**4.2 Channel capacity measurement**

The channel capacity of a 2x2 MIMO system by using the proposed pattern reconfigurable antenna is measured in this section. The experiment setup is as same as Fig. 17. At the TX end, two reference dipoles are arranged perpendicular to XZ plane along Y axis. Another two reference dipoles and two proposed pattern reconfigurable antennas are adopted at the RX end alternatively for comparison. Each port of the two wire dipoles has a bandwidth of 3.9-5.9 GHz with reflection coefficient better than –6 dB, and mutual coupling between the two ports is lower than –25 dB over the frequency band which is achieved by tuning the distance between two elements. Also, the isolation between two proposed pattern reconfigurable antennas is lower than –25 dB.

The measurement was also taken in the Weiqing building of Tsinghua University of Fig. 18. The locations of RX are different from last experiment. The position of RX4 is not measured. Therefore, the LOS scenario includes the RX1 and RX2, and the NLOS scenario includes the RX3 and RX5. The frequency range of measurement is 4-6 GHz, with a step of 10 MHz. A total number of 201 data points/results are obtained as samples. Three configurations (CPW, LS and RS) of each reconfigurable element of the receive end were switched together manually and the highest value signal was selected as the receiving signal. Also, considering the small-scale fading effect, 5x5 grid locations for each RX position were arranged. A total number of 2x201x25=10050 results were measured for LOS and NLOS scenarios respectively. In the measurement, a 2x2 channel matrix H is obtained. The channel capacity is calculated by formula (1) in the last section. We also selected the SNR when the average channel capacity is 5 bit/s/Hz in a 1x1 reference dipole system in LOS or NLOS scenario.

The measured CCDFs of channel capacity of LOS and NLOS scenarios are illustrated in Fig. 26 and 27. The results consist of the channel capacity information of 2x2 multiple antenna system using the proposed pattern reconfigurable antennas, compared with 1x1 and 2x2 systems using reference dipoles. As listed in Table 2, 2.28 bit/s/Hz and 4.13 bit/s/Hz of the average capacity enhancement are achieved in LOS and NLOS scenarios, and 2.51 bit/s/Hz and 3.75 bit/s/Hz enhancement for 95% outage capacities. In the NLOS scenario, the received signal is mainly contributed from reflection and diffraction of the concrete walls and the desk partitions, arriving at the direction of endfire. The diversity gain in the endfire increases the channel capacity. Considering the insertion loss introduced from the non-ideal PIN diodes, better performance of the proposed antenna can be achieved by using high quality switches, such as micro-electro-mechanical systems (MEMS) type switches with less insertion loss and parasitic parameters.

Fig. 26. CCDFs of channel capacity in LOS condition.

Fig. 27. CCDFs of channel capacity in NLOS condition.


Table 2. Average and 95% Outage Channel Capacity (bit/s/Hz).
