Patent Application
20080273478
The invention relates to a method for wireless transmitting data using a contention-based protocol, wherein a plurality of communicating wireless terminals transmit data using a common medium for transmitting. The invention further relates to a terminal for wireless transmitting data packets using a contention-based protocol, including a transmitting and receiving unit and a controller.
The number of different wireless communication standards has increased in the last years, leading to a tremendous boost in sales of wireless devices. The amount of spectrum available for wireless transmission, however, is limited, and every device occupying part of the spectrum temporarily, potentially interferes with other devices and possibly on-going communication.
The number of voices demanding a worldwide “spectrum-etiquette” ruling the use of the scarce resource “spectrum” is increasing. Industry associations such as the IAG (Industry Advisory Group), but also standardization bodies such as ETSI in Europe and IEEE in the US are aware of the up-coming interference problem, and have initiated investigations on how to counteract with it. Specifically in the ISM (Industrial, Scientific and Medical) frequency bands, the problem will become apparent on short term.
Many wireless communication systems rely on contention-based access to the medium: as an example, the IEEE 802.11 protocol is based on the CSMA/CA principle (Carrier-Sense-Multiple-Access/Collision-Avoidance). In the protocol 802.11 every wireless terminal has to wait for the medium to become idle before attempting to access the medium. Starting with the observation of the medium being idle, each wireless terminal has to wait for an individual waiting time, in the following denoted as back-off time before it may start its first attempt to access the medium. If no communication attempt from another wireless terminal or an access point has occurred until the back-off time is elapsed, the medium may be accessed. Obviously, wireless terminals with shorter back-off times have, on average, a larger chance to access the medium than those with longer back-off times. If, however, two wireless terminals access the medium at the same time, be it because their back-off value is identical, or they have a different perception of events in the network, a collision occurs. Once this has happened, each affected terminal backs off again by using its individual back-off time, before it may start a second attempt to access the medium.
In a number of variants to the standard, dedicated use is made of these back-off times: e.g. the QoS (Quality-of-Service) extensions specified in the upcoming IEEE 802.11e standard use shortest back-off time for an enhanced access point, the so-called hybrid coordinator, in order to give this device highest priority in accessing the medium. Sophisticated algorithms are possible to adjust back-off times and back-off time windows, wherein the window specifies the range of possible back-off times for a wireless terminal, including a fixed and a random part, in order to fine-tune priorities. However, none of these methods to adjust the back-off times has been tuned to optimise the use of the spectrum. Although a need for spectrum etiquette has been identified, enforcement rules have not yet been formulated.
Therefore, it is an object of the present invention to provide a method and a system allowing to optimise the use of spectrum in contention-based protocols and to decrease the overall emitted power used for transmitting data packets and decrease the overall interference between wireless communication systems.
This object is solved by the features given in the independent claims.
The invention is based on the thought that by using a rewarding scheme for wireless terminals transmitting with low transmit power, the amount of overall emitted power, and consequently the level of interference, and amount of spent energy as well, could be reduced. Since a transmission of data using high transmit power will interfere other transmission between close wireless terminals the interference of the transmission may result in unsuccessful transmissions between those terminals, which need to be repeated and thereby again occupying the respective frequency band thus rendering an inefficient usage of it. If the wireless terminals transmit with low transmit power the risk of an unsuccessful transmission for other adjacent terminals is decreased. However, also the coverage range of the currently transmitting terminal is decreased, and depending on the distance between this sending terminal and the receiving terminal the chance of successful transmission for this terminal may be reduced. Hence, an optimisation is needed such that only the minimum amount of transmit power required for a message to reach its destination is used.
In particularly it is proposed to register an amount of used transmit power and to adjust features of a wireless terminal to reward the wireless terminal in dependence of the used transmit power.
Thus a rewarding scheme is proposed for those wireless terminals that cause least interference to their environment, by coupling features of the wireless terminals to the amount of transmit power used in transmission. These features will affect the time necessary to access the medium. In the present case the medium is the frequency band.
In a preferred embodiment of the invention the wireless terminals which have been or are using low transmit power are allowed to reduce their back-off time and/or to increase the priority of the data packet to be transmitted, whereas those having used or using high transmit power have to increase their back-off time and/or not to increase the priority of the data packets to be transmitted next.
By this, the inventive method biases the chance of wireless terminals to gain access to the medium towards those with lower transmit power, and thus reduces the overall emitted power. The inventive method using the rewarding scheme results in a new dimension for optimisation. So, depending on the rewarding algorithm, a wireless terminal experiences higher medium access chances from its “well-behaviour” in the past.
A further advantage of reducing of the overall emitted power is the improvement of the quality of service and thereby the increasing successfully transmitted data throughput.
The current inventive method, however, proposes a simple, local enforcement mechanism suitable for selfishly operating wireless terminals. The method is easy to implement in existing and future devices, since only the method for determining the back-off time and/or the priority needs to be changed. The required registration of the amount of used transmit power is available within the wireless terminal. So, this used transmit power value needs to be considered in determining the back-off time or priority of the data packet to be transmitted next.
In a preferred embodiment of the present invention it is advantageously proposed to determine the back-off time of the wireless terminal and/or the priority of the data packet in dependence of the used transmit power of a last transmitted data packet, which is sent within a predefined maximum time interval of e.g. 500 milliseconds. A further criterion could be an average transmit power used for all data packets transmitted within a predefined time interval (e.g. the last 1000 milliseconds). Further the maximum used transmit power for a data packet within a predefined time interval (e.g. the last 500 milliseconds) could be used.
In a further preferred embodiment of the present invention the amount of energy used for transmission is used as further optimising criteria. This will also include the amount of energy used for a retransmission of data packets if a first transmission failed. An example implementation could make use of the so-called energy efficiency, which can be formulated as follows:
k: packet index
G[k]: successful net throughput (e.g. number of information bits in the packet)
E: total energy required for transmission of the currently transmitted packet ‘k’.
Once having the energy efficiency calculated it could be used for determining the back-off time or priority of the data packet. In particularly the energy efficiency J of the last data packet which is sent within a predefined maximum time interval of e.g. 500 milliseconds or an average energy efficiency for all data packets within a predefined time interval (e.g. the last 1000 milliseconds) or the minimum energy efficiency for a data packet within a predefined time interval (e.g. the last 500 milliseconds) could be used for determining the back-off time of the wireless terminal and/or the priority of the data packet.
In a further preferred embodiment the relation between the amount of used transmit power and the back-off time is determined in dependence on the application. For determining the back-off time a linear or a logarithmic or any suited monotone increasing function could be used. It is further possible to use a look up table for assigning the respective back-off times for used transmit power in the past.
In an advantageous embodiment thresholds are used for determining the back-off time of a wireless terminal. If a transmit power is used by a wireless terminal lying over a predetermined threshold the back-off time will be set to a large value to “punish” the wireless terminal in getting access to the medium for the forth-coming transmissions. Contrariwise, in case a transmit power is used by a wireless terminal lying below a predetermined threshold the back-off time will be set to a low value to “reward” the wireless terminal in getting access to the medium for the coming transmissions.
Since each data packet to be transmitted has a priority value assigned a further possibility to reward a wireless terminal could be to increase virtually the priority value of a data packet in dependence of the used transmit power. So the medium will see the higher priority value, wherein the receiving terminal will see the real priority value. By this a data packet having a higher priority value should be earlier transmitted than a data packet having a lower priority value. There is also the possibility to combine the adjustment of the back-off time of a wireless terminal and the setting of the priority value in dependence of the used transmit power.
In a further preferred embodiment of the invention a wireless terminal using a multi-hop communication to reach a faraway wireless terminal is rewarded with a decreased back-off time or an increased priority of the data packets to be transmitted next. The multi-hop communication uses a terminal located between faraway terminals as intermediate terminal. The terminal will act as repeater. Since the transmit power required to transmit data to the intermediate terminal is lower than that needed for transmitting the data directly to the receiving terminal, the amount of overall emitted power and the used transmit power are reduced, thereby decreasing the risk of interference of adjacent communications.
The object of the present invention is also solved by a terminal for wireless transmitting data packets using a contention-based protocol, including a transmitting and receiving unit and a controller, means for registering the used transmit power for transmitting data packets and means for determining features of the wireless terminal to enhance the chance to get access to the medium in dependence of the used transmit power.
Preferred embodiments of the invention are described in detail below, by way of example only, with reference to the following schematic drawings.
The drawings are provided for illustrative purpose only and do not necessarily represent practical examples of the present invention to scale.
In the following the various exemplary embodiments of the invention are described.
Although the present invention is applicable in a broad variety of applications it will be described with the focus put on IEEE 802.11 WLANs, wherein also an implementation within the Bluetooth or the emerging UWB Standard (IEEE 802.15.3a, Wireless PAN with Ultra-Wideband physical layer technology) is possible.
Before embodiments of the present invention are described, the known starting situation will be described in
Thin arrows, e.g. between terminal 2 and 3, indicate low transmit power levels, wherein thicker arrows, e.g. between terminal 1 and 5 indicate higher transmit power levels. Referring to
Referring to
The function shown in
with BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation).
In step S69 the transmit power is selected. In step S70 the packet is sent. In step S71 it is checked whether the time for waiting of the acknowledgement PDU has lapsed or an ACK message has been received. If the acknowledgement time-out is not lapsed or no ACK message has been received a further time slot will be counted in step S72. If the acknowledgement time-out is elapsed or an acknowledgement PDU has been received the total amount of transmission energy spent will be calculated in step S73. The spent energy depends on length of the packet, on the PHY mode and on the transmit power, but may also include the base-band processing energy needed for the last transmission attempt. In step S74 it is checked whether the acknowledge PDU has been received. If the acknowledge PDU has been received the transmission is successfully completed. In that case, the energy efficiency J[k] is calculated in step S75. Once the energy efficiency J[k] has been computed, both, the value for the spent energy and the number of transmissions, n, are reset to zero. The calculated value for the energy efficiency J[k] is used in the next pass of the flow chart to determine the back-off time and/or the priority for a wireless terminal. If the acknowledge PDU has not been received in step S74 the procedure will continue with step S77, wherein the energy efficiency J[k] is set to zero. In step S78 it is checked whether n is equals to the maximal number of tries. If the maximal number of tries has been reached the sending is stopped in step S79. The routing table could be altered, if last J[k] readings are low on average. If the maximal number of tries is not reached the procedure will step back to the start step S61 to restart the calculation of the energy efficiency.
By using this procedure illustrated in the flow chart according
| Number | Date | Country | Kind |
|---|---|---|---|
| 04103057.8 | Jun 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/52075 | 6/23/2005 | WO | 00 | 12/18/2006 |
