This paper proposes a Group Detection (GD) algorithm with Max-Log-MAP Sphere Decoder (MLM-SD) in order to reduce the complexity of signal detection in a Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system. The proposed algorithm divides spatial streams into multiple partial spatial streams by using Minimum Mean Square Error (MMSE) detector, followed by multiple MLM-SDs with reduced number of spatial streams. Although the spatial diversity gain in the MLM-SD degrades because of the lack of the degrees of freedom exploited by the MMSE detector, its diversity gain is recovered by combining the metrics obtained by the multiple MLM-SDs. In a MIMO wireless LAN multipath fading environment, the complexity of the proposed algorithm is 10% of that of the original MLM-SD and the performance degradation in terms of SNR is slightly less than that of the original MLM-SD in 4-by-4 MIMO architecture with 64 QAM achieving 216 Mbps. It is also found that the proposed algorithm is robust against the limitation of the number of searches in sphere decoder.

In a downlink multi-user multiple-input multiple-output (MU-MIMO) system, the vector perturbation (VP) method, which is one of the non-linear precoding methods, is a promising technique with which to maximize the channel capacity. In the VP method, the receiver requires the modulo operation to remove the perturbation signal added at the transmitter. However, owing to noise, the modulo operation may not run correctly which makes the soft demapper produce unreliable log-likelihood ratios (LLRs), resulting in a degradation of system throughput. To enhance the throughput performance, we propose a method that expands the basic perturbation interval and adaptively controls the expansion rate according to the modulation and coding scheme (MCS). The optimum expansion rate is derived by link-level simulation and the system throughput is measured by system-level simulation. The system-level simulation results show that the proposed VP method can obtain higher throughput than the conventional VP method.

This paper investigates the improvement of the radiation pattern design of transmitting array antenna beamforming and transmission power control (TPC) for forward link of DS-CDMA/FDD system. Optimum transmission beamforming and TPC can be derived from the information of the propagation channel for the forward link, in terms of outage probability minimization. It is assumed that the channel is static and then all mobile stations (MS's) report channel characteristics measured in the forward link to a base station (BS) that can control signal to interference plus noise ratios (SINR's) received by individual MS's using that measured information. Then, for the multi-user environment of a single cell, goal programming (GP) is applied to derive an optimum weight vector of the transmitting array antenna and optimum TPC such that outage probability can be minimized.

This paper proposes a preamble structure that raises the throughput of the physical layer of a MIMO-OFDM system that complies with IEEE 802.11n. The proposed preamble is based on two important criteria that have not been considered in conventional preamble structures but have an important bearing on realization of IEEE 802.11n. One is backward compatibility with legacy devices, such as IEEE 802.11a or IEEE 802.11g. The other is accurate AGC for MIMO data payload of the packet. Computer simulations and a laboratory test demonstrate that the proposed preamble is superior to the other preambles with respect to those two important criteria. The results were submitted to the task group N under the IEEE 802.11 working group and contributed to development of the current draft of IEEE 802.11n.

A major drawback with linear precoding in a downlink multi-user MIMO system is the increase in the transmit power when a channel is correlated. On the other hand, nonlinear trellis precoding in downlink multi-user MIMO systems is capable of minimizing the transmit power by adding a shaping sequence to the original transmit sequence. However, conventional trellis precoding cannot be directly applied to existing bit-interleaved coded MIMO-OFDM systems since the trellis precoding and error correcting codes should be designed separately. In this paper, we proposed to embed trellis precoding into the error correcting codes that are used in the original multi-user MIMO-OFDM system employing linear precoding. Major advantage of this approach is that the receiving procedure at user terminals designed for the original system need not be changed up to the error correcting decoder to support our trellis precoding. Computer simulations show that the proposed trellis precoding provides improvements of 2 dB and 2.5 dB in 22 and 33 MIMO configurations, respectively.

The next generation wireless LAN standard IEEE 802.11ax aims to provide improved throughput performance in dense environments. We have proposed an efficient channel sounding mechanism for DL-MU-MIMO that has been adopted as a new sounding protocol in the 802.11ax standard. In this paper, we evaluate the overhead reduction in the 802.11ax sounding protocol compared with the 802.11ac sounding protocol. Sounding is frequently performed to obtain accurate channel information from the associated stations in order to improve overall system throughput. However, there is a trade-off between accurate channel information and the overhead incurred due to frequent sounding. Therefore, the sounding interval is an important factor that determines system throughput in DL-MU-MIMO transmission. We also evaluate the effect of sounding interval on the system throughput performance using both sounding protocols and provide a comparative analysis of the performance improvement.