Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-04-26
2001-07-31
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C370S210000
Reexamination Certificate
active
06269132
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to broadband transmission of information. More specifically, the invention relates to improving data transfer over OFDM channels in the presence of narrowband noise.
Orthogonal Frequency Division Multiplexing (OFDM) is a spread spectrum technology wherein the available bandwidth is subdivided into a number of discrete channels or subcarriers that are overlapping and orthogonal to each other. Each channel has a well defined frequency. Data are transmitted in the form of symbols that have a predetermined duration and encompass some number of subcarrier frequencies. The data transmitted over these channels can be encoded in amplitude and/or phase, using conventional encoding schemes such as Binary Phase Shift Key (BPDK), Quadrature Phase Shift Key (QPSK), m-bit Quadrature Amplitude Modulation (m-QAM).
The OFDM channels frequently experience noise interference from interference sources which can effect both the amplitude and the phase of the subcarriers. Such noise can arise from two sources: random noise wherein the noise energy is randomly distributed in both time and frequency domain; and narrowband noise generated, for example, by a jammer emitting a narrowband signal at one or several frequencies that are localized within the frequency range of the OFDM channels. At the receiver, the data have to be separated from the noise. The receiver typically performs a Fourier transform on the received temporal symbol to recover from the temporal symbol waveform the phase and amplitude of each OFDM channel. The Fourier transform is performed over a finite time interval, e.g., the symbol time T
s
. If a temporal waveform is not strictly periodic during the finite time interval, e.g., due to random noise or narrowband noise, artifacts are introduced in the Fourier-transformed signal. These artifacts may make it more difficult to recover the originally transmitted data.
As is known in the art, processing a noise-distorted temporal waveform with an apodizing window function (e.g., a Hanning window) confines interference from narrowband noise to a relatively small number of OFDM channels. That is, the window prevents the noise signal from spreading substantially to channels beyond the channels closest to that signal. However, the apodizing function itself introduces interchannel interference (ICI), which degrades the orthogonal characteristics of the OFDM channels. Thus, even in the absence of random or narrowband noise, the ICI generated by an apodizing function causes each OFDM channel to “leak” some amount of energy into nearby adjacent channels. In the case of a Hanning window, each of the OFDM channels contain 50% of the original signal and 25% of the signal from each of the adjacent channels.
Related copending U.S. application Ser. No. 09/255,164, in the name of Lawrence W. Yonge III, describes transmitting data only over non-adjacent subcarriers instead of over each one of the subcarriers processed with a Hanning window (or other apodizing window). With this technique, the components of adjacent carriers are removed from each of the transmitted carriers. While this technique eliminates ICI between OFDM channels, it does so at the expense of performance as only half of the available channel bandwidth is utilized.
SUMMARY OF THE INVENTION
In one aspect of the invention, a method of processing an OFDM symbol performs a windowing function on a symbol sample having a symbol period of T and a time-offset symbol sample offset from the symbol sample by T/2 to produce first windowed values for the symbol sample and second windowed values for the time-offset symbol sample. The method further includes applying a time shift to re-align the time-offset symbol sample with the symbol sample and therefore re-align the second windowed values with corresponding first windowed values. Once the time-offset symbol sample is re-aligned with the symbol sample, the method adds the second windowed values to the corresponding first windowed values. The performed windowing function is a “symmetric” apodizing window function W, which is defined as an apodizing window function having the property W
j
+W
(FFTSize/2+j)
=constant, where FFTSize is the size of the FFT sample.
Embodiments of the invention may include one or more of the follwing features.
The symmetric apodizing window function may be performed in the time domain.
The symmetric apodizing window function may be performed in the frequency domain and may be applied to FFT frequency coefficients of the symbol sample and the time-offset symbol sample.
The symmetric apodizing window function may be a Hanning window.
The 1.5*T sample and windowing scheme of the invention offers several advantages. In the frequency domain, the windowing scheme eliminates the effects of a symmetric apodizing window (e.g., Hanning window) on adjacent carriers. Consequently, such an apodizing window may be used to reduce jammer-related signal degradation, i.e., limiting the effect of a non-orthogonal narrowband interferer by tapering the endpoints of a signal smoothly to zero, without introducing added distortion into the OFDM receiver. In the time domain, the windowing scheme, the realignment and combination of the signals restores the original signal without changing the benefit gain (reduction in effects of narrowband interferer) achieved by applying the window. In addition to improved jammer performance, the technique may result in an improved S/N ratio (of approximately 0.6 dB) because it takes a longer sample and, while the two instances of the signal add coherently, the noise does not.
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Synchroniza
Bocure Tesfaldet
Fish & Richardson P.C.
Intellon Corporation
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