Gin-Kyu Choi

Transmit selection diversity technique in the MIMO-OFDM systems for HSDPA

To support the increasing requirement for additional services demanding very high bit rates and higher system capacity, high speed downlink packet access (HSDPA) is provided in the UMTS standard. Orthogonal frequency division multiplexing (OFDM) is a technology that has been shown to be well suited to HSDPA for high data rate transmission. Selective transmit diversity is a well-known technique to combat the detrimental effects of multipath fading and obtain diversity gain from multiple antennas. We consider transmit selection diversity applied to the OFDM-HSDPA system. The proposed OFDM architecture adopts antenna selection diversity and a novel groupwise subcarrier allocation algorithm in closed-loop mode. In the conventional closed-loop mode, user equipment (UE) should feed back channel quality information (CQI) bits for each associated subcarrier, and there may be a lot of feedback information. We try to reduce the feedback bandwidth compared to previous allocation methods while the system performance should not be degraded by grouping adjacent subcarriers. Through simulation results, we found that the throughput, capacity, coverage and reliability can be dramatically increased by employing the proposed scheme.

Theory and design of dyadic subband tree structures with embedded PDF-optimized quantizers

We develop a methodology for the analysis of signal quantization effects in critically sampled dyadic subband tree structures using a nonlinear gain-plus-additive-noise model for the probability density function (PDF)-optimized quantizer. We constrain the two-band nonquantized and uncompensated structure at each level to be perfect reconstruction (PR). We develop an equivalent uniform filter bank followed by its polyphase structure described by primitive submatrices and compute a rigorously correct mean squared error (MSE) in the frequency domain using cyclostationary concepts in terms of: (1) the allocated quantizer bits; (2) the filter coefficients; (3) an embedded compensation parameter vector. This MSE is then minimized over all three items above. Our optimization method is applied to the specific case of a four-channel dyadic tree with average bit rate constraint. This tree is represented by an eight-channel polyphase equivalent whose interchannel signals are correlated. We show how to represent rigorously the correlation of random noise between channels due to the embedded quantizers. Our design of paraunitary and biorthogonal structures with identical and nonidentical stages is performed, compared, and validated by computer simulation under the assumption of uncorrelated cross band noise. The nonidentical stage biorthogonal filter bank turned out to have the best performance in MSE sense, but the most robust structure is the nonidentical stage paraunitary filter bank.

Maximum-likelihood delay estimation for DS-CDMA communication systems

The delay estimation algorithms of a synchronous DS-CDMA system are considered under multipath fading channels. A novel maximum-likelihood delay estimation algorithm is addressed for resolving closely spaced delays in indoor wireless communications. The observation space can be partitioned into a signal and a noise subspace by eigenvector decomposition of the time-averaged correlation matrix generated from the output of a sliding correlator without prior knowledge of the signal vectors. Exploiting known statistical properties of the estimated subspaces to maximize the likelihood of the projections forms the maximum-likelihood delay estimation. The simulation results show that the proposed delay estimation algorithm is superior to the conventional sliding correlator and MUSIC-type algorithm.