Pulse or digital communications – Pulse code modulation – Length coding
Reexamination Certificate
2000-04-07
2003-02-04
Tse, Young T. (Department: 2634)
Pulse or digital communications
Pulse code modulation
Length coding
C370S449000, C341S060000
Reexamination Certificate
active
06516035
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wireless communication. In particular, the present invention relates to wireless communication used in data network and remote control applications.
2. Discussion of the Related Art
Infrared radiation is widely used in data communication and remote control applications. In the prior art, data in infrared communication have been encoded, in sophisticated applications, by modulating amplitude, frequency or both. In simpler applications, data can be encoded simply by the presence or absence of infrared radiation at expected times.
One class of encoding methods is characterized by encoding every N bits of data into a symbol consisting of M binary bits of code (i.e., each bit is represented by the logic value ‘1’ or ‘0’), and transmitting these M bits in M time slots sequentially. In some implementations, each bit of code can be represented within a time slot by the presence or absence of a single pulse of infrared radiation (the “single pulse” approach), or the presence or absence of a sequence of equally spaced pulses of infrared radiation (the “multiple pulses” approach). (Under the multiple pulses approach, the frequency of pulse repetition is known as the sub-carrier frequency.) In either approach, whether the presence or absence of radiation represents logic ‘1’ value or logic ‘0’ value is a matter of convention. For convenience, in the remainder of the present discussion, logic ‘0’ and ‘1’ values are represented, respectively, by the absence and presence of radiation in the designated time slot. The present invention is, however, not limited to either approaches. The single pulse approach is naturally simpler and faster. However, the multiple pulses approach provides a greater range due to a higher signal-to-noise ratio achievable under that approach.
Typically, the number M of bits in each symbol is usually greater than the number N of data bits. This arrangement provides a number of benefits, including (a) allowing better synchronization of clocks on the transmitting and receiving sides, (b) simplifying the design of the transmitter and the receiver, and (c) providing error detection or error correction capabilities.
Several figures of merit have been used to compare encoding schemes. For example, “Code rate” is the ratio N/M. Another example is “run length limits”, which is typically expressed as (d,k), where d and k are, respectively, the minimum and maximum lengths allowed for a run of logic ‘0’ value between two logic ‘1’ values in each symbol of the encoding scheme, or between successively transmitted symbols. Yet another example is “Duty cycle”, which is typically expressed as a product of two factors a and b, where a is the percentage of logic 1 bits out of the total number of bits used in the symbols of the encoding scheme, and b is the percentage of time in a time slot during which radiation representing the data is present.
One encoding scheme in the prior art is the “FIR” or “4PPM” scheme. FIR has a code rate of 2/4 and a run length limit of (0,6). FIR uses the single pulse approach, with the single pulse or logic ‘1’ value using the entire time slot, thus providing a 25%×100% duty cycle. The following is the code dictionary under FIR:
(Data)
{Symbol}
(00)
{1000}
(01)
{0100}
(10)
{0010}
(11)
{0001}
Under FIR, the minimum separation between logic ‘1’ values is zero, which occurs whenever the symbol {0001} is followed by the symbol {1000}. In that instance, the two adjacent logic ‘1’ values form a single pulse of double width. However, such a pulse can be erroneously processed at the receiver and the decoder circuit, as it is often difficult to differentiate a single-width pulse from the occasional double-width pulse due to distortion in the communication channel.
Another encoding scheme is the “MIR” scheme. MIR, which also takes the single pulse approach (with the single pulse occupying one quarter of the time slot), has a code rate of 1/1+ (explained below), a run length limit of (0,5), and a high variable duty cycle of 50%×25% maximum. The code dictionary for MIR is as follows:
(Data)
{Symbol}
(0)
{1}
(1)
{0}
MIR requires that a logic ‘1’ value be inserted after a run of five consecutive logic ‘0’ values. This additional rule provides the variability in the code rate and the duty cycle. The extra logic ‘1’ values are removed by the receiver prior to decoding. As in FIR, the zero value in the run length limit can create error in the receiver. Such errors are reduced by using a single pulse which occupies only one quarter of the time slot, at the expense of bandwidth.
Yet another example is the “SIR” or “HP SIR” coding scheme, which has a code rate of {fraction (8/10)}, a run length limit of (0,9). SIR also uses a single pulse approach, with a pulse width up to {fraction (3/16)} of a time slot, to provide a maximum variable duty cycle of 90%×19%. The code dictionary for SIR is as follows:
(Data)
{Symbol}
(xxxxxxxx)
{1yyyyyyyy0}
where y is the logic value complement of x (i.e., if x has logic ‘0’ value, then y is logic ‘1’ value). In SIR, a logic ‘1’ value is inserted before each 8 bits, and a logic ‘0’ value is always inserted after the 8 bits. SIR also suffers from zero minimum run length limit and a high variable duty cycle, as in MIR and FIR discussed above.
A fourth exemplary encoding scheme is the “DASK” scheme in certain implementations by Sharp Corporation. DASK provides a code rate of {fraction (8/10)}, a run length limit of (0,9). DASK uses a sub-carrier frequency of 500 KHz and a high variable maximum duty cycle of 90%×50%. The code dictionary for DASK is as follows:
(Data)
{Symbol}
(xxxxxxxx)
{1yyyyyyyy0}
where y is the logic complement of x. DASK suffers the same disadvantage as SIR discussed above.
A fifth exemplary encoding scheme is the “IrBUS” or “16PSM” encoding scheme, which provides a code rate of 4/8, a run length limit of (0,10) and a subcarrrier of 1.5 MHz, and providing a high variable maximum duty cycle of 50%×50%. The code dictionary for IrBus is as follows:
(Data)
{Symbol}
(0000)
{10100000}
(0001)
{01010000}
(0010)
{00101000}
(0011)
{00010100}
(0100)
{00001010}
(0101)
{00000101}
(0110)
{10000010}
(0111)
{01000001}
(1000)
{11110000}
(1001)
{01111000}
(1010)
{00111100}
(1011)
{00011110}
(1100)
{00001111}
(1101)
{10000111}
(1110)
{11000011}
(1111)
{11100001}
IrBus is designed to avoid interference with the 38 KHz sub-carrier used by other equipment. However, IrBus also suffers from the zero minimum run length limit, and the variable duty cycle with a 50% maximum in the first factor.
A sixth exemplary encoding method is “RC-5” or “Bi-phase”, which has a code rate of ½, a run length limit of (0,2), a sub-carrier of 36 KHz, and a fixed duty cycle of 50%×50%. The code dictionary for RC-5 is as follows:
(Data)
{Symbol}
(0)
{01}
(1)
{10}
RC-5 also suffers from the zero minimum run length limit and a high duty cycle.
Other encoding methods have data-dependent transmission time, or use complex algorithms, such that they are either unsuitable or uneconomical for many applications. For example, a proposed “IrDA VFIR” or “HHH(1,13)” method has a high code rate of ⅔, a run length limit of (1,13). IrDA VFIR uses a single pulse approach with a pulse width occupying the full time slot, to provide a variable maximum duty cycle of 33%×100%. Under IrDA VFIR, a linear feedback shift register first scrambles the data bits, and then encodes every 2 bits of the scrambled data into three consecutive time slots. The IrDA VFIR uses a symbol dictionary which includes the symbols: {000}, {001}, {010}, {100}, and {101}. A state machine whi
Wang Lichen
Yeh Keming W.
ActiSys Corporation
MacPherson Kwok & Chen & Heid LLP
Tse Young T.
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