Abstract:
A wireless communications system and method controls network load by selectively scaling aggregate base station transmit signals. In one implementation, aggregate in-phase (I) and quadrature (Q) channel transmit signals are multiplied by a scaling coefficient output by an aggregate overload controller based on load levels relative to a threshold. By scaling aggregate I- and Q-channel transmit signals when load level measurements indicate a high load situation, handoff control measurements made at mobile subscriber terminals, such as received signal strength from the base station, bit/frame error rates, and signal-to-noise ratio, will be affected, thereby prompting mobile subscriber terminals at the cell/sector boundaries to request handoff to an adjacent cell/sector. Thus, load is balanced between a number of cells/sector to increase network capacity and prevent overload without relying on a call admission/blocking scheme.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of wireless communications. 
     2. Description of Related Art 
     In wireless communication networks based on spread spectrum technology, such as a Code Division Multiple Access (CDMA) network, a plurality of mobile subscriber terminals (“mobiles”) share the same radio frequency (RF) bandwidth, and are separated by employing different Walsh codes or other orthogonal functions. As compared to communication systems which create multiple channels from a single RF band by assigning different time slots to users, i.e., Time Division Multiple Access (TDMA), or subdividing an RF band into a plurality of sub-bands, i.e., Frequency Division Multiple Access (FDMA), using orthogonal code sequences to form separate channels enables a CDMA system to exhibit “soft” network capacity. In other words, the number of mobiles which can share a given RF bandwidth at one time is not fixed, and instead is typically limited only by the degradation of service quality caused by interference from other users of the same and adjacent cells/sectors. The resulting tradeoff between network capacity and service quality in a CDMA system is typically resolved by reverse link (mobile to base station) power control techniques which adaptively set mobile transmit power to the minimum level needed to maintain adequate performance. 
     Despite the use of reverse link power control techniques to reduce co-channel interference and increase capacity, overload may occur in network cells/sectors when the number of mobiles being served exceeds the maximum number at which target call quality (typically represented as the ratio of energy per bit, E b , to noise and interference, N o , in a given bandwidth) can be maintained, for example when a large number of mobiles attempt to communicate with a single base station at once. One previously implemented technique for avoiding overload relies on a call admission/blocking scheme to guarantee adequate communication quality by blocking service to additional subscribers when load levels exceed a certain threshold. Such call admission schemes, however, may result in unacceptable service outages. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and a method which scales base station transmit signals in a wireless communication network in response to high load levels, thereby affecting handoff control values measured at served mobiles to “push” mobiles to adjacent cells/sectors and avoid overload conditions. In one implementation, a base station overload controller scales the amplitude of aggregate forward link (base station to mobile) transmission signals as a function of the difference between aggregate transmit signal magnitudes and a threshold level. By scaling aggregate base station transmit signals, which include control signal components (e.g., a pilot signal component in a CDMA system), handoff control values, including receive signal strength, bit/frame error rates, and signal-to-noise ratio, measured at mobiles within the network service area are affected. Depending on the location of mobiles and the degree to which the aggregate base station transmit signals are scaled, a percentage of served mobiles, particularly those at cell/sector boundaries, will request handoff to an adjacent cell/sector. As the load level increases relative to the threshold level, the degree of scaling likewise increases, thereby more significantly affecting handoff control values measured at mobiles within the network service area, and causing an increased number of handoffs to balance load between a plurality of cells/sectors. Thus, the present invention increases network capacity and prevents overload without relying solely on a call admission scheme. 
     In one embodiment, the present invention is an aggregate overload controller which samples and sums aggregate in-phase (I) channel and quadrature (Q) channel transmit signal magnitudes over a load measurement period to obtain a load measurement value, and outputs a scaling coefficient as a function of the difference between the load measurement and a threshold. The aggregate overload controller initially sets the scaling coefficient to 1, and maintains the scaling coefficient at 1 as long as the load measurement value remains below the threshold. When the load measurement first exceeds the threshold, the scaling coefficient from the preceding load measurement period (i.e., 1) is decreased by an offset value which is calculated as a function of the difference between the load measurement value and the threshold. In one implementation, the updated scaling coefficient is calculated as: 
     
       
         S M =min{1, S M−1 +μ(E th −E M )},  (1) 
       
     
     where S M−1  is the scaling coefficient from the previous load measurement period, E th  is the threshold, E M  is the load measurement for the current load measurement period, and μ is a constant. The constant μ may be set to a small value, e.g., 0.01, to prevent substantial fluctuations in the scaling coefficient S M , and thereby avoid network instability. 
     I- and Q-channel multipliers multiply the scaling coefficient S M  received from the aggregate overload controller by aggregate I- and Q-channel transmit signals received from a baseband processor. The resulting scaled I- and Q-channel transmit signals are received by an RF processor, which performs digital-to-analog conversion, low-pass filtering, modulates the scaled I- and Q-channel transmit signals onto separate RF carriers, combines the modulated I- and Q-channel carriers, and outputs the combined RF transmit signal to base station antenna for transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the present invention will become e apparent upon reading the following detailed description, and upon reference to the drawings in which: 
     FIG. 1 illustrates an exemplary wireless network configuration suitable for implementing embodiments of the present invention; 
     FIG. 2 is a general block diagram depicting certain components of a base station transmitter according to embodiments of the present invention 
     FIG. 3 is a block diagram depicting an exemplary baseband processor of a base station transmitter which generates aggregate I- and Q-channel transmit signals which are scaled by a scaling coefficient from an aggregate overload controller according to an embodiment of the present invention; 
     FIG. 4 is a flow diagram illustrating an exemplary operation performed by the aggregate overload controller to calculate a scaling coefficient according to an embodiment of the present invention; and 
     FIG. 5 is a block diagram of an exemplary RF processor of the base station transmitter according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is a system and method which scales base station transmit signals in a wireless communications network, such as a CDMA network, to affect handoff control values measured at mobiles within the network area under high load conditions, and thereby prevent overload conditions. An illustrative embodiment of an overload control system and method according to the present invention is described below. 
     Referring to FIG. 1, there is shown a wireless network configuration  10  suitable for implementing embodiments of the present invention. The wireless network  10  includes a plurality of geographic sub-areas (“cells”)  12 - 1 , . . . ,  12 -i. Each cell  12 - 1 , . . . ,  12 -i has a corresponding base station  14 - 1 , . . . ,  14 -i for providing communication service to mobiles located therein, such as mobiles  20 - 1 , . . . ,  20 -j located in cell  12 - 1 . Each of the base stations  14 - 1 , . . . ,  14 -i is connected (e.g., via a trunk line) to a mobile telephone switching office (MTSO)  16 . The MTSO  16  manages communication within the network, and serves as an interface between the wireless network and a public switched telephone network (PSTN)  40 . 
     As will be apparent to those skilled in the art, numerous variations on the wireless network  10  illustrated in FIG. 1 are possible. For example, each of the cells  12 - 1 , . . . ,  12 -i may be divided into a number of sectors. Furthermore, although the cells  12 - 1 , . . . ,  12 -i are shown as hexagonal-shaped areas, different cell shapes are possible. 
     FIG. 2 is a general block diagram illustrating select components of a base station transmitter  100  according to one exemplary implementation of the present invention. As shown in FIG. 2, the base station transmitter  100  includes a baseband processor  110  which receives a plurality of base band communication signals input 1 , input N . These baseband communication signals input 1 , . . . , input N  may include voice/data traffic received from the MTSO  16 , as well as control information, e.g., pilot, paging, and synchronization signals, to be transmitted. For the exemplary implementation illustrated in FIG. 2, the baseband processor  110  utilizes a spectrally efficient modulation scheme, such as Quadrature Phase Shift Keying (QPSK), to output separate aggregate I- and Q-channel transmit signals. It should be realized, however, that principles of the present invention may be applied to base station transmitters which do not form separate I- and Q-channel transmit signals. 
     An I-channel multiplier  130  receives the aggregate I-channel transmit signal from the baseband processor  110 , and multiplies the received aggregate I-channel transmit signal by a scaling coefficient S M  received from an aggregate overload controller  140 . Similarly, a Q-channel multiplier  132  receives an aggregate Q-channel transmit signal output by the baseband processor  110 , and multiplies the received Q-channel transmit signal by the scaling coefficient SM received from the aggregate overload controller  140 . 
     An RF processor  160  receives the scaled aggregate I- and Q-channel transmit signals from the I-and Q-channel multipliers  130  and  132 . As described in more detail below, the RF processor  160  performs well known processing on the scaled aggregate I-and Q-channel transmit signals received from the multipliers  130  and  132 , such as digital-to-analog conversion, band pass filtering, and RF carrier signal modulation, before outputting a combined RF signal to an antenna  170 . The aggregate overload controller  140  also receives the outputs of the I- and Q-channel scaling multipliers  130  and  132  to calculate updated scaling coefficients SM in a manner described in detail below. The aggregate overload controller  140  may be implemented, for example, as an application-specific integrated circuit (ASIC) or as computer-executed software. 
     FIG. 3 is a block diagram depicting select components of an exemplary baseband processor  110  for use in the base station transmitter configuration  100  according an implementation of the present invention. As illustrated in FIG. 3, the baseband processor  110  includes a number of baseband processing units  111 - 1 , . . . ,  111 -N, respectively corresponding to input communication signals input 1 , input N . Each baseband processing unit  111 - 1 , . . . ,  111 -N outputs an I-channel signal I K1 , . . . , I KN  and a Q-channel signal Q K1 , . . . , Q KN . The baseband processor  110  further includes an I-channel summing unit  128  which generates an aggregate I-channel transmit signal from all the I-channel signals I K1 , . . . , I KN  received from the broadband processing units  111 - 1 , . . . ,  111 -N, and a Q-channel summing unit  129  for generating an aggregate Q-channel transmit signal from the Q-channel signals Q K1 , . . . , Q KN  received from the individual baseband processing units  111 - 1 , . . . ,  1   1   1 -N. 
     As will be apparent to those skilled in the art, each baseband processing unit  111 - 1 , . . . ,  111 -N includes conventional components for CDMA communication, such as specified in the CDMA-2000 Standard proposed by the U.S. Telecommunication Industry Association (TIA) to the International Telecommunications Union (ITU). Although a specific baseband processing unit configuration is shown in FIG. 3, it should be realized that principles of the present invention are not limited to a particular baseband processing configuration. 
     Referring again to the exemplary configuration of FIG. 3, each baseband processing unit  111 - 1 , . . . ,  111 -N includes a channel encoder  112 - 1 , . . . ,  112 -N, e.g., a convolutional encoder, which generates encoded blocks of predetermined length from the corresponding input communication signals input 1 , . . . , input N , to protect information bits therein with error correction codes. A first multiplier  113 - 1 , . . . ,  113 -N multiplies the encoded blocks output by the channel encoder  112 - 1 , . . . ,  112 -N with a designated PN code sequence, assigned to the mobile intended to receive the input signal, output by a PN sequence generator  114 - 1 , . . . ,  114 -N. A second multiplier  115 - 1 , . . . ,  115 -N multiplies the output of the first multiplier  113 - 1 , . . . ,  113 -N by a Walsh code sequence, for example containing values from a row of a Walsh function matrix, generated by a Walsh sequence generator  116 - 1 , . . . ,  116 -N. As is well known, combining a communication signal with an orthogonal Walsh code sequence spreads the input data signal over the bandwidth spectrum to prevent co-channel interference. 
     To achieve QPSK modulation, a separator unit  117 - 1 , . . . ,  117 -N divides the output of the second multiplier  115 - 1 , . . . ,  115 -N into even and odd bits. As is well known, QPSK modulation allows two information bits to be transmitted simultaneously on orthogonal carriers. A third multiplier  118 - 1 , . . .  118 -N multiplies the even bits from the separator unit  117 - 1 , . . . ,  117 -N by an I-channel PN sequence output by an I-channel PN sequence generator  119 - 1 , . . . ,  119 -N. Similarly, a fourth multiplier  120 - 1 , . . . ,  120 -N multiplies the odd numbered bits from the separator unit  117 - 1 , . . . ,  117 -N by a Q-channel PN sequence output by a Q-channel PN sequence generator  121 - 1 , . . . , 121 -N. The I- and Q-channel summation units  128  and  129  respectively receive the I- and Q-channel outputs from the individual baseband processing units  111 - 1 , . . .  111 -N to generate aggregate I- and Q-channel transmit signals I Kin  and Q Kin . 
     FIG. 4 is a flow diagram illustrating an exemplary calculation performed by the aggregate overload controller  140  to generate and update the scaling coefficient S M . As illustrated in FIG. 4, the aggregate overload controller  140  initially sets S M  equal to 1 (Step  201 ), and samples the scaled I-channel and Q-channel transmit signals I kout  and Q kout , received from the multipliers  130  and  132 , at a sampling rate t s  (Step  202 ). Next, the aggregate overload controller  140  calculates (I kout   2 +Q kout   2 ) for each sample (Step  204 ), and obtains the sum of (I kout   2 +Q kout   2 ) over a load measurement period T (e.g., 20 milliseconds) to calculate a load measurement, E M  (Step  206 ). Over this load measurement period, several thousand samples of I kout  and Q kout  may be taken. Although the calculation of Step  206  provides a suitable load measurement for controlling scaling, it should be realized that other techniques for obtaining a load measurement may be used. For example, a total Receive Signal Strength Indicator (RSSI) value at the base station, or the number of users being served by the base station, may be relied on to represent load. 
     Next, the aggregate overload controller  140  determines an updated scaling coefficient S M  by calculating: 
     
       
         S M =min{1, S M−1 +μ(E th −E M ),  (1) 
       
     
     where S M−1  is the scaling coefficient from the preceding load measurement period, E th  is a threshold level, and μ is a constant (Step  208 ). The constant μ may be set to a relatively small value, e.g., 0.01, to limit fluctuations in the scaling coefficient S M , and thereby avoid network instability. This operation is repeatedly performed to successively update the scaling factor S M . It should be recognized that equation (1) represents an exemplary calculation for updating the scaling factor S M , and may be modified in various ways without departing from the spirit and scope of the present invention. 
     FIG. 5 is a block diagram depicting select components of an exemplary RF processor  160  used in the base station transmitter  100  shown in FIG.  2 . As shown in FIG. 5, the RF processor  160  includes an I-channel digital-to-analog converter  162  and a Q-channel digital-to-analog converter  170  for respectively converting I kout  and Q kout  to analog form. I-channel and Q-channel filters  164  and  172  respectively low pass filter the analog I- an Q-channel signals received from the digital-to-analog converters  162  and  170 . A first multiplier  166  multiplies the I-channel signal output by filter  164  with an I-channel RF carrier signal Cos(ωt), and a second multiplier  174  multiplies the Q-channel signal outputted by filter  172  with a Q-channel RF carrier signal Sin((ωt). A combiner  178  combines the RF signals output by the first and second multipliers  166  and  176 , and outputs a composite RF transmit signal to the antenna  170  for transmission. 
     By scaling I-and Q-channel transmit signals handoff control values measured at the mobiles, such as receive signal strength from the base station, bit/frame error rate, and signal-to-noise ratio will be affected to alter the cell/ sector boundaries under high load conditions. Depending on the location of mobiles relative to cell/sector boundaries and the degree of scaling, a percentage of mobiles will request handoff to adjacent cells/sectors, thereby balancing load to improve network capacity and avoid overload. Furthermore, by using a relatively small constant μ, fluctuations in the scaling factor SK are limited to avoid network instability. 
     Although the present invention has been described in considerable detail with reference to certain embodiments, it should be apparent to those skilled in the art that various modifications and applications of the present invention may be realized without departing from the spirit and scope of the invention. For example, although the implementation illustrated in FIG. 2 scales Q- and I-channel transmit signals before such signals reach the RF processor  100 , scaling may alternatively be performed as part of RF processing, e.g., after digital-to-analog conversion.