Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-05-27
2004-01-06
Chin, Stephen (Department: 2734)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S260000, C375S285000
Reexamination Certificate
active
06674810
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of telecommunications, and more particularly, to a method and apparatus for reducing the peak-to-average power ratio in a discrete multi-tone signal, such as in a discrete multi-tone transmitter or transceiver.
BACKGROUND OF THE INVENTION
Asymmetric Digital Subscriber Line
Asymmetric Digital Subscriber Line (ADSL) is a communications technology that operates over existing twisted-pair telephone lines between a central office and a residential or business location. It is generally a point-to-point connection between two dedicated devices. ADSL supports bit transmission rates of up to approximately 6 Mbps in the downstream direction (to a subscriber device at the home), but only 640 Kbps in the upstream direction (to the service provider/central office). ADSL connections actually have three separate information channels: two data channels and a POTS channel. The first data channel is a high-speed downstream channel used to convey information to the subscriber. Its data rate is adaptable and ranges from 1.5 to 6.1 Mbps. The second data channel is a medium speed duplex channel providing bi-directional communication between the subscriber and the service provider/central office. Its rate is also adaptable and the rates range from 16 to 640 kbps. The third information channel is a POTS (Plain Old Telephone Service) channel. The POTS channel is typically not processed directly by an ADSL modem. The POTS channel operates in the standard POTS frequency range and is typically processed by standard POTS devices after being split from the ADSL signal.
The American National Standards Institute (ANSI) Standard T1.413, the contents of which are incorporated herein by reference, specifies an ADSL standard that is widely followed in the telecommunications industry. The ADSL standard specifies a modulation technique known as Discrete Multi-Tone modulation.
Discrete Multi-Tone Modulation
Discrete Multi-Tone (DMT) uses a large number of subcarriers spaced close together. Each subcarrier is modulated during training using Quaternary Phase Shift Keying, or QPSK. Training typically consists of adjusting to existing conditions in the communications connection, such as amplitude response, delay distortions, time recovery, and echo characteristics. During normal data transmission mode, the modulation used in ADSL is Quadrature Amplitude Modulation (MQAM). The data bits are mapped to a series of symbols in the I-Q complex plane, and each symbol is used to modulate the amplitude and phase of one of the multiple tones, or carriers.
In some ADSL transceivers, the symbols are used to specify the magnitude and phase of a subcarrier, where each subcarrier frequency corresponds to the center frequency of the “bin” associated with a Discrete Fourier Transform (DFT). The modulated time-domain signal corresponding to all of the subcarriers can then be generated in parallel by the use of well-known DFT algorithms called Inverse Discrete Fourier Transforms (IDFT). There are many well-known forms of the DFT and IDFT, often referred to generically as fast Fourier transforms (FFT) and inverse fast Fourier transforms (IFFT).
The symbol period in ADSL modems is relatively long compared to single carrier systems because the bandwidth available to each carrier is restricted. However, a large number of symbols is transmitted simultaneously, one on each subcarrier. The number of discrete signal points that may be distinguished on a single carrier is a function of the noise level. Thus, the signal set, or constellation, of each subcarrier is determined based on the noise level within the relevant subcarrier frequency band. The appropriate loading of each carrier is determined during initial training and analysis periods.
Because the symbol time is relatively long and is preceded by a guard band, intersymbol interference is a less severe problem than with single carrier, high symbol rate systems. Furthermore, because each carrier has a narrow bandwidth, the channel impulse response is relatively flat across each subcarrier frequency band. The DMT standard for ADSL, ANSI T1.413, specifies 256 subcarriers, each with a 4.3125 kHz bandwidth. Each subcarrier can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to around 60 kbps per tone. DMT transmission allows modulation and coding techniques to be employed independently for each of the subchannels.
The subchannels overlap spectrally, but as a consequence of the orthogonality of the transform, if the distortion in the channel is mild relative to the bandwidth of a subchannel, the data in each subchannel can be demodulated with a small amount of interference from the other subchannels. For high-speed wide-band applications, it is common to use a cyclic-prefix at the beginning, or a periodic extension appended at the end of each symbol to maintain orthogonality. Because of the periodic nature of the FFT, no discontinuity in the time-domain channel is generated between the symbol and the extension. It has been shown that if the channel impulse response is shorter than the length of the periodic extension, subchannel isolation is achieved.
Signal processing is typically performed after the signal waveform is sampled. Processing associated with ADSL modems often includes echo cancellation, equalization, and DMT modulation/demodulation.
Peak-to-Average Power Ratio
Although DMT has been adopted by standards organizations for ADSL modems, DMT has some disadvantages. One of the most significant disadvantages is its large time-domain PAR (Peak-to-Average power Ratio). A large PAR requires higher resolution digital-to-analog conversion to avoid clipping of the signal, which results when a DMT signal exceeds the dynamic range of the DAC (Digital-to-Analog Converter). As resolution requirements increase, the cost of a suitable DAC increases. Similarly, when a transmitted signal having a large PAR is reflected back into the receiving path (a local echo), it causes similar problems for the ADC (Analog-to-Digital Converter). A large PAR requires higher resolution analog-to-digital conversion to avoid signal-clipping. Typically, the cost for increased resolution in an ADC is even greater than for increased resolution in a DAC. As a result of these high resolution DACs and ADCs, ADSL modems for subscriber devices, e.g., personal computers and other remote terminals, are significantly more expensive than traditional analog POTS-type modems.
One prior solution for PAR reduction is described in U.S. Pat. No. 5,787,113. This first solution generally involves limiting and truncating a DMT signal before digital-to-analog conversion occurs. An echo cancellation scheme is described to cancel noise introduced by the limiting process. However, the noise will most likely be a wide-band or white noise, which is difficult to cancel, compared to noise generated from a smaller number of frequencies.
A second prior solution for PAR reduction is described in U.S. Pat. No. 5,835,536 (inventors Michael May, Terence Johnson, and Matthew Pendleton). Variations of this second solution are discussed in Jose Tellado, John Cioffi, and Richard Stuart, “PAR reduction in Multicarrier Transmission Systems,” ITU Contributions, D.150, Geneva, Feb. 9-20, 1998, and in A. Gatherer and M. Polley, “Controlling Clipping Probability in DMT Transmission,” The 31
st
Asilomar Conference on Signals, Systems, and Computers, Nov. 1997. This second solution generally involves using tones within the DMT transmission band to generate a magnitude adjusting symbol to add to the time-domain DMT signal to be transmitted. As the number of tones used to generate the magnitude adjusting signal increases, the reduction in PAR improves. The tones used for PAR reduction in this second solution are often tones within the transmission band that may be insufficient for supporting data transmission. However, tones used for PAR reduction cannot be used for data transmission. Therefore, if it is desired to use more tones to achieve improved PAR reduction, to
3Com Corporation
Chin Stephen
Kim Kevin
McDonnell & Boehnen Hulbert & Berghoff
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