Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2000-07-20
2002-07-16
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C708S323000
Reexamination Certificate
active
06421381
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to simplifying the equalizer needed to combat the intersymbol interference present in a digital communication system.
2. Background
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 LANs. 100BASE-TX Ethernet, using category-5 copper wire, and the newly developing 1000BASE-T Ethernet for Gigabit/s transfer of data over existing category-5 copper wire require new techniques in high speed symbol processing. 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. Twisted pair copper cables are also used in wide-area networking (WAN) and digital subscriber loop data communication applications. With ever increasing need for bandwidth, technologies that support high data transfer rates across twisted pair cables are gaining wide acceptance. 100Base-TX (fast Ethernet), 1000Base-T transmission over long haul copper (also known as Gigabit Ethernet) and digital subscriber loop technologies all transmit data at high transmission rates over twisted copper pairs.
Physically, data is transferred using a set of voltages where each voltage represents one or more bits of data. Each voltage in the set of voltages is referred to as a symbol and the whole set of voltages is referred to as a symbol alphabet.
One system of transferring data at high rates is Non Return to Zero (NRZ) signaling. In 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 (i.e. the positive or negative voltage) is 8 ns.
Another system for high speed symbol data transfer is referred to as MLT3 signaling and involves a three voltage 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 X3.263:199X). The symbol alphabet for MLT3 is {A}={−1, 0, +1}, corresponding to the set of voltages {−V, 0, V}. The voltage V is typically 1 V.
In MLT3 transmission, a logical “1” is transmitted by either a −1 or a +1 symbol while a logic “0” is transmitted as a 0 symbol. 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.
In the ideal MLT3 system, the transmit driver simply sends a voltage pulse corresponding to the symbol being transmitted. The pulse is of duration 8 nanoseconds for each one of the transmit symbols and has a finite rise/fall time of three to five nanoseconds (See American National Standard Information system,
Fibre Distributed Data Interface
(
FDDI
)—
Part: Token Ring Twisted Pair Physical Layer Medium Dependent
(
TP
-
PMD
), ANSI X3.263:199X).
The detection system in the MLT3 standard, however, needs to distinguish between three voltage levels, instead of two voltage levels in a 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 is that the power 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.
A block diagram of a typical digital communication transmission system is illustrated in FIG.
1
. In
FIG. 1
, the transmitted data is represented by the symbol sequence {a
k
}. The transmitted symbols in the sequence {a
k
} are members of the symbol alphabet {A}. In the case of three level MLT3 signaling, the symbol alphabet {A} is given by {−1, 0, +1}. The index k represents the time index for that symbol, i.e., at sample time k, the symbol being transmitted is given by a
k
. The channel response is represented by the channel transfer function f(z). The channel function f(z) is the Z-transformation of the sampled time response of the channel.
In
FIG. 1
, the transmitted symbols {a
k
} enter the channel 1. The signal output from the channel 1, x
k
, is a linear distortion of the transmitted symbols {a
k
}, the distortion being described by the channel transfer function f(z). The signal x
k
is summed in adder 2 with a noise sample n
k
to form the signal y
k
. The noise samples {n
k
} represent the random noise on the transmission line. The signal y
k
, suffering from both the channel distortion and the random noise, is then input to the detector
3
. Detector
3
inputs the distorted signals y
k
, counteracts the effects described by the channel transfer function f(z), and outputs a sequence of detected symbols {â
k
}.
FIG. 2
shows a typical 100Base-Tx transmitter. The transmit data path in a 100Base-TX transceiver (IEEE 802.3u Standard) consists of a physical coding sub-layer (PCS)
11
, and a physical medium dependent (PMD) sub-layer
12
. The PCS
11
contains a medium independent interface (MII) 4 and a 4B5B (rate 4/5) encoder
5
. The medium independent interface
4
is the interface between the transceiver and the media access controller (MAC). The 4B5B encoder
5
guarantees sufficient transitions in the transmit data for the purpose of robust clock recovery in the receiver and generates Ethernet control characters. The data rate at the output terminal of the PCS
11
is 125 Mhz due to the rate penalty associated with the 4B5B encoder
5
. The physical medium dependent portion
12
of the 100Base-TX transmit data path consists of a scrambler
6
, binary to MLT3 converter
7
, and a transmit driver
8
which outputs a 1V peak-to-peak signal onto the twisted pair
10
through an isolation transformer
9
. The transmit symbol sequence {a
k
} is generated in the binary to MLT3 converter
7
.
It is assumed that the channel model represented by f(z) includes the effect of transmit and receive filtering. In addition, the transmission channel is assumed to be linear in that two overlapping signals simply add as a linear superposition. Therefore, the channel transfer function polynomial can be defined as
f
(
Z
)=
f
0
+f
1
Z
−1
+f
2
Z
−2
+ . . . +f
N
Z
−N
, (1)
where f
0
, . . . , f
j
, . . . , f
N
are the polynomial coefficients. The polynomial coefficient f
j
represents the dispersed component of the (k−j)th symbol present in the kth received sample and N is a cut-off integer such that f
j
for j>N is negligible. The polynomial f(Z) represents the Z-transformation of the sampled frequency response of the transmission channel. In Equation 1, Z
−1
is considered to be a one clock period delay. See A. V. Oppenheim & R. W. Schafer, Discrete-Time Signal Processing 1989.
The noiseless output of the channel at samp
Corrielus Jean
Edwards Gary J.
National Semiconductor Corporation
Skjerven Morrill LLP
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