Volume 67, Issue 3-4

A novel 3-dB trans-directional (TRD) coupler with a defected ground structure (DGS) and conductor-backed asymmetric coplanar waveguides (CBACPWs) is proposed. To operate at TRD mode, coupled lines are loaded by periodical shunt capacitors. However, even- and odd-mode phase velocities of coupled microstrip lines are different and capacitances of commercial lumped capacitors usually don't match with theoretical values, which lead to more than 10% drift of operating frequency with maximum coupling level and poor isolation. To solve this problem, DGS is etched under coupled microstrip lines to adjust even- and odd-mode phase velocities, and CBACPW is inserted into each port of the coupler to improve isolation. The proposed coupler exhibits a return loss of better than 20 dB, isolation of better than 20 dB, balanced output power distribution (3.25 ± 0.35 dB) and consistent 90° (± 2°) output signals phase difference over a fractional bandwidth of 15.4% (1.501–1.752 GHz) with a maximum isolation of about 53 dB at 1.635 GHz.

Filters have always remained a field of immense interest in the high frequency domain. Emerging applications such as wireless communications continue to challenge RF/microwave filters with ever more stringent requirements – high performance, smaller size, lightweight, and lower cost. In this paper we propose and investigate the application of electromagnetic bandgap (EBG) structures for improving the performance of a defected ground state (DGS) based low pass filter (LPF) with T-slot on the microstrip line. Whereas the T-slot and DGS are known to improve the roll off factor, EBG substrate further enhances this factor significantly. In our case the LPF with T-slot and square dumbles on DGS give a roll off of −29.3823 dB/GHz, on application of the EBG it improves to −31.7919 dB/GHz. Also, on applying the TE mode boundary condition along the vertical axis, we observe that the maximum attenuation is improved significantly from −60.65321 dB to −69.64911 dB.

A novel approach to generate continuously tunable microwave and millimeter-wave (mm-wave) signals is proposed and experimentally demonstrated. It is realized by a polarization modulator and a Brillouin-assisted sideband-filtering technique. The output optical signal of modulator is controlled to suppress the odd sidebands by a polarization controller and polarizer. After that, by adjusting the power of input microwave signal to remove the optical carrier, and then the strong second-order sideband of the modulated signal can be deeply suppressed by using stimulated Brillouin scattering (SBS) in a fiber ring while the fourth order sidebands keep unchanged. Therefore, only fourth-order sidebands signals exist at the input of the PD, frequency octupling can be realized. If the even sidebands are suppressed, the strong first order sidebands of the modulated signal will be deeply suppressed by the SBS filter, leaving the third sidebands, frequency sextupling can also be realized. As the SBS-assisted sideband-filter is narrow bandwidth and wavelength-independent, a wavelength-flexible way of optical generation mm-wave signals can be successfully realized.

The conversion gain of CMOS active mixers is investigated in terms of several kinds of LO offsets. With the effect of output resistance characterized in submicron process, the effective transconductance of driving stage is rigorously derived based on small-signal cascode equivalence. An analytical conversion gain model for the mixer with several kinds of LO offsets is thus presented. Based on Chartered 0.35 µm CMOS technologies, the Spectre-RF simulations agree with the predictions from the proposed model well, where the maximal error is less than 0.28 dB.

We study the problem of decentralized cooperative spectrum sensing in cognitive radio ad hoc networks without a fusion center, using a consensus based solution. First, the received logarithmic likelihood ratio (LLR) is employed as the local detection outcome; then, it is exchanged between neighboring CR users and is updated based on the LLR received from the neighbors. It is proved that with the proposed decentralized algorithm, the optimal sensing performance is achieved via just local information exchange between neighbors. In addition, it is shown that the proposed algorithm outperforms previous work in which the received energy was employed as the local detection outcome.

Cooperative spectrum sensing, which can greatly improve the ability of discovering the spectrum opportunities, is regarded as an enabling mechanism for cognitive radio (CR) networks. In this paper, we employ a double threshold detection method in energy detector to perform spectrum sensing, only the CR users with reliable sensing information are allowed to transmit one bit local decision to the fusion center. Simulation results will show that our proposed double threshold detection method could not only improve the sensing performance but also save the bandwidth of the reporting channel compared with the conventional detection method with one threshold. By weighting the sensing performance and the consumption of system resources in a utility function that is maximized with respect to the number of CR users, it has been shown that the optimal number of CR users is related to the price of these Quality-of-Service (QoS) requirements.

Time-varying channel estimation for single carrier systems using superimposed training is considered. The variation of a wireless channel is well approximated by a discrete prolate spheroidal basis expansion model (DPS-BEM) as basis functions. A periodic training sequence is superimposed at low power on the information sequence as opposed to being time-multiplexed (TM) with it at the transmitter. A two-step approach is adopted, where, in the first step, we estimate the channel by using only the first-order statistics of the received data. The unknown information sequence acts as interference resulting in a poor signal-to-noise ratio (SNR). We then proposed a low complexity receiving self-interference suppression (RSIS) scheme in the second step where the information interference (termed self-interference) is estimated from detected symbols and iteratively suppressed at the receiver to enhance estimation performance. The RSIS scheme avoids the compromise between self-interference elimination and information integrity of data-dependent superimposed training (DDST) scheme in time-varying channel estimation. Simulation results show that the proposed scheme is better than DDST schemes and competitive with the TM training in term of BER.

It is significant to develop a method to realize efficient broadcast retransmission in wireless networks. Network coding is a promising technique for this purpose. In this paper, we propose a novel Wireless Broadcast Retransmission scheme based on Opportunistic Network Coding (WBRONC) aiming at reducing the number of transmissions, as well as improving the transmission efficiency. This scheme employs a new packet scheduling algorithm which uses Packet Distribution Matrix (PDM) directly to select coded packets. Thus, low computational complexity is obtained by it. It is denoted from the analysis and simulation results that the WBRONC scheme provides a substantial improvement both in transmission efficiency and computational overhead.

Stemming from the fact that the α-μ fading distribution is one of the very general fading models used in the literature to describe the small scale fading phenomenon, in this paper, closed-form expressions for the Shannon capacity of the α-μ fading channel operating under four main adaptive transmission strategies are derived assuming integer values for μ. These expressions are derived for the case of no diversity as well as for selection combining diversity with independent and identically distributed branches. The obtained expressions reduce to those previously derived in the literature for the Weibull as well as the Rayleigh fading cases, which are both special cases of the α-μ channel. Numerical results are presented for the capacity under the four adaptive transmission strategies and the effect of the fading parameter as well as the number of diversity branches is studied.