WO2013127699A1

WO2013127699A1 - A dynamic power control method and system

Info

Publication number: WO2013127699A1
Application number: PCT/EP2013/053545
Authority: WO
Inventors: Diego CARLOS GANDARILLAS; Primitivo MATAS SANZ
Original assignee: Telefónica, S.A.
Priority date: 2012-02-28
Filing date: 2013-02-22
Publication date: 2013-09-06
Also published as: AR090130A1

Abstract

The method comprising adapting the transmit power level in WLAN Access Points (APs) in order to minimize interference generation in WLANs based on local wireless link statistics. The dynamic power adaptation is based on the information provided by input statistical parameters obtained through measurements of wireless link quality over a time period. The method depends on the transmission performance and on physical medium conditions. It takes as input the statistical parameters and compares them with some established thresholds to enable decisions; the decisions adapt the transmission power in order to provide the required throughput with the minimum transmitted power. The system of the invention is adapted to implement the method of the invention.

Description

A dynamic power control method and system

Field of the art

The present invention relates to power management in Wireless Local Area Networks (WLANs), and more particularly, in a first aspect, to a dynamic power control method for adapting the transmit power level in at least WLAN Access Points (APs) in order to minimize interference generation in WLANs.

A second aspect of the invention relates to a dynamic power control system adapted for implementing the method of the first aspect. Prior State of the Art

The proliferation of wireless communications in home environments produces dense networks of Access Points that share the same radio environment, creating a phenomenon known as co-channel interference. Transmission of real time services, like video-streaming or any high data rate services, can be hampered by the interference created by nearby wireless Local Area Network (LAN) transmitting devices. In this type of situation it becomes necessary, or at least advisable, to reduce the transmit signal power of each of the Access Points as much as possible without degrading signal quality at the receivers, because if power is not reduced an excessive level of interference is generated.

Transmit Power Control algorithms (TPC) are critically important in wireless cellular systems (GSM, IS-95 CDMA, 3G-W-CDMA, LTE) in order to ameliorate the near-far problem and minimize the interference to/from other cells and improve the wireless systems performance. The methods for managing transmitted power in Wireless Local Area Network (WLAN) are also very important in order to minimize the interference between neighbor WLANs. Reducing interference levels entails an improvement on WLAN links performance, achieving better received Signal-to-Noise Ratio (SNR) and better communication throughputs.

TPC is a required feature for WLANs to ensure that WLANs comply with the ETSI regulations [1] and limitations on transmitted power in a given region. IEEE specified TPC in the IEEE 802.1 1 h amendment. IEEE 802.1 1 h defines two mechanisms on top of 802.1 1 PHY and MAC layers, namely Dynamic Frequency Selection (DFS), which is outside the scope of this invention, and Transmit Power Control (TPC). The two mechanisms allow extending the 802.1 1 operation in the unlicensed 5 GHz band. The I EEE 802.1 1 h TPC initial objective was to minimize interference between WLANs and radar or satellite systems. But at a later stage another function was added, to provide a transmit power reporting mechanism between AP and STAs in WLAN. The reporting mechanism allows communicating the devices TX power levels, calculating the wireless link path losses and adjusting the devices transmitted power in a feasible way. The added reporting mechanism objective was to minimize interferences between WLANs, in addition to the minimization of interferences between WLAN and external systems.

The problem is that many WLAN network devices transmit at their highest RF output power. Transmitting at maximum power level produces interferences, higher probability of packet collisions, that neighboring transmitters defer their transmission and reduces the overall throughput of the network.

Because of these drawbacks, solutions have been proposed and implemented to adapt the radio emitter to the wireless link conditions at any time, paying in added complexity for adjustable transmission power. For example, many power-management policies have been proposed to force WLAN devices to enter the doze mode at appropriate moments to reduce interference levels, because devices in the doze mode do not transmit signals, and also to reduce battery consumption. Another way to reduce interferences is applying TPC techniques, that allow a WLAN device to use the minimum required power level in the transmit mode. An example of those is the set of adaptive transmit power control methods in 802.1 1 that on a per-link basis reduce interferences, increase network capacity; and improve spatial reuse. It is obviously understood that all transmitted power control mechanism must minimize the power transmission and interferences without compromising the target delivery data rate for each client.

Some of the prior state of the art transmit power link mechanisms in 802.1 1 WLANs are here described:

IEEE Standard:

The TPC feature specified by IEEE in the 802.1 1 h amendment specifies two functions:

• Transmit power limitation: AP transmits the regulatory and local maximum power level for the current frequency channel. The maximum power level is the minimum value between the Region maximum power level and the device constrains maximum output power level. The stations in the BSS can use any transmit power smaller than or equal to this maximum power level assessed by the AP.

• Transmit power reporting mechanism: it defines a TPC Report element that contains a Transmit Power field and a Link Margin field. It is used for minimizing the transmit power level in WLAN devices.

The second function is used to determine the proper transmit power level for a given wireless link. Whenever a wireless station receives a frame containing the TPC Report element, which includes the information of the received signal strength via RSSI (Receiver Signal Strength Indicator) and the sending device transmit power, the wireless station can estimate the link quality (in terms of path loss) from the sending device to itself by performing a simple subtraction. Once the path loss is obtained, the device can adjust its transmitted power.

Several mechanisms have being developed, based on the TPC feature defined in the IEEE 802.1 1 h specification, in order to adjust dynamically the transmitted power in WLANs devices. But there are some drawbacks that TPC for WLANs must to face up to: the receiver side interference (also known as the hidden-terminal problem) and the asymmetric channel access (in Multiple Input Multiple Output (MIMO) systems); both problems lead to unnecessary packet retransmissions and a decrease on the link data rate. Here below are some related methods that cope with these problems for dynamically controlling the transmit power in WLANs.

Methods and transmit power control algorithms

Miser (Minimum-energy transmission Strategy) [2] - is an algorithm based on the link-quality estimation scheme defined for TPC in IEEE 802.1 1 h. The WLAN station (STA) estimates the path loss between itself and the transmitter, updates the data transmission status, and then selects the proper transmission rate and transmits power for the current data transmission attempt by a simple table look-up.

The lower the transmit power or the higher the PHY rate (hence, the shorter the transmission time), the less energy consumed in one single transmission attempt, but more likely the transmission will fail, thus causing re-transmissions and eventually consuming more energy. The key point on Miser is to combine TPC with PHY rate adaptation and pre-establish a rate-power combination table. The table is indexed by the PHY rate data transmission status, the path loss condition, and the frame retries counts. Each entry of the table is the optimal rate-power combination in the sense of maximizing the energy efficiency. The look up table setting is obtained by an initial calibration of the transmitter and receiver devices and depends both on the installation environment and the physical location of devices. A significant change in wireless devices locations involves a new calibration.

At runtime, a wireless station determines the best transmit power as well as the PHY rate for each data transmission attempt based on the rate-power combination table.

Contour- Slotted power control managing [3] - The main goal of this method is minimizing the interference level when different APs and WLANs are working in the same area. The algorithm defines controlled WLANs communication scheme; the APs on different WLANs are synchronized in order to avoid asymmetry links. At any instant of time, all APs in the network operate at the same power level to avoid link asymmetry. Over time, by using different power levels, the system achieves per-client power control to maximize spatial reuse. Each AP can transmit to each of its Clients at the lowest power level that minimizes the interference to other APs wireless communications, while not affecting the performance perceived by the client.

Power control MAC for ad-hoc networks [4] - This method defines a table where 10 transmit power levels are considered for WLAN APs: 1 mW, 2 mW, 3.45 mW, 4.8 mW, 7.25 mW, 10.6 mW, 15 mW, 36.6 mW, 75.8 mW, and 281.8 mW, which roughly correspond to the transmission ranges of 40 m, 60 m, 80 m, 90 m, 100 m, 1 10 m, 120 m, 150 m, 180 m, and 250 m, respectively.

For a random topology, four transmit power levels, 2 mW, 15 mW, 75.8 mW, and 281.8 mW, are considered roughly corresponding to the transmission ranges of 60 m, 120 m, 180 m, and 250 m, respectively. The link range at maximum power level is 250 m.

Depending on the distance between the AP and associated STA the AP transmits at its defined power level.

Once the system is calibrated and deployed, there is an increase on throughput comparing to 802.1 1 transmissions without this TPC method.

Symphonies - The method considers a combination of power level control and rate adjusting for meeting the link quality requirements.

Symphonies defines an interaction with Rate Adaptation. In 802.1 1 a/b/g wireless LANs, senders use one of multiple transmission rates for sending packets. The rate selection is determined by an estimation of the channel conditions like packet loss, delivery ratio, throughput, or Signal to Interference and Noise Ratio (SINR) estimation. Conceptually, a link is expected to have a good performance at a chosen rate if the SINR at the receiver is above a threshold defined by an adaptation table. Rate selection and transmit power control are tied together; power control without considering rate can reduce the SINR, leading to reduction in rate and hence the link and network throughput.

Symphonies considers the above problem and does not compromise the required rate. For instance, for supporting 54 Mbit/s, the transmit power can be reduced until it reaches the SINR threshold limit of 24.56 dB The precise knowledge of receiver SINR is needed in order to adjust the transmit power level.

Device Manufacturers solutions

There are so many systems that adapt transmit power and rate for WLANs on a per-link basis. The most important approaches developed by manufacturers are based on 802.1 1 h, but most of them are based on communications between AP and associated STAs which makes these solutions not interoperable with any other manufacturer wireless devices.

A selection of some manufacturer solutions for TPC is shown below:

- Cisco Aironet controller: this is a dynamic power management solution that automatically adjusts transmitted power according to changes in the RF environment without engineer interaction. The system intelligently modifies access point settings as needed, increasing power for coverage holes detection, decreasing or increasing power such as when a neighbor brings up a new access point. Cisco doesn't provide any more information about the implemented method for adjusting the transmit power on the AP.

- Airties: the DTPC feature is included in their latest WLAN devices.

- Ruckus, in some of its WLAN devices performance list includes a DTPC capability. Only available between APs and STAs of the Ruckus brand. No more information about the method is provided.

Some related patents can be also found in the field, for instance:

Patent US 8010168 'Efficient power management in WLAN' defines a method for managing communications between AP and STAs, controlling the transmitted power, the devices idle states and the data buffering. Patent US 6978151 'Updating path loss estimation for power control and link adaptation in 802.11 h WLAN' discloses a method and apparatus for determining the transmission power level between a set of stations located within the coverage area of a basic service set (AP) in a wireless local area network (WLAN). The receiving station measures a received signal power from the transmitting station, then the path loss estimation is computed based on the difference between the received signal power and the transmit power level extracted from the incoming signal. The computed path loss is updated according to predetermined criteria. Based on the updated path loss information, the transmit power level and/or the transmission rate of a receiving station is adjusted.

Patent US 7944868 'Method and system for dynamic power management in wireless local area networks' defines a method and system for improving spatial reuse in a wireless local area network (WLAN) by per-client dynamic power management. Every AP of the WLAN associates each of its clients with a minimum power level. A central controller of the WLAN generates a schedule for transmitting at different power levels and every access point varies its transmission power level based on this schedule.

Patent US 201 1/0096760 'Method and apparatus for controlling transmission power in WLAN system'; the method is based on measured path loss between STAs and AP. The client station manages transmit power by using path loss information received by the AP. The AP considers the path loss with other associated clients and transmits a frame according to the controlled transmit power. The AP acquires path loss from its clients and control power according to maximum value among path losses collected.

Patent US 201 1/0167291 'Method and apparatus for transmit power control in wireless networks'. This Patent is based on the communication between AP and STA for interchanging information about the path loss, power levels and receivers sensitivity. A method and apparatus are described including receiving, by a transmitter, a report from an associated client, setting and using a downlink transmit power level responsive to the report for data transmissions to the associated client, determining a downlink data loss rate and adjusting the downlink transmit power level responsive to the downlink data loss rate, wherein said report includes received signal strength, client transmit power level and one of link margin and receiver sensitivity. Also described there is a method and apparatus including receiving, by a receiver, a request for a report, transmitting the requested report, receiving an instruction to use an uplink transmit power level, setting and using the uplink transmit power level, determining an uplink data loss rate and adjusting the uplink transmit power level responsive to the uplink data loss rate, wherein said report includes said received signal strength, transmitted power level and one of said link margin and said receiver sensitivity.

Patent US 2007/0238417 ΆΡ multi-level transmission control and protocol control based on the exchange of characteristics'. Each wireless device generates reception characteristics based on transmissions received from the wireless access point and from other devices in the network. In one operating mode, the characteristics gathered by the wireless devices are forwarded to the wireless access point, and, based on all received characteristics, the wireless access point selects its own transmission power for different types of the transmission. In another mode, all characteristics are exchanged between every wireless devices and the access point so that each can independently or cooperatively make the transmission power control decisions.

The problem with the already known solutions is that many of the methods and algorithms proposed for Dynamic TPC are based on the RSSI (SNR) measurements. SNR is directly related to throughput and PHY rate, linked to the packet transmission. Despite the advantages, SNR-based transmit power control has the following drawbacks:

a. The rate and SNR could be variable; this is because of the radio channel fluctuations.

b. In 802.1 1 η, many radio interfaces only provide a signal strength indication (SSI) without any reference value. For instance, a high SSI does not mean high SNR.

c. SNR measurement should be made at the receiver side and sent to transmitter. It might be delivered using the same channel of data transmission so that it implies an additional use of the physical media and reducing date rate. An approach commonly used for measurement of SNR in local mode is to consider the symmetry of transmission channel in uplink and downlink directions and assimilate the measured SNR in the transmitter with SNR of the receiver. This approach is no longer valid in environments with multiple receiving or transmitting antennas where the relationship is different depending on the direction of link communication. For example, the IEEE 802.1 1 h defines TPC with a set of rules for meeting the ETSI regulations and allowing mechanisms for reducing the transmitted power on WLAN devices.

The mechanism for TPC described on IEEE 802.1 1 h amendment considers only the RSSI link measurements as valid parameter for adjusting Tx power. The method defined by the standard can be improved taking into account other parameters of the wireless link in order to reduce the TX power level.

On another hand, the previous methodologies are based on the establishment of a communication between the sender and receiver, using the medium on which it is making the adjustment. When the wireless communication requires a major adjustment the AP will not receive relevant information from the STAs for it.

For instance, Miser needs the communication between the clients and AP in order to complete the table data, and the appropriate software method running on both, AP and STA. The method is not optimized for maintaining the wireless data rate.

Contour needs GPS synchronization between APs and it is made only in order to minimize interferences between APs. The power control is based on SNR, depending on distance between devices and throughput degradation. Its second weakness is the problem to receive GPS signal in indoor deployments.

The power control MAC method is based only on RSSI measurements and calibrated distances between the AP and STAs in a WLAN deployment.

Symphonies considers the relationship between Rate and TPC, making a good approach for dynamically control the TX power. Such an approach, however, makes rate and power selection non-trivial. If the link is in a state of rate and power allocation at a given instant, and the delivery ratio deteriorates, the reaction can either be to reduce rate or increase power. While increasing power appears to be a natural choice, it is possible that even at the maximum power, the current rate cannot be supported; in which case reducing rate is the right choice. In addition to this, the method needs from previous calibration calculations and it doesn't consider real time transmission data rates.

The Device Manufacturers solutions described before are based on properties associated with specific manufacturers, and usually they are not interoperable with other devices and also different firmware environment.

Finally, most of the related patents rely on the communication between AP and STAs in order to obtain the wireless link data and performance. AP-STA communication is associated with the problem of using the medium on which control is performed, which is not always available. In the proposed invention the parameters of the wireless link are obtained only from one WLAN device, without establishing a special communication channel with other devices.

[1] ETSI standard EN 301 893 V1.5.1

[2] 'MiSer: An Optimal LowEnergy Transmission Strategy for IEEE 802.11a/h', Daji Qiao Sunghyun Choi Amit Jain Kang G. Shin The University of Michigan Seoul National University [3] 'Slotted Symmetric Power control in managed WLANs' Vishnu Navda. Ravi Kokku. Samrat Ganguly. Samir Das Stony Brook University. NEC Laboratories America, Inc

[4] Ά Power Control MAC Protocol for Ad Hoc Networks' Eun-Sun Jung & Nitin H. Vaidya U. Texas and U. Illinois.

Summary of the Invention

The present invention contributes to palliate the above and other drawbacks by providing a new dynamic transmit power control mechanism for adapting the transmitted power in wireless LAN (WLAN) devices. It proposes a dynamic iteration based on local wireless link statistics, instead of on the exchange of messages between Access Point (AP) and wireless client devices (STAs) or client.

The invention in a first aspect proposes a dynamic power control method, comprising as commonly in the art, at least one wireless client device (STA) maintaining a connection with an Access Point (AP) through a wireless link composed by a downlink or transmission path from the AP to the at least one STA and an uplink or transmission path from the at least one STA to the AP.

On contrary of the known proposals, and in a characteristic manner, the method comprises the steps of:

- obtaining a set of parameters related to one of the transmission paths of said wireless link operation, said set of parameters providing information of said transmission path of said wireless link at least regarding a modulation and coding scheme index value rate (MCS_C), the percentage of packets sent in one second (PKT_S), the percentage of packets sent without receive acknowledge (PKT_R), the number of bytes sent in one second (ΤΧ_Βγτ) and the channel occupancy, being said obtaining performed through iterative and averaged measurements of the wireless link during a period of time equal to T_AVG;

- computing the number of bytes transmitted during said T_AVG (TXAVG) from the number of bytes transmitted in one second (TX_BYT);

- computing an averaged Percentage of Retries over Packet sent (RET_AVG) as

PKT_R / PKY_R during said T_AVG; - obtaining a plurality of preset threshold values, thus enabling further decisions, said obtained preset threshold values being at least the minimum TX_AVG value which the dynamic power control method remains inactive (TXB_MIN)_> the RET_AVG value that triggers an increase in transmission power (TH RET_MAx), the RET_AVG value that triggers a decrease in transmission power (TH RET_MIN), a maximum occupancy threshold of said channel (TH R_MAX) and a minimum occupancy threshold of said channel (THR_MIN); and

a) increasing said transmitted power level when said TX_AVG is above the minimum threshold (TXB_MIN) and a first set of conditions are met, wherein said first set of conditions are RET_AVG > TH RET_MAX or TX_AVG> THR_MAX; or

b) decreasing said transmitted power level when TX_AVG is above the minimum threshold (TXB_MIN) and a second set of conditions are met, wherein said second set of conditions are RET_AVG < TH RET_MIN and TX_AVG < TH R_MIN,

so that if none of the two previous criteria are met the transmitted power level remains constant.

The transmitted power level is adapted in order to minimize interference generation in WLANs thus providing the required throughput with the minimum transmitted power. The method generally can be applied in a point to point WLAN connection, in a WLAN with a point to multipoint configuration and in a configuration where the wireless client device (STA) is operating as a relay, forwarding packets to a wireless client device (STA) or Access Point (AP).

The increasing or decreasing of the transmitted power level is performed one time in every period of time equal to T_AVG-

In one embodiment, the increasing or decreasing of the transmitted power level is performed, in the downlink direction, by said Access Point (AP). These operations over the transmitted power level are made by the AP independently of each existing wireless link, that is, for each STA connected to that wireless link.

Alternatively, in one embodiment, said increasing or decreasing of the transmitted power level could be performed in the uplink direction by said at least one wireless client device (STA). The STA could then also obtain the same parameters required, the modulation and coding scheme transmission rate and the packet retry and it can calculate the data rate occupancy of the channel in the wireless link uplink direction. The decision of increasing or decreasing said transmitted power level would be taken, optionally, by each wireless client device (STA) depending on their needs. The plurality of obtained preset threshold values: TXB_MIN, TH RET_MAX, TH RETMIN, TH RMAX and TH R_MIN are adjustable parameters either in the Access Point (AP) or in said at least one wireless client device (STA).

The invention in a second aspect proposes a dynamic power control system, comprising an Access Point (AP) and at least one wireless client configured to maintain a connection through a wireless link, the wireless link composed by a downlink or transmission path from the AP to the at least one STA and an uplink or transmission path from the at least one STA to the AP.

The dynamic power control system of the second aspect comprises in a characteristic manner:

- a first module configured to obtain a set of parameters related to said wireless link operation, said set of parameters providing information of said wireless link at least regarding a modulation and coding scheme index value rate (MCS_C), the percentage of packets sent in one second (PKT_S), the percentage of packets sent without receive acknowledge (PKT_R), the number of bytes sent in one second (ΤΧ_Βγτ) and the channel occupancy;

- a second module configured to establish a comparison between the obtained set of parameters with a plurality of threshold values depending on said Access Point (AP); and

- a third module configured to adapt a transmitted power level depending on said comparison.

The parameters and values for controlling the transmitted power level are obtained following rules defined in I EEE 802.1 1 η. This allows the invention to be implemented independently of any other specific I EEE 802.1 1 amendment, independently of the number of wireless clients devices and independent of whether the client is working or not working as a relay.

The invention requires the installation of specific dedicated modules in the WLAN AP for containing the method defined in the invention. Although, and without precluding any other architecture, those modules can also be installed in the associated wireless clients' devices (STAs).

The proposed dynamic transmit power control mechanism in the WLAN allows the establishment of wireless links with the minimum energy cost. For instance, when communicating at a fixed transmission power, nodes waste energy since some links already have a high probability of a successful delivery. Additionally, RF interferences can be generated by the high power level transmission. A transmission control algorithm decreases the transmission power to a level where link reliability is still high, but energy consumption is lower.

It also allows a better reuse of the medium. For instance, when the radio transmission is controlled in TX power, less interference on the STAs connected to it and on other neighbouring WLAN is created. Nodes communicate at the right power needed to ensure a successful communication, decreasing the amount of access attempt collisions in the network. This will also enhance network utilization and lower latency times.

Moreover it determines if the radio channel is able to support, i.e. successfully deliver, the bit rate that is being transmitted by the AP, it is manufacturer independent, because it works on parameters that are common to all IEEE802.1 1 n WLANs and an extension of the method can be installed in STAs, in order to also control their transmission power, also when they are operating as radio relays. Brief Description of the Drawings

The previous and other advantages and features will be more fully understood from the following detailed description of embodiments, with reference to the attached, which must be considered in an illustrative and non-limiting manner, in which:

Figure 1 is a flowchart showing a possible implementation of the proposed method, according to the first aspect of the invention.

Figure 2 is a flowchart showing how the average parameter statistics is obtained and how the calculations are made to determine whether the transmit power level of an AP needs to be changed or not.

Figure 3 is an illustration of a possible generic interference scenario where two A

Ps are transmitting at maximum power causing in-band signal interference.

Detailed Description of Several Embodiments

Concerning some definitions and clarifications, in the invention the term parameter includes input parameters, threshold values and constants. In general the terms are wireless link specific, which in the invention is understood, generally, as STA or client specific (from now on both terms will be independently used), since there is a link per client. In a point to point communication there is only one set of parameters; in a point to multipoint communication there is a number of sets equal to the number of wireless links or clients. It is not precluded that some parameters can have the same value in different links.

Table 1 shows a list of the parameters, together with their description, used in the invention. The list refers to an individual link or client.

Table 1 Names and description of parameters used in the invention

About some of the constants used in the invention, the channel occupancy refers to the number of bits transmitted by the AP during one second. On another hand, the maximum transmitted power (PWMAX) is the minimum power obtained from the characteristics of the power amplifier of associated device front-end and the current regulations of the used link in the regions defined, if the WLAN devices are complying with the regulation. Thus the maximum power would be the minimum between the maximum power amplifier and the power defined by the regulation in the region channel. Finally, the minimum transmitted power (PW_MIN) depends on the wireless client device, initially it would be assumed a typical value of 0 dBm by default, which is the typical value in most of WLAN devices.

Referring first to Figure 1 , there is shown a flowchart procedure of a possible implementation scheme of the described method and its steps. The flowchart shows the possibility of control over multiple connected wireless clients' devices (or clients indistinctly). In this case the statistics parameters are obtained independently per client and the associated power transmission change is performed in the wireless communication links between the AP and each of its associated clients.

Initially the method collects during a time period equal to T_AVG the system statistics: MCS_C, PKT_S, PKT_R and TX_BYT. Afterwards, the statistical averages of these values are calculated, with provisions to avoid extreme or out of range values. Then, for each frequency channel the parameter PW_MAX is defined as input value depending on the WLAN Region and the WLAN device constrains on maximum TX output power level.

Next, the MCS_C rate, the percentage of packet transmission retries (PKT_S,

PKT_R) and the data communication channel occupancy are taken as input parameters. The channel occupancy provides estimation on whether the bit rate on the wireless link is close or far from its maximum physical capacity.

The flow to decide whether the transmitted output power should be increased, decreased or remains constant is as follows: First, a calculation is performed to determine whether the number of bytes sent exceeds a threshold defined in the invention for changing the TX power (TXB_MIN)- Then a first checking is performed in order to determine whether a first set of conditions RET_AVG > TH RET_MAX or TX_AVG> TH RMAX are met. If these conditions are met the transmitted power level is increased in a given step. Next, a second checking is performed to determine whether a second set of conditions RET_AVG < TH RET_MIN and TX_AVG < TH R_MIN are met. If the new conditions are met the transmitted power level is decreased in a given step. If on contrary none of the two previous criteria are met the transmitted power level remains constant.

Referring to Figure 2, there is shown a flowchart procedure on how the step for obtaining the average parameter statistics works and how the calculations are made to determine whether the transmit power level of an AP needs to be changed or not.

One of the most important sub-processes of each iterative process performed during a specified time (T_AVG) is to obtain measurements averaged over this working time and define the collected statistics from the system. The averaging process starts when the method is started. After the period of time TAVG, the average statistics of parameters are obtained. The process continues running, takes new parameters values and every T_AVG the average statistics of parameters are refreshed.

The above mentioned calculations include averaging arithmetic addition and division and a smart verification of unconditional extreme value to avoid unnecessary specific changes on transmission power.

The flow depicted in Figure 2 is divided into two steps. In the first one the flow includes a comparison to determine whether there is a significant difference between the next parameter value and the last received one. If it is not, the value is considered for calculating the average. When another value is received, it is compared again with the previous value, this value is considered for calculation if there is no significant difference between them. In the second step, the calculation of the average values is performed.

The invention may be used in several scenarios, for instance in a saturation scenario where the STA or AP receivers become saturated because of excessive signal levels from nearby Aps, or in a generic interference scenario where the maximum throughput supported by a wireless link is limited by excessive interference. Both scenarios are showed and explained below.

The first scenario, saturation scenario, can be considered as an extreme one. In such scenario, the invention solves problems in a WLAN link where performance is degraded due to saturation in the RF receiver chain of a WLAN device, STA or AP. This scenario takes place when two WLAN devices without any transmission power control are physically close to each other.

When two client devices are close and no transmission power control mechanisms is operating, the transmitter output power, by default at its maximum value (around 20 dBm) can produce the effect known as saturation in a receiver unit. The saturation can be produced in the following points of the receiver chain: Low noise amplifier (LNA) or in an analogue-to-digital (A D) converter in the system digital base- band processor.

Many LNAs in 802.1 1 η systems start saturating at around -30 to -40 dBm input signal levels. In the presence of strong input signals above this level, the LNA will cease showing a linear behaviour and start generating unwanted harmonic distortion, intermodulation distortion and spectral re-growth, cross modulation, SNR degradation, and modulation inaccuracy that adversely affects the WLAN 802.1 1 η OFDM signal's 52 sub-carriers.

Even if the LNA does not saturate, because it is designed to support linear operation in the presence of input levels higher than those mentioned before, another element in the receiver chain can also saturate, i.e., showing non-linear behaviour: the A D converter.

In this environment, if the WLAN transmitter includes a power adjustment solution as the proposed in the invention, the automatic process of adjusting the output power will be enough to solve the problem of saturation, allowing the radio link to operate at its maximum limit of bandwidth and performance.

In the second scenario, less extreme scenario, multiple interconnected devices share the same frequency band, and their performance is limited by interferences due to the fact that transmission powers are higher than the minimum necessary. This scenario, likely in residential areas, is showed in Figure 3.

In Figure 3 two APs are transmitting at maximum power, which for analysis purposes is set at 20 dBm. A link is in operation between AP WiFi 1 and Wi-Fi client 1. AP Wi-Fi 2 maintains communication with other STAs not shown in the drawing. However, since AP Wi-Fi 2 is not far away from Wi-Fi client 1 , its output signal is viewed in Wi-Fi client 1 as interference.

In Table 2 the effect of the above mentioned interference is exemplified when

AP Wi-Fi 2 transmits at maximum power and when its transmission power is reduced by 6 dB because of the operation of a TPC mechanism. When the TPC mechanism is turned on the Signal to Noise Ratio, SNR, at Wi-Fi Client 1 improves by 6 dB, since interference dominates over thermal noise, and the maximum available throughput in the AP Wi-Fi 1 to Wi-Fi 1 client consequently improves. For the calculation of received signal and interference levels in Wi-Fi 1 client the signal propagation loss is assumed to be proportional to D³, where D is the distance between transmitter and receiver. The distance between AP Wi-Fi 1 and AP Wi-Fi 2 is set at 25 m, a representative figure in residential environments.

APs without TPC Distance from Clientl to Access Point 1 (meters)

1.25 2.5 5 7.5 10 12.5

Rx Signal level -22.9 -37.2 -41.0 -46.3 -50 -52.9

Wi-Fi 1 Client RX power

(dBm), AP1 TX Rx interference level without -61.3 -59.8 -59.0 -57.3 -55.3 -52.9 TPC

Wi-Fi 1 Client RX power

(dBm), AP2 TX

SNR(dB) without TPC 38.4 22.6 18.1 1 1 5.3 0.0

SNR(dB) with TPC (6 dB 44.4 28.6 24.1 17 1 1.3 6 reduction in interference level)

Maximum rate supported 54 36 22 22 0 0 without TPC (Mbit/s)

Maximum rate supported with 54 54 36 22 22 0 TPC (Mbit/s)

Table 1 : Effect over SNR with two close APs

To sum up, even though the signal is still limited by in-band interference, TPC procedures in general, and also the one proposed in this invention, are able to improve maximum throughput by reducing interference levels.

The foregoing describes embodiments of the present invention and

modifications, obvious to those skilled in the art can be made thereto, without departing from the scope of the present invention.

Claims

1. A dynamic power control method, comprising at least one wireless client device (STA) maintaining a connection with an Access Point (AP) through a wireless link, said wireless link composed by a downlink or transmission path from the Access Point (AP) to the at least one wireless client device (STA) and an uplink or transmission path from the at least one wireless client device (STA) to the Access Point (AP), the method being characterized in that it comprises the steps of:

- obtaining a set of parameters related to one of the transmission paths of said wireless link operation, said set of parameters providing information of said transmission path of said wireless link at least regarding a modulation and coding scheme index value rate (MCS_C), the percentage of packets sent in one second (PKT_S), the percentage of packets sent without receive acknowledge (PKT_R), the number of bytes sent in one second (ΤΧ_Βγτ) and the channel occupancy, said obtaining being performed through iterative and averaged measurements of said wireless link during a period of time equal to a time used for performing average calculations (T_AVG);

- computing the number of bytes transmitted during said T_AVG (TX_AVG) from the number of bytes transmitted in one second (TX_BYT);

- computing an averaged Percentage of Retries over Packet sent (RET_AVG) as PKT_R / PKY_R during said T_AVG;

- obtaining a plurality of preset threshold values, said obtained preset threshold values being at least the minimum TX_AVG value which the dynamic power control method remains inactive (TXB_MIN), the RET_AVG value that triggers an increase in transmission power (TH RET_MAX), the RET_AVG value that triggers a decrease in transmission power (TH RET_MIN), a maximum occupancy threshold of said channel (THRMAX) and a minimum occupancy threshold of said channel (THR_MIN); and

a) increasing said transmitted power level when said TX_AVG is above the minimum threshold (TXB_MIN) and a first set of conditions are met, wherein said first conditions are RET_AVG > THRET_MAX or TX_AVG> THR_MAX; or

b) decreasing said transmitted power level when TX_AVG is above the minimum threshold (TXB_MIN) and a second set of conditions are met, wherein said second conditions are RET_AVG < TH RET_MIN and TX_AVG < TH R_MIN,

so that if the two previous criteria are not met said transmitted power level remains constant.

2. A dynamic power control method according to claim 1 , characterized in that it comprises performing said step a) or said step b) in the downlink direction by said Access Point (AP) independently for each existing wireless link.

3. A dynamic power control method according to claim 1 , characterized in that it comprises performing said step a) or said step b) in the uplink direction by said at least one wireless client device (STA).

4. A dynamic power control method according to any of claims 1 , characterized in that said increasing or decreasing is performed one time in every period of time equal to

5. A dynamic power control method according to claim 1 or 2, characterized in that said plurality of obtained preset threshold values are adjustable in the Access Point (AP).

6. A dynamic power control method according to claim 1 or 3, characterized in that said plurality of obtained preset threshold values are adjustable in said at least one wireless client device (STA).

7. A dynamic power control method according to claim 1 , characterized in that said set of parameters and said plurality of obtained preset threshold values are obtained according to rules defined in IEEE 802.1 1 η.

8. A dynamic power control system, comprising an Access Point (AP) and at least one wireless client device (STA) configured to maintain a connection through a wireless link, the system being characterized in that further comprises:

- a second module configured to establish a comparison between said obtained set of parameters with a plurality of threshold values depending on said Access Point (AP), and

9. A dynamic power control system according to claim 8, characterized in that said first, second and third modules are dedicated modules installed in said Access Point (AP).

10. A dynamic power control system according to claim 8, characterized in that said first, second and third modules are dedicated modules installed in said at least one wireless client device (STA).