Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
2002-01-04
2003-12-16
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S146000
Reexamination Certificate
active
06665352
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data communication systems and more particularly relates to an overlapping windowing mechanism for smoothing the transition between two consecutive transmit waveform signals in a communications transmitter.
BACKGROUND OF THE INVENTION
The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming increasingly common. Spread spectrum communications transmits data utilizing a spectrum bandwidth that is much greater than the bandwidth of the data to be transmitted. This provides for more reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming received signal.
Spread spectrum communication systems modulate a spreading waveform that is then transmitted onto the channel. Commonly used spreading waveforms may include a chirp signal or a pseudo noise (PN) sequence. A plot of an example PN sequence type spreading waveform is shown in FIG.
1
. The pulse shown is adapted for use in the frequency range of 120 to 400 kHz for use in the United States. The pulse serves as the transmit waveform that is modulated by the transmitter in accordance with the data to be transmitted. Each symbol is transmitted as a pulse that is modulated in accordance with the data.
In order to meet limitations on spectral bandwidth imposed by government bodies, the transmit waveform must meet certain criteria. In the United States, for example, the Federal Communications Commission (FCC) Standard, in Part 15, Section 107 on Conducted limits, specifies for equipment designed to be connected to the public utility (AC) power line, the RE voltage conducted back onto the AC power line for carrier current systems containing their fundamental emission within the frequency band 535-1705 kHz, the limit on conducted emissions is 1000 &mgr;V within the frequency band 535-1705 kHz. This corresponds to an attenuation of 60 dB.
Although each individual transmit waveform may be constructed to meet any such criteria, a problem arises when transmit waveforms are concatenated consecutively into a pulse sequence.
For example, in the case of code shift keying (CSK) modulation, the spreading waveform is circularly shifted in accordance with the data to be transmitted. Discontinuities are created at the point of intersection between two consecutive symbols (i.e. transmit waveforms) which create higher than permitted levels of harmonics in the transmitted signal.
The spectrum plot of a transmit signal comprising a sequence of transmit waveforms is shown in
FIG. 2. A
product using such a transmit waveform to communicate would not be able to meet FCC Part 15 standards for limitations on out of band harmonics.
There is thus a need for a mechanism to reduce the effect of discontinuities between consecutive transmitted symbols, to reduce out of band harmonics of the transmitted output signal.
SUMMARY OF THE INVENTION
Accordingly, the present invention is a novel and useful windowing mechanism for reducing the out of band harmonics of an output transmit signal. The windowing mechanism functions to smooth the transition between two consecutive transmit waveforms. The windowing mechanism of the present invention is operative to reduce the effect of discontinuities to reduce out of band harmonics of the transmitted output signal.
The mechanism of the present invention is useful in communication systems characterized by consecutively transmitted symbols whereby the concatenation of transmitted waveforms causes discontinuities between symbols. The mechanism is particularly useful in spread spectrum communication systems that operate in constrained bandwidth environments such as power line carrier, ISM frequency bands (e.g., 900 MHz, 2.4 GHz, etc.) or any other narrowband environments.
The windowing mechanism is operative to extend each transmit waveform both earlier and later in time thus creating extending waveform portions on either end of the transmit waveform. The extended portions are then multiplied by a suitable windowing function and the result is summed with the previous and next transmit waveforms.
The windowing function was developed in light of the following design goals and tradeoffs. The windowing function was designed to have low computational complexity but with high spectral efficiency wherein the number of multiplications required is reduced to a minimum. In addition, the windowing function has a short time response (i.e. overlap zones) since ISI intersymbol interference (ISI) is introduced in the overlap zones which degrades the overall performance of a modem that incorporates the invention. Further, the windowing function is a constant envelope window which is important for optimal exploitation of the output amplifier since it allows the use of rail-to-rail designed input symbols with the amplifier.
The start of the transmit waveform is extended earlier in time to generate a first overlapping or extended portion while the end of the transmit waveform is also extended later in time to generate a second extended portion. Similarly, the end of the previous transmit waveform is extended to create an extended portion and the start of the next transmit waveform is also extended to create an extended portion.
Once the extended portions are generated, a windowing function is applied to the transmit waveforms. The windowing function is applied not only to the extended portions but also to equivalent length start and end portions of the transmit waveform itself. The windowing function is also applied to the previous and next transmit waveforms as well. The current transmit waveform is multiplied by the amplitude of the windowing function.
Each extended transmit waveform is multiplied by the windowing function. The windowed extended transmit waveforms are overlapped and summed to generate a windowed transmit signal. Thus, the windowing process, causes each transmit waveform to be included somewhat in the previous and next transmit waveforms. The windowing function may comprise any suitable function, such as a linear or non-linear function.
The present invention has applications in systems where it is desired to reduce the out of band harmonics of a transmit signal in order to conform to government imposed regulations on out of band attenuation, an example of which is the FCC Standard, Part 15, Section 107 on Conducted limits which specifies, for equipment designed to be connected to the public utility (AC) power line, a limit of 1000 &mgr;V within the frequency band 535-1705 kHz for carrier current systems containing their fundamental emission within the frequency band 535-1705 kHz for the RF emissions conducted back onto the AC power line, corresponding to an attenuation of 60 dB.
Many aspects of the invention may be constructed as software objects that execute in embedded devices as firmware, software objects that execute us part of a software application on a computer system running an operating system suck as Windows, UNIX, LINUX, etc., an Application Specific Integrated Circuit (ASIC) or functionally equivalent discrete hardware components.
There is therefore provided in accordance with the invention a method of windowing a sequence of symbols, the method comprising the steps of appending zero or more signal continuations to a current symbol so as to generate an augmented symbol and multiplying zero or more portions of the augmented symbol with a window.
There is also provided in accordance with the invention a method of windowing a sequence of transmit waveforms, each transmit waveform having a first end and a second end, the method comprising the steps of continuing the second end of a previous transmit waveform later in time to generate a second continued portion, continuing the first end of a next transmit waveform earlier in time to generate a first continued portion, wherein the next transmit waveform is
Kaufman Oren
Lerner Gregory
Raphaeli Dan
Zarud Boris
Chin Stephen
Itran Communications Ltd.
Kim Kevin
Zaretsky Howard
Zaretsky & Associates PC
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