EP0956650A1 - Signalverarbeitungsvorrichtung die die spektralen eigenschaften des signals ausnutzt - Google Patents

Signalverarbeitungsvorrichtung die die spektralen eigenschaften des signals ausnutzt

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Publication number
EP0956650A1
EP0956650A1 EP96936374A EP96936374A EP0956650A1 EP 0956650 A1 EP0956650 A1 EP 0956650A1 EP 96936374 A EP96936374 A EP 96936374A EP 96936374 A EP96936374 A EP 96936374A EP 0956650 A1 EP0956650 A1 EP 0956650A1
Authority
EP
European Patent Office
Prior art keywords
signal
signals
filter
lcl
interest
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP96936374A
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English (en)
French (fr)
Inventor
William A. Gardner
Stephen V. Schell
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Statistical Signal Processing Inc
Original Assignee
Statistical Signal Processing Inc
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Filing date
Publication date
Application filed by Statistical Signal Processing Inc filed Critical Statistical Signal Processing Inc
Publication of EP0956650A1 publication Critical patent/EP0956650A1/de
Withdrawn legal-status Critical Current

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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006Arrangements for removing intersymbol interference
    • H04L25/03012Arrangements for removing intersymbol interference operating in the time domain
    • H04L25/03019Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception
    • H04L25/03038Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure
    • H04L25/0305Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure using blind adaptation
    • HELECTRICITY
    • H03ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H21/00Adaptive networks
    • H03H21/0012Digital adaptive filters
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/02Arrangements for detecting or preventing errors in the information received by diversity reception
    • H04L1/06Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006Arrangements for removing intersymbol interference
    • H04L25/03012Arrangements for removing intersymbol interference operating in the time domain
    • H04L25/03019Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception
    • H04L25/03038Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006Arrangements for removing intersymbol interference
    • H04L25/03012Arrangements for removing intersymbol interference operating in the time domain
    • H04L25/03019Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception
    • H04L25/03038Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure
    • H04L25/03044Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure using fractionally spaced delay lines or combinations of fractionally integrally spaced taps
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B1/00Details 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
    • H04B1/06Receivers
    • H04B1/10Means associated with receiver for limiting or suppressing noise or interference
    • H04B1/12Neutralising, balancing, or compensation arrangements
    • H04B1/123Neutralising, balancing, or compensation arrangements using adaptive balancing or compensation means
    • H04B1/126Neutralising, balancing, or compensation arrangements using adaptive balancing or compensation means having multiple inputs, e.g. auxiliary antenna for receiving interfering signal
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006Arrangements for removing intersymbol interference
    • H04L2025/03592Adaptation methods
    • H04L2025/03598Algorithms
    • H04L2025/03611Iterative algorithms
    • H04L2025/03617Time recursive algorithms
    • H04L2025/0363Feature restoration, e.g. constant modulus
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006Arrangements for removing intersymbol interference
    • H04L2025/03592Adaptation methods
    • H04L2025/03598Algorithms
    • H04L2025/03611Iterative algorithms
    • H04L2025/03656Initialisation

Definitions

  • the present invention pertains generally to reducing interference in wireless communications systems using Gaussian Minimum Shift Keyed (GMSK) or other Minimum Shift Keyed (MSK) signals, and more specifically to a method and apparatus for (i) rejecting interfering signals received at a communications receiver and occupying the same or adjacent spectral band as one or more desired signals, (ii) separating, if needed, multiple desired communication signals received at a communications receiver, and (iii) correcting distortion that affects one or more desired communication signals received at a communications receiver such as that caused by a multipath radio-frequency (RF) propagation channel.
  • GMSK Gaussian Minimum Shift Keyed
  • MSK Minimum Shift Keyed
  • the background of the present invention falls into two general categories: (i) work pertaining to the filtering structures that perform interference rejection, signal separation, and distortion correction; and (ii) work pertaining to the adaptive algorithms that adjust the coefficients in these filtering structures to allow them to operate in signal environments that are endemic to mobile, portable, and personal wireless communication systems. This work, however, differs substantially from the present invention.
  • work pertaining to the filtering structures that perform interference rejection, signal separation, and distortion correction work pertaining to the adaptive algorithms that adjust the coefficients in these filtering structures to allow them to operate in signal environments that are endemic to mobile, portable, and personal wireless communication systems. This work, however, differs substantially from the present invention.
  • This body of work includes all filtering structures that belong to the class of linear time-invariant (LTI) spatio-temporal filters; this class of filters is defined by the fact that their outputs can be modeled mathematically as sums of amplitude-scaled, phase-shifted, and time-shifted versions of the output signals of the multiple sensors.
  • LTI linear time-invariant
  • the single-sensor FRESH filter has a severe limitation that is transcended by the present invention: it cannot accommodate multiple sensors in any way that allows it to reject and/or separate more than 2 co- channel and 2 adjacent-channel GMSK or MSK signals, in contrast to the present invention.
  • JMLSE Joint Maximum Likelihood Sequence Estimator
  • the filtering structures of the present invention do not suffer from the severe implementational drawbacks of the JMLSE; they accommodate single or multiple sensors to great advantage; and they can reject and/or separate multiple signals even when one or more pairs of these signals cannot be rejected and/or separated by the LTI spatio-temporal filters. More specifically, the filtering structures of the present invention provide capabilities, for rejecting interfering signals and separating desired signals and correcting their distortion, that are not provided by any prior filtering structures.
  • the filtering structures used in this invention, and in other means and apparatus for interference rejection, signal separation, and correction of distortion are controlled through the selection of values of filter parameters or coefficients.
  • these coefficients can include the gains and phases used to weight various signals prior to summing.
  • the values of these coefficients must vary properly because the spatio-temporal characteristics of the received signals vary in time. That is, the values of the coefficients of these filtering structures must be chosen adaptively.
  • the filtering structures can have, potentially, a large number of adjustable coefficients
  • the number of known samples within the embedded training sequence can be insufficient to provide a useful initial setting.
  • a filtering structure initialized in this way would not provide sufficiently high quality estimates of the desired signals to be useful in the communications system.
  • blind methods can make use of the entire record of the received signals over which the spatio-temporal characteristics (i.e., angles of arrival, relative delays, and relative phases of the multiple propagation paths of the multiple signals and carrier and bit phases of the transmitted signals) are approximately constant. This circumvents the limit of conventional methods on the number of filter coefficients that can be adapted.
  • the various blind methods have other severe drawbacks that prevent them from operating successfully in many situations, including the one addressed by the present invention.
  • the class of Constant Modulus Algorithms (CMA) and related methods can require much longer data records, to converge to a high-quality solution, than are available.
  • the direction- inding methods do not provide the types of parameter estimates that are needed to adapt the filtering structures of the present invention.
  • the adaptation methods of the present invention exploit in a novel manner both the embedded training sequences and the constant modulus property of the desired signals, and in doing so circumvent the undesirable aspects of the conventional methods and of the blind methods. That is, the adaptation methods of the present invention, which choose the values of coefficients in filtering structures, such as those of the present invention, provide capabilities that are not provided by prior adaptation methods.
  • the present invention provides a method and apparatus for rejecting undesired interference and separating and correcting the distortion of desired interfering signals of the Gaussian Minimum Shift Keyed (GMSK) or other Minimum Shift Keyed (MSK) type, which arise for example in wireless communication systems based on either the Digital European Cordless Telecommunications (DECT) standard or the Global System for Mobile Communications (GSM) standard and, as a result, improving the quality and quantity of service offered by a wireless communications system that uses GMSK or other MSK signals.
  • GMSK Gaussian Minimum Shift Keyed
  • MSK Minimum Shift Keyed
  • the utility of the present invention may be appreciated, for example but not exclusively, by manufacturers of wireless communications equipment and providers of wireless communications services who wish to improve the quality of communications offered to users of their systems, and/or who wish to increase the number of users of their systems (e.g., to increase revenue in a commercially operated system).
  • Quantity of service improves when a base station is able to support a greater number of users for a fixed frequency allocation by, for example, being able to reduce frequency reuse distance, extend range, possibly reuse frequencies within a cell, and more frequently assign users to adjacent channels.
  • Quality of service improves, for example, when requests made to a central coordinator (hereafter referred to as the base station) by users to initiate calls are rejected less frequently (i.e., blocking probability is reduced), when calls in progress by mobile users are not abnormally terminated (i.e., probability of dropped calls is reduced), and when the intelligibility of the voice messages in the call is better (i.e., the parties to the call hear less noise, interference, abnormal silences, and the like).
  • the present invention addresses these general problems by means of a method and apparatus that perform the following functions: (1) rejection of interfering signals received at a communications receiver and occupying the same (or adjacent) spectral band as one or more desired signals, (2) separation (if needed) of multiple desired communication signals received at a communications receiver, and (3) correction of distortion that affects one or more desired communication signals received at a communications receiver (e.g., distortion caused by a multipath radio-frequency (RF) propagation channel).
  • RF radio-frequency
  • an object of the invention is to provide an array of one or more antennas whose outputs are processed by conventional RF front-ends and are then processed by a filtering structure that rejects interference, separates one or more desired signals, and corrects distortion to obtain high- quality estimates of desired signals.
  • Another object of the invention is to provide a class of filtering structures that can reject interfering signals, separate one or more desired signals, and correct the distortion in these desired signals.
  • Another object of the invention is to provide a class of filtering structures that can operate on the outputs of any number of antennas after these outputs are downconverted by conventional RF front-ends, including the special case when only one antenna is used.
  • Another object of the invention is to provide a linear-conjugate-linear (LCL) frequency-shift (FRESH) filter that uses the particular pattern of frequency-shifts and conjugations to exploit the cyclostationarity properties that are characteristic of GMSK and other MSK signals, thereby providing the desired interference-rejection, signal-separation, and distortion-correction capabilities.
  • LCL linear-conjugate-linear
  • FRESH frequency-shift
  • Another aspect of the invention is to provide a linear-conjugate-linear polyperiodically time-variant (LCL-PTV) filter that uses the particular pattern of conjugations and time-variant filter coefficients to exploit the cyclostationarity properties that are characteristic of GMSK and other MSK signals, thereby providing the desired interference-rejection, signal- separation, and distortion-correction capabilities.
  • L-PTV linear-conjugate-linear polyperiodically time-variant
  • Another object of the invention is to provide an LCL-FRESH fractionally-spaced equalizer (LCL-FRESH-FSE) that operates on higher- than-bit-rate-sampled data to provide bit-rate-sampled output and uses the particular pattern of frequency-shifts and conjugations to exploit the cyclostationarity properties that are characteristic of GMSK and other MSK signals, thereby providing the desired interference-rejection, signal- separation, and distortion-correction capabilities.
  • LCL-FRESH-FSE LCL-FRESH fractionally-spaced equalizer
  • Another object of the invention is to provide an LCL-FSE filter that operates on higher-than-bit-rate-sampled and frequency-shifted (by one- fourth of the bit rate) data and uses the sum of FSE processed data and conjugated data to exploit the cyclostationarity properties that are characteristic of GMSK and other MSK signals, thereby providing the desired interference-rejection, signal-separation, and distortion-correction capabilities.
  • Another object of the invention is to provide a real-imaginary (RI-FSE) filter that provides a computationally efficient alternate embodiment of the LCL-FSE filter.
  • Another object of the invention is to provide an LCL filter that operates on bit-rate-sampled and frequency-shifted (by one-fourth of the bit rate) data and uses the sum of linearly filtered data and conjugated data to exploit cyclostationarity properties that are characteristic of GMSK and other MSK signals, thereby providing the desired interference-rejection, signal- separation, and distortion-correction capabilities.
  • Another object of the invention is to provide a real-imaginary (RI) filter that provides a computationally efficient alternate embodiment of the LCL filter.
  • Another object of the invention is to provide a class of methods for adapting the adjustable coefficients in any of the filtering structures of this invention so that the filter can properly perform the functions of rejecting interference, separating one or more desired signals, and correcting distortion.
  • Another aspect of the invention is to provide a means of adapting the adjustable coefficients, by using both the known training sequence(s) embedded in the desired signal(s) and the knowledge that the desired signal(s) have constant modulus, thus providing training-augmented constant-modulus (TACM) adaptation.
  • TACM training-augmented constant-modulus
  • Another object of the invention is to provide a means of adapting the adjustable coefficients, by using the known training sequence(s) embedded in the desired signal(s) to constrain adaptation that uses the knowledge that the desired signal(s) have constant modulus, thus providing training-constrained constant-modulus (TCCM) adaptation.
  • TCCM training-constrained constant-modulus
  • the quality of service can be improved by using the invention to suppress interference in a particular frequency band and time slot (jointly referred to as a channel) and spatial cell caused by other users in the same or adjacent channels and in the same or adjacent spatial cells and to correct signal distortion, and the quantity of service can be increased by reallocating channels in a manner that intentionally results in more interfering, but separable by this invention, signals per channel and cell, in order to accommodate more users.
  • a channel frequency band and time slot
  • spatial cell caused by other users in the same or adjacent channels and in the same or adjacent spatial cells and to correct signal distortion
  • this invention using only one sensor, can separate multiple spectrally overlapping signals of certain types and can correct even severe frequency-selective fading and other distortion and, because the number of such signals that can be separated by this invention, using multiple sensors increases proportionately with the number of sensors.
  • the allocation of channels can (but need not) be made so that the introduced interference occurs only in up-link channels (from mobile units to base stations) so that the invention need only be implemented in the base station receivers and not in the mobile receivers.
  • the invention can also be used at base stations (or at mobile units, or both) using only a single receiving antenna or multiple antennas.
  • GMSK and other MSK signals in a single channel can be accommodated with one antenna, depending on how many signals arise from users in the same channel and how many arise from adjacent channels. More generally, with a number M of antennas, between 2 and AM signals can be accommodated.
  • FIG. 1 is a graph showing the cross-coherence functions of a sample
  • FIG. 2A is a graph showing the spectral content of a GMSK (0.3) signal, x(t).
  • FIG. 2B is a graph showing the spectral content of a conjugated and frequency shifted version of the signal shown in FIG. 2A,
  • FIG. 2C is a graph showing the spectral density of the degree to which x shown in FIG. 2A and _ , shown in FIG. 2B are correlated.
  • FIG. 3 A is a graph showing the spectral content of a GMSK (0.3) signal, x(t).
  • FIG. 3C is a graph showing the spectral density of the degree to which x shown in FIG. 3 A and - 2 shown in FIG. 3B are correlated.
  • FIG. 4 is a functional block diagram of a 3-path LCL-FRESH filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other
  • FIG. 5 is a functional block diagram of an LCL-PTV filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other
  • FIG. 6 is a functional block diagram of a "type-1" LCL-FRESH-FSE filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 7 is a functional block diagram of a "type-2" LCL-FRESH-FSE filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 8 is a functional block diagram of a "type-1" LCL-FSE filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 9 is a functional block diagram of a "type-2" LCL-FSE filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 10 is a functional block diagram of a "type-1 " RI-FSE filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 1 1 is a functional block diagram of a "type-2" RI-FSE filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 12 is a functional block diagram of an LCL filter in accordance with the present invention for performing interference rejection and/or co- channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 13 is a functional block diagram of an RI filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction for GMSK or other MSK signals.
  • FIG. 14 is a functional block diagram of a 5 -path LCL-FRESH filter in accordance with the present invention for performing interference rejection and/or co-channel separation and/or distortion correction, with improved cancellation of adjacent channel interference for GMSK or other MSK signals.
  • FIG. 15 is a functional block diagram of a 2-input, 2-path LCL-FRESH filter in accordance with the present invention for canceling adjacent upper channel interference.
  • FIG. 16 is a functional block diagram of a 2-input, 2-path LCL-FRESH filter in accordance with the present invention for canceling adjacent lower channel interference.
  • FIG. 17 is a functional block diagram of a 3-input, 3-path LCL-FRESH filter in accordance with the present invention for canceling both adjacent upper and lower channel interference.
  • FIG. 18 is a functional block diagram of a joint space-time multi- sensor FRESH filter in accordance with the present invention.
  • FIG. 19 is a functional block diagram of space-time factorized processor in accordance with the present invention.
  • FIG. 20 is a data structure timing diagram for a time slot in a GSM system, showing the number and location of each type of bit.
  • FIG. 21 is a data structure timing diagram showing the time periods T 0 and T, for the time slot shown in FIG. 20.
  • FIG. 22 is a functional block diagram of a receiver apparatus in accordance with the present invention.
  • FIG. 23 is a functional block diagram of an embodiment of the coherent -antenna collection apparatus block shown in FIG. 22.
  • FIG. 24 is a functional block diagram of an embodiment of the filter apparatus block shown in FIG. 22.
  • FIG. 25 is a functional block diagram of an embodiment of the frequency-shifting, time-shifting, and polarized switching apparatus block of the filter shown in FIG. 24 where the filter is configured as a 3 -path LCL- FRESH filter.
  • FIG. 26 is a functional block diagram of an embodiment of the frequency-shifting, time-shifting, and polarized switching apparatus block of the filter shown in FIG. 24 where the filter is configured as a type-1 LCL- FRESH-FSE filter.
  • FIG. 27 is an alternative embodiment of the frequency-shifting, time- shifting, and polarized switching apparatus shown in FIG. 26 where the filter is configured as a type-2 LCL-FRESH-FSE filter.
  • FIG. 28 is a functional block diagram of an embodiment of the frequency-shifting, time-shifting, and polarized switching apparatus block of the filter shown in FIG. 24 where the filter is configured as a type- 1 LCL- FSE filter.
  • FIG. 29 is an alternative embodiment of the frequency-shifting, time- shifting, and polarized switching apparatus shown in FIG. 28 where the filter is configured as a type-2 LCL-FSE filter.
  • FIG. 30 is a functional block diagram of an embodiment of the frequency-shifting, time-shifting, and polarized switching apparatus block of the filter shown in FIG. 24 where the filter is configured as a type-1 RI-FSE filter.
  • FIG. 31 is a functional block diagram of an embodiment of the frequency-shifting, time-shifting, and polarized switching apparatus block of the filter shown in FIG. 24 where the filter is configured as an LCL filter.
  • FIG. 32 is a functional block diagram of an embodiment of the frequency-shifting, time-shifting, and polarized switching apparatus block of the filter shown in FIG. 24 where the filter is configured as an RI filter.
  • FIG. 33 is a functional block diagram depicting the polarized switch portion of the frequency-shifting, time-shifting, and polarized switching apparatus shown in FIG. 25 through FIG. 32.
  • FIG. 34 is a functional block diagram of an embodiment of the tapped time delay (TDL) apparatus block of the frequency-shifting, time-shifting, and polarized switching apparatus shown in FIG. 25 through FIG. 32, where the input and output vectors can be real or complex.
  • TDL tapped time delay
  • FIG. 35 is a functional block diagram of a TACM adaptation apparatus in accordance with the present invention.
  • FIG. 36 is a functional block diagram of a TCCM adaptation apparatus in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • FIG. 1 through FIG. 36 where like reference numerals denote like parts. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts and that the method may vary as to the steps and their sequence without departing from the basic concepts as disclosed herein.
  • the filtering structures of the present invention exploit the 100% spectral redundancy exhibited by GMSK and other MSK signals. That is, these filtering structures exploit the fact that information conveyed in one subband of the GMSK or other MSK signal is also conveyed in another subband. In fact, for any GMSK or other MSK signal, the waveforms in any two such related subbands are nearly perfectly correlated. Thus, this spectral redundancy augments the more widely known and exploited spatial redundancy in which the waveforms due to a given source and received by two different antennas in an array are perfectly correlated.
  • a receiver without the filter structures of the present invention loses out on the significant performance gains due to the exploitation of spectral redundancy.
  • the filtering structures of the present invention provide at least twice the level of processing gain of an LTI spatio- temporal filtering structure.
  • an -antenna array can separate up to 2 to 4 GMSK or other MSK signals using these filtering structures, or can separate up to the more conventionally attainable number M of GMSK or other MSK signals but can do so with significantly higher reliability and quality than a conventional spatial filter.
  • ⁇ Qty(t) a conjugated and frequency-shifted version of x(t), where the frequency-shift is either f b /2 or -f b /2:
  • S xy (f) denotes the cross spectral density between x(t) and y(t)
  • S ⁇ and S yy denote the power spectral densities o ⁇ x(t) andy(t), respectively.
  • the magnitude ⁇ C xy ⁇ versus frequency /for a GMSK signal is shown in FIG. 1 , where the solid line represents C xy (/) and the dashed line
  • a single antenna receives a desired GMSK signal and a co-channel
  • each of the components numbered -6 to 0 (and the components in the continuum from -150 kHz to 0 kHz) in x ⁇ can be linearly combined with its corresponding component in y t ⁇ to yield a clean estimate of the desired signal while rejecting the co-channel interference.
  • FIG. 3A through FIG. 2C each of the components numbered -6 to 0 (and the components in the continuum from -150 kHz to 0 kHz) in x ⁇ can be linearly combined with its corresponding component in y t ⁇ to yield a clean estimate of the desired signal while rejecting the co-channel interference.
  • each of the components numbered 0 to 6 (and the components in the continuum from 0 to 150 kHz) in x ⁇ can be linearly combined with its corresponding component in y 2 ⁇ to yield a clean estimate of the desired signal while rejecting the co-channel interference.
  • the desired signal can be recovered over the entire band from -150 kHz to +150 kHz, or over just the allocated GSM frequency channel from -100 kHz to +100 kHz if desired.
  • the filtering structures in this invention can reject a GMSK co-channel interfering signal even when only one antenna is used.
  • Adjacent Channel Interference Rejection A single antenna receives a desired GMSK signal and an adjacent channel GMSK signal possibly having much greater signal power. Specifically, let the adjacent channel signal have a carrier frequency that is 200 kHz (per GSM carrier allocation protocols) less than that of the desired signal. Due to spectral leakage, the adjacent channel interferer degrades the desired signal, which cannot be recovered by a linear time- invariant filtering structure. However, with reference to FIG. 3 A through FIG.
  • each of the components numbered 4 to 8 (and the components in the continuum from 100 kHz to 200 kHz) in the adjacent channel interference (now downconverted to zero carrier offset), which comprise the spectral leakage into the desired signal, can be linearly combined with its corresponding component in y j ⁇ to cancel out the effects of the spectral leakage.
  • an adjacent channel interferer whose carrier is 200 kHz above that of the desired signal can be canceled out.
  • the filtering structures in this invention can reject two GMSK adjacent channel interfering signals even when only one antenna is used.
  • the received signal can be a severely distorted version of the transmitted signal, including for example frequency-selective fading (i.e., deep spectral nulls).
  • frequency-selective fading i.e., deep spectral nulls
  • blocks with the symbol ( )* inside denote conjugators, blocks with "Re” inside denote real-part selectors (extractors), and blocks with "Im” inside denote imaginary-part selectors (extractors), each of which operates on a complex signal and each of which is also referred to herein as a polarized switcher.
  • blocks with the symbol ( )* inside denote conjugators, blocks with "Re” inside denote real-part selectors (extractors), and blocks with "Im” inside denote imaginary-part selectors (extractors), each of which operates on a complex signal and each of which is also referred to herein as a polarized switcher.
  • the preceding observations about ⁇ C xy ⁇ and the role of high or nearly perfect correlation in motivating linear filtering structures are taken into account in the 3-path time-domain linear-conjugate-linear (LCL) frequency- shift filter (FRESH filter) 10 in accordance with the present invention whose structure for GMSK and other MSK signals is depicted in block-diagram form in FIG. 4.
  • LCL-FRESH filter When applied to the signal x ⁇ received at a single antenna, two copies of this structure or the other filter structures described below can separate and/or correct the distortion of two co-channel GMSK or other MSK signals.
  • the 3 -path LCL-FRESH filter is appropriate for data that is sampled at a rate higher than the bit rate. Accordingly, the output of the 3-path LCL- FRESH filter must be sampled at the bit rate prior to being processed, for example, by a GMSK demodulator that operates on bit-rate sampled data to recover the bit stream.
  • LCL-PTV Filter As an alternative to the 3-path LCL-FRESH filter, the effect of the two complementary frequency shifts e j ⁇ ⁇ bt and e ⁇ ⁇ bt and the associated filters g_ and g + can be achieved also with a single time-variant filter g ⁇ .
  • the resulting structure is referred to herein as a linear-conjugate-linear periodically time-variant (LCL-PTV) filter 12 shown in block-diagram form in FIG. 5.
  • the filter g ⁇ has a time-variant impulse response according to the possibly periodic time-variations in the possibly cyclostationary statistics of the received signals.
  • the coefficients of time- variant filter g ⁇ it is appropriate for the coefficients of time- variant filter g ⁇ to vary with frequency/, in which case there exists a one-to-one mapping between the LCL-PTV filter and the 3 -path LCL-FRESH filter.
  • the coefficients of filter g ⁇ must change every time one or more of the received signals encounters a time-slot boundary.
  • the LCL-PTV filter is appropriate for data that is sampled at a rate higher than the bit rate. Accordingly, the output of the LCL-PTV filter must be sampled at the bit rate prior to being processed, for example, by a GMSK demodulator that operates on bit-rate sampled data to recover the bit stream.
  • the sampling rate of the filter output exceeds the bit-rate (typically for a linear filter processing GMSK, it is an integer multiple of the bit-rate/, such as 2/), in which case the filter output is typically sampled at the bit-rate prior to being processed by the demodulator (which provides an estimate of the bit stream conveyed in the desired signal).
  • the bit-rate typically for a linear filter processing GMSK, it is an integer multiple of the bit-rate/, such as 2/
  • the filter output is typically sampled at the bit-rate prior to being processed by the demodulator (which provides an estimate of the bit stream conveyed in the desired signal).
  • an alternative to the 3 -path LCL-FRESH filter is to move this bit-rate sampling operation back through the filtering structure to the extent possible without changing the capability of the filter and to eliminate redundant operations.
  • LCL-FRESH-FSE LCL-FRESH fractionally-spaced equalizers
  • LCL-FSE and RI-FSE Filters Straightforward manipulations can be performed on the LCL-FRESH- FSE filter to obtain a filter structure in which the output of each bit-rate- sampler is frequency-shifted by/*.
  • This shifted signal is then split into two paths: the upper path is processed by an LTI filter, and the lower path is conjugated and processed by another LTI filter. That is, the lower path following each bit-rate-sampler no longer contains a frequency shift of (-l) k . However, the output final summer must be frequency-shifted by/.
  • Alternative embodiments of the resulting filter structure 18 (type-1) and 20 (type-2) are shown in FIG. 8 and FIG. 9, respectively.
  • This filter structure will be referred to herein as an LCL-FSE filter rather than an LCL-FRESH- FSE because no path is frequency-shifted any differently than any other path.
  • the LCL-FSE filter can be simplified without any loss of performance by observing that the two paths at the output of each bit-rate sampler and/* frequency-shifter form an LCL filter, which can be equivalently realized by modifying the upper path to operate on the real part (instead of operating on the entire complex signal) and by modifying the lower path to operate on the imaginary part (instead of operating on the conjugated complex signal).
  • RI-FSE filter replaces the two LCL filters by two Real-Imaginary (RI) filter pairs
  • the resulting structure is referred to as the RI-FSE filter and can be interpreted to be an alternate but mathematically equivalent embodiment of the LCL-FSE filter.
  • Alternative embodiments of this filter structure, 22 (type- 1 ) and 24 (type-2), are shown in FIG. 10 and FIG. 11 , respectively.
  • LCL and RI Filters are related to the LCL-FSE and RI-FSE filters, but operate on bit-rate-sampled data. This is possible for GMSK, and for some other MSK signals, because the bit-rate is larger than the RF bandwidth occupied by the signal, and so the bit-rate satisfies Nyquist's sampling criterion for this signal.
  • the result of simplifying the LCL-FSE filter as a result of the input being sampled at the bit-rate is the LCL filter 26, shown in FIG. 12.
  • the LCL filter can be simplified without any loss of performance by observing that it can be equivalently realized by modifying the upper path to operate on the real part (instead of operating on the entire complex signal) and by modifying the lower path to operate on the imaginary part (instead of operating on the conjugated complex signal). Since this structure replaces the LCL filter by a Real-Imaginary (RI) filter pair, the resulting structure 28, shown in FIG. 13, is referred to as an RI filter and can be interpreted to be an alternate but mathematically equivalent embodiment of the LCL filter.
  • RI Real-Imaginary
  • the RI filter offers the same implementation advantage over the LCL filter as the RI-FSE offers over the LCL-FSE: since the data paths being processed by various LTI or FSE filters are real, both the application of the filters to the data and the adaptation of the filters require less arithmetic than if the data paths were complex. Significantly, since the RI filter is mathematically equivalent to the LCL filter, there is no loss of quality in the estimated signal at the output.
  • LCL-FRESH-FSE LCL-FSE. RI-FSE. LCL. and RI
  • the aforementioned filtering structures are ideally suited for the rejection of co-channel interference and separation of co-channel desired signals and removal of signal distortion.
  • variants of these filtering structures can provide rejection of adjacent channel interference by exploiting the spectral correlation properties of the adjacent channel interference in addition to those of the co-channel signals. Capability to reject adjacent channel interference can be especially important when the adjacent channel signals are much stronger than the desired channel signals (e.g., due to the near-far problem).
  • the aforementioned filtering structures can be generalized (or modified) to exploit the cyclostationarity properties of the adjacent channel interference in addition to those of the desired channel signals (or, if desired, only those of the adjacent channel interference).
  • the 3 -path LCL-FRESH filtering structure can be generalized to include two additional paths, one for each of the upper and lower adjacent channels that can be present.
  • the signals present in these two additional paths are highly correlated with the interference, from the corresponding adjacent channels, that contaminates the desired signals; thus, correctly linearly combining these additional paths with the other three paths can significantly reduce the adjacent channel interference.
  • the resulting 5 -path LCL-FRESH filtering structure 30 is depicted in FIG. 14, wherein x_ ⁇ and x + ⁇ are the complex envelope signals from the lower and upper adjacent channels (i.e., downconverted to complex baseband from their respective RF carriers), respectively.
  • the 5-path LCL-FRESH filtering structure 30 shown in FIG. 14 can be simplified as shown in FIG. 15 through FIG. 17 by deleting the two paths feeding into filters g + and g_ can be deleted, yielding a new filtering structure, with only three paths, whose functionality is limited to the cancellation of adjacent channel interference, instead of joint cancellation of adjacent- and co-channel interference, signal separation, and distortion correction.
  • These two-input two-path LCL-FRESH filtering structures 30a, 30b which are shown in FIG. 15 (2 I-LCL-FRESH-U) and FIG.
  • LCL-PTV, LCL-FRESH-FSE, LCL-FSE, RI-FSE, LCL, and RI filtering structures can be obtained from the 3 -path FRESH filter.
  • these filtering structures can also be generalized to improve their ability to reject adjacent channel interference. Such generalization may be accomplished by altering these filtering structures to exploit the cyclostationarity properties of the adjacent channel interference.
  • each of two copies of the 3 -path LCL-FRESH filter can be applied to each of the lower and upper adjacent channels to provide improved estimates of the adjacent channel signals; these estimates can then be appropriately frequency-shifted, adaptively filtered, and then subtracted from the received data in the desired frequency channel, thus reducing the adjacent channel interference.
  • the signal resulting thereby can then be processed by the 3 -path LCL-FRESH filter to separate co-channel signals and correct for distortion.
  • the adjacent channel interference canceling filters can either be adapted jointly with the co-channel separation and distortion correction filters or be adapted separately.
  • each of the foregoing filter structures include filters (comprising time-shifters and linear combiners), frequency shifters, polarized switchers (conjugators or real- or imaginary-part selectors), and summers (linear combiners).
  • filters comprising time-shifters and linear combiners
  • frequency shifters comprising time-shifters and linear combiners
  • polarized switchers conjuggators or real- or imaginary-part selectors
  • summers linear combiners.
  • Table 1 below maps conjugate cycle frequencies and input and output sampling rates into frequency-shift values and polarized switch functions (e.g., conjugation or real- or imaginary-part extraction) defining the specific filter structures described herein.
  • f c + the carrier frequency of the lower adjacent channel
  • f c " the carrier frequency of the lower adjacent channel
  • O oversampled
  • B bit rate sampled.
  • each of the foregoing filtering structures are baseband structures designed for complex baseband signals corresponding to a desired down-converted channel centered at zero frequency, and if the desired channel is centered at a non-zero frequency, then standard baseband-to-passband transformations can be applied to these baseband filtering structures to obtain their passband counterparts for processing passband signals. Joint Space-Time Filtering and Factorized Filtering
  • any of the aforementioned filtering structures can be used in either of two ways: (1) in a joint space- time filtering structure 32, in which each antenna output is processed by one of these filters, and then the filtered outputs are summed, or (2) in a space- time factorized filtering structure 34, in which a purely spatial filter is applied to the received array data, and then the spatially filtered data is processed by one of these filters.
  • the former structure has the capability to attain higher performance at the expense of greater complexity than the latter.
  • the 3 -path LCL-FRESH filter can be replaced by either the LCL-PTV filter or the 5 -path LCL-FRESH filter in either FIG. 18 or FIG.
  • LCL-FRESH, LCL-PTV, LCL- FRESH-FSE, LCL-FSE, RI-FSE, LCL, and RI filtering structures can apply not only to single-sensor receivers (in which case x ⁇ is a scalar signal and the LTI or FSE filters in each signal path are single-input filters) but also to multiple-sensor receivers (in which case x ⁇ is a vector signal and the LTI or FSE filters in each signal path are multiple-input filters).
  • the LCL- FRESH, LCL-PTV, LCL-FRESH-FSE, LCL-FSE, RI-FSE, LCL, and RI LCL filtering structures can be applied to estimate a single desired signal (in which case the LTI or FSE filters in each signal path are single-output filters; in this case, if multiple signals are desired to be estimated, then multiple copies of the filtering structure can be implemented in parallel) as well as to estimate multiple desired signals (in which case the LTI or FSE filters in each signal path are multiple-output filters).
  • the frequency-shifters and conjugators employed in the filters, as well as the real and imaginary part extractors become multiple-input devices.
  • LTI and FSE Filters It will be appreciated by those skilled in the art of signal processing that the linear-time-invariant (LTI) filters and FSE filters describing the various filtering structures of the present invention can be implemented in numerous ways.
  • an LTI filter is implemented as an FIR filter and is applied to its input by using the well-known overlap-and- add or overlap-and-save algorithms that utilize the fast Fourier transform (FFT).
  • FFT fast Fourier transform
  • an LTI filter is implemented using a tapped delay line, multipliers attached to each tap, and a summer.
  • the LTI filter might be implemented using an infinite- impulse response (IIR) filter instead of an FIR filter; however, it is noted that adapting such filters can be computationally expensive and/or unreliable.
  • IIR infinite- impulse response
  • the interaction between the filtering structures (used for interference rejection and/or co-channel signal separation) and the demodulator (used for recovering the bit stream from the estimate of the desired signal) must be considered.
  • demodulators are currently in use for GMSK signals: the maximum likelihood sequence estimator (MLSE) based on the Viterbi algorithm, the multiple-symbol differential detector, and the decision feedback equalizer (DFE). Any one of these demodulators can be used following the filtering structures to demodulate the estimated signal of interest.
  • the DFE is itself a filtering structure, it is reasonable to merge the demodulation function implemented by the conventional DFE and the interference-rejection and/or signal-separation functions implemented by the space-time filtering structure; the resulting filtering structure would have the desirable property of very low complexity relative to the receivers based on MLSE or multiple-symbol differential detection, and may be capable of attaining a higher-quality recovered bit stream due to its smaller number of adjustable coefficients for the same memory length.
  • the result of merging the filtering apparatus with the DFE is that one simply follows the filtering apparatus, be it single- or multiple-sensor, with the feedback part of the DFE.
  • the feedback part alone functions as the demodulator.
  • Algorithms for adaptively adjusting this demodulator are not within the scope of this invention.
  • the complexity of the MLSE demodulator can be reduced and its performance can possibly be increased by the effects of the distortion- removal capability of the FRESH filtering structures.
  • the requirements of the channel estimator that is an integral part of the MLSE demodulator can be eased by virtue of the distortion removal or even just distortion reduction. D.
  • each time slot lasts 577 ⁇ s (156.25 bit periods at 270.833 kbits/sec) and the bits therein are structured as shown in FIG. 20.
  • the three tail bits on either side of the main burst are all zeros, and the guard time lasting 8.25 bit periods prevents consecutive time slots from colliding due to different bulk propagation delays.
  • the 1 16-bit message in each slot can be either digitized speech (after vocoding, convolutional coding, and interleaving) or control information.
  • the 26-bit training sequence embedded in the middle of the slot is used in existing conventional GSM receivers, for example, to perform channel estimation prior to MLSE demodulation.
  • CM constant-modulus
  • the first two adaptation algorithms provide the building blocks for the second two. All four of these algorithms are directed to finding the values of the filter coefficients in a matrix W that controls a linear combiner (the number of columns in this matrix is set to be equal to the number of desired signals to be estimated and the number of rows is set to be equal to the number of weights in the linear combiner to be adaptively adjusted).
  • s(/) W // y(?) , where superscript H denotes conjugate transposition.
  • y(t) is defined in the obvious way. Similar definitions of y(t) can be constructed for the LCL-FRESH-FSE, LCL-FSE, RI-FSE, LCL, and RI filtering structures, as well as for the 5 -path LCL-FRESH filter and any extensions of these in which the number of paths, patterns of conjugations or real-part and imaginary-part selectors, choices of frequency shifts, and origins of input signals for the various paths are altered.
  • the specific dependencies of y ⁇ on x ⁇ is made, since all of the filtering structures of the present invention can be represented using this common framework.
  • W having a single column is considered; to emphasize that the matrix degenerates to a vector in this case, the set of filter coefficients is denoted by w.
  • T 0 is the time interval containing the embedded training signal.
  • This method is simple to implement, with low computational complexity when implemented using the recursive least squares (RLS) algorithm or any of the various fast algorithms (e.g., the modular multichannel method or the fast affine projection algorithm), and it converges reliably and quickly when the length of the training sequence exceeds the number of filter coefficients (i.e., the length of w).
  • this conventionally trained adaptive equalization algorithm may not provide sufficiently reliable or high quality signal estimates.
  • CM Constant Modulus
  • CM Constant Modulus
  • GMSK and other MSK signals have constant modulus (constant envelope), a property which is degraded or destroyed by the presence of co- channel interference and channel distortion.
  • the CM algorithm attempts to solve the following minimization problem:
  • T in our case is the set of all time instants within a particular time slot. Since this is a nonlinear minimization problem with no closed-form solution, and a least-mean-squares (LMS) stochastic gradient descent algorithm would converge much too slowly, an iterative block approach (the so-called least- squares CM algorithm) is used. At the K h iteration, the weight vector w k is found by solving
  • the CM algorithm allows the equalizer to be adapted successfully in the absence of a training signal, although it is known to exhibit unreliable convergence in some situations and its reliability may be lower than desired when the observation interval is short (as it is for GSM time slots).
  • Another complication with the CM algorithm arises when it is applied to environments containing multiple co-channel signals.
  • Multi-target CM algorithms have been developed to allow multiple sets of filter weights to be adapted, each to extract a different signal. A subsequent signal sorting operation must then be performed to identify the correspondence between extracted signals and users; this correspondence can be achieved easily if the users are assigned different training sequences. Notice though that this training signal information might be more profitably used for adaptation of the filter weights themselves rather than simply to sort the output of the blindly adapted filters. This observation is pursued through two other adaptive algorithms, discussed below.
  • T 0 is the time interval occupied by the embedded training signal (FIG. 20).
  • the time period T 1 over which the CM algorithm is applied can be either the entire time slot, or the time slot excluding the training signal. Either way, the closed-form solution is given by
  • the convex linear combiner weight ⁇ controls the influence of the two cost functions and thus also controls the emphasis given to the two distinct types of knowledge used in them. Values of ⁇ closer to unity give more influence to the exploitation of the CM property, whereas values closer to zero give more influence to the exploitation of the known training signal.
  • the TACM yields an algorithm in which the known training signal is used to initialize the
  • s 0 ⁇ is a constant-modulus signal (e.g., as it is for GMSK, CPFSK, and other MSK signals)
  • s 0 ⁇ should be pre-normalized to have unity modulus prior to its use in the TACM algorithm.
  • an adaptive normalization constant must be inserted into the TACM cost function above to yield the adaptively normalized TACM (AN-TACM) algorithm:
  • R 2 2 ⁇ R -Vo '
  • TCM Training-Constrained CM
  • the TCCM differs from the TACM in the way that it attempts to exploit both types of knowledge.
  • the linear system used by the conventionally trained adaptive equalizer (implied in Equation (5)) is under-determined. That is, the training signal is sufficient only to restrict w k to lie in some subspace but not to define it completely. Specifically, in this case all that can be said so far is that w k is given by
  • each of the two versions of the TCCM algorithm may be extended, through the adaptive normalization technique used in deriving the AN-TACM algorithm, to accommodate desired signals having non-constant modulus.
  • RF signals at the outputs of the M antennas 38 are coherently downconverted to complex baseband and digitized by a Coherent M- Antenna Collection Apparatus 40, the output of which is a digitized discrete-time sequence Mx 1 complex vector, denoted elsewhere herein as x ⁇ .
  • This vector of received data is then provided as input to a Filter Apparatus 42, which takes as its other input the filter coefficients (W) provided by an Adaptation Apparatus 44, to produce a digitized discrete-time sequence of d x 1 complex vector signal estimates s k (n) .
  • a switch 46 at the output of the Filter Apparatus 42 directs the output of the Filter Apparatus 42 into the Adaptation Apparatus 44; upon the detection of convergence by the Adaptation Apparatus 44, such as when a prescribed number of iterations are completed, the switch 46 is moved to direct the output of the Filter Apparatus 42, the final signal estimate s(n) , to subsequent processors such as demodulators that will render bit decisions to recover the binary data streams conveyed by the desired signals.
  • FIG. 23 A preferred embodiment of the Coherent M- Antenna Collection Apparatus 40 is shown in FIG. 23.
  • This apparatus is controlled by user- supplied values for the RF tuning frequency and the A/D sampling clock frequency to control the local oscillators 48 used for the downconversion from real RF to complex baseband (also known as in-phase and quadrature- phase signals) and for the A/D converters 50, respectively.
  • the RF front- ends 52 that perform this downconversion can be implemented in a variety of ways known well to those skilled in the art of RF design.
  • the sampling clock frequency of the A/D converters is equal to an integer multiple of the bit rate/.
  • 2/ represents a good compromise between the temporal resolution provided by oversampling and the excess of data and associated computational load induced by oversampling.
  • the digitized discrete-time inphase and quadrature signals at the output of each pair of A/D converters are combined to form a representation of a single complex- valued digitized discrete-time signal.
  • the M signals so formed, one per chain of hardware associated with a single antenna, are grouped into a digitized discrete-time Mx 1 complex vector of received data.
  • the Filtering Apparatus 42 accepts as input the M x 1 complex vector of received data and the filter coefficients output from the Adaptation Apparatus 44.
  • the received data is first processed by a Frequency-Shifting, Time- Shifting, and Polarized Switching (i.e., conjugation, real-part selection or imaginary-part selection) Apparatus 54 whose purpose is to form the vector y ⁇ .
  • the record of such transformed daiay ⁇ is stored in a Signal Buffer 56, which can be implemented using dynamic RAM chips or other high-speed storage media, to allow y ⁇ to be made available to the Adaptation Apparatus 44 over multiple iterations of the adaptation algorithm implemented therein.
  • a Matrix- Vector Multiplier 58 applies the filter coefficients to the received data, producing output s ⁇ . (n) for use in the subsequent iteration (number k + 1 ) performed by the Adaptation Apparatus 44.
  • the apparatus accepts as an input the Mx 1 vector x ⁇ of received data and produces as an output the 3LMx 1 vector y ⁇ according to Equation (4).
  • the Tapped Delay Lines (TDL) 60 produce sets of L time-shifted versions of their respective inputs.
  • the Polarized Switcher 62 which, in this embodiment is a conjugator, and the conjugator 62 and Frequency-Shifters 64 are depicted using standard block symbols from signal processing and communications.
  • the preferred embodiments of the Frequency-Shifting, Time-Shifting, and Polarized Switching Apparatus 54 for the particular filtering structures corresponding to the LCL-FRESH-FSE (2 alternate embodiments), the LCL- FSE (2 alternate embodiments), the RI-FSE (a second alternate embodiment can be obtained for the RI-FSE, similar to the LCL-FRESH-FSE and LCL- FSE), the LCL, and the RI filters are shown in FIG. 26 through FIG. 32, respectively, in which the intermediate signal vector at the output is 4 x 1 or 2LMx 1.
  • the embedded training sequences that appear in the desired signal(s) must also be bit-rate sampled by a Bit-Rate Sampler 66 and frequency-shifted by/* prior to using them in adaptation methods applied to the LCL-FSE, RI-FSE, LCL, and RI filters; it is similarly noted that this frequency shift/* must be compensated for at the filter output by a conjugate frequency shifty* if it is desired to obtain an unshifted estimate of the desired signal.
  • many GMSK demodulators require their inputs to be shifted by/* anyway, and so the conjugate frequency shift and the subsequent frequency shift/* cancel each other.
  • RI- FSE and RI filters can be more efficiently implemented due to their use of real-valued intermediate signal vectors (and consequent arithmetic simplification of filter application and adaptation), instead of the complex- valued intermediate signal vectors that are used in the LCL-FSE and LCL filters.
  • Polarized Switcher 62 is depicted in a functional block diagram form. It can be seen at this point that Polarized Switcher 62 represents a family of filter design parameters wherein the real 68 and imaginary 70 parts of the complex input signal are optionally selected or deselected by selectors 72 according to the conjugate cycle frequencies and input/output sampling rates shown in Table 1. Note also that the sign of the imaginary part of the input signal, if selected, is either unaffected or flipped by switch 74. In other words, the complex input signal is either conjugated, its real part is selected, or its imaginary part is selected. It will be appreciated, however, that Polarized Switcher 62 is preferably implemented as a single function conjugator or real-part selector or imaginary-part selector rather than an apparatus that provides for the alternative selection of each of those functions.
  • TDL Tapped Delay Line
  • TDL 60 can accommodate either a real input or a complex input, with appropriate modification to the underlying unit-sample delay devices and signal paths.
  • Adaptation Apparatus 44 takes as inputs the intermediate signal data y ⁇ , provided by the signal buffer 56 in the Filtering Apparatus 42, and the output signal s k (t) computed by the Matrix- Vector Multiplier 58 in the Filtering Apparatus 42. It provides as output the matrix W k of filter coefficients, and it also controls the output switch 46 shown in FIG. 22. Adaptation Apparatus 44 further moves the switch 46 from its lower position, in which Filtering Apparatus 42 output is directed to the Adaptation Apparatus 44, to its upper position, in which the
  • Filtering Apparatus 42 output is the d x 1 vector s(t) of desired signal estimates, suitable for subsequent use by an appropriate demodulator. It should be understood by those skilled in the art of implementing (e.g., on programmable digital signal processing chips) that alternate embodiments of the Adaptation Apparatus 44 follow directly from the descriptions of the various versions of TACM and TCCM in this document.
  • the present invention can be used to extract a signal of interest from a plurality of spectrally overlapping communications signals, separate and remove distortion from interfering co-channel signals and suppress adjacent-channel interfering signals of the GMSK or MSK type.
  • These signals characteristically have real and imaginary components, exhibit spectral and temporal overlap, exhibit temporal redundancy, have conjugate cycle frequencies equal to twice their carrier frequencies plus and minus one- half of their data bit rate, and exhibit conjugate spectral redundancy for spectral components having frequencies separated by their conjugate cycle frequencies.
  • the present invention provides for time-shifting, frequency- shifting, and conjugation or extraction of real or imaginary parts of complex signals, to produce an estimate of the signal of interest.

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US6944245B2 (en) 2003-10-17 2005-09-13 Motorola, Inc. Multi-pass interference reduction in a GSM communication system
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