US20060277450A1 - Adaptive radio resource management for wireless local area networks - Google Patents

Adaptive radio resource management for wireless local area networks Download PDF

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US20060277450A1
US20060277450A1 US11/495,918 US49591806A US2006277450A1 US 20060277450 A1 US20060277450 A1 US 20060277450A1 US 49591806 A US49591806 A US 49591806A US 2006277450 A1 US2006277450 A1 US 2006277450A1
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data rate
rate
fer
current data
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Maged Zaki
Shamim Rahman
Teresa Hunkeler
Guang Lu
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InterDigital Technology Corp
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InterDigital Technology Corp
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1803—Stop-and-wait protocols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0002—Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the transmission rate
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0015—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the adaptation strategy
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W28/00—Network traffic management; Network resource management
    • H04W28/02—Traffic management, e.g. flow control or congestion control
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W28/00—Network traffic management; Network resource management
    • H04W28/16—Central resource management; Negotiation of resources or communication parameters, e.g. negotiating bandwidth or QoS [Quality of Service]
    • H04W28/18—Negotiating wireless communication parameters
    • H04W28/22—Negotiating communication rate
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W84/00—Network topologies
    • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
    • H04W84/10—Small scale networks; Flat hierarchical networks
    • H04W84/12—WLAN [Wireless Local Area Networks]

Definitions

  • the present invention relates to radio resource management in wireless local area networks (LANs), and more particularly, to a method for adaptively managing the radio resources in a wireless LAN.
  • LANs wireless local area networks
  • Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types: base stations or wireless transmit/receive units (WTRUs), which include mobile units.
  • WTRUs wireless transmit/receive units
  • WTRU includes, but is not limited to, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment.
  • WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections.
  • WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units.
  • the term access point as used herein includes, but is not limited to, a base station, a Node B, a site controller, an access point, or other interfacing device in a wireless environment that provides WTRUs with wireless access to a network associated with the base station.
  • each base station is capable of conducting concurrent wireless communications with appropriately configured WTRUs.
  • Some WTRUs are configured to conduct wireless communications directly between each other, i.e., without being relayed through a network via a base station. This is commonly called peer-to-peer wireless communications.
  • WTRUs can be configured for use in multiple networks with both network and peer-to-peer communications capabilities.
  • WLAN wireless local area network
  • WTRUs equipped with WLAN modems that are also able to conduct peer-to-peer communications with similarly equipped WTRUs.
  • WLAN modems are being integrated into many traditional communicating and computing devices by manufacturers. For example, cellular phones, personal digital assistants, and laptop computers are being built with one or more WLAN modems.
  • a popular wireless local area network environment with one or more WLAN access points is built according to one of the IEEE 802.11 standards.
  • the basic service set (BSS) is the building block of an IEEE 802.11 LAN and consists of WTRUs referred to as stations.
  • the set of stations which can talk to each other can form a BSS.
  • Multiple BSSs are interconnected through an architectural component, called a distribution system (DS), to form an extended service set (ESS).
  • An access point (AP) is a station that provides access to the DS by providing DS services and generally allows concurrent access to the DS by multiple stations.
  • the 802.11 standards allow multiple transmission rates (and dynamic switching between rates) to be used to optimize throughput.
  • Lower transmission rates have more robust modulation characteristics that allow greater range and/or better operation in noisy environments than higher transmission rates, which provide better throughput. It is an optimization challenge to always select the best (highest) possible rate for any given coverage and interference condition.
  • each 802.11 device has a rate control algorithm implemented in it that is controlled solely by that device. Specifically, uplink (UL) rate control is performed in stations and downlink (DL) rate control is performed in APs.
  • UL uplink
  • DL downlink
  • rate switching is not specified by the standards. It is left up to the station and AP implementation. Such rate control algorithms are usually proprietary and thus public information about them is limited. However, several algorithms have been described in academic and industry literature. Generally, they are relatively simple algorithms based on detecting missing acknowledgements (ACKs) and other statistics.
  • ACKs missing acknowledgements
  • the 802.11 standard specifies a common medium access control (MAC) layer, which provides a variety of functions that support the operation of 802.11-based wireless LANs.
  • MAC medium access control
  • the MAC layer manages and maintains communications between stations and APs by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium.
  • the MAC layer uses a physical (PHY) layer, such as defined in 802.11b or 802.11a, to perform the tasks of carrier sensing, transmission, and receiving of data frames.
  • PHY physical
  • every transmitted MAC layer data frame is ACKed by the receiver. This is classically referred to as a “stop and wait” automatic repeat request (ARQ) protocol. If an ACK is not received by the transmitter (lost or never sent), then the original data frame is considered lost and the transmitter will go through the contention process again and try to re-send the data frame. A missing ACK assumes that the receiver does not get the ACK at all. However, a check to determine if an ACK frame can be partially missing (e.g., the CRC of the payload is bad, but the header information is intact) can be made. This can then be used in the decision process as a condition halfway between a missing ACK and a received ACK.
  • ARQ automatic repeat request
  • An example of an existing throughput-based rate control algorithm is as follows. First, 10% of the data is sent periodically at two data rates adjacent to the current data rate. Then, the throughput at each of the three different data rates is periodically evaluated by considering the amount of data that is successfully ACKed versus the amount of data transmitted at a given rate. Finally, a switchover is made to the data rate that provided the best throughput.
  • Such algorithms are one-dimensional in that they only consider their own link quality (via missing ACKs) during a given transmission.
  • APs generally have knowledge of the overall system and can accordingly consider more dimensions. For example, an AP can consider the UL data frame rate that was used by a given station within a given time window (e.g., the previous X seconds) as the starting point rate for its DL rate transmission to that station.
  • An AP can also keep track of the last rate transmitted on the DL to a given station in the AP for a given period of time.
  • FSM finite state machine
  • TDD time division duplex
  • RRM radio resource management
  • a wireless link can suffer from a high frame error rate (FER) compared to a wired link.
  • FER frame error rate
  • the high FER can be due to a high traffic load, which results in more collisions and a consequently high FER; a bad wireless link condition, which can be due to high interference, fading, or a user moving away from an AP; or other reasons.
  • the proposed RRM process manages the radio resources adaptively by acting differently depending on the reason behind the high FER. If the high FER is due to a high traffic load, the RRM will attempt to decrease or regulate the traffic load by triggering congestion control or traffic shaping functions. If the high FER is due to a bad wireless link, the RRM attempts to increase the wireless link robustness by using a more robust modulation scheme.
  • a method for determining whether a data transmission rate in a wireless communication system is appropriate for current system conditions begins by calculating a frame error rate (FER) for a given period of time and obtaining a current data rate. If the current data rate is greater than a minimum data rate and the FER is a high value, then the system switches to a lower data rate. If the current data rate is greater than a minimum data rate and the FER is less than a high value, then the system maintains the current data rate. If the current data rate is less than a maximum data rate and the FER is a low value, then the system determines whether a higher data rate can be used.
  • FER frame error rate
  • FIG. 1 is a flowchart of an adaptive radio resource management procedure in accordance with one embodiment of the present invention
  • FIG. 2 is a flowchart of a rate control procedure at an AP in accordance with one embodiment of the present invention
  • FIG. 3 is a graph of a throughput curve utilized by the rate control procedure shown in FIG. 2 ;
  • FIG. 4 is a flowchart of a missing ACK function used by the rate control procedure shown in FIG. 2 ;
  • FIG. 5 is a diagram of an apparatus constructed in accordance with the present invention.
  • a radio resource management (RRM) adaptive procedure 100 is invoked either periodically or upon detection of a high FER (FER_HIGH, which is an implementation-specific value).
  • the procedure 100 begins by comparing the measured FER value to the high FER threshold (FER_HIGH; step 102 ). If the measured FER value exceeds FER_HIGH, then a comparison is made to determine if the channel utilization is greater than a high channel utilization threshold (CH_UTIL_HIGH; step 104 ). If the channel utilization exceeds CH_UTIL_HIGH, then congestion control is triggered (step 106 ) and the procedure terminates (step 108 ).
  • CH_UTIL_HIGH high channel utilization threshold
  • the goal of congestion control is to reduce the traffic load and channel utilization.
  • the AP can disassociate stations which have one or more of the following characteristics: a high error rate, a low priority MAC address, and excessive channel utilization.
  • CTS clear to send
  • the station sends a request to send (RTS) to an AP if the RTS/CTS mechanism is enabled in the BSS. If the AP holds the CTS, the station cannot transmit packets in the uplink, thereby alleviating the congestion situation. For a user that frequently retransmits, its transmission rate can be reduced when there is congestion, which reduces the contention/collision possibilities.
  • step 104 a determination is made whether the channel utilization is below CH_UTIL_HIGH (CH_UTIL_LOW; step 110 ). If the channel utilization is below CH_UTIL_LOW, then the current data rate is examined to determine if it is greater than the minimum data rate (step 112 ). If the current data rate is greater than the minimum data rate, then rate control is triggered to decrease the data rate (step 114 ) and the procedure terminates (step 108 ). Rate control is performed to reduce the data rate to match the offered traffic load. When an AP is not using all the bandwidth and it experiences a high error rate, its transmission rate can be reduced to increase the quality of transmission. With a lower data rate, a more robust modulation scheme can be used, which in turn improves the FER value.
  • CH_UTIL_LOW low channel utilization threshold
  • step 112 If the current data rate is equal to the minimum data rate (step 112 ), then traffic shaping is used to reduce the FER value (step 116 ), and the procedure terminates (step 108 ). During traffic shaping, excess data can be delayed in order to control the traffic within the allocated bandwidth, and/or additional bandwidth can be allocated for high priority data. If the channel utilization exceeds CH_UTIL_LOW (step 110 ), then traffic shaping is triggered (step 116 ) and the procedure terminates (step 108 ).
  • the measured FER value is compared to a low FER threshold (FER_LOW; step 120 ). If the measured FER value is below FER_LOW, then a comparison is made to determine if the channel utilization is greater than CH_UTIL_HIGH (step 122 ). If the channel utilization exceeds CH_UTIL_HIGH, then the current data rate is examined to determine if it is less than the maximum data rate (step 124 ). If the current data rate is less than the maximum data rate, then rate control is triggered to increase the data rate (step 126 ) and the procedure terminates (step 108 ). By increasing the data rate, the channel utilization will be lowered.
  • FER_LOW low FER threshold
  • step 124 If the current data rate is already equal to the maximum data rate (step 124 ) or if the channel utilization does not exceed CH_UTIL_HIGH (step 122 ), then no further adjustments are made and the procedure terminates (step 108 ).
  • step 120 If the measured FER is above FER_LOW (step 120 ), then a comparison is made to determine if the channel utilization is greater than CH_UTIL_HIGH (step 128 ). If the channel utilization exceeds CH_UTIL_HIGH, then congestion control is triggered (step 106 ) and the procedure terminates (step 108 ). If the channel utilization is below CH_UTIL_HIGH (step 128 ), then the channel utilization is compared to CH_UTIL_LOW (step 130 ). If the channel utilization is below CH_UTIL_LOW, then the current data rate is examined to determine if it is greater than the minimum data rate (step 132 ). If the current data rate is greater than the minimum data rate, then rate control is triggered to decrease the data rate (step 114 ) and the procedure terminates (step 108 ).
  • step 132 If the current data rate is equal to the minimum data rate (step 132 ) or if the channel utilization is above CH_UTIL_LOW (step 130 ), then no further adjustments are made and the procedure terminates (step 108 ).
  • any applicable rate control procedure can be executed; the method 100 does not require the use of any particular rate control procedure. If desired, the method 100 may use a procedure 200 for rate control.
  • a procedure 200 as shown in FIG. 2 is used for rate control at the AP.
  • the procedure 200 begins by determining whether a downlink (DL) transmission to a specific station was made in the last X seconds (step 202 ). If no transmission was made in the last X seconds, then the initial data rate is determined as follows.
  • the cell load is then checked (step 214 ).
  • the cell load statistics are stored in the AP, and is the average channel utilization in the last Y seconds.
  • the cell load is then evaluated (step 216 ).
  • the initial data rate is set to the last data rate considered in step 206 , 210 , or 212 (step 218 ).
  • the initial data rate is determined by using a throughput curve, similar to that shown in FIG. 3 (step 220 ). These curves can be based on experimental results or can be updated dynamically and stored in a database as explained below.
  • the throughput curves shown in FIG. 3 are preferably stored in memory at the AP.
  • the curves are based on collected statistics during AP operation.
  • the x-axis represents channel utilization, which is the current channel utilization plus the data rate for the next transmission.
  • the y-axis is the throughput.
  • Each curve corresponds to a certain FER range, providing the channel throughput as a function of the channel utilization and the frame error rate. The procedure selects the data rate that provides the maximum throughput for the current FER.
  • the data frame is transmitted (step 222 ) and the AP waits for an ACK for the frame (step 224 ). After receiving an ACK or the ACK timeout period expires, the missing ACK count is updated (step 226 ) and the throughput curve is updated (step 228 ). The procedure then returns to step 202 .
  • Rate control is a frame-based procedure; the loop presented by returning to step 202 represents the continuous transmission of frames.
  • step 202 If there has been a downlink transmission made to a specific station in the last X seconds (step 202 ), then the missing ACK count is checked (step 230 ). The cell load is then checked (step 232 ) and evaluated (step 234 ). If the traffic demand is low, a missing ACK function is invoked (step 236 ), which is discussed in detail below.
  • the initial data rate is determined by using a throughput curve (step 238 ), similar to that used in step 220 .
  • the data frame is transmitted (step 222 ) and the AP waits for an ACK for the frame (step 224 ).
  • the missing ACK count is updated (step 226 ) and the throughput curve is updated (step 228 ). The procedure then returns to step 202 .
  • the missing ACK function 400 (from step 236 ) is shown in FIG. 4 .
  • the function 400 begins by calculating the FER for a given period (step 402 ).
  • the function 400 distinguishes if the frame is lost, partially missing (e.g., the CRC of a payload is bad, but the header information is intact), or received in error.
  • the function 400 reacts more quickly when frames are lost than when frames are partially missing or received in error.
  • the difference in how much of the frame is lost can be used to determine how to adjust the rate control. For example, the rate will be decreased less aggressively if there is a partial missing frame versus a completely lost frame.
  • the current data rate is then retrieved (step 404 ).
  • a check is made whether the current data rate is less than or equal to the maximum data rate and whether the FER value is low (step 406 ). If both conditions are met, then the channel is probed at the next highest data rate for a predetermined number of frames (step 408 ). In one embodiment of the present invention, the channel is probed for at least one frame. If all of the frames sent at the higher data rate are ACKed (step 410 ), then the AP switches to the next higher data rate (step 412 ), and the function terminates (step 414 ).
  • step 410 If all of the frames sent at the higher data rate are not ACKed (step 410 ), then no change to the data rate is made (step 416 ) and the function terminates (step 414 ).
  • step 406 If the tests at step 406 are not satisfied, then a further evaluation is made to determine whether the current data rate is greater than the minimum data rate and the FER value is high (step 418 ). If both of these conditions are met, then the AP switches to the next lower data rate (step 420 ) and the function terminates (step 414 ). If these conditions (step 418 ) are not met, then no change to the data rate is made (step 416 ) and the function terminates (step 414 ).
  • FIG. 5 is a diagram of an adaptive RRM apparatus 500 constructed in accordance with the present invention; in a preferred embodiment, the apparatus 500 resides on an AP.
  • the apparatus 500 includes a measurement module (or device) 510 , a RRM decision module 530 , and at least one action module 540 .
  • the measurement module 510 collects measurements from the hardware via a measurement collection module (or device) 512 and calculates performance metrics.
  • the performance metrics calculated by the module 510 include FER 514 , cell load 516 , channel utilization 518 , and missing ACK count 520 . Additional metrics may be calculated by the measurement module 510 based upon collected measurements.
  • the RRM decision module 530 decides which action module 540 to call based upon the performance metrics and predetermined thresholds, as explained above in connection with FIG. 1 .
  • the action modules 540 perform the specific RRM actions, and include a traffic shaping module 542 , a rate control module 544 , and a congestion control module 546 . Additional action modules 540 may be supplied to perform additional RRM functions.
  • the present invention has, for simplicity, been described in the context of wireless LAN type technology, the present invention may be implemented in any type of wireless communication system. Purely by way of example, the present invention may be implemented in wireless LAN, UMTS-FDD, UMTS-TDD, TD-SCDMA, CDMA, CDMA2000 (EV-DO and EV-DV), or any other type of wireless communication system.

Abstract

A method for determining whether a data transmission rate in a wireless communication system is appropriate for current system conditions begins by calculating a frame error rate (FER) for a given period of time and obtaining a current data rate. If the current data rate is greater than a minimum data rate and the FER is a high value, then the system switches to a lower data rate. If the current data rate is greater than a minimum data rate and the FER is less than a high value, then the system maintains the current data rate. If the current data rate is less than a maximum data rate and the FER is a low value, then the system determines whether a higher data rate can be used.

Description

    CROSS REFERENCE TO RELATED APPLICATIONS
  • This application claims priority from U.S. application Ser. No. 10/881,606, filed Jun. 30, 2004, which claims the benefit of U.S. Provisional Patent Application No. 60/516,161 filed Oct. 31, 2003 and U.S. Provisional Patent Application No. 60/518,155 filed Nov. 7, 2003, which are incorporated by reference as if fully set forth herein.
    • FIELD OF INVENTION
  • The present invention relates to radio resource management in wireless local area networks (LANs), and more particularly, to a method for adaptively managing the radio resources in a wireless LAN.
    • BACKGROUND
  • Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types: base stations or wireless transmit/receive units (WTRUs), which include mobile units.
  • The term WTRU as used herein includes, but is not limited to, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units.
  • The term access point as used herein includes, but is not limited to, a base station, a Node B, a site controller, an access point, or other interfacing device in a wireless environment that provides WTRUs with wireless access to a network associated with the base station.
  • Typically, a network of base stations is provided where each base station is capable of conducting concurrent wireless communications with appropriately configured WTRUs. Some WTRUs are configured to conduct wireless communications directly between each other, i.e., without being relayed through a network via a base station. This is commonly called peer-to-peer wireless communications. WTRUs can be configured for use in multiple networks with both network and peer-to-peer communications capabilities.
  • One type of wireless system, called a wireless local area network (WLAN), can be configured to conduct wireless communications with WTRUs equipped with WLAN modems that are also able to conduct peer-to-peer communications with similarly equipped WTRUs. Currently, WLAN modems are being integrated into many traditional communicating and computing devices by manufacturers. For example, cellular phones, personal digital assistants, and laptop computers are being built with one or more WLAN modems.
  • A popular wireless local area network environment with one or more WLAN access points (APs) is built according to one of the IEEE 802.11 standards. The basic service set (BSS) is the building block of an IEEE 802.11 LAN and consists of WTRUs referred to as stations. The set of stations which can talk to each other can form a BSS. Multiple BSSs are interconnected through an architectural component, called a distribution system (DS), to form an extended service set (ESS). An access point (AP) is a station that provides access to the DS by providing DS services and generally allows concurrent access to the DS by multiple stations.
  • The 802.11 standards allow multiple transmission rates (and dynamic switching between rates) to be used to optimize throughput. Lower transmission rates have more robust modulation characteristics that allow greater range and/or better operation in noisy environments than higher transmission rates, which provide better throughput. It is an optimization challenge to always select the best (highest) possible rate for any given coverage and interference condition.
  • The currently specified transmission rates of various versions of the 802.11 standard are as follows:
    Standard Supported Rates (Mbps)
    802.11 (original) 1, 2
    802.11a 6, 9, 12, 18, 24, 36, 48, 54
    802.11b 1, 2, 5.5, 11
    802.11g 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, 54
  • Conventionally, each 802.11 device has a rate control algorithm implemented in it that is controlled solely by that device. Specifically, uplink (UL) rate control is performed in stations and downlink (DL) rate control is performed in APs.
  • The algorithm for rate switching is not specified by the standards. It is left up to the station and AP implementation. Such rate control algorithms are usually proprietary and thus public information about them is limited. However, several algorithms have been described in academic and industry literature. Generally, they are relatively simple algorithms based on detecting missing acknowledgements (ACKs) and other statistics.
  • The 802.11 standard specifies a common medium access control (MAC) layer, which provides a variety of functions that support the operation of 802.11-based wireless LANs. In general, the MAC layer manages and maintains communications between stations and APs by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium. The MAC layer uses a physical (PHY) layer, such as defined in 802.11b or 802.11a, to perform the tasks of carrier sensing, transmission, and receiving of data frames.
  • In general, every transmitted MAC layer data frame is ACKed by the receiver. This is classically referred to as a “stop and wait” automatic repeat request (ARQ) protocol. If an ACK is not received by the transmitter (lost or never sent), then the original data frame is considered lost and the transmitter will go through the contention process again and try to re-send the data frame. A missing ACK assumes that the receiver does not get the ACK at all. However, a check to determine if an ACK frame can be partially missing (e.g., the CRC of the payload is bad, but the header information is intact) can be made. This can then be used in the decision process as a condition halfway between a missing ACK and a received ACK.
  • An example of an existing throughput-based rate control algorithm is as follows. First, 10% of the data is sent periodically at two data rates adjacent to the current data rate. Then, the throughput at each of the three different data rates is periodically evaluated by considering the amount of data that is successfully ACKed versus the amount of data transmitted at a given rate. Finally, a switchover is made to the data rate that provided the best throughput.
  • Such algorithms are one-dimensional in that they only consider their own link quality (via missing ACKs) during a given transmission. Unlike a typical station, APs generally have knowledge of the overall system and can accordingly consider more dimensions. For example, an AP can consider the UL data frame rate that was used by a given station within a given time window (e.g., the previous X seconds) as the starting point rate for its DL rate transmission to that station. An AP can also keep track of the last rate transmitted on the DL to a given station in the AP for a given period of time.
  • Sometimes it is better to transmit at higher rates to all stations (even with relatively high error rate) as transmitting at a low data rate to one user tends to slow down the entire system. The performance and switching points (e.g., how much error rate a typical station application can tolerate) of this type of system can be characterized and used in the AP rate control.
  • A cell-based finite state machine (FSM) type approach (as per 3GPP time division duplex (TDD) radio resource management (RRM)) can also be applied, where rate control can take different actions for different cell states (loads). The cell state can be set, for example, by a congestion control algorithm.
  • In addition, a wireless link can suffer from a high frame error rate (FER) compared to a wired link. The high FER can be due to a high traffic load, which results in more collisions and a consequently high FER; a bad wireless link condition, which can be due to high interference, fading, or a user moving away from an AP; or other reasons.
    • SUMMARY
  • The proposed RRM process manages the radio resources adaptively by acting differently depending on the reason behind the high FER. If the high FER is due to a high traffic load, the RRM will attempt to decrease or regulate the traffic load by triggering congestion control or traffic shaping functions. If the high FER is due to a bad wireless link, the RRM attempts to increase the wireless link robustness by using a more robust modulation scheme.
  • A method for determining whether a data transmission rate in a wireless communication system is appropriate for current system conditions begins by calculating a frame error rate (FER) for a given period of time and obtaining a current data rate. If the current data rate is greater than a minimum data rate and the FER is a high value, then the system switches to a lower data rate. If the current data rate is greater than a minimum data rate and the FER is less than a high value, then the system maintains the current data rate. If the current data rate is less than a maximum data rate and the FER is a low value, then the system determines whether a higher data rate can be used.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example, and to be understood in conjunction with the accompanying drawings wherein:
  • FIG. 1 is a flowchart of an adaptive radio resource management procedure in accordance with one embodiment of the present invention;
  • FIG. 2 is a flowchart of a rate control procedure at an AP in accordance with one embodiment of the present invention;
  • FIG. 3 is a graph of a throughput curve utilized by the rate control procedure shown in FIG. 2;
  • FIG. 4 is a flowchart of a missing ACK function used by the rate control procedure shown in FIG. 2; and
  • FIG. 5 is a diagram of an apparatus constructed in accordance with the present invention.
    • DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • As shown in FIG. 1, a radio resource management (RRM) adaptive procedure 100 is invoked either periodically or upon detection of a high FER (FER_HIGH, which is an implementation-specific value). The procedure 100 begins by comparing the measured FER value to the high FER threshold (FER_HIGH; step 102). If the measured FER value exceeds FER_HIGH, then a comparison is made to determine if the channel utilization is greater than a high channel utilization threshold (CH_UTIL_HIGH; step 104). If the channel utilization exceeds CH_UTIL_HIGH, then congestion control is triggered (step 106) and the procedure terminates (step 108).
  • The goal of congestion control is to reduce the traffic load and channel utilization. In congestion control, the AP can disassociate stations which have one or more of the following characteristics: a high error rate, a low priority MAC address, and excessive channel utilization. From a scheduling perspective, the AP may hold a clear to send (CTS) signal for uplink transmission. In general, the station sends a request to send (RTS) to an AP if the RTS/CTS mechanism is enabled in the BSS. If the AP holds the CTS, the station cannot transmit packets in the uplink, thereby alleviating the congestion situation. For a user that frequently retransmits, its transmission rate can be reduced when there is congestion, which reduces the contention/collision possibilities.
  • If the channel utilization is below CH_UTIL_HIGH (step 104), then a determination is made whether the channel utilization is below a low channel utilization threshold (CH_UTIL_LOW; step 110). If the channel utilization is below CH_UTIL_LOW, then the current data rate is examined to determine if it is greater than the minimum data rate (step 112). If the current data rate is greater than the minimum data rate, then rate control is triggered to decrease the data rate (step 114) and the procedure terminates (step 108). Rate control is performed to reduce the data rate to match the offered traffic load. When an AP is not using all the bandwidth and it experiences a high error rate, its transmission rate can be reduced to increase the quality of transmission. With a lower data rate, a more robust modulation scheme can be used, which in turn improves the FER value.
  • If the current data rate is equal to the minimum data rate (step 112), then traffic shaping is used to reduce the FER value (step 116), and the procedure terminates (step 108). During traffic shaping, excess data can be delayed in order to control the traffic within the allocated bandwidth, and/or additional bandwidth can be allocated for high priority data. If the channel utilization exceeds CH_UTIL_LOW (step 110), then traffic shaping is triggered (step 116) and the procedure terminates (step 108).
  • If the measured FER value does not exceed FER_HIGH (step 102), then the measured FER value is compared to a low FER threshold (FER_LOW; step 120). If the measured FER value is below FER_LOW, then a comparison is made to determine if the channel utilization is greater than CH_UTIL_HIGH (step 122). If the channel utilization exceeds CH_UTIL_HIGH, then the current data rate is examined to determine if it is less than the maximum data rate (step 124). If the current data rate is less than the maximum data rate, then rate control is triggered to increase the data rate (step 126) and the procedure terminates (step 108). By increasing the data rate, the channel utilization will be lowered.
  • If the current data rate is already equal to the maximum data rate (step 124) or if the channel utilization does not exceed CH_UTIL_HIGH (step 122), then no further adjustments are made and the procedure terminates (step 108).
  • If the measured FER is above FER_LOW (step 120), then a comparison is made to determine if the channel utilization is greater than CH_UTIL_HIGH (step 128). If the channel utilization exceeds CH_UTIL_HIGH, then congestion control is triggered (step 106) and the procedure terminates (step 108). If the channel utilization is below CH_UTIL_HIGH (step 128), then the channel utilization is compared to CH_UTIL_LOW (step 130). If the channel utilization is below CH_UTIL_LOW, then the current data rate is examined to determine if it is greater than the minimum data rate (step 132). If the current data rate is greater than the minimum data rate, then rate control is triggered to decrease the data rate (step 114) and the procedure terminates (step 108).
  • If the current data rate is equal to the minimum data rate (step 132) or if the channel utilization is above CH_UTIL_LOW (step 130), then no further adjustments are made and the procedure terminates (step 108).
  • If rate control is triggered at step 114 or step 126, any applicable rate control procedure can be executed; the method 100 does not require the use of any particular rate control procedure. If desired, the method 100 may use a procedure 200 for rate control.
  • In one embodiment of the present invention, a procedure 200 as shown in FIG. 2 is used for rate control at the AP. The procedure 200 begins by determining whether a downlink (DL) transmission to a specific station was made in the last X seconds (step 202). If no transmission was made in the last X seconds, then the initial data rate is determined as follows.
  • A check is made whether the last transmission rate to the station is available (step 204). If the last transmission rate to the station is available, then it is considered (step 206). If the last transmission rate for the station is not available (step 204), then a check is made if the last received rate for the station is available (step 208). If the last received rate is available, then it is considered (step 210). If the last received rate is not available (step 208), then the last transmitted data rate from any other station is considered (step 212).
  • Regardless of the initial data rate that is considered (from step 206, 210, or 212), the cell load is then checked (step 214). The cell load statistics are stored in the AP, and is the average channel utilization in the last Y seconds. The cell load is then evaluated (step 216). In the case of low traffic demand, the initial data rate is set to the last data rate considered in step 206, 210, or 212 (step 218). In the case of high traffic demand (step 216), the initial data rate is determined by using a throughput curve, similar to that shown in FIG. 3 (step 220). These curves can be based on experimental results or can be updated dynamically and stored in a database as explained below.
  • The throughput curves shown in FIG. 3 are preferably stored in memory at the AP. The curves are based on collected statistics during AP operation. The x-axis represents channel utilization, which is the current channel utilization plus the data rate for the next transmission. The y-axis is the throughput. Each curve corresponds to a certain FER range, providing the channel throughput as a function of the channel utilization and the frame error rate. The procedure selects the data rate that provides the maximum throughput for the current FER.
  • Once the initial data rate has been selected, the data frame is transmitted (step 222) and the AP waits for an ACK for the frame (step 224). After receiving an ACK or the ACK timeout period expires, the missing ACK count is updated (step 226) and the throughput curve is updated (step 228). The procedure then returns to step 202. Rate control is a frame-based procedure; the loop presented by returning to step 202 represents the continuous transmission of frames.
  • If there has been a downlink transmission made to a specific station in the last X seconds (step 202), then the missing ACK count is checked (step 230). The cell load is then checked (step 232) and evaluated (step 234). If the traffic demand is low, a missing ACK function is invoked (step 236), which is discussed in detail below.
  • In the case of high traffic demand, the initial data rate is determined by using a throughput curve (step 238), similar to that used in step 220. Once the transmission data rate has been selected, the data frame is transmitted (step 222) and the AP waits for an ACK for the frame (step 224). After receiving an ACK or waiting for the ACK timeout period, the missing ACK count is updated (step 226) and the throughput curve is updated (step 228). The procedure then returns to step 202.
  • The missing ACK function 400 (from step 236) is shown in FIG. 4. The function 400 begins by calculating the FER for a given period (step 402). The function 400 distinguishes if the frame is lost, partially missing (e.g., the CRC of a payload is bad, but the header information is intact), or received in error. The function 400 reacts more quickly when frames are lost than when frames are partially missing or received in error. The difference in how much of the frame is lost can be used to determine how to adjust the rate control. For example, the rate will be decreased less aggressively if there is a partial missing frame versus a completely lost frame.
  • The current data rate is then retrieved (step 404). A check is made whether the current data rate is less than or equal to the maximum data rate and whether the FER value is low (step 406). If both conditions are met, then the channel is probed at the next highest data rate for a predetermined number of frames (step 408). In one embodiment of the present invention, the channel is probed for at least one frame. If all of the frames sent at the higher data rate are ACKed (step 410), then the AP switches to the next higher data rate (step 412), and the function terminates (step 414).
  • If all of the frames sent at the higher data rate are not ACKed (step 410), then no change to the data rate is made (step 416) and the function terminates (step 414).
  • If the tests at step 406 are not satisfied, then a further evaluation is made to determine whether the current data rate is greater than the minimum data rate and the FER value is high (step 418). If both of these conditions are met, then the AP switches to the next lower data rate (step 420) and the function terminates (step 414). If these conditions (step 418) are not met, then no change to the data rate is made (step 416) and the function terminates (step 414).
  • FIG. 5 is a diagram of an adaptive RRM apparatus 500 constructed in accordance with the present invention; in a preferred embodiment, the apparatus 500 resides on an AP. The apparatus 500 includes a measurement module (or device) 510, a RRM decision module 530, and at least one action module 540.
  • The measurement module 510 collects measurements from the hardware via a measurement collection module (or device) 512 and calculates performance metrics. The performance metrics calculated by the module 510 include FER 514, cell load 516, channel utilization 518, and missing ACK count 520. Additional metrics may be calculated by the measurement module 510 based upon collected measurements.
  • The RRM decision module 530 decides which action module 540 to call based upon the performance metrics and predetermined thresholds, as explained above in connection with FIG. 1. The action modules 540 perform the specific RRM actions, and include a traffic shaping module 542, a rate control module 544, and a congestion control module 546. Additional action modules 540 may be supplied to perform additional RRM functions.
  • It should be noted that while the present invention has, for simplicity, been described in the context of wireless LAN type technology, the present invention may be implemented in any type of wireless communication system. Purely by way of example, the present invention may be implemented in wireless LAN, UMTS-FDD, UMTS-TDD, TD-SCDMA, CDMA, CDMA2000 (EV-DO and EV-DV), or any other type of wireless communication system.
  • Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention. While specific embodiments of the present invention have been shown and described, many modifications and variations could be made by one skilled in the art without departing from the scope of the invention. The above description serves to illustrate and not limit the particular invention in any way.

Claims (6)

1. A method for determining whether a data transmission rate in a wireless communication system is appropriate for current wireless communication system conditions, comprising the steps of:
calculating a frame error rate (FER) for a given period of time;
obtaining a current data rate;
switching to a lower data rate if the current data rate is greater than a minimum data rate and the FER is a high value;
maintaining the current data rate if the current data rate is greater than a minimum data rate and the FER is less than a high value; and
determining whether a higher data rate can be used if the current data rate is less than a maximum data rate and the FER is a low value.
2. The method according to claim 1, wherein the lower data rate is a data rate one step lower than the current data rate.
3. The method according to claim 1, wherein the determining step includes the steps of:
probing the transmission channel at a higher data rate for a predetermined number of frames; and
determining whether all of the frames transmitted at the higher data rate have been acknowledged.
4. The method according to claim 3, wherein the higher data rate is one step higher than the current data rate.
5. The method according to claim 3, further comprising the step of:
switching to the higher data rate if all of the frames have been acknowledged.
6. The method according to claim 3, further comprising the step of:
maintaining the current data rate if all of the frames have not been acknowledged.
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