I don't think this is a huge issue outside of Japan, but NTT DoCoMo has developed a system to make mobile phone coverage indoors much easier.

NTT DoCoMo announced o­n the 8th that they have developed an antenna division system for turning the inside of buildings and underground into FOMA service areas. Compared to existing systems, over 8 antennas can be connected, making the creation of indoor service areas simple.

To turn the indoors into a service area, an atenna adjacent to a common base station is divided into multiple parts, then needs to be distributed throughout the location by using optical fiber, or another high speed connection. It was previously difficult to put a large number of these distribution locations throughout buildings, which meant multiple sets of equipment needed to be installed in large-style apartment complexes. This new system reduces the size of the slave station required for each antenna, and also requires o­nly o­ne optical fiber line instead of two, making construction easier.

DoCoMo plans to employ this new system and proactively advance the indoor coverage of FOMA. They seek to increase the September 2005 count of 4,500 indoor FOMA service areas to around 6,000 by March of this year.

Inspired by:
http://plusd.itmedia.co.jp/mobile/articles/0602/08/news038.html

Press Release:
http://www.nttdocomo.co.jp/info/news_release/page/20060208.html