Square-root Raised Cosine Pulse Research Articles

Widely-Linear Nyquist Criteria for DFT-Spread OFDM of Constellation-Rotated PAM Symbols

In this paper, the Nyquist criteria for ordinary linear modulation are generalized to the discrete-Fourier transform-spread (DFT-spread) orthogonal frequency-division multiplexing (OFDM) of constellation-rotated PAM symbols. Unlike the conventional Nyquist criteria with a linear receiver, the new criteria are to remove intersymbol interference in the output of a widely-linear (WL) receiver. It is shown that not only the folded spectrum, but also the shifted and reverse-folded spectrum is crucial in the criteria, where the shift offset is determined by the rotation angle. As a design example, combinations of a constellation rotation angle and a square-root raised cosine pulse are proposed. It is shown that the proposed design can achieve better performance both in spectral efficiency and in peak-to-average power ratio than the conventional DFT-spread OFDM of π/2-BPSK symbols. The optimal one-tap frequency-domain equalizer is also derived under the WL zero-forcing (ZF) constraint. It is shown that, when the rotation angle is properly chosen, the proposed one-tap WL-ZF equalizer significantly outperforms the one-tap linear ZF equalizer in output signal-to-noise ratio. The proposed equalizer also outperforms the conventional equalizer in error vector magnitude measured at the output of a nonlinear power amplifier.

Experimental Demonstration of a 10 Gb/s Subcarrier Multiplexed WDM PON

We experimentally demonstrate a 10 Gb/s bidirectional subcarrier multiplexed (SCM) wavelength-division multiplexed (WDM) passive optical network (PON). We use digital signal processing and square-root raised cosine pulse shaping techniques to generate M-ary quadrature amplitude modulation electrical signals for the downlink and uplink transmissions using simple intensity modulation and direct detection optoelectronics, and 10 GHz transceivers. The design includes spectral preemphasis and offset optical filtering to increase the effective bandwidth of a commercial 2.2 GHz reflective semiconductor optical amplifier based optical network unit. We realize symmetric 10 Gb/s transmission over a 20 km single feeder PON with optical line terminal launch powers from 1 to 9 dBm. We also demonstrate the ability of these SCM WDM PONs to withstand the effects of upstream impairments.

Effect of carrier frequency offset and symbol timing offset on the performance of FMT over time-frequency selective channels

This paper presents the effects of carrier frequency offset (CFO) and symbol timing offset (STO) in filtered multitone (FMT) systems over time and frequency selective multipath fading channels. The analysis is based on an FMT system using a square-root raised cosine pulse. The performance of FMT systems in the presence of CFO and STO over different fading channels are derived and compared with that of orthogonal frequency division multiplexing systems. The offsets degrade the performance of the FMT systems because they cause filter mismatch. The CFO changes the amplitude frequency characteristic of the receiver filter, while the STO changes the phase frequency characteristic of the receiver filter. The simulations are conducted over different fading channels. To achieve signal-to-interference ratio above 20 dB, over a flat static channel, the frequency offset relative to symbol duration must be less than 5% in the orthogonal frequency division multiplexing system, while the upper limit is 8% in the FMT system. The simulation results also show that the FMT system is less sensitive to CFO in time frequency selective channels. The signal-to-interference ratio expressions help predict the bit-error-rate performance of FMT with single tap subchannel equalization. The analysis also helps synchronization algorithm design after analyzing the synchronization offset effect in FMT systems under certain channel conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Analysis and optimization of ds-cdma systems with time-limited partial response chip waveforms

Conventional direct sequence code division multiple access systems (DS-CDMA) using offset quadrature phase shift key (OQPSK) usually employ a strictly bandlimited partial response square-root raised cosine pulse as the chip waveform. They have the disadvantage of large envelope fluctuation that will incur performance degradation due to the intermodulation and bandwidth enlargement caused by post nonlinear processing. To improve the performance of DS-CDMA systems, the chip waveform and receiver should be properly selected. This paper presents a systematic performance analysis of a matched filter receiver and zero-forcing filter (ZF) receiver for DS-CDMA using a time-limited partial response chip waveform. Nevertheless the systematic performance analysis is applicable to bandlimited chip pulse as well. For the zero-forcing filters, we propose to select the frequency responses that satisfy the first Nyquist criterion. With this class of filters, we can choose the roll-off factor to minimize the total power of multiple access interference and noise power. The zero-forcing filter with proper choice of roll-off factor, referred to as optimum ZF, yields a performance better than the matched filter counterpart. The bit error rate (BER) performance of the optimum ZF with superposed quadrature amplitude modulation signal as the time pulse waveform is evaluated. It is shown that the optimum ZF provides better BER performance than conventional OQPSK and minimum shift keying, and its envelope uniformity is much better than that of OQPSK.