GB2461724A - Dynamic spectrum access or cognitive radio networks implementing a dynamic access protocol - Google Patents

Abstract

The application describes medium access coordination and channel selection for establishment of wireless communications in the context of dynamic spectrum access or cognitive radio networks. Facilitating access coordination to a wireless communications medium defining a plurality of channels is achieved by each node in a network storing information about the channels, including whether a channel is available for use by that node, and when the channel is available for use, a value corresponding to a traffic load on the channel. Nodes monitor for control signals relating to data transmissions and, as some of these control signals include traffic flow information, update their stored information accordingly. Such an approach allows nodes to select the least congested channels during channel access negotiation. Moreover, and in addition or in the alternative, can determine available channel capacity based on transmission activity measurements. The invention aims to address the prior art shortfall of not considering dynamic spectrum access or cognitive radio networks implementing a dynamic channel access protocol in which channel selection is based on traffic demand in order to achieve load balancing and better Quality-of-Service.

Description

This application concerns medium access coordination and channel selection for establishment of wireless communications in a network. More particularly, but not exclusively, it relates to aspects of the technology in the context of dynamic spectrum access or cognitive radio networks.

The proliferation of versatile wireless services has increased demand for radio spectrum, a scarce resource much of which is licensed for specific uses or to specific groups of users. However, studies have shown that there exist spectrum holes where a band of frequencies assigned to a primary user is not being utilized by that user at a particular time and specific geographic location. (The term "primary user" generally refers to the licensed user of the spectrum or a user recognised as having high priority for the spectrum band.) The implication of this observation is that efficient spectrum usage can effectively be viewed as a spectrum access problem.

Recently, Dynamic Spectrum Access (DSA), or opportunistic spectrum access, has emerged as a way for secondary users to exploit spectrum holes (also known as "white spaces") to significantly improve spectrum utilization. A secondary user is a user who is authorised to use licensed spectrum opportunistically, without causing unacceptable interference to primary users. Generally, a secondary user accesses spectrum on a temporary basis when primary users are not making use of the spectrum, and should exit once a primary user arrives. In this context, there is an increasing interest in applying the concept of cognitive radio to DSA networks.

Cognitive radio is a field of wireless communications technology in which either a network on a distributed basis or a wireless node in particular can change parameters governing transmission or reception characteristics, such as operating frequency, modulation waveforms, and transmission power. The alteration of parameters can be based on active monitoring of several factors in the external and internal radio environment, such as reservations made of the radio frequency spectrum, user behaviour and network state. In addition, cognitive radio networks and devices are typically implemented such that effective communication is established without creating undue interference to other networks.

Conceptually, the idea behind DSA is simple -when a secondary device identifies spectrum holes (opportunities) in the spectrum it wishes to use, it can transmit using those opportunities in a manner that limits the interference perceived by primary users.

However, the realization of DSA currently faces several challenges. These include wide-band sensing, opportunity characterization and identification, coordinating protocols for multiple nodes, and definition and application of interference-limiting policies.

Known approaches to regulating spectrum allocation can be classified broadly into two strategies: centralized and distributed schemes.

Centralized methods for managing and coordinating spectrum access, in which a central controller allocates spectrum to all nodes of a network, have been put forward by the IEEE 802.22 working group on Wireless Regional Area Networks ("WRANs") (yw.ieee802.orI22/), and in papers such as: "DSAP: a protocol for coordinated spectrum access," (V. Brik, E. Rozner, S. Banerjee, and P. Bahi, First IEEE International Symposium on New Frontiers in Dynamic Spectrum Access Networks, DySPAN 2005, 8-11 Nov. 2005, pp. 611-614), and "DIMSUMnet: New Directions in Wireless Networking Using Coordinated Dynamic Spectrum Access," (M. M. Buddhikot, P. Kolodzy, S. Miller, K. Ryan and J. Evans, Proceedings of the Sixth IEEE International Symposium on World of Wireless Mobile and Multimedia Networks, 2005, pp. 78-85).

For distributed schemes, the literature contains several papers on MAC and network routing protocols for cognitive radio networks and multi-channel networks.

For example, "Distributed Coordination in Dynamic Spectrum Allocation Networks," (J.

Zhao, F!. Zheng; G.-H. Yang, in Proceedings of the first IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN), 2005, pp. 259-268) discusses how to choose control channels among clusters of nodes. Data channel selection entails each user recording the number of successful negotiations by eavesdropping on coordination messages transmitted during a dedicated control window, and selecting the channel with the minimum number of requests.

"Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using a Single Transceiver (J. So, N. H. Vaidya, International Symposium on Mobile Ad Hoc Networking & Computing, Proceedings of the 5th ACM international symposium on Mobile ad hoc networking and computing, 2004, pp. 222-233) proposes a MAC protocol for multi-channel ad hoc networks of nodes, based on channel negotiation using the Ad hoc Traffic Indication Messages (ATIM) windows of the IEEE 802.11 Power Saving Mechanism (PSM).

"C-MAC: A Cognitive MAC Protocol for Multi-Channel Wireless Networks," (C.

Cordeiro, K. Challapali, in Proceedings of the second IEEE International Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySpan), 2007, pp. 147-157) studies a new MAC based on modified superframe structures for cognitive radio networks, wherein nodes determine the load on a particular channel by analyzing the traffic reservation information carried by beacon frames transmitted during a beacon period.

While the present invention may be implemented in networks utilising periodic intervals to provide opportunities to coordinate channel selection activities, such an interval is not a necessity.

Other schemes adopt graph-theoretic and mathematical programming approaches: In "Collaboration and Fairness in Opportunistic Spectrum Access," (H. Zheng, C. Peng, IEEE International Conference on Communications (ICC), May 2005, pp. 3132-3136), available spectrum channel is mapped to a colour, with a set of vertices denoting the users that share the spectrum. The problem then reduces down to colouring each vertex (allocating channels) while trying to maximise a "utility function".

In "MAC-layer Scheduling in Cognitive Radio Based Multi-Hop Wireless Networks," (M. Thoppian, S. Venkatesan, R. Prakash, R. Chandrasekaran, International Workshop on Wireless Mobile Multimedia, Proceedings of the 2006 International Symposium on World of Wireless, Mobile and Multimedia Networks, 2006, pp. 191-201), the optimal MAC-layer schedule problem is formulated as an Integer-Linear Programming (ILP) problem, which is known to be computationally complex. Further, the disclosed approach does not accommodate mobility and dynamic node arrivals and departures since it assumes a static multi-channel wireless network.

"Decentralized Cognitive MAC for Dynamic Spectrum Access," (Q. Zhao, L. Tong, A. Swami, in Proceedings. of the first IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN), 2005, pp. 224-232) discusses a Markov decision formulation of the channel allocation problem, in which secondary users with data to transmit decide which channel to sense based on its past sensing history and channel statistics using either optimal or suboptimal protocols that maximize overall network throughput.

Yet further papers assume that traffic is equally distributed across available channels.

An example of such a paper is: "A Multichannel CSMA MAC Protocol with Receiver-Based Channel Selection for Multihop Wireless Networks," (N. Jam, S. R. Das, A. Nasipuri, in Proceedings of the tenth IEEE International Conference on Computer Communications and Networks, 2001, pp. 432-439), which describes a multi-channel protocol with channel selection based on receiver SINR.

None of the publications discussed above consider dynamic spectrum access or cognitive radio networks implementing a dynamic channel access protocol in which channel selection is based on traffic demand, in order to achieve load balancing and better Quality-of-Service.

According to a first aspect of the invention there is provided a method of establishing wireless communication in a wireless communications medium defining a plurality of channels, the method comprising: receiving a request from a first node at a second node, said request relating to an intended data communication therebetween and including infonnation identifying one or more channels available for use by the first node; determining at the second node whether any of said one or more channels are available for use by the second node and, if said determining is positive, selecting for said data communication at least one channel mutually available for use by the first and second nodes based on an available capacity of said at least one channel; and sending a reply from the second node to the first node, said reply including information identifying said at least one channel.

According to a second aspect of the invention there is provided a method of relinquishing a channel over which first and second nodes have established wireless data communication in a wireless communications medium defining a plurality of channels, the method comprising: sending a notification from either of the first and second nodes to the other node, said notification indicating that the channel should be relinquished; and receiving an acknowledgement responsive to said notification, said acknowledgement including an identifier of the channel and information confirming that the channel will be relinquished.

According to a third aspect of the invention there is provided a method of measuring load in a wireless communications medium defining a plurality of channels by a node opportunistically accessing the medium, the method comprising: identifying one or more channels available for use by the node; sequentially monitoring each of the available channels, the monitoring of a channel comprising measuring transmission activity on the channel over at least a first time period; and calculating an available capacity for each of the monitored channels based on said measuring.

According to a fourth aspect of the invention there is provided a method of facilitating access coordination among a plurality of nodes to a wireless communications medium defining a plurality of channels, the method being performed at each node, the method comprising: storing information about the channels, the information about a channel including an indicator of whether the channel is available for use by the node, and when the channel is available for use, a value corresponding to a traffic load on the channel; monitoring for control signals relating to data transmissions involving the nodes, at least some of said signals including traffic flow information; and updating the stored channel information in accordance with monitored traffic flow information.

According to a fifth aspect of the invention there is provided a wireless communication apparatus operable to establish wireless communication with another apparatus in a wireless communications medium defining a plurality of channels, the apparatus comprising: request receiving means operable to receive a request from the other apparatus, said request relating to an intended data communication therebetween and including information identifying one or more channels available for use by the other apparatus; channel availability determining means operable to determine whether any of said one or more channels are available for use by the wireless communication apparatus; channel selection means operable to select for said data communication, responsive to a positive determination by said determining means, at least one channel mutually available for use based on an available capacity of said at least one channel; and wherein said apparatus is operable to send a reply towards said other apparatus, said reply including information identifying said at least one channel.

According to a sixth aspect of the invention there is provided a wireless communication network comprising a plurality of nodes in accordance with the fifth aspect of the invention.

Aspects of the invention may comprise a computer program product comprising computer executable instructions operable to cause a computer to become configured to perform a method in accordance with any of the above identified aspects of the invention. The computer program product can be in the form of an optical disc or other computer readable storage medium, a mass storage device such as a flash memory, or a read only memory device such as ROM. The method may be embodied in an application specific device such as an ASIC, or in a suitably configured device such as a DSP or an FPGA. A computer program product could, alternatively, be in the form of a signal, such as a wireless signal or a physical network signal.

Further preferred features of these aspects of the invention will now be set forth by the following description of specific embodiments of the invention, provided by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates a schematic diagram of a radio communications network in accordance with a specific embodiment of the invention; Figure 2 depicts an exemplary representation of a spectrum available to an opportunistic user; Figure 3 illustrates a schematic diagram of a radio communications station of the network illustrated in figure 1; Figure 4 is a schematic diagram of message sequences including wireless communication establishment, wireless communication and wireless communication teardown in accordance with a specific embodiment of the invention; Figure 5 is a schematic diagram of a request packet according to the present invention; Figure 6 is a schematic diagram of a reply packet according to the present invention; Figure 7 is a schematic diagram of a channel release packet according to the present invention; Figure 8 illustrates an exemplary data structure including spectrum slot availability information and traffic load in accordance with the present invention; Figure 9 is a flow diagram for determining available channel capacity in accordance with the present invention; Figure 10 illustrates a graph of measured traffic load at a particular node implementing the average occupied bandwidth calculation according to figure 9.

The invention will be described with reference to a specific embodiment comprising a wireless radio communications network 100 as illustrated in figure 1. The network comprises a dynamic channel access or cognitive radio network in which spectrum access is managed in a distributed fashion. Network 100 includes a number of wireless communications stations 102, 104, 106, 108, each of which is enabled to communicate on a channel defined in an available spectrum. Entities in the network, such as the wireless communication stations, will also be referred to simply as stations' or nodes', which is a generic term referring to a point of communication in a network.

Here, the stations comprise a group of secondary users which are opportunistically accessing spectrum licensed to primary users. Indeed, in the following description, the terms "stations", "users", "nodes", or "devices" refer to secondary devices unless otherwise indicated.

However, the skilled reader will appreciate that the use of this concept of division between primary user and secondary user devices is for the purpose of describing the present invention clearly, and an actual implementation of the present invention could be provided without this distinction being made, either explicitly or implicitly. Indeed, this distinction between primary users and secondary users is not an essential element of the claimed invention.

An exemplary representation of a spectrum is shown in figure 2. The available spectrum comprises a wideband spectrum divided into a plurality (eight are illustrated) of distinct frequency bands. The frequency bands can be of fixed bandwidth (such as 20 MHz in the case of 802.lla) or varying bandwidth. For purposes of clarity only, each frequency band is shown as comprising a single channel designated Cn, where n is an integer, though it will be apparent that a frequency band could equally be split into a plurality of sub-bands in any one of the manners known in the art, or any manner yet to be devise, and remain within the scope of the present invention.

Furthermore, although channels are in this particular embodiment of the invention defined in the spectrum by way of frequency, it will be appreciated that channels may be defmed in the medium by any suitable means, such as time, code, space, or any combination thereof, given the nature of the medium and the technology implementation.

It will also be appreciated that, whereas conventional wireless networks are designed for static spectrum access that use a certain fixed spectrum block, in DSA networks there is no statically allocated fixed spectrum for use, such that any available band of spectrum could be utilised by devices having the required capability.

As described previously, because primary users do not always occupy their licensed spectrum (or channel), secondary users can make use of the vacant "white spaces" for transmission. Secondary users can share a channel (spectrum) among themselves, but not with a primary user. So, in figure 2, secondary users can make use of channel C7 during white space 202, but must exit this channel upon the return of a primary user at time t1 (usage of the spectrum by primary users being indicated by filled areas).

Likewise, secondary users can borrow channel C2 during white space 204 (from time to to t3), before having to return this channel to the primary user.

Embodiments of the invention assume that there is a narrowband coordination channel available to secondary users for exchanging control messages. This coordination channel could be in either a licensed or an unlicensed band.

It will be apparent that there may be many different classes of primary users operating in spectrum 200, each of which may have different sensitivity levels to interference and also rate at which sensing by a secondary user is required to avoid this interference.

Detailed algorithms for primary user detection are still an open research problem, which is beyond the scope of this application. Therefore, it is assumed that secondary users can somehow detect primary users. Similarly, it is assumed that secondary users have some mechanism that can distinguish a secondary user from a primary one, though again this is beyond the scope of this disclosure. However, is will be appreciated that the present disclosure is not limited to any particular method of sensing or detecting power level or interference temperature and this detailed description of this function of the apparatus is not described in detail herein.

Figure 3 shows a wireless communication device 300 according to a specific embodiment of the present invention. Here "device" is used in a broad sense to indicate a data processing system with some communication capability and may include (but is not limited to) Pocket PCs, mobile phones and other mobile communications devices, PDAs (J)ersonal digital assistants) and palm-, lap-and desktop computers.

In the embodiment of figure 3, the wireless communications device 300 is implemented by means of a general-purpose computer with communications facilities. In this case, communications facilities are provided by means of hardware, which is in turn configured by means of software. More particularly, the station 300 comprises a processor 302, in communication with the working memory 304 and a bus 306. A mass storage device (which, in this case, is a magnetic storage device, though other such storage devices would suffice) 308, is provided for long term storage of data andlor programs not in immediate use. A medium access controller 310 is connected to a plurality (two in this case) of transceivers 312, 314. One of the transceivers is always tuned to the common control (coordination) channel, while the other transceiver is a data transceiver which is frequency agile and can dynamically change channels (frequencies in this case). The medium access controller 310 will manage the station's access to the communications medium, i.e. the available radio spectrum, including functions such as scanning, adaptive reservation and other functions such as data assembly and transmission.

Channel access and selection will now be discussed in the context of channels with fixed bandwidth and channels with varying bandwidth.

As indicated previously, channel sensing to find a free channel by secondary users is physical layer specific and beyond the scope of this disclosure. For simplicity, it is assumed that secondary users have perfect knowledge of the availability of white spaces (physical layer specific), though it will be understood that for practical application a less than perfect knowledge will suffice.

Looking first at channels with fixed bandwidth, such as the operating channels in 802.1 la (which are specified as 20 MFIz in width), each node that is attempting to opportunistically access an unused channel vacated by an idle primary user maintains a data structure, herein referred to as an Available Channel List (ACL).

The ACL is based on the node's channel sensing and detection, which may be carried out periodically or in an on-demand fashion, though the latter is at the cost of longer delays for connection establishment. Further, it is possible that a group of secondary users cooperatively share spectrum hole information via beacons or Hello messages. By means of such cooperative channel sensing, nodes update their ALCs and can maintain a more accurate picture of spectrum usage.

The ACL typically includes information pertaining to the availability of each channel, which could simply be represented as 0' if a channel is free and 1' if a channel is occupied, and, if a channel is a white space, the current traffic load from secondary users on that channel. The method of obtaining traffic load information will be discussed in more detail in due course.

Table 1 shows an exemplary ACL table stored at a station at time t2 of figure 2, with traffic load as an instantaneous bit rate, though any suitable traffic load indicator and method of obtaining that indicator could be used.

Table 1: Exemplary ACL table Channel 1 2 3 4 5 6 7 8 Identifier Availability (0=free; 1 0 1 1 0 0 1 1.

1 =occupied) _______ _______ ________ Traffic load -100 --300 200 --Establishing a connection between two nodes follows a handshake protocol 406, which is depicted in figure 4 together with the data transfer 408 and channel tear-down 410 processes. Reference is also made to figures 5, 6 and 7, which depict exemplary packet structures.

When a source node 402 (herein also referred to as a sender) wants to communicate with a destination node 404 (herein also referred to as a receiver), it sends a channel request packet (CREQ) (500 shown in figure 5) to the destination node. This CREQ is similar to a request-to-send (RTS) packet in 802.11, with similar fields such as sender address 502 and receiver address 504. In addition, the exemplary CREQ includes extra fields 506, 508 for the current ACL maintained by the sender and the bandwidth requirement (Br) of the forthcoming connection.

In this particular example, it is assumed that the connection in question is of the type of real-time applications with certain bandwidth requirement.

Upon receiving this CREQ, the receiver checks its own ACL and compares it with the sender's ACL. If there are common white space channels, the receiver then selects a channel which is least loaded according to the traffic load information in its own ACL.

This process is described in more detail later. Subsequently, it responds to the sender with a channel reply (CREP) packet (600 shown in figure 6), which is similar to the clear-to-send packet of 802.11, though also including a selected channel field 606 and (optionally) a field 608 for confirming the required bandwidth.

Neighbouring nodes of the receiver overhearing CREP 600 defer their operation and update their ACL tables accordingly. In this way, the surrounding nodes can have up-to-date information regarding the channel load. Upon receiving the CREP, the sender 402 may then reply with a channel confirmation (CCON) message (not depicted, but may be similar in structure to CREP 600) including confirmation of the selected channel and required bandwidth. As before, neighbouring nodes near the sender overhear this CCON and update their ACL tables accordingly.

After this (two-or three-way) handshake 406 is completed, the sender's data transceiver can tune to the selected channel and start transmitting. By this time the receiver's transceiver has also tuned to the selected channel and is ready for receiving the transmission. The data transfer process is depicted as 408 in figure 4, and may take place in accordance with known methods.

It will be apparent from the above description that the proposed approach can act as a dynamic spectrum adaption layer in a protocol stack that shields the higher layer applications from the dynamics and complexity of the underlying physical layer (i.e. dynamically changing channels and frequencies, etc). Once a channel has been selected, various legacy MAC functions (standard DCF, frame aggregation, block ACKs, etc,.) can be used. Further, the control logic of the protocol can be implemented in device drivers and the radio hardware does not necessarily require changing.

When the sender finishes transmitting, it sends a channel release (CREL) packet 700 on the control channel, including the channel it just used and the traffic load. Nearby nodes overhearing this will again update their ACL accordingly. Similarly, upon receiving CREL, the receiver sends a channel release reply (CRIRP) (not depicted) to inform its neighbours of the traffic load decrease on that particular channel. Alternatively, the sender and receiver can include a field of time 510, 610 in the CREQ 500 and CREP 600 (and also the CCON if desired), which indicates the time t that the connection will last. In this case, nodes can additionally maintain a table similar to the NAV in 802.11.

When the NAV timer expires, the nodes update their ACL and remove the traffic load on that particular channel.

Turning now to channels with varying bandwidth, it will be appreciated that spectrum opportunities are more dynamic, in that white spaces are of different sizes. For simplicity, it is assumed that the whole accessible target spectrum is divided into fixed slots and a spectrum opportunity (or an empty channel free of primary users) consists of a number of contiguous slots. For example, if each slot is 1 Ivll-Iz, then a 20 MJ-Iz white space occupies 20 slots. The capacity of a channel is proportional to its bandwidth and can be determined, for example, by the well-known Shannon formula.

For channels with varying bandwidth, the ACL table includes availability information (e.g. is or Os) for each spectrum slot. Also, as a traffic flow from a pair of secondary users may occupy a contiguous segment of the free spectrum consisting of several slots, the ACL records the traffic load in all these slots. An exemplary ACL table of this kind is shown in figure 8. Nodes still follow the handshake process described above.

However, the receiver-based channel selection is different. In particular, upon receiving the CREQ, the receiver looks up its ACL and tries to find several contiguous segments of spectrum opportunity. For example, assuming that the capacity of a spectrum slot is C and a white space channel consists of k slots, if the existing (aggregate) traffic load in these slots is L, then the available bandwidth of this white space can be expressed as: BayCkL.

Thus, the receiver could select the channel with the largest Bay. For example, with reference to figure 8, the white space channel defined by one of the sets of slots { 1,2), (7),.. .,{93,94,95,96} having the largest available bandwidth.

A brief discussion now follows on how a channel is relinquished by a secondary user in accordance with a specific embodiment.

For purposes of this disclosure, it is assumed that a secondary user can detect when a primary user wants to reclaim the channel after leaving it vacant for a while. This could be based on signal detection on-the-fly or some prior knowledge known beforehand. For example, spectrum bands licensed for TV/radio broadcasts may be empty for most of the day, but will be used by the licence holder after a certain time. In such situations, secondary nodes involved in an ongoing transmission should exit the channel in question gracefully and start to find another spectrum opportunity possibly. An EXIT message is initiated on the control channel by the node first detecting that the primary is going to be back (it could be the sender, receiver or any other node in the network).

After receiving this EXIT, the communicating pair starts the CREL/CRRP exchange 410 as discussed previously, to tear down the connection. Since there could be multiple connections exiting the channel, each communicating pair should wait for a random period of time before re-starting the channel negotiation process to avoid collisions.

Load balancing is an important principle of network design and traffic engineering. A simple way of load balancing is for a node to select a channel (among all the available channels) that has the least number of secondary flows (or communicating pairs) using it. However, this is a simplistic approach, as the number of flows may not accurately reflect the usage of a particular channel as the traffic intensity of different flows varies.

The present invention improves upon this situation by considering the actual traffic demand of each flow, for example determined at each node overhearing bandwidth requirement information included in the control messaging (i.e. traffic demands declared by the nodes).

Alternatively, or in addition, more sophisticated and accurate ways to estimate channel usage may be employed, including active measurements. Here, for convenience, the described measurement technique will be considered in the context of load balancing in the handshake mechanism discussed earlier. However, it will be appreciated that the technique can be implemented separately thereof.

Initially, it is assumed that there are N idle channels (the term "idle" means that primary users are not using it, though there may be already be some secondary users on the channel). Secondary users may share the channels using 802.11 CSMA/CA type MAC protocols for example. Further, each channel is assumed to have the same nominal capacity, e.g. 11 Mbps for 802.1 ib, though clearly this need not be the case.

In an exemplary embodiment of the invention, a metric called available bandwidth (of a particular channel), Bavaji, is defined. It can be estimated using on-line measurement. In fact, Bavaji can be determined in accordance with Bavaji = Bm -B0 (1) where Bm is the maximum bandwidth, and B0 is the average occupied bandwidth of existing traffic. For simplicity, Bm comprises a known maximum capacity of the channel.

After the destination receives a CREQ from a sender, it first determines the available channels for use (step S902). As indicated above, the available channels for use in this particular example comprise channels available for use by both the source and the destination. The destination then sequentially tunes to each of the channels (step S904) and stays there for some time (T) to conduct a bandwidth measurement (step S906), performed on a periodic basis if so desired (step S910 YES').

More specifically, the node measures the number (N) of packet transmissions in its carrier sensing range over a specific time interval (t) defining a measurement window j, and calculates the average occupied bandwidth (B) of existing traffic using the following weighted moving average (step S908): B0(j) = B0(j-1) a + (1-a) . Rate(j) (2) where a is a weight (or smoothing factor), B0(j) is the average occupied bandwidth at the measurement window j and Rate(j) is the measured instantaneous bit rate at j: Rate(j) = N*S/t, with S being the packet size in bits.

This measurement does not incur much extra overhead, since the contention access protocol is a CSMA-based MAC protocol which involves physical and virtual carrier sensing mechanisms. Compared to other techniques for channel utilization measurement, this method is simple and efficient. For example, buffer (queue) length or probing packets can be used. It has been shown in previous studies that queue length or buffer occupancy alone is often not an accurate measure of medium utilization.

Congestion or hot spot conditions can be created without any buffer occupancy. On the other hand, round trip time (RTT)-based probing techniques are known in the art for wired networks. However, the disadvantage of such a method is that probing packets need to be sent into the network, hence consuming extra wireless bandwidth.

Based on its calculations, a node can then select the channel that has the maximum Bavaji according to equation (1). This implies that the channel is least utilized and hence likely to provide best quality of service. Alternatively, the node can simply select the channel that can satisfy the bandwidth requirement of the incoming flow, i.e. Bavaji �= Breq.

The online measurement algorithm has been tested with simulations. In particular, figure 10 shows the measured traffic load at a particular node implementing the average occupied bandwidth calculation according to equations (1) and (2).

Further variations, modifications and additional features will be apparent to the skilled person considering the above disclosure and no statement above is intended to limit the scope of protection sought for the invention, which is to be determined by reference to the appended claims, interpreted in the light of, but not specifically limited to, the above

description of specific embodiments.

Claims (20)

1. CLAIMS: I. A method of establishing wireless communication in a wireless communications medium defining a plurality of channels, the method comprising: receiving a request from a first node at a second node, said request relating to an intended data communication therebetween and including information identifying one or more channels available for use by the first node; determining at the second node whether any of said one or more channels are available for use by the second node and, if said determining is positive, selecting for said data communication at least one channel mutually available for use by the first and second nodes based on an available capacity of said at least one channel; and sending a reply from the second node to the first node, said reply including information identifying said at least one channel.
2. 2. A method in accordance with claim 1 further comprising receiving a confirmation from the first node at the second node, said confirmation indicating acceptance of said at least one channel.
3. 3. A method in accordance with claim 1 or 2 wherein at least the request includes information relating a required channel capacity for the data communication.
4. 4. A method in accordance with any one of claims 1 or 3 wherein the channel having the highest available capacity is selected.
5. 5. A method according to any one of claims 1 to 4 wherein each of the channels is defined as one or more channel slots, and wherein said selecting is based on an average available capacity of said at least one channel.
6. 6. A method of relinquishing a channel over which first and second nodes have established wireless data communication in a wireless communications medium defining a plurality of channels, the method comprising: sending a notification from either of the first and second nodes to the other node, said notification indicating that the channel should be relinquished; and receiving an acknowledgement responsive to said notification, said acknowledgement including an identifier of the channel and information confirming that the channel will be relinquished.
7. 7. A method in accordance with claim 6 wherein the acknowledgment further includes information relating to a traffic load on the channel.
8. 8. A method in accordance with claim 6 or 7 wherein the sending is performed in response to one of a determination that the data communication is completed and a determination that the channel is no longer available for use.
9. 9. A method of measuring load in a wireless communications medium defining a plurality of channels by a node opportunistically accessing the medium, the method comprising: identifying one or more channels available for use by the node; sequentially monitoring each of the available channels, the monitoring of a channel comprising measuring transmission activity on the channel over at least a first time period; and calculating an available capacity for each of the monitored channels based on said measuring.
10. 10. A method in accordance with claim 9 wherein the method is performed during establishment of wireless communication with another node, and wherein said monitoring comprises monitoring channels mutually available for use by both nodes.
11. 11. A method in accordance with claim 9 or 10 wherein the monitoring of a channel is performed over a plurality of time periods, and wherein said calculating is performed in accordance with a weighted moving average scheme.
12. 12. A method in accordance with any one of claims 9 to 11 wherein the available capacity of each channel is Bavaji Bm - wherein Bavaji is the available capacity of the channel, Bm is the maximum capacity of the channel, and B0 is the occupied capacity of the channel based on said transmission activity.
13. 13. A method in accordance with claim 16, when dependent on claim 12, wherein the occupied capacity of the channel comprises an average occupied capacity of the channel calculated according to B0(j) = B0(j-1) a + (1-a) Rate(j), wherein B0(j) is the average occupied capacity of the channel for a measurement windowj, B0(j-1) is the average occupied capacity of the channel for a preceding measurement window j-i, a is a smoothing factor, and Rate(j) is the measured instantaneous bit rate for the measurement window j.
14. 14. A method of facilitating access coordination among a plurality of nodes to a wireless communications medium defining a plurality of channels, the method being performed at each node, the method comprising: storing information about the channels, the information about a channel including an indicator of whether the channel is available for use by the node, and when the channel is available for use, a value corresponding to a traffic load on the channel; monitoring for control signals relating to data transmissions involving the nodes, at least some of said signals including traffic flow information; and updating the stored channel information in accordance with monitored traffic flow information.
15. 15. A wireless communication apparatus operable to establish wireless communication with another apparatus in a wireless communications medium defining a plurality of channels, the apparatus comprising: request receiving means operable to receive a request from the other apparatus, said request relating to an intended data communication therebetween and including information identifying one or more channels available for use by the other apparatus; channel availability determining means operable to determine whether any of said one or more channels are available for use by the wireless communication apparatus; channel selection means operable to select for said data communication, responsive to a positive determination by said determining means, at least one channel mutually available for use based on an available capacity of said at least one channel; and wherein said apparatus is operable to send a reply towards said other apparatus, said reply including information identifying said at least one channel.
16. 16. A wireless communication network comprising a plurality of nodes in accordance with claim 15.
17. 17. A storage medium storing computer executable instructions which, when executed on general purpose computer controlled communications apparatus, cause the apparatus to become configured to perform the method of any one of claims 1 to 5, 6 to 8,9to 13, or 14.
18. 18. A signal carrying computer receivable information, the information defining computer executable instructions which, when executed on general purpose computer controlled communications apparatus, cause the apparatus to become configured to perform the method of any one of claims ito 5, 6 to 8, 9 to 13, or 14.
19. 19. A storage medium storing computer executable instructions which, when executed on general purpose computer controlled communications apparatus, cause the apparatus to become configured to a wireless communication device in accordance with claim 15.
20. 20. A signal carrying computer receivable information, the information defining computer executable instructions which, when executed on general purpose computer controlled communications apparatus, cause the apparatus to become configured to a wireless communication device in accordance with claim 15.