ABSTRACT
High data rate coverage, security, and energy efficiency will play a key role in the continued performance scaling of next-generation mobile systems. Dense, small mobile cells based on a novel network architecture are part of the answer. Motivated by the recent mounting interest in free-space optical (FSO) technologies, this paper addresses a novel mobile fronthaul network architecture based on FSO, spread spectrum codes, and graphene modulators for the creation of dense small cells. The network uses an energy-efficient graphene modulator to send data bits to be coded with spread codes for achieving higher security before their transmission to remote units via high-speed FSO transmitters. Analytical results show the new fronthaul mobile network can accommodate up to 32 remote antennas under error-free transmissions with forward error correction. Furthermore, the modulator is optimized to provide maximum efficiency in terms of energy consumption per bit. The optimization procedure is carried out by optimizing both the amount of graphene used on the ring resonator and the modulator's design. The optimized graphene modulator is used in the new fronthaul network and requires as low as 4.6 fJ/bit while enabling high-speed performance up to 42.6 GHz and remarkably using one-quarter of graphene only.
ABSTRACT
An innovative off-chip antenna (OCA) is presented that exhibits high gain and efficiency performance at the terahertz (THz) band and has a wide operational bandwidth. The proposed OCA is implemented on stacked silicon layers and consists of an open circuit meandering line. It is shown that by loading the antenna with an array of subwavelength circular dielectric slots and terminating it with a metamaterial unit cell, its impedance bandwidth is enhanced by a factor of two and its gain on average by about 4 dB. Unlike conventional antennas, where the energy is dissipated in a resistive load, the technique proposed here significantly reduces losses. The antenna is excited from underneath the antenna by coupling RF energy from an open-circuited feedline through a slot in the ground-plane of the middle substrate layer. The feedline is shielded with another substrate layer which has a ground-plane on its opposite surface to mitigate the influence of the structure on which the antenna is mounted. The antenna has the dimensions 12.3 × 4.5 × 0.905 mm3 and operates across the 0.137-0.158 THz band corresponding to a fractional bandwidth of 14.23%. Over this frequency range the average measured gain and efficiency are 8.6 dBi and 77%, respectively. These characteristics makes the proposed antenna suitable for integration in sub-terahertz near-field electronic systems such as radio frequency identification (RFID) devices with high spatial resolution.
ABSTRACT
In this paper, we propose a modulation technique for passive optical networks that harnesses two-dimensional prime hop system optical code division multiplexing access (OCDMA) and optical orthogonal frequency-division multiplexing (OFDM) for intensity modulation with direct-detection (IMDD) to enhance users' signal capacity in a cost-effective manner. The theoretical analysis is built from an analytical formula that takes into account both multiple-access interference and photodetector noise. Results show that OFDM-OCDMA with multiple users has similar performance to single-user conventional OOFDM for low transmitted powers.
ABSTRACT
BACKGROUND: This study was conducted to assess the management of acute upper gastrointestinal bleeding in a district hospital and to compare these results with national guidelines and the published literature. MATERIALS AND METHODS: This prospective and retrospective study included 112 patients, mean age 66 years, who presented with acute upper gastrointestinal bleeding between July 2004 and February 2005. All patients were assigned a Rockall risk assessment score. RESULTS: The surgical on-call teams managed all the patients according to an agreed protocol. Forty-nine patients had a Rockall score > or =4. Endoscopy was performed in all patients, with 60% accomplished within the first 24 hours. The most common cause found was peptic ulcer (30%). Therapeutic endoscopy was undertaken in 10 patients (9%) with a success rate of 70%. Open surgery was performed in 3 patients. One patient died after having surgery and the Rockall score was >5. Of the patients admitted with acute upper gastrointestinal bleeding, 90.2% were discharged without complication. Eleven patients died (9.8%) and all of them from the high risk group with Rockall scores > or =5. Their mean hospital stay was 17.8 days (range, 2-43 days). CONCLUSION: High-standard results in acute upper gastrointestinal bleeding can be achieved in a district hospital. The management, including the use of the operating theater facilities with operative and anesthetic support, was safe and efficient. A 24-hour-a-day endoscopy service is important to achieve early diagnosis and to plan management. A protocol and early endoscopy improve clinical outcome and reduce mortality, which occurred mostly among elderly patients with high risk scores. It is advisable to introduce the Rockall scoring system in practice.