Code shift keying impulse modulation for ultra wideband communications

Ultra wideband impulse radio (UWB-IR) is a promising technology that has been recently investigated for high-rate data communications and low-rate location and ranging systems. Despite its many advantages including low-cost circuitry, use of unlicensed spectrum and multipath resolving capabilities, the current implementation of UWB-IRs exhibits some problems. Firstly, the conventional UWB-IR modulation format, pulse position modulation (PPM), uses consecutive fixed pulse transmit locations that are largely affected by multipath-delayed pulses, which degrades the system performance. Secondly, increasing the data rate of the most commonly used binary PPM (BPPM) format is still an open research problem. Thirdly, periodic time-hopping (TH) codes used for multiple-access generate undesired peaks for the spectrum, which may cause increased interference levels for the systems co-existing in the same bandwidth. Considering above mentioned drawbacks of the current UWB-IRs, a new modulation format is proposed in this dissertation: Code Shift Keying (CSK) Impulse Modulation. M-ary CSK (MCSK), which can be employed by itself or combined with other UWB-IR modulations, achieves data transmission by selecting one of the M orthogonal user-specific TH codes, where these TH codes are used to randomize pulse transmit locations. While the randomized pulse transmit locations reduce the effect of multipath-delayed pulses on the system performance, the random selection of TH codes increases the effective TH code period and suppresses the undesired peaks in the spectrum. In this dissertation, CSK impulse modulation is studied in detail to quantify its performance improvement over the existing systems. Initially, MCSK is compared to M-ary PPM (MPPM) to show the advantage of randomizing pulse transmit locations over fixed locations. It is shown that MCSK can provide about 2dB performance gain over MPPM in a multipath channel. Secondly, MSCK is combined with BPPM (i.e., MCSK/BPPM) in order to increase the data rate of conventional TH-BPPM systems. It is shown that MCSK/BPPM can achieve improved system performance at increased data rate if system design parameters are properly selected. Finally, power spectral density (PSD) characteristics of MCSK based IRs are studied. It is shown that the random selection of TH codes improves the resulting PSD with respect to that of conventional UWB-IRs.