Abstract:
The invention is a CDMA spectral shaping technology that attenuates the out-of-band signal power in the CDMA signal. The in-band CDMA signal is attenuated near the corner frequencies to reduce components that provide a disproportionate contribution to the out-of-band signal power. The power amplifier in the CDMA base station can then operate at higher power levels without exceeding out-of-band signal power limitations. As a result, the power amplifier operates more efficiently and extends the range or capacity of the base station. In some examples of the invention, spectral shaping digital filters are placed between the cell site modem and the digital-to-analog converter in the base station. In other examples of the invention, spectral shaping analog filters are placed between the digital-to-analog converter and the low-pass filter in the base station.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to Code Division Multiple Access (CDMA) systems. More particularly, the present invention includes, but is not limited to, a novel and improved CDMA base station that shapes the frequency spectrum of CDMA signals to reduce out-of-band signal emissions. 
     II. Description of the Related Art 
     Code Division Multiple Access (CDMA) technology is commonly used in communications systems. In a typical CDMA system, a CDMA base station transmits a CDMA signal to numerous CDMA communications devices, such as wireless telephones. The CDMA signal is comprised of numerous individual user signals. The CDMA base station generates the CDMA signal by encoding each individual user signal with a unique spreading sequence, such as a pseudo random sequence. The CDMA base station then adds the encoded user signals together to form the CDMA signal. 
     In a CDMA system, individual user signals are not separated based on frequency or time, but are spread across the entire frequency band. Each CDMA communications device derives its particular user signal based on the unique spreading sequence. Due to this combination of multiple signals encoded with random sequences, the CDMA signal has random signal peaks that cause problems when the CDMA signal is amplified. 
     The CDMA base station uses a power amplifier to amplify the CDMA signal. The power amplifier contributes unwanted noise when operated above a maximum power level. Unfortunately, the random peaks in the CDMA signal force the power amplifier to operate above this maximum power level. In contrast, the typical Frequency Modulated (FM) signal does not have random signal peaks, so the power amplifier is able to continuously operate below the maximum power level. 
     The power amplifier contributes noise in the form of signal power outside of the frequency band of the CDMA signal. This signal power is referred to as out-of-band signal power. Out-of-band signal power is a problem because it interferes with other signals in the neighboring frequency bands. These other signals are disrupted by the interference. Government agencies, such as the Federal Communications Commission in the United States, strictly regulate the interference caused by out-of-band signal power. 
     The existing solution to this problem is to operate the power amplifier in the CDMA base station below its maximum power level. This reduces the amount of out-of-band signal power caused by the random peaks in the CDMA signal. This solution is lacking because the power and range of the base station is reduced. In addition, the power amplifier may operate less efficiently below the maximum power level. 
     CDMA systems would be improved by techniques to reduce the noise contribution of the power amplifier in the base station. The noise reduction would directly increase the power and efficiency of the CDMA base station. 
     SUMMARY OF THE INVENTION 
     The above-described problem is solved with CDMA spectral shaping technology that reduces the out-of-band signal power in the CDMA signal. The in-band CDMA signal is attenuated near the corner frequencies to reduce components that provide a disproportionate contribution to the out-of-band signal power. The power amplifier in the CDMA base station can then operate at higher power levels without exceeding out-of-band signal power limitations. As a result, the power amplifier operates more efficiently and extends the range or capacity of the base station. This improvement is passed on to the wireless communications user in the form of higher quality and lower cost. 
     In some examples of the invention, the corner filters are digital elements that are placed between the cell site modem and the digital-to-analog converter in the base station. In other examples of the invention, the corner filters are analog elements that are placed between the digital-to-analog converter and the low-pass filter in the base station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
     FIG. 1 is a block diagram of a CDMA system with spectral shaping logic; 
     FIG. 2 is a graph illustrating the frequency spectrum of a CDMA signal without spectral shaping; 
     FIG. 3 is a graph illustrating the frequency spectrum of a CDMA signal with spectral shaping; 
     FIG. 4 is a graph illustrating the characteristics of the spectral shaping logic; 
     FIG. 5 is a block diagram of a CDMA system with spectral shaping logic; 
     FIG. 6 is a block diagram of a CDMA base station with spectral shaping logic; and 
     FIG. 7 is a block diagram of a CDMA base station with spectral shaping logic. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Spectral Shaping—FIGS.  1 - 3   
     FIG. 1 depicts a baseband CDMA signal  100 , a CDMA transmitter  101 , an RF CDMA signal  102 , and a CDMA receiver  103 . CDMA is a spread-spectrum communications technology. Some versions of CDMA are specified by standards, such as IS-95 approved by the Telecommunications Industry Association. The CDMA signal  100  could be any CDMA signal, such as the signal produced by a cell site modem in a CDMA base station. The CDMA receiver  103  could be any CDMA device capable of receiving a CDMA signal, such as a wireless CDMA telephone. 
     The CDMA transmitter  101  could be any CDMA transmission device that includes spectral shaping logic  116  to attenuate in-band portions of the CDMA signal  100  adjacent to the corner frequencies. The CDMA transmitter  101  typically amplifies the shaped CDMA signal and transmits the amplified CDMA signal  102  to the CDMA receiver  103 . One example of the CDMA transmitter  101  is a CDMA base station. 
     In operation, the spectral shaping logic  116  in the CDMA transmitter  101  shapes the CDMA signal  100  to form CDMA signal  102 . To shape the CDMA signal  100 , the spectral shaping logic  115  attenuates in-band portions of the signal near the corner frequencies. The attenuation reduces the out-of-band noise caused by amplification of the CDMA signal. The attenuation should not be allowed to degrade the CDMA signal  102  to an unacceptable level. The CDMA transmitter  101  transmits the shaped CDMA signal  102  to the CDMA receiver  103  over the air interface. Although the invention is depicted using an air interface, other transmission media could also be used, such as RF cable, power lines, or telephone lines. 
     FIG. 2 illustrates the frequency spectrum of a CDMA signal that has been amplified without spectral shaping. The vertical axis represents signal power, and the horizontal axis represents frequency. The desired in-band signal power is contained within the bandwidth defined by corner frequencies around a center frequency. A typical example is a 1.25 MHz bandwidth centered about a 1.96 GHz center frequency with corner frequencies at (1.96 GHz−625 KHz) and (1.96 GHz+625 KHz). The signal power drops significantly outside of the bandwidth, but some undesired out-of-band signal power is still present and is shaded on FIG.  2 . Out-of band signal power is undesirable because it represents wasted power that interferes with other signals in neighboring frequency bands. 
     A power amplifier is typically used to amplify CDMA signals. The output of the power amplifier in the time domain can be mathematically modeled as:                y        (   t   )       =         ∑     n   =   0     N                         K   n          [     x        (   t   )       ]       n       =       K   0     +       K   1          x        (   t   )         +       K   2            x   2          (   t   )         +       K   3            x   3          (   t   )                     …                 (   1   )                                
     where x(t) is the input to the power amplifier. If this model is transformed from the time domain to the frequency domain, the mathematical representation is: 
     
       
           Y ( f )= K   0   +K   1   X ( f )+ K   2 ( X ( f ) * X ( f ))+ K   3 ( X ( f )* X ( f )* X ( f ))  (2) 
       
     
     where Y(f) is the Fourier transform of y(t) and the symbol“*” denotes convolution. In the context of the invention, the even terms do not contribute significant power to the in-band signal. 
     Application of the well-known graphical technique for computing the convolution of X(f)*X(f)*X(f) reveals that in-band signal power that is input near the corner frequencies makes a disproportionate contribution to the un-wanted out-of-band signal power that is output from the power amplifier. A reduction of in-band signal power that is input near the corner frequencies causes a disproportionate reduction in the unwanted out-of-band signal power that is output by the power amplifier. The reduction of in-band signal power does degrade the CDMA signal, but the degradation is acceptable given the disproportionate reduction in the out-of-band signal power. 
     FIG. 3 illustrates the frequency spectrum of a CDMA signal that has been amplified after being shaped by spectral shaping logic  116 . The vertical axis represents signal power, and the horizontal axis represents frequency. The desired in-band signal power is contained within the bandwidth defined by corner frequencies around a center frequency. The undesired out-of-band signal power is shaded on FIG.  3 . The dashed lines on FIG. 3 represent the CDMA signal from FIG. 2 that was not shaped by spectral shaping logic  116 . The dashed lines illustrate that attenuation of the in-band signal power near the corner frequencies produces a reduction in the undesired out-of-band signal power. 
     FIG. 4 depicts characteristics of the spectral shaping logic  116 . Those skilled in the art recognize that FIG. 4 represents ideal characteristics, but will recognize how to configure spectral shaping logic  116  based on the ideal characteristics of FIG.  4 . The vertical axis represents signal strength, and the horizontal axis represents frequency. The dashed lines represent the CDMA signal before spectral shaping by the spectral shaping logic  116 . 
     The spectral shaping logic  116  could be comprised of a digital or analog band-pass filter with the following characteristics. The bandpass filter would attenuate the signal strength in the attenuation bandwidths (ABW) by attenuation (A), and pass the signal strength within the passband (PB). The attenuation bandwidths ABW are adjacent to the corner frequencies and in-band, so they are within the CDMA signal bandwidth (BW). In some embodiments, the attenuation bandwidths (ABW) could each be 4.5% of the signal bandwidth BW. Alternatively, the passband PB could be 91% of the signal bandwidth BW and centered on the center frequency. The attenuation A could be 3 decibels. Alternatively, the spectral shaping can be implemented via baseband filtering prior to up-conversion. 
     CDMA Spectral Shaping System—FIGS.  5 - 6   
     FIGS. 5-6 depict a specific example of a CDMA system that uses spectral shaping, but those skilled in the art will recognize numerous other types of CDMA systems that are applicable to the spectral shaping invention described above. FIG. 5 depicts a communications system  504  that is connected to the CDMA communications system  506 . The CDMA communications system  506  communicates with CDMA communications devices  508 . The CDMA communications system  506  is comprised of a switching center  510  and a base station  512 . The communications system  504  exchanges communications signals  505  with the switching center  510 . The switching center  510  exchanges communications signals  511  with the base station  512 . The base station  512  exchanges wireless CDMA communications signals  507  over the air interface with the CDMA communications devices  508 . 
     The communications system  504  could be any communications system capable of exchanging communications signals  505  with the CDMA communications system  506 . The communications system  504  is typically a conventional public telephone network, but could also be many other networks, such as a local area network, wide area network, or internet. 
     The switching center  510  could be any device that provides an interface between the base station  512  and the communications system  504 . Typically, numerous base stations are connected to the communications system  504  through the switching center  510 , but the number of base stations has been restricted for the purpose of clarity. 
     The base station  512  exchanges wireless CDMA signals  507  with the CDMA communications devices  508 . The base station  512  includes spectral shaping logic  516  that attenuates the in-band portion of the CDMA signal near the corner frequencies before amplification and transmission to the CDMA communications devices  508 . Typically, numerous CDMA communications devices exchange signals with the base station  512 , but the number of communications devices has been restricted for the purpose of clarity. Those skilled in the art could adapt the base station  512  from known systems, such asthe base stations provided by Qualcomnm, Inc. of San Diego, Calif. 
     The CDMA communications devices  508  exchange wireless CDMA signals  507  with the base station  512 . The typical CDMA communications device is a mobile telephone, but other CDMA communications devices are also possible, such as fixed wireless devices, data terminals, set-top boxes, or computers. 
     In operation, the CDMA communications devices  508  communicate through the CDMA communications system  506  with the communications system  504  or with each other. On the communications path from the communications system  504  to the CDMA communications devices  508 , the spectral shaping logic  516  attenuates the in-band portion of the CDMA signal near the corner frequencies. The spectral shaping allows the base station  512  to operate more efficiently and with a greater range or capacity. 
     FIG. 6 depicts the base station  512  of FIG. 5 receiving the communications signals  511  and transmitting the CDMA communications signals  507 . The base station  512  is comprised of the following elements connected in series: cell site modems  621 , spectral shaping logic  516 , digital-to-analog conversion and filter  623 , up-converter  624 , power amplifier  625 , and antenna  626 . Aside from the spectral shaping logic  516 , those skilled in the art are familiar with these elements and their operation. 
     The cell site modems  621  produce a CDMA signal comprised of quadrature signals I and Q. Quadrature CDMA signals I and Q are well-known and are the baseband signals to be transmitted using carriers of the same frequency, but in phase quadrature. In other words, the RF CDMA signal can be constructed by modulating I by cosine (2×pi×frequency×time) and by modulating Q by sine (2×pi×frequency×time). In IS-95A for example, quadrature signals carry the same data with different pseudo-random sequence codes. The cell site modems  621  may apply forward error correction coding before transferring the quadrature signals I and Q to the spectral shaping logic  516 . 
     The spectral shaping logic  516  are comprised of digital filters that attenuate the strength of the in-band quadrature signals I and Q near the corner frequencies as depicted in FIG.  4 . The spectral shaping logic  516  provides the shaped I and Q signals to the digital-to-analog conversion and filter  623 . 
     The digital-to-analog conversion and filter  623  converts the shaped I and Q signals to analog and filters out components outside of the desired bandwidth. The digital-to-analog conversion and filter  623  provides the I and Q signals to the up-converter  624 . The up-converter  624  modulates the I and Q signals with intermediate and radio frequencies to form a Radio Frequency (RF) CDMA signal. The power amplifier  625  amplifies the RF CDMA signal. Because of spectral shaping, the power amplifier  625  operates at a higher and more efficient power level without generating intolerable amounts of out-of-band signal power. The antenna  626  transmits the amplified RF CDMA signal  507 . 
     Alternative CDMA System—FIG.  7   
     FIG. 7 depicts an alternative version of the base station  512  of FIG.  5 . The base station  512  receives the communications signals  511  and transmits the CDMA communications signals  507 . The base station  512  is comprised of the following elements connected in series: cell site modems  621 , digital-to-analog conversion  722 , spectral shaping logic  516 , filter  623 , up-converter  624 , power amplifier  625 , and antenna  626 . Aside from the spectral shaping logic  516 , those skilled in the art are familiar with these elements and their operation. 
     The cell site modems  621  produce a CDMA signal comprised of quadrature signals I and Q, and provide the I and Q signals to the digital to analog conversion  722 . The digital-to-analog conversion  722  converts the I and Q signals to analog and provides the analog I and Q signals to the spectral shaping logic  516 . 
     The spectral shaping logic  516  is comprised of analog filters that attenuate the strength of the in-band quadrature signals I and Q near the corner frequencies as depicted in FIG.  4 . The spectral shaping logic  516  provide the shaped I and Q signals to the filter  623 . The filter  623  filters out components outside of the desired bandwidth. The spectral shaping logic  516  and the filter  623  are shown together because it may be desirable in this version of the invention to integrate the spectral shaping logic  516  and the filter  623  into a single analog filter component combining the characteristics of the two. The filter  623  provides the I and Q signals to the up-converter  624 . 
     The up-converter  624  modulates the I and Q signals with intermediate and radio frequencies to form a Radio Frequency (RF) CDMA signal. The power amplifier  625  amplifies the RF CDMA signal. Because of special shaping, the power amplifier  625  operates at a higher and more efficient power level without generating intolerable amounts of out-of-band signal power. The antenna  626  transmits the amplified RF CDMA signal  507 . 
     The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.