Patent Application
20070286122
The accompanying figures, wherein 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.
In overview, the present disclosure concerns distributed scheduling algorithms for transmitting data in a wireless data communication network, such as a wireless communication network according to the IEEE 802.11 standard for mobile ad hoc networks (MANETs), which uses a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) procedure or similar procedures to access the wireless channel. More particularly various inventive concepts and principles embodied in methods and apparatus may be used for automatically selecting and adapting a clear channel assessment (CCA) threshold used in a CSMA/CA procedure in a node in a MANET.
While the CCA threshold adaptation method and system of particular interest may vary widely, one embodiment may advantageously be used in a wireless communication system, or a wireless networking system, operating according to one or more respective standards. For example, one such wireless network standard is the IEEE 802.11b standard for wireless networking, published by the Institute of Electrical and Electronics Engineers (IEEE) and the American National Standards Institute (ANSI). Additionally, the inventive concepts and principles taught herein can be advantageously applied to other wireless communications systems, particularly where a plurality of wireless devices access a radio channel using multiple access and collision avoidance techniques.
The instant disclosure is provided to further explain, in an enabling fashion, the best modes, at the time of the application, of making and using various embodiments in accordance with the present invention. The disclosure is further offered to enhance an understanding and appreciation for the inventive principles and advantages thereof, rather than to limit the invention in any manner. The invention is defined solely by the appended claims, including any amendments made during the pendency of this application, and all equivalents of those claims as issued.
It is further understood that the use of relational terms, if any, such as first and second, top and bottom, and the like, are used solely to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions.
Much of the inventive functionality and many of the inventive principles can be implemented with, or in, integrated circuits (ICs), including possibly application specific ICs, or ICs with integrated processing controlled by embedded software, firmware, or program code. 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. Therefore, in the interest of brevity and minimizing any risk of obscuring the principles and concepts according to the present invention, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts of the various embodiments.
Referring now to
Wireless communication network 100 is preferably a self-organizing and self-configuring, direct (node-two-node) and multi-hop communications network. Multi-hop communication is used when the radio channel separating two nodes is not suitable for direct communication, in which case one or more intermediary nodes relays a data packet until it reaches the final destination node.
Each wireless device 102, 104, and 106 typically includes a processor 114, which is coupled to transceiver 116 and memory 118. Processor 114 can perform many of the functions and operations that occur within wireless device 102 (and 104 and 106) by executing program code and using data stored in memory 118. In one embodiment, processor 114 can include one or more microprocessors, microcontrollers, or digital signal processors, which are each well known and readily available.
Processor 114 can be coupled to transceiver 116 through interface 120. In one embodiment, interface 120 can transfer data for transmission from transceiver 116, data received by transceiver 116, and various command and control signals. Transceiver 116, which is generally known and available, can include baseband logic and radio frequency circuits for encoding, modulating, and transmitting, and conversely receiving, demodulating, and decoding, signals that are transmitted and received using antenna 108. In one embodiment, transceiver 116 can communicate with other wireless devices using IEEE 802.11b protocol (e.g., Wi-Fi), or another similar protocol. Wireless device 102 can communicate wirelessly with other wireless devices 104 and 106 via wireless communication channels 110 and 112.
Processor 114 can be coupled to memory 118 through interface 122, which, in one embodiment, is configured to transfer data and program code for processing and execution in processor 114. In some embodiments, processor 114 can also include internal memory.
Memory 118 can be implemented using some combination of generally known memory technology, such as RAM, ROM, EPROM, magnetic, optical memory, and the like.
Memory 118 can include program code 124 and data storage 126, which are used to execute various algorithms, processes, and methods within processor 114 and wireless device 102. For example, program code 124 can include program code for processes and algorithms that implement busy channel detector 128 and CCA threshold adapter 130. As described in greater detail below, busy channel detector 128 can be used to detect whether another wireless device is transmitting on a channel used by transceiver 116, and CCA threshold adapter 130 can be used to adapt a clear channel assessment (CCA) threshold that is used in deciding whether transceiver 116 can transmit data without colliding with another transmission.
Data storage 126 can be used to store network neighborhood successful transmission data 132 and CCA threshold data 134. As described in greater detail below, network neighborhood successful transmission data 132 relates to the rate of successful transmissions of wireless device 102 and the network neighbors of wireless device 102. CCA threshold data 134 can be used to store a current best CCA threshold, and data related to the use of other CCA thresholds.
While much of the functionality of wireless devices 102, 104, and 106 can, in some embodiments, be attributed to software instructions as executed by processor 114, it will be appreciated that many of these operations can also be performed by hardware or some combination of software and hardware. Additionally, it will be appreciated by those of ordinary skill that a multiplicity of other functions or operations, which are not specifically shown, can be performed in a typical wireless device, and that various of those can be implemented, at least in part, with the processor(s) and various software instructions, etc.
With reference now to
Furthermore, the nodes are typically not synchronized, which means that they can employ a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) procedure to avoid transmitting while another nearby or neighbor node is transmitting, which can cause a collision wherein data reception at the receiving nodes can be corrupted by the interference between the two transmitting nodes.
A wireless device acting as a node in a wireless network can attempt to avoid collisions in the channel by listening for carrier signals from other wireless devices before beginning a transmission. If the wireless device senses the carrier signal, the wireless device delays its transmission until it no longer senses the carrier signal. In one embodiment, the wireless device determines whether a carrier signal is present by measuring radio frequency energy that exceeds a noise floor by a threshold, which is the CCA threshold. As the CCA threshold is lowered closer to the noise floor, the wireless devices become more sensitive to the transmission of other wireless devices, and will therefore determine not to transmit more often due to the greater range of sensing, which can include areas with more wireless devices.
For example,
A benefit of increasing the CCA threshold can come from the ability of wireless device 102 to ignore transmissions between wireless devices 206, 208, and 210, because these devices are now outside of range 214 used by wireless device 102 in determining whether the channel is clear for transmission. When wireless devices 206, 208, and 210 are outside carrier sensing range 214, wireless device 102 can transmit more often because it will not be waiting for the completion of transmissions by devices 206, 208, and 210.
A problem with increasing the CCA threshold, and correspondingly reducing the carrier sensing range from 216 to 214, is that wireless device 106 becomes a “hidden node” with respect to wireless device 102. Wireless device 106 becoming a hidden node with respect to wireless device 102, means that wireless device 102 does not sense or detect whether wireless device 106 is transmitting before beginning its own transmission. This means that wireless device 106 can be transmitting to wireless device 104 at a time when wireless device 102 begins transmitting to device 104, which creates a collision because wireless device 102 did not detect the transmission from “hidden” wireless device 106.
An “exposed node” problem exists when wireless device 102 senses wireless device 206 transmitting to wireless device 208 and then, in response, wireless device 102 delays its own transmission to wireless device 104, wherein the transmission from device 102 to device 104 can coexist without interference with the transmission from device 206 to device 208.
Thus, the “hidden node” problem erroneously fails to postpone a transmission that causes a collision, and the “exposed node” problem erroneously postpones a transmission that would not have caused a collision. Both of these problems should be avoided in order to increase data throughput in the wireless network. This means that it is important for each node to have the correct CCA threshold to increase proper access to the wireless channel and to increase the overall throughput of the wireless network. The correct CCA threshold for each node depends on several factors, including the particular relative placement of nodes and the propagation conditions among them.
Turning now to the operation of one or more of the wireless devices in wireless communication network 100,
In one embodiment, CCA thresholds can range from 1 dB to 9 dB above the noise floor, with several possible CCA thresholds within the range. For example, 2 to 8 possible levels can be used, where each level is spaced apart by, for example, 0.5 dB. Enough levels should be selected to fine tune the CCA threshold without having so many levels that it takes too long to test and determine the correct level.
Next, the process sets the current best CCA threshold to an initial CCA threshold, as shown at 306. In one embodiment, the initial CCA threshold can be near the middle of the range of thresholds. Alternatively, the initial CCA threshold can be set to a value that was stored before the wireless device was shut down after its previous use.
After setting the current best CCA threshold, the process determines a successful transmission rate in the node and in neighboring nodes with the node using the current best CCA threshold, as illustrated at 308. In one embodiment, the successful transmission rate in the node and in neighboring nodes can be determined by counting indications of successful packet transmissions for a period of time and then dividing by the period of time, wherein the indications of successful packet transmissions can be an “ACK” message (i.e., an acknowledgement message) that is transmitted by the node, or a neighboring node (i.e., the node and neighboring nodes collectively) to any other node. Thus, the node is collectively monitoring all indications of successful transmissions generated in its surroundings, including its own transmissions and transmissions not destined for the node, and then dividing by the time of the measurement, which may be hundreds of milliseconds. In order to be representative, the time required to collect indications of successful transmissions can be long enough to reduce the variance and the probability of error. In an alternative embodiment, the process can count packets indicating an unsuccessful transmission, such as an indication that data is being retransmitted, wherein the unsuccessful indication is transmitted from either the node, or from neighboring nodes, to any other node. This allows the process to infer a successful transmission rate that is inversely proportional to the rate of retransmitted or otherwise unsuccessful packets.
In yet another embodiment, the process can estimate an effective throughput of the node and neighboring nodes, wherein the estimated effective throughput corresponds to a rate that data passes through the node and neighboring nodes. Counting indications of successful packet transmissions, or indications of unsuccessful packet transmissions, can be used to estimate effective throughput of the node and neighboring nodes, collectively. In alternative embodiments, specialized messages between nodes (e.g., reports of bit rates) can be used to produce a more accurate estimate of effective throughput of the node and neighboring nodes.
At 310, the process determines whether another CCA threshold may be needed. In one embodiment, the need for a new CCA threshold can be indicated by detecting a quality of service (QOS) degradation. For example, a QOS metric, such as a percentage difference between a desired data rate and an actual data rate, may be declining, which can indicate the need for a new CCA threshold. While the step shown at 310 may be optional, an advantage to having a triggering event for starting the CCA threshold adaptation process is that it can provide a limit to the number of nodes that are simultaneously adapting their CCA threshold. When more than one node simultaneously adapts its CCA threshold, subsequent measurements of rates of successful transmissions can be skewed because two or more nodes changed CCA thresholds at the same time.
As an alternative, the process can determine a new CCA threshold may be needed, or determine to test for a more effective CCA threshold, after a period of using the current CCA threshold. In yet another embodiment, the triggering event for automatically adjusting the CCA threshold can be the detection of a new node in the area, or detecting that a node has left the area.
If there is no triggering event for adjusting the CCA threshold (e.g., a degradation in the quality of service), the process continues using the current best CCA threshold while looping at the “No” branch of 308. If, however, the process detects a QOS degradation, or detects another event that triggers the determining of whether there is a more effective CCA threshold, the process selects a candidate CCA threshold from one of the possible CCA thresholds in the range, as illustrated at 312.
In one embodiment, a candidate CCA threshold can be selected based upon an expected change in busy activity level corresponding to a previously used CCA threshold. The busy activity level is a measurement of the percentage of time in which the channel is sensed busy by a node. The busy activity level varies with the CCA threshold used by a node because it relates to the number of nodes whose transmissions are sensed at a given CCA threshold. Current and historic data (e.g., measurements) can be stored in a table in data storage 126 (see
A node can determine the busy activity level associated with all possible CCA thresholds 402 by using a series of digital averaging filters, one for each possible CCA threshold. Each digital averaging filter can have as an input signal a sequence of numbers valued 0 or 1, and can have as an output the busy activity level for its corresponding possible CCA threshold, which output is stored as busy activity level 404. Whenever the busy channel detector 128 senses the energy contained in the channel, the energy value is compared against each of the possible CCA thresholds 402. When comparing to a possible CCA threshold 402, if the energy sensed is below or above such threshold, a value 0 or 1 is input into the corresponding filter. Thus, when considering a candidate CCA threshold, if a first candidate CCA threshold has a similar busy activity level to a second candidate CCA threshold, then it is probable that the set of nodes present in the sensing region corresponding to the first candidate CCA threshold is similar to the set of nodes present in the sensing region corresponding to the second candidate CCA threshold.
Note that table 400 can also be used to store successful transmission rate measurements 406 that are associated with each CCA threshold 402. The successful transmission rate can be measured in terms of successful packets per second. These measurements 406 can be used when comparing the effects of using CCA thresholds 402. Additionally, column 408 can be used to store additional data associated with CCA thresholds 402, such as, for example, historical successful transmission rates, or averaged, filtered, or otherwise processed successful transmission rates. Such information can also be useful in selecting CCA candidates.
Next, the process determines a successful transmission rate in the node and neighboring nodes with the node using the candidate CCA threshold during a sample period, as illustrated at 314. This successful transmission rate can be determined in a manner similar to that used in 308, which, in one embodiment, uses counting indications of successful transmissions (e.g., counting ACK messages) during the sample period. Here again, the successful transmission rate measurement includes both the successful transmissions of the node and of neighboring nodes so as to measure the effect of changing the CCA threshold on a group of nodes in a manner that promotes fairness of access to the radio channel among the group.
Once a successful transmission rate for the candidate CCA threshold has been determined, the process determines whether the successful transmission rate increased when using the candidate CCA threshold compared to the current best CCA threshold, as depicted at 316. If the successful transmission rate increased, the process passes to 318 wherein the current best CCA threshold is replaced with the candidate CCA threshold, and the node begins using the newly replaced CCA threshold.
If, at 316, the successful transmission rate is not increased, or after the CCA threshold is replaced at 318, the process passes to 320, wherein process determines whether there are additional candidate CCA thresholds that should be tested. If there are additional candidate CCA thresholds, the process selects the next candidate CCA threshold at 322. This selection of the next candidate CCA threshold can be similar to the procedure used at 312.
If, at 320, there are no additional candidate CCA thresholds, and the process has cycled through all the candidates and selected the CCA threshold that produces a high rate of successful transmissions among the node and neighbor nodes, the process returns to 310 to await the next triggering event for adapting the CCA threshold.
The above described functions and structures can be implemented in one or more integrated circuits. For example, many or all of the functions can be implemented in the signal and data processing circuitry that is suggested by the block diagrams shown in
The processes, apparatus, and systems, discussed above, and the inventive principles thereof are intended to produce an improved, more efficient, and more reliable distributed scheduling algorithm for scheduling data transmission, particularly in a single-channel single-transceiver MANET system. In general, the channel access procedures disclosed herein attempt to increase the system utilization by balancing the hidden-node and exposed-node problems. By periodically and automatically adapting the CCA threshold in response to a rate of successful transmissions among a node and its neighboring nodes, the overall throughput of the MANET can be increased. Nodes are able to self-configure their CCA threshold to a value closer to the best CCA threshold for the particular condition and location for the node. The approaches described herein can be implemented without changes to the IEEE 802.11 specification because there is no need for exchanging messages between nodes in order to measure the rate of successful transmissions. However, in alternative embodiments, such new messages can further improve the estimation of effective data throughput of the node and neighboring nodes, and can further coordinate the schedule upon which CCA thresholds are adapted in neighboring nodes. A single node in a wireless network can use the procedure described herein and the node and the network overall will benefit, even if other nodes do not use the procedure. The procedure described herein can be used in both pure ad hoc networks and mesh networks.
This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention, rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) were chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
