission rates according to the wireless channel conditions between the receiver and itself at a specific time One of the rates. In general, the lower the transfer rate, the more reliable the transfer.

In such wireless systems, the propagation and interference environment changes over time and space 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 may be optimal in all situations. Link adaptation (LA) algorithms have been proposed to alleviate this problem, where the performance of the system is improved by modifying the modulation scheme to optimally suit the current link conditions.

In particular, a specific example of the LA algorithm enables the sending station to modify the transmission rate by using Received Signal Strength (RSS) as a link quality indicator, according to the received signal strength (RSS) it received from the access point (AP) Received frame measurements are obtained. Without considering the rapid fluctuation of SNR/SIR due to interference or multipath fading, it is assumed that RSS has a linear relationship with SNR on average. Therefore, received power as a function of distance and path loss conditions is very useful for estimating the most appropriately matching PHY mode for the next transmission. Such a link adaptation algorithm is described in co-pending US Patent Application No. XXXXXXXX, which is hereby incorporated by reference.

A change in RSS indicates that the conditions of the wireless link between the station and the AP are changing and it may be necessary to modify the transmission rate accordingly, possibly due to a larger distance or higher path loss between the station and the AP. However, prior art LA adaptation algorithms always assume that the error is due to low RSS and thus may adapt incorrectly.

For example, other error sources occurring in wireless radio channels are interference noise at the receiving station, collisions and multipath fading at the receiving station. Interfering behavior (like microwaves) due to other stations and other transmitting devices on the same channel is almost impulsive and short-lived. The LA algorithm should not modify its rate for transmission errors caused by interference or collisions and in this case the RSS threshold should not be changed. Accordingly, for an LA algorithm capable of providing dynamic rate adaptation based on Received Signal Strength (RSS) measured from received frames, it is necessary to distinguish between errors due to low RSS (large distance) and errors due to interference mistake.

The present invention relates to a method of modifying the transmission rate for better performance in a wireless network with the ability to distinguish between errors due to increased path loss (due to increased distance), fading and high path loss, and errors due to interference and collisions. Systems and methods for accurately adjusting transmission rates in wireless networks such as wireless local area networks (WLANs).

One aspect of the present invention relates to a method for determining a transmission rate of a mobile station among a plurality of transmission rates, the method comprising the steps of: measuring an average received signal strength (RSS) from a plurality of input frames received by the mobile station ) value; comparing said average RSS value with a predetermined reference table having the minimum RSS required to transmit a particular frame length at one of a plurality of transmission rates; and selecting said mobile station's new transmission rate for subsequent transmission of new frames, wherein in response to a transmission timeout condition in which the frame was sent; using the average RSS value to determine whether the transmission error associated with the timeout condition was caused by increased path loss; if so If , then a new transmission rate for the mobile station is selected for subsequent transmission of new frames.

Another aspect of the present invention relates to a method, such as a link adaptation algorithm, for determining whether a transmission error is caused by increased path loss for determining a transmission rate of a mobile station among a plurality of transmission rates. The method comprises the steps of: responding to a transmission timeout condition of a transmitted frame; using received signal strength (RSS) values from a plurality of incoming frames received by the mobile station to determine a transmission error associated with the timeout condition Is it caused by increased path loss; if so, increase the RSS threshold, such as the link adaptation rate threshold, and retransmit the frame; and if not, retransmit the frame.

Another aspect of the present invention relates to a system for modifying a link adaptive transmission rate of a mobile station among a plurality of transmission rates, comprising: a receiver circuit for demodulating an incoming frame; a power measurement circuit for measuring Received Signal Strength (RSS) of incoming frames received; a processor coupled to said power measurement circuit for determining a transmission timeout condition for a sent frame and using data from multiple incoming frames received by said mobile station Received Signal Strength (RSS) values are used to determine whether the transmission error associated with the timeout condition was caused by increased path loss, and if so, the RSS threshold is increased and the frame is retransmitted or a new frame is sent.

Yet another aspect of the present invention relates to a system for determining said one of a plurality of transmission rates, comprising a receiver circuit for demodulating an incoming frame; a power measurement circuit for measuring a received signal of an incoming frame received therein Strength (RSS); a processor coupled to said power measurement circuit for calculating an average RSS and selecting a new transmission rate for transmitting a new frame based on a comparison with a predetermined reference table comprising a minimum RSS required to transmit a particular frame length at one of a plurality of said transmission rates, wherein in response to a transmission timeout condition for which a frame is transmitted, said average RSS is used to determine whether a transmission error associated with said timeout condition consists of an increased caused by path loss; if so, selecting a new transmission rate of the mobile station for subsequent transmission of a new frame or retransmission of the frame; and a memory coupled to the processor for storing the A reference table and said average RSS are predetermined for subsequent acquisition.

The above and other features and advantages of the present invention will become more apparent from the following more detailed description of preferred embodiments as illustrated in the accompanying drawings, wherein reference numerals refer to like parts throughout the various views.

1 is a simplified block diagram illustrating the architecture of a wireless communication system to which an embodiment of the present invention is applied;

Figure 2 illustrates a simplified circuit diagram of an access point and each station within a particular Basic Service Set (BSS) according to an embodiment of the present invention;

Figure 3 is a graphical illustration of a transmission reference used to adjust a transmission rate in accordance with an embodiment of the present invention; and

4 is a flow chart illustrating operational steps for distinguishing between errors due to increased path loss and errors due to interference and collisions in order to adjust a transmission rate according to an embodiment of the present invention.

Although the invention is described below with particular reference to the system block diagram of FIG. 1, it should be understood that in the ensuing description the apparatus and method according to the invention can be used in other infrastructures where one station is communicating with another station via a wireless medium. .

Figure 1 illustrates a representative 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 that communicate with each other and with the AP via wireless links. The key principle of the present invention is that regardless of receiver performance and channel behavior, the frame error probability depends on the signal-to-noise ratio (SNR) at the receiver, its transmission rate and its length. On average, a transmitting STA can relatively estimate path loss and channel behavior by grasping RSS measured from frames transmitted by a receiving STA. Note that in a typical IEEE 802.11 STA implementation, RSS may be used for the Media Access Control (MAC) protocol. Thus, as long as the receiving STA uses a fixed transmit power level for all its transmissions, the variation in RSS can be used to provide a basis for generating and updating the transmission rate for subsequent transmission frames such that the frames are just at the correct The mechanism by which the transfer rate is sent.

Referring to FIG. 2 , the AP and each STA within the WLAN of FIG. 1 may include a system having the architecture illustrated in the block diagram of FIG. 2 . 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 the description may refer to terminology commonly used to describe a particular mobile station, the description and principles are equally applicable to other processing systems, including systems having different architectures than that shown in FIG. 2 . The processor 20 may represent a microprocessor, a central processing unit, a computer, a circuit card, or an application-specific integrated circuit ASIC. Memory 18 may represent, ie, disk-based optical or magnetic storage units, electronic memory, and portions or combinations of these and other storage devices. In other embodiments, however, hardware may be used instead of or in combination with software instructions for implementing the invention.

In operation, receiver 12 and transmitter 26 are coupled to an antenna (not shown), convert received signals and transmit desired data into corresponding digital data via demodulator 14 and modulator 24, respectively. Power measurement circuit 16 operates under the control of processor 20 to detect the RSS of frames received thereon. The RSS for other stations is estimated and stored in memory 18, which is coupled to processor 20 for subsequent retrieval. The estimated RSS with respect to other stations within the same BSS is updated and later used to generate a reference table which is used to select the correct transmission rate. A timer 22 is used to clear outdated RSS estimates, which are stored in memory 18 . In this embodiment, the RSS is updated when the RSS tends to change due to the time-varying nature of the wireless channel and the potential mobility of the WLAN STAs.

As an example, Figure 3 shows a transmission reference table for selecting an appropriate transmission rate. Whenever the transmitting STA transmits a frame with a certain length and receives a corresponding acknowledgment signal, the transmitting STA generates or updates a threshold boundary for subsequent transmission frames according to the RSS measured in the reference table. Once the RSS threshold boundaries are established for each of the different frame intervals (e.g., 0-100 bytes, 100-1000 bytes, and 1000-2400 bytes), the sending STA based on the RSS to modify the transfer rate. Note that a change in RSS indicates that the condition of the wireless link between the transmitting STA and the receiving STA is changing. As shown in Figure 3, the threshold boundaries for each indicate which is the minimum RSS value required for a particular transmission PHY rate. For example, if a STA that is monitoring the RSS from a frame transmitted by a receiving STA detects that the RSS is becoming lower than one of the thresholds (i.e., due to an increased distance between the receiving STA and the transmitting STA), Then the next transmission attempt may be made at a lower rate in order to ensure that the frames are received correctly.

The following is a list of the variables used in Figure 3-4:

I The subscript of the data rate F: Rate = {1, 2, 5.5, 11} (Mbps) = {1, 2, 3, 4} J The subscript of the frame length G: length j = {0-100, 100-1000, 1000-2400} (bytes) = {1, 2, 3} LA_th[i, j] Link Adaptation Rate Threshold 0<I<5&0<j<4 RSS Most recently received signal length measurement RSS_avg Average received signal length

In short, the frame length intervals, j={1, 2, 3}, represent different frame sizes: 0-100 bytes, 100-1000 bytes and 1000-2400 bytes, respectively. The data rate i = {1, 2, 3, 4} represents one of four available data rates in this example, namely 1, 2, 5.5 and 11 Mbps. Defines the RSS threshold for each interval. Threshold "LA_th[i,j]" represents the minimum "RSS_avg or RSS Threshold" value for which a frame is transmitted at data rate "i" within length interval "j".

Fig. 4 illustrates a diagram for distinguishing between (1) errors due to increased path loss (due to increased distance), fading and high path loss and (2) errors due to interference and collisions in order to adjust the The overall operation of the transfer rate.

Typically, a mobile unit is configured to operate (step 100) in two modes: (1) receive mode; and (2) transmit mode. The STA sends a request signal to send data, and then selects a transmission rate based on an RSS average (RSS_avg) threshold, frame size, and number of retransmission attempts. Here, rate modification occurs when the average RSS measured from received frames passes some threshold in a reference table containing the minimum RSS value required for a particular transmission rate. Thereafter, the STA transmits frames at the selected transmission rate. Depending on whether the transmission was successful and whether the frame error was caused by interference, collision or path loss, the STA updates the corresponding "threshold" in the reference table. Thus, the STA selects the transmission rate according to RSS_avg, frame length and retransmission attempts. Note that this block diagram is shown for an 802.11 STA operating in accordance with the Basic Service Set in Figure 4, in which case all frames are always sent to or received from its AP. Thus the receiving STA referred to here is always its AP.

Whenever a transmitting STA transmits a frame with a certain length, it receives a corresponding acknowledgment signal (ie, an acknowledgment (ACK) frame). When an acknowledgment is received, it indicates that the transfer rate is appropriate. If the acknowledgment signal is not received within the predetermined time, then an acknowledgment signal timeout condition occurs (step 102). In the case of a timeout, it is determined at step 104 whether the error was caused by the performance of the receiver (eg RSS too low). If false, the cause of the error is determined to be interference, collision, multipath, etc., for example. For example, after a timeout condition occurs (step 102), it is determined at step 104 whether a change in received signal strength (RSS) over a predetermined period of time has exceeded a certain RSS threshold.

The change of RSS over time can be calculated according to the following equation, ie, ΔRSS/Δt: ΔRSS/Δt:=RSS_avg−RSS_actual.

In the determination of step 104, as the variation of the RSS during a particular period of time increases, the probability of errors due to too low RSS at the receiver also increases. Furthermore, it shows that the error occurs due to the performance of the receiver (PER) and due to low power at the receiver. Thus, the RSS threshold/link adaptation rate threshold (LA_th[i][j]) is increased at step 108, and then the frame is retransmitted at step 106 after calculating the optimal data rate, as shown in FIG. 5 . If the determination at step 104 shows that there is no or little change in the RSS during the specified time period (e.g. when compared to a predetermined threshold), an error has occurred due to collision or interference at the receiver and channel conditions will not be changed The probability of being bad is high. Thus, in case there is no change in the link adaptation rate threshold (LA_th[i][j]), the frame is retransmitted at step 106 . It should be noted that with each retransmission, the probability of collision is lower, so that one skilled in the art can determine various different retransmission threshold rates for various different conditions.

In FIG. 5, at step 120, a transmission rate is selected. At step 122, the highest possible transmission rate in the LA_th table is checked, eg if (RSSI_avg>=LA_th(I)(J)) is false, then a new (higher) rate is selected. Otherwise in step 124 the selected rate is used.

From the above it can be seen that the present invention only increases the RSS threshold after a channel-related error (eg no update after collision or interference) since it differs from the prior art. The transmission rate is only modified after a relative change in RSS or a long disturbance (after a predetermined number of packets). Furthermore, the present invention is able to select an appropriate transmission rate without making any changes to the current IEEE 802.11 WLAN media access control specification.

While a preferred embodiment of the invention has been illustrated and described, it will be understood by those skilled in the art that various changes and modifications may be made and equivalents may be substituted for elements thereof without departing from the true scope of the invention. . In addition, many modifications may be made to adapt the particular situation and teachings of the invention without departing from the central scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (16)

1. A method for modifying a link adaptive transmission rate of a wireless device in a plurality of transmission rates, said method comprising the steps of: (a) in response to a transmission timeout condition of a transmitted frame; using received signal strength (RSS) values from a plurality of incoming frames received by the wireless device to determine whether a transmission error associated with the timeout condition is caused by increased path loss; and (b) If yes, then increase the RSS threshold and retransmit the frame or send a new frame. 2. The method of claim 1, further comprising the step of retransmitting the frame or sending a new frame if the transmission error associated with the timeout condition was not caused by increased path loss. 3. The method of claim 1, wherein step (a) further comprises the step of: measuring an average received signal strength (RSS_avg) value from a plurality of input frames received by the wireless device, and determining the received signal strength Whether the change in intensity over a predetermined period of time exceeds a predetermined value. 4. The method of claim 1, wherein the RSS threshold is a link adaptation rate threshold. 5. The method of claim 3, wherein if the change in RSS over time is greater than a predetermined value, the transmission error associated with the timeout condition is caused by path loss and the RSS threshold is increased. 6. The method of claim 1, wherein the incoming frame is sent by an access point (AP). 7. The method of claim 1, wherein the incoming frame is sent by another wireless device. 8. A method for determining a transmission rate of a wireless device of a plurality of transmission rates, the method comprising the steps of: (a) measuring an average Received Signal Strength (RSS) value from a plurality of input frames received by the wireless device; (b) comparing said average RSS value to a predetermined reference table having a minimum RSS required to transmit a particular frame length at one of a plurality of transmission rates; (c) selecting a new transmission rate of the wireless device for subsequent transmission of new frames based on the comparison, wherein (d) in response to a transmission timeout condition of the transmitted frame; using the average RSS value to determine whether a transmission error associated with the timeout condition was caused by increased path loss; and (e) If so, selecting a new transmission rate for the wireless device for subsequent transmission of a new frame or retransmission of the frame. 9. A wireless device for modifying a link adaptive transmission rate of a plurality of transmission rates, comprising: receiver circuitry for demodulating incoming frames; a power measurement circuit for measuring a Received Signal Strength (RSS) of an input frame received therein; a processor coupled to the power measurement circuit for determining a transmission timeout condition for a transmitted frame and using received signal strength (RSS) values from a plurality of input frames received by the wireless device to determine a correlation with the Whether the transmission error associated with the timeout condition was caused by increased path loss, and if so, the RSS threshold is increased and the frame is retransmitted or a new frame is sent. 10. The wireless device of claim 9, wherein the processor further comprises retransmitting the frame or sending a new frame if the transmission error associated with the timeout condition is not caused by increased path loss frame. 11. The wireless device of claim 9, wherein the processor further comprises determining an average received signal strength (RSS_avg) value from a plurality of input frames received by the wireless device, and determining that the received signal strength is within a predetermined Whether the change in time period exceeds a predetermined value. 12. The wireless device of claim 9, wherein the RSS threshold is a link adaptation rate threshold. 13. The wireless device of claim 10, wherein if the change in RSS over time is greater than a predetermined value, the transmission error associated with the timeout condition was caused by increased path loss and the RSS threshold is increased. 14. The wireless device of claim 9, wherein the incoming frame is sent by an access point (AP). 15. The wireless device of claim 9, wherein the incoming frame is sent by another wireless device. 16. A wireless device for determining a transmission rate of a plurality of transmission rates, comprising: receiver circuitry for demodulating incoming frames; a power measurement circuit for measuring a Received Signal Strength (RSS) of an input frame received therein; a processor coupled to said power measurement circuit for calculating an average RSS and selecting a new transmission rate for transmitting a new frame based on a comparison with a predetermined reference table comprising a plurality of said one of the transmission rates the minimum RSS required to transmit a particular frame length, wherein the average RSS is used to determine whether a transmission error associated with the timeout condition is caused by increased path loss in response to a transmission timeout condition for the transmitted frame if yes, then selecting a new transmission rate for the wireless device for subsequent transmission of a new frame or retransmission of the frame; and A memory coupled to the processor for storing the predetermined reference table and the average RSS for subsequent retrieval.

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