CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. application Ser. No. 10/968,244, filed Oct. 20, 2004, whose contents are expressly incorporated by reference.
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 802.11a/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.
BRIEF DESCRIPTION OF DRAWINGS
Aspects of the present invention are described in relation to the following drawings.
FIG. 1 shows transmit power control in accordance with aspects of the present invention.
FIGS. 2A and 2B show changing array patterns based on load equalization in accordance with aspects of the present invention.
FIGS. 3A and 3B show changing array patterns based on packet steering in accordance with aspects of the present invention.
FIG. 4 shows a process for reducing power in accordance with aspects of the present invention.
FIG. 5 shows a conventional link adaptation method.
FIGS. 6 and 7 show link adaptation in accordance with aspects of the present invention.
FIGS. 8A and 8B show modifications of antenna parameters in accordance with aspects of the present invention.
FIGS. 9-18 show link adaptation in accordance with aspects of the present invention.
FIG. 19 shows an illustrative example of a base station in accordance with aspects of the present invention.
FIGS. 20-21 show additional illustrative examples of access points in accordance with aspects of the present invention.
FIG. 22 shows a process for determining premium gain in accordance with aspects of the present invention.
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.
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.
a. For a TPC report, 802.11h changes the Probe response for this operation. While the response is changed, no change is made with the Probe Request to initiate TPC. Rather, 802.11h uses an Action frame for a TPC request.
i. The same changes in Probe response in 802.11b/e are possible, because an order number that is used for TPC in 802.11h is currently reserved in 802.11b/e.
ii. In 802.11b, there is no regulation for an Action frame. Thus, it is easier to modify Probe request in this protocol.
iii. In 802.11e, both an Action frame and a Probe request are defined.
b. In 802.11h, a station knows that an AP does TPC if a Spectrum Management slot (inside Beacon or Probe response) is set by 1.
i. The same slot of a Spectrum Management slot is reserved in 802.11b/11e. Aspects of the present invention may use this slot to achieve TPC.
Considering this overview, in 802.11h, TPC may be accomplished as shown in FIG. 1. FIG. 1 shows an access point 101 and a mobile station 102. Transmit power is included in TPC Report from mobile station 102 to access point 101. The TPC Report may be included as part of an Action Frame or part of a Probe Response. This figure shows the situation where the access point 101 wants to adjust a transmit power of mobile station 102. The TPC report is generated in response to a TPC Request from access point 101 to mobile station 102 using an access frame. If mobile station 102 wants to adjust access point 101\'s transmit power, it may by having reciprocal requests and reports.
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 802.11b/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 802.11b/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 FIGS. 2A, 2B, 3A, and 3B.
Referring to FIGS. 2A and 2B, TPC is described. Here, station mobile stations know whether the access point 201 changes the various array patterns.
a. A station 207 sends an RTS (Request to Send) signal 208 to access point 201. A Probe request/response time may be added to a NAV setting timer in the Duration field of the frame. The access point 201 receives the RTS 208 and replies with a CTS (Clear to Send) signal 209 to the mobile station.
b. The station 207 sends a Probe Request 210 and requests access point 201 to use TPC (for instance, by setting a TPC flag).
c. The access point 201 detects the received power from the station and determines the value difference between a received power and a power needed to communicate with the access point 201.
d. The access point 201 sends a Probe Response 211 to the mobile station and informs the mobile station of the value difference.
e. The mobile station then reduces a transmit power and continues operation as normal.
FIGS. 2A and 2B show transition of coverage areas of an array 201 changing automatically to load equalize each beam.
FIGS. 3A and 3B show transition of coverage areas of an array 301 changing automatically by packet steering.
FIG. 4 shows a signal flow chart between a mobile station 401, an access point 402, and other mobile stations 403. An access point 402 sends a beacon or probe response 404 to announce, for instance, that the antenna beam array associated with access point 402 is going to change. Next, mobile station 401 sends an RTS 405 at high power to access point 402. This may be picked up by other mobile stations 403 as signal 406. Of course, the other mobile stations 403 may or may not be in range to be able to pick up signal 406. Next, access point 402 transmits a CTS signal 407 to mobile station 401. The CTS signal 407 may or may not be received by other mobile stations 403.
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. FIG. 5 illustrates a conventional link adaptation method. Each station 502 receives a beacon or control signal 503 from access point 501. The stations 502 may use the beacon or other control signal to determine whether changing power according to the power of the received signal as shown in step 504.
As shown in FIG. 5, these link adaptation methods assume that uplinks and downlinks between access point 501 and stations 502 are reciprocal in terms of link gain. This suggests current approaches to not use smart antennas. This is because, when a system uses an access point with a smart antenna, uplinks and downlinks are not always reciprocal. This is because antenna patterns for receiving is not always the same as that for transmission, especially in packet steering systems as shown in FIG. 3. In addition, link adaptation is currently performed on the supposition that all access points 501 have a constant transmit power in current wireless LAN. However, in the future, access points may not be able to change transmit power using an adaptive array or similar devices to reduce interference. While the link adaptation methods of FIG. 5 may be used with a smart antenna, they will likely be error prone and not provide quality service to users.
FIGS. 6 and 7 show various link adaptation methods that may be used with a smart antenna in accordance with aspects of the present invention. Referring to FIG. 6, access point 601 determines if it antenna parameters are going to be changed in step 603. If yes from step 603, then the parameters of the new antenna pattern and/or the access point 601\'s transmit power are inserted into a beacon (or other control signal) 605. If no from step 603, then step 604 is skipped.
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.
FIG. 7 shows an approach in which an access point 601 only sends only sends change antenna parameter information or change AP\'s transmit power information (inserted in step 604) in the beacon 605. The station may then change the rate up or down per information in the beacon (occurring once per beacon or once per multiple beacons). Each station 602, which receives beacon 605 with any change information, sends a Probe request or Action frame 701 to request power control information from access point 601. The access point 601 then calculates in step 702 the margin or gain difference between a transmit beam and a received beam. Next, access point 601 sends the gain difference or margin in a Probe Response or Action frame 703 to station 602. Alternatively or additionally, access point 601 may send its transmit power using the Probe Response or Action Frame 703 to station 602.
FIGS. 8A and 8B show examples of antenna parameters used in packet steering. In general, for both wide beam 802 (GA, GB, GC) and sharp beams 803-805 (GA′, GB′, GC′) from access point 801, antenna parameters are different according to the azimuth (GA≠GB≠GC, GA′≠GB′≠GC′) for stations A-C. However, access point 801 may be limited as not being able to accommodate all these differences when it sends antenna parameters to all stations (as represented in FIGS. 6 and 7. Two approaches are described that address the situation where less than all antenna parameters are forwarded (including but not limited to no antenna parameters) to all stations with Beacon 605.
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: