Abstract:
A backward compatible enhancement to ATSC-VSB transmissions is provided to enable intra-field channel estimation for better channel equalization. One or more time-frequency waveforms are inserted into the transmitted signal with systematic time and frequency variation synchronized to the currently available field sync. A coherent demodulator may then directly estimate the channel frequency response. The power of the inserted waveform(s) is small compared to the existing 8-VSB signal, such that existing receivers will function as intended with only minimal degradation of performance.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to channel equalization for wireless communications and, more specifically, to providing a time-frequency waveform for frequency domain channel estimation in a manner which allows existing receivers to continue operating as intended. 
     BACKGROUND OF THE INVENTION 
     Within Advanced Television Standards Committee (ATSC) compliant video receivers, trained channel estimation is performed utilizing the field sync available in the ATSC field. However, one trained equalization per field is not sufficiently frequent to estimate even slowly varying channels. Blind equalization techniques applied to intra-field channel estimation have not been entirely successful on all important channels. Other proposals to improve the trained equalization update rate require a new training signal and are therefore not backward compatible with existing systems, and also improve channel estimation at the cost of useful data rate. 
     One alternative to existing equalization schemes which might improve channel estimation without degrading the data rate is frequency domain equalization. However, current proposals for frequency domain equalization for monocarrier systems such as vestigial sideband (VSB) modulation are essentially time-domain equalization performed in the frequency domain. Coefficients, updates, and finite impulse response (FIR) tap calculations are performed in the frequency domain with the assistance of a fast Fourier transform (FFT) of the incoming data. Since the training sequence is defined only in the time domain, the error calculation must be performed on the filtered data after retransformation to the time domain. The error is then retransformed to the frequency domain for coefficient update. Similar restrictions apply to statistical (blind) and decision directed error calculations. 
     There is, therefore, a need in the art for improving the training signal available for channel estimation. It would be desirable to provide an enhancement to the training signal without degrading data rate and allowing existing receivers to continue to operate as intended. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a wireless communications system, a backward compatible enhancement to ATSC-VSB transmissions enabling intra-field channel estimation for better channel equalization. One or more time-frequency waveforms are inserted into the transmitted signal with systematic time and frequency variation synchronized to the currently available field sync. A coherent demodulator may then directly estimate the channel frequency response. The power of the inserted waveform(s) is small compared to the existing 8-VSB signal, such that existing receivers will function as intended with only minimal degradation of performance. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
         FIG. 1  depicts a wireless communications system according to one embodiment of the present invention; 
         FIG. 2  depicts in greater detail an enhanced transmitter producing a signal including a time-frequency waveform for wireless communications according to one embodiment of the present invention; 
         FIG. 3  depicts in greater detail an enhanced receiver utilizing a signal including a time-frequency waveform for channel estimation relating to wireless communications according to one embodiment of the present invention; 
         FIGS. 4A and 4B  are plots of simulation results for actual channel characteristics and estimated channel characteristics according to one embodiment of the present invention; and 
         FIG. 5  is a plot of the spectral power density of the time-frequency waveform inserted into the monocarrier signal according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device. 
       FIG. 1  depicts a wireless communications system according to one embodiment of the present invention. Wireless communications system  100  includes a transmitter  101  providing enhanced signal transmission in accordance with the present invention, with the transmitted signal including a time-frequency waveform as described in further detail below. Wireless communications system  100  also includes a receiver  102 , which may be one of a plurality of such receivers, receiving the signals transmitted by transmitter  101  and capable of demodulating the enhanced signal, including the ability to employ the time-frequency waveform for channel estimation in accordance with the present invention. Wireless communications system  100  may optionally include one or more other “legacy” receivers  103  receiving the signals transmitted by transmitter  101  and, although incapable of employing the time-frequency waveform, able to demodulate the received signal. 
     In the present invention, transmitter  101  and receivers  102  and  103  are preferably video (including associated audio) receivers operating according to the Advanced Television Standards Committee (ATSC) standard for vestigial sideband modulation with eight discrete levels (8-VSB). Alternatively, however, the present invention may be employed within other wireless communications systems including voice (telephony) systems, data (e.g., Internet access) systems, or hybrid video/voice, video/data, voice/data, or video/voice/data systems. 
     True frequency domain equalization is achieved in the present invention through the addition of pilot tones in the transmitted signal. For VSB, the additional pilot tone(s) supplement the tone (field sync) applied to assist carrier recovery. To avoid compromising the carrier recovery function of the original pilot tone while concurrently providing a better measurement of the channel characteristics, the supplemental pilot tone is varied, preferably so that the resulting spectrum of the supplemental pilot tone is flat when measured over several frequency domain transform symbols. The identical frequency distribution is systematically produced and/or replicated in both the transmitter  101  and the receiver  102  using, for example, a linear shift register pseudo-random number sequence to control the frequency position of the supplemental tone. Accordingly, a frequency sweep of the channel is achieved. 
       FIG. 2  depicts in greater detail an enhanced transmitter producing a signal including a time-frequency waveform for wireless communications according to one embodiment of the present invention, and is intended to be read in conjunction with  FIG. 1 . Transmitter  101  includes an 8-VSB baseband signal source  200  producing symbols for the data or information to be transmitted in accordance with the known art, as well as an 8-VSB modulator  201  encoding symbols by modulation of a carrier signal, also in accordance with the known art. 
     In the present invention, transmitter  101  additionally includes a time-frequency waveform generator  202 , the output of which is combined with the output of signal source  200  by signal adder  203 . The 8-VSB transmission signal  204  output by the modulator  201  thus includes modulation of the time-frequency waveform produced by waveform generator  202  as well as the symbols produced by signal source  200 . 
     The time-frequency waveform produced by generator  202  is preferably a set of stepped sinusoids, each of approximately 1024 samples in length with a magnitude 30 decibels (dB) below the baseband signal. The frequency of the waveform is stepped up by a number derived from the bandwidth (e.g., 6 MHz) and the per sweep resolution desired (e.g., 6 MHz/128). To achieve better per-sweep resolution, more than one sinusoid is added at different frequencies per time segment. To achieve better intra-sweep resolution, the frequency position of the sinusoids is continually varied from one time segment to another. In the exemplary embodiment, the time-frequency waveforms produced by generator  202  are frequency-hopping sinusoids with a magnitude 30 dB below the data signal, where the frequency hops every 1024 samples (approximately 100 μs) and sweeps the entire band in 128 segments. The frequency hops follow a predetermined sequence synchronized with the field sync signal. 
       FIG. 3  depicts in greater detail an enhanced receiver utilizing a signal including a time-frequency waveform for channel estimation relating to wireless communications according to one embodiment of the present invention, and is intended to be read in conjunction with  FIGS. 1 and 2 . Enhanced receiver  102  includes a conventional 8-VSB receiver front end  300  performing such functions as carrier recovery and field sync detection. Enhanced receiver  102  receives and 8-VSB received signal  301 , which is the 8-VSB transmission signal  204  after transmission within, and alteration by, the wireless channel between transmitter  101  and receiver  102 . 
     Enhanced receiver  102  also includes a conventional 8-VSB channel equalizer  302  performing channel equalization signal correction or compensation on the output of front end  300  based upon a received channel estimate, and a conventional 8-VSB receiver back end  303  performing such functions as symbol decoding on the channel equalized output of equalizer  302 . 
     Receiver  102  also includes a time-frequency waveform generator  304  producing an identical waveform to that produced by generator  202  within transmitter  101 . Generation of the predefined time-frequency waveform is triggered by detection of a field sync signal. The initial frequency of the sinusoids is already known by design or, alternatively, a cyclic sequence of frequencies may be employed with the initial frequency for a particular field communicated in the reserved bits of the field sync signal. 
     The time-frequency waveform produced by generator  304  is employed, together with data samples from front end  300 , by coherent demodulator  305  to produce a channel estimate. A set of discrete time oscillators, one for each frequency, are initialized and the input data samples are multiplied and integrated over the period of the time segment. That is, a local sinusoid generator is employed to generate the time-frequency waveform, a sample-by-sample multiplication with the received signal is performed, and a running sum is computed by an accumulator. The output of the accumulator is sampled once in every time segment (1024 samples), providing an estimate of the received amplitude and phase of the sinusoid, and then reset. For the next time segment, the same oscillators are initialized with new sinusoids, each with a new frequency systematically varied from the previous frequency. At the end of the sweep, the entire frequency domain characteristic of the channel is known. Because the frequency hops every 1024 samples and sweeps the entire bands in 128 segments, approximately 3 channel estimates may be performed for a given field. The channel is thus estimated approximately every 8 ms, sufficient for use with a channel changing at 50 Hz (20 ms intervals) or less. 
     The output of the accumulator is a matched filter output directly giving a frequency domain estimate of the channel for a single point in the spectrum. Therefore the time series output of the accumulator directly samples the frequency response of the channel and no fast Fourier transform (FFT) is required. However, the channel estimate may be used directly by a frequency domain equalizer, or else transformed to time domain equalizer coefficients. 
     The channel estimate produced by the coherent demodulator  305  may be further refined by the use of a channel estimator post-processor  306 , which smooths the estimate, tracks time varying fades, and makes Doppler estimates. 
     By proper design of the time-frequency waveform, frequent estimation of the channel is feasible utilizing simple time domain processing. By performing coherent demodulation at the receiver, the power of the inserted waveform may be kept very small to permit transmission along with the currently specified system, thereby maintaining the pay load rate. By retaining the existing field sync and synchronizing the inserted time-frequency waveform to that field sync, backward compatability with legacy receiver(s)  103  is achieved while providing a means to dynamically estimate the channel. 
       FIGS. 4A and 4B  are plots of simulation results for actual channel characteristics and estimated channel characteristics according to one embodiment of the present invention.  FIG. 4A  depicts actual channel characteristics, while  FIG. 4B  depicts a channel estimate at the output of a coherent demodulator deriving the channel estimate from a data signal applied to the channel utilizing the process described above. 
       FIG. 5  is a plot of the spectral power density of the time-frequency waveform inserted into the monocarrier signal according to one embodiment of the present invention. The maximum signal power is approximately −30 dB. 
     The present invention enables efficient channel decoding at high throughput wireless communication receivers such as digital television receivers, and may be incorporated into new terrestrial television standards or enhancements of the existing ATSC-VSB standard. 
     Although the present invention has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, enhancements, nuances, gradations, lesser forms, alterations, revisions, improvements and knock-offs of the invention disclosed herein may be made without departing from the spirit and scope of the invention in its broadest form.