The Modares Journal of Electrical Engineering
Year:2015 | Volume:15 | Issue:1|Papers Count:6

In this paper, we consider a cooperative cognitive radio network in which there is an OFDM primary link and multiple single carrier secondary links. The primary link is required to maintain its secrecy rate above a predefined threshold. If the secrecy rate requirement is not satisfied, the secondary system help primary link to maintain its secrecy rate requirement. In doing so, the secondary transmitters work as decode and forward (DF) relays and hence, as a reward, gains access to network resources to transmit its own information. Our objective is to maximize the data rate of secondary system while satisfying the primary secrecy rate constraint and individual transmit power constraints for primary link and secondary system. We solve our proposed optimization problem using dual Lagrange approach to find the set of cooperating secondary link, subcarrier assignments, and transmit power over each subcarrier. Using simulations, we evaluate our proposed scheme in various situations.

In this paper, downlink resource allocation for a sparse code multiple access (SCMA) based heterogeneous cellular networks (HCN) is investigated. The main aim of this paper is to obtain joint power allocation and codebook assignment to achieve the least total system transmit power. In this regard, the proposed objective function is formulated as total transmit power subject to users minimum rate requirement, total available power in each base station (BS), and SCMA constraint. To solve the proposed problem an iterative algorithm considering successive convex approximation (SCA) is applied. Also, convergence of the iterative method is investigated. In the simulation results the effectiveness of proposed resource allocation method are shown.

Base station assisted device-to-device communications (D2D) lets cellular users communicate directly. As D2D connections can operate on a joint frequency band, interference management is essential to boost the spectral efficiency and users’ quality of service. Two different approaches are proposed to tackle the interference in D2D networks. A power control scheme and a frequency-time scheduling are examined to achieve long-term fairness among D2D users. The power control maximizes weighted sum-rate of the users, where the weights indicate priority of the users. The second proposed strategy is to avoid interference when it is high. In this algorithm, the spectrum band is divided among users as interference coupling rises. A metric from the information theory literature has been adopted to measure the coupling. The proposed schemes are tested through multiple time slots and the average rate of a user and network sum-rate have been observed. The results demonstrate the superior performance of the proposed power control scheme.

In this paper, an interference management for multi-beam satellite network to enhance spectral efficiency is studied. First, a two-satellite for future generation satellite network using cognitive radio technology is provided. This network is composed of a secondary satellite and a primary satellite where the primary satellite system does not suffer from severe destructive interferences coming from the secondary satellite system. Upward/downward link power is assumed to be fixed and no power control is used. Therefore, the received data from each beam has a specific level. Also, a novel bandwidth sharing algorithm is provided for the first time. Next, total rate for both primary and secondary satellite systems are computed. Furthermore, Receiver Operating Characteristic (ROC) curve and numerical results based on the number of antennas and threshold levels are given.

Energy detector is one of the non-coherent receivers for impulse radio ultra-wideband (IR-UWB) systems using on-off keying (OOK) modulation. Energy detection for IR-UWB OOK systems gets accomplished based on a threshold value. In these systems, the threshold value depends on several parameters that should be determined in the receiver. In this paper, we propose a blind approach to estimate proper thresholds for the symbol decision. The performance of the proposed method turns out to be asymptotically comparable to that of the optimal threshold selection. The proposed algorithm was also improved using an optimum integration interval. By using a blind estimating of the channel time delay spread, the integration interval was adjusted. Simulation results show that this method improves the performance about 1.5 dB in BER of 10-4.

Pulse or FMCW altimeters are used for height or distance measurement. For both types, a radio frequency signal is sent and after few micro seconds the reflected signal from the ground or sea is received and then by signal analysis the height can be measured. Because of wide frequency bandwidth of mixer in receiver and for better sensitivity, frequency modulation, jamming and noise, a narrowband low insertion loss bandpass filter should be designed. In this paper a new structure lg/4 stepped impedance resonator (SIR) narrowband bandpass filter is proposed that has special specifications such as %4.5 tunable bandwidth (%3 up to %7), sharp and wide stopband frequency characteristic, less than 0.7 dB insertion loss in passband and more than 80 dB rejection in stopband. With new coaxial resonators the seven pole Chebychev bandpass filter is designed and fabricated with more than %15 frequency tuning capability. Good agreement between measured and simulated results validates this design process.