Abstract:
A hybrid spread spectrum system includes a signal combiner for (a) receiving a plurality of spread spectrum encoded data signals and (b) forming a plurality of combined signals. A first set of mixers is included for (a) receiving the plurality of combined signals and at least two carriers, and (b) forming at least a two-carrier, constant envelope signal. A second set of mixers is also included for (a) receiving the at least two-carrier, constant envelope signal and a transmitter hopping signal and (b) forming a hybrid spread spectrum signal. The combined signals include (a) [s 1 (t)−s 1 (t)s 2 (t)s 3 (t)] and (b) [s 2 (t)+s 3 (t)], where s i (t) are three separate spread spectrum encoded data signals. The transmitter hopping signal is a sequence of tones having (a) a duration T c , where T c  is a chip duration, and (b) frequencies f n  taken from a set of 2 k  values, where k is a number of bits from a pseudo-random noise (PRN) sequence.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates, in general, to spread spectrum communications. More specifically, the present invention relates to hybrid spread spectrum systems, which use both direct sequence and frequency hopping operations. 
       BACKGROUND OF THE INVENTION 
       [0002]    Spread spectrum systems operate by spreading the spectrum of the communications signal well beyond the bandwidth of the unspread signal. Motivation for using spread spectrum signals is based on the following facts: (1) These systems have the ability to reject unintentional jamming by interfering signals so that information may be communicated. (2) Spread spectrum signals minimize interference with competing users since the power transmitted is spread over a large frequency bandwidth. (3) Since these signals cannot be readily demodulated without knowing the code and its precise timing, message privacy is attained. (4) The wide bandwidth of spread spectrum signals provides tolerance to multi-path propagation. (5) Multiple access or the ability to send many independent signals over the same frequency band is possible using spread spectrum techniques. Systems employing spread spectrum methods to communicate in a secure and non-interfering manner are well-known in the art. 
         [0003]    Spread spectrum systems may spread the communications signal using direct sequence or frequency hopping methods. In a direct sequence spread spectrum system, data bits are modulated with a spreading sequence before transmission. Each bit of information is modulated with a series of chips from the spreading sequence. The number of chips per bit defines the processing gain. A greater number of chips per bit creates a greater immunity to noise and other interference. 
         [0004]    The band spread is accomplished by means of a code that is independent of the data, and a synchronized reception with a code at the receiver is used for despreading the incoming signal. In operation, a data signal is multiplied by a pseudo-random noise (PN) sequence having a faster data rate than the data signal to be transmitted. 
         [0005]    Prior to application of the spreading code to the information bit stream, the information bits may undergo a series of digital operations which further increase the performance of the system. For example, the information bits may undergo differential encoding in order to be more tolerant to an incorrect phase lock in the receiver&#39;s phase locked loop (PLL). The information bits may be scrambled using a long scrambling sequence in order to further decrease the vulnerability of the system to interception. 
         [0006]    Direct sequence spread spectrum coding may use binary phase shift keying (BPSK). In a phase shift keyed system, information is carried in the phase of the signal. Two different phases are used to denote two different digital values. Whenever the sequence transitions from a “1” to a “0” or from a “0” to a “1”, the phase of the signal transitions. Such a system is referred to as a BPSK system. 
         [0007]    Another form of spread spectrum is called frequency-hopped spread spectrum, where the carrier frequency signal is moved (hopped) around in the band in a pseudo-random fashion. The result is an increase in effective bandwidth over time. Specifically, in a frequency hopped spread spectrum system the carrier frequency is shifted in discrete increments in a pattern generated by a code sequence. In such a system, the signal frequency remains constant for a specified time duration, referred to as a hopping time. The system may be either a fast hop system or a slow hop system. In a fast hop system, the frequency hopping occurs at the rate that is greater than the message bit rate. In a slow hop system, the hop rate is less than the message bit rate. There is also an intermediate situation in which the hop rate and message bit rate are of the same order of magnitude. 
         [0008]    Hybrid spread spectrum systems, which use both direct sequence and frequency hopping to spread the original spectrum, come in different forms and their operation is extensively described in the literature. Spread spectrum systems that use multiple carriers are also known in the literature. What does not appear in any of the literature, however, is that the output signal of hybrid multi-carrier spread spectrum systems may have a constant envelope. 
         [0009]    Since a conventional hybrid spread spectrum system does not transmit a signal having a constant envelope, it has shortcomings. A main shortcoming is low power efficiency, since a high power amplifier cannot operate efficiently when the input signal does not have a constant envelope. Another downside of such a system is its low bandwidth efficiency, since the common way to increase throughput is to increase bandwidth. 
         [0010]    The present invention addresses systems and methods for generating hybrid spread spectrum signals, using both direct sequence and frequency hopping, having constant envelopes. 
       SUMMARY OF THE INVENTION 
       [0011]    To meet this and other needs, and in view of its purposes, the present invention provides a hybrid spread spectrum system. The system includes a signal combiner for (a) receiving a plurality of spread spectrum encoded data signals and (b) forming a plurality of combined signals. A first set of mixers is included for (a) receiving the plurality of combined signals and at least two carriers, and (b) forming at least a two-carrier, constant envelope signal. A second set of mixers is also included for (a) receiving the at least two-carrier, constant envelope signal and a transmitter hopping signal and (b) forming a hybrid spread spectrum signal. The combined signals include (a) [s 1 (t)−s 1 (t)s 2 (t)s 3 (t)] and (b) [s 2 (t)+s 3 (t)], where s i (t) are three separate spread spectrum encoded data signals. The transmitter hopping signal is a sequence of tones having (a) a duration T c , where T c  is a chip duration, and (b) frequencies f n  taken from a set of 2 k  values, where k is a number of bits from a pseudo-random noise (PRN) sequence. 
         [0012]    The two carrier, constant envelope signal includes a first carrier modulated by [s 1 (t)−s 1 (t)s 2 (t)s 3 (t)] and a second carrier modulated by [s 2 (t)+s 3 (t)]. The s i (t) are three separate spread spectrum encoded data signals. The first set of mixers receives at least three carriers and forms at least a three-carrier constant envelope signal, and the second set of mixers receives the at least three-carrier constant envelope signal and forms the hybrid spread spectrum signal. The second set of mixers includes a summer for combining signals outputted by the second set of mixers, and a high pass filter (HPF) for (a) receiving a signal from the summer and (b) producing a single sideband of the hybrid spread spectrum signal. 
         [0013]    Another embodiment of the invention is a hybrid spread spectrum system having a plurality of frequency shift keying (FSK) modulators for forming at least two FSK modulated carriers. A first set of mixers is included for (a) receiving the at least two FSK modulated carriers and a plurality of PRN codes, and (b) forming at least two direct spread carriers. A second set of mixers is also included for (a) receiving the at least two direct spread carriers and a transmitter hopping signal, and (b) forming a hybrid spread spectrum signal having a constant envelope. The plurality of PRN codes includes (a) [c 1 (t)−c 1 (t)c 2 (t)c 3 (t)] and (b) [c 2 (t)+c 3 (t)], where c i (t) are three separate PRN codes. The transmitter hopping signal is a sequence of tones having (a) duration T c , where T c  is a chip duration, and (b) frequencies f n  taken from a set of 2 k  values, where k is a number of bits from a PRN sequence. The at least two direct spread carriers include a first carrier modulated by [c 1 (t)−c 1 (t)c 2 (t)c 3 (t)] and a second carrier modulated by [c 2 (t)+c 3 (t)], where c i (t) are three separate PRN codes. The second set of mixers includes a summer for combining signals outputted by the second set of mixers, and a HPF for (a) receiving a signal from the summer and (b) producing a single sideband of the hybrid spread spectrum signal. 
         [0014]    Yet another embodiment of the present invention is a method for transmitting a hybrid spread spectrum signal. The method includes the steps of: (a) forming at least two FSK modulated carriers; (b) (i) receiving the at least two FSK modulated carriers and a plurality of PRN codes, and (ii) forming at least two direct spread spectrum carriers, and (c) (i) receiving the at least two direct spread carriers and a transmitter hopping signal, and (ii) forming a hybrid spread spectrum signal having a constant envelope. The plurality of PRN codes includes (i) [c 1 (t)−c 1 (t)c 2 (t)c 3 (t)] and (ii) [c 2 (t)+c 3 (t)], where c i (t) are three separate PRN codes. The transmitter hopping signal is a sequence of tones having (i) duration T c , where T c  is a chip duration, and (ii) frequencies f n  taken from a set of 2 k  values, where k is a number of bits from a PRN sequence. The at least two direct spread carriers include a first carrier modulated by [c 1 (t)−c 1 (t)c 2 (t)c 3 (t)] and a second carrier modulated by [c 2 (t)+c 3 (t)], where c i (t) are three separate PRN codes. 
         [0015]    The method further includes the steps of: (d) high pass filtering the hybrid spread spectrum signal having the constant envelope, and (e) transmitting the high pass filtered hybrid spread spectrum signal of step (d). 
         [0016]    It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0017]    The invention is best understood from the following detailed description when read in connection with the accompanying figures. 
           [0018]      FIG. 1  is a block diagram of a coherent hybrid frequency hopping/direct sequence (FH/DS) system for generating an FH/DS signal, in accordance with an embodiment of the present invention. 
           [0019]      FIG. 2A  is a plot of amplitude versus time for the output signal generated by the system of  FIG. 1 . 
           [0020]      FIG. 2B  is a spectral plot of power versus frequency of the output signal generated by the system shown in  FIG. 1 . 
           [0021]      FIG. 3  is a block diagram of a coherent hybrid frequency hopping/direct sequence (FH/DS) system for generating an FH/DS signal, in accordance with another embodiment of the present invention. 
           [0022]      FIG. 4A  is a plot of amplitude versus time for the output signal generated by the system of  FIG. 3 . 
           [0023]      FIG. 4B  is a spectral plot of power versus frequency of the output signal generated by the system shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present invention is a hybrid spread spectrum system, which uses both direct sequence and frequency hopping techniques. The present invention has multipath and jamming resistant properties of similar conventional systems, but overcomes their shortcomings through use of a novel method for combining the spreading codes, and the use of multiple carriers that hop simultaneously. The arrangement of codes that spread the hopping carriers is such that it produces a composite output signal with a constant envelope. This makes the present invention power efficient, since high power amplifiers may be operated in their optimal mode, that is, close to the 1 dB (saturation) point. 
         [0025]    In addition, since multiple carriers hop simultaneously, the throughput of the system is increased in comparison with single carrier hopping systems. Every one of the carriers may be separately modulated by an independent data stream, or a high rate data stream may be inversely multiplexed onto several carriers. Furthermore, multiple carriers may be used to combat multipath or jamming, since redundancy in transmission may be achieved by having the same information repeated on two or more carriers. Finally, the single sideband property of the system included in the present invention achieves high bandwidth efficiency. 
         [0026]    As will be explained, the present invention includes coherent and non-coherent systems. Both systems use a combination of direct sequence and frequency hopping for carrier spreading, but they differ in the way in which the data modulates the carriers. In a BPSK (or DPSK) system, data demodulation has to be coherent, but in an FSK system, data may be demodulated non-coherently, which is a big advantage in wireless systems. 
         [0027]    Referring to  FIG. 1 , there is shown a coherent hybrid frequency hopping/direct sequence (FH/DS) system for generating an FH/DS signal, the hybrid system generally designated as  10 . As shown, two carriers are hopping simultaneously. The carriers are modulated with three data streams, which may either originate from independent sources or be a result of inverse multiplexing of a single high data rate source. 
         [0028]    It will be appreciated that the signal generated by the hybrid system of  FIG. 1  is exemplary and is provided to illustrate the concept. The system may be easily extended to include more than two carriers and more than three data streams. 
         [0029]    The present inventor in U.S. application Ser. No. 11/067,148, filed on Feb. 25, 2005, describes generation of a two carrier, constant envelope signal. Description of the system and method of generating this two carrier constant envelope signal is incorporated herein by reference in its entirety. 
         [0030]    As described in the aforementioned U.S. application, consider a two-carrier, constant envelope signal, such as: 
         [0000]        z ( t )=√{square root over (2 P )}[ s   1 ( t )− s   1 ( t ) s   2 ( t ) s   3 ( t )] cos ω 01   t +√{square root over (2 P )}[ s   2 ( t )+ s   3 ( t )] cos ω 02   t   (1) 
         [0000]    where P is the signal power, and 
         [0000]        s   1 ( t )= d′   1 ( t ) c   1 ( t ) 
         [0000]        s   2 ( t )= d′   2 ( t ) c   2 ( t ) 
         [0000]        s   3 ( t )= d′   3 ( t ) c   3 ( t )  (2) 
         [0031]    Signals d′ i (t) are obtained by differentially encoding data sequences d i (t). Differential encoding, however, is not necessary for the present invention. Signals c i (t) are pseudo-random (PRN) sequences with rates much higher than rates of d i (t), so they act like spreading codes. Both data and PRN signals take on values of ±1. 
         [0032]    To produce the output RF signal, z(t) is used to modulate the transmitter hopping signal, which is a sequence of tones of duration T c  as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0033]    Frequency f n  is taken from a set of 2 k  values, where k is the number of bits from the PRN sequence that is taken at the time to determine the current hopping frequency value. Pulse p(t) has unit amplitude in 0≦t≦T c , where T c  is the chip duration. 
         [0034]    Therefore, the output signal is 
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         [0035]    Referring again to  FIG. 1 , coherent hybrid system  10  includes differential encoders  11   a ,  11   b  and  11   c  for differentially encoding data sequences d′ i (t) to produce signal d′ i (t). (In another embodiment (not shown), the differential encoders are not used.) Mixers  12   a ,  12   b  and  12   c , each multiply d′ i (t) with c i (t), the c i (t) being PRN sequences, to produce the s i (t) signals. 
         [0036]    Signal combiner  14 , which also includes multiplier functions, receives the s i (t) signals and produces the following two combined signals: 
         [0037]    s 1 (t)−s 1 (t)s 2 (t)s 3 (t) and 
         [0038]    s 2 (t)+s 3 (t) 
         [0039]    Each combined signal is then multiplied, using respective mixers  15   a  and  15   b , by a corresponding carrier, cos ω 01 t and cos ω 02 t (where ω=2πf) to produce two modulated carriers, as shown. 
         [0040]    Next, the two modulated carriers are multiplied, using respective mixers  15   c  and  15   d , by the transmitter hopping signal h T (t) (Equation 4). Frequency hopping code generator  19  and frequency synthesizer  18  produce the hopping signal. Summer  16  and high pass filter (HPF)  17 , which are inserted serially in system  10 , produce the final output signal s(t) (Equation 5). It will be appreciated that output signal s(t) may be transmitted via an antenna (not shown). 
         [0041]    The sum-frequency is passed through a transmit high pass filter (HPF). The signal in Equation (5) is a hybrid, direct sequence/frequency hopping spread spectrum (DS/FH-SS) signal. Namely, two direct sequence spread carriers at frequencies f 01 +f n  and f 02 +f n  hop simultaneously. 
         [0042]    In addition, signal s(t) has a constant envelope, because 
         [0000]      [ s   1 ( t )− s   1 ( t ) s   2 ( t ) s   3 ( t )] 2   +[s   2 ( t )+ s   3 ( t )] 2   =const   (6) 
         [0000]      and 
         [0000]      [ s   1 ( t )− s   1 ( t ) s   2 ( t ) s   3 ( t )][ s   2 ( t )+ s   3 ( t )]=0  (7) 
         [0043]    The signal, which is generated using the system shown in  FIG. 1 , and its spectrum are shown in  FIGS. 2A and 2B , respectively. In this example, the number of hopping frequencies is 100; the hopping range is from 40 to 70 MHz; the fixed offset frequencies f 01  and f 02 , are 5 MHz apart; the data and direct sequence spreading modulations are BPSK. The hopping and direct spreading sequences need not be synchronized. The signal has a constant envelope. 
         [0044]    When the signal generated by the system of  FIG. 1 , namely s(t), is transmitted and received by a receiver (not shown), it may be demodulated by any conventional spread spectrum receiver, including a despreader and a demodulator. The demodulator, of course, should be a coherent demodulator. 
         [0045]    A disadvantage in using the signal s(t) shown in  FIG. 1  is that it requires coherent demodulation, which is difficult to accomplish in frequency hopping systems. Therefore, the present invention also provides a system and method for generating signals that may be demodulated non-coherently, but are also of a hybrid FH/DS type and have a constant envelope. 
         [0046]    An exemplary non-coherent hybrid FH/DS system is shown in  FIG. 3 , and is generally designated as  30 . In the example of system  30 , data modulation is binary FSK and two independent data streams are used to modulate two carriers that jointly hop. The method, however, may be expanded to multilevel FSK to achieve greater throughput; and to multiple independent data streams that modulate multiple carriers which hop simultaneously for further increase in information capacity. In addition, instead of independent data streams, an inversely multiplexed high rate data sequence may be used to modulate the hopping carriers. 
         [0047]    For example, consider the following BFSK modulated carriers: 
         [0000]        x ( t )=√{square root over (2 P )} cos 2 π[f   01   +d   1 ( t )Δ] t   (8) 
         [0000]      and 
         [0000]        y ( t )=√{square root over (2 P )} cos 2 π[f   02   +d   2 ( t )Δ] t   (9) 
         [0000]    where 
         [0048]    f 01  and f 02  are fixed FSK center frequencies; 
         [0049]    d 1 (t) and d 2 (t) are data signals taking on values ±1; 
         [0000]    
       
         
           
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         [0000]    is half the distance between orthogonal BFSK signals; and 
         [0050]    T is the data bit duration. 
         [0051]    For simplicity, it is assumed that both data signals have the same rate, but that is not required for the operation of the method and, in general, the rates may be different and do not have to be commensurable. 
         [0052]    By using combinations of PRN codes c 1 (t), c 2  (t), and c 3  (t), the present invention direct-spreads the carriers from Equations (8) and (9) to 
         [0000]        u ( t )=√{square root over (2 P )}[ c   1 ( t )− c   1 ( t ) c   2 ( t ) c   3 ( t )] cos 2 π[f   01   +d   1 ( t )Δ] t   (10) 
         [0000]      and 
         [0000]        v ( t )=√{square root over (2 P )}[ c   2 ( t )+ c   3 ( t )] cos 2 π[f   02   +d   2 ( t )Δ] t   (11) 
         [0053]    The signals u(t) and v(t) shown in Equations (10) and (11) are each further spread by the transmitter hopping sequence of Equation (4), then combined into a composite signal. The sum-frequency components of the composite signal is passed to the antenna via a high pass filter (HPF). The entire output signal is given by 
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         [0054]    Referring still to  FIG. 3 , non-coherent hybrid system  30  includes FSK modulators  31   a  and  31   b  for modulating carrier signals with the data signals. As shown, the FSK modulators provide output signals x(t) and y(t). 
         [0055]    Mixers  32   a  and  32   b  multiply x(t) and y(t), respectively, with combinations of PRN codes, namely, 
         [0056]    c 1 (t)−c 1 (t)c 2 (t)c 3 (t), and 
         [0057]    c 2 (t)+c 3 (t) 
         [0000]    to produce u(t) and v(t), as shown. It will be understood that the combinations of PRN codes are produced by a signal combiner, which may be similar to signal combiner  14  shown in  FIG. 1 . 
         [0058]    Next, mixers  32   c  and  32   d  multiply signals u(t) and v(t), respectively, with the transmitter hopping signal h T (t). Similar to system  10 , the transmitter hopping signal is produced by FH code generator  37  and frequency synthesizer  36 . Finally, summer  34  and HPF  35  combine and filter, respectively, the signals to output s(t) (Equation 12). Output signal s(t) may be transmitted by way of an antenna (not shown). 
         [0059]    Signal s(t), which is generated by non-coherent hybrid system  30 , may be demodulated by any conventional receiver. Coherent detection is, of course, not necessary. 
         [0060]    The signal generated by hybrid system  30 , as well as its spectrum are shown in  FIGS. 4A and 4B , respectively. In this example, the number of hopping frequencies is 1000; hopping range is from 40 to 70 MHz; fixed offset frequencies f 01  and f 02  are 5 MHz apart; data modulation is BFSK; hopping direct sequence spreading and data are not synchronized. As shown in  FIG. 4A , the signal s(t) has a constant envelope. 
         [0061]    The present invention is applicable to all frequency hopped spread spectrum systems in which higher throughput is desired, but the switch to multilevel modulation schemes (e.g. MFSK) is not a good option due to channel characteristics. Although the scheme is multi carrier, the constant envelope property of single carrier frequency hopped systems is preserved, which means that the present invention produces power-efficient systems. The invention is, therefore, especially applicable to mobile and portable systems where preservation of battery power is of special importance. Furthermore, the output signal generated by the present invention is essentially single sideband, making the scheme bandwidth efficient. This makes the present invention applicable to situations where the available spectrum has to be shared by many different wireless systems.