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Abstract— In OFDM systems, it is necessary to estimate the channel to overcome the distortion caused by channel fading’s which can be induced by many phenomena such as: delay spread, mobility and Doppler shift. Most of the channel estimation techniques are proposed in frequency domain using the pilot symbols. One of them that is less complicated is least-squares (LS) method which is widely used in channel estimation but it is more sensitive to noise respected to the other reported techniques. In this paper, a new threshold based method using wavelet decomposition will be proposed which is based on an initial LS estimation technique. The reported simulation results show that the proposed method has better performance compared to the other methods such as Lee Method that has been published recently.

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Recently, Routing in Vehicular Ad hoc Networks (VANETs) has found significant attention due to its unique characteristics such as lack of energy constraints and high-speed vehicles. In addition, since VANET networks are highly dynamic, design of routing algorithms suitable for an urban environment could be extremely challenging. Appropriate algorithms for urban environments should be opportunistic routing (OR) algorithms in which traffic transmission is performed using the store-carry-forward mechanism. Most of the OR algorithms use the flooding mechanism for increasing delivery ratio without considering the overheads of flooding. In this paper, a new algorithm called Reliability-aware Location and History based Opportunistic Routing (RLHOR) is proposed to reduce overhead of message replication . The RLHOR searches for the most reliable route by evaluating the reliability of intermediate links considering their lifetimes, location and direction of intermediate vehicles, and the history of message delivery by intermediate vehicles. Using the ONE simulator, our performance evaluation results show that the RLHOR can operate better than other conventional OR algorithms under different network scenarios. Under sparse networks, the RLHOR not only increases delivery ratio, but also reduces overhead and traffic loss.

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A new multiobjective power control algorithm is presented in this paper in which power update is accomplished using local information. The algorithm benefits a multiobjective optimization technique the objectives of which are designed in a way to not only minimize the users’ transmit power but also maintain the users’ SIR at an acceptable level, on one hand, and reduce as much possible the SIR fluctuations, on the other hand. The convergence characteristics of the proposed method are investigated both theoretically and with simulations. The results indicates that, compared to other existing power control algorithms, the proposed algorithm has a higher convergence speed while reducing user’s transmit power. Introducing the practical version of the proposed algorithm, it is then compared with two existing algorithms including B-BPSC (FSPC) and MOTDPC. Simulation results show significant improvements in the convergence speed and average consumed power.

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In this paper joint effect of transmitter and receiver IQ imbalance under insufficient and sufficient cyclic prefix (CP) is studied. The case of insufficient CP length leads to increase in the rate of symbol transmission, on the other hand it causes inter-symbol-interference. Morever, existence of IQ imbalance in the transmitter and receiver causes distortion in the received signals. So this problem leads to increase of bit error rate. To compensation of this effect and other impairments, simultaneously, per-tone equalization (PTEQ) structure implementation is necessary. Regarding to topology of this structure and required high length filters, the system computations will be very high, even using simplest adaptive algorithms. In this work to reduce the computational burden, an adaptive algorithm based on selective coefficient updating (SCU) has been presented. In addition, to increase of the convergence speed of adaptive algorithms,wavelet packet transform (WPT) is applied to branches of PTEQ structure in second stage. Then SCU method based on the rate of wavelet papcket entropy has been used. It must be mentioned that in this method not only computational complexity has been reduced but also bit error rate has been improved. In the third stage, the combination of both SCU and data selective updating (DSU) to increase of convergence speed and to reduce of computational complexity have been derived. Simultaneous using of two DSU and SCU methods besides WPT, is suitable for compensation ofchannel effect and IQ imbalance. Simulation results show that this algorithm not only causes considerably reduction on amount of calculations, but also have better performance.

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Downlink channel estimation of Massive MIMO is an important challenge is 5G wireless communication.  A classic method for this purpose is training-based method which leads to decreasing in transmitted information rate. To cope with this problem, recently, a blind channel estimation method has been presented in which by assuming that the value of large scale fading coefficient is known, the channel gain is estimated in multiuser Massive MIMO system. In this paper, we propose a new method that simultaneously estimates both channel gain and large scale fading coefficient by applying two different power control gain in the coherence interval. The proposed method is applicable for ZF and MR precoding. The proposed method has higher transmitted information rate (does not need to transmit the large scale fading coefficient) and lower MSE in high SNR values with respect to reference method.

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This paper evaluates the problem of user pairing scenario with similar channel conditions in NOMA with three users per pair. The small difference in the channel gain of the paired users leads to interference in the process of successive interference cancelation (SIC). The incidence of imperfect SIC reduces system capacity. Also, mid users in this scenario will be deprived of the advantages provided by the NOMA without pairing, on the other hand, by pairing,  due to the closeness of the users’ channel conditions to each other, leads to the incidence of imperfect SIC. In this paper, in order to solve this problem, we propose a scenario for users to pair, in which all users, including mid users, will be able to use the benefits of NOMA and the problem of incidence of imperfect SIC is minimized. To evaluate the performance of the NOMA, we examine the sum ergodic capacity in both perfect and imperfect SIC modes. In this paper, the concept of an imperfect SIC has calculated analytically for the first time for pairs containing three users. The calculations performed along with the simulations show that in this case, the system's capacity falls sharply. The simulation results also show that the proposed scenario, in addition to minimizing the imperfect SIC event, provides a higher sum ergodic capacity than other scenarios.

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In spectrum sharing among different users, it is very essential to deal with the jammers in the network. This work is more important when the jammer is intelligent and capable of learning the communication patterns between users. In this paper, a spectrum sharing method has been suggested to deal with the jammers of the network. The proposed method is based on multi agent reinforcement learning. In the first stage, the interfered sub-bands are obtained by the base station. In the next stage, deep reinforcement learning is designed to offer each user a safe and appropriate spectrum. For this purpose, the effect of the jammer in the sub-band is obtained from the correlation between the transmitted pilot and the received feedback in each frequency interval. Then, the discount factor of the reinforcement learning algorithm is adjusted adaptively based on the correlation value. In this way, the sub-bands that are less affected by the jammer are assigned to the users. The proposed method has been evaluated in different scenarios and the results indicate that the sum rate of network converges to the desired level faster and the proposed method shows resistance against the jammer.