Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-12-26
2004-02-10
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S285000, C375S348000, C714S792000
Reexamination Certificate
active
06690739
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods for intersymbol interference compensation in data communications and data storage systems with the objective of improving the reliability of data recovery.
Intersymbol interference (ISI) is found in many communications and recording systems. In order to reduce the frequency spectrum of a modulated signal, ISI may be introduced deliberately at the modulator. An example of modulation schemes that introduce ISI is Gaussian minimum shift keying (GMSK) described in the article: “GMSK Modulation for Digital Mobile Radio Telephony,” by K. Murota and K. Hirade, IEEE Transactions on Communications, vol. COM-29, pp. 1044-1050, July 1981. Filtering at the transmitter to control the signal spectrum can extend the basic modulation pulse beyond one signaling interval, thus introducing ISI. The communications medium is another potential source of ISI. In digital subscriber line (DSL) systems, which support high-rate data transmissions in addition to voice traffic, the severe line attenuation and delay variation with frequency of the telephone lines have been found to cause significant ISI in the received signal, as described in the book:
Understanding Digital Subscriber Line Technology
, by T. Starr et al., Prentice Hall, 1999, pp. 183-186. In mobile communications, ISI associated with multipath fading occurs when the direct-path signal combines with the reflected and delayed signals to form a composite signal at the receiver, as described in the book:
Mobile Communications Engineering: Theory and Applications, Second Edition
, by W. Lee, McGraw-Hill, 1998, pp. 442-445.
In reading data from a magnetic recording medium, a phenomenon referred to as bit crowding may occur if the bit packing density is high, as described in the book:
Electronics Engineers' Handbook, Fourth Edition
, by D. Christiansen, McGraw-Hill, 1997, pp. 27.49-27.54. Bit crowding leads to ISI since a read head, which is a transducer configured to provide an output in response to a change in the polarity of the magnetization pattern, has a limited bandwidth. The transducer output is a pulse whose width is inversely proportional to the transducer's bandwidth. When the bit packing density of the recording is sufficiently high, the response pulse will be become wide enough to have non-zero amplitude in the adjacent bit periods, resulting in ISI. There are precursor and postcursor ISI. Precursor ISI is caused by leading undershoots in the transducer response resulting from stray magnetic fields at the edges of the inductive read heads. Postcursor ISI is caused by trailing undershoots in the transducer response.
Modern high-end disk drive systems employ the partial response maximum likelihood (PRML) technology to achieve better performance. In this scheme a certain amount of ISI is deliberately introduced into the recorded signal in order to reduce constraints on the bandwidth of the electronic circuitry. During data recovery, the ISI is usually removed using a maximum likelihood detection scheme such as the Viterbi algorithm. A popular choice of partial response is the class IV partial response (PR4) characterized by the transfer function 1−D
2
, where D represents a symbol delay, i.e., the current output is equal to the current input minus the input two symbol periods ago.
The block diagrams of a data communications system and a data storage system presented in
FIGS. 1 and 2
, respectively, will be used to define the problem to be solved. In
FIG. 1
, data source
102
provides a data sequence {u
j
}, which is advantageously processed by encoder
104
to provide an encoded sequence {v
k
}. The data sequence is usually a binary digital stream but it could also have elements taken from an M-ary alphabet consisting of M values, where M is an integer. Likewise, the elements of the encoded sequence could be binary or M-ary. The term “symbol” will be used to refer to a binary or M-ary element taken from an alphabet of a set of predetermined values. For is example, a binary alphabet has two symbols, which may be 0 and 1, while an octal alphabet has eight symbols, which may be −7, −5, −3, −1, 1, 3, 5, and 7. Thus, a data sequence is made up of data symbols and an encoded sequence is made up of encoded symbols. With reference to
FIGS. 1 and 2
, the indices j and k may be different since the encoded sequence may have a length different from that of the data sequence. For example, if the encoder is a rate 1/2 error-correction encoder, then the encoded sequence is twice as long as the data sequence. Examples where the encoded sequence is the same length as the data sequence include: (i) the encoder is simply a one-to-one mapper such as mapping “0” to “−1” and “1” to “+1”, as in antipodal signaling; and (ii) the encoder is a binary, rate 1
error-correction encoder followed by a mapping of n bits to a symbol chosen from an alphabet of size 2
n
. With reference to
FIG. 1
, the encoded sequence is supplied to modulator
122
, which converts the discrete-time sequence {v
k
} into a continuous-time modulated signal. The modulated signal is filtered, up-converted to a higher frequency, and amplified by transmitter
124
for transmission over communications medium
126
. The transmitted signal is received by receiver
128
, which usually performs the functions of amplification using a low-noise amplifier, down-conversion to a lower frequency, and filtering to reject out-of-band noise. The receiver output is sampled and digitized using analog-to-digital (A/D) converter
130
to provide a digitized signal, which is advantageously processed by matched filter
132
. In this exemplary arrangement, matched filter
132
is a digital filter. In implementations where matched filter
132
is an analog filter, the positions of A/D converter
130
and matched filter
132
shown in
FIG. 1
are interchanged. Communications channel
120
comprises modulator
122
, transmitter
124
, communications medium
126
, receiver
128
, A/D converter
130
, and matched filter
132
. The channel output sequence {r
k
} is the output of matched filter
132
. Each element of the channel output sequence is represented by a digital word made up of a number of bits dependent on the implementation. The term “sample” will be used to refer to a quantity that is not taken from an alphabet. Thus, the number of possible values assumed by a sample is not predetermined but dependent in part on the length of the digital word representing the sample. With this convention, r
k
is a channel output sample and the channel output sequence {r
k
} is a collection of channel output samples arranged chronologically. In general, the channel generates one or more channel output samples r
k
in response to an encoded symbol v
k
presented to the channel input. To simplify the discussion, the indices of the encoded sequence {v
k
} and the channel output sequence {r
k
} are shown in
FIG. 1
to be identical to signify that there is one channel output sample for each channel input symbol. The use of the same indices for these two sequences in
FIG. 1
should be considered exemplary rather than restrictive. The channel output sequence is supplied to data estimator
106
to recover the data sequence. The output of data estimator
106
is an estimated data sequence {û
j
}, which is composed of estimated data symbols. The estimated data sequence is supplied to data sink
108
.
FIG. 2
, which is a block diagram of a data storage system, shows the encoded sequence being supplied to modulator
222
, which converts the discrete-time sequence {v
k
} into a continuous-time modulated signal. The modulated signal is supplied to data recorder
224
, which writes the modulated signal onto recording medium
226
, which can represent a magnetic tape or disk. Data reader
228
reads recording medium
226
to provide an analog signal, which is sampled and digitized using A/D converter
230
. Recording channel
2
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