It appears that US Mobile SPs are very eager to put GPS modules into their Femtocells (aka FAP). In Europe and elsewhere, however, this is not the case. Why?
Everybody knows that getting reliable GPS lock inside buildings is very hard. The best GPS reception is at the window. But for the best Femto coverage you want it in the middle of a typical McMansion. When SPs embed Femto into other devices such as STB, they won't be by the window. External antennas are pain. Trees interfere with line of sight. And GPS modules are not free. So, why do US providers want to inflict this pain on themselves and their customers?
Let's review why folks *think* they need GPS in a FAP:
1. FAP location verification. This ensures:
(a) FAP does not radiate on the wrong frequency and does not violate FCC rules; this is relevant if an SP has different spectrum channels in different coverage areas;
(b) FAP can provide data to e911 services for UE (phone)
2. FAP oscillator disciplining. Accurate and stable frequency requires expensive crystal hardware - too expensive for FAP. One of the techniques used to sync clock of the cheaper crystals is to continuously obtain precise time from GPS. But there are viable alternatives, such as sync'ing from neighbor macro broadcasts or via NTP from a quality source. These two mechanisms can provide sufficient accuracy for WCDMA and are much cheaper. Certainly, European SPs targeting deployments without GPS think so.
It is possible that 1.a can be more of a problem in the US with SPs there having grown by a lot by acquisitions and covering many different spectrum ranges varying not only by state but also by country and even sub-county level. However, there are likely easier solutions than introducing the GPS for this problem.
For example, the FAP could periodically scan for transmitters on the target channel(s). If it decodes a foreign neighbor (wrong MCC/MNC), we know it is not where it should be or someone else is violating the rules. In the few cases where it finds no neighbors, the FAP may violate FCC rules and transmit on the wrong frequency, but this would require a customer who lied about the placement of their FAP. Therefore, such incidents should be minimal. The damage is limited because the FAP ensures there are no other transmitters around on the same channel (and PSC) and the FAP transmits at relatively small power. But would FCC agree with such interpretation?
Now, e911 gets even more interesting. FCC wireless e911 rules mandated a two phase roadmap. In Phase I, the SPs were required to provide the location of the base station to the emergency services. In Phase II, they were required to provide the location of the UE itself within 50-300 meter accuracy.
For Phase I, the location of FAP would need to be provided. For macro, this is based on its registered address. The idea of movable base station was probably not considered back in 1996 when the regulation was first issued. A more recent precedent in FCC is the e911 rules for VoIP issued in 2005. These rules require SPs to transmit customer's "Registered Location". The customer must maintain his latest location in the SP database. This recognized the fact that it may not always be technically feasible for SPs to automatically determine the location of their end users without users' active cooperation. The same should hold for FAPs IMHO.
For Phase II, we need UE location. The solution has to work on macro as well as on Femto. A number of approaches for UE location determination have been investigated including network-based (such as triangulation of signals), UE based (such as with GPS) and hybrid. None of them are 100% reliable. The current winner appears to be a hybrid approach with GPS and assistance data (AGPS) from the network. Knowledge of the FAP location may help in providing assistance data to UE. But again, it is not clear why registered location won't suffice.
To sum up... the pain was inflicted by FCC & SPs' conservative interpretation of the current rules:
1. The e911 requirements could be addressed with user registering their FAP address with the SP. However, it is not clear if FCC's current rules allow this.
2. The non-interference requirements could be met in the same fashion, plus enforced by having the FAP periodically scan for conflicts and shut itself down.
Should SPs take action at FCC to clarify the rules? I think so. The current FCC rules are ambiguous as they relate to Femto. This leaves SPs to incur extra costs, while providing a less ideal experience to users, some of which have to place Femto devices close to a window, use external antennas, etc. And some customers will never get a GPS lock anyway because of external barriers such as trees and fences.
My take is that FCC rules will eventually be relaxed to be "best-effort only" GPS in Femto. But in the meantime, it is up to each SP to interpret the rules as they see fit.
What do you think?
