Abstract:
A method and apparatus for use in a wireless communications network searches for the best serving access point of a base station as a function of communication link quality and load levels, thereby allowing a subscriber terminal to react to changes in RF conditions and load levels. In one implementation, a wireless modem in a fixed wireless access network initiates an access point searching algorithm in response to a triggering condition, such as initially powering-up the subscriber terminal or degradation of communication link quality or load levels. After detecting beacons for a plurality of neighboring access points, the wireless modem selects the best access point as a function of communication link quality and relative load levels to maintain adequate service quality and to react to changes in load levels. After an initial selection, the wireless modem may continually monitor quality/load conditions to determine whether to select a new access point.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wireless communications, and more particularly to a technique for selecting an access point in a wireless network. 
     2. Description of Related Art 
     Consumer demand for high-speed access to Internet and intranet related services and applications has resulted in several high-bandwidth access network alternatives, such as DSL (Digital Subscriber Line) broadband networks, all fiber networks, ISDN (Integrated Services Digital Network), and fixed wireless networks. 
     Fixed wireless provides a viable alternative to traditional wire-based access, particularly in geographic regions where the costs of upgrading and maintaining wireline connections are high. Essentially, a fixed wireless network is a cellular network which relies on short-range transmitter/receiver (“transceiver”) base stations to serve subscribers in small regions (“cells”) of a larger service area. By dividing a service area into cells with limited range transceivers, the same frequencies can be reused in different regions of the service area, and subscriber terminals which consume relatively little power can be used to communicate with a serving base station. 
     FIG. 1 illustrates a conventional wireless internet access system (WIAS), which is one specific implementation of fixed wireless technology having four major components: (1) multiple data base stations (BS)  100 ( a ) and  100 ( b ) which provide wireless connectivity and radio coverage to subscriber units  102 ( a )-( d ) (for example, residential and corporate terminal equipment as illustrated in FIG.  1 ); (2) wireless modems (“WMs”)  170 ( a )-( c ) which allow the subscriber units  102 ( a )-( d ) to communicate with BS  100 ( a ) or  100 ( b ) via forward (base station to subscriber) and reverse (subscriber to base station) air-interface links  115 ( a )-( c ); (3) a data switching center (DSC)  125  for routing data packets to/from BS  100 ( a ) and  100 ( b ); and (4) a backbone transmission network  135 , such as public IP (Internet Protocol) network, connected to the DSC  125 . 
     Subscriber units may connect to the backbone transmission network  135  in various ways, examples of which are shown in FIG.  1 . Corporate terminals  102 ( c ) and  102 ( d ) are connected to the backbone transmission network  135  via a local area network (LAN), a wireless router and/or firewall (not shown), and a shared WM  170 ( c ), while subscriber units  102 ( a ) and  102 ( b ) each have their own dedicated WM  170 ( a ),  176 ( b ). BS  100 ( a ) and  100 ( b ) may be directly connected to the DSC  125  or communicate with the DSC  125  via a service provider&#39;s private IP network  127 . 
     FIG. 2 illustrates an exemplary cell pattern suitable for implementing fixed wireless access. As seen in FIG. 2, each BS  100 ( a ) and  100 ( b ) provides 360° RF service coverage to subscriber terminals in cells  150 ( a ) and  150 ( b ), respectively, by transmitting and receiving signals over air-interface channels in designated frequency blocks (e.g., 5 MHz wide transmit frequency blocks and 5 MHz receive frequency blocks). Typically, cell coverage is sectorized, such that the frequency block designated for a given cell is distributed among a plurality of sectors (e.g., for a five sector per cell configuration, each sector being assigned a 1 MHz block for transmitting and a 1 MHz block for receiving). Therefore, each BS  100 ( a ) and ( b ) includes a plurality of access points (“APs”, not shown in FIG.  1 ), one per sector. 
     Depending on the location of a subscriber&#39;s WM relative to cell/sector boundaries and the radio frequency (RF) propagation characteristics of the surrounding area, the subscriber may be capable of communicating with multiple APs, i.e., multiple APs for a single cell and/or APs from different cells. For example, a subscriber&#39;s WM may be at or near the boundary of two or more sectors and/or two or more cells. In present implementations of fixed wireless access, the installer of the subscriber&#39;s WM selects a single AP during setup based on forward link signal strength, and the assignment of the AP which transmits/receives to/from the subscriber&#39;s WM does not change. 
     Due to changing RF propagation characteristics of the surrounding area, however, the AP which provides the best performance during installation will often not always be the best or even a suitable AP for ensuring adequate service quality or data throughput rates. For example, temperature and climatic changes, particularly moisture levels which change reflection coefficients, can significantly affect RF propagation between the AP and a subscriber&#39;s WM. Furthermore, degradation of service may result if the AP assigned to the user temporarily fails, or the sector served by the AP becomes overloaded. Still further, a more suitable AP may be subsequently deployed by the service provider (e.g., as a result of growth and “cell-splitting”). 
     Therefore, the need exists for an agile AP selection and assignment technique which allows a subscriber&#39;s WM to select and switch between serving APs in response to network conditions. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for selecting an access point in a wireless communications network which is able to maintain adequate service quality and throughput rates under dynamic network conditions. In one embodiment, the present invention is a technique for selecting and assigning an access point in a fixed wireless network which monitors control signals transmitted by a plurality of neighboring access points and selects the best access point as a function of a communication link quality metric, such as signal strength at the user location, signal quality at the user location, signal strength at the access point, signal quality at the access point, packet error rate (“PER”) at the wireless modem, PER at the access point, or a combination of two or more of these measurements, and relative sector load levels. 
     By dynamically selecting the best serving access point as a function of the communication link quality metric and relative load levels, the subscriber&#39;s wireless modem is able to adapt to changing RF propagation characteristics of the surrounding area to maintain high service quality and throughput rates, and is also able to avoid service outages by directing traffic away from a failed or overloaded access point. 
     In one exemplary implementation, a wireless modem in a fixed wireless network executes an AP search/selection sequence in response to a triggering event, such as when the wireless modem is initially powered-up, when service quality degrades below a threshold level, when sector load exceeds a threshold, or when instructed by the serving AP to do so, to (re)select an access point. When a triggering event has occurred, the subscriber&#39;s wireless modem detects an access control signal, commonly referred to as a “beacon”, transmitted from a plurality of neighboring access points. An access point&#39;s beacon identifies the access point, includes a neighbor list to identify neighboring access points and the frequency channel on which such neighboring access points are transmitting, and includes a field which indicates the access point&#39;s load level. After detecting beacons and obtaining a communication link quality metric for each neighboring access point, the wireless modem selects the best access point based on the communication link quality metric and relative load levels. 
     By considering relative load levels, adequate throughput rates can be maintained by distributing service among a greater number of access points when possible (i.e., achieving load balancing). Furthermore, by initiating access point re-selection when service quality degrades below a threshold level, or when sector load exceeds a threshold, the subscriber&#39;s wireless modem is able to react to changes in RF propagation conditions in the surrounding area, such as temperature and other climatic changes which affect communication quality. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limitative of the present invention, and wherein: 
     FIG. 1 illustrates an exemplary wireless internet access configuration system which is one suitable environment for implementing embodiments of the present invention; 
     FIG. 2 illustrates an exemplary cell pattern layout for a fixed wireless access network; 
     FIG. 3A generally illustrates an exemplary base station configuration suitable for implementing embodiments of the present invention; 
     FIG. 3B generally illustrates an exemplary subscriber terminal configuration suitable for implementing embodiments of the present invention; 
     FIG. 4 is a block diagram depicting select components of a wireless modem in accordance with an embodiment of the present invention; 
     FIG. 5 is a block diagram depicting select components of an access point in accordance with the preferred embodiment; and 
     FIG. 6 is a flow diagram for an access point search/selecting sequence according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is a method and an apparatus for (re)selecting an AP in a wireless communications environment, such as a fixed wireless access network. In one embodiment, the present invention is implemented as a search algorithm performed by a subscriber&#39;s WM to dynamically select an AP as a function of relative communication link quality and load levels to maintain adequate performance and data throughput rates, even when RF propagation characteristics of the service area change and/or AP failure or overload occurs. An embodiment of the present invention will be described with reference to FIGS. 3A,  3 B, and  4 - 6 . Initially, exemplary base station and subscriber terminal architectures will be described. Although specific base station and subscriber terminal configurations are detailed below, it should be recognized that such details are for illustration purposes and that the present invention may be implemented in various wireless network configurations. 
     FIG. 3A generally illustrates an exemplary base station suitable for use in accordance with an embodiment of the present invention. In FIG. 3A, a base station  200  includes a wireless hub  205  and at least one AP  210 . Preferably, the base station  200  includes five APs  210 (1-5) for serving five sectors of 72° coverage each. Assuming each base station in the network service area is assigned the same 5 MHz wide spectrum blocks for transmitting and receiving, each of the five APs  210 (1-5) is assigned a different 1 MHz channel for transmitting and a separate 1 MHz channel for receiving. 
     Wireless hub  205  is preferably a signal router and power supply that supplies each AP  210  with voltage and data (for example, 48V DC and standard 10Base-T LAN data) through cables  211 (1-5), such as 10Base-T cables. All radio and signal processing functions (i.e., transmitting and receiving for BS  200 ) are performed by the APs  210 (1-5). Further, the wireless hub  205  provides connections  213 (1-4) to/from the DSC (not shown). 
     FIG. 3B generally illustrates an exemplary subscriber terminal configuration  202  suitable for use in accordance with an embodiment of the present invention. The subscriber terminal  202  includes a WM  270 , an interface adapter box  275 , and a power supply  280  (for example, a 24V DC power supply). The WM  270  is preferably attached to a subscriber&#39;s home or office near the rooftop to communicate with a selected AP  210 . A PC  290  is connected to the interface adapter box  275  to send/receive data using the WM  270 , e.g., via an ethernet hub  295  and a 10Base-T cable  296  for implementation in a LAN environment. 
     Both the WM  270  and the AP  210  have radio units with receiver and transmitter circuitry, each providing respective transmit and receive functions. A reverse link (subscriber to base station) signal transmitted from the WM  270  to the AP  210  preferably operates in a 1 MHz RF channel between approximately 3450-3500 MHz, whereas a forward link (base station to subscriber) signal transmitted from the AP  210  to the WM  270  preferably occupies a 1 MHz RF channel between approximately 3550-3600 MHz. Further, both radio units have an automatic gain control (AGC) function to provide linear demodulation over a wide dynamic range; a receive signal strength indication (RSSI) function to enable digital control of the AGC and for use in the AP search algorithm discussed below; and both radio units perform modulation and demodulation, for example using quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM) techniques. 
     FIG. 4 is a block diagram depicting an exemplary architecture of the WM  270 . In the exemplary configuration of FIG. 4, the WM  270  includes: (1) a WM antenna  281 ; (2) a radio board  251 ; (3) a digital board  261 ; (4) a power supply  271 ; and (5) an interface  264 . The radio board  251  converts RF signals received from an AP  210  via the WM antenna  281  to digital signals, and converts digital transmit signals to analog RF signals which are then transmitted by the WM antenna  281 . The radio board  251  includes an analog RF/IF processing unit  252  which performs such analog/digital conversion, and which down-converts signals received by the WM antenna  281  to an intermediate frequency (IF) signal. The radio board also includes a digital signal processor (DSP)  253  which demodulates the IF signals output by the analog RF/IF processing unit  252 . The DSP  253  also modulates signals received from the digital board  261  to be transmitted, the modulated signals then being up-converted to RF signals by the analog RF/IF processing unit  252 . 
     The digital board  261  includes a control processor  262  which provides medium access control (MAC) and protocol functions, such as the timing of data transmissions. As discussed in greater detail below, the control processor  262  also executes an AP search/select algorithm according to an embodiment of the present invention. The digital board  261  also includes a format converter  263  which converts the MAC format data output by the control processor  262  to a data stream, such as a standard 10Base-T data stream, for output to the subscriber&#39;s PC  290  (not shown) via an interface  264 . The power supply  271  supplies power to the power radio board  251  and the digital board  261 . 
     FIG. 5 is block diagram of an exemplary architecture of the AP  210  which is suitable for use in accordance with the present invention. In the exemplary configuration of FIG. 5, the AP  210  includes: (1) a horizontally polarized antenna  282 ; (2) a vertically polarized antenna  281 ; (3) a matrix board  241 ; (4) an RX/TX board pair  221 ; (5) a TX board pair  231 ; and (6) a power supply/hub board  212 . 
     Like the WM  270  of FIG. 4, the AP  210  includes respective radio and digital boards which perform the functions discussed above. The AP  210  has both a receiving/transmitting (RX/TX) board pair  221  and a transmission (TX) board pair  231 , each having respective radio boards  222  and  232  and digital boards  223  and  233 . Radio boards  222 ,  232  each include an analog RF/IF processing unit  224 ,  234  and a DSP  226 ,  236 , and each digital board  223 ,  233  includes a control processor  225 ,  235  and a format converter  227 ,  237 . 
     The RX/TX board pair  221  transmits and receives when the AP  210  is used in a half duplex mode (i.e., the AP uses only one board to perform sequential transmit and receive functions), and functions like radio and digital boards  251  and  261  of the WM  270  described above. The TX board pair  231  is used to transmit when the AP  210  is used in full duplex mode (i.e., when the AP is transmitting and receiving simultaneously). 
     The matrix board  241  selects the desired board pair for transmission and/or reception and the best antenna for reception (the vertically polarized antenna  281  or the horizontally polarized antenna  282 ) via switches  242  and  244 . A duplexer  243  isolates receive and transmit frequencies on the vertically polarized antenna  281 , while a separate receive filter (not shown) filters the signals received from the horizontally polarized antenna  282 . Signals are always transmitted on the vertically polarized antenna  281 , whereas reception of signals occurs at both antennas, with the RX/TX board pair  221  determining which of the two signals to select based on performance. The power supply/hub board  212  includes a power supply  214  for providing power to the radio boards  222 ,  232 , the digital boards  223 ,  233 , and the matrix board  241 , and an ethernet hub  213  for sending/receiving data streams to/from the digital boards  223 ,  233 . 
     The operation of an AP search/selection technique according to an embodiment of the present invention will next be described with reference to the flow diagram of FIG.  6 . The AP search/selection described below with reference to FIG. 6 can be performed by the control processor  262  of the WM  270 . 
     The WM  270  performs a AP search/selection sequence upon determining that a triggering event has occurred. A number of different occurrences may constitute a triggering event. For example, the WM  270  may perform AP search/selection each time the subscriber&#39;s terminal is powered-up, at predetermined time intervals, when a deterioration of service quality or an increased load level is detected, or as instructed by a previously selected AP. 
     When the AP search/selection is initiated (step  402 ), the subscriber&#39;s WM  270  selects an initial RF channel to search for APs (Step  404 ). Because the same frequencies are used throughout the wireless network region, only a limited number of channels are available. The WM  270  may select any possible frequency channel within a search range as the initial RF channel (e.g., the lowest frequency channel or the highest frequency channel). 
     Each frame transmitted by an AP includes access control information, commonly referred to as a “beacon.” In accordance with the present invention, each beacon identifies the transmitting AP, indicates the load level for the AP, and includes a neighbor list to identify a number of (e.g.,  8 ) neighboring APs and the channels on which each neighboring AP is transmitting. The beacon may further include control information, such as data packet acknowledgements (“ACKs”). The WM  270  attempts to detect a beacon transmitted by an AP on the initial RF channel (step  406 ). When a beacon is found on the selected channel (step  408 ), the WM  270  extracts the neighbor list from the detected beacon (step  410 ) to identify neighboring APs and their assigned channels. Using the extracted neighbor list information, the WM  270  detects the beacons from each neighboring AP (step  412 ). 
     If the WM  270  does not detect a beacon on the assigned RF channel within a predetermined time period, e.g., 10 seconds (step  418 ), a new RF channel is selected (step  420 ). If the newly selected RF channel is within the channel search range (step  422 ), the WM  270  attempts to detect beacons on the newly selected channel (step  406 ). If the newly selected RF channel is not within the channel search range (i.e., is higher/lower than the highest/lowest channel in the search range), the WM  270  re-selects the initial RF channel (step  404 ) for attempted beacon detection (step  406 ). 
     After retrieving a neighbor list from a detected beacon at step  410  and detecting beacons from neighboring APs (step  412 ), the WM  270  determines if there is at least one acceptable AP based on a communication link quality metric (step  413 ), and, if so, selects a single AP based on the communication link quality metric and relative AP load levels (step  414 ). The communication link quality metric can be one of signal strength at the WM  270  (e.g., RSSI), signal quality at the WM  270 , signal strength at the AP  210 , and signal quality at the AP  210 , or a combination of two or more of these measurements. Signal quality can be represented by any number of measurements, including signal-to-noise ratio, bit error rate, frame error rate, packet acknowledgement percentage (i.e., the percentage of transmitted packages which are acknowledged), etc. 
     Signal strength/quality at the WM  270  indicates forward link quality, while signal strength/ quality at the AP  210  indicates reverse link quality. One or more measurements which indicate forward link quality and one or more measurements which indicate reverse link quality can be combined to obtain a communication link quality metric which represents communication link quality in both forward and reverse directions (i.e., bi-directional link quality). If no AP has an acceptable communication link quality metric (step  413 ), the WM  270  reinitiates the AP search/selection (step  402 ). 
     Relative AP load levels are also considered at step  414  to select the best AP. For example, if the AP with the highest communication link quality metric also has a load level which is below a threshold, that AP is selected. If each AP having an acceptable communication link quality metric also has a load level which exceeds the threshold, then the AP with the best communication link quality metric is selected. If some of the APs which have an acceptable communication link quality metric have a load level which exceeds the threshold but at least one such AP has a load level which is below the threshold, the AP with the lowest load level is selected. Load level may be represented by any number of measurements. For example, the average data throughput per user multiplied by the number of users can be calculated at the AP  210 . Other measurements, such the average number of bits being transmitted by the AP  210  per second, the “CPU up-time” at the hub, and/or data buffer overflow conditions, can be monitored to indicate load. 
     After the WM  270  has assigned the best AP at step  414 , subsequent changes in service quality and/or load levels my cause the WM  270  to re-enter the AP search/selection sequence described above. For example, deteriorating forward link quality may cause the WM  270  to perform AP search/section (step  426 ). Also, the previously selected AP may explicitly instruct the WM  270  to perform the AP search/selection sequence, for example if reverse link quality drops below a threshold. Furthermore, the WM  270  may continuously or periodically monitor the load level for the serving AP and perform AP search/selection when the load level exceeds a threshold (step  428 ). If the WM  270  performs AP search/selection based on degraded service quality or an excessive load level for the previously selected AP (or if instructed by the AP to do so), the WM  270  uses the neighbor list received in the beacon of the previously selected AP to detect the beacons for neighboring APs (step  412 ) so that the best AP may be selected (step  414 ). 
     By implementing the above-described AP search/selection sequence, WMs may switch between APs based on local performance, which can change as a result of changing RF propagation conditions, signal levels, load levels, and network redesign. Furthermore, a subscriber&#39;s WM can select a new AP to avoid temporary service outages by directing traffic away from a failed or overloaded AP. 
     It should be apparent to those skilled in the art that various modifications and applications of the present invention are contemplated which may be realized without departing from the spirit and scope of the present invention. As one example, the subscriber terminal may include a mechanical control mechanism which positions the WM antenna  281 . In this way, when a new AP is selected, the WM antenna  281  may be re-oriented to improve communication link quality for the newly-selected AP. Furthermore, the WM antenna  281  may be controlled to scan for beacons during the AP search/selection when determining the communication link quality metric for each AP in the neighbor list, so that the selection accuracy is improved. Furthermore, although the above-described embodiment specifies that the control processor of the WM  270  performs AP search/selection, it should be understood that the AP search/selection sequence and portions thereof can be implemented in any number of software-driven processing circuitry, application specific integrated circuits, or other arrangements. Still further, although the above-described embodiment specifies that the wireless modem obtains a neighbor list from a detected beacon, the wireless modem may obtain the neighbor list independent of a beacon.