Jongsub Cha

MIMO-OFDM Downlink Channel Prediction for IEEE802.16e Systems Using Kalman Filter

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.

Code-Division Multiplexing Based MIMO Channel Sounder with Loosely Synchronous Codes and Kasami Codes

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.

Design and theoretical analysis of throughput enhanced spatial reuse distributed coordination function for IEEE 802.11

The IEEE 802.11 distributed coordination function (DCF) employs a carrier sensing mechanism, a simple and effective mechanism to mitigate collisions in wireless networks. But the carrier sensing mechanism is inefficient in terms of shared channel use because an overcautious channel assessment approach is used to estimate interference at a receiver. A DCF node simply blocks its transmission when it senses that the channel is busy. However, in many cases this channel assessing nodes own transmission may not generate enough interference to disrupt the ongoing transmission at the receiver. This overcautious channel assessment unnecessarily blocks transmission attempts, and thus degrades the overall network throughput. To avoid this unnecessary blocking, the authors propose a spatial reuse DCF (SRDCF), which utilises location information and transmission parameters to make accurate channel assessments and to permit concurrent transmissions by adjusting the transmission power. SRDCF also resolves the contention between opportunistic concurrent transmissions with a secondary backoff counter. Consequently, the proposed scheme improves the overall network throughput because of more concurrent transmissions. The authors theoretically analyse the performance enhancement of SRDCF over the original IEEE 802.11 DCF by using a Markov chain model and verify it through simulations.

Interference-Aware Channel Assignment with Seamless Multi-Channel Monitoring on Wireless Mesh Network

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.

Spatial Reuse DCF for Enhancing Throughput and Performance Analysis

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.

Low Complexity Tree Searching-Based Iterative Precoding Techniques for Multiuser MIMO Broadcast Channel

In this letter, we propose two computationally efficient precoding algorithms that achieve near-ML performance for multiuser MIMO downlink. The proposed algorithms perform tree expansion after lattice reduction. The first full expansion is tried by selecting the first level node with a minimum metric, constituting a reference metric. To find an optimal sequence, they iteratively visit each node and terminate the expansion by comparing node metrics with the calculated reference metric. By doing this, they significantly reduce the number of undesirable node visit. Monte-Carlo simulations show that both proposed algorithms yield near-ML performance with considerable reduction in complexity compared with that of the conventional schemes such as sphere encoding.

Frequency Domain Equalizer Using Zero-padding for IR-UWB Systems

In this paper, we propose a zero padded single-carrier (SC) block transmission with minimum mean squared error-frequency domain equalization (MMSE-FDE) for impulse radio ultra-wideband (IR-UWB) systems. Moreover, the comparative study among the proposed scheme and the conventional one is presented. The proposed zero padded MMSE-FDE not only has the favorable advantages of the existing cyclic prefixed SC-FDE, but it also overcome the power loss due to the usage of the cyclic prefix. The validity of the proposed algorithm is demonstrated by simulation results with relevant discussions.

Reduced-Complexity Stack-Based Iterative Detection for V-BLAST Systems

In this paper, a computationally efficient stack-based iterative detection algorithm is proposed for V-BLAST systems. To minimize the receivers efforts as much as possible, the proposed scheme employs iterative tree search for complexity reduction and storage saving. After an M-ary tree structure by QR decomposition of channel matrix is constructed, the full tree depth is divided into the first depth and the remaining ones. At tree depth of one, the proposed algorithm finds M candidate symbols. Based on these symbols, it iteratively searches the remaining symbols at second-to-last depth, until finding an optimal symbol sequence. Simulation results demonstrate that the proposed algorithm yields the performance close to that of sphere detection (SD) with significant saving in complexity and storage.

Efficient Modified Fano Detection with Reduced Branches for DSTTD System

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.

Efficient V-BLAST detection using modified fano algorithm

We propose a sub-optimal but computationally efficient Modified Fano Detection algorithm (MFD) for V-BLAST systems. This algorithm utilizes the QR decomposition of the channel matrix and the sequential detection scheme based on tree searching to find the optimal symbol sequence. For more reliable signal detection, the decoder is designed to move backward for the specified value at the end of the tree. This results in significant reduction of the complexity while the performance of MFD is comparable to that of ML detector.