MIMO communication systems have been studied as a way to increase system capacity and to improve spectrum efficiency. This paper proposes a novel branch metric generation method for soft-decision Viterbi decoding in coded OFDM systems employing SDM (OFDM-SDM) systems, one form of MIMO communication, that employ the maximum likelihood decision (MLD) algorithm. In coded OFDM systems, the branch metric for forward error correction (FEC) is normally calculated based on received sub-carrier power level and the constellation point likelihood. This conventional approach fails in OFDM-SDM systems. In the proposed method, branch metric is derived based on the likelihood calculated by MLD algorithm, which improves FEC performance effectively. Then, availability of coded OFDM-SDM to realize wireless LANs with data rate above 100 Mbit/s in a bandwidth of 20 MHz is evaluated in office environments using required CNR calculated by computer simulation. The evaluation indicates that OFDM-SDM has the possibility of realizing 100 Mbit/s in office environments.

This paper proposes a novel space diversity reception scheme suitable for packet-based orthogonal frequency division multiplexing (OFDM) wireless access systems that achieves large diversity gain by improving the accuracy of both carrier frequency synchronization and phase tracking. Phase tracking compensates the phase rotation caused by residual carrier frequency error and phase noise and is necessary for high data rate OFDM systems that use coherent detection. In the proposed scheme, the accuracy of carrier frequency synchronization is improved by combining the information of the carrier frequency offset detected on all diversity branches; the accuracy of phase tracking is improved by using pilot signals whose signal to noise ratio (SNR) is raised by maximal ratio combining of the pilot signals extracted from all branches. Computer simulation results show that the proposed scheme effectively improves the diversity gain even in severe environments such as those with low carrier to noise ratios (CNR) and large delay spreads.

Various planar folded dipole antennas with feed lines are introduced and analyzed. With the added feed line, the planar folded dipole antenna has two resonance modes. Moreover, adjusting the spacing and width of the feed line improves the broadband characteristics of the antenna. The attached feed line has not only an impedance transforming characteristic but also a bandwidth transforming characteristic. The bandwidth transforming characteristic means that the feed line can broaden the bandwidth of folded dipole antenna. A way to reduce the antenna area is also studied, and the characteristics of the resulting compact antenna are analyzed.

The electric fields inside and outside a car must be carefully determined when designing a wireless communication system to be employed in the car. This paper introduces an effective simulation method and a precise measurement method of electric field distributions in a cabin of a simplified scale car model. A 1/3 car model is employed for ease of measurement. The scaled frequency of 2859 MHz, 3 times 953 MHz, is employed. The use of a moment method simulator utilizing the multilevel fast multipole method allows calculations to be performed on a personal computer. In order to judge the accuracy of simulation results, convergence of simulation output in accordance with segment size (triangle edge length) changes is ensured. Simulation loads in the case of metallic body only and a metallic body with window glass are also shown. In the measurements, an optical electric field probe is employed so as to minimize the disturbances that would otherwise be caused by metallic feed cable; precise measurement results are obtained. Comparisons of measured and simulated results demonstrate very good agreement which confirms the accuracy of the calculated results. 3-dimensional electric field distributions in the car model are shown and 3-dimensional standing wave shapes are clarified. Moreover, calculated and measured radiation patterns of the car model are shown so the total electric field distributions around a car are clarified.

This paper proposes a new scheme that can evaluate the cell throughput performance of wireless local area network (LAN) systems, which use carrier sense multiple access/collision avoidance (CSMA/CA) and multiple transmission bit-rates (multi-rate). We extend the interference model of the conventional scheme in order to deal with interference more accurately in multi-cell environments. Unlike the conventional scheme, the proposed scheme is able to handle multi-rate systems. We use the proposed scheme to evaluate the IEEE 802.11a system and systems whose signal-bandwidth is expanded from that of the IEEE 802.11a system. We find that a system with 35(75) MHz signal-bandwidth achieves about 1.3(1.25) times higher cell throughput than the IEEE 802.11a system. Furthermore, the system with 35(75) MHz signal-bandwidth is also shown to have the potential to achieve up to 1.5(1.8) times higher cell throughput performance than the IEEE 802.11a system if the transmission efficiency on the media access control (MAC) layer is assumed to be ideal. It is concluded that the proposed scheme confirms that the approach to expand the signal bandwidth of the IEEE 802.11a system is effective to improve the cell throughput performance. This result is virtually impossible to derive with the conventional scheme.

It has been reported that by adding two folded elements, bow-tie antenna can be miniaturized, but the antenna has VSWR degradation problem. In this paper, the details of the VSWR degradation are investigated and the physical mechanism of the degradation is clarified. The best position for folded element is also shown. Moreover, the bow-tie antenna is bent in half in order to realize more size reduction. When the two folded elements are added to the half bent bow-tie antenna, the lowest operation frequency goes down and the proposed antenna can be more downsized than the previous proposed antenna. The gain is slightly lower than that of the previous model, however, the antenna area is reduced from 31%, which is the antenna area ratio of privious proposed antenna and conventional bow-tie antenna, to 19%. The bandwidth of 92% is obtained for VSWR≤2.

This paper proposes a new vector error measurement scheme for orthogonal frequency division multiplexing (OFDM) systems that is used to define transmit modulation accuracy. The transmit modulation accuracy is defined to guarantee inter-operability among wireless terminals. In OFDM systems, the transmit modulation accuracy measured by the conventional vector error measurement scheme can not guarantee inter-operability due to the effect of phase noise. To overcome this problem, the proposed vector error measurement scheme utilizes pilot signals in multiple OFDM symbols to compensate the phase rotation caused by the phase noise. Computer simulation results show that the vector error measured by the proposed scheme uniquely corresponds to the C/N degradation in packet error rate even if phase noise exists in the OFDM signals. This means that the proposed vector error measurement scheme makes it possible to define the transmit modulation accuracy and so guarantee inter-operability among wireless terminals.