Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1999-08-06
2002-05-07
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S290000, C375S341000, C360S040000, C369S018000
Reexamination Certificate
active
06385255
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to partial response channels, and more particularly to a system and method for equalization and coding on a partial response channel.
2. Related Art
The proliferation of processors and processor-based systems in recent years has led to a tremendous increase in the ability of businesses, industry and individuals to share or otherwise communicate information. Most computers and workstations in today's homes and offices are connected in some manner to another computer or workstation, either locally or remotely. An early form of such inter-connection of computing systems is the local area network (LAN). Using LAN technology, several computers, workstations, peripherals, or other related devices can be connected to share data among one another and to share network resources.
Another technique used for sharing data among a plurality of computing resources is by writing the data to a data storage medium, and physically transporting the data on that medium to another computing system. One such medium that has gained popularity in recent years is the CD (compact disk). CD technology offers relatively high density recording and rapid data access. The cost of a blank recordable CD media has fallen dramatically. In fact, CD media is one of the cheapest mass-storage device available today, at slightly less then $0.01/MB. Additionally, the latest DVDs (Digital Versatile Discs) store 4.7 to 17 GB of data on a single, two-layered, double-sided disc.
Regardless of the communication or storage medium (i.e., channel) used to transmit or store data, system designers strive to increase data densities and access speeds, and to reduce media costs and error-rates. However, these interests are often competing. For example, as data densities and access speeds increase, the likelihood of introducing errors also increases. As such, data transmission and recording requires precise knowledge about the signal sensed by the read head.
When recording density is low, each transition written on the medium results in a relatively isolated voltage peak and conventional peak detection methods can be used to recover written information. However, as densities and readback speeds increase, inter-symbol interference arises and the peak detection channel can not provide reliable data detection. Inter-symbol interference results from the overlap of signal peaks as they stream past the read head at higher and higher rates. As a result, a different detection principle is desired if the density of recording is to be increased. Rather than being based on voltage peaks, the principle should take into account the fact that signals from adjacent transitions interfere. In other words, the method of detection should be aware of the inter-symbol interference in the signal.
Inter-symbol interference has traditionally been combated by encoding the data as a stream of “symbols” as the data is written, allowing the peaks to be separated during read operations. The problem as been that the encoding requires more than one symbol per bit, negatively impacting both disk capacity and drive performance.
To address this issue, Partial Response Maximum Likelihood (PRML) coding was developed in the early 70's by a group of IBM researchers. In fact, PRML may be best known for its application in NASA's Viking Lander project, whose quest was to gather scientific information from the planet Mars. Used to communicate radio signals generated millions of miles from Earth, PRML technology allows engineers to keep data clear from background interference on its long trip back home.
Regardless of the application, the principal effect of PRML is that it can handle more tightly packed data than can peak detection, while improving noise rejection. PRML channels are not required to separate the peaks during read operations, but instead use advanced digital filtering techniques to manage inter-symbol interference. Managing inter-symbol interference, and accounting for it in coding techniques allows greater recording density by allowing media to store more information with some forms of PRML.
PRML employs digital processing and maximum likelihood detection to determine the sequence of bits that was most likely written on the medium. The phrase “Partial Response” refers to the fact that PRML does not directly provide user output information. Instead, additional decoding is required to arrive at the actual data, and is typically accomplished using Viterbi decoding. The phrase “Maximum Likelihood” refers to the step of converting the waveform to data. Viterbi detection represents an algorithm that checks all possible combinations of data and looks for the best match of least error with incoming data. The pattern that has the least error (difference) is the one with the maximum likelihood to be correct.
PRML can be implemented in a remarkably efficient manner, and this contributes to a faster transfer of data. This can be accomplished by using a more efficient Run Length Limited (RLL) coding scheme than, for example, the (1,7) or (2,10) RLL coding used by traditional peak detection schemes.
SUMMARY OF THE INVENTION
The present invention is directed toward systems and methods for reading data from and writing data to multi-level (M-ary) partial response channels. According to one aspect of the invention, an encoder, such as, for example, a trellis coder, is used to encode an input bit stream sequence into a stream of multi-level data symbols. For example, in one application, the input bit stream is a stream of digital data bits and the output is a stream of M-ary multi-level symbols where M>2.
Although various coding techniques can be used, one embodiment utilizes Ungerboeck style trellis codes to code the input bit stream into multi-level data symbols. The data symbols can then be written to the channel using any of a number of techniques. In one embodiment, the multi-level symbols are used to modulate a write laser, which alters the properties of the medium onto which the data symbols are written. The altered properties can then be detected to read data off of the channel.
Preferably, according to one aspect of the invention, a trellis coder anticipates the modulation transfer function of the channel in encoding the data. For, example, for partial response channels, according to this aspect of the invention the encoder is designed taking into consideration the inter-symbol interference introduced by the channel. As such, the performance of the system can be enhanced.
Because the partial response channel has its own transfer function, the relationship between the data read from the channel and the actual input data bits is a function not only of the data encoder but also of the partial response channel. Therefore, according to another aspect of the invention, a decoder specification used to implement the decoder can be designed to take into account not only the effect of the encoder (e.g., trellis or other encoder), but also the effect of the partial response channel.
According to this aspect of the invention, in one embodiment, the decoder considers K*L states where K is the number of states in the trellis code and L is the number of possible states of the partial response channel. This is advantageous because typically the effects of the trellis coder and the partial response channel are not mutually exclusive but are, instead, dependent upon one another. It should be noted that the partial response channel can include, for example, the zero-forcing case (e.g., &agr;
1
=1, &agr;
2
=0, . . . &agr;
n
=0) as well as other partial response channels.
Further features, advantages and aspects of the invention are described in detail below in accordance with one or more embodiments and with reference to the accompanying drawings.
REFERENCES:
patent: 4888779 (1989-12-01), Karabed et al.
patent: 5257272 (1993-10-01), Fredrickson
patent: 5331320 (1994-07-01), Cideciyan et al.
patent: 5497384 (1996-03-01), Fredrickson et al.
patent: 553738
Fan John L.
Lee David C.
McLaughlin Steven W.
Zingman Jonathan A.
Calimetrics, Inc.
Knobbe Martens Olson & Bear LLP
Le Amanda T.
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