Abstract:
A system and method are provided for enabling the use of spatially distributed multichannel wireless access points or base stations. There is enabled a setup which allows frequency reuse whilst still giving many clients around the access points access to the multiple channel capability of each access point. This allows larger overall communication bandwidths to be obtained on average, even within the constraints of frequency reuse.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/349,173, filed Jan. 16, 2002. The entire teachings of the above application are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    A wireless Local Area Network (LAN) protocol allows mobile clients to find other mobile clients and access points, register with the wireless LAN and exchange data with other mobile clients and access points. One such wireless LAN protocol is the Institute of Electrical and Electronics Engineers (IEEE) 802.11b protocol which supports clients roaming within buildings such as, homes, offices, hotels and airports using direct sequence spread spectrum radios with data rates up to 11 Mb/s in the 2.4 GHz band.  
           [0003]    The number of available frequencies for communicating with clients within a wireless LAN band is limited. However, the capacity of the wireless LAN can be increased through a frequency reuse scheme. Instead of having one large base station known as an access point (AP) in a wireless LAN, cover an entire service area, the service area is divided into a plurality of small coverage areas or “cells”, with each cell having an access point at the center. In order to minimize interference between cells, available frequencies in the band are allocated to each cell in a repeating pattern such that adjacent cells are not assigned the same frequency.  
           [0004]    For example, the IEEE 802.11b protocol which operates in the 2.4 GHz band has adequate spectrum to provide three independent channels, each having a different center frequency. Each access point is configured to operate on a channel which does not interfere with the channel assigned to an adjacent access point. A client switches between the available channels in order to communicate with the access point having the best signal strength.  
           [0005]    [0005]FIG. 1 illustrates a prior art wireless Local Area Network  104  having a plurality of single channel access points  102  configured for a conventional frequency reuse scheme. Each access point  102  is shown at the center of a cell  100 . The cell  100  represents the coverage area within which a client can communicate with the access point  102 . There are three available channels having center frequencies of 1, 6 and 11 in the wireless Local Area Network  104 . The available frequencies have been assigned to access points  102  based on the conventional frequency reuse scheme such that access points  102  in adjacent cells  100  are not assigned the same frequency, in order to reduce interference between cells  100 .  
           [0006]    There is a finite bandwidth available for communicating with an access point which is shared by all mobile clients within the cell  100 . The access point  102  communicates with the mobile client based on signal strength detected by the client on the channel assigned to the access point. A client transmits a request to communicate on each available frequency and communicates with the access point that responds based on the strength of the signal received by the client. Thus, each client communicates with the access point having the highest signal strength. As the number of clients close to a particular access point  102  increases, the bandwidth available for clients within the cell  100  decreases accordingly. For example, with two clients communicating with an access point, each gets half the available bandwidth for the channel assigned to the access point.  
         SUMMARY OF THE INVENTION  
         [0007]    A method and apparatus for increasing available bandwidth to mobile clients in a wireless local area network is provided. A wireless access system includes multiple cells. Each cell has one or more primary channels with adjacent cells having different primary channels. An access point within a cell transmits at lesser power on a secondary channel exclusive of the primary channel and assigns channels to wireless clients.  
           [0008]    The access point may sense relative distance of clients and assign secondary channels to closer clients. The access point may sense power to sense relative distance. Clocks in each access point may be synchronized, allowing the access point to expand the secondary channels to fill the cell for a limited time agreed upon with other access points. The access point or the client retransmits, upon detecting a collision of its transmission with transmissions from other clients or access points. Collisions may be reduced by limiting transmission by the client to a pre-determined time slot.  
           [0009]    The wireless access system may include multichannel clients which transmit over all channels simultaneously in the inner region of the cell. A multichannel client is restricted to primary channels in an outer region of the cell. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0011]    [0011]FIG. 1 illustrates a prior art wireless Local Area Network having a plurality of single channel access points configured for a conventional frequency reuse scheme;  
         [0012]    [0012]FIG. 2 illustrates three independent channels within the 2.4 Ghz band;  
         [0013]    [0013]FIG. 3 illustrates a wireless Local Area Network having a plurality of cells, each cell having a multi-channel access point at the center;  
         [0014]    [0014]FIG. 4 illustrates a wireless Local Area Network having a plurality of multi-channel access points configured for a frequency reuse scheme according to the principles of the present invention;  
         [0015]    [0015]FIG. 5A is a graph illustrating a typical distribution of data exchanges between an access point and clients;  
         [0016]    [0016]FIG. 5B illustrates a further increase of available bandwidth by expansion of secondary channels into the outer region of a cell for a limited time period;  
         [0017]    [0017]FIG. 6 illustrates the particular case of a client located on the boundary of two cells;  
         [0018]    [0018]FIG. 7 illustrates a multichannel client located within the inner region of a cell which can perform multichannel communications with the access point;  
         [0019]    [0019]FIG. 8 is a block diagram of a typical access point which performs a bridging function between a wireless network and a wired network; and  
         [0020]    [0020]FIG. 9 is a block diagram of an embodiment of the wireless network interface shown in FIG. 8; and  
         [0021]    [0021]FIG. 10 is a block diagram of another embodiment of the wireless network interface shown in FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    A description of preferred embodiments of the invention follows.  
         [0023]    Several wireless communication protocols and their associated bands allow communication systems to simultaneously use multiple channels within the band. FIG. 2 illustrates three independent channels within a 2.4 GHz band. For example, three IEEE 802.11b channels can be independently transmitted and received at data rates up to 11 Mbps within the 2.4 GHz Industrial, Scientific and Medical (ISM) band in the United States. The IEEE 802.11b standard provides fourteen overlapping channels of 22 MHz in an 83.5 MHz range between 2.4 GHz and 2.48 GHz, with channel centers spaced about 5 MHz apart. The 83.5 MHz range, referred to as the 2.4 GHz band can accommodate three non-overlapping 22 MHz channels simultaneously. The center frequencies for three (1, 6, 11) of the 14 overlapping 22 MHz channels are chosen to provide the three independent non-overlapping 22 MHz channels. Thus, bandwidth to an access point is increased by allowing clients to communicate on three independent channels simultaneously.  
         [0024]    The three channels in the IEEE 802.11b standard provide the minimum number of channels required for a two-dimensional frequency re-use scheme to eliminate interference between adjacent cells. Channels 1, 6, 11 of the 14 channels are used in the United States due to FCC requirements. In Europe, Channels 1, 7 and 13 are typically used.  
         [0025]    In an alternate embodiment, the IEEE 802.11a protocol can be used to communicate between mobile clients and access points. The IEEE 802.11a protocol uses a different portion of the frequency spectrum (the 5 GHz Unlicenced National Information Structure (UNII) band) and provides eight center frequencies with data rates up to 54 Mbps between 5.15 and 5.35 GHz. The eight channels can be distributed among the cells to limit interference between cells and maximize available bandwidth to clients in each cell in a similar way as discussed for the three available channels in the IEEE 802.11b standard.  
         [0026]    The invention is not limited to wireless LANs using the IEEE 802.11a or IEEE 802.11b protocol. The invention can be used in any wireless protocol in which the available bandwidth is shared by clients operating on different frequencies, and which allows clients to change channels.  
         [0027]    [0027]FIG. 3 illustrates a wireless Local Area Network  300  having a plurality of cells  304 , each cell having a multi-channel access point  302  at the center. The cell  304  represents the coverage area within which a client can communicate with a particular multichannel access point. In an IEEE 802.11b wireless Local Area Network, each access point has a small coverage area, typically in the order of about 300 feet radius from the access point if there are no obstructions. There are three available channels with respective center frequencies 1, 6 and 11 in each multichannel access point  302  in the wireless Local Area Network  300 . Each multichannel access point  302  can communicate with clients within the cell  304  on any of the three available channels. For example, with only three clients within the cell, each client can communicate with the multichannel access point on a different channel. However, with each multi-channel access point communicating simultaneously on the same three channels, interference between adjacent cells can result.  
         [0028]    [0028]FIG. 4 illustrates a wireless Local Area Network  400  having a plurality of multi-channel access points  402  configured for a frequency reuse scheme according to the principles of the present invention. One embodiment of the system is illustrated in FIG. 4 in the context of a wireless Local Area Network based on the IEEE 802.11b protocol 2.4 GHz ISM band. However, the invention is not limited to a wireless Local Area Network based on the IEEE 802.11b protocol 2.4 GHz ISM band; the invention is applicable to all wireless networks including cellular.  
         [0029]    Three multichannel access points  402  are shown in the center of cells  400  in FIG. 4. Each client  408  can transmit and receive channels having center frequencies 1, 6 and 11 within the ISM band as discussed in conjunction with FIG. 2.  
         [0030]    The multichannel access point  402  is at the center of each cell  400 . The cell  400  represents the coverage area within which a client  408 ,  410  can communicate with a particular multichannel access point  402 . There are three available channels (1, 6 and 11) in each multichannel access point  402  in the wireless Local Area Network  300 . The multichannel access point  402  controls each channel in terms of power transmitted. A frequency reuse scheme is implemented by configuring each multichannel access point  402  to transmit on full power on a primary channel and on less power on secondary channels. The transmission at different power levels results in the partitioning of each cell  400  into two regions, an outer region  404  and an inner region.  
         [0031]    Clients generally transmit at full-power and use the channel that the multichannel access point  402  assigns for transmitting. The client typically hops among the available channels until it receives a response from an access point. On each hop, the client emits a “beacon” signal to signal its presence to an access point. Access points typically have some form of power detection to determine whether to accept transmission from a client. The access point will not accept transmission from a client if the signal is too weak indicating that the client is too far away from the access point. The client hops to another frequency to attempt to find an access point until assigned a channel.  
         [0032]    The multi-channel access point  402  assigns channels based on detected signal strength. Channel assignment in the IEEE 802.11 protocol can be performed by refusing to communicate with the client on a particular channel and waiting for the client to hop to a better channel before responding. Thus, client  408  in the outer region  404  is assigned to the primary channel. A client  410  in the inner region  410  is assigned the primary channel or any of the secondary channels, but likely one of the secondary channels to free the primary for peripheral clients. Thus more bandwidth is available for clients in the inner region  410  close to the access point. Communication in the outer region  404  is assigned to the primary channel to minimize interference between channels assigned to the outer regions  404  of adjacent cells  400 .  
         [0033]    The two secondary channels in the inner region  406  are transmitted at about half power so as not to cause interference with adjacent cells  400 . The primary channel in the outer region  404  is transmitted at full power to provide coverage for the entire cell. The frequency of the primary channel for each cell  400  is selected so as not to interfere with the primary channels of adjacent multichannel access points. A client  410  located within the inner region  406  can communicate on all three channels. A client in the outer region  404  communicates only with the full power primary channel.  
         [0034]    In the embodiment shown, two secondary channels in each multichannel access point  402  are transmitted at half the maximum power, while a primary channel is transmitted at full power. The primary channel transmitted by each multichannel access point  402  is selected based on a conventional frequency reuse pattern for single channel access points as described in conjunction with FIG. 1.  
         [0035]    Based on the detected power level, the access point determines whether the mobile client is in the inner region  406  or the outer region  404 . If the client is in the inner region  406 , the client may be assigned to channel 1, 6 or 11. The client is likely assigned to channel 1 or 6 in the inner region to give more bandwidth on channel 11 to the outer region  404 . If the client is in the outer region  404  and channel 11 is the primary channel of the cell, the client is assigned to channel 11. Thus, clients in the inner region  406 , with three available channels receive more bandwidth. With two clients, each client can be assigned to a different channel and each receives 100% of the available bandwidth of the assigned channel. Clients in the outer region  404  share the bandwidth of the full-power channel, which will be greater bandwidth than the conventional frequency reuse scheme since some clients (in the inner region) are on channel 1 and channel 6.  
         [0036]    The assignment of primary and secondary channels to access points is performed manually or through intelligent automatic algorithms which synchronize access points through the wired backplane or over wireless channels. Such algorithms are well-known to those skilled in the art.  
         [0037]    Transmitting primary channels at full power and secondary channels at half power minimizes interference between access points as in conventional frequency reuse, with the additional benefit of multiple channels within the inner region  406  for increased bandwidth. In addition, clients within the inner regions  406  around each access point can operate multichannel and take advantage of three times speed downloads and uploads to the access points by bonding channels together if they are equipped to do so. Furthermore, clients in the outer region  404  also benefit because they have reduced bandwidth sharing in general with other clients in the inner region  406  who can be shifted to other channels.  
         [0038]    [0038]FIG. 5A is a graph illustrating a typical distribution of data exchanges between an access point and clients. Data exchanges in wireless LANs are bursty in nature, not continuous, and there are long periods in which there is no activity. Furthermore, in wireless LANs having access points and clients, access points typically transmit 90% or more of the time during wireless exchanges because downloads to clients are typically much longer in duration than uploads. An upload from a client to an access point is typically a request of a short duration to download to the client. Thus, most of the time access points are transmitting (downloading to clients) and it is unlikely than many access points in a contiguous area are transmitting together, so that all channels within an access point can be transmitted at maximum power for some time. The graph illustrates three transmits from the access point to client (TX) and one receive by the access point from a client (RX) over time period T.  
         [0039]    [0039]FIG. 5B illustrates a further increase of available bandwidth by expansion of secondary channels into the outer region  404  of a cell  400  for a limited time period. Typically, each access point is connected to another network, for example, through a wired Local Area Network using the Ethernet communication protocol IEEE 802.1. Transmission can be synchronized between the access points because transmission over the wireless network is bursty in nature as discussed in conjunction with FIG. 5A. In order to synchronize transmission between access points, each access point includes a clock. The clocks in each of the access points are periodically synchronized with clocks in adjacent access points in the wireless network over a wired network or the wireless network.  
         [0040]    As in conventional systems, if collisions occur between access points, a binary exponential back off mechanism is used. The basic access mechanism is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). CSMA/CA works by sensing the medium for activity before every transmission and deferring the transmission if the medium is active. A binary exponential back off mechanism is used to spread transmission opportunities in time and minimize the likelihood of subsequent collisions.  
         [0041]    Alternatively, collisions may be avoided in advance by communication between the access points to set up a slotted scheme. For example, the communication can be over a wired backplane. In the slotted scheme, access points agree to transmit at full power on all channels only during certain time slots, and other access points agree not to interfere during those times.  
         [0042]    Prior to expanding the secondary channels into the outer region  404 , an access point communicates a forecast for a transmission to adjacent access points. The adjacent access points agree not to transmit while the requester access point is transmitting. Having received permission from adjacent access points to transmit, the transmitting access point provides full power to all channels and uses all three channels to simultaneously transmit to the mobile clients. Thus, the secondary channels are transmitted at full power to expand the size of the inner region  406  to cover the entire cell.  
         [0043]    [0043]FIG. 6 illustrates the particular case of a client located on the boundary of two cells  604 ,  602 . Cell  604  defines the coverage area for AP1  610  with primary channel 11 in the outer region  606  and secondary channels 1 and 6 in the inner region  608 . Cell  602  defines the coverage area for AP2  612  with primary channel 1 in the outer region  616  and secondary channels 6 and 11 in the inner region  614 . Cell  618  defines the coverage area of the client  600 . The client  600  receives signals of equal strength from the primary channel 11 for AP1  610  and the primary channel 1 for AP2  612 , and both AP1 and AP2 receive signals of equal strength from the client  600 . Thus the client  600  can communicate with AP1 on primary channel 11 or with AP2 on primary channel 1.  
         [0044]    However, transmission from the client to an access point is infrequent and of short duration. Thus, if a collision is detected, the client can easily retransmit. This procedure is defined in the IEEE 802.11b specification for wireless LANs implementing the IEEE 802.11b standard. The transmission from the client can be synchronized and limited to predetermined time slots; for example, the client can be restricted to transmitting when access point AP2 is not transmitting. Typically, access point transmission uses approximately 90% of the available bandwidth. A client on the periphery of an access point cell (AP 1) causes maximum interference to an adjacent cell (AP 2). This situation can be handled in several ways: (1) Retransmission on AP 2 after the receive from the interfering client is effective; (2) Slotted setups whereby a client waits until a slot is available in which they will not cause interference are an alternative if such delays are acceptable, and advantageous in that they do not involve collisions and retransmission.  
         [0045]    If there are N (where N is a large number) clients distributed equally around an access point, each client would ideally get 3B/N bandwidth (where B is the bandwidth of a single channel). In an embodiment implementing the IEEE 802.1b, the single channel bandwidth is 11 MB/s. Clients in the outer region  404  around each access point only see one channel, and assuming the outer region  404  contains M (M&lt;N, but can be close to N as the outer region  404  is larger than the inner region  406 ) clients, they cannot get more than B/M in bandwidth. Clients in the inner region  406  cannot get more than 2B/(N−M) in bandwidth.  
         [0046]    [0046]FIG. 7 illustrates a multichannel client  802  located within the inner region of a cell  602  which can perform multichannel communications with the access point. Bandwidth to a client can be further increased through the use of multi-channel clients. If a client has the capability of simultaneous transmission on all three channels, transmission to the access point can be up to three times faster. With both a multichannel client and a multichannel access point, downloads to the client and uploads from the client are three times faster when all three channels are used. The three channels can be used throughout the cell, if there is only one cell in the wireless LAN. If there are multiple cells in the wireless LAN, the three channels can be used simultaneously to upload or download when the client is located in the inner region  614  of the cell. A multichannel client  800  in the outer region  618  of the cell  604  is still restricted to using only the primary channel. There is also a network benefit in that the multichannel client completes the download or upload three times faster which compensates for hogging the bandwidth during this download or upload. Such communication is set up on an ad hoc basis with subsequent back off, upon detecting collisions with other access points or clients, or on a slotted basis, whereby the fast download is planned in advance and other access points and clients are informed as necessary in advance.  
         [0047]    [0047]FIG. 8 is a block diagram of a typical access point  820  which performs a bridging function between a wireless network and a wired network. A wireless network interface  800  communicates with remote clients through antenna  812 . Data received from the wireless network by the wireless network interface  800  is stored in memory  804 , by Direct Memory Access (DMA) Controller  802  through control signals  808 , over data bus  810  prior to transferring to the wired network through wired network interface  806 .  
         [0048]    Data received by the wired network interface from the wired network  816  is stored in memory  804 , by Direct Memory Access (DMA) Controller  822  through control signals  824 , over data bus  814  prior to being transmitted by the wireless network interface  800  through antenna  812  to the wireless network.  
         [0049]    Thus, the access point  820  allows wireless clients to download and upload data from/to clients on a wired network. Access points can communicate over the wired network, for example, to synchronize predetermined transfer periods on the wired network.  
         [0050]    [0050]FIG. 9 is a block diagram of an embodiment of the wireless network interface  800  shown in FIG. 8. The wired network interface includes a radio frequency interface  900 , an analog-digital and digital to analog conversion circuit  902  and a multi-channel modem/controller  904 .  
         [0051]    There are three separate receive paths and three separate transmit paths, one receive and transmit path per channel. Signals received by the antenna  812  from mobile clients are coupled to a low noise amplifiers  906 ,  956 ,  958 . The amplified signals are coupled to down convert mixers  914 ,  960 ,  962  which convert high frequency signals to low frequency signals. The amplified and down converted signals are coupled to a respective analog-digital converter  916 ,  964 ,  966  in the signal conversion circuit  902  to convert the amplified analog signals to digital signals.  
         [0052]    The multi-channel controller  904  includes a respective receive modem  924 ,  926 ,  928  per channel and a respective transmit modem  930 ,  932 ,  934  per channel. The digital signals output from the analog-digital converters  916 ,  964 ,  966  is coupled to a respective receive modem  924 ,  926 ,  928  to extract the digital signal from each channel that can be simultaneously transmitted. Each receive modem operates at a different center frequency. For example, in an embodiment for an IEEE 802.11b wireless network, one of the receive modems operates at the center frequency for channel 1, the second operates at the center frequency for channel 6 and the third operates at the center frequency for channel 11.  
         [0053]    Downloads to mobile clients on the wireless network originate in the transmit modems  930 ,  932 ,  934 . Each of the transmit modems  930 ,  932 ,  934  is configured to transmit on a different channel having a respective center frequency. Simultaneous transmission on all three channels is permitted. The output of each of the transmit modems  930 ,  932 ,  934  is coupled to a respective digital to analog converter  918 ,  936 ,  940  for conversion to a respective analog signal to be transmitted over the wireless network through antenna  812 .  
         [0054]    The analog signals output from each of the digital-analog converters  918 ,  936 ,  940  is coupled to a respective up convert mixer  912 ,  950 ,  952 . A pre-amplifier  910 ,  946 ,  948  coupled to the respective up convert mixer  912 ,  950 ,  952  amplifies the respective analog signal. Controllable power amplifiers  908 ,  942 ,  944  coupled to the respective outputs of the pre-amplifiers  910 ,  946 ,  948  further amplify the analog signals based on the value of a respective power control signal PWR CTL CH1-CH3. Each transmit modem  930 ,  932 ,  934  includes power control logic which controls the power of the transmitted analog signal for each channel through the respective power control signal. Thus, in the case of access point (shown in FIG. 4) with channels 1 and 11 transmitted at half power and channel 6 transmitted at full power, the power control signals control the output of power amplifiers  908 ,  942 ,  944  such that the signal strength of analog signals transmitted for channels 1 and 11 is half the power of signals transmitted for Channel 6.  
         [0055]    The radio frequency interface  900  also includes a Received Signal Strength Indication Circuit (RSSI)  954  coupled to the antenna  812  for detecting strength of received signals. The received signal power can also be measured in the receive modems  924 ,  926 ,  928 , if the analog-digital converters  916 , 960 ,  962  have sufficient dynamic range.  
         [0056]    [0056]FIG. 10 is a block diagram of another embodiment of the wireless network interface  800  shown in FIG. 8. The wired network interface includes a radio frequency interface  1000 , an analog-digital and digital to analog conversion circuit  1002  and a multi-channel modem/controller  904 .  
         [0057]    Signals received by the antenna  812  from mobile clients are coupled to a low noise amplifier  906 . The amplified signals are coupled to a down convert mixer  914  which converts high frequency signals to low frequency signals. The amplified and down converted signal is coupled to an analog-digital converter  916  in the signal conversion circuit  902  to convert the amplified analog signal to a digital signal.  
         [0058]    The multi-channel controller  904  includes a respective receive modem  1017 ,  1018 ,  1019  per channel and a respective transmit modem  1020 ,  1021 ,  1022  per channel. The digital signal output from the analog-digital converter  916  is coupled to each of the receive modems  1017 ,  1018 ,  1019  to extract the digital signal from each channel that can be simultaneously transmitted. Each receive modem operates at a different center frequency. For example, in an embodiment for an IEEE 802.11b wireless network, one of the receive modems operates at the center frequency for channel 1, the second operates at the center frequency for channel 6 and the third operates at the center frequency for channel 11.  
         [0059]    Downloads to mobile clients on the wireless network originate in the transmit modems. Each of the transmit modems  1020 ,  1021 ,  1022  is configured to transmit on a different channel having a respective center frequency. Simultaneous transmission on all three channels is permitted. The output of each of the transmit modems  1020 ,  1021 ,  1022  is coupled to the digital to analog converter  1006  for conversion to an analog signal to be transmitted over the wireless network through antenna  812 .  
         [0060]    The analog signal output the digital-analog converter  1006  is coupled to an up convert mixer  1012 . A pre-amplifier  1014  coupled to the up convert mixer  1012 ,  950 ,  952  amplifies the respective analog signal. Power amplifier  1016  coupled to the output of the pre-amplifier  1014  further amplifies the analog signal. The signal power is controlled by varying the output strength of the signals output from the transmit modems  1020 ,  1021 ,  1022 . In the embodiment shown, a separate power control is not needed because the digital-analog converter  1006  has sufficient dynamic range. The radio frequency interface  900  also includes a Received Signal Strength Indication Circuit (RSSI)  954  coupled to the antenna  812  for detecting strength of received signals.  
         [0061]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.