Mehrdad Mirshafiei

All-Optical 500-Mb/s UWB Transceiver: An Experimental Demonstration

We propose and demonstrate experimentally, for the first time, a prototype for all-optical ultra-wideband (UWB) transceiver at 500 Mb/s. We report 1) UWB pulse optimization that takes into account the transmitter RF front end and the U.S. federal communications commission (FCC) spectral mask, 2) a new approximate match filter receiver using optical signal processing, and 3) modulation at 500 Mb/s. Our previous optimization of UWB pulse shapes was based only on the FCC spectral mask, without taking into account the frequency response of the RF components (amplifier and antenna) in the UWB transmitter. Here, we modify our pulse optimization technique to ensure that the equivalent isotropic radiated power (EIRP) from the transmitter meets FCC specifications. For the RF hardware used, we achieve 63.6% efficiency over the FCC mask, which yields an 11.6- and a 5.9-dB improvement over Gaussian monocycle and doublet pulses, respectively. We also introduce simple optical signal processing at the receiver that allows the incoming RF signal to be matched against a square pulse whose duration is adapted to the channel. The exact matched filter would require a new optimized pulse that would include not only hardware frequency response but channel effects that vary with antenna placement as well. The proposed approximation allows a simple variation of the pulse duration: an increase to account for pulse expansion in the channel but an upper limit to combat multipath effects. Finally, we demonstrate the optimized pulse and approximate match filter receiver at 500 M/s. We attain a 10-6bit error rate at a 65-cm separation line of sight (LOS) link with simple on-off keying and no forward error correction.

Generation of Power-Efficient FCC-Compliant UWB Waveforms Using FBGs: Analysis and Experiment

In this paper, we design, analyze, and demonstrate experimentally U.S. Federal Communications Commission (FCC)- compliant power-efficient ultrawideband (UWB) waveforms generated by optical pulse shaping. The time-domain pulse shape is written in the frequency domain, and a single-mode fiber performs the frequency-to-time conversion. The waveform is inscribed in the frequency domain by the fiber Bragg grating (FBG). A significant challenge for this approach is elimination of an unwanted, positive rectangular pulse superimposed on the desired waveform. Our innovative use of balanced photodetection eliminates this pedestal, assuring compliance with the FCC mask at low frequency. Three UWB pulses with duration of 0.3,0.6, and 1.2 ns are designed and tested experimentally. Whereas an excellent match between the optimized and measured pulses is achieved for the simpler, shorter duration waveforms, the noise in the fabrication process of FBGs limits the generation of the more complex, longer duration waveforms.

Ultra-Wideband Waveform Generator Based on Optical Pulse-Shaping and FBG Tuning

We propose and demonstrate experimentally a prototype for ultra-wideband (UWB) waveform generator based on optical pulse shaping. The time-domain pulse shape is written in the frequency domain, and a single-mode fiber performs frequency-to-time conversion. A U.S. Federal Communications Commission (FCC)-compliant power efficient pulse shape is inscribed in the frequency domain by a fiber Bragg grating (FBG) with an excellent match between optimized and measured pulses. Two other popular UWB pulse shapes (Gaussian monocycle and doublet pulses) are achieved by proper tuning of two FBG-based variable optical filters. A balanced photodetector removes an unwanted rectangular pulse superimposed on the desired waveform, assuring compliance at low frequency.

A Silicon Modulator Enabling RF Over Fiber for 802.11 OFDM Signals

We investigate the linearity properties of silicon modulators and show that, contrary to the traditional lithium niobate Mach-Zehnder modulators (MZMs), the third-order intermodulation distortion (IMD3) for silicon modulators is a function of the modulator bias point. The bias point for silicon modulators can be chosen to reduce the IMD3 well below that of standard lithium niobate MZMs. Given the cost and integration advantages of the silicon photonics technology, silicon modulators offer significant advantages for emerging radio over fiber applications. As an example, we examine, for the first time to our knowledge, a silicon modulator for converting analog 802.11 RF signals to the optical domain, achieving an error vector magnitude of -30 dB.

Ultra-wideband pulse shaping: bypassing the inherent limitations of the Gaussian monocycle

UWB impulse radio transmissions are power limited as they must respect a frequency mask with low total permitted power and severe isolation requirements for the GPS band. Shaping of the short UWB impulse maximizes transmit power subject to these constraints. We have previously published results for intricate pulse shaping in the optical domain that achieves record levels of power transmission. The most successful pulse shaping techniques in the electrical domain have been achieved with derivatives of the Gaussian pulse shape. The Gaussian monocycle is a low performance choice in terms of transmit power, but nonetheless the subject of intense research activity. In this paper we compare and contrast these UWB pulses, and quantitatively compare their performance in realistic systems (specific RF and antenna hardware). There is the perception that failings in the pulse shape (especially for the monocycle) can be compensated by simple highpass filter and the bandpass nature of the UWB antenna. We fabricate and characterize two UWB antenna designs available in the literature, as well as one commercial antenna. We use our optical pulse shaping device to experimentally investigate the three different pulse shapes and measure their transmitted power spectral density with each of the UWB antennas. We find that the monocycle is significantly less powerful than the others. The fine pulse shaping available in optical processing provides 1.7 to 2.9 dB gain over the electrical processing methods, depending on the antenna used.

Pulse Shapes That Outperform Traditional UWB Antenna/Waveform Combinations

Traditionally the Gaussian monocycle pulse and its higher derivatives have been proposed and deployed as impulse radio ultra-wideband (IR-UWB) pulses. Although relatively easy to generate in electronics, these pulses are ill-adapted to the sharp cutoffs in the US Federal Communications Commission (FCC) mask. The combination of these pulses with passband UWB antennas with steep roll-off improves power efficiency vis-a-vis the FCC mask. This approach is still quite sub-optimal and pulse shaping can provide marked improvement (to 3 dB) over the best traditional combinations. We show that optimal design of UWB waveforms, when taking into account antenna gain profiles, improves the power efficiency of the pulses. Three typical antennas are considered. A nonlinear optimization process is used to design an efficient pulse for each antenna. The proposed optimization is based on the hybrid genetic algorithm and a sequential quadratic program. We demonstrate that this method finds efficient pulses under severe antenna distortion. Simulation results confirm that the optimally designed pulses have superior performance compared to the more common Gaussian monocycle and the Gaussian fifth-derivative pulse.

Wideband antenna EIRP measurements for various UWB waveforms

We experimentally investigate pulse shaping in the optical domain of ultra-wideband pulses and RF transmission via a wideband antenna. The effective isotropically radiated power (EIRP) is calculated based on the measured transmit power; the resultant power spectral density (PSD) is compared to the FCC (US Federal Communications Commission) power spectral mask for each of three generated pulse shapes: a Gaussian monocycle, a Gaussian doublet and a waveform adapted to the FCC spectral mask.

Upconversion of Gain-Switched Laser Pulses for Optical Generation of UWB Signals

We propose and experimentally demonstrate a simple, low-cost optical ultra-wide band (UWB) pulse generation method. A gain-switched laser pulse is dispersed by optical fiber propagation and upconverted to the center of the US Federal Communications Commission (FCC) mask using an optical intensity modulator. The generated UWB pulses comply with the FCC mask with relatively high power efficiencies. We also show that the optical fiber can be replaced with a fiber Bragg grating filter to make the transmitter compact. Wireless propagation of the UWB impulses is investigated by offline signal processing. A linear receiver and an energy detector are implemented. The linear filter shows a better BER performance, particularly in worse channel conditions. We reach the FEC limit of 10-3 after 3.5 m of wireless propagation at 856 Mb/s.

Optical UWB Waveform Generation Using a Micro-Ring Resonator

Optical generation and transport of ultra-wideband (UWB) pulses extends the reach of these weak signals to several kilometers by exploiting passive optical networks. At the remote node, the UWB transmitter should be low cost and compact in size for indoor communications. We propose and experimentally demonstrate an integrated solution based on a racetrack micro-ring resonator in silicon-on-insulator technology. The resonator filters pulses generated by a gain-switched laser. Frequency-to-time mapping is obtained from fiber propagation. Balanced detection of the pulses ensures FCC-compliant UWB pulse generation.

Equalizer complexity/performance trade-offs for high data-rate IR-UWB linear receivers in multipath channels

We investigate bit rates above 500 Mb/s rate for impulse radio ultra-wideband (IR-UWB) communications. UWB channels exhibit rich multipath leading to intersymbol interference (ISI) at these bit rates. Previous investigations studied decision feedback equalizers (DFE) for moderate bit rates (100 Mb/s and lower).We examine the effectiveness of this solution when ISI is more severe, and compare performance and complexity to that of a Viterbi algorithm (VA) equalizer with a limited (suboptimal) number of states. We consider a complete UWB link composed of a pulse transmitter, antennas, true UWB multipath channel measurements, and a linear receiver. We examine equalizer memory requirements for reliable Gb/s IR-UWB transmission for line-of-sight (LOS) channels. Non-line-of-sight (NLOS) channels are also investigated, however at lower speeds. Bit error rate (BER) simulations show that equalization is effective to varying degrees. The trade-offs in complexity vs. performance of the VA versus the DFE are discussed.