For a wireless communication system to work effectively without interference, the electromagnetic environment needs to be controlled. We experimentally and analytically investigated the requirements for controlling the electrical field strength and delay spread so as to achieve the best communication without electromagnetic interference in selected regions for a 2.4-GHz-band wireless LAN system. To control the coverage, partitions were placed around desks in a test environment and covered on the inside with electromagnetic absorbing board from the top of the desks to the top of the partitions; four indoor environments that combined one of two wall-material types and one of two partition heights were used. The transmission loss and delay spread were measured, then calculated using ray tracing to verify the effectiveness of using ray-tracing calculation. The throughput and BER characteristics were measured for the same environments to clarify the requirements for controlling the coverage. We found that covered and uncovered regions could be created by using partitions with absorbing boards and that the delay spread must be less than 15 ns and the received-signal must be stronger than -75 dBm for a region to be covered. We verified that the delay spread can be calculated to within 5 ns and the received-signal level can be calculated to within 5 dB of the measured data by using ray tracing. Therefore, ray tracing can be used to design antenna positions and indoor environments where electromagnetic environments are controlled for 2.4-GHz-band wireless LAN systems.

2.4-GHz-band mid-speed (1- to 2-Mbit/sec) wireless LAN systems are being widely used in offices and factories. Electromagnetic interference can occur between these systems because they use the same frequency range. In this paper, we investigate the characteristics of the interference between wireless LAN systems that use direct-sequence (DS) systems and frequency-hopping (FH) systems. The interference characteristics were measured for three DS systems and one FH system that meet the IEEE 802.11 and RCR standards and that use different modulation methods. Our results indicate that throughput depends on the system and the modulation method. We have also developed a model that can be used to calculate the interference characteristics between DS and FH systems by considering the bandwidth of their transmission signals, the dwell time of the FH system, and the time that the DS system needs to transmit a data frame. We used this model to calculate the bit error rate (BER) characteristics of the systems used in our experiment, and the results indicate that BER characteristics depend on the modulation method. The throughput characteristics of the systems used in our experiment were also calculated, and agreed with the experiment results within +/- 5 dB. The throughput characteristics of wireless LAN systems based on IEEE 802.11 were also calculated when the signal level was higher than the receiver noise level. The results show that FH systems require a D/U ratio about 7 or 8 dB higher than the ratio required in DS systems because the parameters in the standard differ between FH and DS systems.

Wireless communication systems are affected by several factors in the indoor environment. The complexity of this environment, however, has hampered the development of methods for analyzing it. Reported here is our investigation of the relationship between the propagation characteristics and performance of a 2.4-GHz ISM-band wireless LAN in various indoor environments. Our objective was to develop guidelines for designing ideal indoor environments for wireless LANs. A booth constructed of a ceiling, floor, and wall materials that could be changed was used for our investigation. The transmission loss and delay spread were measured for four environments; they were calculated by using a ray-tracing method to verify the effectiveness of the ray tracing calculation. The throughput and BER characteristics were measured for the same environments. The following results were obtained. (1) The transmission loss and delay spread could be estimated by using this ray tracing method because the deviations between the calculated and measured data were within 5 dB for the transmission loss and within 10 ns for the delay spread. (2) Reflections from the walls caused a serious interference problem: throughput was 0.0 at more than 30% of the positions along the center line of the booth when the walls were constructed of high-reflection-coefficient material. (3) The throughput and BER were closely correlated with the delay spread; the number of positions meeting a certain throughput was estimated by the method based on the delay spread calculated using the ray tracing method. It was within 10% of the number measured. The results obtained can be used to design ideal indoor environments for 2.4-GHz ISM-band LAN systems.

This letter summarizes three talks in the tutorial session of the 13th Asia-Pacific Network Operations and Management Symposium (APNOMS2011), which focused on the disaster recovery and further emergency management regarding the Great East Japan Earthquake of 2011. We present the damage and restoration of communication networks and points to a future disaster-resilient society.