Femto Cells : Challenges that lie ahead ...

In my previous post , I had explained about the value add that a Femto Cell will bring to an operator as well as the subscriber. Though the Femto Box will be similar to a WiFi router, the complexities involved in making the micro and macro cellular ecosystems to jell-well together, will have to be overcome for the product to become the next big thing in the wireless world (or so it is touted to be).

Listed below are some of the problems I could think of that Femtos face as of today. Vendors in conjunction with the operators will need to an elegant solution that will not only solve the above problems but also lay foundation for next generation Femtos.

Frequency/PN Allocation: Frequency allocation and reuse factor (this is related to 3GPP world, for 3GPP2 world, the same thing will translate to PN allocation ) is something that operators will need to ponder over if the target is for every apartment to have a femto but with no interference issues either between femtos or between femto and the macro world.

Ensuring QoS: Since the femto traffic will be carried over existing broadband, operators will need to ensure that QoS is met. If the ISP's observe net neutrality, then it should not be a problem, but it should not happen that with people downloading a video content through ISP, the Femto voice call is blocked or a delay in the voice call is observed. That will defeat the whole purpose.

Near/Far Interference problem (A Myth ?): From what I have read in articles or through discussions, people wonder whether in CDMA femtos, assuming that the femto and macro are on same frequency, would the near/far problem occur in light of the femto asking the handset to bump up the power resulting in the handset causing interference for the macro users. I personally think that the near/far problem won't exist as the reverse link threshold for a femto will be much less than that of the macro (in order of mWatts).

Soft Capacity: Typically Femtos will support 4-8 users (though the actual number touted by different vendors today is variable but generally in the range mentioned). Now in the event that there are 10-15 people in an apartment, there will be contention (no brain er here).  So it remains to be seen if like macro, the femto soft capacity  would allow that much scalability in the reverse link and if it doesn't then the remaining users shouldn't have issues latching onto the macro.

Supporting Users: How to make sure that only legitimate (by this I mean only those who reside) people latch on to the Femto ? The Femto power would be such that the signal (coverage) would be available outside the apartment too (upto few yards beyond).  In such a case is it possible that a by-stander would latch onto the Femto resulting in a resident's call being blocked ?  

Homogeneous and Independent coverage: It is very important that Femto provides a homogeneous coverage throughout the apartment irrespective of the location. It is possible that for a Femto lying in a basement, its signal would experience a penetration loss. So the forward link budget should be balanced taking all the signal strength losses, fading effects into 

account.

HandOffs: Femtos should be able to support Micro->Macro and Macro->Micro handoffs. This is very important from the subscriber point of view as everyone hates dropped calls (the other devil being blocking probability). I think that as the technologies evolve, Femtos should also support inter-technology handoff (UMTS->LTE, 1xRTT->EVDO->LTE) and Femto->Femto handoff. This all will enhance end user experience.

Business Model: I personally think that for Femtos to succeed, operators have to change their business model. For starters, Operators will have to subsidize the cost of Femto box as much as they can. Sprint started selling their Airrave Femto for $50 where as AT&T is rumored to be selling their Femto for $100 though the cost/Femto to these operators is about $250-$300. Other incentive would be free airtime minutes, flat fee/month etc. The more attractive scheme an operator offers, the more demand will be for their Femtos. Also I think that operators will have to collaborate with ISP's for their backhaul use as mentioned above. 

Albeit all of the above challenges (which I term as Birthing Pains for any new product), the Femto technology looks all set to become the Next Big thing in cellular world and a true platform for much touted and eagerly awaited FMC (Fixed Mobile Convergence).