Amir Dezfooliyan

Evaluation of Time Domain Propagation Measurements of UWB Systems Using Spread Spectrum Channel Sounding

Spread spectrum sounding experiments for indoor wireless channel over a frequency band spanning 2-12 GHz, which exceeds the full FCC UWB band, are reported. Experiments were carried out for directional spiral antennas in line-of-sight (LOS) and omnidirectional biconical antennas in nonline-of-sight (NLOS) environments, in the latter case with up to 15 m antenna separation. For biconical antennas, channel multipath dispersion is the main reason for the delay spreads, while for spirals, frequency-dependent delays of the antennas modify signals upon radiation and reception. A special emphasis of this work is to assess the accuracy of the measured impulse responses extracted from the received waveforms by deconvolution in a typical indoor environment. In one scheme to assess accuracy, impulse responses obtained using different spread spectrum waveforms are compared; independent measurements show excellent agreement, with correlation coefficients about 0.99. In a second assessment approach, accuracy is tested by performing time reversal experiments based on the measured channel impulse response. Correlation coefficients between experimental and theoretical time-reversal traces are on the order of 0.98, which further confirms highly accurate measurements.

Photonic synthesis of high fidelity microwave arbitrary waveforms using near field frequency to time mapping.

Photonic radio-frequency (RF) arbitrary waveform generation (AWG) based on spectral shaping and frequency-to-time mapping has received substantial attention. This technique, however, is critically constrained by the far-field condition which imposes strict limits on the complexity of the generated waveforms. The time bandwidth product (TBWP) decreases as the inverse of the RF bandwidth which limits one from exploiting the full TBWP available from modern pulse shapers. Here we introduce a new RF-AWG technique which we call near-field frequency-to-time mapping. This approach overcomes the previous restrictions by predistorting the amplitude and phase of the spectrally shaped optical signal to achieve high fidelity waveforms with radically increased TBWP in the near field region.

Experimental Investigation of UWB Impulse Response and Time Reversal Technique Up to 12 GHz: Omnidirectional and Directional Antennas

An experimental study of the time reversal (TR) technique is presented in a single-input-single-output configuration over the frequency range of 2-12 GHz. A special emphasis of this work is to investigate and compare impulse response (IR) and TR characteristics for omnidirectional biconical and directional spiral antennas over realistic indoor ultrawideband (UWB) channels in both line-of-sight (LOS) and non-line-of-sight (NLOS) environments. We discuss the effects of channel multipath dispersion and antenna frequency-dependant delay distortions on the received responses in both time and frequency domains. The effectiveness of TR for waveform compression is characterized by computing root mean square delay spread and peak-to-average power ratio. Our study suggests that the effectiveness of time reversal is subject to a tradeoff between competing effects-namely, compensation of spectral phase variation (which leads to compression) and aggravation of spectral amplitude structure (which opposes compression). Although TR is a powerful technique for compensation of phase distortions associated with broadband frequency-independent antennas (as shown in LOS experiments with spiral antennas), it shows only modest performance in compressing time spread associated with multipath delays.

Photonic Synthesis of Spread Spectrum Radio Frequency Waveforms With Arbitrarily Long Time Apertures

A photonic technique is proposed to generate spread spectrum radio frequency waveforms with both high RF bandwidth and arbitrarily long temporal period. By switching the polarity of full duty cycle chirped waveforms according to a pseudorandom sequence, we can increase the waveform repetition period under electronic control while preserving RF bandwidth and average power. Proof-of-principle ranging experiments are presented to demonstrate the improvement our technique provides with respect to range ambiguity.

Phase compensation communication technique against time reversal for ultra-wideband channels

Phase compensation (PC) prefiltering is experimentally investigated for multipath channels over the frequency band spanning 2–12 GHz, a topic which to the best of the authors knowledge has not been studied in the literature on ultra-wideband (UWB) communications. The authors emphasis is to assess the capabilities of PC compared to time reversal (TR) prefilters over indoor UWB channels regarding multipath suppression, channel hardening, noise sensitivity and high-speed data transmission. Experiments were carried out for PC and TR prefilters in both line-of-sight (LOS) and non-LOS (NLOS) environments. The multipath compression effectiveness is characterised by computing the root-mean-square delay spread and peak-to-average power ratio for actual measured channels and for the IEEE 802.15.4(a) UWB model. The authors study suggests PC outperforms TR considerably in mitigating the multipath channel dispersion. Bit-error-rate (BER) curves have been simulated for data rates in the range of 125–4000 Mbps based on the measured channel responses. The BER simulations suggest that while the TR performance is prohibitively saturated by its residual intersymbol interference for data rates of 500 Mbps and above (especially in NLOS), PC can be used for high-speed data transmissions as fast as 2 Gbps in both LOS and NLOS environments.

Biphoton manipulation with a fiber-based pulse shaper

We demonstrate spectral shaping of entangled photons in the telecom band with a programmable, fiber-based optical filter. The fine-resolution spectral control permits implementation of length-40 Hadamard codes, through which we are able to verify frequency anticorrelation with a 20-fold increase in total counts over that permitted by the equivalent pair of monochromators at the same input flux. By programming the complex spectral transmission function corresponding to a Mach-Zehnder interferometer, we also construct variations on Franson interferometers that are free from mechanical instabilities, demonstrating spectral phase independence in the slow-detector limit, in which all temporal features are unobservable. Our configuration furnishes a single, compact arrangement for manipulating telecom biphotons and characterizing their quality.

Microwave photonics for space–time compression of ultrabroadband signals through multipath wireless channels

We employed photonic radio frequency (RF) arbitrary waveform generation to demonstrate space-time compression of ultrabroadband wireless signals through highly scattering multipath channels. To the best of our knowledge, this is the first experimental report that explores an RF-photonic transmitter to both characterize channel dispersions in real wireless environments and generate predistorted waveforms to achieve focusing through the multipath channels. Our experiments span a three octave frequency range of 2-18 GHz, nearly an order of magnitude beyond the ~2 GHz instantaneous bandwidth reported in previous spatiotemporal focusing experiments relying on electronic waveform generators.

UWB propagation measurement using spread spectrum channel sounding

We present a new approach for time domain measurements of ultra-wideband channels by using spread spectrum sounding. In this paper, this technique is investigated for non-line-of-sight omni-directional antennas over the frequency range of 3–11.5GHz with ∼15m propagation distance. A special emphasis of our work is to assess the accuracy of our channel response measurements. In one scheme, accuracy of our method is studied by comparing channel responses obtained using different wideband waveforms including pseudo-random (PN) and chirp signals. The independently measured channel responses show correlation coefficient better than 0.99, which proves the excellent accuracy of our method. In another route, time reversal (TR) experiments have been carried out; the results are compared with TR simulations and show excellent agreement.

Experimental Test-Bed for Studying Multiple Antenna Beamforming over Ultra Wideband Channels up to 12 GHz

An experimental setup is designed and implemented to study the performance of transmit beamforming techniques over realistic UWB Multiple-Input Single-Output (MISO) configurations. This setup can be used to investigate practical issues which are not usually addressed in simulation models, such as imperfect channel estimation or coherency between the multiple transmitters. Our implementation is based on the use of optical delay lines to apply appropriate time shifts to the output of an electrical arbitrary waveform generator (AWG). In this paper we assess the experimental accuracy of our setup over a 4 × 1 MISO system using time reversal beamforming.

Spatiotemporal Focusing of Phase Compensation and Time Reversal in Ultrawideband Systems With Limited Rate Feedback

We investigate the performance of phase compensation (PC) and time reversal pre-equalizers in multipath indoor channels when a quantized version of the channel state information (CSI) is available at the transmitter side. We conduct a comprehensive experimental study to assess their spatial and temporal focusing as a function of the number of quantization levels over the frequency band of 2-12 GHz. To characterize multipath compression performance, root-mean-square (RMS) delay spread and peak-to-average power ratio (PAPR) are calculated for different numbers of quantization levels. Bit error rate (BER) curves have been simulated for data rates in the range of 125-4000 Mb/s based on the measured channel responses. To assess spatial focusing, we investigate the decay of the received response peak power as we move away from the intended receiver. Our study suggests that for the same feedback rate, PC has considerably superior performance in suppressing multipath dispersions and focusing the transmitted energy at the intended receiver.