1. Field of the Invention
The present invention relates to a wireless LAN system, and more particularly, a communication control device and a communication control method that use an IEEE 802.11 standard-compliant system and the infrastructure mode of the system.
2. Description of the Related Art
In these days, in a wireless local area network (LAN) system, an IEEE 802.11 standard-compliant system is used. In the IEEE 802.11 standard-compliant system, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used for the access control of a medium (wireless channel).
With CSMA/CA, a node that sends a packet verifies the situation of a medium in order to prevent the loss of transmission data due to packet collisions. That is to say, in a wireless LAN system, before a packet that was sent from a sending node arrives at one receiving node, another packet that was subsequently sent from another sending node sometimes arrives at the receiving node. Packet collisions means that, in such a case, the receiving node cannot distinguish arrived packets or perform demodulation of data.
When packet collisions occurred, the receiving node has captured all the collided packets. Then the receiving node refers to a destination address put on the captured packet, and determines whether or not its own node is a destination. When the receiving node is the destination, it is determined that packet loss occurs, and the need of retransmission arises. Further, if the packet is management data or control data addressed to a plurality of nodes including the receiving node, degradation of communication quality and reduction in system throughput occur.
Note that no problem arises if the packet that triggered the collision was not destined to its own node.
In order to avoid such packet collisions, when a medium is not performing communication, a node waits to send packets (back off) for a random period within specified time (contention window), then sends the packets. On the other hand, when the medium is performing another communication, the node waits for the medium to be released, then sends the packets (transmission control). Then, after the medium is released, the node sends the packets again.
At this time, the node adjusts a modulation scheme, the number of multi-values (M-ary), and an error correcting coding rate depending on the size of the power of a signal, which arrives at a receiving side, and the quality of communication. The adjustment is performed by self-distributed adaptive modulation/demodulation operation at each node.
Further, adjustment by a node allows data transmission speed to be increased (shift-up) or reduced (shift-down). In CSMA/CA, the adjustment of the data transmission speed makes wireless resource utilization more efficient (maximization of data transmission speed, minimization of medium occupancy time of packet, and maximization of the probability of enabling connection of a medium at a node).
Note that transmission power of each node is usually fixed to equal to or less than the specified value of the standard, and that a node usually does not change the level in accordance with the situation of the medium and traffic.
More specifically, the situation of a medium is checked through the measurement of a wireless channel used for transmission by a node, which a node is attempting to send out a packet to. The measurement of the wireless channel allows the size of total received power, and the presence or absence of an arriving packet which reception is identifiable, to be determined. Note that the total received power is the sum of thermal noise power of a receiver, and received signal power of packets which have been sent out from another node, and arrived at its own node at the time of measurement.
When the total received power is larger than a preset threshold (carrier sense threshold), or when there is a arriving packet which reception is identifiable, the node determines that another communication being performed nearby or multiple communications being performed in the surrounding is the situation (Busy).
Note that “nearby” means an inter-node distance for which its own node is determined to be “Busy” as a result of another node sending a packet. In addition, “in the surrounding” means an inter-node distance for which its own node is determined to be “Busy” as a result of a plurality of other nodes sending packets simultaneously. Further, “long distance” means an inter-node distance for which its own node is not determined to be “Busy” even if a plurality of other nodes send packets simultaneously.
If the total received power is equal to or larger than the carrier sense threshold, or if there is an arriving packet which reception is identifiable, when new communication starts, packet collisions occur. In addition, interference of communication by another node causes congestion of a medium due to packet retransmission. Further, transmission error due to interference is promoted, and the throughput of the entire system is reduced.
On the other hand, when the total received power is less than the carrier sense threshold, and there is no arriving packet which reception is identifiable, it is determined that communication is not being performed nearby or in the surrounding (Idle). At that time, the node can start new communication.
However, even when Idle, due to the interference of communications being performed remotely, sometimes packets arrive at its own node with received powers such that the total amount of received signal power is equal to or less than the carrier sense threshold, or with received powers that do not allow identification of reception as packets. Such a state is a factor of shift-down because of avoidance of transmission error based on the adaptive modulation/demodulation operation, even if transmission data loss due to packet collisions does not occur.
As described above, with a carrier sense, packet collisions can be reduced to some extent to facilitate communication.
Prior arts of such a wireless LAN are described in Duck-Yong Yang et al, “Achieving Efficient Channel Utilization Using Dynamic Coverage Control in IEEE802.11,” Proc. IEEE ISPACS, November 2004 (hereinafter referred to as Non-patent Document 1), Hector Velayos et al, “Load Balancing in Overlapping Wireless LAN Cells,” IEEE ICC 2004, June 2004 (hereinafter referred to as Non-patent Document 2) and Olivia Brickley et al, “Load Balancing for QoS Optimisation in Wireless LANs Utilising Advanced Cell Breathing Techniques,” Proc. IEEE VTC 2005-Spring, May 2005 (hereinafter referred to as Non-patent Document 3), for example. An infrastructure mode and an ad hoc mode are defined in the IEEE 802.11 standard-compliant wireless LAN system, and Non-patent Document 1 to Non-patent Document 3 are based on the regulation of the infrastructure mode.
Note that an infrastructure mode is the regulation of a wireless network comprised of two types of nodes: a base station (access point: AP) and a mobile terminal (station: STA). Further, an ad hoc mode is the regulation of a wireless network comprised of only STAs.
A set of one AP and a plurality of STAs attributed thereto in an infrastructure mode is referred to as a Basic Service Set (BSS). A BSS corresponds to a cell of a cellular system.
A technique described in Non-patent document 1 discloses that, in a configuration where infrastructure mode wireless LAN systems are densely deployed, the transmission power of an AP can be controlled to a necessary minimum value that each STA requires based on link margin information that has been reported by each STA. In the technique described in Non-patent document 1, such control allows interference between BSSs (temporal occupancy of the medium of a neighboring BSS) to be reduced, and wireless resources to be used efficiently, thus allowing the system throughput to be increased.
Note that the method disclosed in Non-patent Document 1 is particularly effective for a case where a frequency band assigned to the wireless LAN system is not sufficient for traffic, and interference between BSSs cannot be avoided due to the design of the BSS operating frequency.
In addition, Non-patent document 2 describes that, in the infrastructure mode wireless LAN system, as an AP to be connected to its own device, the STA selects an AP where the received signal is the strongest among a plurality of AP candidates. The technique disclosed in Non-patent Document 2 is a technique that balances, in such a configuration, the loads among self-distributedly overlapping APs, and increases the throughput of the LAN system to reduce a transmission delay.
More particularly, each AP regularly exchanges load information through a wired backbone, and selects an STA requiring hand-off in order to balance the load. At this time, the AP determines the state of its own device (high load, low load, adequate) based on the exchange result of the load information. The load information representing a state is represented by throughput in consideration of uplink and downlink traffics on each AP.
In addition, as a result of determination, an AP whose load was determined to be low actively accepts the roaming and new connection of STAs that a neighboring AP takes charge of. An AP whose load was determined to be adequate accepts the new connection of STAs. An AP whose load was determined to be high does not accept the new connection of STAs, and forces the existing STAs to be handed over in order to reduce the load.
Such a method disclosed in Non-patent Document 2 is effective in increasing the system throughput in a case where STAs in an area are unevenly distributed, and congestion occurs in the AP of a specific BSS, but congestion does not occur in the AP of the adjacent BSS.
In addition, Non-patent Document 3 describes that, in the infrastructure mode wireless LAN system, when the connections of the STAs having data to be transmitted concentrate on a specific AP, the AP whose load (number of connections of SATs) is high reduces transmission power to narrow BSS coverage. In addition, in Non-patent Document 3, the AP whose load is low increases transmission power to expand BSS coverage, in order to distribute the load among APs.
In such Non-patent Document 3, congestion occurs in a specific AP, thus allowing reduction in throughput due to packet collisions and packet retransmission, and the resulting increase in delay and degradation of Quality of Service (QoS) to be avoided. Non-patent Document 3 is effective in increasing the system throughput in a case where STAs in an area are unevenly distributed, and congestion occurs in the AP of a specific BSS, but congestion does not occur in the AP of the adjacent BSS.
However, the above-described carrier sense cannot completely avoid packet collisions when a hidden terminal problem arises. In addition, it is a problem that reduction in throughput due to an exposed terminal problem cannot sufficiently be prevented.
That is to say, a hidden terminal problem occurs when a packet which a sending node “a” sent cannot arrive at another sending node “b”, for example, due to electric wave propagation environment such as obstacles and the distance between sending nodes. This is because, in such a case, the carrier sense cannot be performed properly, and the sending node “b” sends out a packet in spite of the fact that the sending node “a” is sending another packet.
Note that there is a propagation delay time problem as another cause of packet collisions. The propagation delay time problem means that, in spite of the fact that the carrier sense was performed properly, the sending node “b” cannot detect the start of packet sending from the sending node “a” because of the electric wave propagation delay between sending nodes, and starts sending out the packet almost simultaneously with the sending node “a”.
Meanwhile, the exposed terminal problem is a problem that data is sent and received because of an unnecessary carrier sense, resulting in the reduction in system throughput.
For example, when the sending node “a” starts sending out a packet to a receiving node “a”, the sending node “b” detects the packet of the sending node “a” with a carrier sense, and determines that the situation of the medium is Busy. At this time, when there is a packet which the sending node “b” is about to send to the receiving node “b”, the sending node “b” is controlled to not send the packet but keep it.
However, in the case described above, a situation can also be considered, where the receiving node “a” and the receiving node “b” are distanced enough such that packet collisions do not occur even if the sending node “b” sends a packet to the receiving node “b”. In such a situation, the carrier sense is unnecessary, reducing the system throughput.
The hidden terminal problem and the exposed terminal problem occur to predict packet collisions depending on the presence or absence of the packet sending in the sending node (verification of the situation of the medium with a carrier sense).
That is to say, when the sending node “a” and the receiving node “a” are at close enough spots, the node “b” detects the packet sending in the sending node “a” with the carrier sense and suppresses the sending, allowing packet collisions in the receiving node “a” to be avoided.
However, when the sending node “a” and the receiving node “a” are far from each other, even if the packets are arriving at the receiving node “a”, sometimes the situation of the medium is not verified with the carrier sense of another sending node “b” due to shielding objects in the surrounding and attenuation of packets. In such a case, when the node “b” sends a packet to the receiving node “b”, packet collisions occur in the receiving node “b” due to the hidden terminal problem.
In addition, when the sending node “a” and the receiving node “a” are far from each other, packets sent out from another sending node “b” are attenuated to a received signal power that does not cause packet collisions in the receiving node “a” while packets are arriving at the receiving node “a”. In the present circumstances, even in such a case, the sending node “b” detects the sending of the packet to the receiving node “a” with a carrier sense and suppresses the sending. Accordingly, an exposed terminal problem arises, and efficient utilization of a wireless resource is prevented.
The Non-patent Document 1 exchanges control data (RTS: Request To Send, CTS: Clear To Send) among APs or among STAs prior to data transmission in order to avoid a hidden terminal problem. A technique described in such Non-patent Document 1 causes a possibility that a substantial throughput which can be assigned to user data is reduced by the exchange of control data.
In addition, in the technique described in Non-patent Document 1, only the transmission power of an AP is controlled, but transmission power of an STA is not controlled. Accordingly, in Non-patent Document 1, there is room for improvement on an interference reduction effect in a BSS. In addition, the technique described in Non-patent Document 1 assumes the Direct-sequence Spread Spectrum (DSSS) scheme in the physical layer, and is not applicable to the IEEE 801.11a and IEEE 802.11g standards which use the Orthogonal Frequency Division Multiplexing (OFDM) scheme. The OFDM scheme is a scheme which obtains high throughput in the same frequency occupancy band by adopting adaptive modulation/demodulation operation. The fact that the standard using such a scheme cannot be applied reduces the flexibility of the design of a wireless LAN system.
Further, techniques described in Non-patent Document 2 and Non-patent Document 3 can prevent connections from concentrating on a portion of nodes in a BSS, and reduce a probability that packet collisions occur. However, the techniques described in Non-patent Document 2 and Non-patent Document 3 do not consider the effect of interference, which occurs when a plurality of BSSs use the same frequency for communication. Accordingly, it is not unrealistic because it is required that there is a sufficient frequency band assigned to the traffic of the wireless LAN system, and that all the BSSs in an area are operated in different frequencies.
Furthermore, Non-patent Document 3 does not specifically describes a method for detecting the load of a BSS and the degradation of QoS, a relationship between a detected value and the control of transmission power, and a method for coordinating and controlling BSSs.
The present invention is made in light of the above points, and has an object to provide a communication control device and a communication control method that can efficiently use wireless resources to improve communication quality and system throughput by setting and performing node transmission power properly.