Abstract:
A wireless spread spectrum communication system comprises a base station and at least one subscriber unit. The base station has a plurality of modems producing at least one channel signal and a global signal. A signal combiner combines the at least one channel and global signals. A radio frequency transmitter transmits the combined signal. A global power control processor determines a desired transmit power level of the combined signal. A power controller adjusts a transmit power level of the global signal to the desired transmit power level. The at least one subscriber unit receives a channel signal of the at least one channel and global signal.

Description:
[0001]    This application is a continuation of application Ser. No. 09/665,865, filed Sep. 20, 2000, which is a continuation of application Ser. No. 09/196,808, filed Nov. 20,1998, now U.S. Pat. No. 6,181,919, which is a continuation of application Ser. No. 08/797,989, filed Feb. 12, 1997, now U.S. Pat. No. 5,842,114. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to wireless local loop and cellular communication systems. More particularly, the present invention relates to a wireless communication system which dynamically adjusts the power of signals transmitted over global channels from a base station to minimize power spillover to adjacent communication cells.  
           [0004]    2. Description of the Related Art  
           [0005]    Wireless communication systems have rapidly become a viable alternative to wired systems due to their inherent advantages. Wireless systems enable subscribers to freely move throughout the operating range of a service provider and even into the territory of other service providers while using the same communication hardware. Wireless communication systems are also utilized for applications where wired systems are impractical, and have become an economically viable alternative to replacing aging telephone lines and outdated telephone equipment.  
           [0006]    One of the drawbacks with wireless communication systems is the limited amount of available RF bandwidth. There is a constant desire to improve the efficiency of these systems in order to increase system capacity and meet the rising consumer demand. A factor that degrades the overall capacity of wireless communication systems is signal power spillover between adjacent cells or base stations. This occurs when the power of signals transmitted by a base station in a particular cell exceeds the boundary of that cell, otherwise known as the operating range. The spillover becomes interference to adjacent cells and degrades the efficiency of the system. Accordingly, minimizing spillover is one of the most important issues in wireless communications system design.  
           [0007]    Forward power control (FPC) is used to minimize spillover by adjusting the power level of signals transmitted from the base station to subscriber units on assigned channels. The FPC operates in a closed loop wherein each subscriber unit continuously measures its received signal-to-noise ratio and transmits an indication back to the base station of whether the base station should increase or decrease the transmit power to that subscriber unit. The closed loop algorithm assists in maintaining the transmit power level from the base station at a minimum acceptable level, thereby minimizing spillover to adjacent cells.  
           [0008]    FPC, however, cannot adjust the power level for global channels such as the pilot signal, broadcast channel or paging channel. Since there is no closed loop algorithm that operates on these channels, the global channel transmit power level for the worst case scenario is typically used. The power level is generally more than what is required for most subscriber units, resulting in spillover to adjacent cells.  
           [0009]    There have been prior attempts to overcome the problem of spillover. U.S. Pat. No. 5,267,262 (Wheatley, III) discloses a power control system for use with a CDMA cellular mobile telephone system including a network of base stations, each of which communicates with a plurality of subscriber units. Each base station transmits a pilot signal which is used by the mobile units to estimate the propagation loss of the pilot signals. The combined power of all base station transmitted signals as received at a mobile unit is also measured. This power level sum is used by the mobile units to reduce transmitter power to the minimum power required. Each base station measures the strength of a signal received from a mobile unit and compares this signal strength level to a desired signal strength level for that particular mobile unit. A power adjustment command is generated and sent to the mobile unit which adjusts its power accordingly. The transmit power of the base station may also be increased or decreased depending upon the average noise conditions of the cell. For example, a base station may be positioned in an unusually noisy location and may be permitted to use a higher than normal transmit power level. However, this is not performed dynamically, nor is the power correction based upon the total transmit power of the base station.  
           [0010]    Accordingly, there exists a need for an effective method for controlling the power level of global channels transmitted from a base station.  
         SUMMARY  
         [0011]    A wireless spread spectrum communication system comprises a base station and at least one subscriber unit. The base station has a plurality of modems producing at least one channel signal and a global signal. A signal combiner combines the at least one channel and global signals. A radio frequency transmitter transmits the combined signal. A global power control processor determines a desired transmit power level of the combined signal. A power controller adjusts a transmit power level of the global signal to the desired transmit power level. The at least one subscriber unit receives a channel signal of the at least one channel and global signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a communication network embodying the present invention;  
         [0013]    [0013]FIG. 2 is the propagation of signals between a base station and a plurality of subscriber units;  
         [0014]    [0014]FIG. 3 is a base station made in accordance with the present invention; and  
         [0015]    [0015]FIG. 4 is a flow diagram of the method of dynamically controlling the transmit power of global channels in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    The preferred embodiment will be described with reference to the drawing figures wherein like numerals represent like elements throughout.  
         [0017]    A communication network  10  embodying the present invention is shown in FIG. 1. The communication network  10  generally comprises one or more base stations  14 , each of which is in wireless communication with a plurality of fixed or mobile subscriber units  16 . Each subscriber unit  16  communicates with either the closest base station  14  or the base station  14  which provides the strongest communication signal. The base stations  14  also communicate with a base station controller  20  which coordinates communications among base stations  14  and between base stations  14  and the subscriber units  16 . The communication network  10  may optionally be connected to a public switched telephone network (PSTN)  22 , whereupon the base station controller  20  also coordinates communication between the base stations  14  and the PSTN  22 . Preferably, each base station  14  is coupled with the base station controller  20  via a wireless link, although a land line may also be provided. A land line is particularly applicable when a base station  14  is in close proximity to the base station controller  20 .  
         [0018]    The base station controller  20  performs several functions. Primarily, the base station controller  20  provides all of the operation, administration and maintenance (OA&amp;M) signaling associated with establishing and maintaining the communications between the subscriber units  16 , the base stations  14  and the base station controller  20 . The base station controller  20  also provides an interface between the wireless communication system  10  and the PSTN  22 . This interface includes multiplexing and demultiplexing of the communication signals that enter and exit the system  10  via the base station controller  20 . Although the wireless communication system  10  is shown as employing antennas to transmit RF signals, one skilled in the art should recognize that communications may be accomplished via microwave or satellite uplinks. Additionally, the functions of a base station  14  may be combined with the base station controller  20  to form a master base station. The physical location of the base station controller  20  is not central to the present invention.  
         [0019]    Referring to FIG. 2, the propagation of certain signals in the establishment of a communication channel  18  between a base station  14  and a plurality of subscriber units  16  is shown. Forward signals  21  are transmitted from the base station  14  to a subscriber unit  16 . Reverse signals  22  are transmitted from the subscriber unit  16  to the base station  14 . All subscriber units  16  located within the maximum operating range  30  of the cell  11  are serviced by that base station  14 .  
         [0020]    Referring to FIG. 3, a base station  100  made in accordance with the present invention is shown. The base station  100  includes an RF transmitter  102 , an antenna  104 , a baseband signal combiner  106  and a global channel power control (GCPC) algorithm processor  108 . The base station  100  also includes a plurality of modems  110 , one for each channel, for generating a plurality of assigned channels  112  and a plurality of global channels  114 . Each modem  110  includes associated code generators, spreaders and other equipment for defining a communication channel as is well known by those skilled in the art. Communications over assigned and global channels  112 ,  114  are combined by the combiner  106  and upconverted by the RF transmitter  102  for transmission. The power of each assigned channel  110  is individually controlled by the FPC. However, in accordance with the present invention, the power of the global channels  114  is simultaneously and dynamically controlled by the GCPC processor  108 .  
         [0021]    The total transmit power of all channels  112 ,  114  is measured at the RF transmitter  102  and this measurement is input into the GCPC processor  108 . As will be described in detail hereinafter, the GCPC processor  108  analyzes the total transmit power of all channels  112 ,  114  and calculates the desired transmit power level of the global channels  114 . Preferably, the power level is measured prior to outputting the RF signal to the antenna  104 . Alternatively, the power level may be: 1) measured at the combiner  106 ; 2) sampled at each assigned and global channel  112 ,  114  and summed; or 3) received as an RF signal just after transmission using a separate antenna (not shown) co-located with the base station antenna  104 . Those skilled in the art should realize that any method for monitoring the total transmit power at the base station  100  may be employed without significantly departing from the spirit and scope of the present invention.  
         [0022]    Dynamic control of the power of global channels  114  is performed by using several assumptions in analyzing the total transmit power. It is assumed that the FPC for the assigned channels  112  is working ideally and the power transmitted to each subscriber unit  16  is adjusted so that all subscriber units  16  receive their signals at a particular signal-to-noise ratio. Since changing the transmit power to a particular subscriber unit  16  affects the signal-to-noise ratio seen at other subscriber units  16 , the analysis of transmit power by FPC for each assigned channel  112  is preferably performed continuously. Alternatively, the analysis may be performed on a periodic basis, as appropriate, to adjust the power for each assigned channel  112 .  
         [0023]    Prior to the analysis of the total transmit power, several factors must be defined: γ denotes the signal-to-noise ratio required at a subscriber unit  16 , N o  the white noise power density, W the transmit bandwidth and N the processing gain. The propagation loss is such that if the transmit power is P, the power level P r  of a subscriber unit  16  located at distance r is:  
           P   r   =P *β( r )  Equation (1)  
         [0024]    Different propagation models may be utilized depending upon the size of the cell, such as a free space propagation model, a Hata model or a break-point model. Those of skill in the art should realize that any empirical or theoretical propagation model may be used in accordance with the teachings of the present invention. For example, the free space propagation model is used in small cells. In this model the propagation loss is:  
               β        (   r   )       =     α     r   2               Equation                   (   2   )                                 
 
         [0025]    where  
             α   =       λ   2         (     4      π     )     2               Equation                   (   3   )                                 
 
         [0026]    and λ is the wavelength of the carrier frequency. Accordingly, if the transmit power is P, the power seen at distance r is inversely proportional to the square of the distance. Thus, the power P r  seen at distance r is:  
               P   r     =     P   *     α     r   2                 (     from                 Equations                 1                 and                 2     )                               
 
         [0027]    When the FPC is operating on assigned channels  112 , the power transmitted P i  from the base station  100  to a subscriber  16  that is located at a distance r i  from the base station  100  is:  
               P   i     =         N     N   +   γ            a        (     r   i     )         +       γ     N   +   γ            P   T                 Equation                   (   4   )                                 
 
         [0028]    where P T  is the total transmit power and:  
               a        (     r   i     )       =       γ                   N   0        W       N                   β        (     r   i     )                   Equation                   (   5   )                                 
 
         [0029]    Since a global channel  114  must be received adequately throughout the operating range  30  of the cell  11 , the transmit power requirement PG for a global channel  114  becomes:  
               P   G     =         N     N   +   γ            a        (   R   )         +       γ     N   +   γ            P   T                 Equation                   (   6   )                                 
 
         [0030]    where R is the operating range  30  of the cell  11 . The value of a(R) can be calculated easily for any propagation model. Accordingly, P G  is a constant plus a fraction of the total transmit power P T . Since the total transmit power P T  is continuously monitored at the base station  100 , the global channel transmit power P G  is updated dynamically instead of transmitting it for the worst case, which corresponds to the maximum transmit power P T  that the base station  100  can transmit.  
         [0031]    For example, for the aforementioned free space propagation model, the propagation loss is:  
               β        (   r   )       =     α     r   2               (     from                 Equation                 2     )                               
 
         [0032]    where  
             α   =       λ   2         (     4                 π     )     2               (     from                 Equation                 3     )                               
 
         [0033]    and λ is the carrier frequency of the signal. In this model, at distance r i :  
               β        (     r   i     )       =     α     r   i   2               (     from                 Equation                 2     )                               
 
         [0034]    and  
               a        (     r   i     )       =         γ                   N   0        W       α                 N              r   i   2     .               (     from                 Equations                 2                 and                 5     )                               
 
         [0035]    Substituting R for the operating range  30  of the cell  11 :  
                 a        (   R   )       =         γ                   N   0        W       α                 N            R   2         ,           (from Equations 2 and 5)                               
 
         [0036]    we have  
               P   G     =         γ     γ   +   N                           N   0        W     α          R   2       +       γ     γ   +   N            P   T                 (from Equation 6)                               
 
         [0037]    Therefore, using the free space propagation model, the optimum global channel transmit power is given by a constant term, which is proportional to the square of the cell radius, plus a variable term which is a function of the total transmit power P T .  
         [0038]    The significance of the present invention can be further illustrated by the following numerical example. Suppose system parameters are given as:  
         [0039]    γ=4 (desired signal to noise ratio)  
         [0040]    N=130 (processing gain)  
         [0041]    W=10×106 (transmit bandwidth)  
         [0042]    N 0 =4×10 −21  (white noise density)  
         [0043]    R=30×10 3  m (30 km cell radius)  
         [0044]    λ=0.1667 m (corresponding to 1.9 GHz carrier frequency).  
         [0045]    Using the free space propagation model:  
             α   =           (   0.1667   )     2         (     4      π     )     2       =     1.76   ×     10     -   4                   (from Equation 3)                               
 
         [0046]    Therefore, when the total power P T  transmitted from the base station is 100 watts, the global channel transmit power P G  should be:  
                     P   G     =                  [     4     130   +   4       ]     ×     [       4   ×     10     -   21       ×   10   ×     10   6         1.76   ×     10     -   4           ]     ×                                  (     3   ×     10   4       )     2     +       [     4     130   +   4       ]     ×   100                   =                3                 Watts                   (from Equation 6)                               
 
         [0047]    Referring to FIG. 4, the method  200  for dynamically controlling the global channel transmit power P G  is shown. Once all of the system parameters have been defined (step  202 ) and several constants are calculated (β(R), a(R)) (step  204 ), the processor  108  then calculates A and B, which are used to determine the global channel power level P G  (step  206 ). The total transmit is power is measured at the base station  100  (step  208 ) and the desired global channel power level P G  is calculated (step  2   10 ) using the formula:  
           P   G   =A+B*P   T   Equation (7)  
         [0048]    Once the desired global channel power level PG is calculated (step  210 ), all of the global channels  114  are set to the calculated power level (step  212 ). This process is then repeated (step  214 ) to continually monitor the total transmit power at the base station  100  to dynamically control the power level of the global channels  114 .  
         [0049]    The required transmit power for a global channel  114  can change by as much as 12 dB depending on the traffic load of the cell  11 . As a result, in an application where the global channel power level P G  is set such that it is sufficient under the highest traffic load expected (i.e., worst case), the global channel transmit power level P G  will exceed the required power level necessary most of the time. The method of the present invention controls the global channel transmit power level PG optimally by reducing it when the traffic load is light and increasing it when the traffic load is high such that reliable communications are maintained at all times. In this manner, the spillover to neighboring cells is kept at minimum possible levels and overall system capacity is increased.  
         [0050]    Although the invention has been described in part by making detailed reference to certain specific embodiments, such details is intended to be instructive rather than restrictive. It will be appreciated by those skilled in the art that many variations may be made in the structure and mode of operation without departing from the spirit and scope of the invention as disclosed in the teachings herein.