WO2003010917A1 - Modulation de frequence chaotique en bande etroite utilisant des applications en forme de tente asymetrique - Google Patents

Modulation de frequence chaotique en bande etroite utilisant des applications en forme de tente asymetrique Download PDF

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Publication number
WO2003010917A1
WO2003010917A1 PCT/US2002/022396 US0222396W WO03010917A1 WO 2003010917 A1 WO2003010917 A1 WO 2003010917A1 US 0222396 W US0222396 W US 0222396W WO 03010917 A1 WO03010917 A1 WO 03010917A1
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signal
data
map
signal communication
frequency
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PCT/US2002/022396
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English (en)
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Chandra Mohan
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Atlinks Usa, Inc.
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Publication of WO2003010917A1 publication Critical patent/WO2003010917A1/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/001—Modulated-carrier systems using chaotic signals

Definitions

  • the present disclosure relates to signal transmission and reception.
  • approaches to the design of communication systems based on chaos such as those suggested by Kocarev (1992), Belsky and Dmitriev (1993), Cuomo (1993), Pecora and Carrol (1993), and Dmitriev and Starkov (1997).
  • These prior approaches have been focused on analog spread-spectrum types of systems and, hence, were inherently broadband.
  • linearity refers to Euclidean objects such as lines, planes, (flat) three-dimensional space, and the like. These objects appear the same no matter how they are examined.
  • a nonlinear object such as a sphere, for example, looks different for different scales. When viewed closely, it looks like a plane; and from afar, it looks like a point.
  • Nonlinearity is defined as the negation of linearity. This means that the result f(x+y) may be out of proportion to the inputs x and/or y.
  • a dynamical system has an associated abstract phase space or state space with coordinates that describe the dynamical state at any instant; and a dynamical rule that specifies the immediate future trend of all state variables, given the present values of those state variables.
  • a dynamical system can be defined with respect to discrete or continuous time.
  • F(z) is called a vector field, which gives a vector pointing in the direction of the velocity at every point in a phase space.
  • a phase space or state space is the collection of possible states of a dynamical system.
  • a state space can be finite (e.g., for the ideal coin toss, there are two states, heads and tails), countably infinite (e.g., where the state variables are integers), or uncountably infinite (e.g., where the state variables are real numbers). Implicit in the notion of state or phase space is that a particular state in phase space specifies the system completely. It is all one needs to know about the system to have complete knowledge of the immediate future.
  • phase space of the planar pendulum is two-dimensional, consisting of the position or angle and velocity.
  • time e.g., a model for plant growth that depends on solar flux
  • time must be included as a phase space coordinate since one must specify a specific time (e.g., 3 pm on Tuesday) to know the subsequent motion.
  • the path in phase space traced out by a solution of an initial value problem is called an orbit or trajectory of the dynamical system. If the state variables take real values in a continuum, the orbit of a continuous-time system is a curve; while the orbit of a discrete-time system is a sequence of points.
  • degrees of freedom means one canonical conjugate pair: a configuration, q, and its conjugate momentum, p.
  • Hamiltonian systems always have such pairs of variables and so the phase space is even-dimensional.
  • phase space is often used differently to designate a single coordinate dimension of phase space.
  • a map is a function f on the phase space that gives the next state f(z) (i.e., the "image") of the system given its current state z.
  • a function must have a single value for each state, but there may be several different states that give rise to the same image. Maps that allow every state of the phase space to be accessed onto and which have precisely one pre-image for each state (i.e., a one-to-one correspondence) are invertible. If, in addition, the map and its inverse are continuous with respect to the phase space coordinate z, then it is called a homeomorphism. Iteration of a map means repeatedly applying the consequents of the previous application. Thus producing the sequence:
  • this sequence is the orbit or trajectory of the dynamical system with initial condition z 0 .
  • Every differential equation gives rise to a map.
  • the timel map advances the flow one unit of time. If the differential equation contains a term or terms periodic with time T, then the time T map in a system represents a Poincare section.
  • This map is also called a stroboscopic map as it is effectively looking at the location in phase space with a stroboscope tuned to the period T. This is useful as it permits one to dispense with time as a phase space coordinate.
  • Poincare section In autonomous systems (i.e., no time dependent terms in the equations), it may also be possible to define a Poincare section to reduce the phase space coordinates by one.
  • the Poincare section is defined not by a fixed time interval, but by successive times when an orbit crosses a fixed surface in the phase space. Maps arising out of stroboscopic sampling or
  • Poincare sections of a flow are necessarily invertible because the flow has a unique solution through any point in phase space.
  • the solution is unique both forward and backward in time.
  • An attractor is simply a state into which a system settles, which implies that dissipation is needed. Thus, in the long term, a dissipative dynamical system may settle into an attractor.
  • An attractor can also be defined as the phase space that has a neighborhood in which every point stays nearby and approaches the attractor as time goes to infinity. The neighborhood of points that eventually approach the attractor is the "basin of attraction”.
  • Chaos is defined as the effective unpredictable long term behavior arising in a deterministic dynamic system due to its sensitivity to initial conditions. It must be emphasized that a deterministic dynamical system is perfectly predictable given knowledge of its initial conditions, and is in practice always predictable in the short term. The key to long-term unpredictability is a property known as sensitivity to initial conditions. For a dynamical system to be chaotic, it must have a large set of initial conditions that are highly unstable. No matter how precisely one measures the initial conditions, a prediction of its subsequent motion eventually goes radically wrong.
  • Lyapunov exponents measure the rate at which nearby orbits converge or diverge. There are as many Lyapunov exponents as there are dimensions in the state space of the system, but the largest is usually the most important. Roughly speaking, the maximal Lyapunov exponent is the time constant ⁇ in the expression for the distance between two nearby orbits. If ⁇ is negative, the orbits converge in time and the dynamical system is insensitive to initial conditions. If ⁇ is positive, then the distance between nearby orbits grows exponentially in time and the system becomes sensitive to initial conditions.
  • Lyapunov exponents can be computed in two ways. In one method, one chooses two nearby points and evolves them in time measuring growth rates of the distance between them. This method has the disadvantage that growth rate is not really a local effect as points separate. A better way to measure growth is to measure the growth rate of the tangent vectors to a given orbit.
  • the Minimum Phase Space dimension for Chaos is a slightly confusing topic, since the answer depends on the type of system considered.
  • a flow or a system of differential equations is considered first.
  • the Poincare-Bendixson theorem indicates that there is no chaos in one or two- dimensional phase space.
  • Chaos is possible only in three-dimensional flows. If the flow is non-autonomous (i.e., dependent on time), then time becomes a phase space co-ordinate. Therefore, a system with two physical variables plus a time variable becomes three-dimensional and chaos is possible.
  • M-ary phase shift keying PSK
  • M-ary quadrature amplitude modulation QAM
  • FCC Federal Communications Commission
  • M-ary QAM or PSK architectures have a "set-top” box to decode the high-speed sub-carrier signals because normal receivers use FM demodulators for recovering the baseband information.
  • M-ary systems suffer from power loss associated with higher levels of bandwidth compression brought about by the modulation scheme that employ multiple bits per symbol. The M-ary systems become too lossy for practical implementation beyond an upper limit.
  • the system includes a forward error correction encoder for receiving input data, a data packetizer in signal communication with the forward error correction encoder, a compression encoder in signal communication with the data packetizer, a radio frequency link in signal communication with the compression encoder, a compression decoder in signal communication with the radio frequency link, a data de-packetizer in signal communication with the compression decoder, and a forward error correction decoder in signal communication with the data de-packetizer for recovering the input data by controlling the orbits of chaos.
  • the associated method includes the steps of transmitting a signal indicative of chaotic frequency modulated data, propagating the transmitted signal within a narrow frequency band, and receiving the propagated signal substantially without degradation of the indicated data by controlling the orbits of chaos.
  • Figure 1 shows a block diagram of a system for Narrow Band Chaotic Frequency Modulation Using Skewed Tent Maps
  • Figure 2 shows a block diagram for a narrow band chaotic modulation transmitter in accordance with the system of Figure 1 ;
  • Figure 3 shows a block diagram for an encoder in accordance with the system of Figures 1 and 2;
  • Figure 4 shows a block diagram for a narrow band chaotic communication receiver in accordance with the system of Figure 1 ;
  • Figure 5 shows a block diagram for a decoder in accordance with the system of Figures 1 and 4;
  • Figure 6 shows a schematic diagram for an exemplary Colpitts oscillator for use in describing the operation of a system in accordance with Figure 1 ;
  • Figure 7 shows a plot of a skewed tent map for use with the system of Figure 1 ;
  • Figure 8 shows a plot of another skewed tent map for use with the system of Figure 1 ;
  • Figure 9A shows a schematic diagram for a shift register as a Bernoulli shift process for use with the system of Figure 1 ;
  • Figure 9B shows a function diagram for a shift map to tent map conversion in accordance with the shift register of Figure 9A for use with the system of Figure 1 ;
  • Figure 10 shows a plot for a chaotic limit cycle for use with the system of Figure 1
  • Figure 11 shows an alternate plot for a chaotic limit cycle for use with the system of Figure 1 ;
  • Figure 12 shows a block diagram for a high-speed phase change detector in accordance with the system of Figure 4.
  • Figure 13 shows a plot of oscillator output in accordance with the system of Figure 1.
  • the present disclosure relates to signal transmission and reception using a composite information and chaos signal that is modulated onto a carrier signal by narrow band chaotic frequency modulation using skewed tent maps.
  • the disclosure addresses chaos generation using the exemplary nonlinear operation of a Colpitts oscillator driven by a tent map to aid synchronization between a transmitter and a receiver.
  • the disclosure sets forth how a non-linear generator can be made by mapping a bit-interval into itself using asymmetric maps, such as, for example, tent maps, Baker maps and/or Bernoulli maps.
  • the modulator is a wideband voltage-controlled crystal oscillator ("VCXO") that enables a wide pulling range without sacrificing too much of the high tank circuit quality "Q".
  • VCXO wideband voltage-controlled crystal oscillator
  • FIG. 1 shows a block diagram of a system 110 for Narrow Band Chaotic Frequency Modulation Using Skewed Tent Maps in an illustrative embodiment of the present disclosure.
  • the system 110 includes a transmitter portion 112, a radio frequency ("RF") link 114 in signal communication with the transmitter 112, and a receiver portion 116 in signal communication with the RF link 114.
  • RF radio frequency
  • the transmitter portion 112 includes a forward error correction (“FEC") encoder 118 that uses Reed-Solomon (“RS”) error correction code for receiving input data, a data packetizer 120 in signal communication with the FEC encoder 118 for receiving FEC coded data, and a compression encoder 122 in signal communication with the data packetizer 120 for providing encoded data for modulation to the RF link 114.
  • the receiver portion 116 includes a compression decoder 124 for receiving encoded data from the RF link, a data de-packetizer 126 in signal communication with the compression decoder 124, and an FEC decoder 128 that uses RS error correction code in signal communication with the data de-packetizer 126 for receiving FEC coded data and providing output data.
  • FEC forward error correction
  • RS Reed-Solomon
  • a narrow band chaotic modulation transmitter 210 represents an exemplary embodiment of the transmitter 112 of Figure 1.
  • the transmitter 210 receives a high-speed data bit stream 212 at an encoder 214.
  • a signal shaper 217 is in signal communication with the encoder 214, and feeds a first local oscillator 218 comprising a wideband Colpitts VCXO.
  • the first local oscillator 218 feeds an amplifier 222.
  • An intermediate frequency (“IF") filter 224 is in signal communication with the amplifier 222, and feeds an amplifier section 226.
  • the amplifier section 226 feeds a first 90-degree phase delay unit 227 and a multiplier 228.
  • a second local oscillator 230 also feeds the multiplier 228 and feeds a second 90-degree phase delay unit 232.
  • the first and second 90-degree phase delay units 227 and 232 are each received as inputs at a multiplier 234.
  • the multiplier 234 is connected, in turn, to a first positive input of a summer 236.
  • the summer 226 receives a second positive input from the multiplier 228 and feeds a power amplifier 238.
  • a plot 250 shows the output of an exemplary embodiment modulator comprising a wideband VCXO that enables a wide pulling range without sacrificing too much of the high tank circuit quality "Q".
  • the plot 250 shows that this results in a clean modulation process with sidebands well below 50 dBc with respect to the carrier. Since the modulating signal is based on a level-to-time conversion process, the width of the signal changes with respect to the incoming data stream in a predetermined manner. When this signal is fed into a varactor diode connected to the VCXO tank circuit, the stationary chaotic dynamics of the oscillator are changed due to the change in the effective inductance presented by the tank circuit.
  • the output from the oscillator is clean with modulation components at least 50-60 dB down.
  • the signal can be limited and detected with a phase-locked loop ("PLL") or a high-speed detector.
  • PLL phase-locked loop
  • Exemplary embodiments provide chaos generation using the exemplary non-linear operation of a Colpitts oscillator driven by a tent map to aid synchronization between a transmitter and a receiver.
  • an encoder 310 represents an exemplary embodiment of the encoder 214 of Figure 2.
  • the encoder 310 includes a by 4.5 ("x4.5") divider 312 for receiving a local 18 MHz clock signal, and providing a 4.096 MHz signal.
  • the local clock signal is also received by a divider 314.
  • An edge detector 316 receives input data and provides an edge signal to the divider 314 as well as to a first sequence generator 318 and a second sequence generator 320.
  • the divider 314 provides a CLK9 signal to the first and second sequence generators 318 and 320, respectively.
  • a MUX 322 receives a SEQ1 and SEQ2 signals from the sequence generators 318 and 320, respectively, and further receives the input data signal at its output select terminal.
  • the MUX 322 provides an encoded data signal output.
  • a narrow band chaotic communications receiver 410 represents an exemplary embodiment of the receiver 116 of Figure 1.
  • the receiver 410 includes an incoming communications buffer 412 in signal communication with a low-noise amplifier ("LNA") and/or mixer 414.
  • a local oscillator 416 also feeds the LNA/Mixer 414.
  • a tunable band-pass filter 418 passes the 10.7 MHz center frequency band with a bandwidth of 2 KHz to an IF amplifier 420, which feeds a demodulator 422.
  • a level translator/filter section 424 is in signal communication with the demodulator 422, which comprises a PLL, discriminator and high-speed detector.
  • the demodulator 422 feeds a high-speed detector 425, which, in turn, feeds a data decoder 426 that outputs to a high-speed data channel.
  • the detector 425 includes a limiter 430 for receiving the output of filter 424 of Figure 4, which indicates an IF input from the IF amplifier 420 of Figure 4.
  • the limiter 430 feeds an injection amplifier 432, which, in turn, feeds a modulation stripper 434 and a phase comparator 436.
  • the phase comparator 436 may be substituted with a D-type flip-flop or an EX-OR device in alternate embodiments.
  • the phase comparator 436 feeds a template insertion block 438, which, in turn, feeds a second input to the modulation stripper 434.
  • the output of the modulation stripper 434 feeds a second input to the phase comparator 436.
  • the output of the phase comparator 436 feeds first and second monoshots 438 and 440, respectively.
  • the monoshot 438 feeds a clock generator 442, which, in turn, feeds a first input of a D-type flip- flop 444.
  • the monoshot 440 feeds a second input of the D-type flip-flop 444.
  • the D-type flip-flop 444 produces a data signal coupled to the data decoder 426 of Figure 4.
  • a decoder 510 represents an exemplary embodiment of the decoder 426 of Figure 4.
  • the decoder 510 includes an edge detector 512 for receiving encoded data and providing an EDGE signal to first and second 7-bit binary counters 516 and 520, respectively.
  • a clock divider 514 receives the 18 MHz clock signal and provides a CLKx72 signal to the first and second counters 516 and 520.
  • a count-64 detector 518 is in signal communication with the first counter 516, and feeds a reset signal to the second counter 520.
  • a count-64/80 detector 522 is in signal communication with the second counter 520, feeds each of a load value function 524 and a synchronization function 526, and provides decoded data to a latch 528.
  • the load value function 524 feeds the second counter 520.
  • the synchronization function 526 receives a CLKxl signal from the clock divider 514, and provides a SYNC clock to the latch 528 at a x1 bit rate.
  • the latch in turn, provides output data.
  • an exemplary Colpitts oscillator is indicated generally by the reference numeral 610.
  • the oscillator 610 includes a diode 612 having its anode grounded and its cathode coupled to a Cvar capacitor 614, which is coupled, in turn, to a CO capacitor 616.
  • the capacitor 616 is coupled to an L inductor 618, which is then grounded.
  • the capacitor 616 is also coupled to a Cc capacitor 620.
  • the capacitor 620 is coupled to a C1 capacitor 622 as well as to the base of a transistor 624.
  • the transistor 624 has its collector coupled to Vcc, and its emitter coupled to the output of capacitor 622, which output is then coupled to a grounded C2 capacitor 626, a grounded Iq current source 628, and an output buffer 630.
  • the base of the transistor 624 is coupled to an Rb resistor 632 to provide a Vbias voltage bias output.
  • a first exemplary skewed tent map function f(x) is generally indicated by the reference numeral 710 of Figure 7
  • a second exemplary skewed tent map function 810 is generally indicated by the reference numeral 810 of Figure 8.
  • Transmission of information utilizing Skewed Tent Maps is considered with iteration of a skewed tent map 710 or 810 centered on 0. Since the only state variable of this dynamical system is directly transmitted, it is a very straightforward approach to a nonlinear dynamical system. Since the partitioning of the period is asymmetric, the skewed tent map is always non-invertible. This means that if the time dimension is also factored into this system, a 1-D skewed tent map that exhibits flow is produced.
  • the encoding algorithm is defined as follows:
  • the whole bit duration is partitioned into 9 equal sub-intervals by using a x9 clock for the encoding process. If the incoming bit stream has a "0" to "1" transition, then the encoder outputs a width of 10 equal intervals of x9 clock. When there is a "1" to "0" transition, a width corresponding to 8 cycles of x9 is used for encoding. If there is no change for the data, a width corresponding to a width of "9” clock pulses is encoded. For a tent map to exhibit flow, there should be asymmetry in the partition. Hence, when a width of "9" is output, there will be no flow.
  • the decoding algorithm can be forced into synchronism by an initial pattern followed by periodic resetting of the master clock. This way the orbits of chaos are controlled. This method of synchronization is based on recovered trajectories.
  • the skew-tent map f:[0,1]->[0,1] is given by:
  • a unique feature of the presently disclosed approach is to manipulate the encoding process to generate the skewed tent map such that the map will aid in both synchronization as well as data transmission. For this to happen, we need to iterate the map through many cycles before synchronism can be achieved. To aid the iteration process, the skewed tent map is first converted to a higher intermediate frequency, which is then used for iterating the lower frequency signals out of the map since this concept does not work at the lower base-band map frequencies without incurring delays due to buffering the iterated samples.
  • ⁇ gives the average rate of divergence, if ⁇ > 0, or convergence, if ⁇ ⁇ 0, of the two trajectories from each other. Simulations show that about 10 to 15 cycles are typically needed for the convergence of the trajectories. This implies that the Intermediate frequency on which the Skewed tent map is being transferred is preferably between 50 and 100 times the map frequency to obtain clean, burst free synchronism. The number of limit cycles needed for stabilization by the synchronizing loop influences the choice for the intermediate frequency.
  • ⁇ (x) a p(ax) +(1 -a)p(1 -(1 -a)x)
  • C W log 2 (P+NVN, where C is the channel capacity, W is the channel bandwidth and (P+N)/N is the signal- to-noise ratio.
  • the presently disclosed method allows the average information processing capability to go up by 83 times, assuming no change in channel bandwidth. If, however, the channel bandwidth is reduced by 83 times, our signal-to-noise ratio essentially remains the same as that of a linear communication system. It is a significant advantage of the Narrow band Chaotic modulation system that we do not pay a penalty in signal-to-noise ratio in order to narrow the bandwidth.
  • Amplitudes and all other variables of the dynamical system are locally transversal to the cycle corresponding to the negative Lyapunov exponents.
  • the description in terms of the Lyapunov exponent demonstrates why the phase is an exceptional variable of the dynamical system. Since it corresponds to the sole neutrally stable direction, the phase, in contrast to amplitudes, can be controlled by a very weak external action. A weak perturbation to the amplitude will relax to its stable value. However, a small perturbation to phase will neither grow nor decay. Thus, even small perturbations on phase can be accumulated.
  • the circuit for a Colpitts oscillator can be used for analysis.
  • This model contains three dynamical elements: C-i, C 2 and L. Since the statistical transistor model contains only 3 elements, the dynamical system is of third order.
  • the constants l b , mV t and ⁇ are constants of the transistor. From the network analysis of the Colpitts oscillator, two independent node equations and a mesh equation are obtained. Together with the voltage-current relationship of the network devices, the following set of equations can be derived.
  • first and second portions 910 and 912 respectively, of an encoding algorithm are indicated.
  • modulation is impressed on the tank circuit by connecting a varactor diode across the tank circuit.
  • an encoded signal having either a tent Map, Bernoulli map, Henon map or a Baker's map is impressed upon the varactor, the resonant frequency changes, resulting in a change to the effective value of the inductance. This will alter the value of Y slightly, giving rise to different limit cycle patterns. Since ⁇ is always less than
  • embodiments of the present disclosure include the ability of the encoding scheme, signal conditioning and modulating elements of the transmitter to restrict the bandwidth required to send information over a channel.
  • Demodulation is accomplished by various methods such as PLL, FM Discriminator or a higher gain phase detector.
  • Most of the popular modulation schemes employing M-ary techniques keep the modulator “on” or “off” over the whole duration of the symbol times.
  • Certain bandwidth efficient schemes such as, for example, Feher's modulation scheme, Minimum Shift Keying (“MSK”), and raised cosine shaped Offset Quadrature Phase Shift Keying (“OQPSK”) attempt to restrict the bandwidth of the modulation components.
  • MSK Minimum Shift Keying
  • OFQPSK Offset Quadrature Phase Shift Keying
  • the waveforms used are periodic, with well- defined boundaries and, hence, interpolation techniques can be applied to reconstruct the signal back in the receiver.
  • Minimum bandwidth required to send information across a channel is a strong function of the energy/bit and the noise bandwidth.
  • the exemplary system that is disclosed here integrates the limiting conditions for maximizing the channel capacity by reducing the noise bandwidth, employing a VCXO that is forced by a map, and is able to transfer the modulation onto the operating frequency of the crystal by mode locking the oscillator frequency with the frequency of the forcing frequency at a particular amplitude of the forcing frequency.
  • the encoding process ensures that the periodicity of the phase changes happen within a window of 0.44 to 0.55 of the bit rate.
  • the local oscillator is chosen to be at 10.7MHz in this exemplary embodiment. Sidebands are at least 55-60db down.
  • the output of the oscillator is taken from the modulating point on the varactor, buffered and passed through a semi-lattice filter for further improvement in carrier-to-noise ratio at the transmitter. This signal is further up-converted to an appropriate band for transmission using Image reject mixing.
  • the above method can be used at various frequencies.
  • Choice of the 1 st local oscillator (“LO") is based on the criteria that the 1 st LO should have at least 10-20 carrier cycles between modulation signal bit boundaries.
  • NRZ Non-Return-To-Zero
  • This type of mapping maps a unit period into itself, and is a non- invertible map that has a flow.
  • Increasing and decreasing the pulse width to accommodate 8 clock cycles, 9 clock cycles or 10 clock cycles are exemplary embodiments. Higher numbers of encoded clock cycles are possible, but system performance in view of multi-path effects coupled with zero-crossing detection accuracy will be elements that may influence the maximum frequency that an encoding and/or decoding clock should have in a given system.
  • the coding technique is unique in that, if the encoded waveform is observed closely, depending on whether there was a 1 to 0 transition or a 0 to 1 transition on the original NRZ waveform, the encoded waveform would have the phase change points later or earlier than the phase change points associated with the bit boundaries.
  • the output of the encoder is fed into the varactor on the VCXO.
  • the received signal is converted into the intermediate frequency ("IF") and given a 90-degree phase shift so as to align phase transition boundaries (e.g., the phase of the transmitted signal had a phase shift of 90 degrees due to the integration process during the frequency/phase domain mapping at the varactor).
  • the IF is chosen in such a manner as to facilitate low cost design for the system.
  • Other embodiments may use filters at 6 MHz, 10.7 MHz, 21.4 MHz, 70 MHz, 140 MHz, etc., due to their ready availability.
  • a linear phase, band pass filter is required to filter out all "out of band” Fourier components.
  • the output of the filter has information embedded in it in the form of phase or frequency modulation. This signal is frequency translated to the 900 MHz frequency band.
  • the transmitter frequency is chosen to be at 902.77 MHz.
  • the digital sub carrier is at 10.7 MHz.
  • the 2 nd LO is chosen to be 892.07 MHz. Therefore, the entire digital signal transmission spectrum is within a bandwidth of about 10 kHz.
  • the receiver includes a down converter at 892.07 MHz.
  • the output from the down converter is at 10.7MHz.
  • This signal is processed after sufficient band-pass filtering and amplification by a limiter.
  • the 10.7 MHz filter should possess minimal group delay characteristics.
  • the narrowband filter used is very similar to the semi-lattice filters used in amateur radios as well as reported in Filter Design Handbook by Zverev.
  • An FM discriminator or a PLL is included that, as a phase change detector, can be used detect and regenerate the variable bit-width signal similar to the one in the transmitter. Since we are employing the spatio-temporal characteristics of the signal to make an unambiguous detection, a higher gain in the detector will enhance the minimum signal detection capability of the system.
  • PLLs need adequate signal-to-noise ratios for proper detection. Moreover, it is very difficult to have fast tracking capability with narrow tracking range. FM discriminators suffer from very low detector gains and high detector bandwidths. Since a self similar replica of the original signal of proper amplitude and phase is needed at the receiver for detection, both the above systems work with poor efficiencies at low signal to noise ratios.
  • the new high-speed detector 425 of Figures 4 and 12 is provided.
  • a signal from a limiting section is fed into an injection amplifier that helps to maintain a steady output level for varying input levels.
  • the injection amplifier acts as a fast tracking filter with minimal group delay in its tracking range.
  • the output from the injection amplifier is split into two branches. One branch is passed through an oscillator at 10.7 MHz with a high Q tank circuit. This operation virtually strips all modulation from the signal out of the limiter and serves as a reference signal. We call this branch the modulation stripper branch.
  • the other branch is fed into either a type D flip-flop or an EX-OR gate. The output of the EX- OR gate or the D flip-flop will indicate phase change points.
  • the output of the detector will vary in periodicity like the transmitted encoded wave form. Since most of the signal has been filtered off, both at the transmitter as well as the receiver, the energy of the spikes out of the detector is very low and there will be other spikes between the responses. To mitigate this problem, a one shot is used. The output from the detector will have only two pulse widths instead of the three widths that were originally used to modulate the transmitter. This is because the phase detectors are basically having a range of 0 to ⁇ . If the modulated bit width is "5", signal phase can exceed ⁇ .
  • a clock signal is generated out of this signal so that the decoder can be clocked.
  • Another output from the phase change detector is processed through a mono- shot that will capture both of the pulses, but will filter out all the spikes in between.
  • the chosen signal is fed into a D flip-flop and clocked by the clock signal. The output of this flip-flop will be the reproduced original data stream.
  • plain data from an external source is first block encoded for Forward Error Correction ("FEC") using a Reed-Solomon (“RS”) error-correction code.
  • FEC Forward Error Correction
  • RS Reed-Solomon
  • a packetizer adds header and other redundant bits to FEC blocks and forms data packets. Packetized data is subjected to compression channel encoding. The encoded data is then modulated by a radio frequency (“RF”) circuit to be transmitted over an RF link.
  • RF radio frequency
  • encoded data received over the RF link is demodulated to a base band signal and passed to decoder.
  • the decoded data is de- packetized and FEC data blocks are fed to an RS decoder circuit.
  • the RS decoder circuit validates and corrects errors.
  • the error free original plain data stream is delivered to a destination. The steps involved in the encoding and decoding processes are further described below.
  • a transmitter 112 includes an FEC Encoder 118 wherein the raw user data is grouped into blocks of 235 bytes each. RS coding (255, 235) is applied to each block. For a block of 235 bytes, 20 bytes of error check bytes are added.
  • a packetizer 120 adds header and trailer bits to the FEC encoded data blocks to send as packets.
  • a compression encoder 122 encodes data packets for compression using a unique encoding scheme.
  • a receiver 116 includes a compression decoder 124 where a received data stream from an RF stage is decoded to retrieve original data packets.
  • a de-packetizer 126 removes header and other pre-ambles from received data packets.
  • an FEC decoder 128 processes FEC coded received data blocks for error correction.
  • the encoding scheme is based on input data edge transitions.
  • the encoded data width varies depending upon the input data transition.
  • a higher clock is used at nine times the data bit rate. This clock is referred to as CLK_9 or CLKx9.
  • the resulting code has three phase positions: 8, 9 or 10 times width of CLK_9 depending upon the input data transitions.
  • a low to high transition is represented by 8 clock periods
  • a high to low transition is represented by 10 clock periods
  • no transition is represented by 9 clock periods.
  • Encoded data has a transition for each input data bit. This allows the code to achieve the advantages of bi-phase coding where a base-band spectrum is clustered about two bands. Using further carrier suppression, high bandwidth efficiency is achieved. In addition, the output code transition is at the center of each bit. This improves bandwidth efficiency.
  • an MP3 audio source needs a clock of 4.096 MHz for its operation.
  • an 18.432 MHz crystal is used as frequency source. This requires an odd division ratio of 4.5.
  • the division is implemented using finite state machine design.
  • a basic counter counts 000 to 111. The most significant bit is XOR'ed with the clock to give an additional edge at the transition from count 011 to 100.
  • the counting sequence is controlled using a state machine. Effectively, count 4 is extended to count 4.5.
  • An input data edge is detected using a digital technique.
  • edge detection unlike conventional edge detection, external resistor and capacitor use is avoided.
  • a second and more important advantage is that the edge remains visible until the rising edge of the local clock, which is used as reference for all phase transitions. This avoids the problem of missing an edge due to a race condition, and also allows straightforward synchronous digital design implementations.
  • the local clock is divided to generate the CLK_9 clock, which is used for encoding. This clock generation is synchronized with the input data edges, and is used as an input to sequence generators. Two sequence generators are used to generate the encoded output. One generates 5 1 's and 4 0's, the other generates 4 1's and 5 0's. The choice of 5 and 4 allows a change of encoded waveform at the center of each data bit.
  • a multiplexer selects an output of the sequence generators depending on the input data state.
  • the output of the multiplexer is compression-encoded data.
  • the output from the encoder is low-pass filtered. This renders the varying-width pulses to tent and inverted tent maps, which are then used to modulate the 10.7 MHz sub-carrier.
  • Decoding is accomplished as shown in Figures 4 and 5.
  • the decoding scheme works based on the encoded data input edge and width.
  • Two 7 bit binary counters are used to measure incoming data width, and a decision for the output data is based on these counters.
  • the input edge detection circuit synchronizes the counter values.
  • a soft decision technique is applied with a variable threshold to decide the state of the output data.
  • a clock at the data bit rate is generated locally and synchronized with the input data stream. Output data is then latched with the synchronized clock.
  • An input data edge is detected using a digital technique. This is the same as done in the encoder. There are two edge-triggered latches used to capture positive and negative edges of the input data, respectively. These latches are cleared with a 'clearjatch' signal that is generated at the rising edge of the clock in a synchronous fashion.
  • a local crystal clock generator at 18 MHz is divided to generate two clock signals.
  • CLK_144 is 144 times the data rate. This is used for data sampling and input data width counting.
  • the other clock is same as the data rate and is used for latching the final data output.
  • the first counter (“counterl”) is used to track an input data width of 4 from the encoder CLK_9 clock. Since, in the decoder, CLK_144 is used for a counter, a count of 64 indicates the event of counting a width 4 pulse.
  • Counter2 is a 7-bit binary counter that counts 0 to 127 in free running mode.
  • the count64 detector circuit controls the counting sequence of counter2.
  • a count 64/80 detector is implemented as follows: On every detected edge, a constant is loaded into the counter. The constant value is decided based upon the count at the edge and can be one of the following values: If the edge occurs 64 (i.e., 8 x 8) counts after reset, the constant value is 48 (i.e., 128 - 80). If the edge occurs 80 (i.e., 10 x 8) counts after reset, the constant value is 64 (i.e., 128 - 64). The constant values are chosen so that at the beginning of next incoming encoded data bit, the counter reaches a count of zero. This is used for output data transition generation. The constant as decided by the count64/80 detector circuit is stored in this circuit and loaded to counter2 at the rising edge of encoded data.
  • the event of counter2 reaching a count of zero is recorded in the synchronization circuit and used for synchronizing the locally generated clock.
  • Decoded data is latched with the synchronized clock, and the output of the latch is the final decoded data.
  • the present disclosure provides a new type of secure, synchronized communication system using chaotic frequency modulation.
  • a novel feature of this system is that the information bit stream is encoded in such a manner as to generate a coupled skewed tent map of the possible trajectories of the wave- form in order to stay within a bounded state-space region.
  • An incidental advantage is that while transformation is to an Intermediate RF frequency, the whole chaotic perturbation can be contained in a very narrow bandwidth, thus reducing the noise in the system.
  • the technique used here enables analog signals to be encoded into the digital realm and processed as digital signals within the same confinements of bandwidth.
  • a similar algorithmic generator is used to synchronize the receiver to the transmitter.
  • chaotic synchronization happens due to the skewed tent maps arising out of the encoder modifying the stationary chaotic process of the Colpitts oscillator, which can be detected by a fast detector that looks for phase perturbations.
  • the encoding algorithm automatically allows synchronization as well as information transmission.
  • a combination of the symbolic algorithm and the non-linear chaos generation automatically generates an RF Tent map. This process is unique in that it allows for sharp noise reducing filters on both transmitter and receiver, resulting in superior noise performance.
  • the relative orbits of the spatial orbits can be controlled very precisely. This means that the look-up window is extremely constrained due to the symbolic selection. As long as the band-pass filter can pass through the chaotic signal faithfully, decoding can be accomplished at the receiver by ensuring that the Lyapunov exponent is negative.
  • architectures in accordance with the present disclosure address the limitations of the existing systems in terms of cost and complexity. Furthermore, system embodiments are capable of realizing multiple high-speed digital services over bandwidth constrained RF bands. Flexible architecture enables any radio, television or cellular station to be able to send separate digital information on separate sub-carriers on either side of the fundamental transmitting frequency, without violating FCC power spectral templates. Modulation and demodulation processes are very similar to conventional radio receivers, and hence, can be integrated with existing radio architectures.
  • embodiments of the present disclosure provide for low- complexity and high data-rate communication systems that are inherently secure, MAC free, and easily interfaced with existing systems. Multi-path effects are minimal because the embodiments use a timed modulation scheme wherein the energy occupancy for detection is a miniscule portion of the entire bit width.
  • the teachings of the present disclosure may be implemented as a combination of hardware and software.
  • the software is preferably implemented as an application program tangibly embodied on a program storage unit.
  • the application program may be uploaded to, and executed by, a machine comprising any suitable architecture.
  • the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPU"), a random access memory (“RAM”), and input/output ("I/O") interfaces.
  • CPU central processing units
  • RAM random access memory
  • I/O input/output
  • the computer platform may also include an operating system and microinstruction code.
  • the various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU.
  • various other peripheral units may be connected to the computer platform such as an additional data storage unit and an output unit.

Abstract

L'invention concerne un système (110) et un procédé d'émission et de réception de signal par modulation de fréquence chaotique en bande étroite utilisant des applications en forme de tente asymétrique. Ce système comprend un codeur (118) de correction aval d'erreur destiné à recevoir des données d'entrée, un dispositif de mise en paquets de données (120) en communication, via signal, avec le codeur de correction aval d'erreur, un codeur de compression (122) en communication, via signal, avec le dispositif de mise en paquets de données, une liaison de fréquence radio (114) en communication, via signal, avec le codeur de compression, un décodeur de compression (124) en communication, via signal, avec la liaison de fréquence radio, un dispositif de libération de paquets de données (126) en communication, via signal, avec le décodeur de compression, ainsi qu'un décodeur de correction aval d'erreur (128) en communication, via signal, avec le dispositif de libération de paquets de données permettant de récupérer les données d'entrée par la commande des orbites de chaos. Le procédé consiste à émettre un signal indiquant des données modulées par déplacement de phase chaotique, à propager le signal émis dans une bande de fréquence étroite et à recevoir le signal propagé quasiment sans dégradation des données indiquées par la commande des orbites de chaos.
PCT/US2002/022396 2001-07-24 2002-06-15 Modulation de frequence chaotique en bande etroite utilisant des applications en forme de tente asymetrique WO2003010917A1 (fr)

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