For differential space-time block coding (DSTBC), this paper proposes a simple decision-feedback block differential detection (DFBDD) scheme with small computational complexity, in order to improve tracking capability on fast time-varying fading channels. Decisions can be derived directly without computation of metrics in the proposed DFBDD. It can also employ higher-order channel prediction, which needs neither complex trellis search algorithms nor complex channel prediction recursive algorithms. Finally, computer simulation results confirm that the proposed DFBDD with higher-order channel prediction has higher tracking capability at low computational complexity than the conventional block differential detection without channel prediction.

A frequency domain equalization for multipath fading is widely used for OFDM systems and accurate channel estimation is required. In conventional OFDM systems, pilot symbols are inserted periodically in time and frequency domain by a transmitter for channel estimation. Channel estimation error is quite large, especially in frequency-selective fading channel, because the estimates are interpolated with the restricted pilot subcarriers. The channel estimation error can be decreased by assigning pilot symbols to all subcarriers in specified OFDM blocks. In that case, however, no data symbols can be transmitted in OFDM blocks where pilot symbols are transmitted. In this paper we propose a new channel estimation scheme in which OFDM signal is transmitted after spread by chirp sequences, and channel estimation of all subcarriers is achieved with only one pilot symbol without interpolation. Simulation results show that the MSE performance of channel estimation and the BER performance can be significantly improved by the proposed scheme.

SC-OFDM (single carrier orthogonal frequency division multiplexing) has received considerable attention in recent years due to its low peak power and robust performance against multipath fading channels. There are emerging demands in a variety of wireless communication systems to transmit multiple SC-OFDM signals in different channels to improve bandwidth efficiency. However, the peak power of such multi channel SCOFDM (MCH-SC-OFDM) signals increases as more signals are multiplexed. In this paper, we present two techniques which can lower the peak power of MCH-SC-OFDM signals by applying phase rotation to SC-OFDM signals in the frequency domain. It is shown through simulation results that the proposed techniques do not introduce any penalties on error performance of the receivers.

In a multiple-input multiple-output (MIMO) system, cross-polarized antenna selection yields significant reduction in cost and hardware size. However, actual benefits of the technique are dependent on the propagation characteristics including channel polarization. To accurately characterize the target 2 GHz-band MIMO channels, the authors conduct 2 GHz cross-polarized channel measurement campaigns. Based on the measured data, novel channel models specifically for the 2 GHz bands are established. In addition, we evaluate the performance improvement obtained with cross-polarized antenna selection using the channel models. Simulation results reveal that antenna selection is particularly useful in the low SNR regime, and that the system capacity at cell edges can be increased up to 13%.

This paper introduces novel frame synchronization methods which require lower computational complexity than conventional methods. There are two contributions in this paper. First, a low-complexity method to compute signal estimation error is introduced. Second, an algorithm is proposed to search frame timing candidates efficiently for an estimation error minimizing timing candidate. The simulation results indicate that the proposed algorithm yields a computation reduction of nearly 80% when compared to the conventional method, while it maintains the error performance of a conventional method.

This paper proposes new over-the-air inter-node carrier phase synchronization to maintain constant inter-node carrier phase offset among wireless nodes participating cooperative coherent transmission over slow Rayleigh fading channels. In the proposed method, one of the two nodes updates its carrier frequency and carrier phase periodically using the measured bi-directional channel parameters. In OFDMA systems, bi-directional channels are measured in distributed subcarriers to attain frequency diversity effect. In the presence of phase noise, one node can track the other node's phase error using the proposed synchronization. Numerical results show that the proposed method maintains inter-node carrier phase alignment error less than 5 degrees. It is also revealed that the proposed method achieves accurate synchronization with feasible amount of signaling and complexity in LTE-advanced.

This paper proposes an ultra accurate inter-node carrier frequency synchronization which keeps carrier frequency offset between wireless nodes less than 1Hz in slowly time-varying Rician fading channels. In the proposed method, one of the two nodes adjusts its carrier frequency to be close to the other node's carrier frequency based on bi-directional channel measurements. We demonstrate that the proposed scheme achieves very small inter-node carrier frequency offset, e.g. less than 0.01 Hz. This synchronization scheme is useful in improving performance of wireless applications, such as cooperative coherent transmission or mobility measurements.

This paper proposes an ultra accurate carrier frequency synchronization for OFDMA/TDD systems, which keeps carrier frequency offset between two wireless nodes less than 0.01Hz in frequency-selective Rayleigh fading environments. In the proposed method, bi-directional channels are measured in multiple subbands of an OFDMA system. One of the two nodes corrects its carrier frequency with high accuracy using the bidirectional channel parameters corresponding to subbands of good channel quality. Numerical results show that appropriate weighting and averaging of relative phases of bi-directional channel parameters over multiple subbands improves performance of inter-node carrier frequency syhcoronization. The proposed method is a promising techology towards cooperative coherent transmission or low-power wireless communications.

This paper proposes an efficient multiuser relaying scheme in OFDMA systems. In the proposed scheme, multiple terminals transmit packets simultaneously in the same subband and multiple relay stations retransmit their received signals in different subbands after subband conversion. The base station (BS) extracts each packet from multi-dimensional signals received at the different subbands. In advance of data transmission, the BS's scheduler selects appropriate terminals for packet transmission such that the BS can detect each packet correctly. Numerical results show that the proposed relaying scheme achieves much higher system throughput than the conventional relaying scheme.

This paper investigates an amplify and forward (AF) relaying scheme compatible with a legacy wireless system in cellular environments. In the proposed scheme, terminals and relay stations use different frequency bands for signal transmission. The two frequency bands for terminals and for relay stations are used by individual frequency reuse factors in cellular environments. We study frequency planning and a relay station's location for high spectral efficiency. The numerical results show that the spectral efficiency of a relaying scheme is enhanced by deploying a relay station close to the base station (BS) and using frequency reuse factor 1 for a relay station to BS link.

This paper proposes cooperative control for antenna array calibration to exploit channel reciprocity in OFDMA/TDD systems. The proposed scheme seeks a cooperative supporter which is another terminal or a fixed station close to the terminal. In cooperative calibration mode, the terminal compensates mismatch of analog gains between antennas, based on bidirectional channel measurements between the terminal and the supporter. The proposed scheme selects adaptively cooperative or normal calibration mode through a base station, depending on the terminal's situation. The numerical results show that the proposed scheme improves calibration accuracy significantly compared to the case of no cooperative calibration. The proposed scheme keeps phase error less than 5deg in any position of terminal in a cell, maintaining practical amount of control signalling and complexity.

This paper proposes a new antenna array calibration using frequency selection in OFDMA/TDD systems. In the proposed method, subbands or frequencies of good channel conditions are initially selected for accurate channel measurement. Next, the relative calibration is performed at the selected subbands, in which mismatch of analog gains in multiple antennas is compensated by use of measured channel parameters on uplink and downlink. Third, the calibration parameters are interpolated in frequency domain for whole bandwidth. The proposed method keeps more accurate channel reciprocity for whole bandwidth than the conventional method which does not use the frequency selection. The proposed method is effective in exploiting channel reciprocity at both base station and terminals keeping feasible feedback amount and low-cost operation.

This paper proposes a novel channel estimation method for iterative equalization in MIMO systems. The proposed method incorporates co-channel interference (CCI) cancellation in the channel estimator and the channel estimation is successively performed with respect to each stream. Accuracy of channel estimation holds the key to be successfully converged the iterative equalization and decoding process. Although the channel estimates can be re-estimated by means of LS (Least Square) channel estimation using tentative decisions obtained in the iterative process, its performance is severely limited in a MIMO system because of erroneous decisions and ill-conditioned channel estimation matrix. The proposed method can suppress the above effects by means of CCI cancellation and successive channel estimation. Computer simulation confirms that the proposed channel estimation method can accurately estimate the channel, and the receiver with iterative equalization and the proposed method achieves excellent decoding performance in a MIMO-SM (Spatial Multiplexing) system.

This paper evaluates reduced-complexity maximum-likelihood (ML) detectors with soft-decision outputs in a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system. The reduced-complexity ML detectors, e.g., a sphere decoder (SD) or QR decomposition (QRD)-M algorithm, are very promising candidates as a MIMO detector because they can estimate the ML or quasi-ML symbol with very low computational complexity. These detectors, however, have a difficulty in producing soft-decision outputs because they may lose likelihood required for calculating log-likelihood ratio (LLR), which is related to reliability of detector outputs. We have proposed a simple likelihood estimation method which estimates the lost likelihood by means of a combination of the ML estimation and spatial-filtering method. This paper evaluates the reduced-complexity ML detectors with the proposed method in a spatially correlated MIMO channel to prove its availability in a realistic environment. Computer simulation confirms that the proposed method provides excellent decoding performance and achieves very high system capacity.

This paper proposes a likelihood estimation method for reduced-complexity maximum-likelihood (ML) detectors in a multiple-input multiple-output (MIMO) system. Reduced-complexity ML detectors, e.g., sphere decoder (SD) and QR decomposition (QRD)-M algorithm, are very promising candidates as a MIMO detector because they can estimate the ML or quasi-ML symbol with very low computational complexity. However, they may lose likelihood information about signal vectors having the opposite bit to hard decisions. Therefore, bit error rate performances of the reduced-complexity ML detectors are inferior to that of the ML detector when soft-decision decoding is employed. This paper proposes a simple estimation method of the lost likelihood information suitable for the reduced-complexity ML detectors. Computer simulation confirms that the proposed method provides excellent decoding performance, keeping the advantage of computational cost of the reduced-complexity ML detectors.

This paper proposes a frequency-domain equalizer (FEQ) which utilizes not only guard interval but also redundancy in the frequency domain to eliminate inter-symbol and intercarrier interferences. The proposed FEQ employs the hybrid criterion, i.e., the zero-forcing (ZF) criterion for compensating desired subcarriers and the minimum mean square error (MMSE) criterion for suppressing interference. The proposed hybrid-FEQ achieves a good equalization performance, because it can suppress the noise enhancement caused by the ZF criterion with relatively small computational complexity exploiting soft-decision forward error correction (FEC). In this paper, we show its equalization performance and complexity compared with the conventional FEQs

This paper proposes performance improvement schemes for multiple-input multiple-output (MIMO) communication systems employing per transmit antenna differential encoding (PADE), which generates an independent differentially encoded sequence for each of the multiple transmit antennas by means of space-time coding and mapping. PADE can operate without channel state information (CSI) at both the transmitter and the receiver. This paper proposes the following schemes suitable for PADE: 1) novel mapping schemes for transmit diversity (TD) and spatial multiplexing (SM); 2) introduction of the state-reduction algorithm for per-survivor processing (PSP), if the channel is estimated by a large number of observation symbols of the received signal. Next, computer simulation confirms that the proposed schemes can improve bit error rate (BER) performance and can mitigate floor of BER on slowly time-varying fading channels, keeping small hardware complexity.

This paper proposes a demodulator employing data detectors which do not need a known sequence for its startup, i.e., a parallel blind demodulator, in the presence of intersymbol interference (ISI). This demodulator operates as follows: (1) it includes multiple adaptive maximum-likelihood sequence estimations with per-survivor processing (PSP-MLSEs) respectively for different estimated channel memory lengths and different sampling timing thanks to over-sampling; (2) it selects the most reliable PSP-MLSE sequence according to reliability information. Thus, it uses the known sequence not for data detection and timing/parameter estimation but for the frame synchronization. Finally, computer simulations confirm that performance of this parallel blind demodulator is similar to that of the demodulator with the perfect timing/parameter information in the presence of ISI.

This paper evaluates a parallel blind demodulator combined with a Reed-Solomon (RS) decoder in the presence of intersymbol interference (ISI) by means of hardware experiments. Our target systems are point to multi-point (P-MP) communication systems for business use, of which bit rates in the air are 9.6 kbit/s. In such narrow frequency band systems, fast time-varying fading is a serious problem. The implemented parallel blind demodulator employs adaptive maximum-likelihood sequence estimation with per-survivor processing (PSP-MLSE), and the RS code is used for a forward error correction (FEC) code. Finally, experimental results confirm that the parallel blind demodulator combined with the RS decoder achieves excellent BER performance.

This paper compares the following two maximum likelihood sequence estimation schemes incorporating carrier phase estimation for coded MPSK: (1) the respective-state channel estimation and (2) the multiple differential detection. This paper confirms by analysis and computer simulation that both schemes have similar performance in the absence of intersymbol interference.

This paper proposes an adaptive maximum-likelihood sequence estimation (MLSE) by means of combined equalization and decoding, i.e., adaptive combined MLSE, which employs separate channel estimation for respective states in the Viterbi algorithm. First, an approximate metric including channel estimation is derived analytically for this proposed adaptive combined MLSE. Secondly, procedures to accomplish blind equalization are investigated for the proposed MLSE. Finally, its excellent BER performance on fast time-varying fading channels is confirmed by computer simulation, when the proposed MLSE operates as a blind equalizer.>

This paper proposes an adaptive maximum-likelihood sequence estimator (MLSE), which is a suboptimum approximation to the adaptive maximum-likelihood detector and is capable of tracking fast time-varying intersymbol interference (ISI) channels. The adaptive MLSE consists of a channel estimator and an MLSE implemented by the Viterbi algorithm (VA). The novel feature of the proposed adaptive MLSE is a channel estimation scheme, where the channel estimation is accomplished for each state in the VA along the surviving path connected to each state. This MLSE makes it possible to estimate a channel impulse response without an influence of a decision delay inherent in the VA, and the performance and complexity of the proposed procedure is controlled by the memory length of the VA. The bit error rate performance of the proposed MLSE is confirmed by experimental results. It is shown that the proposed MLSE has a capability of excellent tracking performance in a severe environment caused by fast time-varying ISI, for example frequency selective multipath fading in digital mobile radio communications.>

In robot vision the detection of edges of an image serves to simplify the data, in the early stages, into a form which is more easily processed. But because of noise or due to the inherent weaknesses of the chosen edge detector, gaps or interruptions in the edges may occur. These gaps must be filled or linked to form a more continuous edge. A new method of edge linking is proposed that makes use of a weighting system based on edge segment position, direction, and magnitude in deciding the optimal choice of which edge segments to link. Edge pixels are labeled and formed into edge segments, and the edge segments are then linked based on the decision of the weighting system. As a result of the unique way in which the gaps between edge segments are filled, distortion of the edge image is kept minimal, the proposed edge linker can be combined with different edge detection schemes. Comparisons with a method previously proposed by the authors shows an improvement in the quality of the linked edge map produced as well as a savings in computational time.