Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-02-29
2004-05-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S343000, C375S360000, C370S506000, C370S510000
Reexamination Certificate
active
06735255
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to a method and device for recovering timing information in a multi-carrier communication system. Specifically the invention relates to a method and structure for correcting for the effects of a frequency offset between a transmitter clock and a receiver clock in a multi-carrier transmission system, as found, for example, in ADSL transceivers.
B. Description of the Related Art
1. Asymmetric Digital Subscriber Lines
Asymmetric Digital Subscriber Line (ADSL) is a communication system 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, as opposed to multi-point, where numerous devices share the same physical medium.
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 upstream channel providing bidirectional 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 the ADSL modems—the POTS channel operates in the standard POTS frequency range and is 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. A similar standard, Recommendation G.992.1 from the ITU, is also incorporated herein by reference. A variation of the standard that accomodates POTS service without the use of a signal splitter is set forth in specification G.Lite, or Recommendation G.992.2, the contents of which are incorporated herein by reference. The ADSL standards specify a modulation technique known as Discrete Multi-Tone modulation.
2. Discrete Multi-tone Modulation
Discrete Multi-Tone (DMT) uses a large number of subcarriers spaced close together. Each subcarrier is modulated using a type of Quadrature Amplitude Modulation (QAM). Alternative types of modulation include Multiple Phase Shift Keying (MPSK), including BPSK and QPSK, and Differential Phase Shift Keying (DPSK). 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. 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 algorithm called Inverse Fast Fourier Transforms (IFFT).
The symbol period 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.
Because the symbol time is relatively long and follows 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 sub-carrier can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to 60 kbps per tone. DMT transmission allows modulation and coding techniques to be employed independently for each of the sub-channels.
The sub-channels 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 sub-channel, the data in each sub-channel can be demodulated with a small amount of interference from the other sub-channels. For high-speed wide-band applications, it is common to use a cyclic-prefix at the beginning, or a periodic extension at the end of each symbol, in order to maintain orthogonality, and more specifically, to eliminate inter-symbol-interference.
3. Frequency Domain Equalization
In standard DMT modulation, each N-sample encoded symbol is prefixed with a cyclic extension to allow signal recovery using the cyclic convolution property of the discrete Fourier transform (DFT). Of course, the extension may be appended to the end of the signal as well. If the length of the cyclic prefix, L, is greater than or equal to the length of the impulse response, the linear convolution of the transmitted signal with the channel becomes equivalent to circular convolution (disregarding the prefix). The frequency indexed DFT output sub-symbols are merely scaled in magnitude and rotated in phase from their respective encoded values by the circular convolution. It has been shown that if the channel impulse response is shorter than the length of the periodic extension, sub-channel isolation is achieved. Thus, the original symbols can then be recovered by transforming the received time domain signal to the frequency domain using the DFT (implemented using, e.g., the FFT), and performing equalization using a bank of single tap frequency domain equalizer (FEQ) filters. The FEQ effectively deconvolves (circularly) the signal from the transmission channel response. This normalizes the DFT coefficients allowing uniform QAM decoding.
Such an FEQ is shown in FIG.
1
. The FFT calculator
20
accepts received time domain signals from line
10
, and converts them to frequency domain representations of the symbols. Each frequency bin (or output) of the FFT
20
corresponds to the magnitude and phase of the carrier at the corresponding frequency. In
FIG. 1
, each bin therefore contains a separate symbol value X(i) for the i
th
carrier. The frequency domain equalizer FEQ
40
then operates on each of the FFT
20
outputs with a single-tap filter to generate the equalized symbol values X′(i). The FEQ
40
inverts the residual frequency response of the effective channel by a single complex multiplication. The FEQ outputs are then decoded by a slicer, or data decision device (not shown). The FEQ taps can be updated, and can make use of the slicer output in this regard. That is, the FEQ taps may be updated so as to minimize the error between the FEQ output and the slicer output. This is commonly referred to as decision feedback equalization, or decision-directed adaptation.
4. Timing Recovery
Also shown in
FIG. 1
is a clock recovery and control circuit
30
. The clock recovery circuit
30
analyzes the pilot tone that is embedded in the transmitted DMT signal in ADSL communication systems.
A typical hardware solution is shown in FIG.
2
. The clock recovery components are indicated with dashed lines. Control words from a clock recovery algorithm running in a DSP
10
are converted to voltage levels by a digital-to-analog converter (DAC)
12
which controls the receive sampling rate of an ADC
14
through a voltage-controlled oscillator (VCO)
16
. With a pure software timing recovery solution, the DAC
12
and VCO
16
(marked in dotted line) and any as
Bevan Scott A.
Christiansen Mark W.
Dobson William Kurt
Smart Kevin J.
Stoddard Trent
3Com Corporation
Chin Stephen
Ha Dac V.
McDonnell Boehnen & Hulbert & Berghoff LLP
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