Due to the importance of maintaining the orthogonality among subcarriers, the estimation of carrier frequency offset (CFO) is a crucial issue in orthogonal frequency division multiplexing (OFDM) systems. The CFO estimation becomes complicated in OFDM direct-conversion receivers (DCRs), where additional analog impairments such as I/Q imbalance and time-varying DC offset (TV-DCO) exist. In this paper, we propose a novel joint estimation method for CFO and I/Q imbalance in the presence of TV-DCO. By using the linear property of the TV-DCO and employing a periodic pilot sequence, the desired estimates can be obtained in closed-form. Simulation results confirm the validity of the proposed method.

Recently, a new multi-carrier CDMA (MC-CDMA) system with cyclic-shift orthogonal keying (CSOK) has been proposed and shown to be more spectral and power efficient than conventional MC-CDMA systems. In this paper, a novel extension called the multiplexed CSOK (MCSOK) MC-CDMA system is proposed to further increase the data rate while maintaining a low peak-to-average power ratio (PAPR). First, the data stream is divided into multiple parallel substreams that are mapped into QPSK-CSOK symbols in terms of cyclic shifted Chu sequences. Second, these sequences are repeated, modulated, summed, and placed on IFFT subcarriers, resulting in a constant-modulus multiplexed signal that preserves the desired orthogonality among substreams. The receiver performs frequency-domain equalization and uses efficient demultiplexing, despreading, and demapping schemes to detect the modulation symbols. Furthermore, an alternate MCSOK system configuration with high link quality is also presented. Simulations show that the proposed MCSOK system attains lower PAPR and BER, as compared to conventional MC-CDMA system using Walsh codes. Under a rich multipath environment, the high link quality configuration exhibits excellent performance with both diversity gain and MCSOK modulation gain.

In this paper, a sidelobe suppression technique for orthogonal frequency division multiplexing (OFDM)-based cognitive radios (CR) is proposed. In the OFDM-based CR systems, after the CR terminal executes spectrum sensing, it transmits a CR packet by activating the subcarriers in the frequency bands where no signals are detected (hereinafter, these subcarriers are called "active subcarrier") and by disabling (nulling) the subcarriers in the frequency bands where the signals are detected. In this situation, a problem arises in that the signals that leak from the active subcarriers to the null subcarriers may interfere with the primary systems. Therefore, this signal leakage has to be minimized. In many OFDM-based wireless communication systems, one packet or frame consists of multiple OFDM symbols and the discontinuity between the consecutive OFDM symbols causes the signal leakage to the null subcarriers. In the proposed method, signal leakage to the null subcarriers is suppressed by regenerating null subcarriers in the frequency-domain signal of the whole packet as follows. One CR packet consisting of multiple OFDM symbols having null subcarriers and guard interval (GI) is buffered and oversampled, and then the oversampled signal is Fourier transformed at once and consequently the frequency-domain signal of the packet is obtained. The null subcarriers in the frequency-domain signal are zeroed again, and then the signal is inverse Fourier transformed and transmitted. The proposed method significantly suppresses the signal leakage. The spectral power density, the peak-to-average power ratio (PAPR) and the packet error rate (PER) performances of the proposed method are evaluated by computer simulations and the effectiveness of the proposed method is shown.

In this paper, a novel decentralised dynamic sub-carrier assignment (DSA) algorithm for orthogonal frequency division multiple access (OFDMA)-based adhoc and cellular networks operating in time division duplexing (TDD) mode is proposed to solve the hidden and exposed node problem in media access control (MAC). This method reduces the co-channel interference (CCI), and thus increases the overall throughput of the network. Reduced CCI and increased throughput can be achieved, if time and frequency selectivity of the multi-path fading channel and the channel reciprocity offered by the TDD are fully exploited. The time and frequency selectivity of the channel are usually the main problem in mobile communication. However, in the context of channel assignment for OFDMA-based networks in TDD mode, the time and frequency selectivity of the channel are the key to reduce the interference. In the proposed channel assignment mechanism, several clusters of sub-carriers are assigned for data transmission between a transmitter and a receiver only if the corresponding channels of those sub-carriers linking this transmitter to potential victim receivers are deeply faded. In addition, the proposed algorithm works in a fully decentralised fashion and, therefore, it is able to effectively support ad hoc and multihop communication as well as network self-organisation. Numerical results show that the throughput obtained by the proposed approach for a given quality of service is higher than those of the conventional methods in any precondition of adhoc geographic scenario.

UHF band passive RFID systems are being steadily adopted by industries because of their capability of long range automatic identification with passive tags. For an application which demands a large number of readers located in a limited geographical area, referred to as dense reader mode, interference rejection among readers is important. The coding method, baseband or subcarrier coding, in the tag-to-reader communication link results in a significant influence on the interference rejection performance. This paper examines the frequency sharing of baseband and subcarrier coding UHF RFID systems from the perspective of their transmission delay using a media access control (MAC) simulator. The validity of the numerical simulation was verified by an experiment. It is revealed that, in a mixed operation of baseband and subcarrier systems, assigning as many channels as possible to baseband system unless they do not exploit the subcarrier channels is the general principle for efficient frequency sharing. This frequency sharing principle is effective both to baseband and subcarrier coding systems. Otherwise, mixed operation fundamentally increases the transmission delay in subcarrier coding systems.

This paper proposes an architecture of an interference canceller for satellite communications with super-posed transmission, which is applicable not only to QPSK but also to 16QAM transmission to get higher satellite capacity. We implement it as an FPGA-based prototype and verify its performance. We propose here to use a new method to measure the satellite round-trip delay using an extended matched filter (EMF), which can work in low C/N conditions such as 0 dB and under. Given this performance, our canceller can work in a network in which forward and reverse links have the same power level. The results of the laboratory tests for QPSK show that interference can be suppressed by about 30 dB and that the BER degradation due to the canceller was small enough for operation.

In this paper, the issue of carrier frequency offset (CFO) compensation in interleaved orthogonal frequency division multiple access (OFDMA) uplink system is investigated. To mitigate the effect of multiple access interference (MAI) caused by CFOs of different users, a new parallel interference cancellation (PIC) compensation algorithm is proposed. This scheme uses minimum mean square error (MMSE) criterion to obtain the estimation of interference users, then circular convolutions are employed to restore MAI and compensate CFO. To tackle the complexity problem of circular convolutions, an efficient MAI restoration and cancellation method is developed. Simulations illustrate the good performance and low computational complexity of the proposed algorithm.

A Generalized Symbol-rate-increased (GSRI) Pragmatic Adaptive Trellis Coded Modulation (ATCM) is applied to a Multi-carrier CDMA (MC-CDMA) system with bi-orthogonal keying is analyzed. The MC-CDMA considered in this paper is that the input sequence of a bi-orthogonal modulator has code selection bit sequence and sign bit sequence. In, an efficient error correction code using Reed-Solomon (RS) code for the code selection bit sequence has been proposed. However, since BPSK is employed for the sign bit modulation, no error correction code is applied to it. In order to realize a high speed wireless system, a multi-level modulation scheme (e.g. MPSK, MQAM, etc.) is desired. In this paper, we investigate the performance of the MC-CDMA with bi-orthogonal keying employing GSRI ATCM. GSRI TC-MPSK can arbitrarily set the bandwidth expansion ratio keeping higher coding gain than the conventional pragmatic TCM scheme. By changing the modulation scheme and the bandwidth expansion ratio (coding rate), this scheme can optimize the performance according to the channel conditions. The performance evaluations by simulations on an AWGN channel and multi-path fading channels are presented. It is shown that the proposed scheme has remarkable throughput performance than that of the conventional scheme.

Carrier aggregation is a potential technology for the LTE-Advanced system to support wider bandwidth than the LTE system. This paper analyzes the performance of carrier aggregation under elastic traffic, and compares it to that of a simpler approach for the same purpose, referred to as the independent carrier approach. The queueing behaviors of these two approaches are formulated as one fast versus multiple slow state-dependent Processor Sharing servers, respectively. Both analytical and simulation results show that when there are L component carriers with uniform bandwidth in the system, the performance of the carrier aggregation approach is L times better than that of the independent carrier approach in terms of the average user delay and throughput under the same traffic load.

In this paper, an error correction scheme suitable for MC-DS-CDMA system with bi-orthogonal modulation is proposed. The input sequence of a bi-orthogonal modulator consists of n - 1 bit code selection bit sequence and 1 bit sign bit sequence. In order to apply an efficient error correction code, the following points should be considered; (1) if the code selection bits can be protected sufficiently, the sign bit error can also be reduced sufficiently, (2) since a code selection bit sequence consists of n - 1 bits, employing a symbol error correcting code is more effective for encoding code selection bits, (3) the complexity of the error correction encoder and decoder implementations should be minimum. Based on these conditions, we propose to employ Reed-Solomon (RS) code for encoding the code selection bits and no error correction code for the sign bit. Additionally, detection algorithm at the bi-orthogonal demodulator is modified for compensating degradations of the sign bit error rate performance. The performance in an Additive White Gaussian Noise (AWGN) channel is evaluated by both theoretical analysis and computer simulations. The performance evaluations by simulations on multi-path fading channels are also shown. It is shown that the proposed scheme has remarkable improvement.

In this paper, we propose a novel frequency-hopping scheme in order to improve the BER (Bit Error Rate) performance of the Partial Block MC-CDMA (PB/MC-CDMA) system. The joint carrier distribution and frequency hopping (JDFH) scheme achieves the optimal frequency diversity gain while avoiding interference. By contrast, the conventional FH scheme only avoids interference, and the frequency interleaving scheme achieves only frequency diversity. The JDFH scheme thus performs better than conventional schemes, such as carrier FH, block FH, or frequency interleaving. Through computer simulations, we confirmed the superior performance of the PB/MC-CDMA system when using the JDFH scheme.

Recently, frequency-domain equalization (FDE) has been attracting much attention as a way to improve single-carrier (SC) signal transmission in a frequency-selective wireless channel. Since the SC signal spectrum is spread over the entire signal bandwidth, FDE can take advantage of channel frequency-selectivity and achieve the frequency diversity gain. SC with FDE is a promising wireless signal transmission technique. In this article, we review the pioneering research done on SC with FDE. The principles of simple one-tap FDE, channel estimation, and residual inter-symbol interference (ISI) cancellation are presented. Multi-input/multi-output (MIMO) is an important technique to improve the transmission performance. Some of the studies on MIMO/SC with FDE are introduced.

In OFDM systems, the estimation/correction of carrier frequency offset (CFO) is crucial to maintain orthogonality among subcarriers. However, the CFO estimation suffers from DC offset (DCO) generated in low-cost direct-conversion receivers (DCRs). More seriously, in practice, DCO is time-varying due to the automatic gain control. In this paper, a novel CFO estimator in the presence of time-varying DCO is proposed. It is shown the residual DCO after high-pass filtering varies in a linear fashion. Based on this observation and the periodicity of the training sequence, we derive a CFO estimator independent of DCO. Also, the residual DCO can be estimated, using the obtained CFO. The validity of the proposed estimation method is demonstrated by simulations.

Single-carrier ultra-wideband (SC-UWB) is weak due to the problem of serious inter-symbol interference (ISI), which is generated in dense multipath with a long root-mean-square (RMS) delay spread. The selective RAKE (SRAKE) based RAKE-decision feedback equalizer (RAKE-DFE) receiver is usually employed to combat ISI in practical SC-UWB systems. Considering the system complexity, however, the number of RAKE fingers is usually small. In this case, conventional RAKE-DFE receivers can hardly collect enough energy to achieve a good performance. In this paper, the optimum SRAKE based RAKE-DFE receiver was proposed as a solution. Theoretical analysis and simulations are presented. Results and conclusions show that the proposed SRAKE scheme is optimum to collect energy of multipath. Moreover, the proposed RAKE-DFE receiver outperforms conventional RAKE-DFE receiver by about 1 dB, but the complexity for them both is almost the same.

In this letter, we propose a novel frequency-domain equalizer (FDE) for single-carrier systems characterized by severe inter-symbol interference (ISI) channels; it consists of a linear FDE and an iterative block noise-predictor (IBNP). Unlike the FDE with time-domain noise predictor (FDE-NP), the proposed scheme allows the feedback equalizer being an uncausal filter, and performs the noise prediction in an iterative manner. For this reason, FDE-IBNP can remove both precursor and postcursor ISI, and alleviate the impact of error-propagation. Besides, our scheme has lower computational complexity than the present iterative block equalizers.

Multicarrier code division multiple access (MC-CDMA) systems are well suited for high data rate wireless multimedia services, due to their ability to convert frequency-selective fading channels to distinct flat fading channels with low complexity fast Fourier transform (FFT) devices. However, when multiple users are present, the performance of MC-CDMA systems is degraded by the multiuser interference (MUI) when the channel is frequency-selective. In order to mitigate MUI, we present a joint algorithm that combines transmit power control, antenna array processing and multiuser detection at the receiver. Interestingly, the frequency-selectivity that entails the MUI also provides multipath diversity which can help suppress the MUI. Performance of the algorithm in a number of MC-CDMA system models is evaluated in terms of the average transmit power to achieve the target signal to interference plus noise ratio (SINR). Simulations confirm the outstanding performance of this algorithm compared with the existing ones in MC-CDMA systems.

In this letter, we present the impact of carrier frequency offset (CFO) in dual-hop orthogonal frequency division multiplexing (OFDM) systems with a fixed relay for frequency-selective fading channels. Approximate expressions of the average signal-to-noise ratios (SNRs) for both downlink and uplink are obtained and validated by simulations. It is shown that dual-hop systems have slightly worse average SNR degradation than single-hop systems. We also show that the average SNR degradation due to the CFO varies according to the gap between average received SNRs for the first and the second hop.

A single-carrier multiple-input multiple-output (MIMO) system with frequency-selective channels suffers from the inter-symbol interference (ISI) and the co-channel interference (CCI). To eliminate both type of interference, we propose in this letter a hybrid two-stage decision-feedback equalizer (HTS-DFE), which performs the frequency-domain equalization (FDE) in the first stage and the layered serial interference-cancellation (SIC) in the second stage. Since the decision-feedback (DF) or noise-prediction (NP) architecture can be employed in FDE or SIC, the proposed equalizer actually can have four variations that achieve the same mean square error (MSE) under the assumption of perfect feedback. Further, we combine HTS-DFE with the decoded decision-feedback (DDF) scheme to mitigate the error-propagation encountered in the practice. Simulation results confirm that the proposed HTS-DFE can outperform the existing equalizers significantly.

In direct-conversion orthogonal frequency division multiplexing (OFDM) receivers, the impact of frequency-dependent I/Q mismatch (IQ-M) with carrier frequency offset (CFO) must be considered. A preamble-assisted estimation is developed to circumvent the frequency-dependent IQ-M with CFO. The results of a simulation and an experiment show that the proposed method could provide good estimation efficiency and enhance the system performance. Moreover, the proposed scheme is compatible with current wireless local area network standards.

In this letter, a relay-assisted transmission scenario over frequency-selective fading channels perturbed by different random carrier frequency offsets is considered. OFDM and block-double differential (BDD) design are implemented to overcome the problem of intersymbol interference (ISI) and carrier frequency offsets (CFOs). We analyze the symbol error rate (SER) performance of decode-and-forward relaying with BDD design in wireless cooperative communications over frequency-selective fading channels and derive a theoretical upper bound for average SER when the relay (R) is error free. It can be seen from our analysis that the system performance is influenced by the ability of R to decode, and when R decodes without error, both spatial and multipath diversity can be obtained without requiring any knowledge of channel state information and CFO information at the receivers. Numerical examples are provided to corroborate our theoretical analysis.