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WO2001093519A1 - Systeme de communication a bande passante extra large avec dispositif a faible reception de bruit - Google Patents

Systeme de communication a bande passante extra large avec dispositif a faible reception de bruit Download PDF

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Publication number
WO2001093519A1
WO2001093519A1 PCT/US2001/014834 US0114834W WO0193519A1 WO 2001093519 A1 WO2001093519 A1 WO 2001093519A1 US 0114834 W US0114834 W US 0114834W WO 0193519 A1 WO0193519 A1 WO 0193519A1
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WIPO (PCT)
Prior art keywords
signal
uwb
data
receiver
code
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PCT/US2001/014834
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English (en)
Inventor
John W. Mccorkle
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XtremeSpectrum Inc
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XtremeSpectrum Inc
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Priority claimed from US09/684,782 external-priority patent/US6859506B1/en
Application filed by XtremeSpectrum Inc filed Critical XtremeSpectrum Inc
Priority to AU2001261278A priority Critical patent/AU2001261278A1/en
Publication of WO2001093519A1 publication Critical patent/WO2001093519A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/7163—Spread spectrum techniques using impulse radio
    • H04B1/719—Interference-related aspects
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/0209—Systems with very large relative bandwidth, i.e. larger than 10 %, e.g. baseband, pulse, carrier-free, ultrawideband
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/023—Interference mitigation, e.g. reducing or avoiding non-intentional interference with other HF-transmitters, base station transmitters for mobile communication or other radar systems, e.g. using electro-magnetic interference [EMI] reduction techniques
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/7163—Spread spectrum techniques using impulse radio
    • H04B1/717—Pulse-related aspects
    • H04B1/7172—Pulse shape
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/38—Synchronous or start-stop systems, e.g. for Baudot code
    • H04L25/40—Transmitting circuits; Receiving circuits
    • H04L25/49—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems
    • H04L25/4902—Pulse width modulation; Pulse position modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0004—Modulated-carrier systems using wavelets
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/7163—Spread spectrum techniques using impulse radio
    • H04B1/7176—Data mapping, e.g. modulation

Definitions

  • the present invention relates to ultra wideband (UWB) radio communication systems, methods and devices used in the system for generating and receiving UWB waveforms that include wavelets that are modulated to convey digital data over a wireless radio communication channel using ultra wideband signaling techniques.
  • radio communication techniques to transmit digital data over a wireless channel. These techniques include those used in mobile telephone systems, pagers, remote data collection systems, and wireless networks for computers, among others. Most conventional wireless communication techniques modulate the digital data onto a high-frequency carrier that is then transmitted via an antenna into space.
  • Ultra wideband (UWB) communications systems transmit carrierless high data rate, low power signals. Since a carrier is not used, the transmitted waveforms themselves contain the information being communicated. Accordingly, conventional UWB systems transmit pulses, the information to be communicated is contained in the pulses themselves, and not on a carrier.
  • PPM pulse position modulation
  • a single time frame could be used encode a single bit of data, rather than the two time frames (i.e., early and late) that would be required by a PPM system.
  • the present UWB communications system and related co-pending application Serial No. 09/209,460 filed May 14, 1998, entitled ULTRA WIDE BANDWIDTH SPREAD SPECTRUM COMMUNICATIONS SYSTEM (Attorney Docket No. 10188-0001-8), information is carried by the shape of the pulse, or the shape in combination with its position in the pulse-sequence.
  • non-ideal device performance can cause tones to pass through to the antenna and to be radiated.
  • switches, gates, and analog mixers that are used to generate pulses are well known to be non-ideal devices.
  • leakage is a problem.
  • a signal that is supposed to be blocked at certain times, for example, can continue to leak through.
  • non-ideal symmetry in positive and negative voltages or current directions can allow tones be generated or leak through.
  • the output of a mixer can include not only the desired UWB pulse stream, but also spikes in the frequency domain at the clock frequency and its harmonics, as well as other noise, due to leakage between the RF, LO, and IF ports.
  • This is problematic since one of the design objectives is to generate a pulse stream that will not interfere with other communications systems.
  • Similar problems to those discussed above regarding transmitters are also encountered in UWB receivers.
  • Mixers are used in UWB receivers to mix the received signal with matching waveforms so that the data transmitted may be decoded.
  • the spectral spikes (DC and otherwise) introduced by the non-ideal analog mixers can make decoding of only moderately weak signals difficult or impossible.
  • UWB receivers often suffer from leakage of the UWB signal driving the mixer. These UWB drive signals can radiate into space and be received by the antenna where it can jam the desired UWB signal due to its very close proximity and large amplitude. This reception of the drive signal being used to decode the received signal can therefore cause a self-jamming condition wherein the desired signal becomes unintelligible.
  • the challenge is to develop a highly integratable approach for generating shape-modulated wavelet sequences that can be used in a UWB communications system to encode, broadcast, receive, and decode a data stream. It would be advantageous if the data stream to be transmitted could be encoded by changing a shape of the UWB pulse rather than a position of the UWB pulse as with conventional systems.
  • the challenge is to build such a wavelet generator where the smooth power spectrum calculated by using ideal components, is realized using non- ideal components.
  • an approach to generating and receiving UWB waveforms that does not generate unwanted frequency domain spikes as a by-product, spikes that are prone to interfere with other communications devices or cause self- jamming would be advantageous. It would also be advantageous if the UWB waveform generation approach were to minimize the power consumption because many of the targeted applications for UWB communications are in handheld battery-operated mobile devices.
  • one object of this invention is to provide a novel receiver for use in a UWB communication system that addresses the above-identified and other problems with conventional devices.
  • the inventors of the present invention have recognized that by implementing a two-mixer approach to receiving UWB waveforms, that the noise leakage from the non-ideal analog mixers can be whitened, thereby avoiding the interference problems caused by conventional single-mixer approaches.
  • the present inventors have provided a contrarian approach of suppressing mixer-created interference by using a second mixer.
  • the UWB receiver uses a conventional differential mixer to mix a received waveform of a sequence of UWB wavelets not with a correlated and synchronized sequence of wavelets as has been done in conventional systems, but rather, with a synchronized «-bit user polarity code, the same user code that was used to encode the data stream at the UWB transmitter.
  • the n-bit user polarity code is a non-return-to zero code, not wavelets. Accordingly, by mixing the received signal with this synchronized code, the received wavelets will be either passed through the mixer non-inverted (if mixed with a NRZ '1'), or inverted (if mixed with a NRZ '0').
  • the output of the first differential mixer is a waveform that has sequences of n wavelets, all in an upright orientation, or all in an inverted orientation, according to the data stream transmitted.
  • the UWB receiver has a synchronized UWB wavelet generator that generates wavelets according to the same shape coding that was used by the transmitter, except always having the same polarity.
  • the output of this receiver wavelet generator is mixed with the output signal of the first mixer using a second mixer.
  • the output of the second differential mixer will be a waveform that has sequences of coherently detected wavelets, where each group of n wavelets has only positive components (e.g., corcesponding to a data '1'), or only negative components (e.g., conesponding to a data '0'), depending on the data being sent.
  • the output waveform has this characteristic because mixing a positive-negative wavelet with itself will produce a positive-positive wavelet, and conversely, mixing a positive- negative wavelet with an inverted representation of itself (i.e., a negative-positive wavelet) will result in a negative-negative wavelet.
  • These all-positive or all-negative wavelets are then integrated and sampled in order to decode the transmitted data stream.
  • the inventors of the present invention have recognized that by using two non- ideal mixers, the interference produced by the first mixer due to imbalance, non- linearity, and leakage between ports, is whitened (i.e., spread over a wide range) by mixing it's output with the output of the wavelet generator. Furthermore, by mixing the received waveform with a NRZ user code, the present inventors have recognized that leakage (e.g., radiated through the air and coupled into the receive antenna, or coupled via the substrate or wiring due to the close proximity of parts in a miniaturized radio) from the wavelet generator in the receiver is whitened because it is no longer coherent to itself by the time it reaches the second mixer.
  • leakage e.g., radiated through the air and coupled into the receive antenna, or coupled via the substrate or wiring due to the close proximity of parts in a miniaturized radio
  • the coupled leakage becomes pseudo-randomly inverted and non-inverted in the first mixer by the NRZ code such that its contribution to the output of the second mixer integrates toward zero in the integrator.
  • any similar leakage of the NRZ user code could self-mix to produce a positive or negative output at the first mixer.
  • This low frequency component is blocked by a coupling network between the two mixers, shown, for example, as a DC blocking capacitor in Figure 4. Both these forms of leakage are particularly troubling since they dynamically change with the environment, which affects the coupling.
  • the leakage from the NRZ signal that passes through the second mixer tend to be zero-mean and spiky, but most importantly are synchronized with the integrator and A/D timing.
  • the enor is constant and can be estimated and removed by the receiver controller and interface, by a servo- loop that sets the A/D zero-reference voltage, or by setting up the coding so as to add and subtract chips such that these leakage terms cancel.
  • the conventional mixer is a Gilbert cell mixer.
  • the mixer is, for example, a diode bridge mixer, or any electrically, optically, or mechanically-driven configuration of switching devices including, for example, an FET, a bulk semiconductor device, or a micro-machine device. Consistent with the title of this section, the above summary is not intended to be an exhaustive discussion of all the features or embodiments of the present invention. A more complete, although not necessarily exhaustive description of the features and embodiments of the invention is found in the section entitled "DESCRIPTION OF THE PREFERRED EMBODIMENTS" as well as the entire document generally.
  • FIG. 1 A is a block diagram of an ultra-wide band (UWB) transceiver, according to the present invention.
  • UWB ultra-wide band
  • Figure IB is a diagram for illustrating the operation of the transceiver of Figure 1 A, according to the present invention.
  • Figure 2 is a block diagram of the transceiver of Figure 1A, that manipulates a shape of UWB pulses, according to the present invention
  • Figure 3 is a schematic diagram of a general-purpose microprocessor-based or digital signal processor-based system, which can be programmed by a skilled programmer to implement the features of the present invention
  • Figure 4 is a schematic diagram of an ultra wideband receiver and waveform corcelator according to one embodiment of the present invention.
  • Figure 5 is an exemplary timing chart illustrating the signals at the various inputs and outputs of the components in Figure 4;
  • Figure 6A is a schematic diagram of a generalized single stage mixing circuit susceptible to self-jamming
  • Figure 6B is a schematic diagram of a generalized two-stage mixing circuit for avoiding self-noise in an ultra wideband receiver according to the present invention.
  • FIG. 1A is a block diagram of an ultra-wide band (UWB) transceiver.
  • the transceiver includes three major components, namely, receiver 11, radio controller and interface 9, and transmitter 13.
  • the system may be implemented as a separate receiver 11 and radio controller and interface 9, and a separate transmitter 13 and radio controller and interface 9.
  • the radio controller and interface 9 serves as a media access control (MAC) interface between the UWB wireless communication functions implemented by the receiver 11 and transmitter 13 and applications that use the UWB communications channel for exchanging data with remote devices.
  • the receiver 11 includes an antenna 1 that converts a UWB electromagnetic waveform into an electrical signal (or optical signal) for subsequent processing.
  • the UWB signal is generated with a sequence of shape-modulated wavelets, where the occurrence times of the shape-modulated wavelets may also be modulated.
  • the shape control parameters is modulated with the analog signal. More typically, the wavelets take on M possible shapes. Digital information is encoded to use one or a combination of the M wavelet shapes and occurrence times to communicate information.
  • each wavelet communicates one bit, for example, using two shapes such as bi-phase.
  • each wavelet may be configured to communicate nn bits, where M ⁇ 2 nn .
  • four shapes may be configured to communicate two bits, such as with quadrature phase or four-level amplitude modulation.
  • each wavelet is a "chip" in a code sequence, where the sequence, as a group, communicates one or more bits. The code can be M-ary at the chip level, choosing from M possible shapes for each chip.
  • UWB waveforms are modulated by a variety of techniques including but not limited to: (i) bi-phase modulated signals (+1, -1), (ii) multilevel bi- phase signals (+1, -1,+al, -al, +a2, -a2, ..., +aN, -aN), (iii) quadrature phase signals (+1, -1, +j, -j), (iv) multi-phase signals (1, -1, exp(+j ⁇ /N), exp(-j ⁇ /N), exp(+j ⁇ 2/N), exp(-j ⁇ 2/N), ..., exp(+j(N-l)/N), exp(-j ⁇ (N-l)/N)), (v) multilevel multi-phase signals (a; exp(j2 ⁇ /N)
  • the present invention is applicable to variations of the above modulation schemes and other modulation schemes (e.g., as described in Lathi. "Modern Digital and Analog Communications Systems," Holt, Rinehart and Winston, 1998, the entire contents of which is incorporated by reference herein), as will be appreciated by those skilled in the relevant art(s).
  • Some exemplary waveforms and characteristic equations thereof will now be described.
  • the time modulation component for example, can be defined as follows.
  • the signal 2 ⁇ could be encoded for data, part
  • Equation 1 can be used to represent a sequence of exemplary transmitted or received pulses, where each pulse is a shape modulated UWB wavelet,
  • the subscript i refers to the i pulse in the sequence of UWB pulses transmitted or received.
  • the wavelet function g has M possible shapes, and therefore B ⁇ represents a mapping from the data, to one of the M-ary modulation tin shapes at the i pulse in the sequence.
  • the wavelet generator hardware e.g., the UWB waveform generator 17
  • the wavelet generator hardware has several control lines (e.g., coming from the radio controller and interface 9) that govern the shape of the wavelet. Therefore, B ⁇ can be thought of as including a lookup-table for the M combinations of control signals that produce the M desired wavelet shapes.
  • the encoder 21 combines the data stream and codes to generate the M-ary states.
  • Demodulation occurs in the waveform conelator 5 and the radio controller and interface 9 to recover to the original data stream.
  • Time position and wavelet shape are combined into the pulse sequence to convey information, implement user codes, etc.
  • function h is simply the Hubert transform of the function/
  • the parameters ⁇ i 2 ,B i 3 ,... ⁇ represent a generic group of parameters that control the wavelet shape.
  • An exemplary waveform sequence x(t) can be based on a family of wavelet pulse shapes/ that are derivatives of a Guassian waveform as defined by Equation 3 below.
  • the function ⁇ () normalizes the peak absolute value of f B (t) to 1.
  • the parameter B_ __ controls the pulse duration and center frequency.
  • the parameter B_ 3 is the number of derivatives and controls the bandwidth and center frequency.
  • Another exemplary waveform sequence x(t) can be based on a family of wavelet pulse shapes /that are Gaussian weighted sinusoidal functions, as described by Equation 4 below.
  • bi controls the pulse duration
  • ⁇ controls the center frequency
  • k ⁇ controls a chirp rate.
  • Other exemplary weighting functions, beside Gaussian, that are also applicable to the present invention include, for example, Rectangular, Hanning, Hamming, Blackman-Harris, Nutall, Taylor, Kaiser, Chebychev, etc.
  • Another exemplary waveform sequence x(t) can be based on a family of wavelet pulse shapes/that are inverse-exponentially weighted sinusoidal functions, as described by Equation 5 below.
  • the leading edge turn on time is controlled by ;
  • the turn-on rate is controlled by tr.
  • the trailing edge turn-off time is controlled by t2, and the turn-off rate is controlled by tf.
  • the starting phase is controlled by ⁇
  • the starting frequency is controlled by ⁇
  • the chirp rate is controlled by k
  • the stopping frequency is controlled by ⁇ +kT D .
  • tl tr 0.51
  • t2 T D - tr/9.
  • a feature of the present invention is that the M-ary parameter set used to control the wavelet shape is chosen so as to make a UWB signal, wherein the center frequency/ and the bandwidth B of the power spectrum of g(t) satisfies 2f>B>0.25f c .
  • conventional equations define in-phase and quadrature signals (e.g., often referced to as I and Q) as sine and cosine terms.
  • I and Q in-phase and quadrature signals
  • This conventional definition is inadequate for UWB signals.
  • the present invention recognizes that use of such conventional definition may lead to DC offset problems and inferior performance.
  • a key attribute of the exemplary wavelets is that the parameters are chosen such that neither /nor h in Equation 2 above has a DC component, yet/and h exhibit the required wide relative bandwidth for UWB systems.
  • the matched filter output of the UWB signal is typically only a few cycles, or even a single cycle.
  • the parameter n in Equation 3 above may only take on low values (e.g., such as those described in co-pending U.S. Patent Application Serial No. 09/209,460).
  • TFT inverse Fourier transform
  • the width of the compressed pulse p(t) is defined by T & which is the time between the points on the envelope of the compressed pulse E(t) that are 6 dB below the peak thereof, as shown in Figure IB.
  • the envelope waveform E(t) may be determined by Equation 6 below.
  • the above-noted parameterized waveforms are examples of UWB wavelet functions that can be controlled to communicate information with a large parameter space for making codes with good resulting autoconelation and cross- conelation functions.
  • each of the parameters is chosen from a predetermined list according to an encoder that receives the digital data to be communicated.
  • at least one parameter is changed dynamically according to some function (e.g., proportionally) of the analog signal that is to be communicated. Referring back to Figure 1 A, the electrical signals coupled in through the antenna 1 are passed to a radio front end 3.
  • the radio front end 3 processes the electric signals so that the level of the signal and spectral components of the signal are suitable for processing in the UWB waveform conelator 5.
  • the UWB waveform conelator 5 conelates the incoming signal (e.g., as modified by any spectral shaping, such as a matched filtering, partially matched filtering, simply roll-off, etc., accomplished in front end 3) with different candidate signals generated by the receiver 11, so as to determine when the receiver 11 is synchronized with the received signal and to determine the data that was transmitted.
  • the timing generator 7 of the receiver 11 operates under control of the radio controller and interface 9 to provide a clock signal that is used in the conelation process performed in the UWB waveform conelator 5.
  • the UWB waveform conelator 5 conelates in time a particular pulse sequence produced at the receiver 11 with the receive pulse sequence that was coupled in through antenna 1 and modified by front end 3.
  • the UWB waveform conelator 5 provides high signal to noise ratio (SNR) data to the radio controller and interface 9 for subsequent processing.
  • SNR signal to noise ratio
  • the output of the UWB waveform conelator 5 is the data itself.
  • the UWB waveform conelator 5 simply provides an intermediate conelation result, which the radio controller and interface 9 uses to determine the data and determine when the receiver 11 is synchronized with the incoming signal.
  • the radio controller and interface 9 when synchronization is not achieved (e.g., during a signal acquisition mode of operation), the radio controller and interface 9 provides a control signal to the receiver 11 to acquire synchronization. In this way, a sliding of a conelation window within the UWB waveform conelator 5 is possible by adjustment of the phase and frequency of the output of the timing generator 7 of the receiver 11 via a control signal from the radio controller and interface 9. The control signal causes the conelation window to slide until lock is achieved.
  • the radio controller and interface 9 is a processor-based unit that is implemented either with hard wired logic, such as in one or more application specific integrated circuits (ASICs) or in one or more programmable processors.
  • ASICs application specific integrated circuits
  • the receiver 11 provides data to an input port ("RX Data In”) of the radio controller and interface 9.
  • An external process via an output port (“RX Data Out”) of the radio controller and interface 9, may then use this data.
  • the external process may be any one of a number of processes performed with data that is either received via the receiver 11 or is to be transmitted via the transmitter 13 to a remote receiver.
  • the radio controller and interface 9 receives source data at an input port ("TX Data In”) from an external source. The radio controller and interface 9 then applies the data to an encoder 21 of the transmitter 13 via an output port ("TX Data Out”). In addition, the radio controller and interface 9 provides control signals to the transmitter 13 for use in identifying the signaling sequence of UWB pulses.
  • the receiver 11 and the transmitter 13 functions may use joint resources, such as a common timing generator and/or a common antenna, for example.
  • the encoder 21 receives user coding information and data from the radio controller and interface 9 and preprocesses the data and coding so as to provide a timing input for the UWB waveform generator 17, which produces UWB pulses encoded in shape and/or time to convey the data to a remote location.
  • the encoder 21 produces the control signals necessary to generate the required modulation.
  • the encoder 21 may take a serial bit stream and encode it with a forward enor conection (FEC) algorithm (e.g., such as a Reed Solomon code, a Golay code, a Hamming code, a Convolutional code, etc.).
  • FEC forward enor conection
  • the encoder 21 may also interleave the data to guard against burst enors.
  • the encoder 21 may also apply a whitening function to prevent long strings of "ones" or “zeros.”
  • the encoder 21 may also apply a user specific spectrum spreading function, such as generating a predetermined length chipping code that is sent as a group to represent a bit (e.g., inverted for a "one" bit and non-inverted for a "zero” bit, etc.).
  • the encoder 21 may divide the serial bit stream into subsets in order to send multiple bits per wavelet or per chipping code, and generate a plurality of control signals in order to affect any combination of the modulation schemes as described above (and/or as described in Lathi .
  • the radio controller and interface 9 may provide some identification, such as user ID, etc., of the source from which the data on the input port ("TX Data In") is received.
  • this user ID may be inserted in the transmission sequence, as if it were a header of an information packet.
  • the user ID itself may be employed to encode the data, such that a receiver receiving the transmission would need to postulate or have a priori knowledge of the user ID in order to make sense of the data.
  • the ID may be used to apply a different amplitude signal (e.g., of amplitude "f ') to a fast modulation control signal to be discussed with respect to Figure 2, as a way of impressing the encoding onto the signal.
  • the output from the encoder 21 is applied to a UWB waveform generator 17.
  • the UWB waveform generator 17 produces a UWB pulse sequence of pulse shapes at pulse times according to the command signals it receives, which may be one of any number of different schemes.
  • the output from the UWB generator 17 is then provided to an antenna 15, which then transmits the UWB energy to a receiver.
  • the data may be encoded by using the relative spacing of transmission pulses (e.g., PPM, chirp, etc.).
  • the data may be encoded by exploiting the shape of the pulses as described above (and/or as described in Lathi). It should be noted that the present invention is able to combine time modulation (e.g., such as pulse position modulation, chirp, etc.) with other modulation schemes that manipulate the shape of the pulses.
  • FIG. 2 is a block diagram of a transceiver embodiment of the present invention in which the modulation scheme employed is able to manipulate the shape and time of the UWB pulses.
  • the energy when receiving energy through the antenna 1, 15 (e.g., conesponding antennas 1 and 15 of Figure 1A) the energy is coupled in to a transmit/receive (T R) switch 27, which passes the energy to a radio front end 3.
  • the radio front end 3 filters, extracts noise, and adjusts the amplitude of the signal before providing the same to a splitter 29.
  • the splitter 29 divides the signal up into one of N different signals and applies the N different signals to different tracking conelators 31 ⁇ - I -
  • Each of the tracking conelators 31 i-3 I N receives a clock input signal from a respective timing generator 7I-7 N of a timing generator module 7, 19, as shown in Figure 2.
  • the timing generators 7 I -7 N for example, receive a phase and frequency adjustment signal, as shown in Figure 2, but may also receive a fast modulation signal or other control signal(s) as well.
  • the radio controller and interface 9 provides the control signals, such as phase, frequency and fast modulation signals, etc., to the timing generator module 7, 19, for time synchronization and modulation control.
  • the fast modulation control signal may be used to implement, for example, chirp waveforms, PPM waveforms, such as fast time scale PPM waveforms, etc.
  • the radio controller and interface 9 also provides control signals to, for example, the encoder 21, the waveform generator 17, the filters 23, the amplifier 25, the T/R switch 27, the front end 3, the tracking conelators 31 i-3 IN (conesponding to the UWB waveform conelator 5 of Figure 1A), etc., for controlling, for example, amplifier gains, signal waveforms, filter passbands and notch functions, alternative demodulation and detecting processes, user codes, spreading codes, cover codes, etc.
  • the radio controller and interface 9 adjusts the phase input of, for example, the timing generator 7 ⁇ , in an attempt for the tracking conelator 31 ⁇ to identify and the match the timing of the signal produced at the receiver with the timing of the arriving signal.
  • the radio controller and interface 9 senses the high signal strength or high SNR and begins to track, so that the receiver is synchronized with the received signal.
  • the receiver will operate in a tracking mode, where the timing generator 7 ⁇ is adjusted by way of a continuing series of phase adjustments to counteract any differences in timing of the timing generator 7 ⁇ and the incoming signal.
  • a feature of the present invention is that by sensing the mean of the phase adjustments over a known period of time, the radio controller and interface 9 adjusts the frequency of the timing generator 7 ⁇ so that the mean of the phase adjustments becomes zero. The frequency is adjusted in this instance because it is clear from the pattern of phase adjustments that there is a frequency offset between the timing generator 7 ⁇ and the clocking of the received signal. Similar operations may be performed on timing generators 7 -7N, SO that each receiver can recover the signal delayed by different amounts, such as the delays caused by multipath (i.e., scattering along different paths via reflecting off of local objects).
  • a feature of the transceiver in Figure 2 is that it includes a plurality of tracking conelators 31 y!> 1 N .
  • a plurality of tracking conelators 31 y!> 1 N By providing a plurality of tracking conelators, several advantages are obtained. First, it is possible to achieve synchronization more quickly (i.e., by operating parallel sets of conelation arms to find strong SNR points over different code-wheel segments). Second, during a receive mode of operation, the multiple arms can resolve and lock onto different multipath components of a signal. Through coherent addition, the UWB communication system uses the energy from the different multipath signal components to reinforce the received signal, thereby improving signal to noise ratio. Third, by providing a plurality of tracking conelator arms, it is also possible to use one arm to continuously scan the channel for a better signal than is being received on other arms.
  • the communications system dynamically adapts to changing channel conditions.
  • the radio controller and interface 9 receives the information from the different tracking conelators 31 1 -3 I N and decodes the data.
  • the radio controller and interface 9 also provides control signals for controlling the front end 3, e.g., such as gain, filter selection, filter adaptation, etc., and adjusting the synchronization and tracking operations by way of the timing generator module 7, 19.
  • the radio controller and interface 9 serves as an interface between the communication link feature of the present invention and other higher level applications that will use the wireless UWB communication link for performing other functions. Some of these functions would include, for example, performing range- finding operations, wireless telephony, file sharing, personal digital assistant (PDA) functions, embedded control functions, location-finding operations, etc.
  • a timing generator 7 0 also receives phase, frequency and/or fast modulation adjustment signals for use in encoding a UWB waveform from the radio controller and interface 9.
  • Data and user codes (via a control signal) are provided to the encoder 21, which in the case of an embodiment of the present invention utilizing time-modulation, passes command signals (e.g., ⁇ t) to the timing generator 7 0 for providing the time at which to send a pulse. In this way, encoding of the data into the transmitted waveform may be performed.
  • command signals e.g., ⁇ t
  • the encoder 21 When the shape of the different pulses are modulated according to the data and/or codes, the encoder 21 produces the command signals as a way to select different shapes for generating particular waveforms in the waveform generator 17. For example, the data may be grouped in multiple data bits per channel symbol.
  • the waveform generator 17 then produces the requested waveform at a particular time as indicated by the timing generator 7 0 .
  • the output of the waveform generator is then filtered in filter 23 and amplified in amplifier 25 before being transmitted via antenna 1, 15 by way of the T/R switch 27.
  • the transmit power is set low enough that the transmitter and receiver are simply alternately powered down without need for the T/R switch 27.
  • neither the filter 23 nor the amplifier 25 is needed, because the desired power level and spectrum is directly useable from the waveform generator 17.
  • the filters 23 and the amplifier 25 may be included in the waveform generator 17 depending on the implementation of the present invention.
  • a feature of the UWB communications system disclosed is that the transmitted waveform x(t) can be made to have a nearly continuous power flow, for example, by using a high chipping rate, where the wavelets g(t) are placed nearly back-to-back.
  • This configuration allows the system to operate at low peak voltages, yet produce ample average transmit power to operate effectively.
  • sub- micron geometry CMOS switches for example, running at one- volt levels, can be used to directly drive antenna 1, 15, such that the amplifier 25 is not required. In this way, the entire radio can be integrated on a single monolithic integrated circuit.
  • the system can be operated without the filters 23. If, however, the system is to be operated, for example, with another radio system, the filters 23 can be used to provide a notch function to limit interference with other radio systems. In this way, the system can operate simultaneously with other radio systems, providing advantages over conventional devices that use avalanching type devices connected straight to an antenna, such that it is difficult to include filters therein.
  • the UWB transceiver of Figures 1A or 2 may be used to perform a radio transport function for interfacing with different applications as part of a stacked protocol architecture.
  • the UWB transceiver performs signal creation, transmission and reception functions as a communications service to applications that send data to the transceiver and receive data from the transceiver much like a wired I/O port.
  • the UWB transceiver may be used to provide a wireless communications function to any one of a variety of devices that may include interconnection to other devices either by way of wired technology or wireless technology.
  • the UWB transceiver of Figure 1A or 2 may be used as part of a local area network (LAN) connecting fixed structures or as part of a wireless personal area network (WPAN) connecting mobile devices, for example.
  • LAN local area network
  • WPAN wireless personal area network
  • all or a portion of the present invention may be conveniently implemented in a microprocessor system using conventional general purpose microprocessors programmed according to the teachings of the present invention, as will be apparent to those skilled in the microprocessor systems art.
  • Appropriate software can be readily prepared by programmers of ordinary skill based on the teachings of the present disclosure, as will be apparent to those skilled in the software art.
  • FIG. 3 illustrates a processor system 301 upon which an embodiment according to the present invention may be implemented.
  • the system 301 includes a bus 303 or other communication mechanism for communicating information, and a processor 305 coupled with the bus 303 for processing the information.
  • the processor system 301 also includes a main memory 307, such as a random access memory (RAM) or other dynamic storage device (e.g., dynamic RAM (DRAM), static RAM (SRAM), synchronous DRAM (SDRAM), flash RAM), coupled to the bus 303 for storing information and instructions to be executed by the processor 305.
  • a main memory 307 may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor 305.
  • the system 301 further includes a read only memory (ROM) 309 or other static storage device (e.g., programmable ROM (PROM), erasable PROM (EPROM), and electrically erasable PROM (EEPROM)) coupled to the bus 303 for storing static information and instructions for the processor 305.
  • ROM read only memory
  • PROM programmable ROM
  • EPROM erasable PROM
  • EEPROM electrically erasable PROM
  • a storage device 311 such as a magnetic disk or optical disc, is provided and coupled to the bus 303 for storing information and instructions.
  • the processor system 301 may also include special purpose logic devices (e.g., application specific integrated circuits (ASICs)) or configurable logic devices (e.g, simple programmable logic devices (SPLDs), complex programmable logic devices (CPLDs), or re-programmable field programmable gate anays (FPGAs)).
  • ASICs application specific integrated circuits
  • SPLDs simple programmable logic devices
  • CPLDs complex programmable logic devices
  • FPGAs re-programmable field programmable gate anays
  • Other removable media devices e.g., a compact disc, a tape, and a removable magneto-optical media
  • fixed, high density media drives may be added to the system 301 using an appropriate device bus (e.g., a small system interface (SCSI) bus, an enhanced integrated device electronics (IDE) bus, or an ultra-direct memory access (DMA) bus).
  • SCSI small system interface
  • IDE enhanced integrated device electronics
  • DMA ultra-direct memory access
  • the processor system 301 may be coupled via the bus 303 to a display 313, such as a cathode ray tube (CRT) or liquid crystal display (LCD) or the like, for displaying information to a system user.
  • the display 313 may be controlled by a display or graphics card.
  • the processor system 301 includes input devices, such as a keyboard or keypad 315 and a cursor control 317, for communicating information and command selections to the processor 305.
  • the cursor control 317 for example, is a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to the processor 305 and for controlling cursor movement on the display 313.
  • a printer may provide printed listings of the data structures or any other data stored and/or generated by the processor system 301.
  • the processor system 301 performs a portion or all of the processing steps of the invention in response to the processor 305 executing one or more sequences of one or more instructions contained in a memory, such as the main memory 307. Such instructions may be read into the main memory 307 from another computer-readable medium, such as a storage device 311. One or more processors in a multi-processing anangement may also be employed to execute the sequences of instructions contained in the main memory 307. In alternative embodiments, hard- wired circuitry may be used in place of or in combination with software instructions. Thus, embodiments are not limited to any specific combination of hardware circuitry and software.
  • the processor system 301 includes at least one computer readable medium or memory programmed according to the teachings of the invention and for containing data structures, tables, records, or other data described herein.
  • the present invention includes software for controlling the system 301, for driving a device or devices for implementing the invention, and for enabling the system 301 to interact with a human user.
  • Such software may include, but is not limited to, device drivers, operating systems, development tools, and applications software.
  • Such computer readable media further includes the computer program product of the present invention for performing all or a portion (if processing is distributed) of the processing performed in implementing the invention.
  • the computer code devices of the present invention may be any interpreted or executable code mechanism, including but not limited to scripts, interpretable programs, dynamic link libraries, Java or other object oriented classes, and complete executable programs. Moreover, parts of the processing of the present invention may be distributed for better performance, reliability, and/or cost.
  • the term "computer readable medium” as used herein refers to any medium that participates in providing instructions to the processor 305 for execution.
  • a computer readable medium may take many forms, including but not limited to, nonvolatile media, volatile media, and transmission media.
  • Non-volatile media includes, for example, optical, magnetic disks, and magneto-optical disks, such as the storage device 311.
  • Volatile media includes dynamic memory, such as the main memory 307.
  • Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise the bus 303. Transmission media may also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications.
  • Common forms of computer readable media include, for example, hard disks, floppy disks, tape, magneto-optical disks, PROMs (EPROM, EEPROM, Flash EPROM), DRAM, SRAM, SDRAM, or any other magnetic medium, compact disks (e.g., CD-ROM), or any other optical medium, punch cards, paper tape, or other physical medium with patterns of holes, a carrier wave, carrierless transmissions, or any other medium from which a system can read.
  • Various forms of computer readable media may be involved in providing one or more sequences of one or more instructions to the processor 305 for execution.
  • the instructions may initially be carried on a magnetic disk of a remote computer.
  • the remote computer can load the instructions for implementing all or a portion of the present invention remotely into a dynamic memory and send the instructions over a telephone line using a modem.
  • a modem local to system 301 may receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal.
  • An infrared detector coupled to the bus 303 can receive the data carried in the infrared signal and place the data on the bus 303.
  • the bus 303 carries the data to the main memory 307, from which the processor 305 retrieves and executes the instructions.
  • the instructions received by the main memory 307 may optionally be stored on a storage device 311 either before or after execution by the processor 305.
  • the processor system 301 also includes a communication interface 319 coupled to the bus 303.
  • the communications interface 319 provides a two-way UWB data communication coupling to a network link 321 that is connected to a communications network 323 such as a local network (LAN) or personal area network (PAN)323.
  • a communications network 323 such as a local network (LAN) or personal area network (PAN)323.
  • the communication interface 319 may be a network interface card to attach to any packet switched UWB-enabled personal area network (PAN)323.
  • the communication interface 319 may be a UWB accessible asymmetrical digital subscriber line (ADSL) card, an integrated services digital network (ISDN) card, or a modem to provide a data communication connection to a conesponding type of communications line.
  • ADSL asymmetrical digital subscriber line
  • ISDN integrated services digital network
  • the communications interface 319 may also include the hardware to provide a two-way wireless communications coupling other than a UWB coupling, or a hardwired coupling to the network link 321.
  • the communications interface 319 may incorporate the UWB transceiver of Figure 2 as part of a universal interface that includes hardwired and non-UWB wireless communications coupling to the network link 321.
  • the network link 321 typically provides data communication through one or more networks to other data devices.
  • the network link 321 may provide a connection through a LAN to a host computer 325 or to data equipment operated by a service provider, which provides data communication services through an IP (Internet Protocol) network 327.
  • IP Internet Protocol
  • the network link 321 may provide a connection through a PAN323 to a mobile device 329 such as a personal digital assistant (PDA) laptop computer, or cellular telephone.
  • PDA personal digital assistant
  • the LAN/PAN communications network 323 and IP network 327 both use electrical, electromagnetic or optical signals that carry digital data streams.
  • the signals through the various networks and the signals on the network link 321 and through the communication interface 319, which carry the digital data to and from the system 301, are exemplary forms of carrier waves transporting the information.
  • the processor system 301 can transmit notifications and receive data, including program code, through the network(s), the network link 321 and the communication interface 319.
  • the encoder 21 and waveform generator 17 of the transceiver of the present invention function together to create a UWB waveform from a digital data stream by first, multiplying each bit of data in the data stream by an identifying code (e.g., an n- bit user code), thereby expanding each bit of data into a codeword of data bits equal in length to the length of the identifying code.
  • the codeword is then further processed to create two derivative codewords that are that are sent to the UWB waveform generator 17 where they are mixed with a pulse generator and recombined through a two-stage mixing process prior to being transmitted via the antenna 15.
  • the encoder 21 receives a digital data stream from an external source via the radio and controller interface 9.
  • the encoder 21 multiplies each bit of the digital data stream by a user code, which in one embodiment is a unique sequence of bits conesponding to a particular user. For example, multiplying a user code of '1101 0110' by a data bit of '1' results in an 8-bit representation of the '1 ' that is identical to the user code, or '1101 0110.' On the other hand, multiplying that same user code by a data bit of ⁇ 0' results in an 8-bit representation of the '0' that is the 8 bits of the user code inverted, or '0010 1001.'
  • the encoder 21 multiplies the user code by each bit of the digital data stream to create a sequence of n-bit codewords, where n is the length of the user code.
  • the UWB waveform generator 17 further processes the sequence of codewords in creating an UWB waveform that can be transmitted.
  • Figure 4 illustrates the details of the UWB waveform conelator 31 of Figure 2, according to the present invention.
  • Figure 5 is a timing diagram conesponding to the signals discussed with respect to Figure 4. As shown in Figure 4, a propagated signal SI is coupled to the antenna 1, 15, and is amplified and filtered by the front end 3. The front end 3 outputs a signal S2, which is input to a first mixer 31a.
  • the first mixer 31a mixes the incoming signal S2 with a Code A signal to produce an output signal S3.
  • the signal S3 passes through a simple DC blocking capacitor 31b, or other DC blocking filter network which will block any DC bias component of the signal S3, resulting in a new signal S4.
  • the signal S4 is mixed via mixer 31c with a sequence of UWB wavelets W from the wavelet generator 3 le.
  • the wavelet generator 31 e is triggered by the signal 322/422 of the timing generator 7 to generate the UWB wavelets W.
  • the output of the mixer 31c is signal S5, which may include a DC component.
  • Signal S5 is passed to an integrator 3 Id.
  • the integrator 3 Id integrates signal S5 for a predetermined number of clock pulses, outputting signal S6, as shown in Figure 5. In the exemplary timing diagram of Figure 5, the integrator has integrated signal S5 for four clock pulses, resulting in an output level signal S6 of four volts.
  • the integrator 3 Id is reset by the signal Reset I. After being reset, the integrator 3 Id continues to integrate signal S5 for a second predetermined number of clock pulses. Continuing with the example of Figure 5, the integrator 31 d will continue to integrate the signal S5 for three clock pulses from the point indicated as Al in Figure 5 to the point indicated as A2. Since the signal S5 being integrated is made up of negative-amplitude small pulses for the period of time beginning at point Al and ending at point A2, the integrator 3 Id will integrate down for those three clock pulses, as shown by signal S6 of Figure 5. In this example the resultant output level signal S6 is -3 volts.
  • the A/D converter 31g samples the signal S6 after both the first predetermined number of clock pulses (e.g., the first four clock pulses) indicated as point Al in Figure 5, and after the second predetermined number of clock pulses (e.g., the second three clock pulses) indicated as point A2 in Figure 5.
  • the A/D converter 3 lg continues to sample the signal S6 at points A3, A4, and so on.
  • the output of the A/D converter 31g is signal S7, which is multiplied with a Code B signal by a digital multiplier 3 lh.
  • the Code B signal is used to invert the signal S7 for every second sample (i.e., the signal S6 sampled at points A2 and A4 where the integrator had integrated negative amplitude pulses).
  • Latch 3 lj latches the value of signal S9 as signal S10, then summer 31i is reset via signal Reset S. The latch 31j ensures proper alignment of the signal S10, which is provided to the radio controller and interface 9.
  • Control signals are also provided to the waveform conelator 31 from the radio controller and interface 9.
  • the Control signals communicate the parameters (e.g., code length, code values, etc.) of the actual codes generated by the code generator 3 If (e.g., Code A, Code B, Xmit Code, etc.).
  • a transmit code, Xmit Code is shown, for example, as a seven-bit length code in Figure 5.
  • the Control signals also program the wavelet generator 31e via the code generator 3 If for different wavelet styles (e.g., odd symmetry, even symmetry, different center frequency wavelets, different amplitudes, different phases, wavelet width, etc).
  • the control signals can also program code B, for example, to always be a positive value (e.g., +1), and the A/D converter 31g and integrator 31d to integrate and sample only once per bit.
  • the control signals might also program code B, for example, to be an L-length sequence of plus and minus ones, code A to repeat L times for each bit, and the A/D 31g and integrator 3 Id to integrate and sample once per Code A repetition. In this way, if Code A were, for example, an M-length sequence, then a bit would be comprised of M*L chips.
  • a NRZ data source has been encoded prior to transmission with an w-bit user code.
  • the received signal is mixed with Code A at a first mixer 31a.
  • Code A conesponds to the first four bits of the user code (Xmit Code) used to encode the data source and the last three bits of the user code inverted.
  • Figure 6A is a schematic diagram of a generalized single stage mixing circuit for use in an ultra wideband receiver.
  • the receiver includes an antenna 700, a wavelet generator 701, and mixer 702.
  • the receiver of Figure 6 A will be susceptible to self-jamming and self-noise since the output of the wavelet generator 701, being mixed with the received signal at mixer 702 has the same characteristics as the signal being looked for by the receiver. Due to the leakage by the mixer 702 the antenna 700 may receive not only the signal being looked for, but also the leaked signal having similar properties to the signal being looked for resulting in a self jamming of the receiver.
  • Figure 6B is a schematic diagram of a generalized two-stage mixing circuit for achieving noise cancellation and avoiding self-jamming in an ultra wideband receiver according to the present invention.
  • the receiver includes an antenna 703, a de-jam code generator 704, a first mixer 705, a network 706, a wavelet generator 707, and a second mixer 708.
  • the present invention uses a two-stage mixing to cancel self-noise caused by the non-ideal analog mixers 705, 708 and to avoid self-jamming.
  • the network 706 is used to block any DC bias produced at the first mixer 705.
  • the received UWB wavelets coupled to the antenna 700 may, for example, be bi-phase wavelets, multi-level bi- phase wavelets, quad-phase wavelets, multi-level quad-phase wavelets, or other shapes used to encode a NRZ data source at the transmitter.
  • Decoding is achieved by providing the de-jam code generator with the transmit code used by the transmitter to generate two signals that are mixed with the received signals via a two-stage mixing approach. As described in the context of Figures 4 and 5, the receiver can avoid self- jamming by mixing the received waveform with a waveform having different characteristics than the signal being looked for. As shown in Figure 6B, the de-jam code generator generates two codes A, B that are mixed with the received signal at the first mixer 705 and the second mixer 708, respectively. Signal B is used to shape the wavelets conesponding to the wavelet shaping scheme used by the transmitter. Wavelet shaping schemes are described in co-pending application Serial No.
  • the output generated by the wavelet generator 707 (signal D) is mixed with the received signal at mixer 708.
  • the two signals that are mixed with the received signal in Figure 6B i.e., A and D
  • Signals A and D that are mixed with the received signal have properties such that if A and D were mixed together, the resultant waveform would be the same as signal C generated by the wavelet generator in the single mixer scheme of Figure 6A.
  • the de-jam code generator 701 in combination with the wavelet generator 704 can implement a variety of schemes for decoding the received UWB waveform and achieving the advantageous results described herein.

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Abstract

Cette invention concerne un récepteur à bande passante ultra-large (UWB) avec suppression de bruit qui s'utilise dans un système de communication numérique (UWB). Le récepteur UWB fait appel à une méthode de mélange bi-étagée pour supprimer le bruit et les distorsions internes. On empêche l'auto-brouillage en inversant une partie du signal reçu dans le premier mélangeur, puis en détectant de façon cohérente le signal partiellement inversé de façon synchrone dans le second mélangeur. Comme les signaux de commande des deux mélangeurs ne correspondent pas au signal requis, les fuites de l'un ou l'autre signal ne viennent pas brouiller le signal recherché, ce qui empêche le récepteur de détecter et de décoder un signal faible.
PCT/US2001/014834 2000-05-26 2001-05-25 Systeme de communication a bande passante extra large avec dispositif a faible reception de bruit Ceased WO2001093519A1 (fr)

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Non-Patent Citations (4)

* Cited by examiner, † Cited by third party
Title
AINSLEIGH P L ET AL: "A B-WAVELET-BASED NOISE-REDUCTION ALGORITHM", IEEE TRANSACTIONS ON SIGNAL PROCESSING, IEEE, INC. NEW YORK, US, vol. 44, no. 5, 1 May 1996 (1996-05-01), pages 1279 - 1284, XP000598086, ISSN: 1053-587X *
DANESHGARAN F ET AL: "Clock synchronisation without self-noise using wavelets", ELECTRONICS LETTERS, IEE STEVENAGE, GB, vol. 31, no. 10, 11 May 1995 (1995-05-11), pages 775 - 776, XP006002781, ISSN: 0013-5194 *
KUCUR ET AL: "Performance of scale-code division multiple access (SCDMA) over the asynchronous AWGN channel", VEHICULAR TECHNOLOGY CONFERENCE, 1999 IEEE 49TH HOUSTON, TX, USA 16-20 MAY 1999, PISCATAWAY, NJ, USA,IEEE, US, 16 May 1999 (1999-05-16), pages 2204 - 2208, XP010342297, ISBN: 0-7803-5565-2 *
WANG RONG ET AL: "Performance of MC-CDMA based on wavelet packets in Rayleigh multipath fading channel", ELECTRONICS LETTERS, IEE STEVENAGE, GB, vol. 36, no. 12, 8 June 2000 (2000-06-08), pages 1070 - 1072, XP006015320, ISSN: 0013-5194 *

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