Communications systems employ various means of allowing multiple users to send data streams within an allocated portion of the communications frequency spectrum in a shared manner. These means include time-division multiple access (TDMA); frequency-division multiple access (FDMA); orthogonal frequency division multiplexing (OFDM), with carrier sense multiple access (CSMA); and code division multiple access (CDMA). Performance of these systems can be limited by the canonical algorithmic approaches for frequency spectral analysis and temporal correlation analysis/matched filtering.
Major advances in communications, especially related to cellular telephone systems, have been achieved as a result of spread-spectrum, Code Division Multiple Access (CDMA).
FIG. 1 shows a known generalized CDMA system for explanatory purposes. A binary information sequence of “1s” and “0s” (data bits) is converted to phase-shift forms, e.g., a “1” corresponds to no phase shift, and a “0” corresponds to a 180-degree phase shift. This is referred to as binary phase shift keying (BPSK), although other methods that use more phase shifts and multiple amplitudes, such as QPSK or QAM-64, could be used. These phase shifts can be applied with a particular keying rate to an intermediate frequency subcarrier signal. Next, orthogonal or quasi-orthogonal binary codes (e.g., Walsh Codes and/or Pseudo-Noise PN Codes) are applied in a similar fashion to channelize signals (especially via the Walsh code) and to spread the signal spectrally across a much wider bandwidth than the signal itself requires (e.g., 1.25 MHz for 3GPP2 specifications, and 5 MHz for 3GPP specifications). The signal is modulated onto a radio-frequency carrier signal, amplified, and fed to a transmitting antenna system. Each code is assigned to a different communications channel. With proper orthogonality of the codes, each channel can occupy the same frequency space 100% of the time and still be decoded and despread by receivers. At the receiving end, the RF carrier is demodulated and code correlated with a correlator or a matched filter to despread the signal, leaving an information sequence of 1's and −1's that are re-interpreted as binary ones and zeros. While shown with two steps of frequency conversion, often one step (direct conversion) is used from baseband to RF and vice versa.
USPTO Backgrounds
USPTO Backgrounds