1. Field of the invention
The invention generally relates to the field of signal processing. More specifically the invention is related to effective and efficient algebraic projections of signals for the purpose of reducing the effects of interference.
2. Discussion of the Related Art
Digital filtering may be used to separate undesired components of a digital signal from desired signal components. For example, a digital filter may be used to pass frequency components of a desired signal while substantially blocking frequency components of an undesired signal.
In order to efficiently utilize time and frequency in a communication system, multiple-access schemes are used to specify how multiple users or multiple signals share a specified time and frequency allocation. Spread-spectrum techniques may be used to allow multiple users and/or signals to share the same frequency band and time interval simultaneously. Code division multiple access (CDMA) is an example of spread spectrum that assigns a unique code to differentiate each signal and/or user. The codes are typically designed to have minimal cross-correlation to mitigate interference. However, even relatively slight multipath effects can introduce cross correlations between codes and cause CDMA systems to be interference-limited. Digital filters that only pass or block selected frequency bands of a signal to filter out unwanted frequency bands are not applicable since CDMA signals share the same frequency band.
Multiple-access coding specified by CDMA standards provides channelization, or channel separability. In a typical CDMA wireless telephony system, a transmitter may transmit a plurality of signals in the same frequency band by using a combination of scrambling codes and/or covering (i.e., orthogonalizing) codes. For example, each transmitter may be identified by a unique scrambling code or scrambling-code offset. For the purpose of the exemplary embodiments of the invention, a scrambler (which is typically used in a W-CDMA system to scramble data with a scrambling code) is functionally equivalent to a spreader, which is typically used in CDMA2000 and IS-95 systems to spread data using short pseudo-noise (PN) sequences.
A single transmitter may transmit a plurality of signals sharing the same scrambling code, but may distinguish between signals with a unique orthogonalizing code. Orthogonalizing codes encode the signal and provide channelization of the signal. In W-CDMA, orthogonal variable spreading factor (OVSF) codes are used as multiple-access orthogonalizing codes for spreading data. CDMA2000 and IS-95 employ Walsh covering codes for multiple-access coding.
While CDMA signaling has been useful in efficiently utilizing a given time-frequency band, multipath and other channel effects cause these coded signals to interfere with one another. For example, coded signals may interfere due to similarities in codes and consequent correlation. Loss of orthogonality between these signals results in interference, such as co-channel and cross-channel interference. Co-channel interference may include multipath interference from the same transmitter, wherein a transmitted signal propagates along multiple paths that arrive at a receiver at different times, thereby degrading reception of a particular signal. Cross-channel interference may include interference caused by signal paths originating from other transmitters, thus degrading reception of a selected signal.
Interference can degrade communications by causing a receiver to incorrectly detect received transmissions, thus increasing a receiver's error floor. Interference may also have other deleterious effects on communications. For example, interference may diminish capacity of a communication system, decrease the region of coverage, and/or decrease maximum data rates. For these reasons, a reduction in interference can improve reception of selected signals while addressing the aforementioned limitations due to interference.
A received communication signal comprises a signal of interest, as well as interfering signals and noise. One or more of the interfering signals may be selected for removal. Systems and methods described and illustrated herein provide for filtering by projecting a received signal onto a subspace that is orthogonal to a signal selected for removal.
In one embodiment of the invention, a confidence weight may be applied to at least one projection operator configured to cancel one or more interfering signals. A confidence weight may be based on any of various parameters or signal measurements, including the relative strengths of desired and interfering signals, or estimation errors for each interfering signal. Weighted interfering signals or weighted interference code spaces may be used to generate an interference matrix or a combined interference vector from which the orthogonal projection operator may be constructed.
Receiver embodiments of the invention may be configured for receiving signals from a transmit-diversity system. Furthermore, receiver embodiments comprising a plurality of receiver antennas may be configured to provide both interference cancellation and diversity combining.
One embodiment of the invention provides for constructing a projection operator from linear transformations of the row or column space of an interference matrix or a combined interference vector. A projection operator PS⊥ may take the form PS⊥=(I−S(SHS)−1SH), wherein I is an identity matrix, S is an interference matrix, and SH is a Hermitian transpose of the interference matrix. If the received signal and the interference matrix are separated into real and imaginary parts, the projection operation may be expressed by a combination of up to eight real algebraic operations. Embodiments of the invention may provide for making at least one simplifying approximation to the projection operator PS⊥ in order to reduce the number of operations, and thereby reduce the complexity of the projection operator.
In some embodiments of the invention, an oblique projection operator QS(R−1)=S(SHR−1S)−1SHR−1 or QC(PS⊥)=C(CHPS⊥C)−1CHPS⊥ may be constructed. The term R denotes a shaping matrix, and C denotes a code matrix. An oblique projection may be advantageously configured to preserve at least one desired property of a signal of interest.
Embodiments disclosed herein may be advantageous to systems employing CDMA (e.g., cdmaOne, cdma2000, 1xRTT, cdma 1xEV-DO, cdma 1xEV-DV, and cdma2000 3x), W-CDMA, Broadband CDMA, Universal Mobile Telephone System (UMTS) and/or GPS signals. However, the invention is not intended to be limited to such systems, as other coded signals may benefit from similar advantages.
These and other embodiments of the invention are described with respect to the figures and the following description of the preferred embodiments.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form disclosed, but rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.
In
Transmission signals comprising a plurality of subchannels are typically spread with a covering or scrambling code, such as a PN sequence. Scrambling codes may include real or complex codes. Such scrambling codes may be specific to particular transmitters in order to mitigate inter-sector or inter-cell interference. Particular types of scrambling codes may be favored due to their auto-correlation properties. For example, preferred scrambling codes may have a sharp (1-chip wide) autocorrelation peak to facilitate code synchronization. Received transmission signals are typically characterized by differential delays and complex gains due to multipath and/or transmit diversity.
The receiver system 101 may include a single antenna, or a plurality of antennas that may be used for receiver diversity and/or beam-forming operations. The receiver system 101 may provide for any well-known RF front-end operation, such as amplifying a received signal, filtering a received signal, adjusting phase or delay of a received signal, and/or combining signals received from a plurality of receiver chains. Other well-known RF front-end operations may be performed.
An RF-to-baseband processor 102 is configured to convert a received RF signal to a baseband signal, such as a digital baseband signal. The RF-to-baseband processor 102 may include various well-known receiver-processing components, such as a mixer, a local oscillator, IF processing circuitry, filters, a direct-conversion system, an ADC, etc. An optional matched pulse-shaping (PS) filter 103 or an equalizer (not shown) may be matched to at least one corresponding pulse-shaping filter in the transmitter.
An output of the matched pulse-shaping filter 103 is coupled to a Rake receiver 106 and a projection module 105. The Rake receiver 106 includes a plurality M of Rake fingers configured to demultiplex and demodulate the baseband signals with respect to the signal information. For the purposes of the present invention, the term “finger” refers to a signal processing entity in a Rake receiver that may be capable of tracking and demodulating a signal. A Rake receiver is comprised of multiple fingers, each of which is assigned to either a unique source or a multipath version of an assigned source. The purpose of a Rake receiver is to combine multipath signals in order to increase the SNR.
Each finger of the Rake receiver 106 made include one of a plurality of PN generators (not shown) for providing a timing offset corresponding to the finger's assigned multipath component. Timing offsets may preferably account for any system latency, such as may be introduced by the projection module 105. The Rake fingers may be configured to supply PN codes, symbol boundaries, and chip boundaries to the projection module 105.
The projection module 105 is configured to cancel inter-symbol interference (ISI), inter-channel interference (ICI), and/or co-channel interference, which typically arises from pulse shaping, multipath in the channel, multiple carriers, and/or interference from multiple base stations (e.g., during a hand off). The projection module 105 is also configured to receive signal-timing information from the searcher/tracker module 104. In this embodiment, the projection module 105 produces an interference-canceled signal for each Rake finger (not shown) in the Rake receiver 106. The Rake receiver 106 typically includes a combiner (not shown) to produce linearly combined demodulated baseband signals. Alternatively, non-linear (e.g., iterative) combining may be performed.
The projection module 105 may include a Rake receiver structure (not shown). However, unlike the Rake receiver 106, which typically performs reception with respect to only one orthogonalizing (e.g., Walsh) code, the projection module 105 may be configured to perform Rake reception for a plurality of orthogonalizing codes. One or more of the codes may be identified as interfering signals, which are subsequently provided with baseband transmission processing, channel emulation, and baseband receiver processing prior to being used to construct at least one projection operator.
In an exemplary embodiment, the projection module 105 may be configured to orthogonally project at least one Rake finger output relative to at least one interfering signal space derived from at least one other finger. Alternatively, the projection module 105 may orthogonally project the filter 103 output relative to at least one interfering signal space derived from at least one of the fingers. Furthermore, the projection module 105 may select between a Rake finger output and an interference-cancelled (i.e., projected) output, and route the selected signal for further processing. For example, the selected signal may be decoded with an orthogonalizing code corresponding to at least one signal of interest, combined in an MRC, and processed by a detector.
Embodiments of the invention may provide for various arrangements of combining and interference cancellation. For example, the projection module 105 may be configured to cancel interference on Rake signals prior to combining and/or following combining. Further embodiments of the invention may place the projection module 105 at any of various positions within the Rake receiver 106, such as illustrated in
The channel-compensated receive signal is processed by a descrambler 206 (which removes the scrambling code), a demultiplexer 208 (which removes at least one of the orthogonal channelization codes), and an optional gain-correction module 210 (which may compensate for gain applied to one or more user channels by a transmitter). A projection module 212 is also provided, which may be included in the Rake finger at position 205 (coupled between the channel compensator 204 and the descrambler 206), position 207 (coupled between the descrambler 206 and the demultiplexer 208), and/or position 209 (coupled between the demultiplexer 208 and the gain-correction module 210). The projection module 212 receives as a control signal a digital baseband signal that undergoes the same signal-processing operation(s) as the interference signals selected to be projected out of the digital baseband signal. The projection module 212 may optionally receive delay information from the searcher/tracker 104.
In a space-time transmit diversity (STTD) system, a primary path is provided with a P-CPICH and a diversity path is provided with an S-CPICH. Either the S-CPICH or the P-CPICH signal may be used by the channel estimator 222 depending on whether a primary path or a multipath component is being processed by the Rake finger. Similarly, pilot bits on the dedicated physical channel (DPCH) may be used for antenna weight determination, as well as other receiver-processing functions that are well known in the art. If closed loop transmit diversity is employed, channel compensation includes compensating for transmit-antenna weights in addition to channel effects.
A descrambler 226 may perform an inner product operation employing a vector derived from a complex conjugate of a transmitted Gold code corresponding to a signal path of interest. A demultiplexer 228 may be configured to perform an operation employing a complex conjugate of a channelization matrix W that was used to encode transmitted signals. An optional inverse space-time processor 230 may be configured to process a received diversity path of a signal transmitted in an STTD system. The receiver also may include an optional inverse-gain operator 232.
A projection module 242 may be included in the Rake finger at one or more positions, such as position 225 (coupled between the channel compensator 224 and the descrambler 226), position 227 (coupled between the descrambler 226 and the demultiplexer 228), position 229 (coupled between the demultiplexer 228 and the inverse space-time processor 230), and/or position 231 (coupled between the inverse space-time processor 230 and the gain-correction module 232). The projection module 242 receives as its control signal a digital baseband signal that undergoes the same signal-processing operation(s) as the interference signals selected to be projected out of the digital baseband signal. The projection module 242 may optionally receive delay information from the searcher/tracker 104
The at least one symbol estimate may be passed through at least one of a threshold detector 315 (such as a P-CCPCH threshold detector) and a multiple-access interference (MAI) selection module 316. The threshold detector 315 may use a channel known to be present, such as a common channel (e.g., the P-CCPCH), to generate a threshold value. For example, P-CCPCH symbols may be separated into in-phase (I) and quadrature-phase (Q) parts, and then a function of these parts, such as a sum of the absolute values of the I and Q parts may be averaged over a plurality of symbols. Similarly, other common channels (or combinations thereof), including P-CPICH, PICH, AICH, S-CCPCH, S-PICH, etc., may be used. In some embodiments of the invention, one or more traffic channels may be used as thresholding references to produce one or more threshold values. Alternatively, a predetermined constant value may be selected as a threshold. In some embodiments, a combination of thresholding references (e.g., a threshold derived from one or more traffic channels and a predetermined constant-value threshold) may be employed.
The MAI selection module 316 typically identifies a plurality of user (i.e., traffic) channels present in a particular path. Furthermore, signal distortions due to channel effects and/or diversity processing may be accounted for either directly or indirectly. A decision to include or exclude a particular channel may be made by examining the associated power resolved by that channel. If a channel is to be excluded from the interference space, then the power of that Walsh channel may be set to zero or simply ignored. This operation will result in that channel being excluded from the construction of an interference matrix.
In one embodiment, the MAI selection module 316 may use a sum of absolute values of I and Q components of the at least one symbol estimate for a particular sub-channel. The sum may be compared to at least one threshold for determining the presence or absence of that particular channel. Data values corresponding to measurements that don't pass the threshold criterion may optionally be forced to zero. Data that pass the threshold criterion is spread by an FWT 318. An optional channel emulator (not shown) may be employed to approximate channel distortions observed in the digital baseband signal. The resulting spread (and optionally distorted) signal is scrambled in a Gold code scrambler 319 to produce at least one selected interference signal.
An optional sync-code insertion module 320 may be employed for inputting synchronization codes, such as P-SCH and S-SCH codes in a W-CDMA system. An interpolating filter 321 processes each selected interference signal prior to processing by a weighted-decision combiner 323. In one embodiment, the interpolating filter 321 closely models the combined function of transmit and receive pulse-shaping filters (not shown). The weighted-decision combiner 323 may select and sum a plurality of the selected interference signals from various interfering multipaths to produce a composite interference vector (CIV).
A CIV may refer to an interference vector formed as a linear combination of interference vectors scaled according to each channel's relative amplitude. One advantage to producing a CIV is that it provides for rank reduction of the S matrix while still enabling cancellation of multiple interfering channels. This rank reduction allows for a single rank interference matrix (i.e., the CIV) to cancel a plurality of signal vectors.
An interference-cancelled signal is produced by a projection operator 311 configured to orthogonally or obliquely project the digital baseband signal onto a subspace that is substantially orthogonal to an interference subspace determined from the CIV. Interference cancellation may be performed over a data-symbol interval, or some integer multiple or a fraction of the data-symbol interval. Interference cancellation may be performed over a sample interval in which there is a plurality of samples per chip. The interference-cancelled signal may be coupled into an optional power-scaling block 312 to adjust the power of the interference-cancelled signal to match that of the digital baseband signal. Optionally, a signal selection block (not shown) may be configured to select either the interference-cancelled signal or the received digital baseband signal based on at least one signal-quality criterion.
The invention may employ various selection criteria to determine which channels may produce MAI and determine which projections to use. The interference selectors 302.1-302.M are typically configured to produce a symbol-level output for one or more MAI channels. The aforementioned techniques are described more fully in a co-pending U.S. Pat. Appl. entitled “Interference Selection and Cancellation for CDMA Communications,” and assigned to the assignee of the present invention. The contents of this U.S. Patent Application are incorporated herein by reference.
In an exemplary embodiment of the invention, interference selectors 302.1-302.M may employ threshold detection in which the instantaneous or averaged signal power for individual receive channels is compared to a predetermined threshold to determine which channels should be considered to be active. In another embodiment of the invention, the interference selectors 302.1-302.M may employ a signal-processing algorithm that uses correlations and principles of multi-variate statistical inference to identify active MAI channels and their complex gains. The number of MAI channels identified may be bounded by a predetermined maximum number, the number of channels determined to exceed a predetermined power threshold, and/or the number of channels required to optimize at least one predetermined measure of performance. In some other embodiments of the invention, the interference selectors 302.1-302.M may be configured to identify common (e.g., control) channels that are known to be present. Alternative interference-selection procedures may be implemented without departing from the scope and spirit of the invention.
In a preferred embodiment, the interference selectors 302.1-302.M are configured to identify a common channel that is always present and use an average function of the common channel's complex amplitude as a comparison metric for other channels. For example, the average function may include the magnitude-squared of the absolute value of the real and imaginary parts of the complex amplitude. In IS 95/CDMA 2000, the synchronization channel can be used as the common channel. In W-CDMA, the P-CPICH or P-CCPCH may be used. Those skilled in the art should appreciate that there are other channels and functions that may be used in conjunction with the embodiments of the invention, and that the scope of the invention should not be limited by the constraints of the channel-selection procedure employed.
Two or more of the interference selectors 302.1-302.M may be coupled together such that decisions for Walsh selection of one path may be influenced by a detection process for at least one other path. For example, when two or more multipath components from a base station are processed in a Rake receiver, it may be advantageous to use the strongest multipath for interference selection. Thus, the interference selector 302.1-302.M corresponding to the strongest multipath may be used to determine the MAI channels for each path.
Optional channel emulators 305.1-305.M may provide complex gains to the selected channel outputs such as to reproduce the effects of channel distortion resulting from the propagation channel between the transmitter(s) and the receiver. A baseband signal reconstruction module 303.1-303.M processes the MAI channel symbols to produce a signal that is substantially in the same form as the transmitted baseband signal. For example, each baseband signal reconstruction module 303.1-303.M may provide scaling, orthogonalizing codes, and scrambling codes to the MAI channel symbols.
Outputs of the baseband signal reconstruction module 303.1-303.M may be coupled to an optional pulse-shaping filter 304.1-304.M. In an alternative embodiment, an interpolating filter may be used, such as an interpolating filter configured to approximate the combined effects of transmit and receive pulse-shaping filters. In an exemplary embodiment, a linear interpolator may be used. Another exemplary embodiment may employ a raised-cosine interpolating filter having a predetermined roll-off factor.
A weighted-decision combiner 306 provides confidence weights to input MAI-channel signals to produce a weighted MAI-channel output that is coupled to a projection operator 307. The received baseband signal is also coupled into the projection operator 307, which produces at least one interference-canceled signal. Canceled signals produced by the projection operator 307 are output to Rake fingers of a receiver.
The confidence-weight generator 401 may comprise any of various DSP algorithms and correlation functions to determine the weights. In an exemplary embodiment of the invention, the confidence-weight generator 401 may be configured to determine relative strengths of the received paths and determine the weights from the binary set of {0,1}. For example, if a desired signal path is stronger than an interfering path by a given factor, then the interferer is assigned a weight of 0, else the interferer is assigned 1. Because signal paths that are below a predetermined threshold may not be considered reliable for cancellation, they may be excluded from the cancellation process.
In other embodiments of the invention that employ large weight constellations, relative weights may be assigned to the interfering paths such that paths with high estimation errors may be given a lesser weight. For example, a path 6dB below the desired signal path may be assigned a smaller confidence measure than a path that is only 3dB below the desired signal path. A combiner 402 weights and combines the MAI-channel signals to output a combined interference matrix. Alternatively, the combiner 402 may perform a vector addition of the weighted interference vectors to produce an output interference vector, such as a CIV.
The output of the combiner 402 may include a CIV or an S-matrix of interference vectors selected for cancellation. An S-matrix may be coupled to an optional left linear transformation 403 (LLT) or to an optional right linear transformation 404 (RLT). The LLT 403 and RLT 404 differ in that the former is a linear transformation applied to the row space of an interference matrix S, and the latter is a linear transformation applied to the column space of the interference matrix. Embodiments of the invention may advantageously employ any type of linear transformation on the column space of the interference matrix. Such embodiments may exploit the fact that complex projection operations performed by the projection operator 307 are invariant to non-singular RLTs. If the output of the combiner 402 is a vector, the LLT 403 and/or the RLT 404 provide a real or complex scaling factor.
Singular RLTs may be used to construct low-dimensional interference sub-spaces to be used in producing low-dimensional projections. A CIV is a one-dimensional interference vector constructed from a higher-dimensional interference matrix S. A projection PS⊥ constructed from the CIV is an (N-1) dimensional projection, whereas a projection PS⊥ constructed from an interference matrix S is (N-M) dimensional, where S is a matrix comprising M interfering vectors. The construction of CIVs and S-matrices is well known in the art, such as described in U.S. patent Application Ser. No. 10/294,834, entitled “Construction of an interference matrix for a coded signal processing engine,” which is incorporated by reference in its entirety.
For systems configured to produce a vector from S, the RLT is a vector. A single-column RLT may be used to provide a linear transformation, such as (but not limited to) channel emulation. The projection is invariant to complex scaling of the RLT. Some embodiments of the invention may employ RLT matrices, such as to provide multiple linear combinations, or multiple CIVs, of the active Walsh Channels.
{circumflex over (y)}=(I−S)(SHS)−1SH)y,
where I is an identity matrix.
If S and y are complex-valued, then the projection can be decomposed into eight real algebraic operations 405. For each of the operations 405, an input comprises some combination of the input signal's in-phase and quadrature components yi and yq, and the in-phase and quadrature components Si and Sq. The operations performed on these inputs are represented algebraically by the eight operations 405, where the term A is:
A=SHS,
where S=Si+iSQ is the complex representation of in-phase and quadrature interference matrices. The terms in-phase and quadrature may include any two channels in a two-channel processing system, whether or not there is an associated quadrature demodulator.
Outputs from the operations 405 are multiplied by weights 411-418. In the case in which the weights 411-418 have values of 1 or 0, the weights 411-418 merely represent a selection process with respect to which operations 405 are performed. Thus, in some embodiments of the invention, the weights 411-418 are not intended to provide a literal interpretation of how signal processing is performed. For example, in some embodiments of the invention, it is not desirable to perform one or more of the operations 405 only to have the corresponding output zeroed out by the weighting process 411-418. Rather, it may be preferable to simply avoid performing the corresponding one or more operations 405. A weight-value of one may correspond to an implementation of a particular one of the operations 405.
The outputs from the operations 405 are combined in a combiner 407. For example, the real in-phase outputs are summed 406 to produce a combined real output. The quadrature outputs are summed 408 to produce a combined quadrature output. The combined in-phase and quadrature outputs are combined in a combiner 409 to produce a combined interference vector, which may be represented as a complex vector. In some embodiments, the complex vector may be expressed in terms of magnitude and phase angles. The combined interference vector is subtracted 419 from the input signal y to produce the interference-cancelled signal ŷ, which may be coupled into a receiver.
In some embodiments of the invention, the operations 405 may be simplified without a significant loss in performance by making the following assumptions:
In one embodiment of the invention, the matrix A may be simplified to:
A=SiTS1+SqTSq
Furthermore, correlation properties of the scrambling codes may sometimes be exploited to provide the following approximations:
SiTSi=SqTSq and SiTyi=SqTyq
In such cases, the system shown in
In some embodiments of the invention, S may be a vector representing a combination of interfering paths and MAI channels in a CDMA system. Thus, Si and Sq are vectors. In CDMA, the scrambling codes (e.g., PN codes for CDMA 2000/IS 95 and Gold Codes in W-CDMA) have excellent auto-correlation properties. This enables approximations of SiTSi=SqTSq and SiTyi=SqTyq to be accurate for certain conditions. For example, the assumption SiTyi=SqTyq is typically valid when a received signal has a relatively high signal-to-noise ratio. Furthermore, cross correlations between in-phase and quadrature components can often be regarded as relatively small. Thus, the aforementioned assumptions enable a simplified projection operation.
(SiTSi)−1SiTyi.
When Si is a vector, the first operation can be expressed by
The values yq and Sq are input to a second operator 511 to perform a second operation:
(SqTSq)−1SqTyq,
which can be expressed by
when Sq is a vector.
The output of the first operator 501 is multiplied 502 by Si and then subtracted 503 from yi to produce an interference-cancelled in-phase signal ŷi. Similarly, the output of the second operator 511 is multiplied 512 by Sq and then subtracted 513 from yq to produce an interference-cancelled quadrature signal ŷq. The interference-cancelled in-phase and quadrature parts may be combined 508 in a complex algebra to produce a complex interference-cancelled signal ŷ=ŷi+iŷq.
(SiTyi)/(SiTSi) and (SqTyq)/(SqTSq)
The output of operator 510 is multiplied 514 by Si and then subtracted 516 from yi to produce an interference-cancelled signal ŷi. Similarly, the output of operator 510 is multiplied 534 by Sq and then subtracted 536 from yq to produce an interference-cancelled signal ŷq. Although many of the preferred embodiments of the invention have been illustrated and described with respect to a vector version of the interference S, those skilled in the art will recognize that appropriate adaptations and variations of the above-recited embodiments may be provided for any matrix version of the interference S. Furthermore, each of the previously described embodiments may include means for selecting which of a set of signals, including an interference-cancelled signal ŷ and an input signal y, may be coupled to further processing means, such as a Rake receiver.
As an alternative to the orthogonal projection operator PS⊥, the embodiments of the invention may provide an oblique projection operator QS⊥. An orthogonal projection typically transforms the signal of interest. However, an oblique projection may be advantageously configured to preserve at least one predetermined property of the signal of interest by accounting for the angle between the interference and the signal of interest. Thus, an oblique projection may avoid decision errors in a signal-of-interest estimate that can result from an orthogonal projection.
In one embodiment of the invention, an interference-rejecting oblique projection operator QS⊥(R−1) may be expressed by
QS⊥(R−1)=(I−S(SHR−1S)−1SHR−1),
where S is an interference matrix, and matrix R may represent correlation of the signal of interest, correlation between the signal of interest and the interference, or correlation of the received signal. One may obtain R by using correlation functions and other elements of statistical signal processing that are well known in the art. One of ordinary skill in the art should appreciate that an oblique projection operator may be constructed from a CIV, which is a vector t of the form t=Sb, where b is a vector-valued RLT.
In one embodiment of the invention, the projection receiver may be configured to produce a matrix R that describes the correlation between the signal of interest and the interference. A perfect estimate of R makes QS⊥(R−1)y a best linear unbiased estimator of the interference. An accurate (but less than perfect) estimate of R produces an empirical best linear unbiased estimator, which substantially projects interference out of the direction of the desired code space. In yet another embodiment of the invention, the projection receiver may be reconfigured to produce an orthogonal projection by setting the matrix R to be an identity matrix I.
In another embodiment of the invention, an oblique projection operator QC⊥(PS⊥) may be expressed by
QC(PS⊥)=C(CHPS⊥C)−1CHPS⊥,
where PS⊥ is an orthogonal projection operation and C is a signal matrix of interest (e.g., a spread-spectrum code matrix). In this case,
QC⊥(PS⊥)C=C and QC⊥(PS⊥)S=0,
thus removing the interference and passing the signal of interest undistorted.
In one Rake receiver embodiment of the invention, interference cancellation for a particular (i.e., selected) Rake finger, or multipath, includes determining interference from one or more non-selected Rake fingers (e.g., multipaths). In the case where the signal of interest is a traffic channel, the signal matrix of interest C may be a corresponding Walsh code c scrambled with a particular PN code. The interference space S will comprise a compound vector emulating interference from paths assigned to the one or more non-selected Rake fingers that are likely to interfere with the signal of interest.
A weighted projection is a scaling of a projection based on at least one reliability estimate of the projection. For example, an orthogonal projection operator that fails to meet a predetermined reliability threshold may be weighted by a factor β<1. Thus, some embodiments of the invention may provide a pseudo-projection operation of the form
PS⊥=I−βPS.
Another embodiment of the invention may provide for a weighted combination of y and PS⊥y based on reliability estimates.
y⊥=(α1I+α2PS⊥)y,
where α1 and α2 represent reliability weights. Those skilled in the art should appreciate that many different techniques may be used to calculate reliability weights. For example, the reliability weights may be determined from signal measurements, such as SNR or probability of error. Reliability weights are also known in the art as confidence measures. In some embodiments of the invention, a weighted oblique projector may be expressed by
y⊥=(α1I+α2Q)y
The invention is not intended to be limited to the preferred embodiments. Furthermore, those skilled in the art should recognize that the method and apparatus embodiments described herein may be implemented in a variety of ways, including implementations in hardware, software, firmware, or various combinations thereof. Examples of such hardware may include Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), general-purpose processors, Digital Signal Processors (DSPs), and/or other circuitry. Software and/or firmware implementations of the invention may be implemented via any combination of programming languages, including Java, C, C++, Matlab™, Verilog, VHDL, and/or processor specific machine and assembly languages.
Computer programs (i.e., software and/or firmware) implementing the method of this invention may be distributed to users on a distribution medium such as a SIM card, a USB memory interface, or other computer-readable memory adapted for interfacing with a consumer wireless terminal. Similarly, computer programs may be distributed to users via wired or wireless network interfaces. From there, they will often be copied to a hard disk or a similar intermediate storage medium. When the programs are to be run, they may be loaded either from their distribution medium or their intermediate storage medium into the execution memory of a wireless terminal, configuring an onboard digital computer system (e.g. a microprocessor) to act in accordance with the method of this invention. All these operations are well known to those skilled in the art of computer systems.
In an exemplary embodiment of the invention, a first weighted MAI-channel output is coupled into a first stage of a first Rake finger (i.e., Rake Finger1) 607.1. An Mth weighted MAI-channel output is coupled into a first stage 607.M of an Mth Rake finger (i.e., Rake FingerM). A projection module 608.1 is coupled between the first stage 607.1 of Rake Finger1, and a second stage 609.1 of Rake Finger1. Similarly, a projection module 608.M is coupled between the first stage 607.M of Rake FingerM and a second stage 609.M of Rake FingerM. Those skilled in the art will recognize that a projection module (e.g., the projection modules 608.1-608.M) can be placed anywhere in a receiver chain of a Rake finger, such as shown in
Each of the projection modules 608.1-608.M is typically configured to receive a digital baseband signal including a signal of interest, and at least one selected interfering signal. A preferred embodiment of the invention provides for processing the digital baseband signal in substantially the same manner (e.g., with respect to descrambling, despreading, de-multiplexing, space-time processing, etc.) as the selected interfering signals.
Outputs from the projection operators 705.1-705.M or the pulse shaping filters 706.1-706.M may optionally be coupled to one or more Rake fingers, such as Rake fingers 701.1-701.M. Alternatively, auxiliary Rake fingers (not shown) may be employed. The receiver shown in
In some embodiments of the invention, the projection operators 705.1-705.M may be placed at any of various positions within the baseband signal reconstruction modules 703.1-703.M. The baseband signal reconstruction modules 703.1-703.M may be separated into discrete baseband-reconstruction components configured to perform various operations, such as spreading, scrambling, channel emulation, etc. Thus, the projection operators 705.1-705.M may be configured to process at least one selected interference signal and at least one digital baseband signal comprising at least one signal of interest in a manner corresponding to where the projection operators 705.1-705.M are located within each baseband signal reconstruction module 703.1-703.M.
In some embodiments of the invention, a successive approximation method may be employed to construct the projection operator. For example, the number of columns in the S matrix may be progressively increased or decreased with each iteration of the successive approximation method. That is, Si=[Si−1,Si] or Si−1=[Si,Si]. In embodiments of the invention configured to produce a CIV s from S, successive approximation may include progressively increasing or decreasing the number of MAI channels (e.g., Walsh channels) in the linear combination. For example, si=si−1+sibi or si−1=si+sib1. Those skilled in the art will recognize other successive approximation methods that may be applied to embodiments of the present invention.
Any of various metrics may be used to control the iterative process. A preferred metric is a coherence measure that indicates the strength of the signal of interest relative to the total power in the base-band signal after performing each projection. A coherence measure ξ for a one-dimensional code space c is given by
where Si is an interference matrix or vector for an ith iteration of the successive approximation step, c is a desired code vector, and y is a complex base-band signal. In the case of a predetermined maximum number of iterations being reached, a choice of Si may be made that maximizes ξ. In a preferred embodiment, a pilot channel is selected as c. Alternatively, a traffic channel may be used to construct c.
Various embodiments of the invention may include variations in system configurations and the order of steps in which methods are provided. In many cases, multiple steps and/or multiple components may be consolidated. Successive approximations of a projection shown herein may also include performing only a single iteration for a selected interference matrix S or a CIV s.
The method and system embodiments described herein merely illustrate particular embodiments of the invention. It should be appreciated that those skilled in the art will be able to devise various arrangements, which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are intended to be only for pedagogical purposes to aid the reader in understanding the principles of the invention. This disclosure and its associated references are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.
It should be appreciated by those skilled in the art that the block diagrams herein represent conceptual views of illustrative circuitry, algorithms, and functional steps embodying principles of the invention. Similarly, it should be appreciated that any flow charts, flow diagrams, signal diagrams, system diagrams, codes, and the like represent various processes that may be substantially represented in computer-readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.
The functions of the various elements shown in the drawings, including functional blocks labeled as “processors” or “systems,” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, the function of any component or device described herein may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.
Any element expressed herein as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a combination of circuit elements which performs that function, or software in any form, including, therefore, firmware, micro-code or the like, combined with appropriate circuitry for executing that software to perform the function. Embodiments of the invention as described herein reside in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the operational descriptions call for. Applicant regards any means that can provide those functionalities as equivalent to those shown herein.
This application is a continuation-in-part of commonly owned and co-pending U.S. patent applications Ser. No. 10/773,777 (filed Feb. 6, 2004), Ser. No. 10/686,829 (filed Oct. 15, 2003), Ser. No. 10/686,359 (filed Oct. 15, 2003), Ser. No. 10/294,834 (filed Nov. 15, 2002), and Ser. No. 10/247,836 (filed Sep. 20, 2002), all assigned to the assignee hereof and hereby expressly incorporated by reference herein. This application incorporates by reference co-pending U.S. Pat. Appl. entitled “Interference Selection and Cancellation for CDMA Communications,” filed on, the entire disclosure and contents of which is hereby incorporated by reference.
Number | Date | Country | |
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Parent | 10247836 | Sep 2002 | US |
Child | 11100935 | Apr 2005 | US |
Parent | 10773777 | Feb 2004 | US |
Child | 11100935 | Apr 2005 | US |
Parent | 10686829 | Oct 2003 | US |
Child | 11100935 | Apr 2005 | US |
Parent | 10686359 | Oct 2003 | US |
Child | 11100935 | Apr 2005 | US |
Parent | 10294834 | Nov 2002 | US |
Child | 11100935 | Apr 2005 | US |