Kapseok Chang

Interference Analysis and Subchannel Allocation Schemes in Tri-Sectored OFDMA Systems

This paper analyzes the interference aspects of tri-sectored OFDMA systems and evaluates the performance of some subchannel allocation schemes to mitigate inter-sector interference. To maximize spectral efficiency, it is preferred to adopt frequency reuse of 1. But, carrier to interference and noise ratio (CINR) values at the boundary of sectors are extremely deteriorated. Two subchannel allocation methods are compared and evaluated in this environment. One is to maximize disparity, which is to maximize the number of subchannels in good CINR values, and the other is to minimize disparity, which is to maximize the number of subchannels in moderate CINR values. These schemes are compared under proportionally fair scheduling algorithm. Simulation results regarding to throughput show that the schemes outperform random allocation schemes in 802.16 based systems.

Signal Design for Reduced Complexity and Accurate Cell Search/Synchronization in OFDM-Based Cellular Systems

This paper proposes a variant of the frequency-domain synchronization structure specified in the long-term evolution (LTE) standard. In the proposed scheme, the primary synchronization signal used in step-1 cell search is the concatenation of a Zadoff-Chu (ZC) sequence and its conjugate (as opposed to only the ZC sequence in LTE). For step-2 cell search, we propose a complex scrambling sequence requiring no descrambling and a new remapped short secondary synchronization signal that randomizes the intercell interference (as opposed to the first/second scrambling sequence and swapped short signals in LTE). Through a combination of analysis and simulation, we demonstrate that the proposed synchronization signals lead to lower searcher complexity than LTE, a lower detection error rate, a shorter mean cell search time, and immunity toward a frequency offset.

Reduction of Doppler effects in OFDM systems

This paper observes the inter-carrier interference (ICI) by Doppler effect in time domain in orthogonal frequency division multiplexing systems. This observation allows us to propose a new time domain two-stage equalization scheme that iteratively cancels the ICl. The proposed scheme consists of the following two stages: First, we apply low-complexity linear minimum mean squared error (MMSE) employing partial rank of time impulse response matrix to shorten the ICI components into diagonal region. Second, we propose new parallel interference cancellation with hard decision feedback to remove the residual ICI. Performance evaluations and complexity analyses show that the bit error rate and the signal-to-interference-plus-noise ratio performances of the proposed scheme are very close to those of the classical frequency domain linear MMSE equalization scheme with much reduced complexity

A 2 GHz Synthesized Fractional-N ADPLL With Dual-Referenced Interpolating TDC

This paper presents a synthesized 2 GHz fractional-N ADPLL with a dual-referenced interpolating time-to-digital converter (TDC). The proposed TDC measures fractional phase by referencing adjacent two integer phases and achieves gain matching without any calibration scheme. It also improves linearity with little sensitivity to process, voltage, and temperature variations by averaging nonlinearity errors of opposite polarities. Except for digitally controlled oscillator (DCO), the PLL is designed only by RTL-level behavioral descriptions and synthesized with a standard cell library. The PLL is implemented in 65 nm CMOS with an active area of 0.047 mm2 and achieves a stable in-band phase noise of lower than -100 dBc/Hz in a wide range of supply voltage from 1 to 1.4 V.

Frequency-immune and low-complexity symbol timing synchronization scheme in OFDM systems

In this paper, we propose a simple time-domain replica correlation (TDRC) based symbol timing synchronization scheme in orthogonal frequency division multiplexing systems. By allocating a locally concatenated sequence of a base sequence and its modified sequence to total feasible frequency resources, we can achieve both hardware complexity and frequency offset reductions. The hardware complexity of a searcher is less than halved over existing TDRC-based schemes, such as long-term evolution (LTE) scheme [3] and a performance-efficient scheme [8]. Also the proposed scheme provides significant frequency offset immunity over the LTE scheme. The performance analysis and comparisons are made in terms of complexity and detection error probability.

A distributed multiple spectrum pricing scheme for optimality support in multiaccess systems

This paper focuses on a distributed multiple spectrum pricing scheme to maximize system capacity in next generation multiaccess systems, where multimode user equipments (MUEs) can connect simultaneously to multiple base stations (BSs) with multiple radio access technologies (RATs). The multi-price based scheme provides a distributed decision making for an optimal solution where radio resource allocations are determined by each MUE, unlike most centralized mechanisms where BS controls the whole radio resource. By the proposed optimal solution, MUEs can decide their share of spectrum bands and power allocation according to the spectrum price of each RAT, and at the same time the multiaccess system can achieve maximized total throughput. Numerical analysis shows that the proposed scheme achieves the maximal capacity by distributed resource allocation for the multiaccess system.

Highly scalable fair contention resolution scheme based on idle time

We propose a new contention resolution scheme designed for large scale autonomous wireless systems and fully distributed D2D (device-to-device) communications networks. The proposed contention resolution scheme is designed for scalability, efficiency, and fairness. Unlike traditional contention resolution schemes, the proposed contention resolution scheme uses idle time of channel as a measure of the channel condition. Idle time of channel is not only a much more reliable and versatile measure of channel condition than the traditional way of collision detection using ACK, it also makes it possible to use the proposed contention resolution scheme in situations where ACK is not expected such as with broadcast and multicast messages or in fully distributed synchronization procedures. The simulation study confirms that the proposed scheme exhibits high degree of scalability, efficiency, and fairness.

Achievable Rates of Cooperative Relaying Schemes Applied in Beamforming Mode

This paper studies six half-duplex cooperative relaying schemes with one multi-hop beamforming scenario in the 60 GHz band. Five of them are known. One new scheme is modified based on superposition coding. Under this scenario, we make the achievable rate expressions of the studied relaying schemes. By using these expressions, the comparison is made in terms of achievable rate versus relay-node position with the fixed received signal-to-noise ratio.

Distributed space-time block coding scheme in in-band full duplex relaying networks

This paper proposes a cooperative scheme for in-band full duplex (IFD) in a one-way two-hop relaying system. The proposed scheme realizes collocated space-time block coding by applying both of the superposition coding at the source and the IFD mode at the relay. The achievable rate of the proposed scheme is derived for fixed relay channel gains and compared to those of the upper-bound IFD as well as existing IFD relaying schemes. Our achievable-rate results elucidate that our scheme is close to the upper-bound IFD than any other scheme.