Pulse or digital communications – Pulse position – frequency – or spacing modulation
Reexamination Certificate
1999-02-18
2001-09-25
Le, Amanda T. (Department: 2734)
Pulse or digital communications
Pulse position, frequency, or spacing modulation
C375S362000, C332S112000, C329S313000, C370S213000, C370S503000, C359S199200
Reexamination Certificate
active
06295318
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to the field of digital data communications, and more particularly relates to a method of increasing the rate of data transmitted over a limited bandwidth medium such as twisted copper pairs by using a modified pulse position modulation scheme and a synchronization signal transmitted over a separate-path communications channel.
BACKGROUND OF THE INVENTION
The amount of digital information that can be transmitted over a communications medium such as copper telephone wires, typically provided as a pair of twisted copper wires, is limited by the inherent bandwidth of the medium. Information to be transmitted digitally is first typically converted into digital pulses. The number of pulses that can be transmitted in one second is limited by the bandwidth of the medium. Limiting the number of pulses that can be transmitted limits the amount of data that can be transmitted. To increase the amount of data that can be transmitted, data encoding schemes were created.
One such scheme is pulse position modulation (PPM). A PPM scheme creates a plurality of symbol positions, with each symbol position corresponding to a predefined value representing the information. Placing a pulse in one of the symbol positions transmits the predefined value. In n-PPM, information is converted into n-bit data, where n is greater than zero. The information is then converted into one of 2
n
possible pulse values. Each of the values at which a pulse may be placed is referred to as a “symbol position.” The unit indicating the n-bit information formed by the 2
n
symbol positions is referred to as a “frame.”.
The number of frames that can be transmitted per unit time is related to the bandwidth of the communications medium. For example, the bandwidth of a twisted copper pair is typically no more than 64 kHz, which corresponds to a channel capacity of 64,000 bits per second. Therefore, there can be up to 64,000 frames available. As an example, in a 3-PPM system transmitted over twisted copper pairs, there are eight possible combinations of 3-bit binary data: 000, 001, 010, 011, 100, 101, 110, and 111. Each frame is divided into eight symbol positions, having the values 0, 1, 2, 3, 4, 5, 6, 7 (2
3
=8), which correspond to the 3-bit binary data.
For transmission of the 3-bit data with a PPM scheme, a pulse is placed in the corresponding symbol position. In this manner, 3 bits of information are transmitted by a single pulse, as opposed to a single bit of information represented by a single pulse. Therefore, the amount of data transmitted increases by a factor of 3.
Simply increasing the factor n can increase the amount of data transmitted. However, increasing the factor n increases the number of symbol positions, but the number of frames remains the same because of the limited bandwidth of the channel. Necessarily, the width of the individual pulses decreases. This leads to an increase in the required bandwidth of the communications medium used to transmit the PPM signal. However, communications media have a fixed bandwidth. As stated above, for twisted copper pairs, this is typically about 64 kHz for lengths of about 18,000 feet to 24,000 feet. Once the bandwidth limitation of the communications medium is reached, the individual pulses can occur so closely together that the receiver will not be able to resolve one pulse from another.
One possible solution is to switch to a communications medium with a greater bandwidth capacity. However, media with greater bandwidth are typically more expensive than twisted copper pairs and also require more complex transmitters and receivers.
Thus, there is a general need in the art for a method of increasing the amount of data that can be transmitted over a bandwidth-limited communications medium, such as twisted copper pairs, without increasing the required bandwidth and thereby providing improved telecommunications services to the home and office.
SUMMARY OF THE INVENTION
The present invention meets the above-described need by providing a method of increasing the amount of data that can be transmitted over a bandwidth-limited communications medium, such as twisted copper pairs, without increasing the bandwidth requirement of the communications medium. The invention may be referred to as “SPAD,” an acronym for Separate Paths to Amplify the Data.
Information that is to be transmitted is first converted into n-bit digital data. A separate communications path is employed for transmitting a synchronization clock pulse (SCP) signal. The number 2
n
of symbol positions available for transmitting the data is related to the frequency of the SCP signal. The frequency of the pulse synchronization signal and the number of symbol positions are arbitrary and may be determined through testing methods that are beyond the scope of the present invention. The symbol positions are grouped into a plurality of frames, where each frame transmits a single pulse at a rate not exceeding the bandwidth of the channel. Therefore the number of frames is equal to the bandwidth of the system.
For example, in a twisted copper pair having a maximum bandwidth of 64 kHz, the number of frames is equal to 64,000. Each frame is then divided into 2
n
symbol positions. A pulse is then placed in a symbol position that corresponds to the value of the data contained in the pulse. However, the width of the pulse does not correspond to the width of the symbol position. To ensure that the pulse in a given frame can be resolved from a pulse in an adjacent frame, the pulse width is provided at a predetermined value, preferably no more than one half of the frame width with a duty cycle of 50 percent. This means that the pulse will be “high” for a period equal to one half the frame width and “low” for a period equal to one half the frame width. Using SPAD-3 as an example, there will be eight symbol positions available for each frame. Each pulse will be “high” for four symbol positions and “low” for four symbol positions.
To ensure that the period of a pulse in a given frame does not encroach on the period of a subsequent pulse in a subsequent frame, the subsequent pulse may be inverted. Inverting a pulse means that the value of the pulse is negated relative to zero voltage, and the value is changed to its the complementary value. For SPAD-3, for example, a position of
0
would have an inverted position of
4
, a position of
1
would have an inverted position of
5
, a position of
2
would have an inverted position of
6
, a position of
3
would have an inverted position of
7
a position of
4
would have an inverted position of
0
, a position of
5
would have an inverted position of
1
, a position of
6
would have an inverted position of
2
, and a position of
7
would have an inverted position of
3
.
Inverting pulses in this manner increases the data rate without decreasing the duty cycle. Because inverted pulses have negative voltages, the receiving equipment must be capable of detected three voltage levels, a negative voltage level, a zero voltage level, and a positive voltage level. Although the invention is described as negating a pulse relative to a zero voltage reference level, those skilled in the art will appreciate that a pulse may be “negated” relative to a non-zero reference voltage. In this case, a “negative” voltage will be some voltage level less than the reference voltage and a “positive” voltage will be a voltage level greater than the reference voltage.
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Dougherty & Clements LLP
Le Amanda T.
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