Systems and methods for providing adaptive pulse position modulated code division multiple access for ultra-wideband communication links
View Patent ↗An adaptive pulse position modulated CDMA scheme for use in UWB communications systems is disclosed. A stream of input bits is encoded with spreading code sequence. The spreading code sequence consists of a number of code elements that may have one of two values. Each bit is encoded into a symbol consisting of a predefined number of chips, which are transmitted during a symbol period. Next, at least one code element is associated with each chip in the symbol. Then, an encoded pulse is generated in each chip. Each encoded pulse has a value determined by multiplying the data bit with the code value associated with the particular chip. The encoded pulse is then pulse position modulated by shifting the position of the pulse to a first position or a second position within the chip depending upon the encoded value of the pulse.
1. A method of modulating a communications signal within an ultra-wideband (UWB) communications system, comprising:
receiving a plurality of bits in an input data stream;
receiving at least one spreading code sequence comprising a plurality of code elements having either a first value or a second value;
encoding each bit of the stream of input data bits into a symbol having a finite symbol period, and comprising a plurality of chips, wherein each chip has a chip period;
associating at least one code element with each chip in the symbol;
generating an encoded pulse for each chip of the symbol, wherein each encoded pulse has a value equal to the product of a bit and the code value associated with the particular chip; and
modulating the position of the encoded pulse within each chip by placing the pulse at a first position if the encoded value associated with the pulse is a first value and placing the pulse at a second position if the encoded value is a second value, wherein the first position and second position are separated by a time shift equal to one-half of a chip period, the period being greater than a delay spread associated with the Ultra wideband (UWB) communications system.
2. The method of claim 1 , wherein the spreading code sequence is an orthogonal code sequence.
3. The method of claim 1 , wherein the spreading code sequence is a pseudorandom code sequence.
4. The method of claim 3 , wherein the time shift is a predefined time shift other than one-half a chip period.
5. A transmitter for transmitting a pulse position modulated spread spectrum signal in a communications system, comprising:
a spreading code sequence generator for generating at least one spreading code sequence, each spreading code sequence comprising a plurality of elements having either a first value or a second value;
a spread spectrum encoder, operable for:
receiving a stream of input data bits;
receiving at least one spreading code sequence from the spreading code generator; and
encoding each bit of the stream of input data bits into a symbol having a finite symbol period, and comprising a plurality of chips, wherein each chip comprises a chip period;
associating at least one code element with each chip in the symbol; and
generating an encoded pulse in each chip of the symbol, wherein the encoded pulse has a value equal to the product of the bit and the code value associated with the particular chip;
a pulse position modulator operable for modulating each pulse by shifting the position of the pulse within each chip by placing the pulse at a first position if the encoded value associated with the pulse is a first value amd placing the pulse at a second position if the encoded value is a second value, wherein the first position and second position are separated by a time shift eciual to one-half of a chip period, the period being greater than a delay spread associated with an Ultra wideband (UWB) communications system; and
an antenna for transmitting the pulse position modulated signal over an ultra-wideband communications channel.
6. The transmitter of claim 5 , wherein the spreading code sequence is an orthogonal code sequence.
7. The transmitter of claim 5 , wherein the spreading code sequence is a pseudorandom code sequence.
8. The transmitter of claim 5 , wherein the time shift is a predefined time shift other than one-half a chip period.
9. A receiver for receiving a pulse position spread spectrum signal in an ultra-wideband (UWB) communications system, the pulse position modulated spread spectrum signal comprising a plurality of symbols, each symbol representing one data bit and consisting of a plurality of chips, each chip having a chip period and each chip period containing an encoded pulse, comprising:
a demodulator operable to demodulate the symbols in the pulse position modulated spread spectrum signal by detecting the position of the encoded pulse within each chip period to determine whether the encoded pulse is located at a first position or a second position, wherein the first position and second position are separated by a time shift eQual to one-half of the chip period, the period being greater than a delay spread associated with the ultra wideband (UWB) communications system; and
a spread spectrum decoder operable for: receiving at least one spreading code sequence; and decoding each symbol by combining the appropriate code value of the spreading code sequence with a value associated with the position of the encoded pulse in each chip of the symbol to extract the data bit.
10. The receiver of claim 9 , wherein the spreading code sequence is an orthogonal code sequence.
11. The receiver of claim 9 , wherein the spreading code sequence is a pseudorandom code sequence.
12. The receiver of claim 9 , wherein detecting the position of the encoded pulse within each chip period comprises determining a multi-level value of the likelihood that the pulse is located in either the first position or the second position within each chip period.
13. The receiver of claim 12 , wherein the time shift is a predetermined time shift other than one-half a chip period.
14. A method of receiving a pulse position modulated spread spectrum signal, the pulse position modulated spread spectrum signal comprising a plurality of symbols, each symbol representing one data bit and consisting of a plurality of chips, each chip having a chip period and each chip containing an encoded pulse, comprising:
demodulating the pulse position modulated spread spectrum signal by detecting the position of the encoded pulse within each chip period to determine whether the pulse is located at a first position or a second position, wherein the first position and second position are separated by a time shift equal to one-half of the chip period, the period being greater than a delay spread associated with the ultra wideband (UWB) communications system;
receiving at least one spreading code sequence; and
decoding each symbol by combining the appropriate code value of the spreading code sequence with a value associated with the position of the encoded pulse in each chip of the symbol to extract the data bit.
15. The method of claim 14 , wherein detecting the position of the pulse within each chip comprises determining a multi-level value of the likelihood that the pulse is located in either a first position or a second position within each chip period.
16. The method of claim 14 , wherein the time shift is a predetermined time shift.