Fetmo Access Point: Is the interference problem a big Red Herring?

If Femtocells were to operate on a frequency that is shared with the macro network in a restricted mode (i.e. only specific users can access a particular Femtocell), there is a concern that interference between the two layers may create a doomsday scenario.

For some reason, many think that the uplink interference from Femto users will jam the macro recievers and cause macro uplink coverage to deterioate. Some technologists and vendors have responded to this challange by developing extremely complex techniques. I have seen documents on both FemtoForum and 3GPP talking about things like:
- an interface between the macro layer and femtocells to exchange radio specific contexts.
- A "cognitive pilot channel" proposal that seems to be at least 15 years ahead of its time.
- Proposals to use uplink interference cancellation technology in femtocells.
- Suggestions that only LTE access points and release 8 (or 9) handsets will eliminate the interference problem.

All of the above is very interesting for a technologist. Ofcourse there is always a way to make things better, but there are also other considerations: value added, cost, market segment, technology application, market drivers ...etc, otherwise we would be all driving a Ferrari or a Porsche.

Looking at this interference issue, I can't help point out a number of things:
- I believe that more than half of femtocell deployments will end up using a dedicated carrier.
- In the absence of a femtocell, the user will go on the macro network and will create an amount of noise rise anyway. When you put a femtocell close-by the user will radiate less power and create less noise rise to the macro site. There is at least a 25-30dB reduction in path loss, so the femto users will radiate much less power than if they are using the nearest macro cell.
- A residential femtocell will only support a very limited number of users, so the interfernce limit ishardly ever reached and additional uplink interference from macro users operating in the vicinity will hardly block the femtocell uplink.

Is is possible that theso called interference problem is nothing but a big red herring?

Advanced Cellular Technologies Episode 5: Interference Cancellation for W-CDMA

In a conventional WCDMA system, each user is detected non-cooperatively where other users’ signals are treated as noise. This basic scheme has a low implementation complexity but a limited overall spectral efficiency due to multiple-access interference (MAI).
Higher spectral efficiency can be achieved by reducing the MAI. In contrast to conventional RAKE receivers, the Multi-user detection (MUD) receiver does not treat intra-cell user signals as noise, but tries to eliminate their interfering effect.
There are two main variants of The MUD receiver structures: interference cancellation (IC) and joint detection (JD). The first category involves a subtractive scheme where the receiver produces estimates of the interference and then subtracts them from the received signal. The second category (JD) involves linear transformation of the received signal using, depending on the algorithm, information regarding the signals' time offsets, codes, amplitudes and phases.
The major advantage of this scheme is its relatively low complexity compared to other MUD techniques. Interference cancellation receivers belong to the class of non-linear suboptimal MUD techniques and their complexity is a linear function of the number of users compared to polynomial or exponential complexity for other types of MUD receivers. In effect, IC enhances the interference limited uplink capacity of WCDMA cellular systems.