Search

Space-time block code and spatial multiplexing are promising approaches that exploit the multiple input multiple output (MIMO) channel to provide high diversity and multiplexing gain without sacrifice in bandwidth. In this paper, a hybrid MIMO scheme for orthogonal frequency division multiplexing systems is proposed to achieve various multiplexing gains according to the channel condition. The existing hybrid MIMO schemes are appropriate for limited integer multiplexing rates without taking into account the channel condition, whereas the proposed scheme can attain various multiplexing rates according to the channel condition. [ABSTRACT FROM AUTHOR]

In this paper, a joint estimation of carrier frequency offset and sampling frequency offset is suggested for multi-band ultra-wideband orthogonal frequency division multiplexing (UWB-OFDM) systems. The basic idea of our approach is based on the fact that two adjacent OFDM symbols carry the identical information in the UWB-OFDM system, thus removing the need of pilot symbols. The system performance of using our algorithm is demonstrated in terms of mean square error and bit error rate. [ABSTRACT FROM AUTHOR]

In orthogonal frequency division multiplexing (OFDM) systems, the loss of orthogonality between the subcarriers due to carrier frequency offset (CFO) can degrade the performance of the system. In this paper, a residual CFO synchronization method is proposed for ultra wideband OFDM (UWB-OFDM) systems. The proposed fine frequency synchronization scheme uses a channel estimation sequence and physical layer convergence protocol (PLCP) header information which are equipped with frequency-domain spreading. A UWB-OFDM receiver with algorithm for fine frequency error synchronization is simulated and is evaluated in terms of mean square error. [ABSTRACT FROM AUTHOR]

In this paper, a fine carrier frequency offset synchronizer is presented for ultra-wideband multi-band orthogonal frequency division multiplexing (UWB MB-OFDM) systems. The proposed scheme uses a channel estimation (CE) sequence provided in the current UWB MB-OFDM system and is designed to be insensitive to a sampling clock frequency offset. To demonstrate the efficiency of the proposed fine frequency synchronizer, comparisons are made with the conventional frequency synchronizer in terms of mean square error. [ABSTRACT FROM AUTHOR]

A combination of orthogonal frequency division multiplexing (OFDM) with multiband frequency hopping (FH) is regarded as a promising solution for realizing ultra-wideband (UWB) systems. This paper deals with a modified maximum likelihood approach for frequency ambiguity resolution in the multiband UWB system. By using computer simulation, the synchronization performance of the traditional and proposed methods is evaluated in terms of probability of synchronization failure. [ABSTRACT FROM AUTHOR]

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called OFDMA is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This work focuses on the performance of OFDM when used as a modulation and access technique for the uplink of an interactive wireless broadcasting system. In such an application, we present our investigations of the effects of symbol timing misalignments and peak-to-average power ratio (PAR). Under the condition of the same side information bits, the selective mapping (SLM) approach can be implemented by decimating OFDM samples, which is less complex compared to the ordinary SLM incurring a slight degradation of the PAR performance. Taking into account the effects of clipping operation and symbol timing misalignment, the bit error rate (BER) and throughput performances are investigated for a typical OFDMA uplink scenario. [ABSTRACT FROM AUTHOR]

Distributed-loss gyrotron traveling-wave amplifiers (gyro-TWTs) with high-gain, broadband, and millimeter-wave capabilities have been demonstrated. Most structures with distributed wall losses are stabilized in gyro-TWTs that operate at low beam currents. Attenuating severs are added to the interaction circuit of a distributed-loss gyro-TWT to prevent high beam currents that result in mode competition. Simulation results show that gyrotron backward-wave oscillations (gyro-BWOs) are not effectively suppressed by the lossy section; in contrast, the severed sections can effectively enhance the start-oscillation threshold of gyro-BWOs in the proposed gyro-TWT. Meanwhile, localized reflective oscillations seem not to occur in the gyro-TWT unless it operates at a high magnetic field or with a high interaction length. The stable gyro-TWT, operating in the low-loss TE01 mode, is predicted to yield a peak output power of 405 kW at 33 GHz with an efficiency of 20%, a saturated gain of 77 dB and a 3 dB bandwidth of 2.5 GHz for a 100 kV, 20 A electron beam with an axial velocity spread of Δvz/vz=5%. [ABSTRACT FROM AUTHOR]

An inductor-less wideband (6-18 GHz) 3-bit attenuator with low phase variations is presented based on octagonal SiGe p-type intrinsic n-type (pin) diodes. To achieve low insertion loss and high linearity, a DC bias scheme has been designed to minimise the leakage from the parasitic diodes between the P-sub and N-well of the pin diodes. The attenuator is fabricated in a standard 0.18 µm SiGe BiCMOS process without using post-processed transmission lines. It has a maximum attenuation range of 7 dB. The minimum measured insertion loss is 7.9, 9.4, 10.6 dB at 6, 12, 18 GHz, respectively. Phase variation is lower than ±2.5° and the chip size, including pads, is 0.85×0.412 mm2. [ABSTRACT FROM AUTHOR]

Broadband terahertz generation via two-color femtosecond laser filamentation is studied with laser input energies up to 60 mJ. In the small f-number focusing regime, the output THz energy strongly saturates, which is attributed to ionization-induced laser defocusing in filamentation. This saturation effect can be minimized by elongating the plasma filament with weak focusing. A conversion efficiency of >10-4 is achieved in elongated filamentation. [ABSTRACT FROM AUTHOR]