Chang-Soon Choi

All-optical signal up-conversion for radio-on-fiber applications using cross-gain modulation in semiconductor optical amplifiers

The authors present a novel scheme of up-converting optical intermediate frequency (IF) signals with an optical local oscillator (LO) signal using cross-gain modulation in a semiconductor optical amplifier. This scheme provides high conversion efficiency and is independent of the incident light wavelength and polarization. It can be useful for radio-on-fiber transmission system applications in which one remote LO signal is provided for several wavelength-division-multiplexing IF signals.

SOA-EAM frequency up/down-converters for 60-GHz bi-directional radio-on-fiber systems

We investigate a frequency up/down-converter based on a single cascaded semiconductor optical amplifier (SOA)-electroabsorption modulator (EAM) configuration for bi-directional 60-GHz-band radio-on-fiber (RoF) system applications. SOA cross-gain modulation and photodetection in EAM are used for frequency up-conversion, and EAM nonlinearity is used for frequency down-conversion. In our scheme, both 60-GHz local-oscillator (LO) signals and IF signals are optically transmitted from a central station to base stations. We characterize the dependence of frequency up/down-conversion efficiencies on EAM bias and optical LO power. For frequency up-conversion, maximum conversion gain of approximately 8 dB was obtained and, for frequency down-conversion, more than approximately 18-dB conversion loss was measured. Utilizing this frequency up/down converter, we demonstrate a 60-GHz bi-directional RoF link. Optically transmitted downlink 10-Mb/s quadrature phase-shift keying (QPSK) data at 100-MHz IF are frequency up-converted to the 60-GHz band at the base station, and uplink 10-Mb/s QPSK data in the 60-GHz band are frequency down-converted to 150-MHz IF and transmitted to the central station. In addition, the dependence of error vector magnitudes on IF signal power and wavelength is investigated.

Characterization of phototransistor internal gain in metamorphic high-electron-mobility transistors

We characterize the phototransistor internal gain of metamorphic high-electron-mobility transistors (mHEMTs). When the mHEMT operates as a phototransistor, it has internal gain provided by the photovoltaic effect. To determine this internal gain, photoresponse characteristics dominated by the photoconductive effect as well as the photovoltaic effect are investigated. When the device is turned off, it acts as a photoconductor, and by calculating photoconductor gain, the primary photodetected power can be determined, which indicates the absorbed optical power. The ratio between this and the photodetected power due to the photovoltaic effect represents phototransistor internal gain. It is demonstrated that the phototransistor internal gain is function of optical modulation frequency.

A design of single carrier based PHY for IEEE 802.15.3c standard

This paper proposes an air interface for ultra high-speed millimeter wave (60 GHz) systems which have been under standardization process at IEEE 802.15.3c: the proposed channel plan permits ease of portable device implementation employing the major crystal oscillator used in CDMA cellar phones. The proposed transmission modes offer much greater scalability, covering from several tens Mbps to over 4 Gbps. The proposed preamble employs Golay codes, providing sufficient robustness against wireless environment being subject to the characteristics of millimeter wave with reduced hardware complexity. Common mode is a novel technique to communicate with single carrier (SC) and OFDM camps with multi rates up to around 1.5 Gbps. This technique gives the proposed systems easy expandability from SC to OFDM or other SCs and vice versa.

Phototransistors based on InP HEMTs and their applications to millimeter-wave radio-on-fiber systems

Phototransistors based on InP high electron-mobility transistors (HEMTs) are investigated for millimeter-wave radio-on-fiber system applications. By clarifying the photodetection mechanism in InP HEMTs, the phototransistor internal gain is determined. We present their use as millimeter-wave harmonic optoelectronic mixers and characterize them at the 60-GHz band. In order to evaluate the InP HEMT optoelectronic mixer performance, internal conversion gain is introduced and a maximum of 17 dB is obtained for 60-GHz harmonic optoelectronic up-conversion. Utilizing them, we construct a 60-GHz radio-on-fiber system and demonstrate 622-Mb/s data transmission over 30-km single-mode fiber and 3-m free space at 60-GHz band.

60-GHz bidirectional radio-on-fiber links based on InP-InGaAs HPT optoelectronic mixers

We demonstrate 60-GHz band bidirectional radio-on-fiber (RoF) links based on InP-InGaAs heterojunction phototransistor (HPT) optoelectronic mixers. They employ remote up/down conversion scheme with optical local oscillator signals distributed from the central office and intermediate frequency (IF) fiber transmission for both up- and down-links. Since frequency up/down conversions and photodetection are carried out by a single HPT optoelectronic mixer, base station architecture is greatly simplified. In order to validate its feasibility, both up- and down-link RoF transmissions of 16 quadrature amplitude modulator data are successfully demonstrated at 60-GHz band using 1.25-GHz IF for down-link and 2.0-GHz IF for up-link.

High optical responsivity of InAlAs-InGaAs metamorphic high-electron mobility transistor on GaAs substrate with composite channels

The high optical responsivity of the InAlAs-InGaAs metamorphic high-electron mobility transistor on GaAs substrate with composite channels is reported. Experimental results verify that the photovoltaic effect causing the effective decrease of threshold voltage is responsible for the photoresponse to a 1.55-/spl mu/m optical illumination.

A Modified SV-Model Suitable for Line-of-Sight Desktop Usage of Millimeter-Wave WPAN Systems

A modified Saleh-Valenzuela (SV) model, which is useful in designing a millimeter-wave WPAN (mmW WPAN) system for line-of-sight (LoS) desktop environments, is proposed. It is demonstrated that a well-known two-path model component is dominantly observed in a typical LoS desktop channel along with the components of the other non-line-of-sight (NLoS) paths. Then, the deterministic two-path model is extended to a statistical two-path model by introducing random variables for the antenna position and merging with the conventional SV-model which is suitable to express NLoS path components. The proposed channel model parameters are extracted in a typical LoS desktop environment, and then the cumulative distribution of total path power gain is discussed using a simulation technique that assumes a specific antenna location scenario. It is shown that the cumulative distribution of total path power gain obtained using the proposed model is useful to estimate the outage probability of the mmW WPAN link.

70-GHz-Band OFDM Transceivers Based on Self-Heterodyne Scheme for Millimeter-Wave Wireless Personal Area Network

70-GHz-band orthogonal frequency-division multiplexing (OFDM) transceivers were developed by combining self-heterodyne transmission with two-element diversity reception. The transceivers were used to study and demonstrate a millimeter-wave wireless personal network that enables cost-effective broadband data transmissions in a multipath channel environment. A 100-MHz sampling OFDM modulator/demodulator was developed for the baseband part. It has a payload data rate of 100 Mb/s using quadrature phase-shift keying (QPSK) modulation and a coding rate of 3/4 (many other modulations and coding rate are available). The bit error rate was experimentally evaluated when a pair of devices was placed on a wooden table under line-of-sight path conditions. The results showed that the combined use of the self-heterodyne technique and two-element diversity receiver successfully avoids serious signal fading at unpredictable transmission distances. The transceiver with QPSK modulation and coding rate of 1/2 for forward error correction achieves error-free data transmission over a distance of up to 3.4 m. In addition, a successful transmission in 64 quadrature amplitude modulation mode was demonstrated, although the communication range was quite short

Millimeter-Wave Fiber-Fed Wireless Access Systems Based on Dense Wavelength-Division-Multiplexing Networks

This paper describes new 60-GHz-band bidirectional fiber-fed wireless access systems, which provide simple antenna base-station architecture and full compatibility with dense wavelength-division multiplexing (DWDM) fiber-optic networks. For downlink millimeter-wave signal generation, the proposed scheme utilizes uplink optical transmitters as downlink optical heterodyne sources, which promises wavelength allocations of downlink and uplink signals that are fully compatible with those of DWDM networks. The frequency instability and poor phase-noise characteristics of these optically heterodyned signals do not affect transmission quality at all by using self-heterodyne wireless transmission techniques. Experimental results demonstrate that the proposed scheme provides not only strong immunity to phase-noise degradation coming from both fiber and millimeter-wave links, but also less sensitivity to fiber transmission loss than conventional remote optical heterodyne approaches. With the help of these attractive features, we successfully demonstrate both downlink and uplink orthogonal frequency-division multiplexing data transmissions in both 10-km fiber-optic and 60-Hz links.