Ho-Kyu Choi

Preamble design for channel estimation in MIMO-OFDM systems

A maximum-likelihood (ML) channel estimation and preamble design rules for the multiple-input-multiple-output (MIMO) channels are described when orthogonal frequency division multiplexing (OFDM) is employed with null subcarriers at both DC and high frequencies. The ML channel estimator in the time domain is derived assuming the knowledge of the maximum length of channel. To reduce the mean square error (MSE) of the estimation, three design rules are proposed: orthogonality between the preambles of different antennas, orthogonality between the circular shifted preamble sequences, and the condition that the number of subcarriers is larger than the maximum length of channel multiplied by the number of transmit antennas. In addition, we prove that the use of Golay complementary sequence (GCS) for preamble limits the peak to average power ratio (PAPR) by 3 dB although there are null subcarriers at high frequencies. Numerical results show that the MSE of the proposed method approaches that of optimum ML estimation when the number of null subcarriers and maximum length of channel are small.

Comparative study of time-domain and frequency-domain channel estimation in MIMO-OFDM systems

The time-domain channel estimator is compared with the frequency-domain channel estimator when multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) is employed. The time-domain maximum likelihood channel estimation (TMLE) and its mean square error (MSE) are derived assuming the knowledge of the maximum channel length. If we use only one OFDM symbol for the estimation, the preamble sequence of each transmit antenna uses a different set of subcarriers in MIMO-OFDM systems. As a result, the MSE of the frequency-domain least square channel estimation (FLSE) is derived assuming that linear interpolation is employed, numerical results show that the MSE of TMLE is smaller than that of FLSE and that the performance difference increases as the number of transmit antenna increases.

Interference Level Control in Mobile WiMAX Uplink System

Interference over thermal noise (IoT) level control is crucial to system performance and link quality in interference limited scenarios, especially for cell edge users. This paper proposes a simple and novel technique for IoT control by adopting the concept of load control where load is defined as the total interference impacting on neighbor sectors. The proposed technique is able to control IoT level accurately and is very robust against impairments in practical implementations, while minimizing the required signaling overhead.

Adaptive pilot pattern for multi-carrier spread-spectrum (MC-SS) transmission systems

In this paper, we address the issue of the design of link adaptation mechanisms in the context of multi-carrier spread-spectrum (MC-SS) transmission systems. By optimally varying the frequency pilot pattern, we present two original adaptation strategies that manage to preserve the requested QoS parameter, in our case the minimum BER target. The dynamic sub-carrier allocation policy is first detailed and then adaptive pilot pattern results are presented for different channel conditions and QoS. Moreover, an original geometric interpretation of the theoretical BER expression in terms of Bezier curves allows complexity reduction of the overall adaptation scheme. Based on the metric of the ratio between pilot and data subcarriers (pilot overhead), we thus propose a generalizing adaptive scheme, taking into account this loss of bandwidth parameter, and varying both the spreading factor and the pilot tone pattern. Allocation results of such an optimal adaptation policy are finally presented.

Effective MAC state transition for the mobile station power saving in 3G mobile communication systems

The control hold mode from cdma2000 1x systems has been adapted to the 1xEV-DV (evolution data and voice) systems because it has a lot of benefits such as power saving, reducing of overload and channel interferences, etc. However d1xEV-DV system does not fully capture the benefits from control hold mode, because the MS has to do continuous monitoring the F-PDCCH indicating whether the BS sends packet data or not. That is, the MS has to do monitoring the F-PDCCH (forward packet data control channel) continuously to capture the MAC ID while in control hold mode like as an operation in active mode. This continuous monitoring of the F-PDCCH causes higher power consumption and gives an overload to the decoder block at the MS side. In this paper to resolve these problems, we propose a new channel, F-WUCH (forward wake-up channel), to maximize the power saving of the MS, to minimize the interferences and to reduce the overhead from the continuous monitoring of the F-PDCCH in the control hold mode.