Jae Young Ahn

Synchronization and Cell-Search Technique Using Preamble for OFDM Cellular Systems

In this paper, a novel preamble structure, including a synchronization field (S-field) and a cell-search field (C-field), is proposed for orthogonal-frequency-division-multiplexing-based cellular systems. An efficient algorithm for downlink synchronization and cell search using the preamble is also proposed. The synchronization and cell-search process includes the initial symbol-timing estimation using continuously or, at least, periodically transmitted downlink signal, the frame detection, the fine symbol-timing estimation, the frequency-offset estimation using the preamble S-field, and the cell identification using the preamble C-field. Performance of each synchronization and cell-search step is analyzed and verified by computer simulation. The overall performance of the synchronization and cell search is then analyzed in terms of the mean acquisition time. It is shown that the proposed preamble with the corresponding synchronization and cell-search algorithm can provide a very robust synchronization and cell-search capability, even in bad cellular environments.

Iterative estimation and decoding for an LDPC-coded OFDMA system in uplink environments

In this paper, we propose an LDPC-coded OFDMA system with a resource allocation method suitable for the uplink of mobile cellular environments. At the receiver of the proposed system, an iterative channel estimation and decoding (IED) method with a Wiener filter adapting to the channel variation rate is employed to support high mobility without boosting the pilot power. For practical applications, the channel variation rate is also estimated with pilot symbols to select Wiener filter coefficients for every received packet dynamically allocated. It is shown that the proposed system supports the mobility up to 250 km/h and exhibits more than 1 dB performance gain over the system with conventional non-iterative receiver.

An Inter-Cell Interference Cancellation Method for OFDM Cellular Systems Using a Subcarrier-Based Virtual MIMO

In this paper, a new method of inter-cell interference (ICI) cancellation in the downlink is proposed for users at the cell boundary in OFDM-based cellular systems, by using the concept of a subcarrier-based virtual multiple-input multiple-output (SV- MIMO) where multiple antenna techniques are performed on a set of subcarriers, not on the actual antenna array. In the proposed SV-MIMO, subcarrier signatures, obtained at the mobile station (MS) with a single antenna, are exploited to separate the desired signal from ICIs. The concept of a virtual signature randomizer (VSR) is introduced to improve channel separability in the SV-MIMO approach. Also, the concept of a virtual smart antenna, in which the steering vector is formed by a set of subcarriers with the estimated symbol timing offset (STO) between BSs, is compared with conventional ICI reduction methods. It is shown by simulation that the proposed method is effective in reducing ICI and inter-sector interference for fully- loaded OFDM cellular systems with a frequency reuse factor equal to 1, compared with conventional methods.

Performance of an LDPC-coded frequency-hopping OFDMA system based on resource allocation in the uplink

An LDPC-coded frequency hopping OFDMA system is proposed for the uplink of cellular systems, where the frequency hopping is based on a resource block (RB) for coherent demodulation. For the system, two different RB types are considered, either for better intercell interference averaging capability or for better channel estimation performance. A turbo receiver with iterative channel estimation and decoding is also proposed to provide better channel estimation performance without boosting the pilot power. The performance is evaluated both in single- and multi-cell environments to investigate the effect of the interference on the overall system performance.

An intercell interference cancellation method for OFDM-based cellular systems using a virtual smart antenna

In this paper, a concept of virtual smart antenna (SA) is introduced for OFDM-based cellular systems with a frequency reuse factor equal to 1. An estimation method of intercell symbol timing offsets (STOs) from received OFDM signals impinging on virtual antenna is proposed for users at a cell boundary. Also, adaptive beamforming methods for virtual SA are proposed to reduce intercell interference (ICI) from adjacent base stations (BSs)

Design of an LDPC code with low error floor

A search algorithm for stopping sets in a Tanner graph is proposed for designing good low-density parity-check (LDPC) codes. By applying the belief-propagation algorithm with messages containing the information of originated variable nodes and their connected edges, the stopping sets can be detected. Furthermore, a code design method using the algorithm is presented and the performances of the designed code over several channels are shown

A Cycle Search Algorithm Based on a Message-Passing for the Design of Good LDPC Codes

A cycle search algorithm based on a message-passing in a Tanner graph is proposed for designing good LDPC codes. By applying the message-passing algorithm with a message alphabet composed of only two messages to a cycle search, we can perform a cycle search with less computational complexity than tree-based search algorithms. Also, the proposed algorithm can be easily implemented by using an existing message-passing decoder and can easily adopt different kind of criteria for an LDPC code design with a slight modification in the node update equations.

Cumulative-TIM method for the sleep mode in IEEE 802.16e wireless MAN

The IEEE 802.16e WMAN (Wireless Metropolitan Area Networks), which has been designed for fixed or mobile broadband wireless access, is getting into the spotlight as the base technology for the mobile Internet As most mobile stations are battery-powered in wireless environment, energy efficient protocols are essential for their practical use Although many systems, including the IEEE 802.16e, adopt the sleep mode, little attention has been paid to the traffic characteristics or status of the mobile station In this paper, we propose a new scheme called Cumulative-TIM, in which BS(base station) does not wake up a mobile station until a sufficient amount of data for the station is stored, thus guaranteeing mobile stations sleep enough amount of time In this way, mobile stations can use their energy more efficiently than the standard sleep mode: Our simulation result shows that proposed scheme has a better performance compare with the current standard of the WMAN.

An adaptive MIMO transmission technique for LDPC coded OFDM cellular systems

An adaptive MIMO transmission scheme using QAM and LDPC code is proposed for an OFDM cellular system employing FDD. By approximating the LLR distribution to a Gaussian distribution, only two parameters, the mean and the normalized standard deviation, are required to be sent to the transmitter, which requires only a few more bits than those required in currently used single carrier cellular systems, such as cdma2000-1x EV-DO. It is shown by computer simulation that the proposed adaptive transmission scheme can provide up to 2/spl sim/3 dB gain over the conventional system using the mean SNR only, at the expense of only 3 more bits in the feedback information.

A Synchronization and Cell Searching Technique Using Pilot Tones for OFDM Cellular Systems

SUMMARY In this letter, a pilot structure and an efficient algorithm for downlink synchronization and cell searching in OFDM-based cellular systems are proposed. The pilots, randomly allocated in the frequency domain, allow us to minimize inter-cell interference (ICI) as well as to increase cell searching capability, estimation range of integer carrier frequency offset (CFO), and estimation accuracy of symbol timing offset (STO). The proposed low-complexity joint algorithm for integer CFO estimation, cell searching, and downlink detection is robust to ICI, multipath channel, STO and fine CFO.