In this paper, we propose a computationally efficient decoding algorithm for space-time trellis codes in slow Rayleigh fading channels. The proposed scheme is based on a stack algorithm with two key ideas: (i) a variable stack size depending upon the signal-to-noise ratio to avoid the exhaustive search of paths and (ii) a normalized metric, which is defined as each cumulative path metric divided by its own length in the stack, to provide an appropriate comparison of the paths with different lengths. Simulation results demonstrate that the proposed algorithm achieves near-ML performance with significant reduction in complexity, compared with the conventional Viterbi algorithm.

The wireless mesh networks (WMNs) are statically deployed on heterogeneous areas and are operating in open wireless media, and thus it coexists with other networks operating on the same frequency with the same or different radio access technology (RAT). The WMNs experience two types of interferences according to the source of interference. Coexisted networks with WMN induce the interference called external interference and nodes in WMN experience interference each other, which is called internal interference. The increased interferences can degrade the performance significantly. To resolve this coexistence problem, we propose three channel assignment schemes for hybrid multi-channel protocol (HMCP) by recognizing the states of each channel and selecting its operating channel.

The wireless mesh networks (WMNs) are statically deployed on heterogeneous areas and are operating in open wireless media, and thus it coexists with other networks operating on the same frequency with the same or different radio access technology (RAT). The WMNs experience two types of interferences according to the source of interference. Coexisted networks with WMN induce the interference called external interference and nodes in WMN experience interference each other, which is called internal interference. Most of existing protocols strive for dealing with internal interference. But the increased external interferences can degrade the performance of WMN significantly. To resolve this coexistence problem, we propose three channel assignment schemes for hybrid multi-channel protocol (HMCP) by capturing the states of each channel and selecting its operating channel. We also devise a seamless multi-channel monitoring method to recognize channel states without performance degradation. Intensive simulation results demonstrate that our proposed channel assignment schemes outperform existing HMCP's channel assignment scheme in terms of aggregate throughput, delay, and fairness.

The stack-based iterative precoding (SBIP) is an efficient algorithm that achieves a tradeoff between bit error rate (BER) performance and computational complexity in multi-user MIMO downlink systems. In this paper, we propose an adaptive SBIP algorithm with variable candidate set and statistically derived threshold for pruning. By analyzing the upper bound of accumulated metric, the range of candidate set is calculated. The probability distribution is also derived for statistical pruning of the tree search. Applying these ideas, the number of undesirable node visit is significantly reduced. Monte-Carlo simulations show that the proposed algorithm considerably reduces the computational complexity whereas the performance is close to that of sphere encoder (SE).

In this paper, we propose a new MAC protocol to effectively exploit spatial reuse of IEEE 802.11 distributed coordination function (DCF). The existing DCF node simply blocks its transmission if the medium is sensed to be busy. Thus, it is inefficient in terms of the network throughput due to overcautious estimation of the interference. The proposed spatial reuse DCF (SRDCF) employs the information of locations and the required signal to interference ratio (SIR) to predict the interference more accurately. Then, it determines whether the node blocks its data transmission or not. Based on the interference estimation (i.e., feasibility analysis), we extract the appropriate transmission power for successful concurrent transmissions. By doing this, the node of the SRDCF can adjust its transmission power. Consequently, the SRDCF increases the network throughput by allowing concurrent transmissions. Theoretical analysis and simulation results show that the proposed SRDCF works better than original IEEE 802.11 DCF.

In this paper, a computationally efficient iterative detection algorithm (IDA) for D-STTD systems is presented. To find an optimal sequence by iterative searching, the proposed scheme consists of two stages. Firstly, the IDA establishes the candidate sequences. Then, it iteratively finds an optimal sequence by examining the candidate sequences until the early termination condition is satisfied. The adaptation of the number of candidate sequences makes the proposed IDA more efficient in computational complexity. The proposed detector is shown to achieve the near-ML performance with significantly reduced complexity.

In this paper, we propose a computationally efficient stack-based iterative precoding (SBIP) algorithm that achieves a performance comparable to the sphere encoder for multi-user MIMO downlink systems. The proposed algorithm is basically based on stack algorithm aided lattice reduction in the closest lattice point search of vector precoding. The first full expansion is tried by selecting the first level node with a minimum metric, constituting a reference metric. To find the best sequence, it iteratively visits each node and terminates the expansion by comparing node metrics with the calculated reference metric. In addition, we derive the range of candidate set size by using a characteristic of vector precoding. All things considered, the number of undesirable node visit is significantly reduced. Monte-Carlo simulations show that the proposed algorithm considerably reduces the computational complexity whereas the performance is close to that of sphere encoder.

RFID is a generic term for technologies which use RF waves to identify, track, or categorize any object. One of the research areas in RFID systems is a tag anti-collision protocol; how to reduce identification time with a given number of tags in the field of an RFID reader. There are two types of tag anti-collision protocols for RFID systems: tree based algorithms and slotted aloha based algorithms. Since the tree based tag anti-collision protocols achieve 100% read rate, we consider how to improve the performances of the tree based RFID tag anti- collision protocols. This paper proposes bi-slotted tree based tag anti-collision protocols, bi-slotted query tree algorithm (BSQTA) and bi-slotted collision tracking tree algorithm (BSCTTA), which reduce both prefix overhead and iteration overhead by time divided responses depending on whether the collided bit is '0' or '1'. According to the simulation results, the bi-slotted tree based RFID tag anti-collision protocols require less time consumption for tag identification than the present tag anti-collision protocols.

The closed-loop multiple-input multiple-output (MIMO) systems can provide considerably improved system performances by taking advantage of channel information at the transmitter. In this paper, we propose an efficient transmit power allocation utilizing a fraction of channel feedback for the sorted QR decomposition (SQRD) based vertical Bell labs layered space-time (V-BLAST) systems. With the proposed transmit power allocation, the bit error rate (BER) performance can be improved significantly compare to the conventional open-loop SQRD based V-BLAST systems. Furthermore, the feedback overhead of the system can be reduced efficiently by employing the feedback reduction technique. Simulation results with some numerical analysis demonstrate the validity of the proposed algorithm.

V-BLAST (Vertical Bell Labs Layered Space-Time) system is considered as a key enabling technology for achieving high spectral efficiency in wireless communication. To meet the requirements of low complexity and high constant throughput in practical V-BLAST system design, we propose a computationally efficient stack-based tree searching algorithm (SBSA) from a hardware implementation point of view. To reduce the complexity, we introduce sub-space division method, which divides the constellation search space into four or nine sub- spaces. Stack size is adapted to eliminate redundant tree searching and early termination is employed to ensure a constant throughput. Consequently, the SBSA increases the system throughput with significant complexity reduction while achieving near-ML performance, compared with ML detector.

In this paper, we propose an efficient transmit strategy utilizing the partial channel information feedback for the QRD based V-BLAST systems. In the proposed scheme, after QR decomposition (QRD) of the channel matrix, positive real-valued diagonal elements of R are forwarded to the transmitter through the feedback channel. Based on the feedback information, the transmitter allocates the optimum transmit power to minimize the bit error rate (BER) of the system. With small feedback overhead, the proposed scheme shows significantly improved BER performance compare to the conventional sorted QR decomposition (SQRD) based V-BLAST systems. Moreover, we can reduce the receiver complexity by removing the unnecessary sorting process at the receiver.

MIMO-OFDM system is a promising way to achieve high data-rate wireless transmission. In this paper, loosely synchronous (LS) codes assisted multipath channel interference canceller (MCIC) is proposed for the channel estimation of MIMO-OFDM systems. Since LS codes have perfect autocorrelation and cross-correlation functions within certain vicinity of the zero shifts, LS codes assisted MCIC can substantially reduce multipath channel interference. Simulation results show that the MCIC estimator outperforms least square estimator and has slightly inferior performance as compared with linear minimum mean square error (LMMSE) estimator. Moreover the MCIC estimator has the advantage of not requiring any information about the channel statistics like auto-correlation of channels.

An orthogonal frequency division multiplexing (OFDM) system combined with space division multiple access (SDMA) is considered as a spectral efficient transmission scheme. However, the computational effort is very intensive to achieve the near ML performance. In this paper, we propose a computationally efficient sequential multiuser detector using a stack algorithm for uplink OFDM/SDMA systems. The proposed scheme has a structure with three interrelated procedures, namely preprocessing, stack-based tree searching, and optimal sequence selection. In order for complexity to be reduced with achieving appropriate performance, the proposed scheme is designed to search branches with constrained stack size. Simulation results show that the proposed algorithm yields the average bit error rate (BER) performance close to that of sphere decoding (SD) applied to OFDM/SDMA systems with significant saving in complexity.

Channel state information (CSI) at the transmitter in multiple-input multiple-output (MIMO) downlink systems is indispensable to beamforming or precoding to take advantage of MIMO capacity. In time-division duplex (TDD) systems, CSI for downlink can be obtained from uplink channel using reciprocity. However, the CSI from uplink is not accurate enough to keep track of the continuously varying channel characteristic in downlink period. In this paper, a MIMO-OFDM downlink channel prediction technique based on Kalman filter is proposed for IEEE802.16e systems. The proposed method consists of three procedures: MMSE channel estimation, Kalman filtering and prediction, and linear interpolation. Kalman filter is employed to filter the estimated channel and to predict the next channel sample to determine the precoding weights. Simulation results demonstrate that the proposed method improves the bit error rate (BER) performance significantly.

This paper proposed partial linear and sorted QR decomposition (SQRD)-based decision feedback detection (DFD) schemes for double space time transmit diversity (DSTTD)-orthogonal frequency division multiplexing (OFDM) systems to overcome severely time-varying multipath channels. The rapid channel variation deteriorates BER performance due to the introduction of intra-STTD-unit interference and intercarrier interference (ICI). The partial linear approaches mitigate the ICI effects with banded submatrices, instead of full channel frequency response (CFR) matrix. For further improvement of BER performance, the partial SQRD-based DFD with a modified ordering criterion is developed, which effectively removes above two types of interferences including inter-STTD-unit interference simultaneously. Computer simulation results show that the performance of the partial SQRD-based DFD is superior to those of the other linear approaches, with comparable complexity.

A sub-optimal but computationally efficient modified Fano detection (MFD) algorithm for DSTTD system is presented. The proposed algorithm utilizes the sequential detection scheme based on tree searching in order to find the optimal symbol sequence. For more reliable signal detection and complexity reduction, the decoder is designed to move backward for the specified value at the end of the tree and to compute the reduced branch metrics. Simulation results show that the performance of MFD is comparable to that of ML detector while reducing the computational effort of ML method significantly.

This paper presents iterative sequential detection (ISD) scheme using the modified Fano algorithm (MFA) for a double-STTD system. The proposed scheme can be decomposed into three interrelated steps to overcome the limitation of sequential detection for MIMO systems, which are preprocessing, iterative tree searching, and optimal decision. At the first step, preprocessing is performed to decide the number of iterations and to enumerate the candidate symbols. Then, the tree searching step is followed, where a modified version of Fano algorithm is performed to find the candidate symbol sequences. Finally, the optimal sequence is selected among the candidate sequences. In particular, the variable number of candidate symbols determined in the first and second steps plays an important role of reducing the receiver complexity significantly. Simulation results show that the proposed algorithm has a significant reduction in complexity with a small performance degradation

In this paper, the code-division multiplexing (CDM) based MIMO channel sounder with loosely synchronous (LS) codes and Kasami codes is presented for real time measurement of MIMO radio channel. Since this scheme has drawback of reduced dynamic range depending on the number of transmit antennas, it is important that low correlation codes should be utilized to get reliable performance. This paper adopts two efficient codes, LS codes and Kasami codes, of which performances are investigated through Monte-Carlo simulation in 2 x 8 and 4 x 8 MIMO channel environment, respectively. Basically, LS codes are noise-limited and Kasami codes are interference-limited in relation with cross-correlation characteristic. According to the simulation, LS codes are the best in 2 x 8 MIMO channel measurement and modified LS codes approach to this performance of LS codes in both 2 x 8 and 4 x 8 MIMO channel measurement. Kasami codes achieve somewhat less performance than the others, but are not nearly affected by the number of transmit antennas. Thus, Kasami codes are effective when transmit antennas are more than 4.

MIMO-OFDM system is one of the promising schemes for achieving high data rate in communication systems. In this paper, channel estimation techniques based on loosely synchronous (LS) codes are proposed for MIMO-OFDM systems. Since LS codes have perfect auto-correlation and cross-correlation functions within certain vicinity of the zero shifts, multiple antenna and multipath signal interference can be reduced substantially. Through simulations, it is shown that the proposed methods outperform both of least square and linear minimum mean square error (LMMSE) estimators.