Reverse-recovery modeling for p-i-n diodes in the high current-density conditions are discussed. With the dynamic carrier-distribution-based modeling approach, the reverse recovery behaviors are explained in the high current-density conditions, where the nonquasi-static (NQS) behavior of carriers in the drift region is considered. In addition, a specific feature under the high current-density condition is discussed. The proposed model is implemented into a commercial circuit simulator in the Verilog-A language and its reverse recovery modeling ability is verified with a two-dimensional (2D) device simulator, in comparison to the conventional lumped-charge modeling technique.

This letter presents a robust receiver using the generalized sidelobe canceller aided with the high-order derivative constraint technique for multicarrier code-division multiple-access (MC-CDMA) uplink against carrier frequency offset (CFO). Numerical results demonstrate the efficacy of the proposed receiver.

This paper proposes a novel satellite channel allocation algorithm for a demand assigned multiple access (DAMA) controller. In satellite communication systems, the channels' total bandwidth and total power are limited by the satellite's transponder bandwidth and transmission power (satellite resources). Our algorithm is based on multi-carrier transmission and adaptive modulation methods. It optimizes channel elements such as the number of sub-carriers, modulation level, and forward error correction (FEC) coding rate. As a result, the satellite's transponder bandwidth and transmission power can be simultaneously used to the maximum and the overall system capacity, i.e., total transmission bit rate, will increase. Simulation results show that our algorithm increases the overall system capacity by 1.3 times compared with the conventional fixed modulation algorithm.

Orthogonal frequency-division multiplexing (OFDM) is a popular air interface technology that is adopted as a standard modulation scheme for 4G communication systems owing to its excellent spectral efficiency. For OFDM systems, synchronization problems have received much attention along with peak-to-average power ratio (PAPR) reduction. In addition to frequency offset estimation, frame synchronization is a challenging problem that must be solved to achieve optimal system performance. In this paper, we present a maximum likelihood (ML) frame synchronizer for OFDM systems. The synchronizer exploits a synchronization word and cyclic prefixes together to improve the synchronization performance. Numerical results show that the performance of the proposed frame synchronizer is better than that of conventional schemes. The proposed synchronizer can be used as a reference for evaluating the performance of other suboptimal frame synchronizers. We also modify the proposed frame synchronizer to reduce the implementation complexity and propose a near-ML synchronizer for time-varying fading channels.

This paper investigates the non-data-aided (NDA) carrier frequency estimation of amplitude and phase shift keying (APSK) signals. The true Cramer-Rao bound (CRB) for NDA frequency estimation of APSK signals are derived and evaluated numerically. Characteristic and jitter variance of NDA Luise and Reggiannini (L&R) frequency estimator are analyzed. Verified by Monte Carlo simulations, the analytical results are shown to be accurate for medium-to-high signal-to-noise ratio (SNR) values. Using the proposed closed-form expression, parameters of the algorithm are optimized efficiently to minimize the jitter variance.

Power consumption due to transmissions in base stations (BSs) has been a major contributor to communication-related CO2 emissions. A power optimization model is developed in this study with respect to radio resource allocation and activation in a multiple Component Carrier (CC) environment. We formulate and solve the power-minimization problem of the BS transceivers for multiple-CC networks with carrier aggregation, while maintaining the overall system and respective users' utilities above minimum levels. The optimized power consumption based on this model can be viewed as a lower bound of that of other algorithms employed in practice. A suboptimal scheme with low computation complexity is proposed. Numerical results show that the power consumption of our scheme is much better than that of the conventional one in which all CCs are always active, if both schemes maintain the same required utilities.

In this study, a precoding scheme based on QR-decomposition is proposed for mitigating the inter-carrier-interference (ICI) in orthogonal-frequency-division-multiplexing (OFDM) systems. The proposed approach first subjects the ICI matrix to QR decomposition so that the ICI effect is transformed into its spectrally causal equivalent. With this causality, the precoding can then be conducted based on the resultant spectrally causal matrix. In addition, by using the stationary property of the ICI factors, in conjunction with zero padding, we implement the QR-based precoding in a segmentation manner which can significantly alleviate the computational complexity imposed by QR decomposition while eliminating ICI within each segment. This study also analyzes the residue interference power induced by the segmentation. The residue interference power is then accordingly used to determine the order of zero padding. Computer simulations support the validity of the proposed approach.

A receiver architecture and a digital IQ imbalance compensation method for dual-carrier reception are newly proposed. The impact of IQ imbalance on the baseband signal is mathematically analyzed. Based on the analysis, IQ imbalance parameters are estimated and the coupling effect of IQ imbalance is compensated using digital baseband processing alone. Simulation results show that the proposed IQ imbalance compensation successfully removes IQ imbalance. The deviation from the ideal performance is less than 1 dB when it is applied to the 3GPP-LTE carrier aggregation.

In an orthogonal frequency division multiplexing (OFDM) system, carrier frequency offset (CFO) causes intercarrier interference (ICI) which significantly degrades the system error performance. In this paper we provide a closed-form expression to evaluate the exact error probabilities of arbitrary 2-D modulation OFDM systems with CFO, and analyze the effect of CFO on error performance.

Maximum likelihood block signal detection employing QR decomposition and M-algorithm (QRM-MLBD) can significantly improve the bit error rate (BER) performance of single-carrier (SC) transmission while significantly reducing the computational complexity compared to maximum likelihood detection (MLD). However, its computational complexity is still high. In this paper, we propose the computationally efficient 2-step QRM-MLBD. Compared to conventional QRM-MLBD, the number of symbol candidates can be reduced by using preliminary decision made by minimum mean square error based frequency-domain equalization (MMSE-FDE). The BER performance achievable by 2-step QRM-MLBD is evaluated by computer simulation. It is shown that it can significantly reduce the computational complexity while achieving almost the same BER performance as the conventional QRM-MLBD.

We present a carrier and sampling frequency offset estimation and compensation scheme for a multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) modem. We first perform initial carrier frequency offset (CFO) estimation and compensation during the preamble period, and then conduct the estimation and compensation of the residual CFO and sampling frequency offset (SFO) during the payload period. The proposed design scheme reduces the logic gate count of the frequency offset compensation block by about 10%, while it gives almost the same performance at the packet error rate (PER) of 10-4 in the CM1 channel. The frequency offset estimation and compensation block is implemented using 90 nm CMOS technology and tested.

In orthogonal frequency division multiple access (OFD-MA) uplink, the distortions introduced by both multiple carrier frequency offsets (CFOs) and in-phase and quadrature-phase (IQ) imbalances will severely degrade the system performance. With both CFOs and IQ imbalances, signal detection at the receiver becomes hard, if not impossible. In this letter, a linear receiver is proposed to cope with the distortions at a slight drop in system transmission rate. The analysis and simulations demonstrate the effectiveness of the proposed approach.

Conventional algorithms for the joint estimation of carrier frequency offset (CFO) and I/Q imbalance no longer work when the I/Q imbalance depends on the frequency. In order to correct the imbalance across many frequencies, the compensator needed is a filter as opposed to a simple gain and phase compensator. Although, algorithms for estimating the optimal coefficients of this filter exist, their complexity is too high for hardware implementation. In this paper we present a new low complexity algorithm for joint estimation of CFO and frequency dependent I/Q imbalance. For the first part, we derive the estimation scheme using the linear least squares algorithm and examine its floating point performance compared to conventional algorithms. We show that the proposed algorithm can completely eliminate BER floor caused by CFO and I/Q imbalance at a lesser complexity compared to conventional algorithms. For the second part, we examine the hardware complexity in fixed point hardware and latency of the proposed algorithm. Based on BER performance, the circuit needs a wordlength of at least 16 bits in order to properly estimate CFO and I/Q imbalance. In this configuration, the circuit is able to achieve a maximum speed of 115.9 MHz in a Virtex 5 FPGA.

In Long-Term Evolution-Advanced (LTE-A), which is currently in the process of standardization in the 3rd generation partnership project (3GPP), carrier aggregation (CA) was introduced as a main feature for bandwidth extension while maintaining backward compatibility with LTE Release 8 (Rel. 8). In the CA mode of operation, since two or more component carriers (CCs), each of which is compatible with LTE Rel. 8, are aggregated, mobility management is needed for CCs such as inter/intra-frequency handover, CC addition, and CC removal to provide sufficient coverage and better overall signal quality. Therefore, the signaling overhead for Radio Resource Control (RRC) reconfiguration for the mobility management of CCs in LTE-A is expected to be larger than that in LTE Rel. 8. In addition, CA allows aggregation of cells with different types of coverage. Therefore, the signaling overhead may be dependent on the coverage of each CC assumed in a CA deployment scenario. Furthermore, especially in a picocell-overlaid scenario, the amount of signaling overhead may be different according to whether the aggregation of CCs between a macrocell and a picocell, i.e., transmission and reception from multiple sites, is allowed or not. Therefore, this paper investigates the CC control overhead with several CC management policies in some CA deployment scenarios, including a scenario with overlaid picocells. Simulation results show that the control overhead is almost the same irrespective of the different management policies, when almost the same coverage is provided for the CCs. In addition, it is shown that the increase in the control overhead is not significant even in a CA deployment scenario with overlaid picocells. We also show that the amount of signaling overhead in a picocell-overlaid scenario with the CA between a macrocell and a picocell is almost twice as that without the CA between a macrocell and a picocell.

In multi-cell OFDMA-based networks, co-channel interference (CCI) is inevitable when the frequency reuse scheme is used. The CCI affects the performance of users, especially that of cell edge users. Several frequency reuse schemes and subcarrier allocation algorithms have been proposed to solve the CCI problem. Nevertheless, it is difficult to improve both the cell capacity and the performance of cell edge users since they have a trade-off. In this paper, we propose a new balanced frequency reuse (BFR) as a new frequency partitioning scheme that gives more power to the users in the outer region and allocates more subcarriers to the users in the inner region. In addition, we propose ordering and directional subcarrier allocation (ODSA) for our frequency partitioning proposal to mitigate the CCI effectively when cells have heterogeneous traffic loads. The performance of the proposed BFR with the ODSA algorithm is investigated via analyses and simulations. Performance evaluation shows that the proposed BFR with the ODSA algorithm can increase both the spectral efficiency and the performance of cell edge users if the transmission power is appropriately handled.

Secrecy on the physical layer is receiving increased research interest due to its theoretical and practical importance. In this letter, a subcarrier allocation scheme is proposed for physical-layer security in cooperative orthogonal frequency division multiple access (OFDMA) networks that use the Amplify-and-Forward (AF) strategy. We consider the subcarrier pairing and assignment to maximize overall system rates subject to a secrecy level requirement. Monte Carlo simulations are carried out to validate our analysis.

A combination of single-carrier frequency-division mult-iple-access (SC-FDMA) and relay transmission is effective for performance improvement in uplink transmission. In SC-FDMA, a mapping strategy of user's spectrum has an enormous impact on system performance. In the relay communication, the optimum mapping strategy may differentiate from that in direct communication because of the independently distributed channels among nodes. In this letter, how each link should be considered in subcarrier mapping is studied and the impact of mapping strategies on the average bit error rate (BER) performance of single-user SC-FDMA relay communications will be given.

Carrier aggregation (CA) is one of the most important techniques for LTE-Advanced because of its capability to support a wide transmission bandwidth of up to 100 MHz and heterogeneous networks effectively while achieving backward compatibility with the Release 8 LTE. In order to improve the performance of control information transmission in heterogeneous networks, cross-carrier scheduling is supported, i.e., control information on one component carrier (CC) can assign radio resources on another CC. To convey the control information efficiently, a search space is defined and used in Release 8 LTE. In cross-carrier scheduling, the optimum design for the search space for different CCs is a paramount issue. This paper presents two novel methods for search space design. In the first method using one hash function, a user equipment (UE)-specific offset is introduced among search spaces associated with different CCs. Due to the UE-specific offsets, search spaces of different UEs are staggered and the probability that the search space of one UE is totally overlapped by that of another UE can be greatly reduced. In the second method using multiple hash functions, a novel randomization scheme is proposed to generate independent hash functions for search spaces of different CCs. Because of the perfect randomization effect of the proposed method, search space overlapping of different UEs is reduced. Simulation results show that both the proposed methods effectively reduce the blocking probability of the control information compared to existing methods.

This letter proposes a method for choosing the best quantized beamforming vector that represents a subcarrier group, for coordinated beamforming in the downlink of multiuser multiple input multiple output-orthogonal frequency division multiplexing systems. The correlation between subcarriers is exploited for reducing the feedforward overhead, while maximizing the sum rate.

This paper proposes a single-carrier transmission method based on an overlap frequency-domain equalizing (FDE) and a coherent averaging. FDE is a block-based equalizing technique using discrete Fourier transform. A cyclic prefix is often used to avoid inter-block interference under multipath channel conditions, which reduces transmission efficiency. An overlap FDE is a technique to avoid the cyclic prefix insertion, but the residual interferences often exist after the FDE processing according to the channel conditions. The method proposed in this paper suppresses the residual interferences by applying a coherent averaging to the FDE outputs and improve the equalization performances. Computer simulation shows the effect of the proposed technique over the multipath channels.