Beyond design, there's the issue of co-channel interference. When cells are placed much closer to one another, output power must be adjusted. When output power is kept high to assure higher link rates, it's much more likely that two APs on the same channel will introduce co-channel interference. This means that CCAs (Clear Channel Assessments) don't pass, which reduces effective throughput. Even if the AP has lowered transmit power, the client likely has not; this leads to power asymmetry and a situation in which the cell size of the client is much larger than that of the AP.
Cisco has its own standard, CCX (Cisco Compatible Extensions), which includes hooks for CCX-compatible clients to turn down their power, but it requires a Cisco WLAN infrastructure.
Client load balancing is another issue in microcell environments. Ensuring clients are associated with the AP that provides the best service levels increases the system's aggregate throughput. But because of some vendors' idiosyncratic roaming algorithms, clients may never voluntarily move. There are vendor-specific mechanisms to load balance APs and perhaps encourage lower-rate clients to associate to better APs. For example, some WLAN infrastructure vendors issue disassociates to kick the client off, and then refuse the client when it tries to re-associate. Others load balance as clients associate the first time, refusing associations when another AP has less load. This is where Meru and Extricom have an advantage, because they use a single BSSID.
How might 802.11n fit into all of this? Besides extended coverage, there's also extended throughput and an implicit commitment to the 5-GHz band. Even with the lowest count of nonoverlapping channels (eight) and channel bonding, there will be four nonoverlapping channels to choose from. At the maximum, using the newly allocated middle 255-MHz range plus the upper portion of the 5-GHz band would result in 23 nonbonded channels, or 11 nonoverlapping channels when bonding is in use. This will dramatically simplify channel planning and facilitate insertion of new APs (along with any channel adjustment) for capacity reasons.
Of course, MIMO will increase both data transmission rates and coverage, which could also up the potential for co-channel interference. It will take some time to see how this works in practice.
