Patent Application
20090168930
This invention relates to methods for channel estimation in data communications, and, in particular to, methods for Doppler shift estimation and adaptive channel filtering in a data communication system.
Orthogonal frequency division multiplexing is a multi-carrier transmission technique that uses orthogonal subcarriers to transmit information within an available spectrum. Since the subcarriers may be orthogonal to one another, they may be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (FDM) system.
In an OFDM system, the subcarriers may be modulated with a low-rate data stream before transmission. It is advantageous to transmit a number of low-rate data streams in parallel instead of a single high-rate stream since low symbol rate schemes suffer less from intersymbol interference (ISI) caused by multipath propagation of the transmitted streams. For this reason, many modem digital communications systems are turning to the OFDM system as a modulation scheme for signals that need to survive in environments having multipath or strong interference. Many transmission standards have already adopted the OFDM system, including the IEEE 802.11a standard, the Digital Video Broadcasting—Handheld (DVB-H), the Digital Video Broadcasting Terrestrial (DVB-T), the Digital Audio Broadcast (DAB), and the Digital Television Broadcast (T-DMB).
Although the OFDM system is advantageous in combating intersymbol interference, it is quite sensitive to frequency deviations. The frequency deviations may be caused by the difference in the oscillator frequency of the receiver and the transmitter, or by the Doppler shift of the signal due to the movement of either the receiver or the transmitter. Frequency deviations cause significant interference between signals at different subcarriers, hence result in dramatic performance degradation. Therefore, channel estimation to correct the frequency deviations is critical for delivering good transmission quality.
Therefore, it is desirable to provide methods for estimating frequency deviations for a transmitted signal caused by a Doppler shift.
An object of this invention is to provide methods for a power spectrum based Doppler estimation in a data communication system that can correctly estimate Doppler shifts to within 20 Hz at more than 95 percent probability.
Another object of this invention is to provide methods for channel estimation in a data communication system, where Doppler estimation and an adaptive channel filter in the time domain are used to improve performance.
Yet another object of this invention is to provide methods for Doppler estimation in a data communication system that is not sensitive to phase noise.
Briefly, a method for Doppler shift estimation for channel estimation of a received signal, comprising the steps of: calculating time domain correlations; providing a Hamming window over the calculated time domain correlations; calculating a power spectrum by using FFT; calculating an adaptive threshold based on a noise floor and an average power density calculated from the power spectrum; and estimating a Doppler shift based on the adaptive threshold.
An advantage of this invention is that Doppler shifts in a data communication system can be correctly estimated to within 20 Hz at more than 95 percent probability.
Another advantage of this invention is that performance is improved for channel estimation in a data communication system by using Doppler estimation and an adaptive channel filter in the time domain.
Yet another advantage of this invention is that methods for Doppler estimation in a data communication system that are not sensitive to phase noise are provided.
The foregoing and other objects, aspects, and advantages of the invention will be better understood from the following detailed description of the preferred embodiment of the invention when taken in conjunction with the accompanying drawings in which:
a-2b illustrate a block diagram of an embodiment of the present invention for Doppler estimation and adaptive channel filtering.
a-4c illustrate a flow chart of an embodiment of the present invention for an adaptive threshold based Doppler estimation.
The present invention will be described using a DVB-H system. However, it will be appreciated by one skilled in the art that the present invention can be applied to other communication systems.
Due to the motion of a receiver or a transmitter, the frequency response of a DVB-H channel is a two-dimensional random process that can be characterized by a correlation function, H(f, t). The correlation function of the frequency response at different times and at different frequencies, rH[k, m], can be expressed as the product of a time domain correlation, rt[m], and a frequency domain correlation, rf[m], given in Equation (1) and Equation (2).
where k is the carrier index, m is the symbol index, Δf is the subcarrier space, and T is the symbol time.
It is well known that the time domain correlation and the frequency domain correlation are related to a Doppler spread, fDmax, and a time delay spread, τmax, respectively, in the following manner
rt[m]∝fDmax (3)
rf[m]∝τmax (4)
For example, in the typical urban 6-paths (TU6) channel model, the time domain correlation function and the frequency domain correlation function are zero-order Bessel functions, J0(.).
In order to support estimating the Doppler value in RICE and AWGN spectrum channel models, it is not enough to consider only the time domain correlation. Thus, spectrum analysis is necessary for Doppler estimation in various spectrum channels.
By transferring the time domain correlation function to the frequency domain by using a FFT, the channel power spectrum can be generated. Based on the power spectrum analysis, the Doppler bandwidth can be achieved. In various channel models including DVB-TU6, DVB-RA6, DAB-RA4, DAB-TU6 and self-defined 2-ray RICE channel model, a common feature is a sharp slope at the edge of the Doppler bandwidth (i.e. 10-20 dB higher than a noise floor). In a RICE channel model, the channel power spectrum can be shifted according to the frequency offset adjustment in a DVB-H receiver, such that the power spectrum may not be symmetrical, as would be in a DVB-TU6 channel model. Furthermore, if more than one Ray RICE channel has occurred, not only would the power spectrum be shifted, but the spectrum power at the frequency center for the different RICE channels may also have large variations due to channel fading.
According to these features, a novel power spectrum based on a Doppler estimation algorithm is proposed. By considering the implementation complexity and diversity gain, a total of ten equally spaced continual pilots for located sub-carriers are used for time domain correlation calculations. If a selected carrier is erased due to co-channel interference, then that carrier will not be used. Ten first-in-first-out (FIFO) buffers with a length of a 150 for each buffer are used for generating a correlation value. By considering the minimum 10 Hz or 20 Hz resolution Doppler bin in the 8K mode, 50 correlation values can be enough for Doppler estimation.
In order to smooth the noise floor in the high frequency range of the power spectrum, a Hamming window can be applied before a FFT is performed on the received signal. An adaptive threshold is calculated based on the noise floor and the average power density within a previously estimated Doppler bandwidth. Based on the adaptive threshold, the edge of the Doppler bandwidth can be clarified and estimated.
An adaptive threshold is then selected based on a noise floor and an average power density based on the FFT of the received signal within the previous Doppler estimation (18), where the initial Doppler value can be a pre-defined number. Finally, the Doppler estimation based on the adaptive threshold is performed (20).
a-2b illustrate a block diagram of an embodiment of the present invention for Doppler estimation and adaptive channel filtering. A FFT is applied to a received signal (30). Next, ten equally spaced continual pilots for located sub-carriers can be selected for time domain correlation calculations (32). If a selected carrier is erased due to co-channel interference, then that carrier will not be used. Ten FIFO buffers are used for generating a correlation value (10). By considering the minimum 10 Hz or 20 Hz resolution Doppler bin in 8K mode, 50 correlation values can be enough for Doppler estimation (12). In order to smooth the noise floor in the high frequency range of the power spectrum, a Hamming window is applied (14) before the FFT is performed (16). An adaptive threshold is calculated based on the noise floor and the average power density within the previously estimated Doppler bandwidth (18). Based on the adaptive threshold, the edge of Doppler bandwidth can be clarified (20).
The Doppler estimation can then be performed using the adaptive threshold (20). In particular, the adaptive threshold is used for a right spectrum Doppler estimation and a left spectrum Doppler estimation. The maximum of which is outputted, Fd_est.
a-4c illustrate a flow chart for an embodiment of the present invention for an adaptive threshold based Doppler estimation. In the first step, a FFT is applied to 128 taps for a received signal, where the output of the FFT can be divided into a left part (negative frequency component) and a right part (positive frequency component). Since the power values of the received signal are of concern, the real values of the received signal are analyzed (50). A left part average power and a part right average power can then be calculated (52).
A noise floor can also be found for the received signal (56).
Adaptiv_th=Noise_floor*Th_factor1 (5)
The threshold factors used in
If the minimum power value is not greater than the adaptive threshold (Apaptive_th) (60), the Adaptiv_th can be halved to lower such Adaptive_th (62). If the minimum power value is greater than the Adaptive_th, the adaptive threshold is achieved (64).
If the adaptive threshold is greater than the noise floor multiplied by a second threshold factor (Th_factor2) (66), referred to as the good cases, a highest index on the left part (indices from 0 to 63), denoted index_left, where a condition that the FFT_real of the index_left is greater than the Adaptive_th is met, can be found (70). A lowest index on the right side (indices from 64 to 127), denoted index right, where the condition that the FFT_real of the index right is greater than the Adaptive_th, can also be found (72).
Next, the index_left and 128 minus the index right are compared by finding the maximum value, denoted Max_Bin (74). Max_bin corresponds to the Doppler shift. Since this is just an index, the Max_bin can be multiplied with the bin_Hz (the width of the bin) to get the Doppler shift (76). The Th_bin can be generated based on the Max_bin with some protection gap and can be fed back to calculate the left part average power and the right part average power (52). The Noise_bin can then be generated based on the noise, and be fed back to determine a noise floor (56). An infinite impulse response (IIR) filter is then applied for monitoring purposes, where 1/16 is the IIR factor (78).
If the adaptive threshold is not greater than the noise floor (noise_floor) multiplied by a second threshold factor (Th_factor2), referred to as the bad cases, and if the minimum power (min_pwr) is not greater than the noise_floor (90), the Adaptive_th is readjusted as a function of the max_pwr, the noise_floor, and a th_factor4_doppler (92) in accordance with Equation (6).
Adaptive—th=MAX(Floor(max_pwr*3/32),Floor(noise_floor*th_factor4_doppler)) (6)
If the minimum power (min_pwr) is greater than the noise_floor (90), the Adaptive_th is set to the noise_floor multiplied by Th_factor2 (94). If the maximum power is less than the noise_floor multiplied by Th_factor2 (96), the Adaptive_th is readjusted as a function of the max_pwr, the noise_floor, and a th_factor3_doppler (98), according to Equation (7); else, the Adaptive_th is achieved (100).
Adaptiv—th=MAX((int)(max_pwr*3/32),(int)(noise_floor*th_factor3_doppler) (7)
Then, similarly to the good cases, a maximum (or highest) index on the left side (indices from 0 to 63), where FFT real is greater than the Adaptive_th, is located (102). A minimum index on the right side (indices from 64 to 127), where FFT_real is greater than the Adaptive_th, is also located (104). Then, an overall max, Max_bin, the Doppler shift, and the IIR are calculated (along with Th_bin and Noise_bin) (106, 108, and 110, respectively).
While the present invention has been described with reference to certain preferred embodiments or methods, it is to be understood that the present invention is not limited to such specific embodiments or methods. Rather, it is the inventor's contention that the invention be understood and construed in its broadest meaning as reflected by the following claims. Thus, these claims are to be understood as incorporating not only the preferred methods described herein but all those other and further alterations and modifications as would be apparent to those of ordinary skilled in the art.
This application claims priority from a provisional patent application entitled “Doppler Estimation and Adaptive Channel Filtering in Time Domain” filed on Dec. 28, 2007 and having an Application No. 61/017,425. Said application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61017425 | Dec 2007 | US |
