Pulse or digital communications – Receivers – Automatic baseline or threshold adjustment
Reexamination Certificate
1998-09-11
2002-07-02
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Receivers
Automatic baseline or threshold adjustment
C327S307000, C375S232000
Reexamination Certificate
active
06415003
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to digital communication systems and, more particularly, to correction of baseline wander in baseband transceiver systems.
2. Background of the Invention
The dramatic increase in desktop computing power driven by intranet-based operations and the increased demand for time-sensitive delivery between users has spurred development of high speed Ethernet local area networks (LANs). 100 BASE-TX Ethernet (see IEEE Std. 802.3u-1995 CSMA/CD Access Method, Type 100 Base-T) using existing category 5 (CAT-5) copper wire, and the newly developing 1000 BASE-T Ethernet (see IEEE Draft P802.3ab/D4.0 Physical Layer Specification for 1000 Mb/s Operation on Four Pairs of Category 5 or Better Twisted Pair Cable (1000 Base-T)) for Gigabit/s transfer of data over category 5 data grade copper wiring, require new techniques in high speed symbol processing. On category 5 cabling, gigabit per second transfer can be accomplished utilizing four twisted pairs and a 125 megasymbol/s transfer rate on each pair where each symbol represents two bits.
Physically, data is transferred using a set of voltage pulses where each voltage represents one or more bits of data. Each voltage in the set is referred to as a symbol and the whole set of voltages is referred to as a symbol alphabet.
One well-known system of transferring data at high rates is Non Return to Zero (NRZ) signaling. In binary NRZ signaling, the symbol alphabet {A} is {−1, +1}. A logical “1” is transmitted as a positive voltage while a logical “0” is transmitted as a negative voltage. At 125 M symbols/s, the pulse width of each symbol (the positive or negative voltage) is 8 ns.
An alternative well-known modulation method for high speed symbol transfer is MLT3 and involves a three level system. (See American National Standard Information system, Fibre Distributed Data Interface (FDDI)—Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), ANSI X3.263:1995). The symbol alphabet for MLT3 is {A}={−1, 0, +1}. In MLT3 transmission, a logical 1 is transmitted by either a −1 or a +1 while a logic 0 is transmitted as a 0. A transmission of two consecutive logic “1”s does not require the system to pass through zero in the transition. A transmission of the logical sequence (“1”, “0”, “1”) would result in transmission of the symbols (+1, 0, −1) or (−1, 0, +1), depending on the symbols transmitted prior to this sequence. If the symbol transmitted immediately prior to the sequence was a +1, then the symbols (+1, 0, −1) are transmitted. If the symbol transmitted before this sequence was a −1, then the symbols (−1, 0, +1) are transmitted. If the symbol transmitted immediately before this sequence was a 0, then the first symbol of the sequence transmitted will be a +1 if the previous logical “1” was transmitted as a −1 and will be a −1 if the previous logical “1” was transmitted as a +1. The actual voltage levels that are transmitted are typically +1 V, 0 V and −1 V for the +1 symbol, the 0 symbol and the −1 symbol, respectively.
The detection system in the MLT3 standard, however, needs to distinguish between 3 levels, instead of two levels in a more typical two level system. The signal to noise ratio required to achieve a particular bit error rate is higher for MLT3 signaling than for two level systems. The advantage of the MLT3 system, however, is that the energy spectrum of the emitted radiation from the MLT3 system is concentrated at lower frequencies and therefore more easily meets FCC radiation emission standards for transmission over twisted pair cables. Other communication systems may use a symbol alphabet having more than two voltage levels in the physical layer in order to transmit multiple bits of data using each individual symbol. In Gigabit Ethernet over twisted pair CAT-5 cabling, for example, 5-level pulse amplitude modulated (PAM) data with partial response shaping is transmitted at a baud rate of 125 Mbaud. (See IEEE Draft P802.3ab/D4.0 Physical Layer Specification for 1000 Mb/s Operation on Four Pairs of Category 5 or Better Twisted Pair Cable (1000 Base-T)).
FIG. 1A
shows a typical transmission system
100
for transmitting data at high rates over conventional twisted copper pair wiring. Transmission system
100
includes a transmitter
101
, a transmit coupler
102
, a transmission channel
103
, a receive coupler
104
and a receiver
105
. The transmitter
101
receives data in the form of a symbol stream from a host
111
through a medium independent interface (MII)
112
and couples the modulated data into transmission medium
103
through transmit coupler
102
. Receive coupler
104
receives a modulated waveform from transmission medium
103
and couples the modulated waveform into receiver
105
. The modulated waveform received in receiver
105
suffers from the effects of intersymbol interference (ISI) caused by channel distortion, transmit and receiver filters in transmitter
101
and receiver
105
, and couplers
102
and
104
. Receiver
105
outputs the received data, after correcting for channel distortion, to host
113
, via a medium independent interface
114
.
Intersymbol interference can be compensated for by equalization in receiver
105
. However, some of the effects resulting from couplers
102
and
104
, which are typically transformers, are not compensated adequately by equalization in receiver
105
. These effects include baseline wander and killer packets.
Baseline wander refers to the result of a transmission, in baseband transceiver systems, of symbols where most of the symbols are of identical polarity, for example, in MLT-3 transmission a long series of ones or negative ones. In that case, the output signal from transmitter
101
appears to be a DC signal (a constant 1 V is transmitted by transmitter
101
if a long series of +1 symbols is transmitted). In general, the baseline of the transmit signal is shifted up or down based on the polarity of the transmitted data. Couplers
102
and
104
are typically inductors and, therefore, do not pass DC voltages. The net effect is that the input signal to receiver
105
suffers an exponential decay, called droop or “baseline wander”, eventually resulting in increased error rates in the receiver if the baseline wander effect is not adequately compensated.
In addition, some particular data sequences result in peak-to-peak voltage levels at the receiver that are much higher than other data sequences. For example, even though transmitter
101
outputs a signal having a peak-to-peak voltage of 2 V, because of the effects of couplers
102
and
104
the input signal at receiver
105
can be as high as about 4 V peak-to-peak in response to certain sequences of symbols. A sequence of transmitted symbols that results in particularly high peak-to-peak voltages at receiver
105
is referred to as a “killer packet.” An example of a killer packet satisfying the transmission constraints of a 100 BaseTX system is given in American National Standard for Information Systems, ANSI X3.263:1995, Fibre Distribued Data Interface (FDDI)—Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), March 1995.
In order to process symbol streams that include killer packets, analog-to-digital converters in receiver
105
are required to receive even the statistically less likely, but higher voltage level, signals that result from such packets. This results in either an increased cost for analog-to-digital conversion (i.e., utilization of higher resolution analog-to-digital converters), a loss of resolution of the receiver detection circuitry by setting the resolution of the analog-to-digital converter low enough to include the higher range of voltages, or allowing the analog-to-digital converter to clip the input signal resulting from killer packets. All of the above solutions are, therefore, undesirable.
Corrections for baseline wa
Corrielus Jean
Edwards Gary J.
National Semiconductor Corporation
Skjerven Morrill LLP
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