Zhizhang Chen

Experimental studies of the 2.4-GHz ISM wireless indoor channel

Experimental results from indoor measurements of the 2.4-GHz wireless channel are presented. These measurements include both quasi-static and time-varying channel conditions in a number of indoor environments. Four channel propagation characteristics of interest are evaluated. For the time-varying environment, the Doppler spread of the wireless channel is measured. For the quasi-static environment, mean excess delay, rms delay spread, and coherence bandwidth of the channel are determined. The effect each of these propagation characteristics has on multiple antenna wireless systems and their suitability for indoor use is discussed.

RF channel modeling of a WSN testbed for industrial environment

Industrial applications of wireless sensor networks often involve radio-harsh environments. These industrial settings typically include an abundance of metallic structures which obstruct, reflect, refract and scatter signals. As a result, RF channel modeling for these environments becomes necessary and important. In this paper, a WSN testbed emulating realistic industrial environments was constructed and developed. With it, RF channel propagation properties were measured and analyzed in 2.4GHz and 5.8GHz ISM bands using vector network analyzer.

A direct down-conversion receiver for coherent extraction of digital baseband signals using the injection locked oscillators

In todays digital wireless communications direct down-conversion receiver is becoming a favorite choice due to its overall simplicity and low cost. However, carrier recovery of the received digitally modulated RF signal for clear IQ signal extraction at high symbol rates is not so simple. Until now, phase-locked loop (PLL) and/or direct-digital synthesizers (DDS) or combination of both have been used for better performance but at the expense of high complexity or cost. In this paper for the first time we propose a direct down- conversion receiver that uses an injection-locked oscillator (ILO) for carrier recovery and frequency synthesizing. The results are shown for carrier recovery of a 2.1 GHz 20 Msps 8 PSK signal and coherent demodulation of a 2.78 GHz 160 Msps QPSK signal. The receiver presented is of low-cost and low-complexity and applicable to any kind of digitally modulated signals reception. It can be used in many wireless applications as WiMax, RFID and sensor networks.

A direct RF-undersampling retrodirective array system

A retrodirective antenna system is a smart antenna system where signals received from a source are automatically sent back in the direction of the source. Undersampling (also named bandpass sampling) is a signal processing technique with which an RF signal is directly sampled at a rate related to its bandwidth rather than its highest frequency. In this paper, we incorporate the undersampling technique into a retrodirective antenna system, resulting in a new and simplified smart RF transceiving architecture: no analog RF mixers and local oscillators for frequency down-conversion are required. This has lead to a significant reduction in the number of the RF components required and therefore in the cost of the retrodirective system. Both simulation and experiments are presented and they show the effectiveness of the proposed design.

Injection-Locked Phase-Locked Loop for BPSK Coherent Demodulation: Theory and Design

In this paper for the first time the use of injection-locked phase-locked loop (ILPLL) for the BPSK to ASK coherent conversion is explored. Two circuit architectures are proposed, one using two second-harmonic ILPLL (SH-ILPLL) and the other using a single SH-ILPLL. In the case of two SH-ILPLL, feedback loop amplifiers are used and an increase of 20 times in the locking range of the converter is achieved. In the case of the single SH-ILPLL, direct BPSK to ASK conversion is also achieved but found with smaller locking and dynamic ranges. However, it is very simple and easy to develop. With the addition of an envelop detector, both circuit architectures can be used as a complete and compact RF front-end receiver that can direct demodulate digital RF/microwave signals without many intermediate components and devices. Such a low-cost and low-complexity receiver can be employed in many wireless applications such as RFID and sensor networks.

A study of retrodirective array performance in the presence of multipath

The paper attempts to analyze the performance of a heterodyne phase conjugate retrodirective transceiver for full duplex communications in the presence of multipath signals and amplitude and phase perturbations across a four element antenna array. Simulation results are given and it is shown that the retrodirective array improves multipath fading in a full duplex scheme even under amplitude and phase imperfections across the array.

Retrodirective array using direct down-conversion and fixed point DSP for duplex digital communications

Adaptive and multi-antenna systems will be an integrated part of the future wireless world. In this paper, a novel low cost implementation of a self phased retrodirective array using direct RF conversion and coherent digital modulation in the uplink and downlink is presented. The transceiver is designed to receive and demodulate digital signals of arbitrary modulation type up to 100 ksymbols/s while at the same time directs its main beam back in the direction of the source using any vector digital modulation scheme at speeds up to 20 ksymbols/s. A direct conversion receiver is employed here in combination with a low-power fixed point digital signal processor, eliminating the need for an IF modulator and carrier recovery unit, this makes the design particularly attractive for a commercial application. Measured results of the uplink antenna pattern are presented and show the system has good retro directivity with beam pointing error less than three degrees in the angular range of -45 degrees to +45 degrees.

Finite-resolution digital receiver for high rate ultra-wideband weighted-transmitted reference system

Having adequate robustness to variations of both weight coefficient and integration interval, the weighted-transmitted reference (WTR) system can achieve a superior performance and support high data rate ultra-wideband (UWB) communications as well. To benefit from these excellent advantages without using ultra-wideband analog delay lines, in this paper, a finite-resolution digital receiver is proposed. The performance of the proposed receiver is evaluated at a data rate of 114Mbps averaged over 100 realizations of IEEE 802.15.3a channel model 1 (CM1) for different quantization levels of finite-resolution analog-to-digital converters (ADCs) operating at Nyquist rate, taking into account noise, inter-path/pulse interference (IPaI/IPI), and inter-symbol interference (ISI). The effects of sampling rates, number of reference pulses, and the ADC threshold(s), weight coefficient, and summation interval on the receiver performance are also investigated. The evaluation results show that the proposed receivers using finite-resolution ADCs only suffer a performance degradation of several dB as compared to the full-resolution ADC and analog receivers. Therefore, employing finite-resolution ADCs could be a promising approach to implement the WTR receivers without using ultra-wideband analog delay lines.

Performance analysis of code-shifted reference UWB radio

In this paper, the bit-error rate (BER) performance of the code-shifted reference (CSR) transceiver for impulse radio ultra-wideband (IR-UWB) systems is analyzed theoretically, and the validation of the theoretical analysis results are verified with computer simulations. Furthermore, by comparing the BER performance of the CSR, TR and FSR transceivers, this paper shows that as a low complexity transceiver employing no delay element and no analog carrier, the CSR transceiver can achieve the same BER performance as the TR transceiver and much better BER performance than the FSR transceiver.

Experimental Study of the Retrodirective Antenna Array System in a Multipath Environment

The concept of retro directive arrays was originated decades ago and various system architectures associated with it have been studied and developed in the past several years. However, the focus of the research so far has been on implementation algorithms and potential applications; although the retrodirective systems have been proposed for uses in communications, little has been carried out in evaluation of its communication performance metric. In this work, we first developed the software radio based retrodirective system and then placed it under test for its communication performance in an indoor environment that is rich of multipath. The test results demonstrate for the first time that the retrodirective system has superior performance in the two important performance parameters, the error-vector- magnitude (EVM) and bit-error-rate (BER). In a non-line-ofsight environment, the BER performance of a four-element retrodirective system can be three decades over that of a single antenna element system and two decades better than that of a conventional non- retro directive array system with the same number of antenna elements.