Abstract:
A method and apparatus for enhancing the data transmission capacity of a wireless communication system includes a smart antenna array for forming a radiation pattern to communicate with a plurality of mobile devices and control circuitry for adaptively modifying the radiation pattern to increase the carrier-to-interference (C/I) ratio of the wireless communication system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit of U.S. provisional patent application serial No. 60/305,240, filed Jul. 13, 2001, which is herein incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to wireless communication systems and, more particularly, to a method and apparatus for enhancing the data transmission capacity of a wireless communication system.  
           [0004]    2. Description of the Related Art  
           [0005]    In general, wireless communication systems use hexagon-shaped cells to divide a given geographical area to a more manageable size, given constraints such as carrier frequencies, base station power output, number of users, and local terrain. This approach is commonly known as a “cellular” approach and is applicable for cellular telephone and personal communications service (PCS) applications using the 800 MHz, 900 MHz, 1800 MHz, and 1900 MHz frequency bands. Each of the cells may, in turn, be sub-divided into sectors that are commonly (but not necessarily) 1200 wide along the azimuth.  
           [0006]    The transmit and receive antennas used by the base stations in each cell are typically omni-directional for covering the entire cell, or have a beamwidth of 120° for covering an individual cell sector. Currently, the antenna gain and beam direction for each base station is fixed and cannot be varied dynamically. As such, base station antennas typically receive signals from users of other cell sites who occupy the same channel. This co-channel interference reduces the carrier-to-interference (C/I) ratio and, hence, the capacity of the system. In addition, other intentional or unintentional electromagnetic emissions in the same frequency band will give rise to signal interference. Due to co-channel interference, wireless communication systems often operate below their theoretical data transmission capacity and do not use the scarce frequency spectrum optimally.  
           [0007]    Therefore, there is a need in the art for a method and apparatus that enhances the capacity of a wireless communication system.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is a method and apparatus for enhancing the data transmission capacity of a wireless communication system. The wireless communication system comprises a base station in communication with a plurality of mobile devices over a respective plurality of channels. Each of the plurality of channels is defined in a frequency band by a particular identifying attribute. The present invention comprises an antenna array for forming a beam in the radiation pattern. For example, the antenna array can comprise a phased array. The present invention further comprises control circuitry for switching the direction of the beam towards a location or an incoming signal of each mobile device when each mobile device is communicating over a respective channel. In one embodiment, the location of each mobile device is determined by a location sensing unit that utilizes the particular identifying attribute for each mobile device to determine the direction of the strongest transmitted signal from each mobile device. In another embodiment, the location each mobile device is determined via a physical location of each mobile device received from the wireless communication system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.  
         [0010]    It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0011]    [0011]FIG. 1 depicts a cell of a wireless communication system in which the present invention can be employed;  
         [0012]    [0012]FIG. 2 depicts a block diagram showing one embodiment of the smart antenna of the present invention as used in a time division multiple access (TDMA) wireless communication system;  
         [0013]    [0013]FIG. 3 depicts a block diagram showing one embodiment of a phased array of the smart antenna of FIG. 2;  
         [0014]    [0014]FIG. 4 depicts a block diagram showing one embodiment of a location sensing unit of the smart antenna of FIG. 2; and  
         [0015]    [0015]FIG. 5 depicts a block diagram showing another embodiment of a smart antenna of the present invention as used in a code division multiple access (CDMA) wireless communication system. 
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 1 depicts a cell  101  of a wireless communication system  100  in which the present invention can be employed. As shown, the cell  101  is divided into three sectors  102   1 ,  102   2 , and  102   3 . A base station  112  provides wireless communication service to mobile devices within the sector  102   1 . In particular, mobile devices  104   1  and  104   2  are present within the sector  102   1  and are in communication with the base station  112 . Interferers  110  are also present within the sector  102   1 , which interfere with communication between the base station  112  and the mobile devices  104   1  and  104   2 . Mobile device  108  is present within an adjacent cell (not shown) and is not in communication with the base station  112 . Likewise, mobile device  106  is in the sector  102   3  and is also not in communication with the base station  112 . Mobile devices  106  and  108  can also interfere with communication between the base station  112  and the mobile devices  104   1  and  104   2 . Interference from mobile devices  106  and  108  is known as co-channel interference. Interference from the interferers  110  as well as co-channel interference can reduce the C/I ratio and, consequently, reduce the available number of channels, and hence, the data transmission capacity of the wireless communication system  100 . Those skilled in the art will appreciate that the cell  101  can be divided into any number of sectors or can remain undivided (i.e., the base station  112  serves the entire cell).  
         [0017]    The C/I ratio of a received signal at the base station  112  depends on the gain of the antenna used and the location (distance and angle) of the mobile device. In accordance with the present invention, the C/I ratio for the wireless communication system  100  is improved by employing a smart antenna at the base station  112 . As described below, the smart antennas of the present invention are used directly with the existing base stations (e.g, base station  112 ) of the wireless communication system  100 , obviating the need for substantial modifications to the base stations.  
         [0018]    As described more fully below, each mobile device communicates with the base station  112  over a channel defined in a frequency band by a particular identifying attribute. The present invention will first be described with reference to time division multiple access (TDMA) wireless communication systems, such as the Global System for Mobile Communications (GSM) or the IS-136 system, where the particular identifying attribute is a time slot. The present invention will then be described with reference to code division multiple access (CDMA) wireless communication systems, such as IS-95A and IS-95B systems, wideband CDMA (W-CDMA) systems, or CDMA2000© systems, where the particular identifying attribute is an orthogonal code.  
         [0019]    To best explain the invention as it applies to TDMA wireless communication systems, it is useful to understand the operation of such systems. In TDMA systems, the total available frequency range for the service is sub-divided into frequency bands that are characterized by the channel carrier frequency and the bandwidth. For example, the GSM system has a total bandwidth of 25 MHz for the uplink (i.e., mobile device to the base station) and a total bandwidth of 25 MHz for the downlink (i.e., base station to the mobile device). Both the uplink and the downlink total bandwidths are divided into 125 frequency channels each having a 200 kHz bandwidth.  
         [0020]    A “channel” in a TDMA system is defined in a particular frequency band by a particular time slot. Each mobile device is allotted a specific time slot in which the mobile device is allowed to transmit and receive traffic. As used herein, “traffic” means data or speech signals. The time slots can be separated by a guard period to account for lack of perfect synchronization at the mobile device due to its mobility. The maximum number of time slots supported in a given frequency band are grouped together as a frame. In the GSM system, for example, each frequency band supports 8 time slots for full-rate communication and 16 time slots for half-rate communication. For full-rate communication, there are 8 time slots per frame. Each time slot is spaced from the next by a 30.46 μs guard period.  
         [0021]    [0021]FIG. 2 depicts a block diagram showing one embodiment of the present invention as used in a TDMA wireless communication system. Although the present invention will be described with reference to a GSM wireless communication system, those skilled in the art will appreciate that the present invention can be used with any TDMA wireless communication system. The present invention comprises a smart antenna  202  coupled to the base station  112 . The base station  112  is coupled to a base station controller (BSC)  222 . The BSC  222  is coupled to a mobile switching center (MSC)  224  of the wireless communication system.  
         [0022]    Briefly stated, the smart antenna  202  generates radiation patterns to transmit and/or receive traffic to and from the mobile devices within the service area of the base station (e.g., a sector of a cell). The base station  112  modulates and demodulates the traffic and performs other data processing functions under control of the BSC  222 . The BSC  222  manages radio resources for a plurality of base stations, including base station  112 , and facilitates hand-overs therebetween. The MSC  224  is typically coupled to the publicly switched telephone network (PTSN) and provides the functionality needed to handle the mobile devices, including registration, authentication, inter-MSC hand-overs, and the like.  
         [0023]    More specifically, the-base station  112  illustratively comprises a radio unit  214 , a frequency hopping unit  216 , a baseband processing unit  218 , and a BSC interface  220 . The radio unit  214  comprises a carrier unit  215 , a transmitter  217 , and a receiver  219 . The radio unit  214  is coupled to the frequency hopping unit  216 , which implements a frequency hopping matrix in a well known manner. The frequency hopping unit  216  is coupled to the baseband processing unit  218 , which forms TDMA frames, encodes and encrypts signals to be transmitted, and decodes and decrypts received signals. The baseband processing unit  218  is coupled to the BSC interface  220  for transmitting and receiving signals to and from the BSC  222 . The BSC interface  220  can comprise, for example, a microwave link between the base station  112  and the BSC  222 . Those skilled in the art will appreciate that the base station  112  can comprise additional and/or different components depending on the wireless communication system in use.  
         [0024]    In accordance with one embodiment of the present invention, the smart antenna  202  comprises a phased array  204  and a location sensing unit  206 . The phased array is a multi-beam, beam-switching antenna array capable of generating narrow, high-gain beams in its radiation patterns for transmission and/or reception of traffic to and/or from the mobile devices. Briefly stated, the phased array  204  dynamically changes its radiation pattern to direct beams towards individual mobile devices when the mobile devices are communicating with the base station  112  over their respective channels (i.e., during their respective time slots). The phased array  204  generates both transmit and receive beams for each frequency band in use by the particular base station  112 . For a given frequency band, the beam is switched from one mobile device to the next for each time slot in use. The beams are switched towards the location of each mobile device for maximizing the C/I ratio for each mobile device. In the present embodiment, the location of each mobile device is defined as the direction of the strongest transmitted signal from each mobile device, which is determined by the location sensing unit  206 .  
         [0025]    [0025]FIG. 3 depicts a block diagram showing an illustrative embodiment of the phased array  204 . As shown, the phased array  204  comprises a plurality of antenna elements  302   1  through  302   k  (where k is an integer greater than 1), directional couplers  304 , transmission amplifiers  306 , a low-noise amplifier (LNA) bank  308 , a transmission beamforming network  310 , a reception beamforming network  312 , and an adaptive controller  314 . The antenna elements  302   1  through  302   k  are arranged in an array and are coupled to the directional couplers  304 . The directional couplers  304  couple transmission signals from the transmission amplifiers  306  to the antenna elements  302   1  through  302   k , and coupled received signals from the antenna elements  302   1  through  302   k  to the LNA bank  308 . The transmit beams are formed by the transmission beamforming network  310  under control of the adaptive controller  314 . Likewise, the receive beams are formed by the reception beamforming network  312  also under control of the adaptive controller  314 . The adaptive controller  314  controls the direction and gain of each beam formed by the beamforming networks  310  and  312  in a known manner.  
         [0026]    In an alternative embodiment, the phased array  204  can comprise two separate antenna arrays, one for reception and one for transmission. In this embodiment, the directional couplers  304  are removed and the transmission amplifiers  306  and the LNA bank  308  are couple directly to the respective transmission and reception antenna arrays.  
         [0027]    [0027]FIG. 4 depicts a block diagram showing an illustrative embodiment of the location sensing unit  206 . The location sensing unit  206  utilizes the particular identifying attribute associated with each mobile device (i.e., a time slot in the present embodiment) to determine the direction of the strongest transmitted signal from each mobile device. The location sensing unit  206  comprises antenna elements  208   1  through  208   m  (where m is an integer greater than 1), receivers  402   1  through  402   m , analog-to-digital (A/D) converters  404   1  through  404   m , a processor  408 , and memory  409 . The antenna elements  208   1  through  208   m  are spatially separated to receive spatially diverse versions of an RF signal transmitted by a mobile device. In an alternative embodiment, the antenna elements  208   1  through  208   m  are part of the array of antenna elements  302   1  through  302   k  in the phased array  204 . In either case, the antenna elements  208   1  through  208   m  receive a transmitted signal from a mobile device  406  at different times T 1  and T 2  for a given location of the mobile device  406 . The location sensing unit  206  determines which mobile device is transmitting by identifying the particular identifying attribute of the mobile device (i.e., the time slot assigned to the mobile device). This may involve the wireless communication system providing frequency and time slot information, or the determination of either or both of these parameters by the location sensing unit  206 . By analyzing amplitude and phase characteristics of the received signals, the angle-of-arrival of the transmitted signal can be determined.  
         [0028]    More specifically, the outputs of the antenna elements  208   1  through  208   m  are coupled to receivers  402   1  through  402   m , respectively, for demodulation. The demodulated outputs from the receivers  402   1  through  402   m  are digitized by analog-to-digital (A/D) converters  404   1  through  404   m , and are then coupled to the processor  408 . The processor  408  executes an algorithm stored within the memory  409  to determine the angle-of-arrival and signal strength of the received signals from the mobile device  406  using the phase and amplitude relationship between the received signal paths. Given the angle-of-arrival and strength for each of the received signals, the processor  408  can determine the direction of the strongest transmitted signal from the mobile device  406  during a particular time slot. Such algorithms for determining the angle-of-arrival and received strength of RF signals are well-known in the art.  
         [0029]    Returning to FIG. 2, in operation, the phased array  204  receives information from the location sensing unit  206  regarding the direction of the strongest transmitted signal in a particular time slot from a given mobile device (not necessarily the direct signal in multipath environments). Given the direction of the strongest transmitted signal for a mobile device in a particular time slot, the phased array  204  directs a beam in this direction when the mobile device is communicating with the base station during its time slot. The phased array  204  then switches the direction of the beam to communicate with the mobile device assigned to the next time slot, and so on. The beam is switched from one direction to another within the guard period between time slots (e.g., 30.46 μs in GSM systems) to remain in communication with each of the mobile devices in a given frequency band. Each time slot is of a short enough duration that the phased array  204  can transmit traffic to a mobile device via the path of the strongest received signal even if this path is not the direct path to the mobile device. For example, in a GSM system, each time slot has a 0.577 ms duration over which the present invention assumes an approximately static channel corresponding to the strongest received signal from the mobile device.  
         [0030]    As described above, the phased array  204  is capable of forming many beams  212   1  through  212   n  for communicating with the mobile devices over many frequency bands. The direction of each of the beams  212   1  through  212   n  is switched as described above. Traffic received by the phased array  204  is coupled to the radio unit  214  of the base station  112 . Likewise, traffic to be transmitted by the phased array  204  is received from the radio unit  214 . In an alternative embodiment, the phased array  204  can direct only the receive beams to the mobile devices during their respective time slots, while the phased array  204  transmits signals to the mobile devices omni-directionally, or by sector.  
         [0031]    In addition, the phased array  204  can also form a broad beam  210  for broadcasting signals (e.g., control messages, paging messages, and the like) to the mobile devices within the sector. The broad beam  210  can also be used to service mobile devices in an “idle” state (i.e., not transmitting or receiving traffic). In an alternative embodiment, the broad beam  210  for broadcasting signals is generated by a supplemental antenna  207 , such as an omni-directional antenna.  
         [0032]    In this manner, the smart antenna  202  of the present invention couples directly to the base station  112  and requires no changes to the architecture of the base station  112 . This allows the present invention to be used with existing base stations in current TDMA wireless communication systems without substantial modification thereto.  
         [0033]    In an alternative embodiment, the smart antenna  202  comprises only the phased array  204 . The physical location of each mobile device is received from the MSC  224  via dashed path  226 . The beams of the phased array  204  are switched towards the location of each mobile device. In the present embodiment, the location of each mobile device is defined as the physical location of each mobile device, which is received from the MSC  224  of the wireless communication system. In particular, the wireless communication system employing the present invention may be adapted to determine the physical location of each mobile device using, for example, the Global Positioning System (GPS). Given the physical location of each mobile device, the present invention can determine the required beam direction for each mobile device. The phased array  204  then operates as described above.  
         [0034]    The present invention can also be used in wireless communication systems employing CDMA. To best explain the invention as it applies to CDMA wireless communication systems, it is useful to understand the operation of such systems. In CDMA systems, the term “channel” refers to a specific RF carrier frequency, bandwidth, and a unique code, which distinguishes the channel from other channels that use different codes. For a given frequency band, each mobile device is assigned a code that is orthogonal to the other codes used in the frequency band. In this manner, a base station can support a plurality of channels to communicate with the mobile devices within its service area (e.g., a sector).  
         [0035]    In CDMA systems, it is desirable that all signals from mobile devices arrive at the base station with equal powers. If perfect power control is not maintained over each mobile device, then the detection deteriorates quite rapidly, thereby reducing the number of mobile devices in the cell and the capacity of the wireless communication system. CDMA systems are generally limited in capacity by interference. This is particularly true for the uplink (mobile to base station), where maintaining perfect power control for all mobile devices operating in a dynamic multipath environment is difficult. Since an increase in mobile output power is not desired (drains the battery), and increase in CDMA capacity must be achieved by increasing the antenna gain for particular mobile devices and/or reducing the gain of interfering sources.  
         [0036]    [0036]FIG. 5 depicts a block diagram showing another embodiment of the present invention as used in a CDMA wireless communication system. The present invention comprises a smart antenna  502  coupled to a base station  112 . As described above, the base station  112  is coupled to a BSC  512 , which is in turn coupled to a MSC  514 . The base station  112 , BSC  512 , and MSC  514  operate substantially as described above, with the exception that CDMA communication techniques are employed, rather than TDMA. Thus, each frequency band supports a plurality of orthogonal codes, which are assigned to particular mobile devices. The smart antenna  502  produces radiation patterns to transmit and.or receive traffic to and/or from the mobile devices over their respective channels.  
         [0037]    In the present embodiment, the smart antenna  502  comprises a phased array  504  and a location sensing unit  506 . Operation of the phased array  504  is described above with respect to FIG. 3. The phased array  504  is capable of dynamically modifying its radiation pattern in order to reduce the signal power level from interferers and to boost the signal power level from mobile devices with low received powers at the base station. For example, placement of a beam peak in the direction of a mobile device experiencing a temporary fade will ensure the receipt of equal power levels at the base station  112 . Similarly, placement of a null in the direction of an interferer will reduce the noise power level at the base station  112 .  
         [0038]    The location sensing unit  506  can be configured as shown in FIG. 4. The location sensing unit  506  determines angle-of-arrival information and the received signal strength from the mobile devices. As described above, the antennas  208   1  through  208   m  receive spatially diverse signals from the mobile devices communicating with the base station  112  (e.g., mobile device  406 ). The receivers  402   1  through  402   m  receive the spatially diverse signals from the mobile devices. The received signals are digitized by the A/D converters  404   1  through  404   m  and are coupled to the processor  408 .  
         [0039]    The processor  408  uses the particular identifying attribute of each mobile device to determine the location thereof. In the present embodiment, the location of a mobile device is the direction of the strongest transmitted signal. In CDMA systems, the particular identifying attribute is an orthogonal code. More specifically, the processor  408  decodes the signals using the orthogonal codes assigned to each of the mobile devices currently communicating with the base station  112  (i.e., mobile devices within the sector) in a known manner using code searching and correlation techniques. The orthogonal codes assigned to each of the mobile devices that are currently communicating with the base station  112  are received from the MSC  514  of the wireless communication system. Alternatively, the location sensing unit  206  can store the orthogonal codes in the memory  409 .  
         [0040]    Once the received signals from the mobile devices have been decoded, the processor  408  can store the code searching and correlation results in the memory  409 . The processor  409  then only has to decode the signals from new mobile devices that initiate communication with the base station  112  for the first time. In this manner, the location sensing unit  506  can differentiate among the various mobile devices transmitting signals using the same frequency but different orthogonal codes. Using the decoded signals, the processor  408  can then determine the angles-of-arrival and the received signal strengths as described above for determining the direction of the strongest transmitted signal.  
         [0041]    In the present embodiment, the location sensing unit  506  also determines the direction of interfering out-of-cell or out-of-sector mobile devices. As described above, CDMA systems use power control to receive signals from all mobile devices with the same power level at the base station. While CDMA systems provide power control for all mobile devices within the sector of a base station, the relative power levels between sectors or other cells will vary. Thus, out-of-cell or out-of-sector mobile devices can cause interference with the mobile devices communicating with the base station  112 . The location sensing unit  506  can differentiate between the mobile devices within the service area of the base station  112  and out-of-cell or out-of-sector mobile devices by using the orthogonal codes received from the MSC  514 . For example, in IS-95 wireless communication systems, the short sequence offset can be used to differentiate among the mobile devices within the service area from the mobile devices outside the service area. The MSC  514  can be used to indicate which orthogonal codes are assigned to the mobile devices within the service area of the base station  112  (e.g., a sector), and which are assigned to the mobile devices outside of the service area. Alternatively, the location sensing unit  206  can store this information in the memory  409 .  
         [0042]    The phased array  504  receives the direction of the strongest transmitted signal for each of the mobile devices from the location sensing unit  506 . The phased array  504  also receives the direction the interfering out-of-cell or out-of-sector mobile devices. Alternatively, the phased array  504  can receive the physical locations of the mobile devices from the wireless communication system as described above via dashed path  516 . In addition, the phased array  504  can received the received signal strengths using received signal strength indicator (RSSI) information from the base station  112 .  
         [0043]    In any case, if the received signal from a mobile device within the sector of the base station  112  has a less than desired power level, the phased array  504  modifies its radiation pattern to place a beam peak  508  in the direction of the mobile device (or the strongest received signal from the mobile device). A beam peak increases the gain, and thus maintains power control at the base station. If there is an out-of-cell or out-of-sector interferer, the phased array  504  modifies its radiation pattern to place a attenuation  510  in the direction of the interferer to reduce the gain, and thus reduce the noise at the base station.  
         [0044]    When the phased array  504  forms an attenuation in the radiation pattern to reduce noise from interferers, the signal strength of mobile devices communicating with the base station  112  that happen to be in the same direction of the null will also be affected. In this instance, the present invention causes the base station  112  to instruct the affected mobile devices to increase signal power. By increasing signal power, the received signal strength from these affected mobile devices will remain constant as required in CDMA wireless communication systems.  
         [0045]    The phased array  504  is capable of forming a plurality of transmit and receive beams for the frequency bands used by a particular cell or sector. In addition, the phased array  504  is capable of forming a broad beam for broadcast signals. Alternatively, the smart antenna  502  can comprise a supplementary antenna  507  (e.g., an omni-directional antenna) for producing the broadcast beam. In this manner, the smart antenna  502  of the present invention couples directly to the base station  112  and requires no changes to the architecture of the base station  112 . This allows the present invention to be used with existing base stations in current CDMA wireless communication systems without substantial modification thereto.  
         [0046]    While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.