Patent Application
20070060155
The present invention relates generally to clear channel assessment in a wireless communication network and more specifically to controlling the clear channel assessment threshold in a wireless local area network.
A communication channel, e.g., a radio frequency (RF) channel, comprises a receiver and a transmitter. The transmitter sends a communication signal in the form of a data packet (also known as a datagram in the art) to the receiver across the channel. A preamble, used for receiver packet detection, synchronization, channel estimation, etc., typically heads the data packet. The signal, which may be impaired by a channel response, additive noise and possibly interference, arrives at the receiver. In a multi-user communications system using a shared random access channel topology, a carrier sense multiple access/collision avoidance (CSMA/CA) methodology or protocol may be employed in order to avoid collisions on the channel. The receiver, which is monitoring the channel for the presence of energy, may indicate the status of the RF channel by setting the state of an indicator according to the presence of energy in the channel. There are several ways to indicate whether the channel is busy (unavailable) or clear (available) to permit transmission of data. A clear channel assessment CCA mechanism (also referred to herein simply as a CCA) provides one such indication.
For example, the Institute of Electrical and Electronics Engineering (IEEE) 802.11 standards for a wireless local area network (WLAN) implements a CSMA/CA access methodology, which is a contention-based protocol. In the receiver, a signal or indicator known as clear channel assessment (CCA) is used to indicate the status of the channel. A received signal strength (RSS) is further used to indicate whether the channel is occupied, and is also referred to as CCA sensitivity. According to the 802.11 standards, the CCA sensitivity depends on the detection of a start of a valid Orthogonal Frequency Division Multiplexing (OFDM) transmission, e.g., a preamble. When a valid preamble is detected with an RSS at or above a CCA threshold that is a minimum RSS value (e.g., 6 Mbps, −82 dBm), the CCA indicates a channel busy status. In the absence of a detected preamble, the CCA threshold for indicating a busy channel is 20 dB above the 6 Mbps sensitivity or −62 dBm. In the case of preamble detection, the CCA will indicate a clear channel when either the RSS falls below −82 dBm, or the preamble is rejected. In the case of no preamble detection with an RSS above −62 dBm, the CCA indicates a busy channel even if a valid OFDM signal is not identified or in the presence of noise, until the energy falls below −62 dBm. The specified thresholds for CCA, thus, assume that signals with energy above −62 dBm are valid OFDM signals, which may enable a malicious intruder to cause a serious disruption of service. For example, an intruder can repeatedly transmit a preamble at just above −82 dBm, causing the CCA to indicate busy until the preamble is rejected. The intruder may also transmit a simple jamming signal with received energy just above −62 dBm, causing the CCA to indicate busy continuously. This may reduce the quality of service (QoS) to an unacceptable level.
Thus, there is a need for a method to dynamically adapt or adjust the CCA threshold in response to external conditions.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to a method and apparatus to dynamically adapt a CCA threshold Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood elements that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments.
In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” “has”, “having,” “includes”, “including,” “contains”, “containing” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a”, “has . . . a”, “includes . . . a”, “contains . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. The terms “a” and “an” are defined as one or more unless explicitly stated otherwise herein. The terms “substantially”, “essentially”, “approximately”, “about” or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1% and in another embodiment within 0.5%. The term “coupled” as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is “configured” in a certain way is configured in at least that way, but may also be configured in ways that are not listed.
It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and apparatus to dynamically adapt the CCA threshold described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method to perform the function of dynamically adapting the CCA threshold described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.
Various embodiments of making a clear channel assessment (CCA) threshold adaptable in response to external conditions in a wireless network, for example in a wireless network based on the 802.11 standards, are described. However, skilled artisans will realize and appreciate that the embodiments described herein are not limited to 802.11 implementations but may be applied within various other types of communication networks. The CCA threshold can be varied based on one or more parameters. For example, a time interval or a received signal strength can serve as such parameters upon which the adjustment of the CCA threshold can be based. The methods disclosed herein for adapting the CCA threshold are complimentary, can be used to address different types of jamming, and can be implemented on a single communication system. The communication system may comprise one or more access points and/or one or more stations. The embodiments disclosed are generally implemented at a Physical Layer level, for instance of the well known Open System Interconnection (OSI) networking model. However, those skilled in the art will appreciate that varied implementations of the methods disclosed are possible and all such methods are within the scope of the present invention.
One embodiment of the present invention adjusts a CCA threshold based on a busy channel indicator of a CCA. A communication channel is said to be busy when the busy channel indicator indicates a busy status, for instance when a RSS is greater than the CCA threshold. Similarly, the communication channel is said to be clear when the busy channel indicator indicates a clear status, for instance when the RSS is less than the CCA threshold. The clear status indication permits a user to use the communication channel to transmit data. However, a user is not permitted to use the channel for transmitting data when the communication channel is in use, e.g., when the busy channel indicator indicates a busy status.
Turning now to
There are several ways to determine whether the channel is busy or clear to permit transmission of data. A clear channel assessment (CCA) represents one such mechanism. To avoid having a status of a busy channel indicator remaining busy for long periods of time, a CCA threshold can be made adaptable and varied based on the status of the busy channel indicator. In general, one embodiment of the present invention comprises determining a status of a busy channel indicator for one or more parameters, step 105, and changing the CCA threshold based on the status of the busy channel indicator, step 110. The CCA threshold can be decreased or increased based on the status of the busy channel indicator. In the embodiment illustrated by reference to
Turning now to
In one illustrative embodiment, a time period Y can be measured. Further, a time period Z can be measured, wherein Z represents a time during which the busy channel indicator indicates a busy status within time period Y. A ratio Y/Z, e.g., a first time interval, may then be calculated and compared with a first predetermined time value. If the ratio Y/Z is greater than the first predetermined time value, the CCA threshold can be increased by a first predetermined value. If after the CCA threshold has been increased, the ratio Y/Z, e.g., a second time interval, is less than a second predetermined time value, the CCA threshold can be reduced by a second predetermined value.
The CCA threshold is varied based on the timings of the busy channel indicator. To begin with, the Y_cnt is set at zero and the CCA threshold is set at the default CCA threshold (305). The Y_cnt increases as the time interval indicating the busy status of the CCA increases. The Y_cnt is set at zero when the CCA indicates the clear status. It remains in that status as long as the CCA indicates a clear status. Once the busy channel indicator changes its status from the clear status to the busy status Y_cnt is enabled (310). The Y_cnt on being enabled is constantly compared with Z[Y/Z]incr. Y_cnt is set back to zero when the busy channel indicator stops indicating the busy status and if the Y_cnt is less than Z[Y/Z]incr. It remains in this status as long as the busy channel indicator indicates the busy status and Y_cnt is less than Z[Y/Z]incr. Once the Y_cnt crosses the Z[Y/Z]incr value the CCA threshold is incremented by a predetermined value X (315) to an increased threshold value.
The CCA threshold remains at the increased threshold value (with Y_cnt being set at zero) as long as the busy channel indicator indicates a clear status. The Y_cnt is only enabled when the busy channel indicator indicates a busy status. Once the busy channel indicator indicates the busy status the Y_cnt is compared with Z[Y/Z]decr. Checking again for the CCA status leads to four conditions. Where the CCA is not busy, and Y_cnt is greater than Z[Y/Z]decr, the Y_cnt is set at zero and the CCA threshold at an increased threshold value as shown at the third state 315. Next, where the CCA is not busy and Y_cnt is less than Z[Y/Z]decr, the CCA threshold is decremented by a predetermined value X to a decreased threshold value that leaves the CCA threshold back to the default CCA threshold, and Y_cnt set at zero as shown at the first state 305. Further, where the CCA indicates busy, Y_cnt is compared with Z[Y/Z]decr and if the Y_cnt is greater than Z[Y/Z]decr, the Y_cnt is set back to zero as shown at the third state 315. Lastly, where the Y_cnt is less than Z[Y/Z]decr then Y_cnt continues counting with CCA threshold set at default CCA threshold value as shown in the fourth state 320.
For the state diagram shown in
Turning now to
Prior art systems maintain a constant CCA threshold. According to one embodiment of the present invention the CCA threshold is varied based on a time interval parameter associated with the busy channel indicator, for example, the amount of time the busy channel indicator maintains a busy status. The status of the busy channel indicator indicates a busy status when the RSS exceeds the CCA threshold. As per the embodiments disclosed, the busy channel indicator would now indicate a busy status for a more limited period of time owing to the adaptive nature of the CCA threshold represented using the signal diagram. As soon as the CCA threshold is increased, the busy channel indicator switches back to a clear status allowing transmission of data. This facilitates the usage of the communication channel by more users by reducing the amount of time the channel is considered to be occupied. This in turn increases the system efficiency and also the quality of service (QoS) offered by the communication channel.
As shown in
Turning now to
In this embodiment, for instance, the CCA threshold is maintained at a positive predetermined offset from a low-pass filtered or “leaky-integrated” RSS, step 505. Summers may be used for performing integration of the RSS signal. A time constant for the RSS filter may be determined by the state of the CCA, for example, channel busy or channel clear. For instance, when the CCA indicates channel busy (e.g., as determined at step 510), the time constant may be slow so as to allow for the channel to be occupied by a non-malicious user for a nominal period of time. If, however, the CCA state indicates the channel busy for too long of a time interval, the filtered RSS output value may be increased to a value that, when added to the predetermined offset, will result in a threshold that causes the CCA status to toggle to the channel clear state, step 515. This increased filtered RSS output may be stored as the new CCA threshold, step 520.
The time interval for which the channel is allowed to be considered busy is determined by the RSS filter's “slow” time constant. When the CCA indicates channel clear (e.g., as determined at step 525), the RSS filter time constant may be increased to allow for a fast discharge of the RSS filter's output which may be decreased by subtracting a predetermined offset if the RSS is less than the CCA threshold, step 530. This provides a quick recovery from the channel busy to the channel clear operation, so that the CCA will properly indicate channel busy when the channel becomes busy, with a lower RSS than say a previous user, after a short channel clear period. The decreased filtered RSS output may be stored as the new CCA threshold, step 535.
In one embodiment, a slow increase in the CCA threshold is desired when there is an increase in the RSS. Raising the CCA threshold slowly during the time during which CCA indicates “busy” ensures that there would be no sudden cancellation of the “busy” status in the case of a legitimate packet step 515. The CCA threshold however, tracks the RSS more quickly in the time interval during which the CCA indicates a “clear” status step 530. Using a faster filter time constant causes the system to discharge the accumulated RSS energy within a very short time. This allows for asymmetric RSS duty cycles. As a result, the operating point of the CCA determination is modulated according to the background channel energy.
When the busy channel indicator, CCA 620, indicates busy (e.g., a one), weighting factor a_slow may be used, resulting in a slow filter response to changes in the RSS input. Similarly, when the busy channel indicator, CCA 620, indicates a clear channel (e.g., a zero), weighting factor a_fast may be used, resulting in a fast filter response to changes in the RSS input. Upon comparing the RSS to the CCA threshold T using a suitable comparator 618, the busy channel indicator 620 (output from comparator 618) indicates a busy channel state when the RSS is greater than the CCA threshold and a clear channel state when the RSS is less than the CCA threshold. The filtered RSS, and hence the CCA threshold, tracks the RSS input at a rate determined by the weighting factors, a_slow and a_fast. Those of ordinary skill in the art will realize that apparatus 600 is merely exemplary apparatus for implementing the methods described herein and that different apparatus for enabling such implementation is well within the scope of the various embodiments described herein
Turning now to
Based on the status of the CCA, one of the two factors (a_fast and a_slow) is switched to the integrator. When the CCA indicates busy, a first factor a_slow is used to raise the CCA threshold, which is generally selected to yield a slow RSS filter time constant. A slow increase in the CCA threshold is desired to avoid a sudden increase in the CCA threshold due to a sudden burst of RSS. A significant increase in the CCA threshold for a period of time determined by the first factor, cancels the busy status of the CCA where the signal may be the due to a malicious intruder. If the RSS remains at an elevated level for an excessively long period of time, the CCA threshold settles to a higher value. A second factor a_fast, resulting in a faster RSS filter time constant, is employed when the CCA indicates a channel clear status. In one embodiment of the present invention the faster time constant causes the CCA threshold to track the RSS value more quickly.
As mentioned above, device 802 can be an access point or a station. Device 802 may also be a separate server, where the CCA threshold values can be determined based on the parameters disclosed above. Where the CCA threshold is determined on the separate server, the calculated increase or decrease in threshold can be communicated to the access point. According to one embodiment, the CCA threshold is calculated at the access point, the processor 810 and memory of the access point can work in conjunction to determine the status of a busy channel indicator for a predetermined parameter and correspondingly change the CCA threshold based on the status of the busy channel indicator. Those skilled in the art will appreciate that software and other components present on the communication system 800 can be configured to perform the tasks disclosed above and such software and components are within the scope of the present invention. The processor 810 can be configured to handle the overall functioning of a comparator and integrator and determining the status of the busy channel indicator. The processor 810 generally performs the operations at a physical layer level. The transceiver 805 may be a combined transmitter and receiver configured to handle the transmission and reception of communication signals.
In this disclosure various embodiments are provided that are complementary, and can be used for different types of jamming. For instance, the method to adapt the CCA threshold based on time interval can be used for intermittent preamble jamming. The method for adapting the CCA threshold based on RSS values and comparing the RSS value to the CCA threshold can be executed for continuous noise jamming. Keeping the CCA threshold constant can cause prolonged disruption of service due to the presence of intruders or noise or transmission of data that is not legitimate. Adapting the CCA threshold dynamically enables efficient and uninterrupted transmission of data on the transmission channel.
While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.
