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Multimode quantized precoding (QP) can provide full diversity gain or high capacity gain by adapting the number of substreams, as well as the precoding matrix, according to the instantaneous channel condition with low-rate feedback. Conventional multimode QP (MM-QP), however, does not consider the adaptive rate allocation among substreams; thus, it cannot have the additional gain by adaptive modulation. Furthermore, it is computationally complex since exhaustive matrix inversions are required to determine the optimal mode. In this paper, we propose an efficient MM-QP system that improves the performance of a conventional system in terms of error rate and has a lower computational complexity than the conventional system. First, we define the rate-partitioning vector as the mode and control the rate among substreams and the number of substreams according to the channel instantaneous condition. Second, to reduce the computational complexity for the receiver to determine the optimal mode, the simplified mode-selection technique using estimates of the modal metric is proposed. In the proposed mode-selection technique, the optimal mode can be obtained by several multiplication and division operations. Finally, the mode-reduction technique eliminating the less-frequently used modes is proposed, which leads to a significant reduction of the feedback information with negligible performance loss. In numerical experiments, it was verified that the proposed MM-QP system gives a better error-rate performance than the conventional system, with much less computational complexity for the same amount of feedback information. Index Terms--Closed-loop multiple-input-multiple-output (MIMO) systems, limited feedback channel, modal metric, mode reduction, multimode, quantized precoding (QP).

This paper proposes new algorithms to improve the bit error rate (BER) and data rate of link-adaptive multiple-input--multiple-output (MIMO) systems with ordered successive interference cancellation (OSIC) receivers in a multiuser environment. The proposed algorithms are as follows: 1) the power adaptation (power allocation--equal rate) scheme; 2) the rate adaptation (equal power--adaptive modulation) scheme; and 3) the power and rate adaptation (power allocation--adaptive modulation) scheme. Additionally, as the OSIC receiver is exploited in the proposed algorithms, the detection order of each user's receiver is considered since the performance of the system using it is seriously affected by the detection order. Finally, the proposed algorithms are executed to minimize the BER or to maximize the data rate of the system by simultaneously considering link adaptation techniques and the detection order of the OSIC receiver. Index Terms--Adaptive modulation, bit error rate (BER), data rate, multiple input--multiple output (MIMO), multiuser diversity, ordered successive interference cancellation (OSIC), power allocation.

Two efficient algorithms are proposed for estimating code timing in a code-division multiple-access system in which multiple antennas are used at the transmitter and receiver. One of the algorithms is a pilot data transmission technique; while pilots in a conventional method are transmitted continuously, they are cyclically transmitted over each transmit antenna in the proposed algorithm. Hence, when compared to the conventional method, code allocation efficiency and more powerful transmission for unmodulated pilot channels could be achieved. The other is a weighted average (WA) method in which code-timing acquisition is achieved by averaging the estimated delays by weight. In addition, we propose two efficient code-timing estimators based on the proposed algorithms. Based on the analysis in this paper, the performance of the WA method is better than the equal-gain average method. Simulation results show that the two code-timing estimators based on the proposed methods provide a better code-timing acquisition performance than the conventional method. Index Terms--Code-division pilot (CDP), code-timing acquisition, equal-gain average (EGA), multiple-input--multiple-output code-division multiple access (MIMO-CDMA), time-division pilot (TDP), weighted average (WA).

An interference-cancellation-based idle period downlink (IC-IPDL) method is proposed to improve hearing in mobile-positioning systems. While the IPDL method ceases all transmissions in the serving base station during short intervals, the proposed IC-IPDL method maintains common pilot channel (CPICH) during an idle period to provide better stability to the wireless communication systems. Instead, CPICH is cancelled out in a mobile by using interference-cancellation techniques. The performances of the proposed method and the IPDL method are theoretically analyzed and the analyses are verified through computer simulations. Index Terms--Common pilot channel (CPICH), hearability problem, interference-cancellation-based idle-period downlink (IC-IPDL), idle-period downlink (IPDL), position location.

In multi-input-multi-output orthogonal frequency-division multiplexing systems, conventional channel-estimation techniques using comb-type training symbols give relatively large mean squared errors (MSEs) at the edge subcarriers. To reduce the MSEs at these subcarriers, a cyclic comb-type training structure is proposed. In the proposed cyclic training structure, all types of training symbols are transmitted cyclically at each antenna. At the receiver, the channel frequency responses that are estimated using each training symbol are averaged with weights obtained from the corresponding MSEs. Computer simulations showed that the proposed cyclic training structure gives more signal-to-noise ratio gain than the conventional training structure. Index Terms--Channel estimation, cyclic training structure, multi-input-multi-output orthogonal frequency-division multiplexing systems (MIMO-OFDM), training symbols.

An integrated acoustic echo and noise canceler is proposed for hands-free telephony applications. The proposed system includes a computationally efficient double-talk detection (DTD) method and a residual echo cancellation algorithm. The DTD method uses two cross-correlation coefficients and is robust in adverse conditions. In the residual echo cancellation approach, a linear prediction error filter removes the short-term correlation of the residual echo and the resulting whitened residual echo is canceled by a noise-reduction filter. The proposed system was implemented using a general purpose digital signal processor (DSP) with 16-b arithmetic. Index Terms--Double-talk detection (DTD), echo canceler, hands-free telephony, noise reduction, residual echo cancellation.