JP2008512025A - Method and system for error identification in a wireless network - Google Patents

Abstract

  A method and system for determining whether a transmission error is caused by channel conditions is provided for use in determining transmission characteristics of a mobile station in a wireless local area network (WLAN). The method and system detect transmission time-out situations of transmitted frames and use transmission signal strength (RSS) values from a plurality of input frames received by the mobile station to detect transmission errors related to the time-out situations. And whether it was caused by channel conditions.

Description

  The present invention relates to a communication system. More particularly, the present invention identifies errors due to channel conditions and errors due to interference and collisions, and more accurately adjusts station characteristics in a wireless network such as an IEEE 802.11 wireless local area network (WLAN). And a method.

  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, the LA algorithm enables the transmission station to adapt the transmission rate by using 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 prior art LA adaptation algorithms always assume 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. It is therefore necessary to distinguish between errors due to low RSS (bad channel conditions) and errors due to interference.

  The present invention is based on errors due to bad channel conditions such as distance, shading and high path loss, interference and collisions to adjust the characteristics of stations in a wireless network such as a wireless local area network (WLAN). It is directed to a system and method for identifying errors. The information is then used to determine the type of error that is occurring, and various responses, for applications such as power control or any other application that needs to determine the reason for lost frames. Used for

  One aspect of the present invention relates to a method for determining whether a transmission error is caused by a channel condition, which can be used later to determine a transmission rate of a mobile station from among a plurality of transmission rates, for example. . The method includes detecting a transmission timeout situation of a transmitted frame and whether a transmission error associated with the timeout situation is caused by a channel situation from a plurality of input frames received by the mobile station. Determining by utilizing the received signal strength value.

  Another aspect of the present invention is a mobile station for identifying an error between two mobile stations in a wireless network, the receiver circuit for demodulating an input frame, and the received signal strength of the received input frame. A power measurement circuit to measure and a processor coupled to the power measurement circuit to determine a transmission timeout situation of a transmitted frame and whether a transmission error related to the timeout situation is caused by a channel condition; And a processor that determines received signal strength values from a plurality of input frames received by the mobile station.

  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 (ie 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 the overall operation of identifying (1) errors due to bad channel conditions such as distance, shading and high path loss and (2) errors due to interference and collisions in order to adjust the transmission rate in the wireless network. .

  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, 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 can be calculated by the following equation.
ΔRSS = 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. Accordingly, in step 108, the RSS threshold / link adaptation rate threshold (LA_th [i] [j]) is increased, and then in step 106 the frame is transmitted. 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.

  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 bad channel conditions and errors due to interference and collisions for adjusting the transmission rate according to an embodiment of the present invention.

Claims (10)

A method for error identification between two mobile stations of a wireless network, comprising:
Detecting a transmission timeout situation of the transmitted frame;
Determining whether a transmission error associated with the timeout situation is caused by a channel condition by utilizing received signal strength values from a plurality of input frames received by the mobile station; and
Having a method.   Further comprising measuring an average received signal strength value from a plurality of input frames received by the mobile station and determining whether the received signal strength has changed more than a predetermined value during a predetermined time; The method of claim 1. The change ΔRSS of the received signal strength over time is
ΔRSS = the average received signal strength−RSS
The method of claim 1, wherein RSS represents the currently received signal strength.   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 mobile station. A mobile station for error identification between two mobile stations in a wireless network,
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 receiving by the mobile station whether a transmission error associated with the timeout situation is caused by a channel situation A processor that determines a received signal strength value from a plurality of received input frames;
A mobile station.   The processor further measures an average received signal strength value from a plurality of input frames received by the mobile station and determines whether the received signal strength has changed more than a predetermined value during a predetermined time. The mobile station according to claim 6, comprising: The change ΔRSS of the received signal strength over time is
ΔRSS = the average received signal strength−RSS
The mobile station according to claim 7, wherein RSS represents a currently received signal strength.   The mobile station according to claim 6, wherein the input frame is transmitted by an access point.   The mobile station according to claim 6, wherein the input frame is transmitted by another mobile station.

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