Orthogonal frequency division multiplexing (OFDM) communication systems have great advantages, such as high spectrum efficiency and robustness against multipath fading. In order to enhance the advantages, this paper investigates an efficient utilization of both diversity combining and higher-level modulation (adaptive modulation) with a repetition code on the frequency domain in the OFDM systems. The repetition coded OFDM systems can achieve an improvement of performance with such a simple structure as one pair of transmit/receive antennas. In this paper, we derive simple closed-form equations for bit error probability (BEP) and throughput, and then improvements of those performances in the proposed OFDM systems are verified by both theoretical analysis and Monte Carlo simulation.

In this paper, multiple-symbol differential detection (MSDD) is applied to the differential unitary space-time-frequency coding (DUSTFC) scheme over frequency selective fading multiple-input multiple-output (MIMO) channels. The motivation of applying MSDD is to compensate for the performance loss of conventional (two-symbol observation) differential detection comparing with coherent detection, by extending the observation interval and considering the fading autocorrelations. Since the differential coding of DUSTFC can be performed in time or frequency domain, both the time-domain and frequency-domain MSDD are investigated. After calculating the frequency-domain fading autocorrelation, the decision metrics of MSDD considering appropriate fading autocorrelations are derived in time and frequency domain respectively. Bit error rate (BER) performances of the two kinds of MSDD are analyzed by computer simulations. Simulation results demonstrate that a considerable performance gain can be got by applying MSDD in both cases, and the transmit diversity gain can also be enhanced by applying MSDD. So that it is proved that full advantage of transmit diversity with DUSTFC can be taken by applying MSDD.

In the massive machine-type communication scenario, aiming at the problems of active user detection and channel estimation in the grant-free non-orthogonal multiple access (NOMA) system, new sets of non-orthogonal spreading sequences are proposed by using the zero/low correlation zone sequence set with low correlation among multiple sets. The simulation results show that the resulting sequence set has low coherence, which presents reliable performance for channel estimation and active user detection based on compressed sensing. Compared with the traditional Zadoff-Chu (ZC) sequences, the new non-orthogonal spreading sequences have more flexible lengths, and lower peak-to-average power ratio (PAPR) and smaller alphabet size. Consequently, these sequences will effectively solve the problem of high PAPR of time domain signals and are more suitable for low-cost devices in massive machine-type communication.

The complexity and scale of Networks-on-Chip (NoCs) are growing as more processing elements and memory devices are implemented on chips. However, under strict power budgets, it is also critical to lower the power consumption of NoCs for the sake of energy efficiency. In this paper, we therefore present three novel input unit designs for on-chip routers attempting to shrink their power consumption while still conserving the network performance. The key idea behind our designs is to organize buffers in the input units with characteristics of the network traffic in mind; as in our observations, only a small portion of the network traffic are long packets (composed of multiple flits), which means, it is fair to implement hybrid, asymmetric and reconfigurable buffers so that they are mainly targeting at short packets (only having a single flit), hence the smaller power consumption and area overhead. Evaluations show that our hybrid, asymmetric and reconfigurable input unit designs can achieve an average reduction of energy consumption per flit by 45%, 52.3% and 56.2% under 93.6% (for hybrid designs) and 66.3% (for asymmetric and reconfigurable designs) of the original router area, respectively. Meanwhile, we only observe minor degradation in network latency (ranging from 18.4% to 1.5%, on average) with our proposals.

In this paper, we propose a novel noncoherent maximum likelihood detection (NMLD) method for differential spatial multiplexing (SM) multiple-input multiple-output (MIMO) systems. Unlike the conventional maximum likelihood detection (MLD) method which needs the knowledge of channel state information (CSI) at the receiver, NMLD method has no need of CSI at either the transmitter or receiver. After repartitioning the observation block of multiple-symbol differential detection (MSDD) and following a decision feedback process, the decision metric of NMLD is derived by reforming that of MSDD. Since the maximum Doppler frequency and noise power are included in the derived decision metric, estimations of both maximum Doppler frequency and noise power are needed at the receiver for NMLD. A fast calculation algorithm (FCA) is applied to reduce the computational complexity of NMLD. The feasibility of the proposed NMLD is demonstrated by computer simulations in both slow and fast fading environments. Simulation results show that the proposed NMLD has good bit error rate (BER) performance, approaching that of the conventional coherent MLD with the extension of reference symbols interval. It is also proved that the BER performance is not sensitive to the estimation errors in maximum Doppler frequency and noise power.

In this letter, a construction of sparse measurement matrices is presented. Based on finite fields, a base matrix is obtained. Then a Hadamard matrix or a discrete Fourier transform (DFT) matrix is nested in the base matrix, which eventually formes a new deterministic measurement matrix. The coherence of the proposed matrices is low, which meets the Welch bound asymptotically. Thus these matrices could satisfy the restricted isometry property (RIP). Simulation results demonstrate that the proposed matrices give better performance than Gaussian counterparts.

In this letter, we propose a novel kind of uncertain query, top (k1,k2) query. The x-tuple model and the possible world semantics are used to describe data objects in uncertain datasets. The top (k1,k2) query is going to find k2 x-tuples with largest probabilities to be the result of top k1 query in a possible world. Firstly, we design a basic algorithm for top (k1,k2) query based on dynamic programming. And then some pruning strategies are designed to improve its efficiency. An improved initialization method is proposed for further acceleration. Experiments in real and synthetic datasets prove the performance of our methods.

Text categorization, especially short text categorization, is a difficult and challenging task since the text data is sparse and multidimensional. In traditional text classification methods, document texts are represented with “Bag of Words (BOW)” text representation schema, which is based on word co-occurrence and has many limitations. In this paper, we mapped document texts to Wikipedia concepts and used the Wikipedia-concept-based document representation method to take the place of traditional BOW model for text classification. In order to overcome the weakness of ignoring the semantic relationships among terms in document representation model and utilize rich semantic knowledge in Wikipedia, we constructed a semantic matrix to enrich Wikipedia-concept-based document representation. Experimental evaluation on five real datasets of long and short text shows that our approach outperforms the traditional BOW method.

Smart TVs are expected to play a leading role in the future networked intelligent screen market. Currently, many operators are planning to deploy it in large scale in a few years. Therefore, it is necessary for smart TVs to provide high quality services for users. Packet loss is one critical reason that decreases the QoS in smart TVs. Even a very small amount of packet loss (1-2%) can decrease the QoS and affect users' experience seriously. This paper applies stochastic differential equations to analyzing the queue in the buffer of access points in smart TV multicast systems, demonstrates the reason for packet loss, and then proposes an end-to-end error recovery scheme (short as OPRSFEC) whose core algorithm is based on Reed-Solomon theory, and optimizes four aspects in finite fields: 1) Using Cauchy matrix instead of Vandermonde matrix to code and decode; 2) generating inverse matrix by table look-up; 3) changing the matrix multiplication into the table look-up; 4) originally dividing the matrix multiplication. This paper implements the scheme on the application layer, which screens the heterogeneity of terminals and servers, corrects 100% packet loss (loss rate is 1%-2%) in multicast systems, and brings very little effect on real-time users experience. Simulations demonstrate that the proposed scheme has good performances, successfully runs on Sigma and Mstar Moca TV terminals, and increases the QoS of smart TVs. Recently, OPRSFEC middleware has become a part of IPTV2.0 standard in Shanghai Telecom and has been running on the Mstar boards of Haier Moca TVs properly.

In the Coordinated Multi-Point (CoMP) system under the condition of limited feedback, a reasonable coordinated set relies heavily on the splitting factor that is used to divide the total feedback bits into channel direction information (CDI) feedback bits and channel quality information (CQI) feedback bits. The relation of splitting factor and coordinated set is examined in this paper. After defining a penalty factor, we derive the net ergodic capacity optimization problem, whose variables to be optimized are the number of coordinated BSs, the divided area's radius and the splitting factor. According to an existing codebook and the quantized channel error model, the downlink received signal model is updated after adding the splitting factor. Through random matrix knowledge, the stochastic property of this model is obtained. A close approximate expression including the splitting factor to be optimized related to coordinated set is given. In addition, a revised adaptive feedback scheme is proposed to split the feedback bits. Simulation results show that the proposed scheme provides a significant performance gain, especially as the user velocity is high.