Abstract:
A CDMA transmitter that limits the peak-to-average ratio to a selected maximum level. The CDMA transmitter comprises: 1) a first baseband combiner for receiving N baseband chip streams, each of the N baseband chip streams comprising a sequence of chips, each chip having one of a positive amplitude value and a negative amplitude value, wherein the first baseband combiner combines chips from corresponding time slots in each of the N baseband chip streams to thereby generate a first composite baseband chip sequence; 2) a data processor for detecting a first peak amplitude in the first composite baseband chip sequence that exceeds a pre-determined maximum threshold and determining an amplitude and a polarity of a first compensation pulse associated with the first peak amplitude; and 3) a pulse generator for generating the first compensation pulse having the amplitude and the polarity determined by the data processor. The first baseband combiner then combines the first compensation pulse with the first composite baseband chip sequence during a chip time slot corresponding to the first peak amplitude.

Description:
The present invention claims priority to U.S. Provisional Application Ser. No. 60/273,272 filed Mar. 2, 2001. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention is directed, in general, to code division multiple access (CDMA) wireless communication devices and, more specifically, to an apparatus for controlling the peak-to-average power ratio of the input signal of a CDMA transmitter. 
   BACKGROUND OF THE INVENTION 
   Wireless communication systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the end-user&#39;s total cost. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service in order to make wireless services cheaper and better. 
   To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination of these factors. 
   CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations and other access terminals (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data on the same frequency in assigned channels that correspond to specific unique orthogonal codes. For example, a mobile station may receive multiple forward channel data signals from a base station. Each forward channel signal transmitted by the base station is formatted, repeated, encoded, formatted, interleaved, spread with a Walsh code assigned to that channel, and separated into an in-phase (I) digital stream and a quadrature (Q) digital stream. The I-digital stream is further spread by an in-phase pseudo-noise (PN) code. The Q-digital stream is spread by a quadrature pseudo-noise code. The PN-modulated I-digital stream and the PN modulated Q-digital stream are filtered by a finite impulse response (FIR) filter and then used for subsequent quadrature phase-shift keying (QPSK) modulation of a radio frequency (RF) carrier signal. 
   In another embodiment, a base station may receive reverse channel data signals from mobile stations that are formatted, repeated, encoded, block-interleaved, and spread prior to transmission by the mobile station with a pseudo-noise code based on the mobile station identification number. In another embidiment, the data signals may be encoded with 64-ary or M-ary modulation prior to block interleaving. Those skilled in the art will recognize that a mobile station may employ quadrature phase shift keying (QPSK) modulation, binary phase shift keying (BPSK) modulation, quadrature amplitude modulation (QAM) or other digital modulation format for modulation of a radio frequency (RF) carrier for transmission of the data signals. One such implementation, is found in the TIA/EIA-95 CDMA standard (also known as IS-95). Another implementation is the TIA/EIA-2000 standard (also known as IS-2000). 
   Power amplifiers for amplification of CDMA signals have traditionally been some of the most expensive components of a wireless communication system and have often resisted attempts aimed at lowering their cost. A key design specification for power amplifiers used for amplification of CDMA signals used in cellular radio systems is limitation on adjacent channel power (ACP). ACP is a result of signal distortion caused by the non-linearity of the devices used in power amplifiers that produces undesired spectral components in adjacent transmission channels. Usually, devices are more linear with lower input signal levels. That is, for a given device, signals with less strength experience less distortion and, thus, have lower ACP. A transmitter (or power amplifier) may need to operate in back-off mode (reduced input signal level) to reduce signal distortion in order to meet the ACP requirements. However, the required amplifier back-off to accommodate large peak power signals leads to higher cost, inefficient operation and excessive heat dissipation—all of which combine to raise system cost significantly. It has therefore been the goal of designers to reduce peak-to-average power ratios as much as possible without degrading other performance parameters. 
   For example, U.S. Pat. No. 5,930,299 discloses a digital modulator with compensation that reduces the peak-to-average power ratio of a modulated signal to allow an increase in power amplifier efficiency. In the digital modulator with compensation, a digital bit stream is sent to an encoder which translates bit sequences into I and Q digital pulses. These target symbol sequences have been predetermined to cause excessive amplitude peaking in the modulated signal. If a target symbol sequence is encountered, it is adjusted using amplitude or filter compensation. The compensation is implemented by adjusting the coefficients of I and Q finite impulse response (FIR) pulse-shaping filters for one or more symbols in each target symbol sequence. Non-target symbol sequences in the digital bit stream are largely unaffected. The I and Q FIR pulse-shaping filters are followed by I and Q digital-to-analog converter (DAC) and reconstruction filters. Together, the filters shape the I and Q pulses according to communication system specifications. The filtered I and Q signals are then sent to a quadrature modulator for RF modulation, amplified, and transmitted over a communication channel. Howver, the device disclosed in U.S. Pat. No. 5,930,299 requires repeated adjustment of the I and Q FIR pulse-shaping filter coefficients, which is processor intensive and which may not yield adequate filtering of ACP for all combinations of Walsh or spreading codes. 
   U.S. Pat. No. 5,838,732 discloses a wideband digital combiner that generates a composite signal as a frequency multiplexed combination of many narrowband modulated digital carrier signals. The technique used by the digital combiner involves introducing predetermined phase shifts into each of the digital channel signals after a baseband modulation step. The wideband composite signal thus exhibits a reduced peak-to-average signal power, despite the fact that the phases of the digital carrier signals cannot be directly controlled. This permits the use of a power amplifier, which may have a much smaller peak-to-average rating. However, those skilled in the art will recognize that the phase shifting of the digital channel signals degrades the detection and demodulation of the QPSK signals at the receiver by introducing phase jitter or zero-crossing jitter. Furthermore, it decreases the auto-correlation magnitude of desired signals and increasing the cross-correlation magnitude of undesired signals thereby increasing the level of interference. 
   U.S. Pat. No. 5,606,578 discloses a communication device in which a digital processor responds to an information generator by alternately mapping the digital information onto a first or a second constellation diagram to produce data symbols. The processor processes the data symbols alternately from the first and second constellation diagrams in order to minimize the peak-to-average power ratio at the amplifier. This alternating scheme allows the power amplifier to operate more efficiently. However, this mapping to different constellations to produce data symbols changes the Walsh modulation sequence at the receiving station and thereby removes the unique identification for the traffic channel. 
   Therefore, there is a need in the art for improved CDMA-based wireless devices that minimize peak-to-average power ratio without suffering the other performance degradations associated with the prior art systems. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and method for reducing the peak-to-average ratio, or crest factor, of the transmitted waveform for direct sequence spread spectrum (DSSS) code division multiple access (CDMA) signals. Reduction of the peak-to-average ratio (i.e., crest factor) allows improved power amplifier efficiency, reduced heat dissipation requirements, smaller heat sinks, lower system size and weight, reduced power supply load, and lower cost for CDMA cellular base stations. 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a CDMA transmitter that limits the peak-to-average ratio to a selected maximum level. According to an advantageous embodiment of the present invention, the CDMA transmitter comprises: 1) a first baseband combiner capable of receiving N baseband chip streams, each of the N baseband chip streams comprising a sequence of chips, each chip having one of a positive amplitude value and a negative amplitude value, wherein the first baseband combiner combines chips from corresponding time slots in each of the N baseband chip streams to thereby generate a first composite baseband chip sequence; 2) a data processor coupled to the first baseband combiner capable of detecting a first peak amplitude in the first composite baseband chip sequence that exceeds a pre-determined maximum threshold and determining an amplitude and a polarity of a first compensation pulse associated with the first peak amplitude; and 3) a pulse generator coupled to the data processor capable of generating the first compensation pulse having the amplitude and the polarity determined by the data processor, wherein the first baseband combiner receives the first compensation pulse and combines the first compensation pulse with the first composite baseband chip sequence during a chip time slot corresponding to the first peak amplitude. 
   According to one embodiment of the present invention, the data processor determines the amplitude and the polarity of the first compensation pulse as a function of a maximum peak-to-average power ratio associated with the first composite baseband chip sequence. 
   According to another embodiment of the present invention, each chip in each of the N baseband chip streams has one of a +1 relative amplitude value and −1 relative amplitude value. 
   According to still another embodiment of the present invention, the CDMA transmitter further comprises: 4) a second baseband combiner capable of receiving M baseband chip streams, each of the M baseband chip streams comprising a sequence of chips, each chip having one of a positive amplitude value and a negative amplitude value, wherein the second baseband combiner combines chips from corresponding time slots in each of the M baseband chip streams to thereby generate a second composite baseband chip sequence, wherein the data processor coupled to the second baseband combiner and is further capable of detecting a second peak amplitude in the composite baseband chip sequence that exceeds a pre-determined maximum threshold and determining an amplitude and a polarity of a second compensation pulse associated with the second peak amplitude; and 5) a second pulse generator coupled to the data processor capable of generating the second compensation pulse having the amplitude and the polarity determined by the data processor, wherein the second baseband combiner receives the second compensation pulse and combines the second compensation pulse with the second composite baseband chip sequence during a chip time slot corresponding to the second peak amplitude. 
   According to yet another embodiment of the present invention, the data processor determines the amplitude and the polarity of the second compensation pulse as a function of a maximum peak-to-average power ratio associated with the second composite baseband chip sequence. 
   According to a further embodiment of the present invention, the chip in each of the M baseband chip streams has one of a +1 relative amplitude value and −1 relative amplitude value. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
       FIG. 1  illustrates an exemplary wireless network, in which base stations implement power amplifiers according to the principles of the present invention; 
       FIG. 2A  illustrates selected portions of the transmitter circuitry that generate in-phase digital streams and quadrature digital streams according to one embodiment of the present invention; 
       FIG. 2B  illustrates selected portions of the transmitter circuitry in a base station for processing of in-phase digital streams and processing of quadrature digital streams for quadrature phase shift keying modulation (QPSK) of an RF carrier according to one embodiment of the present invention; 
       FIG. 3A  illustrates a timing diagram of an exemplary series of sequential combined chips that form an exemplary composite I chip sequence in the in-phase baseband combiner; 
       FIG. 3B  illustrates a timing diagram of an exemplary series of sequential combined chips that form an exemplary composite Q chip sequence in the quadrature baseband combiner; 
       FIG. 4  illustrates a QPSK power envelope of a modulated carrier in which the chip sequences in  FIGS. 3A and 3B  are used to modulate the carrier signal; 
       FIG. 5A  is a timing diagram of a compensation pulse of amplitude −2 that is synchronously combined with the composite I chip sequence in  FIG. 3A  during chip time slot  5 ; 
       FIG. 5B  is a timing diagram of a compensation pulse of amplitude +2 that synchronously combined with the composite Q chip sequence in  FIG. 3B  during chip time slot  23 ; and 
       FIG. 6  is a flow diagram illustrating the operation of the peak-to-average power ratio limiting circuitry in the transmit path of an exemplary base station. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 6 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged code division multiple access (CDMA) transmitter. 
     FIG. 1  illustrates exemplary wireless network  100 , in which base stations implement power amplifiers according to the principles of the present invention. Wireless network  100  comprises a plurality of cell sites  121 – 123 , each containing one of the base stations, BS  101 , BS  102 , or BS  103 . Base stations  101 – 103  are operable to communicate with a plurality of mobile stations (MS)  111 – 114  over CDMA channels. Mobile stations  111 – 114  may be any suitable cellular devices, including conventional cellular telephones, PCS handset devices, portable computers, metering devices, and the like. Other types of access terminals, including fixed access terminals, also may be present in wireless network  100 . However, for the sake of simplicity, only mobile stations are shown. 
   Dotted lines show the approximate boundaries of the cell sites  121 – 123  in which base stations  101 – 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions. 
   In one embodiment of the present invention, BS  101 , BS  102 , and BS  103  may comprise a base station controller (BSC) and a base transceiver station (BTS). Base station controllers and base transceiver stations are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver station, for specified cells within a wireless communications network. A base transceiver station comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces, and RF transmitters and RF receivers. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver station in each of cells  121 ,  122 , and  123  and the base station controller associated with each base transceiver station are collectively represented by BS  101 , BS  102  and BS  103 , respectively. 
   BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public telephone system (not shown) via communications line  131  and mobile switching center (MSC)  140 . Communications line  131  may be any suitable connection means, including a T 1  line, a T 3  line, a fiber optic link, a network backbone connection, and the like. Mobile switching center  140  is well known to those skilled in the art. Mobile switching center  140  is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the public telephone system. In some embodiments of the present invention, communications line  131  may be several different data links, where each data link couples one of BS  101 , BS  102 , or BS  103  to MSC  140 . 
   In one advantageous embodiment, BS  101 , BS  102  and BS  103  transfer data signals between each other and the Internet or other packet data network (not shown) via communications line  145  and data core network (DCN) server  150 . Data core network (DCN) server  150  is well known to those skilled in the art. Data core network (DCN) server  150  is a packet data switching or routing device that provides services and coordination between the subscribers in a wireless network and external packet data networks, such as a corporate Ethernet system and/or the Internet. Communications line  145  may be any suitable connection line, including an Ethernet link, a T 1  connection, a T 3  line, a fiber optic link, a network backbone connection, and the like. In some embodiments, communications line  145  may comprise several different data links, where each data link couples one of BS  101 , BS  102 , or BS  103  to data core network (DCN)  150 . 
   In the exemplary wireless network  100 , MS  111  is located in cell site  121  and is in communication with BS  101 , MS  113  is located in cell site  122  and is in communication with BS  102 , and MS  114  is located in cell site  123  and is in communication with BS  103 . The MS  112  is also located in cell site  121 , close to the edge of cell site  123 . The direction arrow proximate MS  112  indicates the movement of MS  112  towards cell site  123 . At some point, as MS  112  moves into cell site  123  and out of cell site  121 , a handoff will occur. 
   As is well known, the handoff procedure transfers control of a call from a first cell to a second cell. As MS  112  moves from cell  121  to cell  123 , MS  112  detects the pilot signal from BS  103  and sends a Pilot Strength Measurement Message to BS  101 . When the strength of the pilot transmitted by BS  103  and received and reported by MS  112  exceeds a threshold, BS  101  initiates a soft handoff process by signaling the target BS  103  that a handoff is required as described in TIA/EIA-95 or TIA/EIA-2000. BS  103  and MS  112  proceed to negotiate establishment of a communications link in the CDMA channel as described in TIA/EIA-95 or TIA/EIA-2000. Following establishment of the communications link between BS  103  and MS  112 , MS  112  communicates with both BS  101  and BS  103  in a soft handoff mode. Those acquainted with the art will recognize that soft handoff improves the performance on both forward and reverse links. When the signal from BS  101  falls below a threshold, MS  112  may then drop the link with BS  101  and only receive signals from BS  103 . MS  112  then transfers on-going voice, data, or control signals through BS  103 . The call is thereby seamlessly transferred from BS  101  to BS  103 . An idle handoff is a handoff between cells of a mobile device that is communicating in the control or paging channel, rather than transmitting voice and/or data signals in the regular traffic channels. 
     FIG. 2A  illustrates selected portions of the transmitter circuitry for CDMA quadrature generator  200  of each CDMA channel used for quadrature phase shift keying (QPSK) modulation in base station  101  according to one embodiment of the present invention. Each input forward channel signal  205  to quadrature generator  200  and transmitted by the base station is convolutionally encoded, formatted, interleaved and spread with a Walsh code assigned to that channel according to one embodiment as described by TIA/EIA-95 or TIA/EIA-2000. Those skilled in the art will recognize that the forward traffic channel is comprised of the pilot channel, the synchronization channel, one or more paging channels, and one or more traffic channels. Demultiplexer  210  separates the digital components of each input forward channel signal  205  into an in-phase I-Branch component  215 A and a quadrature-phase Q-Branch component  215 B. Those skilled in the art will recognize that the mapping of the input digital signal into quadrature components may occur by duplicating each digital bit on I and Q branches  215 A and  215 B, respectively, or by alternating each digital bit on the I-Branch  215 A and the Q-Branch  215 B, respectively. Pseudo-noise encoder  225  spreads the I-Branch digital stream with pilot in-phase pseudo-noise code (I-PN) according to TIA/EIA-95 or TIA/EIA-2000 in one embodiment of the invention to produce a PN-coded, in-phase component  230 A. Pseudo-noise encoder  225  spreads the Q-Branch digital stream with pilot quadrature-phase pseudo-noise code (Q-PN) according to TIA/EIA-95 or TIA/EIA-2000 in one embodiment of the invention to produce a PN-coded, quadrature-phase component  230 B. Those skilled in the art will also recognize that the signal amplitude for each signal channel in the forward traffic channel may have a different amplitude. 
     FIG. 2B  illustrates selected portions of the transmitter circuitry in base station  101  according to one embodiment of the present invention. Base station  101  contains quadrature phase-shift keying (QPSK) circuitry that combines a plurality of in-phase (I) chip sequences (I-DATA)  230 A and a plurality of quadrature (Q) chip sequences (Q-DATA)  230 B. The I-DATA chip sequences  230 A are processed by in-phase (I) synchronous pulse generator  250 A, in-phase baseband combiner  255 A, in-phase finite impulse response (I-FIR) filter  260 A, digital-to-analog converter (DAC) and filter block  265 A. The Q-DATA chip sequences  230 B are processed by quadrature (Q) synchronous pulse generator  250 B, quadrature (Q) baseband combiner  255 B, quadrature finite impulse response (Q-FIR) filter  260 B, digital-to-analog converter (DAC) and filter block  265 B. Those skilled in the art will recognized that the transmitter circuitry in BS  101  also comprises a quadrature modulator formed by mixer  270 A, mixer  270 B and combiner  275 . The transmitter circuitry in BS 101  also comprises data processor  280 , which controls the operation of I-synchronous pulse generator  250 A and Q-synchronous pulse generator  250 B. 
   The present invention synchronously adds compensation pulses (Compensation I pulse and Compensation Q pulse) to the combined, multi-user CDMA chip sequences in such a manner that it reduces the peak-to-average ratio of the QPSK modulated signal applied to a CDMA amplifier. The technique introduces minimal degradation in the signal detection by a CDMA receiver since the de-correlation process spreads the energy in the applied pulse over all chips in the spread spectrum modulation symbol. It is well known to those skilled in the art that a direct-sequence spread spectrum (DSSS) waveform is resilient to the application of a single pulse to a sequence of chips. 
   In multi-access CDMA communication systems, a QPSK transmitted signal is generated by quadrature modulation of a carrier with I and Q chip sequences which are generated according to the user data. Because the channels may be activated independently of each other, the composite signals for the I and Q components are a sum of digital channel signals having non-deterministic phases. Thus, even when the modulation used for each channel has a constant amplitude envelope over time, the composite signal will not exhibit a constant envelope. Rather, the composite signal may be described as having uniform random phase distribution and a Rayleigh probability density amplitude envelope. The resulting Rayleigh-distributed envelope has undesirably high peak-to-average power requirements that places demanding linearity and dynamic range requirements on the power amplifier in the transmit signal path. 
   Typically, the QPSK signal streams exhibit a peak-to-average power ratio that ranges from 10 dB to 14 dB. Large peaks in the transmitted signal lead to inefficient usage of the power amplifier. This, in turn, leads to large heat dissipation. Considering the theoretical efficiency limit of a Class AB amplifier, these ratios would require the transmitter amplifier to operate at an efficiency below 15% for single-carrier amplification and below 10% for multi-carrier amplification. This compares rather poorly with the 65% efficiency that can be attained with constant envelope modulation. 
   In  FIG. 2B , the transmitter circuitry combines the multiple bipolar I and Q signals from the transmit segments of multiple channel elements in a multiaccess digital communications system. In-phase baseband combiner  255 A clocks in, buffers, and sums the sequentially corresponding chips from each of the I-DATA baseband chip sequences  230 A at the incoming chip rate to form a composite I chip sequence. Likewise, quadrature baseband combiner  255 B clocks in, buffers, and sums the sequentially corresponding chips from each of the Q-DATA baseband chip sequences  230 B at the incoming chip rate to form a composite Q chip sequence. 
   In-phase baseband combiner  255 A also receives a Compensation I pulse from in-phase synchronous pulse generator  250 A and combines it with the composite I chip sequence to produce a compensated I chip sequence in which the peak-to-average ratio is held at or below a pre-determined maximum threshold established by the system operator. The compensated I chip sequence is filtered by I-FIR filter  260 A and DAC and filter block  265 A and applied to one input of mixer  270  of the quadrature modulator. Similarly, quadrature baseband combiner  255 B also receives a Compensation Q pulse from quadrature synchronous pulse generator  250 B and combines it with the composite Q chip sequence to produce a compensated Q chip sequence in which the peak-to-average ratio is held at or below a pre-determined maximum threshold established by the system operator. The compensated Q chip sequence is filtered by Q-FIR filter  260 B and DAC and filter block  265 B and applied to one input of mixer  270 B of the quadrature modulator. 
   I-FIR filter  260 A and Q-FIR filter  260 B are pulse shaping filters and DAC and filter block  265 A and DAC and filter block  265 B contain reconstruction filters which shape the I and Q pulses according to communication system specifications. The filtered I and Q signals are applied to the inputs of the quadrature modulator (i.e., mixer  270 A, mixer  270 B, and combiner  275 ) for RF modulation of a carrier signal and are amplified and transmitted over a communication channel. 
   To obtain reduction of the crest factor (i.e., peak-to-average ratio), each of in-phase baseband combiner  255 A and quadrature baseband combiner  255 B reports the uncompensated composite (or combined) chip amplitude to data processor  280 . If the composite chip amplitude exceeds a specified value corresponding to the predetermined maximum threshold of the crest factor, data processor  280  determines the amplitude and polarity of a compensation pulse which, when sequentially combined with the corresponding I or Q composite chip sequences, produces a combined signal that is within allowed limits. The synchronously added compensation pulses have the same pulse width as the CDMA chip. The resulting compensated I and Q baseband chip streams are then filtered and applied to the quadrature modulator. 
   Consider a set of twelve CDMA direct sequence chips of unity relative amplitude (+1 or −1) as shown in Table 1 below that are applied as the I-DATA chip sequences in  FIG. 2 . 
   
     
       
             
             
           
             
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Chip Sequence number 
             
           
        
         
             
               Channel 
               1 
               2 
               3 
               4 
               5 
               . . . 
               24 
               25 
               . . . 
             
             
                 
             
           
        
         
             
               1 
               −1 
               −1 
               −1 
               +1 
               −1 
               . . . 
               +1 
               −1 
               . . . 
             
             
               2 
               +1 
               −1 
               +1 
               +1 
               +1 
               . . . 
               −1 
               +1 
               . . . 
             
             
               3 
               −1 
               −1 
               −1 
               +1 
               +1 
               . . . 
               −1 
               +1 
               . . . 
             
             
               4 
               −1 
               −1 
               +1 
               −1 
               +1 
               . . . 
               −1 
               −1 
               . . . 
             
             
               5 
               +1 
               +1 
               −1 
               +1 
               +1 
               . . . 
               −1 
               −l 
               . . . 
             
             
               6 
               +1 
               −1 
               +1 
               +1 
               +1 
               . . . 
               −1 
               +1 
               . . . 
             
             
               7 
               −1 
               +1 
               +1 
               +1 
               +1 
               . . . 
               +1 
               −1 
               . . . 
             
             
               8 
               +1 
               +1 
               +1 
               +1 
               +1 
               . . . 
               −1 
               +1 
               . . . 
             
             
               9 
               −1 
               −1 
               +1 
               −1 
               +1 
               . . . 
               −1 
               −1 
               . . . 
             
             
               10  
               +1 
               −1 
               +1 
               +1 
               +1 
               . . . 
               −1 
               −1 
               . . . 
             
             
               11  
               −1 
               +1 
               +1 
               −1 
               +1 
               . . . 
               +1 
               −1 
               . . . 
             
             
               12  
               −1 
               −1 
               −1 
               +1 
               −1 
               . . . 
               −1 
               +1 
               . . . 
             
             
               Comp. I: 
               −2 
               −4 
               +4 
               +6 
               +10 
               . . . 
               −6 
               −2 
               . . . 
             
             
                 
             
           
        
       
     
   
   Note that while the actual chip sequence for each channel may have a different amplitude, an example with unity amplitude chip sequences does not change the basic concept or operation of the present invention. The combining process produces the composite I signal chip sequence in the last row of Table 1. 
     FIG. 3A  illustrates a timing diagram of an exemplary series of sequential combined chips that form an exemplary composite I chip sequence in in-phase baseband combiner  255 A. Exemplary combined chips  1 ,  5 ,  10 ,  15 ,  20  and  25  are labeled. A signal maximum occurs at chip time slot  5 . Similarly,  FIG. 3B  illustrates a timing diagram of an exemplary series of sequential combined chips that form an exemplary composite Q chip sequence in quadrature baseband combiner  255 B. Exemplary combined chips  1 ,  5 ,  10 ,  15 ,  20  and  25  are labeled. A signal maximum occurs at chip time slot  23 .  FIG. 4  illustrates QPSK power envelope  400  of a modulated carrier in which the chip sequences in  FIGS. 3A and 3B  are used to modulate the carrier signal. The corresponding peak-to-average ratio for this signal is 12.4 dB. 
     FIG. 5A  illustrates timing diagram  500 , in which a compensation pulse of amplitude −2 is synchronously combined with the composite I chip sequence in  FIG. 3A  during chip time slot  5 .  FIG. 5B  illustrates timing diagram  550 , in which a compensation pulse of amplitude +2 is synchronously combined with the composite Q chip sequence in  FIG. 3B  during chip time slot  23 . The compensation pulses in  FIGS. 5A and 5B  keep each combined chip amplitude below a relative threshold amplitude of 8. The calculated peak-to-average is 8.4 dB. If the pulses shown in  FIGS. 5A and 5B  are applied to the chip sequences in  FIGS. 3A and 3B , respectively, the peak-to-average ratio is reduced to 5.5 dB. 
     FIG. 6  depicts flow diagram  600 , which illustrates the operation of the peak-to-average power ratio limiting circuitry in the transmit path of exemplary base station according to one embodiment of the present invention. Initially, in-phase baseband combiner  255 A combines multiple in-phase baseband chip streams to form an uncompensated composite I chip sequence, and quadrature baseband combiner  255 B combines multiple quadrature baseband chip streams to form an uncompensated composite Q chip sequence (process step  605 ). Data processor  280  determines the peaks in the uncompensated composite I and Q chip sequences which exceed the specified maximum threshold value. Data processor  280  then calculates the amplitude and polarity of the compensation I and Q pulses, if any (process step  610 ). In-phase baseband combiner  255 A combines the compensation I pulse(s), if any, with the uncompensated composite I chip sequence during the correct chip time slot(s). Quadrature baseband combiner  255 B also combines the compensation Q pulse(s), if any, with the uncompensated composite Q chip sequence during the correct chip time slot(s) (process step  615 ). The compensated I and Q composite chip streams are filtered and applied to the quadrature modulator to form a QPSK modulated carrier signal, which is later amplified (process step  620 ). 
   The present invention requires less processing power and fewer components than the prior art devices for peak-to-average power ratio reduction. The present invention also requires less processing power and fewer components than amplifier linearization techniques for ACP reduction. Furthermore, the present invention does not rely on amplitude compensation with FIR filter techniques. 
   An alternate embodiment of the present invention operates in a closed loop form, as opposed to an open loop configuration described above. In such an embodiment, the data processor generates a spectral representation of the amplified output of the pulse compensated signal from samples taken at the amplifier output. The spectral representation of the amplifier output provides an estimate of the adjacent channel power. The level of ACP terms is used to adjust the amplitude of the pulses generated by the synchronous pulse generators in  FIG. 2B . 
   The present invention introduces the use of compensation pulse generators to reduce the peak-to-average power ratio (or crest factor) of direct sequence spread spectrum signals. Reduction of the peak-to-average ratio allows a CDMA power amplifier to operate with less back-off and, therefore, with greater efficiency. The present invention allows a wireless service provider to operate power amplifiers in wireless network base stations or access points with greater efficiency (i.e., less power). Hence, smaller amplifiers and power supplies are required for a given RF output power. In addition, heat dissipation requirements are reduced, as are the size of heat sinks and cooling requirements. 
   Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.