Abstract:
In a wireless communication system, a chip time is selected in a complex pseudonoise (PN) sequence generator. For a next chip time following the selected chip time, a phase difference between a previous complex PN chip and a next complex PN chip is restricted to a preselected phase angle. In one embodiment, every other chip time is selected and the preselected angle is 90 degrees.

Description:
FIELD OF THE INVENTION 
     The present invention is related in general to wireless communication systems, and more particularly to a method and system for processing code division multiple access signals with a complex pseudonoise sequence. 
     BACKGROUND OF THE INVENTION 
     In power amplifiers used to transmit modulated radio frequency signals, is desirable to operate with an input signal having a low peak-to-average ratio. Signals with high peak-to-average ratios are undesirable because the power amplifier produces extraneous side bands when a peaking signal causes it to operated in a nonlinear portion of its operating range. These extraneous side bands are produced by a mechanisms called AM-to-PM conversion and AM-to-AM conversion when passing a signal with large amplitude fluctuations. Furthermore, these side bands deprive the information signals of some of their portion of the transponder power, and also can interfere with nearby channels (adjacent channel interference). 
     In a communications system using quartenary phase shift keying (QPSK) the signal phase can be any of one of four phases for the duration of each phase shift interval. This is shown in the signal space diagram in FIG. 1, wherein phase  30  illustrates the phase of constellation point  32 , which is one of the constellation points  32 - 38 . Transitions  40 - 46  illustrate the permitted phase changes between phase shift intervals. A zero degree transition is shown at reference numeral  40 . Examples of π/2 radians or 90° transitions are shown at reference numerals  42  and  44 , and a 180° or π radian transition is shown at reference numeral  46 . 
     In a code division multiple access (CDMA) system, such as a CDMA system implemented according to American National Standards Institute (ANSI) J-STD-008, user data is spread and modulated by a pseudorandom noise (PN) sequence, which is periodic and has noise-like properties. For example, with reference to FIG. 2, in direct sequence QPSK transmitter  60 , real-valued user data  62  is split and multiplied by 2 PN sequences: a PN I  sequence  64  and a PN Q  sequence  66 , using multipliers  68  and  70 , respectively. The PN sequences are generated by PN I  and PN Q  sequence generators  72  and  74 , respectively. The duration of the output of these PN sequence generators may be referred to as a chip time or chip interval, which is the duration of a single pulse in a direct sequence modulated signal. 
     After in-phase (I) and quadrature (Q) components of user data  62  have been multiplied by PN I  sequence  64  and PN Q  sequence  66 , the signals output by multipliers  68  and  70  are each separately filtered by pulse shaping filters  76 . Pulse shaping filters  76  may be implemented with finite impulse response filters that filter higher frequency components from the signal. 
     Next, the filtered I and Q signal components are multiplied by quadrature carrier components  78  and  80  using multipliers  82  to produce I and Q radio frequency (RF) signals  84  and  86 . Signals  84  and  86  are then added together in summer  88 . The output of summer  88  is RF modulated signal  90 , which is then amplified by power amplifier  92 . The output of power amplifier  92  is then coupled to antenna  94  for transmitting the signal to a receiving unit. 
     As shown in FIG. 2, PN sequence generators  72  and  74  are typically implemented with a maximal-length linear feedback N-bit shift register, wherein selected stages are tapped and exclusive ORed with the shift register output to form a signal that is fedback to the shift register input. Other ways of implementing PN sequence generators may be used. For example, nonlinear feedback shift registers may be used to generate the PN sequences. 
     A combination of the outputs of PN I  and PN Q  generators  72  and  74  may be referred to as having a complex value that corresponds to a phase. For example, referring again to FIG. 1, if PN I  equals 1 and PN Q  equals 1 the complex PN value of (1, 1) corresponds to phase  30 , which is π/4 radians. Other values output by the complex PN generator correspond to constellation points  34 - 38 . Transitions  40 - 46  from one constellation point to another are determined by the difference between a previous complex PN chip and a next complex PN chip generated by the complex PN sequence generator in the next chip time. 
     When RF modulated signal  90  peaks and causes power amplifier  92  to operate in a non-linear region, extraneous side bands are created in the transmitted signal. These side band signals may be eliminated by reducing the occurrence of peaks in RF modulated signal  90 , hence the desirability of reducing the peak-to-average ratio. 
     Peaks in RF modulated signal  90  occur as a result of receiving a sequence of chip values in pulse shaping filter  76  that highly correlates with the impulse response of pulse shaping filter  76 . Furthermore, the peaking of signal  90  is greater when peaks are formed in pulse shaping filters  76  in both the I and Q channels at the same time. 
     In the prior art, π/2 BPSK modulation has been used to reduce the peak-to-average in signals sent to the power amplifier. However, π/2 BPSK modulation produces BPSK spreading, which is inferior because signals from other users are not easily rejected. 
     QPSK spreading, on the other hand, provides superior rejection between user&#39;s signals, but produces a signal with an inferior peak-to-average ratio. For a more detailed discussion regarding spreading methods, see the book “CDMA, Principles of Spread Spectrum Communications,” by Andrew J. Viterbi, published by Addison Wesley in 1995, pages 26-32. 
     Thus, it should be apparent that a need exists for an improved method and system for generating a complex pseudonoise sequence for processing a code division multiple access signal wherein the complex pseudonoise sequence helps reduce the peak-to-average ratio of a modulated communications signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts a QPSK signal space diagram according to the according to the prior art; 
     FIG. 2 is a direct sequence spread spectrum modulator in accordance with the method and system of the prior art; 
     FIG. 3 is a direct sequence spread spectrum modulator incorporating a method and system for generating a complex pseudonoise sequence in accordance with the method and system of the present invention; and 
     FIG. 4 is a high-level logic flowchart which illustrates the method and system of generating a complex pseudonoise sequence according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to the figures, and in particular with reference to FIG. 3, there is depicted a direct sequence spread spectrum transmitter incorporating the method and system for generating a complex pseudonoise sequence in accordance with the method and system of the present invention. As illustrated, direct sequence QPSK transmitter  110  receives real-valued user data  62 , which is split and multiplied by two PN sequences generated according to the present invention. While generation of PN I  and PN Q  sequences  112  and  114  is new according to the present invention, many remaining portions of the transmitter operate in the manner discussed above. For example, multipliers  68  and  70  operate in much the same way as described with reference to FIG.  2 . Similarly, pulse shaping filters  76  are used to filter high frequencies components from signals output from multipliers  68  and  70 . The I and Q signals are then modulated in multipliers  82  by quadrature carrier components  78  and  80 . I and Q RF signals  116  and  118  are added together in summer  88  to produce RF modulated signal  120 , which is then amplified by power amplifier  92  and coupled to antenna  94  for transmitting the signal to a receiving unit. Note that signals  116 ,  118 , and  120  are new because they are modified according to the present invention using the new complex PN sequence. 
     In a preferred embodiment, the generation of the improved complex pseudonoise sequence begins with C 1  and C 2  sequence generators  130  and  132 , which may be implemented in much the same manner as PN I  and PN Q  sequence generators  72  and  74  shown in FIG.  2 . The outputs of sequence generators  130  and  132  have values C 1  and C 2  during any given chip time. Signals C 1  and C 2  are both coupled to last phase register  134 , and inputs to one side of multiplexer  136 . Last phase register  134  converts the values of C 1  and C 2  into a phase angle and stores such a phase angle for one chip time. 
     Last phase information output from last phase register  134  is coupled to phase adjuster  138 , which also receives the current chip value of C 1  from PN I  sequence generator  130 . As shown in FIG. 4, phase adjuster  138  is a ±90 degree phase adjuster wherein the determination of whether to add or subtract 90 degrees depends upon the current value of C 1 . In one implementation of phase adjuster  138 , the sign of either C 1  or C 2 , which are inferred from the phase input from last phase register  134 , is changed depending upon whether the current value of C 1  is a +1 or a −1. Phase adding or phase subtracting in phase adjuster  138  may be controlled according to any sequence that may be determined or preset in the receiver. 
     The outputs of phase adjuster  138 , PN I  and PN Q , are coupled to inputs of multiplexer  136 , as shown. 
     The values output from multiplexer  136  are selected from the pairs of inputs based upon a signal from chip selector  140 . Chip selector  140  is clocked by a clock signal that is common to both PN I  sequence generator  130  and PN Q  sequence generator  132 , wherein the period of the clock is a chip time. In a preferred embodiment, chip selector  140  causes multiplexer  136  to select the output of phase adjuster  138  during every other chip time. When the output of phase adjuster  138  is not selected, the unmodified, current values of C 1  and C 2  are output from multiplexer  136 . Thus, in the preferred embodiment, at every other chip time, the phase of the next complex PN chip differs from the phase of the previous complex PN chip by 90 degrees. 
     PN I  and PN Q  sequences  112  and  114 , which are the outputs of multiplexer  136 , are coupled to multipliers  68  and  70 , respectively, and are thereby used to process or spread a code division multiple access signal that carries user data  62 . 
     With reference now to FIG. 4, there is depicted a high-level logic flowchart that illustrates the method of generating a complex pseudonoise sequence according to the present invention. As illustrated, the process begins at block  200  and thereafter passes to block  202  wherein the process stores a current PN chip phase. This may be implemented by converting the current values of C 1  and C 2  to a phase, wherein C 1  and C 2  have values of +/−1. 
     Next, the process determines whether or not a phase change for a next chip should be restricted to a predetermined angle, as depicted at block  204 . If the next chip is not selected as a chip for which the phase change will be restricted, the process reads C 1  and C 2  from the outputs of the complex PN sequence generator, as illustrated at block  206 . The process then equates PN I  with C 1  and PN Q  with C 2 , as illustrated at block  208 . Finally, the process outputs the PN I  and PN Q  values, as depicted at block  210 . Because the process had not selected this chip time to restrict the phase change of the next PN chip, the PN I  and PN Q  values are output as the next PN chip without modification. 
     With reference again to block  204 , if the next chip is selected for restricting the phase change, the process recalls the last PN chip phase, as illustrated at block  212 . Next, the process examines code C 1  and determines whether or not it is equal to 1, as depicted at block  214 . If C 1  is equal to 1, the process adds 90 degrees to the last PN chip phase to compute the next PN chip phase, as illustrated at block  216 . However, if code  1  is not equal to 1, the process subtracts 90 degrees from the last PN chip phase to compute the next PN chip phase, as depicted at block  218 . 
     After adding or subtracting 90 degrees from the last PN chip phase to compute the next PN chip phase, the process converts the next PN chip phase to PN I  and PN Q  values, as illustrated at block  220 . Thereafter, the PN I  and PN Q  values are output, as depicted at block  210 . The process then iteratively returns to block  202 , wherein the current PN chip phase is stored. 
     While the present invention generates a complex PN sequence used to process or spread a CDMA signal in a transmitter, this method and system for generating the complex PN sequence must also be used in a receiving unit to process or despread the received CDMA signal. Therefore, those persons skilled in the art should recognize that CDMA receivers must also practice the method and system of the present invention. 
     The present invention has been described in reference to a system that transmits real user data  62 . Persons skilled in the art should recognize that user data may be complex data and that multipliers  68  and  70  may be implemented in a complex manner. 
     Those persons skilled in the art should recognize that the spreading scheme that uses the complex PN generator of the present invention is neither a QPSK spreading scheme nor a π/2 BPSK spreading scheme; the spreading scheme produced by using the present invention is a hybrid wherein selected chip times behave like a π/2 BPSK spreading scheme and the remaining chip times behave like a QPSK spreading scheme. This hybrid spreading scheme avoids the low interference rejection of the π/2 BPSK spreading and avoids the high peak-to-average ratio of the QPSK spreading. 
     The foregoing description of a preferred embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.