US 20030076900 A1 Abstract A system and method facilitate estimating noise in a received signal. The received signal includes a plurality of data tones. Noise is estimated for a selected subset of data tones for each data burst, such that noise estimates for all data tones can be computed over a plurality of data bursts. As a result of spreading the noise estimates over more than one burst, the overall computations associated with obtaining the noise estimates can be reduced based on the size of the respective subsets. The updated data tone noise estimates further can be employed in subsequent processing, such as in beamforming calculations.
Claims(46) 1. A signal processing system for use in a receiver, comprising:
a data tone selector that identifies a subset of data tones from a received data signal having a plurality of identifiable data tones; and a noise estimator that computes a noise estimate for each data tone in the subset of data tones. 2. The system of 3. The system of 4. The system of 5. The system of 6. The system of 7. The system of 8. The system of 9. The system of 10. The system of 11. The system of 12. The system of 13. The system of 14. The system of 15. A noise estimation system comprising:
a data tone identifier operative to identify a subset of data tones from a plurality of data tones for processing with a given data burst; a hard decision component operative to compute hard decisions for the subset of data tones in the given data burst; and a noise estimator operative to compute a data tone noise estimate for each data tone in the desired subset of data tones based on the respective hard decisions, the noise estimator being operative to provide an indication of the computed data tone noise estimate for each data tone of the subset of data tones in the given data burst. 16. The noise estimator of 17. A beamforming system comprising:
an aggregator that includes stored noise estimates for a plurality of data tones, the noise estimates for a selected subset of the plurality of data tones being updated for a received data signal based on noise estimates computed for the selected subset of the plurality of data tones in the received data signal; and a beamformer that computes at least one of a soft decision and noise-to-signal ratio estimate for each of the plurality of data tones in the received data signal based on the stored noise estimate for each respective data tone. 18. The system of 19. The system of 20. The system of 21. The system of 22. The system of where
H
_{n }is a channel estimate for the n^{th }antenna; R
_{avg }is the average noise estimate; Y
_{n }is the received data signal for the n^{th }antenna; r is a data tone index functionally related to k ε{0,1, . . . , N
_{data}−1}, where
N
_{data }is the number of data tones; and n is a number of antennas at which the received data signal is received.
23. The system of the noise estimation system comprising:
a data tone identifier operative to identify the selected subset of data tones from a plurality of data tones for processing with the received data signal;
a hard decision component which computes hard decisions for the selected subset of data tones in the received data signal; and
a noise estimator operative to compute a data tone noise estimate for each data tone in the selected subset of data tones based on the respective hard decisions, the computed data tone noise estimates being provided to update the noise estimates of the aggregator.
24. The system of 25. A wireless communications system, comprising:
at least one antenna operative to receive a wireless signal and convert the received signal into a corresponding electrical signal; a preprocessing system operative to process the electronic signal and convert the electrical signal from the at least one antenna into a digital signal and perform desired preprocessing of the digital signal to provide a preprocessed digital signal in the frequency domain having a plurality of data tones and training tones for each data burst; and a decimated noise estimator operative to estimate noise for a subset of the plurality of data tones for a given data burst. 26. The system of 27. The system of 28. The system of 29. The system of 30. The system of 31. The system of 32. The system of 33. The system of 34. A receiver system, comprising:
means for receiving at least one digitized signal in the frequency domain corresponding to a data burst having a plurality of data tones; means for identifying a subset of the plurality of data tones from the data burst in the at least one digitized signal; means for estimating noise for the identified subset of the plurality of data tones; and means for providing an indication of the data tone noise estimates for the identified subset of the plurality of data tones. 35. The system of 36. The system of 37. The system of 38. The system of 39. A method for processing a digitized frequency domain signal having a plurality of data tones at different carrier frequencies, comprising:
decimating a received data burst to provide a subset of a plurality of data tones associated with the received data burst; estimating noise for each of the data tones in the subset of data tones; and providing an indication of the estimated noise for each data tone in the subset. 40. The method of 41. The method of 42. The method of 43. The method of 44. The method of 45. A digital signal processor having computer-executable instructions for performing the method of 46. An application specific integrated circuit configured to implement the method of Description [0001] The present invention relates generally to processing of communications signals and, more particularly, to a system and method to perform decimated noise estimation and/or beamforming for use in a communications system. [0002] Various techniques have been developed to facilitate communication of data signals over an associated communications path. The particular communications protocol employed generally depends on the transmission medium, the available bandwidth resources, and other design considerations. Regardless of the type of communications system being employed, noise and distortion often are introduced into data signals transmitted over an associated communications path, including both wired and wireless systems. [0003] Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. Accordingly, multiple access protocols have been developed and used in wireless communications systems to increase bandwidth efficiency. Examples of multiple carrier protocols include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Space Division Multiple Access (SDMA). A particular type of FDMA protocol is Orthogonal Frequency Division Multiplexing (OFDM). Features from these and other multiple-access techniques also have been combined in an effort to make highly efficient use of the scarce bandwidth resource to increase the quality of service. [0004] In a wireless system, in which data signals are transmitted through space as modulated electromagnetic waves, distortion and noise can result from interference with other signals within the same frequency range and path as well as multipath dispersions. Multipath dispersions occur when signals propagate along different or reflected paths through a transmission medium to a receiving destination. For example, radio transmissions or data signals that travel through the space can be reflected off trees, vehicles, houses, larger buildings, and terrain features such as mountains and hills. The radio transmissions further are subjected to fading and interference. Fading involves fluctuations in amplitude of a signal, while interference involves unwanted frequencies in the same frequency band. [0005] Multipath scattering effects alter or distort the signal spectrum when compared to the spectrum as transmitted. The effects are different at different frequencies across the signaling band. At some frequencies, the multipath signals add constructively to result in an increased signal amplitude, while at other frequencies the multipath signals add destructively (out of phase) to cancel or partially cancel the signal, resulting in reduced signal amplitude. [0006] A wireless communication system is designed to compensate for interference due to noise and the deleterious effects of multipath dispersion. However, in order to compensate for the noise, an indication of the noise in the signal must first be determined. Thus, it is desirable to know how much noise and/or interference is present in the system. Generally, noise can be measured relative to the signal as a ratio, such as a signal-to-noise ratio (SNR) and a signal-to-interference-to-noise ratio (SINR). The noise/interference levels are monitored and used to facilitate demodulation of the incoming signal, such as by assigning a confidence level to the data tones of the received signal. Accordingly, it is desirable to efficiently and accurately estimate the noise/interference in a received signal. [0007] The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. [0008] One aspect of the present invention provides a noise estimation system for a communications system capable of improving system performance. The noise estimation system includes a selector that selects a subset of data tones (or carrier tone frequencies) from a given data burst. Noise estimates are computed for the data tones in the selected subset. The noise estimates can be used to update previously computed noise estimates. By controlling how the data tones in each subset are selected, such as based on a burst counter that monitors the received data bursts or other criteria, one can reduce the number of calculations required for each data burst. [0009] For example, the noise estimates can be performed at every other, every fourth, or every n [0010] In a particular aspect of the present invention, the noise estimates for the data tones can be employed in beamforming computations. Because the noise estimates have been decimated in accordance with the present invention, the overhead associated with signal processing requirements for beamforming also are reduced, thereby improving the level of system performance. [0011] The following description and the annexed drawings set forth in certain illustrative aspects of the invention. These aspects are indicative, however, of but a few ways in which the principles of the invention may be employed. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. [0012]FIG. 1 is a functional block diagram of a receiver that includes a noise estimator in accordance with an aspect of the present invention. [0013]FIG. 2 is an example of a data burst showing different types of tones in a transmission signal. [0014]FIG. 3 is an example of a data burst, similar to that of FIG. 2, in which the order of the tones has been modified. [0015]FIG. 4 is a functional block diagram of a receiver that includes a noise estimator and beamformer in accordance with an aspect of the present invention. [0016]FIG. 5 is a functional block diagram illustrating an example of noise estimation and beamforming in accordance with an aspect of the present invention. [0017]FIG. 6 illustrates an example of a wireless communications system that can employ a receiver in accordance with an aspect of the present invention. [0018]FIG. 7 is a flow diagram illustrating a methodology for estimating noise in accordance with an aspect of the present invention. [0019]FIG. 8 is a flow diagram illustrating a methodology for beamforming in accordance with an aspect of the present invention. [0020] The present invention provides a system and method to estimate noise in a received signal, such as received over a wired (e.g., electrically conducting or optical) or wireless medium. For purposes of brevity, the following description is described with respect to a wireless signal. The received signal can be partitioned into bursts, which contain a specified number of data tones. Noise is estimated for a selected subset of data tones of the received signal by decimating the received signal, such as to compute a partial noise estimate. The results of the computed partial noise estimate, for example, can be combined with one or more noise estimates for other frequency portions of the received signal to provide an aggregated indication of estimated noise for the received signal. In this way, the noise estimation process can be distributed over g data bursts, where g is a natural number, so that noise estimation for all frequencies in the received signal or burst will be updated every g data bursts. Accordingly, a larger value of g results in partial noise estimates being performed for a smaller subset of the frequencies associated with each data burst, which can significantly reduce the number of system computations and improve system performance. [0021]FIG. 1 illustrates a schematic block diagram of a wireless receiver system [0022] The analog front end [0023] The A/ID converter [0024] The digital preprocessor [0025] The digital preprocessor [0026] The particular implementation of the digital preprocessor [0027] By way of example, the data signal or burst can be transmitted in a multiple carrier (or multiple access) format having a plurality of carrier frequencies or tones, such as shown in the examples of FIGS. 2 and 3. It is to be understood and appreciated that any arrangement and modulation for the tones can be implemented in accordance with an aspect of the present invention. A multiple carrier format allows multiple users to reuse the same communications resources while maintaining the identity of each user. [0028] By way of further example, one special type of multiple carrier protocol is referred to as Orthogonal Frequency Division Multiplexing (OFDM). In general, OFDM is a block-oriented modulation scheme that maps a number of data symbols into a number of orthogonal carriers separated by a distance 1/T, where T is a block period. OFDM is a technique by which data is transmitted at a high rate by modulating several low bit rate carriers in parallel rather than one single high bit rate carrier. OFDM is particularly useful in the context of Digital Audio Broadcasting, High Definition Television (HDTV), and Asymmetric Digital Subscriber Lines (ADSL), all of which can benefit from the present invention. OFDM can also be useful in satellite television systems, cable television, video on demand, interactive services, mobile communication devices, voice services and Internet services. [0029] An advantage of the OFDM is that intersymbol interference due to signal dispersion in the transmission channel can be reduced by inserting a guard time interval, known as the cyclic prefix, between the transmission of a subsequent symbol. This helps remove intersymbol interference, thereby alleviating the need for equalizers as required by most single carriers. The guard time allows delayed copies (e.g., reflections) of the main symbol to die out before the receiver receives the succeeding symbol. Due to this property, OFDM has gained popularity in high data rate systems including wireless broadband applications. [0030] In transmission of a data signal, an OFDM modulator converts a serial data stream into a block of N complex carriers, which can include zero tones, training tones and data tones. The sum of the individual carriers, of which amplitude and phase can be modulated, correspond to a time domain waveform that can be generated using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) is a well known efficient implementation of the IDFT that performs an N-point IDFT transform, where N is the number of tones. The guard time is inserted into the output of the IDFT and filled in with a copy of the symbol (called a cyclic prefix) to preserve the orthogonality between carriers. At the receiver end, a data signal or data burst is received in the time domain and converted back into the frequency domain employing an FFT for extraction of the symbols from the data burst. The frequency domain signal is comprised of a plurality of data tones, training tones and zero tones. The training tones are transmitted at known magnitude and phase and employed in determining the channel estimate for use in compensating the data tones due to the effects of the channel on the tones. [0031] In an OFDM based communications system, data bursts divide the OFDM carriers into a plurality of N tones or carrier frequencies. FIG. 2 illustrates an example of a frequency-domain representation of a Quadrature Amplitude Modulation (QAM) data burst [0032] The scrambled indexing of the data tones represents the frequency interleaving obtained through bit reversal. The input that is provided to the digital preprocessor [0033] Referring back to FIG. 1, the digital preprocessor [0034] For example, it may be desirable to perform noise estimates for
[0035] data tones in each data burst, where g is a natural number that defines the decimation level. That is, a higher value of g results in increased level of decimation. The decimation function [0036] data tones for each count value between 0 and g−1, where N [0037] data tones on which noise estimates are to be performed for the current data burst. [0038] Alternatively, the decimation function [0039] While the foregoing example determines to estimate noise for a different subset of data tones based on receiving a data burst, it is to be understood and appreciated that the noise estimator can be programmed and/or configured to decimate noise estimation computations in response to criteria different from a data burst in accordance with an aspect of the present invention. For example, it might be desirable to compute and update noise estimates for some or all data tones after receiving a certain number of data bursts or based on a count cycle that increments more slowly than every data burst. Additionally or alternatively, the decimation level can be variable so as to change as a function of the detected noise level, such as employing a higher decimation level when a relatively low level of noise is present and lower decimation level when a relatively high level of noise exists. Those skilled in the art will further understand and appreciate that whatever basis is used to control decimation at the noise estimator, the decrease in noise computations should be balanced with maintaining an acceptable level of system performance. [0040]FIG. 4 illustrates an example of another wireless receiver system [0041] The front end [0042] The A/D converters [0043] By way of example, the channel estimator of the preprocessor [0044] The hard decision component [0045] The decimation function [0046] The noise estimator [0047] The noise estimation also can include a variance and correlation computation performed on the initial noise estimates for each of the antenna signals to provide a covariance matrix indicative of the noise estimates for each of the selected data tones in the respective received signals. Averaging also can be employed to smooth the respective noise estimates [0048] The noise estimator [0049] By way of example, the beamformer [0050] Because noise estimates are computed for data tones according to the decimation rate, in accordance with an aspect of the present invention, the overall number of computations required to obtain values needed to perform beamforming computations is significantly reduced. The additional overhead associated with controlling the decimation is less than that required to compute a corresponding noise estimate for each data tone in every data burst. Those skilled in the art will understand and appreciate that the computational efficiency can be increased as a function of the decimation rate, but that this should be balanced with maintaining an acceptable level of system performance. Further, it may be desirable to employ feedback from the noise estimator to the decimation function to adjust the decimation level according to the amount of noise computed. [0051] The beamformer [0052] Additional aspects of the present invention will be better appreciated with respect to FIG. 5, which depicts a functional representation for part of a receiver system [0053] By way of example, the channel estimator [0054] The intermediate channel responses B [0055] A training tone index [0056] By way of further illustration, Eq. 1 represents a computation that can be performed by the noise estimator [0057] where: [0058] B corresponds to the intermediate channel responses; [0059] b is an index for the current data burst; [0060] k is an index for each of the training tones, e.g., k ε{0,1, . . . , ν−1}; [0061] s is a slip value for identifying slip characteristics (e.g., s=0 indicates no [0062] slip; s=−1 indicates forward slip; and s=1 indicates backward slip); and [0063] r is the bit reversed value of the training tone index k. [0064] The training tone noise estimator [0065] By way of illustration, the covariance of the training tone noise can be computed from the noise vectors W [0066] where k ε{0,1, . . . , ν−1} [0067] Because of symmetry, only four real elements are actually computed; namely, the two diagonal elements and the real and imaginary parts of either off-diagonal element. [0068] The covariance function [0069] Following the example of using a single pole filter for the averager [0070] where [0071] k ε{0,1, . . . , ν−1}; [0072] b is the index for the current data burst, and [0073] β=2 [0074] Once the training tone noise estimates have been computed, they can be employed by a beamformer or other subsequent processing that might require an estimation of the training tone signal noise. [0075] In the example of FIG. 5, the averager [0076] By way of example, the hard decisions Z [0077] where: [0078] k ε{desired data tones identified by the selector [0079] r is the bit reversed index for data tone k [0080] l is the linear index for data tone k; and
[0081] In accordance with an aspect of the present invention the desired data tones can be any subset of the data tones, such that hard decisions are computed for different subsets of data tones over a plurality of data bursts. The hard decisions for the desired subset of data tones can be stored in memory operatively associated with the hard decision estimator (e.g., RAM, SDRAM, data registers, etc.). [0082] By way of example, the data tone selector [0083] For example, if there are g subsets, then each data burst can result in the hard decision estimator [0084] data tones (where g is a natural number), which are shifted into the hard decision estimator [0085] In accordance with an aspect of the present invention, the data tone noise estimator [0086] By way of illustration, the data tone noise estimator [0087] where: [0088] k ε{desired data tones identified by the selector [0089] r is the bit reversed index for data tone k [0090] The data tone noise estimator [0091] where: [0092] k ε{desired data tones identified by the selector [0093] r is the bit reversed index for data tone k [0094] After the noise on the desired data tones has been computed, the processes associated with Eqs. 4-6 can be repeated for desired zero tones. However, hard decisions are not required for the zero tones because no data is being transmitted at these particular frequencies. The covariance function [0095] The averager [0096] The operation performed by the averager [0097] where: [0098] k ε{desired data tones identified by the selector [0099] [0100] r is the bit reversed index for data tone k [0101] n is the bit reversed index for data tone m [0102] N [0103] β=└N [0104] Eq. 7 thus can provide a sliding window averager that is updated for every data tone, although it does not need to be updated for every data tone. It is to be understood and appreciated that the averager [0105] The averager [0106] For example, it may be desirable to perform noise estimates for
[0107] data tones in each data burst, where g is a natural number that defines the decimation level. That is, a higher value of g results in increased level of decimation (e.g., smaller subsets of data tones). The data tone selector [0108] data tones selected for each count value between 0 and g−1, where N [0109] The updated average data tone noise estimates of the register [0110] By way of further illustration, the soft decisions and NSR estimates {circumflex over (Z)} and {overscore (q)} can be computed for all data tones according to the following equations:
[0111] where [0112] k ε{0,1, . . . , N [0113] r is the bit reversed value of the data tone index k. [0114] Because the denominator in Eqs. 8 and 9 contains the average noise estimates, whose values are decimated, it is to be appreciated that the denominator calculations for estimates {circumflex over (Z)} and/or {overscore (q)} (e.g., {circumflex over (Z)} [0115] By way of example, {circumflex over (Z)} [0116] In view of the foregoing, those skilled in the art will understand and appreciate that decimation, in accordance with an aspect of the present invention, enables reduced computations and processing steps, as only a portion of the data being decimated needs to be updated with each data burst. It is to be also understood and appreciated that equations [0117] where n denotes the number of antennas. [0118]FIG. 6 illustrates one example of a possible implementation of the present invention as it applies to a wireless communications system [0119] For each data burst from the antenna system [0120] The transceiver [0121] The modem [0122] When the modem receives wireless signals from more than one antenna, such as in the example of FIG. 6, noise estimates are performed on signals from each of the antennas. Further, the signal processor [0123] The modem [0124] The computer device [0125] In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the present invention will be better appreciated with reference to FIGS. [0126]FIG. 7 illustrates an example of a methodology to perform noise estimation on a signal in accordance with an aspect of the present invention. The methodology can be implemented as hardware and/or software, such as by a DSP or an ASIC. The methodology begins at [0127] Next, at [0128] data tones in each data burst can be updated, thereby reducing the number of computations performed on each received signal as a function of the value of g. It is to be understood and appreciated that the value of g could be programmable and/or vary as a function of the estimated noise level so as to provide a dynamic system that can be optimized according to its environment. Additionally, the burst counter can be implemented in other forms. [0129] From [0130] Next, at [0131] At [0132] As a result, noise estimates will be updated for a selected subset of the data tones each iteration through the methodology for a given data burst. Because noise estimates are not required to be performed for all data tones in each data burst, computational efficiency can be increased so as to improve overall system performance. It further is to be appreciated that additional computational savings can be afforded by decreasing the number of data tones in each selected subset, although the benefits should be balanced with overall system performance. [0133]FIG. 8 illustrates a beamforming methodology that can be implemented relative to a data signal or burst in accordance with an aspect of the present invention. The methodology begins at [0134] Next, at [0135] At [0136] From [0137] The hard decisions are employed to compute noise estimates for the selected data tones at [0138] At [0139] Next, at [0140] Beamforming computations are performed ( [0141] At [0142] At [0143] The burst counter can implement a cyclic count, which resets after reaching its maximum count value. In this way, a different subset can be associated with each count value so as to provide a desired decimation of the received signal for each data burst as a function of the count value. [0144] By way of further illustration, the burst counter can be programmed and/or configured to count from zero to g−1, where g is a natural number that defines the decimation level. The value of g further can be adjustable, such that a desired level or decimation can be provided. The burst counter thus enables noise estimates to be updated at [0145] What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Referenced by
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