Patent Grant
8463308
Aspects of the present invention relate to wireless communications. More particularly, aspects of the present invention relate to controlling power used to transmit wireless signals.
The growth of wireless communications and integration with the internet continues to influence the growth of local area networks. Since the expansion of IEEE 802.11-based communication protocols and related devices, wireless local area networks (WLANs) are appearing with regular frequency. WLANs provide high speed wireless connectivity between PCs, PDAs and other equipment in corporate, public and home environments. WLAN users have come to expect access to WLANs and wanting larger coverage areas and higher throughputs. For portable users power consumption concerns are also an issue.
Currently, IEEE 802.11-series protocols are the leading WLAN standards. Some standards (ex: IEEE 802.11 a/b/g) have finished standardization. Some of these standards include the ability to modify power on a link to a unit.
At the same time, wireless providers are experimenting with adaptive antenna arrays (also referred to as smart array antennas). Current approaches to adaptive antenna arrays do not address power control issues. Rather, adaptive arrays concentrate on beam steering techniques.
Aspects of the present invention address one or more of the issues identified above, thereby providing an improved power control system for use with wireless communications.
Aspects of the present invention are described in relation to the following drawings.
Aspects of the present invention relate to controlling power in access points for us with wireless local area networks. The following has been divided into sections to assist the reader: power control; transmit power control in IEEE 802.11h; transmit power control in IEEE 802.11b, 802.11e, and other standards; link adaptation methods; and transmit power control with link adaptation.
It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect.
Power Control
Aspects of the present invention may be used with non-reciprocal uplink and downlink systems in terms of link gain. For instance, aspects of the present invention may be used with WLAN systems using access points (APs) with smart antennas. Here, aspects of the present invention address at least one of the stations transmit rate but also the stations power consumption. Transmit power control (TPC) capabilities and link adaptation may be used with various environments or expectations. For example, aspects of the present invention may be used in systems where stations transmit with their highest data rate or where stations transmit with their lowest power.
To realize the reduction in power consumption while maintaining usefulness of the system, methods and systems that function with TPC and compliant wireless LAN APs and stations may be used.
Power reduction does not mean that all devices will always be connected to an access point. Rather, hidden terminals exist where every station's transmit power isn't enough to reach every other station or back to an access point. In the 802.11b or 802.11e specification, stations transmit with a constant power and have no TPC functionality. The following describes various approaches to allow TPC in 802.11 protocols.
Transmit Power Control In IEEE 802.11h
IEEE 802.11h is a specification for Europe in 5-GHz band. This specification mainly deals with TPC and Dynamic Frequency Selection (DFS). The primary reason for TPC in 802.11h is that TPC (which means maximum regulatory transmit power setting in 802.11h) is required for operation on a 5 GHz band in Europe. Concerning TPC, 802.11h defines only the frame structure. It describes no method to achieve TPC.
Aspects of the present invention relate to using IEEE 802.11h specification's Probe Request/Response or Action commands to send some TPC information. These features may help other IEEE 802.11 specifications use TPC. These commands may or may not be used to transmit control signals to help avoid any hidden terminals. If control signals are used, they may be set to transmit with normal power to avoid the hidden terminal problem. This may include some modification of both AP and stations. However, aspects of the present invention may use any slot or frame that is reserved in 802.11b/e specification to allow for TPC based on a technique similar to that used with 802.11h.
While both 802.11h and 802.11b have frame structures, they are not identical. The following describes various observations in 802.111h and how to achieve TPC in non-802.11h protocols.
Considering this overview, in 802.11h, TPC may be accomplished as shown in
However, there is no availability for mobile station 102 to adjust its own transmit power. The current transmit power information for TPC is contained in the Probe response frame. This means that any calculation must be done at a receiver.
Aspects of the present invention include the ability of a mobile station 102 to adjust its own transmit power. The access point 101 may calculate the difference between a current mobile station 102's transmit power, update this information, and forward this information to the mobile station 102.
Transmit Power Control in IEEE 802.11b, 802.11e, and other Standards
To achieve TPC in 802.11 b/e, a minor modification of the slot structure of 802.11h may be used. Various TPC approaches may be constrained by the ability to modify 802.11 b/e protocol's frame structure. The access point 101 and mobile station 102 may also need to be modified to allow for TPC. TPC may be realized as a method of using Probe Request and Probe Response signals. Both types of situations (fixed array and changing array) may be used with TPC. This is shown with respect to
Referring to
Access point 402 may then send a Probe Request or Action signal 408 to access point 402. The same signal may or may not be received by other mobile stations 403 (shown here as broken signal 409. The access point 402 next determines in step 410 the power to be reduced with respect to mobile station 401.
Access point 402 then sends a Probe Response 411 to mobile station 401 that includes the new power setting or the amount by which mobile station 401 may reduce power. Using the new low power setting, mobile station 401 transmits data at signal 412 to access point 402. The access point 402 then acknowledges (ACK signal 413) the receipt of the data. The transmission of signal 413 may be performed at high power to ensure that mobile station 401 knows that the access point 402 has received the data signal 412. Alternatively, ACK signal 413 may be transmitted at low power to save energy at access point 402.
One benefit of transmitting ACK signal 413 at high power is that other stations 403 may then recognize that mobile station 401 has completed transmitting data and now other mobile stations 403 may start the process of transmitting data with access point 402.
Two navigation setting intervals may occur. A first 414 may occur from RTS signal 405 through acknowledgement signal 413. A second 415 may occur from CTS signal 406. through acknowledgement signal 413.
Link Adaptation Methods
The following describes various link adaptation methods in accordance with aspects of the present invention. Here, each station may check a received power and change a data rate according to a received power from an access point. These methods may minimize or eliminate the need to send any control information from/to AP.
A practical method for link adaptation is not defined in current IEEE 802.11 specifications. Nonetheless, most of the current IEEE 802.11 chipsets or relate equipment perform a type of link adaptation with traditional approaches. Considerations include setting a transfer rate at a highest rate first then decrease it according to channel condition, setting a transfer rate at a lowest rate then increasing it, how often should link adaptation be performed, should a received power and an error detection result be used for link adaptation, and the like.
As shown in
Next, the beacon or other control signal 605 is sent to station 602. The station 602 then changes in step 606 its transmission rate up or down according to the information in the beacon (or other signal) 605. The modifications may occur once per beacon or once per multiple beacons. The access point 601 and station 602 then wait (paths 607 and 608, respectively) for a next transmission of the beacon or other signal 605. Also link adaptation may be performed with the transmission of every beacon signal, may be done periodically, or may only be performed when the antenna parameters change.
Antenna parameters may be, for example, the gain difference between transmit beam and receive beam. This may be applicable in a system that uses packet steering as the transmit beams are wide to cover a larger area.
In a first approach, access point 801 calculates and informs the minimum gain difference ((δG)min) as antenna parameters. Access point 801 next sends control information with the wide beam (GA, GB, GC) 802 and receives each station's signal with the sharp beam (GA′, GB′, GC′) 803-805. (δG)min may be represented by the following equations:
(δG)min=Min[(GA′−GA),(GB′−GB),(GC′−GC)] Eq. (1)
or
(δG)min=Min[GA′, GB′, GC′]−Max [GA, GB, GC] Eq. (2)
This method is easy to implement. However, not every station may achieve an individual optimum gain with this approach. The process for the equations is shown in
In a second approach, access point 801 knows a direction of each station and sends this information to each station in advance. Each station A-C memorizes or stores the direction information. Next, when access point 801 changes its antenna radiation pattern, access point 801 calculates the relationship between antenna directivity and a radiation characteristic, and send this information to stations as an estimated radiation characteristic of antenna beam (or beam pattern). Stations A-C receive this information and calculate a premium gain by using the new beam using current condition and an estimated radiation characteristic of antenna beam.
For example, as shown in
Generally, each station has some information about the relationship between received power and affordable transmit rate to be used for link adaptation. If a station complies with one of the above link adaptation methods, it may modify a received power using the following equation:
Received power=actual received power+antenna parameter Eq. (3)
Then, if the case that an access point 801 changes its power, stations may need received power and transmit rates and the transmit power of access point 801 to perform link adaptation as described above.
Tables 1 and 2 show various relationships between transmit power, received power, and data rates tables. Using information similar to that shown in table 1, stations may adjust their power to achieve a useful transfer rate.
Transmit Power Control with Link Adaptation
TPC and link adaptation may be used together as a systematic control, because both of them use a received power level of station. Both methods may be combined based on different priorities or adopted policies for TPC.
The following lists various possible policies for TPC methods with combined link adaptation:
Next, a TPC interval performed by a station is related to system throughput as well as control complexity. The following three situations are considered:
The combinations of control policies and message frequency for TPC are shown in the following table 3. Various examples are shown in the following figures as well. The examples described herein include examples 1-9. The number in the following table shows the example number to which it corresponds.
In the case of EDCA, the method is similar to that of DCF. One difference for TPC between DCF and EDCA is that Block ACK mode exists in EDCA. In the Block ACK mode, ADDBA request/ADDBA response commands are used instead of RTS/CTS and they can replace RTS/CTS in
In cases of PCF or HCF, a Point Coordinator (PC) (Hybrid Coordinator (HC) in 802.11e) controls these signals. The PC (HC) may be located in an access point. The PCF scheme may be initiated by stations requesting that the PC (HC) registers them on a polling list, and the PC (HC) then regularly polls the stations for traffic while also delivering traffic to the stations. Stations may be controlled by the PC (HC) and allows transmitting one (or several) frame(s) for each polling signal from PC (HC). (See IEEE 802.11 specification.)
Thus, in PCF (HCF), a station should enclose TPC requests in DATA+CF ACK frames and PC (HC) should enclose TPC responses in DATA+CF Poll frames. Currently, slots for address 4 are N/A in 802.11/802.11e (according the specification, this is for the case of transmit between an access point and another access point). It can be used for the TPC signals as described herein. Alternatively, any other reserved slots can be used. One may also use RTS/CTS.
In future specifications, some or all of the modes will generally be backwards compatible and interoperable with IEEE 802.11 a/b/g. Thus, the TPC and link adaptation described herein may likely suit every standard in the 802.11 family.
To enable TPC, the access point may use tables showing transmission rate and required received power levels to maintain a link with specified rate. Most stations have such tables to perform link adaptation. Table 4-1 and 4-2 are the sample tables. “b” is a variable that represents the required power for 11 Mb/s. Here for example, a station sending a signal with 11 Mb/s and its received power is (b+4) dBm. The access point checks and knows from the table that the required rate 11 Mb/s needs b dBm power. Thus, the access point tells the mobile station to reduce power by 4 dB. In response, the station reduces its transmit power by 4 dB.
Example 2 shows a example where the system attempts to reduce the frequency of TPC message exchange. Two approaches are described with respect.
In a first approach with an access point 601 and a station 602, the station 602 examines whether access point 601 has changed its antenna radiation pattern or other characteristic at every transit opportunity. When the access point 601 uses a smart antenna (adaptive antenna) and changes its array width, for instance, reception conditions of station 602 are also changed. Thus, station 602 inquires whether a change has occurred. If a change has occurred, the station invokes TPC.
This approach also applies where access point 601 changes its transmit power for other reasons. Stations 602 with antenna parameter signals can respond where an access point 601 changes its condition more precisely. If an access point 601 changes an array width or transmits power on a large scale and, if link adaptation is done only at every several control signals, for instance, the rate which is changed by link adaptation may not be updated as well as it should be. Thus, under this condition, having an antenna parameter is useful.
In a second approach, TPC is described with an additional control margin to reduce its frequency. This margin is set so that a usual fading depth by typical multi-path and shadowing are impacted by a little change of an antenna parameter. Here, when the antenna parameters do not exceed the margin, the station does not need TPC at every transmission time.
This second approach has two advantages. First, this approach may reduce the transmission of additional signals being transmitted only for TPC between a station and an access point. One reason why decreasing the frequency of transmission of signals only for TPC is because redundant signals waste bandwidth. This may also be referred to as throughput degradation. This is noticeable in the situations that use RTS/CTS. (See Table 5.) One may use reserved slots in RTS/CTS for TPC. However, the maximum reserved slots are 3 bits only in RTS/CTS slots in the current 802.11 standard. These 3 bits may not be enough to inform the power value to be reduced with a sufficient range and accuracy.
Second, this approach provides advantages for channel conditions between access points and stations that are not changed and where the station (or access point) wants to send signals almost constantly (like voice etc). The reduction of unnecessary processing for TPC can avoid dissipating signal processing resources as well as consuming power.
If TPC is required then it is performed in step 908 and the information transmitted between the station 602 and access point 601 using a probe response 909, data signals 910, and ACK 911. If no TPC is required, then the process steps to probe response, data and ACK signals 909-911. Finally the new rate is stored in step 912.
If there was no change in antenna parameters from step 902, then the stations 602 determines if the difference of rate or/and power between a current rate (ratec) and a previous rate (ratep) is greater than 2 times the rate level in step 903. It is noted that power information may be used in conjunction or in place of the rate information.
If yes from step 903, then the system proceeds as above. If no, the system begins a new cycle.
In step 907, the access point 601 calculates the value of a difference based on the signal data and a margin information which may be taken from the tables shown for instance as Tables 4-1 and 4-2. For example, using Table 4-1, when the received rate is 2 Mb/s (required power is (b−6)), access point 601 calculates the difference between received power and required power for transmission at rate 5.5 Mb/s, which is one-level higher than current rate, and required power at this rate is (b−m3). In this case, the value of difference is “received power—(b−3)”. This 3 dB is the margin. The margin level in this example is 1 level, but it can be changed according to a control policy. Also, if Table 4-2 is used, the power difference between rates 2 Mb/s and 5.5 Mb/s is little and it is possible to group them together in such a case.
Example 3 may be useful under one or more of the following conditions:
The following examples are described with respect to one of the above approaches. For the following examples, one may substitute steps 902-903 with step 1001 as well as step 1001 with steps 902-903 for the reasons specified above.
When station 602 wants to send a payload, it checks a transmit payload category according to traffic or content and its required rate using table like that shown in Table 6 below, for instance. A margin may be set at an access point 601 as shown in
In step 1201, the station 602 checks the data transmit category and its required rate. Various rates are shown in Table 6. In step 1202, the station 602 checks to see if the required rate is less than the current rate. If yes, then in step 1203, the system sets the required rate as the current rate. If no from step 1202, then the process continues with step 903 where station 602 checks to see if TPC is needed with or without a margin.
For example, using Table 6, if the transmit data category is “voice” (the required rate being 2 Mb/s according to this table) and current rate is 7 Mb/s, station updates the rate to 2 Mb/s. The advantage of this case is that each station can transmit with sufficiently high rate for desired traffic or content and lower power.
The values used shown in Table 6 are for example purposes only. They may be altered based on system preferences.
Station 602 can use antenna parameter change information for examination as shown in Example 2. The process at the access point 601 is the same as that of Examples 2 and 3.
Example 5 is shown with respect to
One advantage is that station 602 does not need to have Table 6. Also, station 602 is not required to set the appropriate rate. This example may be beneficial where station 602 is desired to have less processing functions so as to minimize power consumption for the station 602. However, in this example, the access point 601 needs to send not only a value difference but also rate information. Current Probe response or similar signals can be used to send both power and rate with a little modification.
Table 7 shows a sample of a table that may be used with Example 6. “b” shows the required power for 2 Mb/s. In this case, access point 601 has both traffic category-rate and rate-required power information. Station 602 may or may not use an antenna parameter for examination like that shown in Example 2. Because this process is shown in
The policy for Example 6 is an emphasis on WLAN management. Here “WLAN resource” means how much wireless resource of access point 601 is occupied. It mainly depends on a number of stations which have payload to transmit/receive in each AP or in each array, a size of load from/to each station and so on. Note that AP sends a binary signal as “WLAN resource management signal” in this figure but any other signals can be also used. For example, “Station Count” and “Channel Utilization” signals are defined as a Beacon by IEEE 802.11e specification and we can use these signals as WLAN resource management signal. Here, “Station Count” indicates a total number of stations currently associated in each AP (or array), and “Channel Utilization” indicates a percentage of time AP (or array) senses the medium busy, as indicated by either physical or virtual carrier sense mechanism. In these cases, AP or stations sets a threshold. If the value of these signals becomes larger than the threshold, AP or stations consider the WLAN resource to be full. When stations examine whether the value becomes larger than the threshold, AP sends the value of threshold signal to station in advance. For example, if the maximum number of VoIP stations in each AP (or array) is x+2, AP sets the threshold x−1, and the current number of VoIP stations is x, AP or station consider the WLAN resource to be full.
When the process of
Here, each array in an access point sends resource information to a master resource controller in the access point or in a backbone network. Next. A master resource controller examines the WLAN resource considering information from all arrays, and sends this result to each array. It is also possible that each array examines WLAN resource associated with itself. The same scheme can be used even if AP is not a smart antenna and only has one array.
The AP may send WLAN resource information with control signals like the Beacon. Then, station considers modifying the rate considering WLAN resource. If this WLAN resource is full, each station sends signals at its maximum rate. However, if WLAN resource is not full, each station is not needed to send with its maximum power. In such case, station updates the rate into the required rate shown in Table 7 shown above to reduce the power consumption.
Example 7 relates to where the AP calculates a transmit rate for each station considering the WLAN resource.
The process of
Here, a station requires TPC at every transmission in these figures but the station may function with only a sparser interval. When the station requires TPC, the AP calculates the value of difference. If WLAN resource is not full, it also calculates a transmit rate for each station. The advantages of this approach includes the station does not need to do WLAN load examination as well as to calculate the transmit rate.
Here, the WLAN load information is used for control. Of course, other relevant information may also be available to achieve control with an emphasis on WLAN resource management.
Optionally, it is possible to combine the flow charts of
Example 8 shows a process where a station reduces the frequency of TPC using the margin shown in examples 2 and 3 above. Here,
Here, in
That process of example 9 is shown in
Here, the AP instructs all stations the minimum required rate when WLAN resource is full or almost full. When a station wants to send a payload, but the WLAN resource is full or almost full, the AP sends a required rate. The station compares the current rate with this AP's required rate. If the current rate is higher than the required rate, this station can send. But if the current rate is lower than the required rate, this station cannot send any data.
Optionally, the AP requires the minimum rate not only when the resource is full but also for other reasons. For example, even if the resource is not full, if one station transmits large scale of data with very low rate, it affects other stations and reduces the number of VoIP stations.
Further, it is also possible in this case that AP does not send the minimum required rate and AP examines the station's transmit rate. In this way, station sends RTS at first, but when the AP determines that a station's transmit rate is lower than the required rate, the AP does not send the CTS.
However, in this way, other stations in the same AP or in the same array must to set the NAV and may be prevented from sending any data for a while.
Link adaptation may generally be performed by done by “Tx MAC” using information from CCA (Clear Channel Assessment) 1917 or 1912. At first when the station wants to send a payload and, if the TPC is required at every transmitting opportunity (see examples 1-3 above), Tx MAC 1914 sends a TPC request signal using a Probe request or Action or any other frame. If TPC is required at every several opportunities, Tx MAC 1914 or TPC controller logic 1918 examines the requirements for TPC using at least one of transmit rate and received power information, which may be derived from link adaptation unit in Tx MAC 1914 or CCA 1907 or 1912.
When a station receives a TPC response from an AP, the station picks up a value of difference information at Rx MAC 1913 and sends this information to TPC controller logic 1918. TPC controller logic 1918 controls PA 1910 to change the transmit power. It is also possible that Rx MAC 1913 controls PA directly. In the cases where station checks a transmit data category and its required rate, the necessary tables are located in the memory, which is in MAC or host memory. Then TPC controller logic 1918 or Tx MAC 1914 accomplishes the control using information from both link adaptation unit and memory.
When each channel 2005-2007 in AP receives a signal, that received power information may be noted and stored. When each channel receives a signal which includes a TPC required slot, receiver MAC 1913 sends a control signal to TPC controller logic unit 2002 indicating it that should initiate a TPC calculation. In
Next, a value of difference information may be sent to Tx MAC 1914 and conveyed in the transmit signal. Various tables may be stored in memory, which is located in MAC 1911 or host memory 1917.
When AP controls link adaptation and TPC considering the WLAN resource, Master resource controller 2001 controls the WLAN resource.
The following provides examples of various policies described above.
If AP selects 3 or 4 mentioned above and each station decides how to select policy, the following examples may further be considered:
The present invention has been described in terms of preferred and exemplary embodiments thereof. Numerous other embodiments, modifications and variations within the scope and spirit of the appended claims will occur to persons of ordinary skill in the art from a review of this disclosure.
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