GB2462614A - Determination of a contention period for access to a channel in a wireless communication device - Google Patents

Abstract

A wireless device applies a distributed coordination function for medium access control which involves deferring transmissions by a contention period to avoid collisions. Additionally, the contention period is adjusted to take account of current and predicted channel conditions. In one embodiment the device acquires data on the favourability of conditions on the channel between transmitter and receiver for transmissions and determines a contention period dependent on that data. The data is acquired from a number of transmissions received by the device over a time interval. A comparison of data received in the number of transmissions is used to weight the contention period determined and used by the device. The favourability data may relate to signal strength measurements of received acknowledgement transmissions.

Description

WIRELESS COMMUCATION DEVICE AND METHOD

The present invention concerns a device for, and a method, of efficiently utilising time varying channels for wireless transmissions on a channel between transmitter and receiver, for instance in a wireless LAN with distributed media access control which uses contention periods to avoid collisions.

Various known wireless systems involve multiple devices sharing wireless medium.

The basic requirement of these systems is the avoidance of collisions of transmissions from different devices sharing the medium. Some of these systems rely upon distributed Media Access Control (MAC) protocols for collision avoidance.

Commonly, distributed MAC protocols take the approach of avoiding collisions by sensing transmissions and deferring transmissions by a randomly selected, or probabilistic, channel contention period or window. One protocol taking this approach is the Distributed Coordination Function (DCF) MAC protocol within the 802.11 Standard.

This approach enables fair sharing of resources between wireless devices by using a randomly selected collision avoidance, or back-off, period based on the perceived level of channel contention. This period may also be known as a contention period or a contention window.

In the DCF MAC and similar protocols the contention period increases by a binary exponent each time there is an absence of an acknowledgment frame ACK received by a device. As the contention period increases binary exponentially, the protocol is sensitive to frame errors. Therefore transmission performance over the medium is highly dependent on conditions relating to transmission channels.

This binary exponential algorithm is effective in providing stability to the wireless system when there is excessive contention between different devices. It also provides a degree of adaptability to dynamic variations in channel conditions. This is because poor channel conditions, or a poor channel state, results in a tendency towards long contention periods and there is a probability that the channel will have improved at a later point in time. According to some protocols, the rate of transmission can be reduced in response to successive frame errors to further improve the responsiveness of the system to poor channel conditions.

Deferring transmissions by a contention period that increases binary exponentially increasing, each time a transmission is not successful, provides collision avoidance with some degree of performance. However, more optimal performance may be possible.

One approach to improving performance is to focus on selecting a more optimal contention period for sharing resources between different traffic classes. The 802.1 1E enhancement and another adaptive MAC technique given in "Distributed MAC Adaptation for WLAN QoS Differentiation", Zhao et al, IEEE GLOBECOM 2003.

These provide different traffic classes with different contention periods. This approach provides priority differentiation of services rather than overall optimisation of services.

Also, these approaches do not take advantage of awareness of channel state within an adaptive MAC protocol.

Another approach to optimising system performance in general relates to exploiting awareness of channel state to improve channel utilisation and energy efficiency. This approach is given in "Transport Level Performance -Energy Trade-off in Wireless Networks and Consequences on the System-Level Architecture and Design Paradigm".

B. Bougard et al, IEEE Workshop on Signal Processing Systems (SIPS'04), 13-15 October 2004 Austen, Texas, USA, pp.77-82. This document describes lazy scheduling' techniques which seek to exploit the dynamic nature of radio channels. This work has shown that the greatest energy savings can be made with a combination of lazy scheduling' and device shutdown such that the lowest transmission rate is used within the necessary delay constraints.

By assigning devices to use the channel when quality or state of the channel is most advantageous, performance of the system can be improved.

A potential limitation of this approach arises because determination of the channel state can be an involved process and may, in itself, consume power and add system overhead.

The applicant has observed a need for reduced complexity solution to applying channel state awareness to optimise the selection of contention periods in systems with distributed MAC protocols.

The document WO 2007/09 1930 Al "Link Adaptation and Power Control with Consumed Energy Minimisation", by Larson and Zhang, proposes adaptive power level optimisation to ensure the best use of energy by wireless devices. This document formulates an optimisation objective based on minimisation of energy using transmission rate and power control, otherwise known as link adaptation.

The problem of selecting an optimal link rate under time varying channel conditions is a well known problem with an information theoretic derivation which can be solved for time varying channels with lazy scheduling using water filling. Water filling in time involves scheduling transmissions on a dynamically varying channel based on real-time channel knowledge. Normally channel knowledge is required at the transmitter to achieve an optimal solution, which means continuous feedback is required from the receiver. The optimisation involves a trade-off between latency and energy efficiency and so a threshold is set at which transmissions are scheduled so as to meet the latency objectives while attempting to minimise the energy consumed to transmit the data.

One treatment of this problem is provided in by the well-known Shannon's theory which has been referenced and applied extensively in patents and publications, such as WO 2007/091930 Al.

The energy required to transmit a given individual unit s of size S can be expressed as E(ri) at a given average link rate r1. Selection of an optimal link rate under time varying channel conditions is a well known problem and is described in this document as well as "Transport Level Performance-Energy Trade-off and Wireless Networks and Consequences on the System-Level Architecture and Design Paradigm".

A common characteristic of solving this problem is that a lower energy is required if a larger delay can be tolerated and channel variability is exploited. An expression for the theoretical minimum energy is given below: Min[E] S (aP + Po) [S B'Ilg2 (1 + PW) + T0], where P0 is the constant, quiescent, power consumption, T0 is the constant fixed time overhead for transmission or reception of data unit s, S is the number of bits in the data unit s, B is the bandwidth of the channel c, P is transmit power, a is related to power amplifier efficiency and W is path gain to noise ratio.

The theoretical energy minimisation expression of equation 1 can be used in a number of ways. One approach may be to adapt transmit power (P) as the channel state (W) changes to maintain operation at the optimal energy point. Another way may be to increase channel bandwidth to shift the entire curve in order to operate at a lower energy level. Another way may be to use a combination of power and bandwidth adaptation to give a wider range of possibilities. A further approach may be to use lazy scheduling to use the channel only when the channel state (W) corresponds to the optimal point under the assumption that the channel state (W) is highly dynamic. These approaches are all dependent on estimating the dynamic channel state (W). In general, this is time-varying with a pseudo-cyclic pattern caused by multi-path and shadow fading. The channel variation pattern can be exploited to avoid requiring detailed channel knowledge, as shown in the example channel characteristic in figure 1.

As shown in Figure 1, when the channel is in a null, or deep fade, it is possible to estimate the time at which the channel will be next at its peak based on the pseudo cyclic behaviour pattern. However, estimating the cyclic periodicity is difficult to achieve in distributed MAC schemes due to the fact that transmission are not continuous but occur at irregular intervals depending on the device activity arid level of contention on the medium. This problem is particularly prevalent in wireless (WLAN) or cognitive radio overlay systems in which devices contend for access to a medium with other devices. In these scenarios devices will not be able to determine whether a failed transmission was due to the state of the channel, when fading occurs, or whether it was due to collisions with transmissions from other devices.

The patent US6877043 describes a method of contention by devices for access to a channel between transmitter and receiver. This method involves distributing sets of collision resolution parameters among devices in a non-centralised media access control network. A set of collision resolution parameters provides a sequence of fixed numbers used to resolve a single access contention event. This approach assumes that a centralised node is elected to determine collision resolution. This approach considers only efficient resource allocation and not the problem of dynamic channel state exploitation. Such a centralised approach suffers disadvantages when transmissions are not all directed towards the central controller. The applicant has observed the need for a more generally applicable approach which is assumes no centralised controller or contention resolution.

The patent US68 59831 describes a Tiered Contention Multiple Access (TCMA) distributed media access protocol which schedules transmission of different types of traffic based on the service quality specifications. This approach does not require centralised coordination, but does not consider channel variation. Therefore, it cannot exploit the variations in channel favourability between devices.

The patent US 6954800 describes a method of enhancing network transmission between stations on a priority-enabled frame-based communications network. The network has multiple transmit priorities and transmitting frames so that a network access time to transmit a frame with a lower priority is longer than a network access time to transmit a frame of a higher priority. Again, this approach does not consider the dynamic channel variation between devices and so cannot exploit this variability in the method of providing priority based access to the channel.

Aspects of the invention provide a wireless device, and method, for use with a medium with distributed contention where contention periods for transmissions over a wireless channel are determined using a weighting mechanism dependent on observed variations in channel conditions.

When multiple devices share the medium the mechanism favours those experiencing better channel conditions on the assumption that the pseudo cyclic variation in channel conditions will on average favour all nodes equally.

Aspects of the invention provide an apparatus, and a corresponding method, which uses a distributed media access control protocol, the apparatus using collision avoidance and a weighted awareness of transmission conditions over the medium, such as channel state for example, to determine contention periods.

The apparatus may determine the contention period based on awareness acquired from a signal level observed in a received transmission frame.

The apparatus may determine the contention period based on a minimum signal strength of frames received in a time interval.

The apparatus may determine the contention period based on predictions of the favourability of the medium for transmissions. The favourability may relate to channel fading which may be indicated or predicted by variations in signal strength of received frames. The predictions may include an estimation of a period for variations based on an assumption of pseudo cyclic behaviour.

This approach could be combined with a TCMA distributed multiple access approach to provide both traffic based priority, such as using different contention windows for different traffic classes, and channel favourability variation exploitation. In such a combined solution approach, the traffic classes with differing priority and latency requirements would be associated with different weighting factors taking account of channel favourability variation pattern and the traffic priority / latency requirement.

Another aspect of the invention provides a wireless device operable to contend with other devices for access to a medium said contention comprising deferring transmissions over a wireless channel by a contention period, the device being operable to: acquire transmission favourability data relating to the favourability for transmissions over the channel; and determine a contention period dependent on the transmission favourability data.

As used herein a "contention period" relates broadly to a time period by which transmissions may be deferred to avoid collisions with transmissions from other devices sharing a medium.

The contention period may be determined dependent on a weighting provided by the favourability data. The favourability data may be derived from channel awareness. The favourability data may indicate the state of a channel. The favourability data may indicate the gain of a channel. The gain may be negative to indicate attenuation.

The "contention period" may be determined dependent on further weighting according to estimated channel conditions to additionally account for pseudo cyclic channel variations observed in the transmission favourability data. This weighting may be dependent on an estimate of the period of pseudo cyclic channel variations. This may permit both collision avoidance and channel fading avoidance.

The device may be operable to receive transmissions and acquire transmission favourability data from said received transmissions.

The device may be operable to acquire transmission favourability data relating to the signal strength of received transmissions.

The device may be operable to acquire transmission favourability data relating to signal-to-noise ratio of received transmissions.

The device may be operable to receive acknowledgement transmissions. The acknowledgement transmissions may be acknowledgement frames.

The device may be operable to acquire a received signal strength indication value, or RSSI, from the acknowledgement frame.

The device may be operable to receive a number of transmissions. The number of transmissions may be defined in terms of the number received in a given time interval preceding estimation of a channel favourability variation period and using this favourability variation period as one of the factors in determining an appropriate contention window weighting factor.

The weighting term may be based on an estimation of the current operating point within the pseudo cyclic channel state pattern. Therefore, the formulation of the time to wait is based on the favourability data and the estimated period of the pseudo cyclic pattern in that data relative to the size of the contention period. The favourability data may be signal strength or Received Signal Strength Indication (RSSI), The weighting term may be multiplicative (A) or additive (B). In the case of multiplicative weighting the period of the channel variation may be normalised to the contention window and so A = cyclic period estimate * latency tolerance factor / CP, where CP is the contention period. In the case of additive weighting the contention period cannot be reduced and so the additive weighting extends the contention window to the next channel state peak, as indicated by the favourability data. Therefore, B = cyclic period estimate * latency tolerance factor -CP. The latency tolerance factor may be based on the traffic latency tolerance requirements and could be different for different traffic classes. The tolerance factor may be normalised to the highest priority traffic class, such as voice class.

The device may be operable to determine the contention period dependent on transmission favourability variation data relating to variations in transmission favourability data acquired from said number of transmissions. The variation of favourability data such as the RSSI of ACK' s can be used to predict fading.

The device may be operable to determine the contention period dependent on the variation data.

The device may be operable to acquire transmission favourability variation data by computing the difference between a minimum transmission favourability datum and the last transmission favourability datum of transmissions received in said time interval.

The device may be operable to determine the contention period by adjusting a contention period determined according to a predetermined method. The method may be provided by a given communication protocol. The device may therefore use the protocol but benefit from more optimal contention periods by an adjustment. The protocol may provide binary exponentially increasing contention periods for each time a transmission is not successful.

The device may be operable to adjust the predetermined contention period by addition of an adjustment value.

The device may be operable to adjust the contention period by multiplication by an adjustment value.

The adjustment value may comprise a quotient said quotient being dependent on the variation data.

The adjustment value may be equal to C/(FVD + 1), wherein C is derived from the charmel favourability variation period estimate and FVD is based on the most recent received favourability data.

Another aspect of the invention provides a wireless network comprising a wireless device in accordance with any preceding claim.

Another aspect of the invention provides a method of controlling a wireless device to contend for access to a medium, the method comprising deferring transmissions over a wireless channel by a contention period, wherein the method includes: acquiring transmission favourability data relating to the favourability for transmissions over the channel; and determining a contention period dependent on the transmission favourability data.

The method may involve receiving a number of transmissions and acquiring transmission favourability data from the received transmissions.

The method may include acquiring said data from the signal strength of the received transmissions.

The method may include determining the contention period by first predetermining a contention period according to a given protocol and then adjusting the predetermined contention period dependent on the transmission favourability data.

The adjustment maybe dependent on variation of the signal strength of a number of received transmissions.

The number of transmissions may be received in a time window which moves with the timing of deferment of transmissions so that the adjustment to the contention period is dependent on variations in the signal strength observed substantially immediately preceding the determination of channel favourability variation period and using this favourability variation period as one of the factors in determining the most appropriate contention window weighting factor by the contention period.

Another aspect of the invention provides a computer program product operable to configure a computer to perform the method above.

Figure 1 depicts the signal strength of transmissions received over a period of time; Figure 2 is a flow diagram of a method of acquiring data according to a specific embodiment of the invention; Figure 3 is a flow diagram of a method of contending for access to a channel between transmitter and receiver according to a specific embodiment of the invention; Figure 4 is a flow diagram of a method of contending for access to a channel between transmitter and receiver according to an alternative embodiment of the invention to that described in reference to Figure 3; Figure 5 depicts a comparison between the performance of a wireless device, in terms of signal energy required, where the contention period is determined in accordance with a specific embodiment of the present invention and where the contention period is determined according to a given protocol.

A simple flow diagram illustrating operation of a wireless device according to a preferred embodiment of the present invention is shown in Figure 2. Prior to contending for access to a channel between transmitter and receiver, the wireless device acquires data to allow it to predict fading for a transmission channel over the channel between transmitter and receiver. The wireless device of the present embodiment forms part of a wireless local area network (WLAN) using 802.11 distributed coordination

function (DCF) protocol specification.

Prior to contention for access to the medium the wireless device begins reception of transmissions from a reciprocal device. According to this embodiment, the transmissions are acknowledgement frames (ACKs), data frames or management frames (such as beacons). The start of the reception is depicted by step Si -1.

At step S1-2 the received signal strength indication (RSSI) of each received frame is monitored, or acquired and stored.

The skilled reader will understand that if the wireless device acquires the RSSI data sent from a distant transmitter and assumes that the transmission channel over the channel between transmitter and receiver is symmetric, or reciprocal, then the RSSI will provide information on how favourable the state of the transmission channel or transmissions is for successful transmissions from the wireless device to the distant device.

At step S1-3 the wireless device computes the minimum RSSI acquired over a number of received frames N or all the frames received within a time interval T. At step Si -4 a wireless device computes and stores data relating to the variation in the RSSI. In the case of the present embodiment, this variation is characterised by the difference between the minimum RSSI computed in step Sl-3 and the RSSI of the last received frame. The RSSI data can be used to estimate the pseudo cyclic penodicity pattern of the channel state and hence select multiplicative (A) of additive (B) weighting values based on this estimate. A simple method of computing the period is to measure the time between nulls, or minima, in the RSSI. However, as wireless transmissions are not necessarily continuous in time and may not occur at regular intervals, minima may be missed and so a more accurate measure is to take the average time between nulls over a longer period of time.

Clearly, the longer the period over which the channel is monitored, the more accurate the estimation of channel variation period becomes. However, as the channel variation pattern will also change over time, for instance, as a result of the wireless device moving, then there is a trade-off between period estimation accuracy and tracking of the period changing over time. This specific embodiment of the present invention overcomes this by not only adjusting estimation of the pseudo-cyclic channel variation period, but also adapting to the instantaneous changes in channel state, as determined by the latest RSSI measurement obtained. In this manner longer term periodic variations, such as caused by shadow fading, are captured as well as shorter term more rapid fluctuations, such as caused by multi-path fading.

Alternative specific embodiments may take the approach of measuring time between nulls, or minima, to estimate periodicity would be the apply techniques such as Fourier analysis to the measured channel state data. This enables computation of all of the periodic components of the measurement data and so the strongest, largest magnitude, periodic components can be selected to estimate the period and hence appropriate A and B values. It is also possible to measure time between peaks, rather than nulls, and adjust the weighting formula such that the öRSSI value is based on the difference between the peak value and the latest value and thus it will appear on the numerator of the weighting factor. This approach is a completely dual solution to the proposed approach. It may have benefits if the RSSI measurement device has better measurement accuracy and precision at high RSSI values than at low RSSI values and thus the peak detection may become more reliable.

Figure 3 shown a simple flow diagram illustrating transmission from a wireless device using a contention period. Figure 3 specifically relates to an embodiment of the present invention which determines a contention period by adjusting a contention period determined using a given contention protocol. Additionally, this particular embodiment adjusts the contention period determined by the protocol by multiplying the contention period by a term.

The process S2-1 begins with the wireless device attempting a transmission.

As in all collision avoidance schemes, each device selects a back-off period based on a random number selected from the contention window and waits this time before attempting transmission. If another transmission is detected during the back-off period the transmission is deferred and the back-off counter is paused until the channel is detected as being free again. In this manner the collision probability is directly related to the length of the contention period, or window, and the number of contending devices. By adjusting or weighting the contention window the probability of collision is changed. In the case of multiplicative weighting, if more than one device observes a good channel condition at the same time, then the probability of a collision occurring is increased as both will apply a weighting factor that reduces the contention window, but conversely if both observe poor channel conditions the probability of collisions occurring is reduced. Therefore, the weighting factor is designed to, on average, maintain a similar and acceptable level of collisions on the channel when averaged over time, especially if the channel conditions observed by different devices are not correlated.

In the case of additive weighting, a more conservative adjustment of the contention window is adopted. In this case the contention window is only expanded and never reduced. This means that the collision is probability always lower than if the weighting term was not applied. This is important if the possibility of a higher collision probability is unacceptable, and this can also cope with the case of highly correlated channel conditions, by ensuring the collision probability is reduced compared with the multiplicative approach.

Detecting whether a channel state is correlated between devices could be performed by observing the pattern of the collisions. For instance, if the probability of collisions is higher when the channel state is deemed to be good, this may indicate that other devices also observe good channel state at the same time. In this case use of the additive weighting factor may be more suitable.

At step S2-2 the wireless device determines a contention period by which to defer transmission, or to back-off from transmitting. This embodiment calculates a contention period by multiplying a contention determined by a multiplication term. The term is dependent on the RSSI values acquired previously, as illustrated in Figure 2. In this particular embodiment, the multiplication term is dependent on the variations seen in the data defining the favourability of the channel for a transmission. Specifically, in the case of this embodiment, this is the difference between the minimum RSSI seen over the N frames or time T proceeding the attempted data transmission and the final RSSI value seen the same time interval. More specifically, this particular embodiment calculates a back-off period equal to CP = CWprotocoi x AI@ RSSI + 1) where 6 RSSI is the variation between the minimum and final values of RSSI and A channel cyclic period estimate * latency tolerance factor I CW. Also, CW0001 is a contention window, or period, provided by a protocol or standard to which the device conforms.

Channel cyclic period estimate is the estimate of the pseudo cyclic variation in channel favourability (time between nulls or peaks), and the latency tolerance factor is the latency tolerance of the traffic class normalised to the channel cyclic period, for instance voice traffic. Therefore, a voice stream traffic class could have a latency tolerance factor of 0.1 and e-mail data traffic will have a higher latency tolerance factor, such as 1.

The 6 RSSI is obtained from a data store where it was entered during the process illustrated in Figure 2. Alternative embodiments envisaged may determine the contention period based on signal-to-noise ratio (SNR) or bit error rate (BER) in place of RSSI.

At step S2-3, the wireless device transmits a frame that is deferred by using a given contention protocol for which the contention period has been adjusted.

Step S2-4 is a decision point. The process returns to step S2-1 if more frames are to be transmitted. The process ends at step S2-5 if no more frames are to be transmitted.

Figure 4 shows a flow diagram illustrating a process of transmitting frames according to an alternative embodiment of the invention to that which follows the process illustrated in Figure 3.

In this embodiment also the contention period is determined by adjusting a contention period determined by a given protocol. However, in the case of this particular embodiment, the adjustment is performed by addition of a term to the contention period.

Transmission begins at step S3-1.

At step S3-2 a contention period is calculated using a RSSI value representing the variation in the acquired data relating to the favourability of the transmission channel.

In this case again, the RSSI value is used to indicate favourability of the channel.

The contention period is determined a CP = CWprotocoi + B/( RSSI + 1) where B = channel cyclic period estimate * latency tolerance factor -CW and 8 RSSI is the variation in RSSI determined in the process illustrated by Figure 2.

At step S3-3 a frame is transmitted according to a given contention protocol but using the adjusted period in step S3-2.

Step S3-4 is a decision point where transmission is repeated, by return to step S3-1, or the process ends at step S3-5.

Figure 5 depicts the required energy for successful transmission where a device in accordance with an embodiment of the present invention adjusts a contention period determined by a binary exponential. The performance is contrasted by the performance of a device where the contention period is not adjusted.

The back-off period is recalculated for each frame, but the contention window is only increased if the previous frame was unacknowledged. The values of A and B can be recalculated either periodically or when deemed necessary if, for instance, no transmission occurs for an extended period.

Other embodiments ideally also have alternatives to data acquired from received frames only. For example, alternative embodiments may estimate channel favourability on the bit error rate or frame error rate. However, it is generally necessary to determine from which device the signal is being sent in order to determine the channel between these two devices. Knowing the channel state for devices to which you are not transmitting, or intending to transmit to, is not useful. Determining from which device the signal is being sent the device can be performed explicitly by sending probe frames to the device of interest, in which case some overhead is incurred by sending and receiving probes that are not used for actual data transmission.

Claims (18)

1. CLAIMS: 1. A wireless device operable to contend with other devices for access to a medium, said contention comprising deferring transmissions over a wireless channel by a contention period, the device comprising: means for acquiring transmission favourability data relating to the favourability for transmissions over the channel; and means for determining a contention period dependent on the transmission favourability data.
2. 2. A wireless device in accordance with claim 1 wherein the means for acquiring transmission favourability data is operable to receive transmissions and acquire transmission favourability data from said received transmissions.
3. 3. A wireless device in accordance with claim 2 wherein the means for acquiring transmission favourability data is operable to acquire transmission favourability data relating to the signal strength of received transmissions.
4. 4. A wireless device in accordance with claim 2 or claim 3 and operable to receive acknowledgement transmissions.
5. 5. A wireless device in accordance with claim 4 wherein the means for acquiring transmission favourability data is operable to acquire a received signal strength indication value from said acknowledgement transmissions.
6. 6. A wireless device in accordance with any one of the preceding claims wherein the means for determining a contention period is operable to determine a contention period dependent on an estimate of a period for variations in the transmission favourability data.
7. 7. A wireless device in accordance with claim 6 wherein the means for determining a contention period is operable to determine the contention period dependent on a period estimated assuming a pseudo cyclic channel pattern for variations in the transmission favourability data.
8. 8. A wireless device in accordance with claim 6 or claim 7 wherein the means for acquiring transmission favourability data is operable to acquire transmission favourability variation data by computing the difference between a minimum transmission favourability and the last transmission favourability datum of transmissions received in said time interval.
9. 9. A wireless device in accordance with any one of the preceding claims wherein the means for determining a contention period is operable to determine the contention period by adjusting a contention period determined according to a given method.
10. 10. A wireless device in accordance with claim 9 and operable to adjust a contention period determined by the means for determining a contention period by addition of an adjustment value.
11. 11. A wireless device in accordance with claim 10 operable to adjust a contention period determined by the means for determining a contention period by multiplication by an adjustment value.
12. 12. A wireless device in accordance with claim 10 or claim 11, wherein the adjustment value comprises a quotient, said quotient being dependent on the variation data.
13. 13. A wireless device in accordance with claim 12 wherein the adjustment value is equal to C/(FVD + 1), wherein C is derived from the channel favourability variation period estimate and FVD is based on the most recent received favourability data.
14. 14. A wireless network comprising a wireless device in accordance with any preceding claim.
15. 15. A method of controlling one or more wireless devices to contend for access to a channel between transmitter and receiver, the method comprising deferring transmissions by a contention period, wherein the method includes: receiving a number of transmissions prior to deferring transmission; acquiring transmission favourability data relating to the favourability of the channel between transmitter and receiver for transmissions, said data acquired from said received transmissions; and determining a contention period dependent on the transmission favourability data.
16. 16. A computer program product operable to configure a computer to perform the method ofclaim 15.
17. 17. A computer program in accordance with claim 16 stored on a computer readable medium.
18. 18. A computer program product in accordance with claim 16 carried on a transmission medium.