Patent Grant
8300803
Digital subscriber line (DSL) implementations, including very high speed digital subscriber line (VDSL) implementations may be utilized as a last mile solution for high speed data communications. While such technologies have increased the data exchange rate between a customer and a central office, such implementations may experience noise that affects the data transmission rate. More specifically, VDSL implementations may be impaired by any number of disturbances, including self-crosstalk, impulse noise, alien noise, etc.
Included are embodiments for reducing alien crosstalk. At least one embodiment of a method includes receiving noise data associated with a first user signal on a first tone, receiving noise data associated with a second user signal on the first tone, and receiving at least one alien crosstalk canceller coefficient for the first user on the first tone. Some embodiments include applying the at least one alien crosstalk canceller coefficient to the second user signal to reduce alien crosstalk for the first user signal.
Also included are embodiments of a system. At least one embodiment of a system includes a first receiving component configured to receive noise data associated with a first user signal on a first tone, a second receiving component configured to receive noise data associated with a second user signal on the first tone, and a third receiving component configured to receive at least one alien crosstalk canceller coefficient for the first user on the first tone. Some embodiments include an applying component configured to apply the at least one alien crosstalk canceller coefficient to the second user signal to compute a correction signal for the first user and a reducing component configured to combine the correction signal for the first user and the first user signal to reduce alien crosstalk into the first user signal.
Other embodiments and/or advantages of this disclosure will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description and be within the scope of the present disclosure.
Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. While several embodiments are described in connection with these drawings, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents.
Dynamic spectrum management level 3 (DSM3) is an initiative taken to mitigate the limitations posed by self-FEXT in DSL environments. DSM3-compliant devices may be configured to use signal cooperation (e.g., signal vectoring) at a central office (CO), and employ CO-centric processing. However, alien-crosstalk mitigation, in both upstream and downstream directions, may not fall under the purview of the DSM3 standard.
Alien crosstalk may include an additive noise (apart from the additive Gaussian thermal noise) at a receiver device of vectored users in a VDSL DSM3 configuration. Alien crosstalk may stem from the services outside the vectored system that share the same cable. This additive noise may decrease the achievable capacity for the VDSL. Owing to the inception of the noise from the same sources, the alien noise experienced at the receivers of the different vectored users exhibits correlation. The vectored VDSL configuration may not have access to the transmitted or received data of the alien disturbance sources. Nevertheless, in the upstream direction (client to central office), the vectored VDSL configuration can take advantage of simultaneous access to the received signals of vectored users at the CO to effectively mitigate alien crosstalk.
At least one embodiment discussed herein addresses alien crosstalk mitigation in the upstream direction for a vectored group of VDSL users. Similarly, some embodiments employ a software and/or hardware based low complexity and adaptive alien-crosstalk canceller that may be initialized, engaged, and adapted non-disruptively while the vectored users operate in data mode.
Similarly, embodiments of the alien-crosstalk canceller may be configured to leverage the correlation across vectored users that is present in the additive noises on each tone. This may be accomplished by directly acting on error signals that include alien-crosstalk. Similarly, in some embodiments, at the central office, this alien cancellation methodology may be applied subsequent to the self-FEXT cancellation algorithm that mitigates the self-far end cross talk (self-FEXT, which is FEXT originating from users within the vectored group). Similarly, in some embodiments, initialization and/or engagement of the alien canceller may be performed during a warming-up phase of the VDSL line (pre-data-mode), in conjunction with a corresponding pre-data-mode self-FEXT canceller initialization.
Further, some embodiments may be configured to process signals on a per-tone basis, which may be justified since such a configuration may be dedicated to DMT modulated implementations. Such an implementation may be configured to mitigate the error signal of one or more tones across multiple channels, where the error is computed at the output of the slicer subsequent to frequency-domain equalization and self-FEXT cancellation.
Additionally included are embodiments of a blind whitening algorithm for alien cross talk mitigation, which may be configured to operate on the residual errors at the output of the slicer subsequent to frequency-domain equalization and self-FEXT cancellation. Such embodiments may be configured with a low complexity by virtue of not requiring any matrix factorization or inversion which is typically employed by previously suggested solutions. Similarly, some embodiments may be configured without the need for modifications to current hardware.
In implementation, after undergoing self-FEXT cancellation, received symbols in the frequency domain may be infected with a mixture of the alien-noise and the white Gaussian noise. In many VDSL environments, the bit-loading may be designed such that a bit error rate (BER) a every tone stays at or below 10A(31 7), even when the signal to-noise-ratio SNR on the tone is lowered by an amount equal to the noise margin (e.g., nominally 6 dB). Thus, when the bit-loading is primarily based on the signal-to-noise ratio (SNR) at the output of the self-FEXT canceller (coding gain being ignored), the residual error vector can be evaluated using an estimate of the transmitted symbol for each vectored user at the output of the self-FEXT canceller. Each element of the residual error vector may contain the additive white Gaussian noise (AWGN) at the receiver as well as the alien cross-talk. The variance of residual error for each user may be concurrently minimized by removing the correlation in the alien cross-talk.
To facilitate non-disruptive operation in data-mode, at least one embodiment may be configured with a linear recursion for the above cancellation scheme. For each of the N vectored users (where N can be any integer), a stochastic gradient based algorithm may be used to adaptively estimate the N−1 coefficients of the alien-canceller that operate on the residual errors of the remaining users computed prior to alien-crosstalk cancellation (e.g., at the output of the self-FEXT canceller). The stochastic update paradigm is described below and facilitates operation of the software alien canceller in the data mode. Additionally, the update paradigm may be configured to adapt the canceller in a changing noise environment and/or when new users join or leave the vectored group.
The alien-crosstalk canceller may be inserted into operation (engaged) with an initial null canceller and/or with an initial estimate based on the iterative estimation described in equation below. Similar to the adaptations with the canceller in operation, the iterative estimation during initialization may also be based on the stochastic gradient paradigm and may not require any matrix inversion or factorization. However, one may note that during the initial iterative estimation, the alien-crosstalk canceller may not be in active operation.
Engaging the alien-crosstalk canceller followed by the least mean square (LMS) adaptations of the canceller may result in an improvement of the SNR at the output of the alien-crosstalk canceller. An example of the possible SNR improvements due to alien-crosstalk cancellation obtained via simulations is presented in
One should note that once the new bit-loading table is computed based on the improved SNR due to the alien-crosstalk cancellation, the bit-error rate based on the received signal on a tone at the output of the self-FEXT canceller (e.g., the input to the alien-crosstalk canceller) is no longer guaranteed to be always below 10A(−7). In other words, the residual error at the output of the self-FEXT canceller may no longer be guaranteed to be reliable. This phenomenon may limit the realistic improvement in SNR to a value equal to the noise-margin of the system (nominally 6 dB).
Embodiments disclosed herein may be utilized in discrete multitone (DMT) modulated systems and is per-tone based, where the frequency domain error signal for one particular tone may be processed across multiple channels (users).
Referring now to the drawings,
Additionally, the CPE 102 may be coupled to a DSL modem 103a, 103b, 103c or other CPE (not shown). The DSL modem 103 may be configured as a recipient and/or provider of information between the CPE 102 and a central office (CO) 104. The central office 104 may include any equipment and/or logic configured to provide and/or receive data from the customer premises equipment 102, 103. More specifically, the central office 104 may include a DSL access multiplexor (DSLAM), server, personal computer, and/or other equipment. The central office 104 may also be coupled to a network 106. The network 106 may include the Internet, a public switched telephone network (PSTN), an integrated services digital network (ISDN) or other wide area network or local area network. Similarly, while the components of
As discussed above, such a configuration of wireline components may introduce alien crosstalk among cables, indicated at ellipse 108. More specifically, the cables between the customer premises 101 and the central office 104 may include lines for a plurality of communications protocols. As a nonlimiting example, DSL may be included, as well as a telephone line, and/or other lines. In such a configuration, alien crosstalk may affect the DSL data in another cable. As discussed above, such alien crosstalk may reduce the quality of communicated data and/or reduce speed of transmission.
The processor 282 can be any custom made or commercially available processor, a central processing unit (CPU), an auxiliary processor among several processors associated with the central office 104, a semiconductor based microprocessor (in the form of a microchip or chip set), a macroprocessor, or generally any device for executing instructions.
The memory component 284 can include any one or combination of volatile memory elements (e.g., random access memory (RAM, such as DRAM, SRAM, SDRAM, VRAM, etc.)) and nonvolatile memory elements (e.g., ROM, hard drive, tape, CD-ROM, etc.). Moreover, the memory component 284 may incorporate electronic, magnetic, optical, and/or other types of storage media. One should note that the memory component 284 can also have a distributed architecture, where various components are situated remotely from one another, but can be accessed by the processor 282.
The software in the memory component 284 may include one or more separate programs, each of which includes an ordered listing of executable instructions for implementing logical functions. In the example of
Additionally, while the logic components 286, 299 are each illustrated in this nonlimiting example as a single piece of logic, these components can include one or more separate software, hardware, and/or firmware modules. Similarly, one or more of these logical components can be combined to provide the desired functionality. Additionally, the operating system 286 may be configured to control the execution of other computer programs and may be configured to provide scheduling, input-output control, file and data management, memory management, and communication control and related services.
A system component embodied as software may also be construed as a source program, executable program (object code), script, and/or any other entity that includes a set of instructions to be performed. When constructed as a source program, the program is translated via a compiler, assembler, interpreter, or the like, which may or may not be included within the volatile and nonvolatile memory 284, so as to operate properly in connection with the operating system 286.
The input/output devices that may be coupled to system I/O interface(s) 296 may include input devices, for example but not limited to, a keyboard, mouse, scanner, microphone, camera, proximity device, receiver, etc. Further, the input/output devices may also include output devices, for example but not limited to, a printer, display, transmitter, etc. The input/output devices may further include devices that communicate both as inputs and outputs, for instance but not limited to, a modulator/demodulator (modem for accessing another device, system, or network), a radio frequency (RF) or other transceiver, a telephonic interface, a bridge, a router, WIFI communications device, WiMAX communications device, bluetooth communications device, etc. Similarly, a network interface 298, which is coupled to local interface 292, can be configured to communicate with a communications network, such as the network 100 from
More specifically, the network interfaces 298 may be configured for facilitating communication with one or more other devices. The network interface 298 may include any component configured to facilitate a connection with another device. While in some embodiments, among others, the central office 104 can include the network interface 298 that includes a Personal Computer Memory Card International Association (PCMCIA) card (also abbreviated as “PC card”) for receiving a wireless network card, this is a nonlimiting example. Other configurations can include the communications hardware within the client device 102, such that a wireless network card is unnecessary for communicating wirelessly. Similarly, some embodiments may include the network interfaces 298 for communicating via a wired connection. Such interfaces may be configured with universal serial bus (USB) interfaces, serial ports, and/or other interfaces. In operation, the wireless network interfaces 298 may be configured to communicate with other CPEs 102, 103, and other wireless devices via a wireless local area network (WLAN) or other wireless network.
If the central office 104 includes a personal computer, workstation, or the like, the software in the memory component 284 may further include a basic input output system (BIOS) (omitted for simplicity). The BIOS is a set of software routines that initialize and test hardware at startup, start the operating system 286, and support the transfer of data among the hardware devices. The BIOS is stored in ROM so that the BIOS can be executed when the central office 104 is activated.
When the central office 104 is in operation, the processor 282 can be configured to execute software stored within the memory component 284, to communicate data with the memory component 284, and to generally control operations of the central office 104 pursuant to the software. Software in memory 284, in whole or in part, may be read by the processor 282, perhaps buffered within the processor 282, and then executed. Additionally, one should note that while the above description is directed to a central office 104, other devices can also include the components described in
One should note that the CPEs 102, 103 can be configured with one or more of the components and/or logic described above with respect to the central office 104. Additionally, the CPEs 102, 103, the central office 104, and/or other components of
The respective signals z1[q,t],zm[q,t],zN[q,t] may then be analyzed to generate {circumflex over (x)}1[q,t],{circumflex over (x)}m[q,t],{circumflex over (x)}N[q,t], the estimates of the signals transmitted from the CPEs 102, 103 (see 307a, 307m, 307n). The transmitted signal estimates {circumflex over (x)}1[q,t], {circumflex over (x)}m[q,t],{circumflex over (x)}N[q,t] can be subtracted from z1[q,t],zm[q,t], zN[q,t] at adder 309a, 309m, 309n to provide the slicer errors w1[q,t],wm[q,t], wN[q,t]. The slicer errors are sent to a slicer error buffer 311, as well as to alien crosstalk mitigators 313a, 313m, and 313n. The slicer error buffer 311 may also be configured to send a subset of the slicer errors to the alien crosstalk mitigators 313. The alien crosstalk mitigator for the user m 313m (as illustrated in the exploded view) can receive the slicer errors wm[q,t] and w−m[q,t]=(w1[q,t] . . . wm−1[q,t] wm+1[q,t] . . . wN[q,t])T at an initialization computation component 319, where T represents the matrix transpose operator. The initialization computation component 319 may be configured to implement an initialization computation, such as described below. The result from this computation is the initialization coefficient for user m and is a vector labeled as αm and stored in a component 327 (when the alien crosstalk mitigator is set in initialization mode). One should note that, while slicer errors are provided in this nonlimiting example, any noise data may be provided.
Similarly, the slicer error w−m[q,t] may be sent to an adaptation computation component 321. The alien crosstalk mitigator 313m may also be configured to receive alien canceled signal z′m[q,t] in a feedback loop. A transmitted signal may be estimated at block 323 and may be subtracted from the alien canceled signal zm[q,t] (block 325). The resulting signal (em[q,t]) is sent to the adaptation computation component 321. The adaptation computation component 321 may be configured to implement an adaptations computation, as described below. The resulting data comprise the adaptation coefficients and may be sent to the αm component 327. The alien crosstalk mitigator 313m may also be configured to apply the adaptation coefficients αm to the slicer error of the other users w−m[q,t] via a scalar product operation (αmTw−m[q,t]) to generate a resulting signal which may be subtracted from the original signal zm[q,t] to yield the alien canceled signal z′m[q,t]. The alien canceled signals z′1[q,t],z′m[q,t],z′N[q,t] from the alien crosstalk mitigator 313 may be sent to a trellis coded modulation (TCM) demodulator 317 for demodulation.
More specifically, in operation, the exemplary embodiment of
z=x+w (1)
In a system with R alien disturbers. One may let A denote the N×R alien coupling matrix for tone q and φ denote the R×1 alien symbol transmitted vector on tone q. The noise data w may be infected with the AWGN (denoted as v) and alien-cross talk and therefore, its elements wm may be correlated. Therefore, equation (2) may yield:
w=v+Aφ (2)
In at least one exemplary embodiment, the alien disturbers may transmit with substantially the same power, E[φφh]=ρ2I and similarly for the AWGN noise, E[vvh]=σv2I. Therefore, the total noise for a mth channel and its variance σbac,m2 (the subscript bac denotes before-alien-canceller) may be given by the following equations:
wm=vm+aTmφ (3)
σbac,m2=E[wmw*m]=σv2+ρ2∥am∥2 (4)
Here, amT is the mth row of the Alien coupling matrix A. One may define {circumflex over (x)}=demapped(z), that is the sliced output of the received signal vector on tone q post self-FEXT cancellation. Since the operating BER during the data-mode in the VDSL may be 10−7, one can assume that {circumflex over (x)} acts as a reliable estimate of the actual transmitted symbol x.
Since, wm, 1≦m≦N may be correlated, one may find a (N−1)×1 vector αm such that the resulting variance σm2=var(wm−w−mTαm) is minimized for user m. The optimal αm using the orthogonality principle is given by Equation (5):
αmopt=Γ−m−1pm (5),
where, Γ−m=E[w*−mw−mT], is the noise covariance matrix of all the users except, user m, pm=E[wmw*−m] and w−m[q,t]=(w1[q,t] . . . wm−1[q,t] wm+1[q,t] . . . wN[q,t])T is an (N−1)×1 noise data comprising of all elements but wm. This linear combination can be done using the following equations:
z′m=zm−αm(opt)Tw−m (6)
The resulting alien cancelled symbol for channel m, z′m then passes through the TCM demodulation block as shown in the figure. This procedure can be applied concurrently for all the users.
Now, one can evaluate the expected reduction in the noise variance due to the above blind whitening methodology. Substituting the expression for the optimal N−1 dimensional coefficient amT obtained in equation (5) in the expression for variance σm2 yields the following:
σm2=E[(wm−wT−mαm(opt))w*m] or σm2=E[|wm|2]−pmTΓ−m−1*p*m (7)
Hence, the residual total noise (alien+Gaussian) variance post alien-cancellation is given by Equation (8):
σm2=σv2+amT(I+λA−mHA−m)−1)a*mρ2, (8),
where
The Cramer-Rao lower bound (CRLB) for an estimator of xm, e.g., the minimum achievable variance for an estimator of xm can be computed as:
CRLB({circumflex over (x)}m)=σbac,m2−pmTΓ−m−1*p*m
It is clear from (7) and the above equation that low complexity blind whitening algorithm may be configured to achieve the CRLB.
A linear recursion of the scheme may be utilized, as well. More specifically, Equation (5) utilizes a knowledge of the joint statistics of Γ−m and pm. In seeking to avoid the need to learn these statistics before estimating the optimal αm, one can replace the equation (5) by an iterative method of estimating αm by using the stochastic gradient approach. As an error vector w[tn] at DMT symbol index tn is received, αm[tn] is updated in the direction opposite of the gradient of the instantaneous quadratic error subsequent to applying the canceller, ∥em[tn]|2 where
em[tn]=z′m[tn]−demapped(z′m[tn]) (9)
The above principle gives the following recursion for the channel m
αm[tn]=αm[tn−1]+μw−m[tn]e*m[tn] (10)
The stochastic update paradigm described in Equation (10) may allow for operation of the software alien canceller in the data mode and also adapt the canceller in a changing noise environment or when new users join or leave the vectored group. The alien-crosstalk canceller may be inserted into operation (engaged) with an initial null canceller, or alternatively, with an initial estimate based on the iterative estimation described in Equation (11):
αm[tn]=αm[tn−1]+μw−m[tn](wm[tn]−αmT[tn−1]w−m[tn])* (11).
Just like the adaptations with the canceller in operation Equation (10), the iterative estimation during initialization Equation (11) may also be based on the stochastic gradient paradigm and does not require any matrix inversion or factorization. However, one may note that during the initial iterative estimation described by Equation (11), the alien-crosstalk canceller is not in active operation.
While these gains may improve signal quality, utilizing the LMS alien canceller described herein may improve the SNR to 49.77 for the first user, 56.33 for the second user, and 63.40 for the third user. Based on the Cramer-Rao lower bound (CRLB) the highest SNR achievable may be 50.03 for user 1, 56.41 for user 2, and 63.40 for user 3.
The embodiments disclosed herein can be implemented in hardware, software, firmware, or a combination thereof. At least one embodiment disclosed herein may be implemented in software and/or firmware that is stored in a memory and that is executed by a suitable instruction execution system. If implemented in hardware, one or more of the embodiments disclosed herein can be implemented with any or a combination of the following technologies: a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate combinational logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc.
One should note that the flowcharts included herein show the architecture, functionality, and operation of a possible implementation of software. In this regard, each block can be interpreted to represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that in some alternative implementations, the functions noted in the blocks may occur out of the order and/or not at all. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved.
One should note that any of the programs listed herein, which can include an ordered listing of executable instructions for implementing logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device. More specific examples (a nonexhaustive list) of the computer-readable medium could include an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (electronic), a read-only memory (ROM) (electronic), an erasable programmable read-only memory (EPROM or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). In addition, the scope of the certain embodiments of this disclosure can include embodying the above described functionality in hardware and/or software-configured mediums.
One should also note that conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more particular embodiments or that one or more particular embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment.
It should be emphasized that the above-described embodiments are merely possible examples of implementations, merely set forth for a clear understanding of the principles of this disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure.
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