PILOT ASSISTED CHANNEL ESTIMATION
Systems and methods are described for the implementation of a receiver that includes a channel estimation block that uses known pilots to estimate the value of channel gain and phase at data subcarrier indexes. Time interpolation as well as an auto regression filter can be to estimate the channel gain and phase at the “missing” pilot indexes as well as frequency interpolation to estimate the value of the channel at data subcarrier indexes.
This application is a continuation of U.S. patent application Ser. No. 14/088,145, filed on Nov. 22, 2013 (20400.0011.NPUS00) and titled “PILOT ASSISTED CHANNEL ESTIMATION”, of which the full disclosure of this application is incorporated herein by reference for all purposes.
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FIELD OF THE INVENTIONThis invention relates generally to the field of communications, and more specifically to pilot-aided channel estimation for orthogonal frequency division multiplexed (OFDM) transmission.
BACKGROUNDIn the age of rapid innovations in the field of telecommunications, the requirements for communication devices that enable faster, cheaper and more reliable data transfer is escalating. Orthogonal frequency-division multiplexing (OFDM) has developed as a method for reliable, high-volume data transfer in both wired and wireless mediums to transfer data and compensate for the effects of distortion at the receiver side. Wideband digital applications such as digital television, audio broadcasting, wireless networking, and broadband internet have become popular applications for OFDM transmission. When a signal travels through a transmission medium, such as a cable or air, the signal is affected and distorted due to multipath effects. This distortion is generally considered as the “channel”. Several approaches have been proposed to estimate the channel. In one such approach, cross-talk between subchannels can be eliminated by selecting subcarrier frequencies such that the subcarrier frequencies are orthogonal to each other. If the channel is accurately estimated, its effects can be compensated and the transmitted signal can be recovered more accurately. However, a solution is needed for the estimation of channel when “pilots” of pre-defined amplitude and phase are inserted into the signal at regular intervals in both time and frequency, where the pilots can be used by the receiver to estimate changes in channel response in both time and frequency dimensions.
Various embodiments in accordance with the present disclosure will be described with reference to the drawings, in which:
In the following description, various embodiments will be illustrated by way of example and not by way of limitation in the figures of the accompanying drawings. References to various embodiments in this disclosure are not necessarily to the same embodiment, and such references mean at least one. While specific implementations and other details are discussed, it is to be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without departing from the scope and spirit of the claimed subject matter.
Systems and methods in accordance with various embodiments of the present disclosure may overcome one or more of the foregoing or other deficiencies experienced in conventional approaches for wireless and/or wired communication. In particular, various embodiments describe systems and methods for pilot-aided channel estimation for orthogonal frequency division multiplexed (OFDM) communication systems, such as digital video broadcasting (DVB), audio broadcasting, and cable transmissions, among other. In general, a receiver including a channel estimation system can use known pilot subcarriers to estimate the channel (i.e., channel gain and channel phase) at data subcarrier indexes. By estimating the channel and applying an inverse function of the channel to a received OFDM transmission, an original transmitted OFDM transmission can be equalized (i.e., recovered) from an affected received OFDM transmission.
In accordance with various embodiments, a channel estimation system can utilize, for example, at least a time interpolation component as well as an auto regression filter to estimate the channel of “missing” pilot indexes as well as a frequency interpolation component to estimate the value of channel at data subcarrier indexes for an OFDM symbol. For example, at every OFDM symbol in an OFDM transmission, there can exist some indexes that carry data subcarriers, while in previous or next OFDM symbols of the OFDM transmission, the same indexes can carry pilot subcarriers. Time interpolation can be used to estimate the channel at these pilot subcarriers from pilot subcarriers of previous OFDM symbols and future OFDM symbols relative to a reference OFDM symbol and frequency interpolation can be used to estimate the value of channel at data subcarrier indexes for an OFDM symbol. In accordance with various embodiments, an OFDM symbol corresponds to a symbol rate (also known as baud or modulation rate), which is the number of symbol changes (waveform changes or signalling events) made to the transmission medium per second using a digitally modulated signal or a line code. The Symbol rate can be measured in baud (Bd) or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second. Each symbol can represent or convey one or several bits of data.
For example, the channel estimation system can receive an OFDM transmission that includes a plurality of OFDM symbols, where each of the OFDM symbols can include data subcarriers and pilot subcarriers. In this example, the plurality of OFDM symbols can be stored in one or more memory components upon being received. The number of memory components can depend on a repetition period of the pilot subcarrier patterns of the OFDM transmission. In various embodiments, the repetition period can be set by a transmitter of the OFDM transmission. For example, the transmitter can determine which one of a plurality of pilot patterns is suitable for a specific transmission and can store information indicated of the pilot pattern in a header portion of the OFDM transmission. The receiver, upon receiving the OFDM transmission, can determine the repetition period by the selected pattern. In this way, if pilot pattern number three was selected, the receiver can index an appropriate interpolation filter (as well as weights for the interpolation filter) for the channel estimation system.
In various embodiments, upon receiving the OFDM transmission at the receiver, a Fast Fourier Transmission (FFT) of the transmission is computed. As described, the OFDM transmission includes a plurality of OFDM symbols. Accordingly, a FFT of the OFDM transmission includes computing a FFT of each OFDM symbol in the transmission. Upon computing the FFT of an OFDM symbol, the FFT of the symbol is stored to one of the memory components. For example, for an OFDM transmission that includes at least a first, a second, and a third OFDM symbol, upon computing a FFT of the first OFDM symbol, the output can be stored to one of the memory component. In this example, the output of the FFT for the first OFDM symbol can be stored in the N-dth memory component, where d is the repetition period and N is the number of the current OFDM symbol received in the OFDM transmission (e.g., the tenth OFDM symbol of the OFDM transmission). The output of the FFT for the second OFDM symbol can be stored in the N-1th memory component and the output of the FFT for the first OFDM can be stored in the Nth memory component. An equalizer can use a de-multiplexer to extract data subcarriers from the output of the N-dth memory component and can use the result of a channel estimation block that includes information from pilot subcarriers of the Nth until N-dth OFDM symbol, as well as information from N-d-1th OFDM symbol until N-2dth OFDM symbol. In accordance with various embodiments, determining the result of the channel estimation block can include estimating a channel value at pilot subcarrier indexes for a current symbol “N” of the plurality of OFDM symbols, estimating a first channel value at pilot subcarrier indexes for a first grouping of OFDM symbols (i.e., nth until n-d) and a second channel value at pilot subcarrier indexes for a second grouping of OFDM symbols (n-d-1th until n-2d) based at least in part on the estimate of a channel value of the pilot subcarrier indexes for the current symbol, estimating a channel value at pilot subcarrier indexes for a reference symbol (n-d) by taking a weighted average of the first grouping of OFDM symbols and the second grouping of OFDM symbols, and estimating, using at least one frequency interpolation function, a channel value at data subcarrier indexes for the reference symbol based at least in part on the estimated channel at pilot indexes of the reference symbol.
Various other applications, processes, and uses are presented below with respect to the various embodiments.
OFDM is a method of encoding digital data on multiple carrier frequencies where a large number of closely spaced orthogonal subcarrier signals are used to carry data on several parallel data streams or channels. Each subcarrier signal is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying, etc.) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. As described, a solution is needed for channel estimation when OFDM is in use and known pilots of pre-defined amplitude and phase are inserted into the signal at regular intervals in both time and frequency directions. Accurate channel estimation in an OFDM receiver is important for the recovery of the transmitted information data at the receiver. If the receiver makes a significant error in its channel estimation, the original modulation symbol can be decoded in error because each subcarrier in the OFDM symbol is multiplied by fading coefficients that have different amplitudes and phases. Further, a solution is needed for efficient estimation of channel when different pilot spacing may be in place based at least in part on the guard interval to provide a range of efficiency and to increase capacity by increasing the pilot spacing for small channel delay spreads. Further still, the solution should consider efficient channel estimation by using pilot subcarriers from previous and next OFDM symbols with minimum memory requirements when channel is not heavily suffered from the Doppler Effect. Accordingly, in accordance with various embodiments, a channel estimation block at a receiver can use known pilots to estimate the value of channel gain and phase at data subcarrier indexes. A time interpolation component as well as an auto regression filter or other similar filter can be used to estimate the channel gain and phase of the “missing” pilot indexes and a frequency interpolation component can be used to estimate the value of the channel at data subcarrier indexes. By estimating the value of the channel at the data subcarrier indexes and applying an inverse function of the channel to a received OFDM transmission, an original transmitted OFDM transmission can be equalized from an affected received OFDM transmission.
In accordance with various embodiments, a channel estimation block at a receiver can use known pilot subcarriers to estimate the value of channel gain and phase at data subcarrier indexes. Time interpolation as well as an auto regression filter or other similar filter can be used to estimate the channel gain and phase of the “missing” pilot indexes as well as frequency interpolation to estimate the value of the channel at data subcarrier indexes. To increase capacity when channel delay spread is small, known pilot subcarriers can have different patterns in time and frequency. The pilot patterns can be repeated in time where the repetition period can depend on the channel characteristic and its variation in period with time. At every OFDM symbol, there can exist some indexes that carry data subcarriers, while at the previous or next OFDM symbols, the same indexes can carry pilot subcarriers. The channel gain and phase at these pilot indexes can be estimated using pilot values from previous and future/next OFDM symbol by means of time interpolation. The channel in the remaining, non-pilot subcarriers can be estimated based on the estimated channel in the pilot subcarriers. Once the channel is estimated, channel equalization can be performed to compensate the data signals for the channel to recover the original transmitted data.
For example,
As shown in example 300 of
An input OFDM symbol is received at a FFT unit 324. The FFT unit computes the Fourier Transform of the input OFDM symbol and the output of the FFT unit is provided to the series of memory components, e.g., FFT output memory components (306, 308, 310). In accordance with various embodiments, the number of memory components, d, can depend on a repetition period of the pilot patterns in time. The N-dth previous OFDM symbol is stored in the last memory component (i.e., the n-dth memory unit). The equalizer can use a de-multiplexer to extract data subcarriers from the output of the last memory component and can use the result of the channel estimation block that had information from pilots of the Nth until N-dth OFDM symbol, as well as information from N-d-1th until N-2dth OFDM symbol.
For example, de-multiplexer 326 can extract data subcarriers from the output of memory component 310 and the data subcarriers can be provided to the equalizer. De-multiplexer 328 can extract pilot subcarriers from the output of N until N-dth memory components. The equalizer can use the result of the channel estimation block (e.g., components 312-320) that had information from pilot subcarriers of the Nth until N-dth OFDM symbol, as well as data subcarriers from N-d-1th until N-2dth OFDM symbol to estimate the channel. The equalized data symbol can be provided to the forward error correction (FEC) 330 or channel coding component to decode the input data and control errors in data transmission over unreliable or noisy communication channels. Accordingly, embodiments provide a solution to perform interpolation in time by considering pilot subcarrier information from the previous d OFDM symbols as well as the next or future d OFDM symbols. The equalized data symbol enters the FEC or channel coding component to decode the input data and control errors in data transmission over unreliable or noisy communication channels.
As described, the grouping of previous OFDM symbols and the grouping of next/future OFDM symbols can be used to interpolate the value of channel estimates for pilot subcarriers over time to determine the value of channel for data subcarriers.
In accordance with various embodiments, as OFDM symbols are processed, the channel estimation system can update the channel value of the pilot subcarriers. As described, channel estimation can be performed to measure the channel in an OFDM transmission. The channel in an OFDM transmission can be estimated by estimating the channel in the subcarriers. In various embodiments, the channel in subcarriers in an OFDM transmission can be estimated based at least in part on pilot subcarriers. The channel in pilot subcarriers can be estimated at the receiver based at least in part on a measured subcarrier value and an expected, known subcarrier value. For example, the channel in the pilot subcarriers can be based on expected values, for example, by dividing the measured value of the pilot subcarriers by the expected value of the pilot subcarrier. The channel in the remaining, non-pilot subcarriers can be estimated using methods of interpolation based on the estimated channel in pilot subcarriers. Accordingly, the channel can be estimated in every subcarrier of every OFDM symbol. Once the channel is estimated, channel equalization can be performed to compensate the data subcarriers for the channel to recover the original transmitted data.
In this example, a pilot sign correction component (e.g., component 312 of
As described, the previous OFDM symbols and the next/future OFDM symbols can be used to interpolate the value of channel estimates on pilots over time to determine the value of channel of data subcarriers. Upon updating the channel block for previous OFDM symbols and updating the channel block for future/next OFDM symbols, a weighted average of the channel block can be taken. For example,
The channel value can be determined 808 at pilot subcarrier indexes for a reference OFDM symbol (i.e., the n-dth received OFDM symbol) by computing a weighted average of the channel value of the pilot subcarrier indexes for the first grouping of OFDM symbols and the channel value of the pilot subcarrier indexes for the second grouping of OFDM symbols. In various embodiments, the reference OFDM symbol is received prior to the current OFDM symbol, the reference OFDM symbol being received an amount corresponding to the predetermined number of OFDM symbols. In various embodiments, the predetermined number of OFDM symbols (i.e., “d”) can correspond to a repetition period. As described, the number of memory components can depend on a repetition period of the pilot subcarrier patterns of the OFDM transmission. In various embodiments, the repetition period can be set by a transmitter of the OFDM transmission. For example, the transmitter can determine which one of a plurality of pilot patterns is suitable for a specific transmission and can store information indicated of the pilot pattern in a header portion of the OFDM transmission.
The channel value for the pilot subcarrier indexes of the reference symbol is time filtered 810. Using at least one frequency interpolation function, the channel value at data subcarrier indexes for the reference symbol can be estimated 812 based at least in part on the channel value for the pilot subcarrier indexes of the reference symbol. Thereafter, at least a portion of the data subcarriers in the reference OFDM symbol is divided 814 by the channel value of the data subcarrier indexes to equalize 816 the OFDM transmission and reduce a portion of channel distortion.
Various embodiments discussed or suggested herein may be conveniently implemented using a conventional general purpose or a specialized digital computer or microprocessor programmed according to the teachings of the present disclosure. These devices also can include other electronic devices, such as dummy terminals, thin-clients, gaming systems, and other devices capable of communicating via a network. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art.
Such devices also can include a computer-readable storage media having instructions stored thereon/in which can be used to program a computer to perform any of the processes of the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disks, optical discs, DVD, CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, DRAMs, VRAMs, flash memory devices, magnetic or optical cards, nanosystems (including molecular memory ICs), or any type of media or device suitable for storing instructions and/or data. The computer-readable storage media can be connected with, or configured to receive, a computer-readable storage medium, representing remote, local, fixed, and/or removable storage devices as well as storage media for temporarily and/or more permanently containing, storing, transmitting, and retrieving computer-readable information. The system and various devices also typically will include a number of software applications, modules, services, or other elements located within at least one working memory device. It should be appreciated that alternate embodiments may have numerous variations from that described above. For example, customized hardware might also be used and/or particular elements might be implemented in hardware, software (including portable software, such as applets), or both. Further, connection to other computing devices such as network input/output devices may be employed. Based on the disclosure and teachings provided herein, a person of ordinary skill in the art will appreciate other ways and/or methods to implement the various embodiments.
The foregoing description of embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to one of ordinary skill in the relevant arts. For example, steps preformed in the embodiments of the invention disclosed can be performed in alternate orders, certain steps can be omitted, and additional steps can be added. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular used contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents.
Claims
1. A method, comprising:
- receiving an Orthogonal Frequency Division Multiplexing (OFDM) transmission that includes a plurality of OFDM symbols, each one of the plurality of OFDM symbols including data subcarriers and pilot subcarriers;
- determining a channel value at pilot subcarrier indexes for a current OFDM symbol of the plurality of OFDM symbols;
- updating, based at least in part on the channel value at the pilot subcarrier indexes of the current OFDM symbol, the channel value of pilot subcarriers indexes for a first grouping of OFDM symbols of the plurality of OFDM symbols and a second grouping of OFDM symbols of the plurality of OFDM symbols;
- determining a channel value at pilot subcarrier indexes for a reference OFDM symbol by computing a weighted average of the channel value of the pilot subcarrier indexes for the first grouping of OFDM symbols and the channel value of the pilot subcarrier indexes for the second grouping of OFDM symbols;
- time filtering the channel value for the pilot subcarrier indexes of the reference symbol;
- estimating, using at least one frequency interpolation function, the channel value at data subcarrier indexes for the reference symbol based at least in part on the channel value for the pilot subcarrier indexes of the reference symbol; and
- dividing at least a portion of the data subcarriers in the reference OFDM symbol by the channel value of the data subcarrier indexes to equalize the OFDM transmission and reduce a portion of channel distortion.
Type: Application
Filed: Jan 26, 2015
Publication Date: Sep 24, 2015
Inventors: Marzieh Veyseh (Los Altos, CA), Mahdi Khoshgard (Los Gatos, CA)
Application Number: 14/605,903