Takki Yu

OFDM Channel Estimation With Jammed Pilot Detector Under Narrow-Band Jamming

This paper presents a channel estimation scheme using a new jammed pilot detection algorithm for orthogonal frequency-division multiplexing (OFDM) systems under narrow-band jamming (NBJ).signal-to-jamming ratio The jammed pilot subcarrier can be detected and excised by using the proposed algorithm after least squares estimation. The average mean square error (mse) on one OFDM symbol both under a jammed and a removed pilot subcarrier are derived analytically. Moreover, the symbol error rate (SER) performance for the channel estimation scheme using the proposed algorithm is evaluated by simulation. From the results, we can show that the channel estimator with the proposed algorithm improves the mse and SER performances at a low signal-to-jamming ratio.

Spectrum Leasing via Distributed Cooperation in Cognitive Radio

The concept of cognitive radio (or secondary spectrum access) is currently under investigation as a promising paradigm to achieve efficient use of the frequency resource. In this paper, we consider a decentralized cognitive radio model based on spectrum leasing, whereby a primary (licensed) user leases its bandwidth for a fraction of time to a network of independent secondary (unlicensed) terminals in exchange for cooperation. On one hand, the primary user decides whether to exploit (space-time coded) cooperation from the network of secondary terminals in order to maximize its own transmission rate. On the other hand, secondary terminals accept to cooperate with the primary only if compensated with a large enough fraction of time for their own transmission, towards the goal of maximizing their rate discounted by the overall cost of transmitted power. The considered model is studied in the framework of Stackelberg games, with the primary and the set of secondary users modelled as the (Stackelberg) game leader and the follower, respectively. Numerical results show that spectrum leasing based on trading secondary spectrum access for cooperation is a promising framework for cognitive radio.

Packet-wise vertical handover for unlicensed multi-standard spectrum access with cognitive radios

In this letter, packet-by-packet vertical handover is investigated as a means to improve the performance of unlicensed spectrum access. The analysis focuses on the optimization of a channel access strategy for a cognitive multi-standard radio node that has the capability to switch between two orthogonal uplink radio interfaces, based on long-term channel state information. The radio interfaces differ for coverage and quality of service (QoS) requirements of primary users. The proposed strategy prescribes a cross-layer selection of physical-layer (transmitting powers) and medium access control layer (handover probability) parameters and is designed to maximize secondary throughput while guaranteeing the primary QoS constraints. Analysis and numerical results bring insight into the impact of network topology, measurement errors and primary QoS constraints on the optimal system design and corresponding performance.

Joint Doppler-Frequency Diversity for OFDM Systems Using Hybrid Interference Cancellation in Time-Varying Multipath Fading Channels

In time-varying fading channels, orthogonal frequency division multiplexing systems suffer severe performance degradation caused by interchannel interference (ICI). We utilize this ICI as a source of diversity by using a new ICI-cancellation algorithm and Wiener filter. The proposed algorithm can support joint Doppler-frequency diversity gain. In addition, both the computational complexity and total detection time are reduced.

A modified covariance-based mobile velocity estimation method for Rician fading channels

This paper contains our proposal for a novel velocity estimation method based on the auto-covariance of the received signal and that of the squared envelope. This modified covariance-based (MCOV) estimator exhibits unbiased characteristics in a Rician fading environment. Moreover, it requires no other channel information, such as Rician factors or the angle of arrival (AoA). The authors also propose a scaling technique which can improve the performance of the MCOV method in a Rayleigh fading environment. When coupled with the scaling technique, the new method demonstrates robust performance in both the Rayleigh and Rician fading environments.

A modified covariance-based velocity estimation method for Rician fading channel

In wireless mobile communication systems, the mobile velocity or, equivalently, the maximum Doppler frequency, is very useful information to optimize the system performance. A novel velocity estimation method, based on the autocovariance of the received signal and that of the squared envelope, is proposed. The proposed estimator provides unbiased characteristics. Moreover, the proposed method does not need any other channel information such as Rician factor and angle of arrival (AoA). The performance of the proposed estimator is verified by tracking simulation in realistic cellular mobile communication scenarios.

Effective Small Cell Deployment with Interference and Traffic Consideration

We examine small cell deployment strategies in two- tier cellular networks. Specifically, we propose a joint interference and load deployment (JD) strategy, reflecting a realistic interference environment and traffic load. In this JD, the location of a small base station (s-BS) is sequentially decided in greedy manner by exploiting both the spatial user equipment (UE) distribution and the received signal strength indicator (RSSI). Furthermore, we propose a deterministic deployment (DD), which does not require this RSSI measurement. In DD, the deployed pattern of s-BSs is the same for each macro cell. System level simulation (SLS) results demonstrate that the proposed schemes yield a cell throughput gain of up to 61%, when the UEs are densely placed near a m-BS. Moreover, 5%-tile UE throughput and median UE throughput are improved by up to 134% and 76%, respectively, when UEs are densely distributed far from the m-BS. Furthermore, the SLS results provide intuition on between load and interference in the s-BS deployment as follows: 1) load is more important than interference in the macro edge region, 2) interference is more significant than load when the s-BS is deployed close to the m-BS.

Noncoherent adaptive code synchronization for DS/CDMA systems

In this paper, a noncoherent adaptive code acquisition scheme is proposed by combining an adaptive filter with a differential detector. The carrier phase offset in the PN signal component of the filter input is removed by a differential detector. The probabilities for detection and false alarm are derived, and the mean acquisition time is evaluated under AWGN channel. The performance of the proposed system is compared with the conventional system using a matched filter. Simulation results show that the proposed system has a good performance under overall channel conditions, especially at low SNR. And the proposed system outperforms the matched filter by 4-6 dB under AWGN channel for 16 taps, as measured by the mean acquisition time.

A New Adaptive Code-Acquisition Algorithm Using Parallel Subfilter Structure

This paper presents a new adaptation algorithm using parallel subfilter structure, which enables fast-code acquisition for direct-sequence spread-spectrum systems. The proposed algorithm adapts the filter, based on a concept of dividing it into several subfilters, thus reducing the adaptation time. The detection and false-alarm probabilities are derived analytically. Simulation results show that the adaptive code-acquisition scheme, using the proposed algorithm, improves the mean-acquisition-time performance by 4-5 and 3-4 dB on additive white Gaussian noise and multipath Rayleigh fading channels for 64 taps, respectively

Improved adaptive code acquisition scheme for practical DS/SS systems

This paper presents our proposals to overcome two problems in applying the adaptive code acquisition scheme to practical systems. At first, a system parameter setting method based on SNR estimation is proposed to obtain near-optimum performance in real environments. Secondly, we propose a novel adaptation algorithm using a sub-filter structure, which enables the fast acquisition required in the practical systems. Simulation results show that the first proposal gives reliable acquisition performance in real situations without severe degradation, and the proposed adaptation algorithm offers 5-6 dB gain compared to the conventional adaptation algorithm for a number of taps of 64.