EP1787418A1

EP1787418A1 - A method and system for link adaptation in wireless networks

Info

Publication number: EP1787418A1
Application number: EP05781240A
Authority: EP
Inventors: Patrick Wienert; Javier Del Prado Pavon
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2004-08-30
Filing date: 2005-08-24
Publication date: 2007-05-23
Also published as: JP2008512024A; WO2006024993A1; CN101010900A; KR20070041591A

Abstract

A method and system of adapting the transmission rate, with the capability of determining whether a transmission error is caused lay increased path loss, of a mobile station among a plurality of transmission rates between a plurality of stations in a wireless local area network (WLAN) are provided. In response to a transmission time out condition of a transmitted frame, a determination is made whether a transmission error associated with the time out condition was caused by increased path loss using a Received Signal Strength (RSS) value from a plurality of incoming frames received by a mobile station. If the determination is true, an RSS threshold is increased, for example, a link adaptation rate threshold, and the frame is retransmitted.

Description

A METHOD AND SYSTEM FOR LINK ADAPTATION IN WIRELESS NETWORKS

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

The IEEE 802.1 1 standard specifies the medium access control (MAC) and physical characteristics for a wireless local area network (WLAN) to support physical layer units. The

IEEE 802.11 standard is defined in International Standard ISO/IEC 8802-11, "Information

Technology—Telecommunications and information exchange area networks," 1999 Edition, which is hereby incorporated by reference in its entirety.

The IEEE 802.11 Physical Layers (PHYs) define a plurality of transmission rates based on different modulations and channel-coding schemes so that the transmitter of a frame can choose one of the multiple rates based on the wireless channel condition between the receiver and itself at a particular time. In general, the lower the transmission rate, the more reliable the transmission.

In such wireless systems, the propagation and interference environments change 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 can be optimal under all scenarios. Link adaptation (LA) algorithms have been proposed to alleviate this problem, wherein the efficiency of a system is improved by adapting the modulation scheme optimal to the current link condition. In particular, a particular example of LA algorithm enables a transmitting station to adapt the transmission rate by using the Received Signal Strength (RSS), measured from the frames it received from the access point (AP) as an indicator for the link quality. Without considering rapid fluctuations of the SNR/SIR due to interferences or multipath fading, it is assumed that the RSS has a linear relationship in average with the SNR. Therefore the received power as a function of distance and path loss conditions is very useful to estimate the most suitable matching PHY mode for the next transmission. Such a Link Adaptation algorithm is described in co-pending U. S. Patent Application No. XXXXXXXX, which is hereby incorporated by reference. Changes in the RSS indicate that the conditions in the wireless link between the station and the AP are changing, and it might be necessary to adapt the transmission rate accordingly due to greater distance or a higher path loss between the station and the AP. However, the prior art LA adaptation algorithms always assume that the errors are due to low RSS, thus, the adaptation may be done incorrectly. For example, another source of errors, which appear in wireless radio channels, are interference noise at the receiving station, collisions and multipath fading at the receiving station. The behavior of interference due to other stations and other transmitting devices on the same channel like microwaves is almost pulsed and short-lived. The LA algorithm should not adapt its rates for transmission errors caused by interference or collisions and in this case not change the RSS thresholds. Accordingly, there is a need to differentiate between errors due to low RSS (large distance) and errors due to interferences for an LA algorithm capable of providing a dynamic rate adaptation based on the Received Signal Strength (RSS) measured from the received frames.

The present invention is directed to a system and method for adapting the transmission rate with the capability of differentiating between errors due to increased path loss (due to increased distance), shading and high path loss and errors due to interference and collisions for better adjusting the transmission rate in a wireless network, such as a wireless local area network (WLAN).

One aspect of the invention relates to a method for determining the transmission rate of a mobile station among a plurality of transmission rates, the method comprising the steps of: measuring an averaged Received Signal Strength (RSS) value from a plurality of incoming frames received by said mobile station; comparing said averaged RSS value to a predetermined reference table having a minimum RSS required for a particular frame length to transmit at one of the plurality of said transmission rates; and selecting a new transmission rate of said mobile station for a subsequent transmission of a new frame based on said comparison outcome, wherein in response to a transmission time out condition of a transmitted frame; determining whether a transmission error associated with the time out condition was caused by increased path loss using the average RSS value; if yes, selecting a new transmission rate of said mobile station for a subsequent transmission of a new frame.

Another aspect of the invention relates to a method of determining whether a transmission error is caused by increased path loss for use with determining the transmission rate of a mobile station among a plurality of transmission rates, for example a link adaptation algorithm. The method includes the following steps: in response to a transmission time out condition of a transmitted frame; determining whether a transmission error associated with the time out condition was caused by increased path loss using a Received Signal Strength (RSS) value from a plurality of incoming frames received by said mobile station; if yes, increasing a RSS threshold, for example, a link adaptation rate threshold, and retransmitting said frame; and if no, retransmitting said frame. Another aspect of the invention relates to a system for adapting a link adaptation 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 a Received Signal Strength (RSS) of said incoming frame received therein; a processor, coupled to said power-measurement circuit, for determining a transmission time out condition of a transmitted frame and determining whether a transmission error associated with the time out condition was caused by increasing path loss using a Received Signal Strength (RSS) value from a plurality of incoming frames received by said mobile station and if yes, increasing a RSS threshold and retransmitting said frame or transmitting a new frame.

Yet another aspect of the invention relates to a system for determining the transmission rate among a plurality of transmission rates, comprising a receiver circuit for demodulating an incoming frame; a power-measurement circuit for measuring a Received Signal Strength (RSS) of said incoming frame received therein; a processor, coupled to said power- measurement circuit, for computing an average RSS and selecting a new transmission rate for the transmission of a new frame based on a comparison outcome to a predetermined reference table, said predetermined reference table including a minimum RSS required for a particular frame length to transmit at one of the plurality of said transmission rates, wherein in response to a transmission time out condition of a transmitted frame, determining whether a transmission error associated with the time out condition was caused by increased path loss using the average RSS; if yes, selecting a new transmission rate of said mobile station for a subsequent transmission of a new frame or retransmission of said frame; and a memory, coupled to said processor, for storing said predetermined reference table and said average RSS for a subsequent retrieval.

The foregoing and other features and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views.

FIG. 1 is a simplified block diagram illustrating the architecture of a wireless communication system whereto embodiments of the present invention are applied; FIG. 2 illustrates the simplified circuit diagram of an access point and each station within a particular basic service set (BSS) according to the embodiment of the present invention;

FIG. 3 is a graphical illustration of the transmission reference used to adjust the transmission rate according to an embodiment of the present invention; and FIG. 4 is a flow chart illustrating the operation steps of differentiating between errors due to increased path loss and to interference and collisions for adjusting the transmission rate according to an embodiment of the present invention.

While the present invention is described hereinafter with particular reference to the system-block diagram of FIG. 1, it is to be understood at the outset of the description which follows that the apparatus and methods in accordance with the present invention may be used with other infrastructures, in which a station is communicating with another station via the wireless medium.

FIG. 1 illustrates a representative network whereto the embodiments of the present invention are applied. As shown in FIG. 1, an access point (AP) 2 is coupled to a plurality of mobile stations (STA;) 10, which, through a wireless link, are communicating with each other and with the AP. A key principle of the present invention is that irrespective of the 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, the transmitting STA can estimate the path loss and channel behavior relatively by keeping track of the RSS measured from the frames sent by a receiving STA. Note that in a typical IEEE 802.11 STA implementation, the RSS is available to the Medium Access Control (MAC) protocol. Hence, as long as the receiving STA uses a fixed transmission-power level for all its transmissions, the changes in the RSS can be used to provide a mechanism to generate and update a transmission-rate reference for the subsequent transmission of frames, such that frames are transmitted at just the right transmission rate.

Referring to FIG. 2, the AP and each STA within the WLAN of FIG. 1 may include a system with an architecture that is illustrated in the block diagram of FIG. 2. Both the AP and 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 terms commonly used in describing particular mobile stations, the description and concepts equally apply to other processing systems, including systems having architectures dissimilar to those shown in FIG. 2. The processor 20 may represent, i.e., a microprocessor, a central processing unit, a computer, a circuit card, an application-specific integrated circuit (ASICs). The memory 18 may represent, i.e., disk-based optical or magnetic storage units, electronic memories, as well as portions or combinations of these and other memory devices. In other embodiments, however, hardware circuitry may be used in place of, or in combination with, software instructions to implement the invention.

In operation, the receiver 12 and the transmitter 26 are coupled to an antenna (not shown) to convert received signals and transmit desired data into corresponding digital data via the demodulator 14 and the modulator 24, respectively. The power-measurement circuit 16 operates under the control of the processor 20 to detect the RSS of the frame received thereon. The RSS with respect to other stations is estimated and stored in the memory 18, which is coupled to the 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 that is used to select the right transmission rate. The timer 22 is used to eliminate the outdated RSS estimation, which is stored in the memory 18. In the embodiment, the RSS is updated as it tends to change due to the time-varying nature of the wireless channel as well as the potential mobility of WLAN STAs. FIG. 3, by way of example, represents a transmission-reference table to select an appropriate transmission rate. Each time a transmitting STA sends a frame having a particular length and receives a corresponding acknowledgement signal, the transmitting STA generates or updates the threshold boundary based on the measured RSS in the reference table to be used in the subsequent transmission of frames. Once the RSS threshold boundary is established for each of the different frame intervals (for example, 0-100 bytes, 100-1000 bytes, and 1000- 2400 bytes), the transmitting STA adapts the transmission rate depending on the RSS measured from the frames it receives from the receiving STA. Note that changes in the RSS indicate that the conditions in the wireless link between the transmitting STA and the receiving STA are changing. As shown in FIG. 3, the respective threshold boundaries indicate which is the minimum RSS values required for a particular transmission PHY rate. For example, if an STA, that is monitoring the RSS from frames sent by the receiving STA, detects that the RSS is becoming lower than one of the thresholds (i.e., due to an increasing distance between the receiving STA and the transmitting STA), the next transmission attempt may be at a lower rate to ensure the correct reception of the frame. Below is a list of variables used in FIGS. 3-4:

Briefly, the frame-length interval, j = {1,2,3}, represents different frame sizes of 0-100 bytes, 100-1000 bytes, and 1000-2400 bytes, respectively. The data rate i = {1,2,3,4} represents one of the four available data rates in this example, i.e., 1, 2, 5.5 and 11 Mbps. The RSS thresholds will be defined for each of the intervals. The threshold "LA_th[i,j]" represents the minimum "RSS_avg or RSS threshold" value to transmit a frame within the length interval "j" at a data rate "i".

FIG. 4 illustrates the overall operation of differentiating between (1) errors due to increased path loss (due to increased distance), shading and high path loss and (2) errors due to interference and collisions for adjusting the transmission rate in a wireless network.

In general, the mobile unit is configured to operate (step 100) in two modes: (1) the receiving mode; and, (2) the transmitting mode. The STA transmits a request signal to transmit data, then selects a transmission rate based the values of RSS average (RSS_avg) thresholds, frame size, and number of retransmission attempts. Here, the rate adaptation occurs when the average RSS measured from the received frame passes some thresholds in the reference table, which contains the minimum RSS values required for a particular transmission rate. Thereafter, the STA transmits the frame at the selected transmission rate. Depending on whether the transmission was successful and whether a frame error was cause by interferences, collisions or due to path loss, the STA updates the corresponding "threshold" in the reference table. Thus, the STA chooses a transmission rate depending on the RSS_avg, Frame length and retransmission attempts. Note that the block diagram is shown for an 802.11 STA operating in a Basic Service Set in FIG. 4, in which case all the frames are always transmitted/received to/from its AP. Hence the receiving STA here mentioned is always its AP. Each time a transmitting STA sends a frame having a particular length, it receives a corresponding acknowledgement signal (i.e., an acknowledgement (ACK) frame). When an acknowledgement signal is received it indicates that the transmission rate was appropriate. If the acknowledgement signal is not received in a predetermined time an acknowledgement signal timeout condition occurs (Step 102). In a timeout condition, it is determined whether the error was caused by the performance of the receiver e.g. RSS too low, in step 104. If false, the cause of the error is determined to be, for example, interference, collision, multipath, etc. For example, after a time out condition (step 102) occurs, in step 104 a determination is made whether the Received Signal Strength (RSS) has changed more than a certain RSS threshold in a predetermined period of time. The change in RSS over time, or ΔRSS/Δt, can be calculated according to the following equation:

ΔRSS/Δt: = RSS_avg - RSS_actual.

In the determination in step 104, as the change in the RSS during a particular period of time increases, the probability that the error is caused because the RSS at the receiver is too low also increases. Moreover, it indicates that the errors occur due to the performance of the receiver (PER) and caused by low power at the receiver. Thus, the RSS threshold/ link adaptation rate threshold (LA_th[i][j]) is increased in step 108, and then the frame is retransmitted in step 106 after calculating the optimal data rate, as shown in FIG. 5. If the determination in step 104 indicates a no or a small change (for example when compared to a predetermined threshold) in the RSS during a particular period of time, the probability is high that the error occurred due to a collision or interference at the receiver, and that the channel conditions are not degraded. Thus, the frame is retransmitted in step 106, without a change in the link adaptation rate thresholds (LA_th[I][j]. It is noted that with every retransmission the probability of a collision is lower, thus, one skilled in the art can determine various retransmission threshold rates for various conditions.

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

As is apparent from the foregoing, the present invention is advantageous in that, unlike the prior art the RSS thresholds are increased only after channel related errors (e.g. no update after collision or interference). The transmission rate are only adapted after relevant changes in the RSS, or long interference (after a predetermined number of packets). Moreover, the invention enables selection of the appropriate transmission rate without making any change in the current IEEE 802.11 WLAN Medium Access Control specification. While the preferred embodiments of the present invention have 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 present invention. In addition, many modifications may be made to adapt to a particular situation and the teaching of the present invention without departing from the central scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS:

1. A method for adapting a link adaptation transmission rate of a wireless device among a plurality of transmission rates, the method comprising the steps of:

(a) in response to a transmission time out condition of a transmitted frame; determining whether a transmission error associated with the time out condition was caused by increased path loss using a Received Signal Strength (RSS) value from a plurality of incoming frames received by said wireless device; and

(b) if yes, increasing a RSS threshold and retransmitting said frame or transmitting a new frame.

2. The method of claim 1, further including the step of retransmitting said frame or transmitting a new frame if the transmission error associated with the time out condition was not caused increased path loss.

3. The method of claim 1, wherein step (a) further includes the steps of: measuring an average Received Signal Strength (RSS_avg) value from a plurality of incoming frames received by the wireless device and determining whether the Received Signal Strength has changed more than a predetermined value in a predetermined period of time.

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 the RSS over time is more than the predetermined value, the transmission error associated with the time out condition was caused by the path loss and the RSS threshold is increased.

6. The method of claim 1, wherein said incoming frames are transmitted by an access point (AP).

7. The method of claim I₅ wherein said incoming frames are transmitted by another wireless device.

8. A method for determining the transmission rate of a wireless device among a plurality of transmission rates, the method comprising the steps of:

(a) measuring an averaged Received Signal Strength (RSS) value from a plurality of incoming frames received by said wireless device;

(b) comparing said averaged RSS value to a predetermined reference table having a minimum RSS required for a particular frame length to transmit at one of the plurality of said transmission rates;

(c) selecting a new transmission rate of said wireless device for a subsequent transmission of a new frame based on said comparison outcome, wherein

(d) in response to a transmission time out condition of a transmitted frame; determining whether a transmission error associated with the time out condition was caused by increased path loss using the average RSS value; and

(e) if yes, selecting a new transmission rate of said wireless device for a subsequent transmission of a new frame or retransmission of said frame.

9. A wireless device for adapting a link adaptation transmission rate among a plurality of transmission rates, comprising: a receiver circuit for demodulating an incoming frame; a power-measurement circuit for measuring a Received Signal Strength (RSS) of said incoming frame received therein; a processor, coupled to said power-measurement circuit, for determining a transmission time out condition of a transmitted frame and determining whether a transmission error associated with the time out condition was caused by increased path loss using a Received Signal Strength (RSS) value from a plurality of incoming frames received by said wireless device and if yes, increasing a RSS threshold and retransmitting said frame or transmitting a new frame.

10. The wireless device of claim 9, wherein the processor further includes retransmitting said frame or transmitting a new frame if the transmission error associated with the time out condition was not caused by increased path loss.

11. The wireless device of claim 9, wherein processor further includes determining an average Received Signal Strength (RSS_avg) value from the plurality of incoming frames received by the wireless device and determining whether the Received Signal Strength has changed more than a predetermined value in a predetermined period of time.

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 the RSS over time is more than the predetermined value, the transmission error associated with the time out condition was caused by increased path loss and the RSS threshold is increased.

14. The wireless device of claim 9, wherein said incoming frames are transmitted by an access point (AP).

15. The wireless device of claim 9, wherein said incoming frames are transmitted by another wireless device. ,

16. A wireless device for determining the transmission rate among a plurality of transmission rates, comprising: a receiver circuit for demodulating an incoming frame; a power-measurement circuit for measuring a Received Signal Strength (RSS) of said incoming frame received therein; a processor, coupled to said power-measurement circuit, for computing an average RSS and selecting a new transmission rate for the transmission of a new frame based on a comparison outcome to a predetermined reference table, said predetermined reference table including a minimum RSS required for a particular frame length to transmit at one of the plurality of said transmission rates, wherein in response to a transmission time out condition of a transmitted frame, determining whether a transmission error associated with the time out condition was caused by increased path loss using the average RSS; if yes, selecting a new transmission rate of said wireless device for a subsequent transmission of a new frame or retransmission of said frame; and a memory, coupled to said processor, for storing said predetermined reference table and said average RSS for a subsequent retrieval.