Research On Waveform Of Impulse UWB Systems Based On Power Spectrum Shaping

In recent years, Ultra Wideband (UWB) technology has become one of the candidates for short-range high-speed wireless communications with its advantages of spectrum sharing, large capacity, low power consumption, low cost and high multipath resolution, etc. By utilizing a narrow-pulse as its carrier, impulse UWB system occupies an ultra-wide frequency band, which naturally results in a considerable difference with conventional sinusoidal-function-based communication systems in both characteristics and processing methods. The requirements on the pulse of UWB system include no DC component, meeting the FCC frequency mask, adaptively controllable center frequency and bandwidth, orthogonal, suppressing narrow-band interference and higher power spectrum utilization,etc.Firstly, the formulas of power spectrum for IR and DS-IR systems are derived after thorough study. The analysis proves that the whole shape of power spectrum is determined by the spectrum of narrow pulse, but also related to the signal structure, TH code, DS code and the modulation format. The impact of these factors is reflected in code spectrum. As the power spectrum of DS-IR systems almost entirely depends on the spectrum of narrow pulse, obviously, the pulse plays an important role in the power spectrum. The interference between UWB systems and narrow band systems is also analyzed theoretically and the formulas of signal-to-noise ratio at receivers are derived.Secondly, considering the requirements on the pulse of UWB system, several common-used pulses are studied theoretically, such as Gaussian pulses, pulses based on Hermite polynomials and pulses based on Prolate Spheroidal Wave Functions (PSWF), besides, their waveforms and PSD are simulated. The 5th derivative of Gaussian pulse satisfying FCC frequency mask is obtained via optimizing its parameters. Hermite orthogonal pulses are derived by modifying Hermite polynomials, however, these pulses have DC component and can not meet FCC frequency mask immediately. PSWF double-orthogonal pulses obtained by numerical solution can meet FCC frequency mask directly, and have adaptively controllable center frequency and frequency band. Based on PSWF pulses, the mutual interference with narrowband systems at random frequency can also be suppressed by spectrum avoiding and notch.Finally, to solve the problem of spectrum utilization, the normalized effective signal power (NESP) of PSWF pulse is improved via optimizing parameters. Besides, the idea of designing composite pulse is proposed to obtain pulses with higher NESP value. Based on the 1st derivative of Gaussian pulse, two kinds of composite pulses with high NESP values are derived via minimum mean square error principle and semi definite programming (SDP) arithmetic. Pulse with notch at UNII band is designed using the SDP arithmetic, whose NESP value is improved obviously compared to PSWF pulse. Finally, four orthogonal pulses with high NESP values are derived based on Walsh orthogonal code and SDP arithmetic.