On 2$\,\times\,$ 2 MIMO Observable Capacity in Subway Tunnels at $C$-Band: An Experimental Approach

The demand of video and Internet services in underground transport systems has triggered the need to increase wireless transmission capacity in tunnel environments. The current construction of the new L9 subway line in Barcelona, Spain, has provided the opportunity to characterize a 2 × 2 MIMO channel at C-band with the objective of increasing the capacity to provide video surveillance inside subway coaches. In this letter, the authors present the measurement campaign that took place at the L9 tunnels, the statistical study derived from the raw data obtained during that campaign, and a set of conclusions focused on determining the design tradeoffs inside tunnels in terms of antenna spacing and polarization diversity.