Briefing
Technical Briefing
Technical Briefing
Technical
Femtocell Concepts Providing reliable in-building coverage has always required a special effort from mobile-network operators. While there have been some advanced dedicated in-building solutions installed in high-profile buildings, most in-building coverage is still provided from outdoor cells.
The Problem Providing indoor coverage from outdoor cells can be problematic for UMTS operators. On one hand the higher frequency and greater dependence on accurate power control and interference management mean that providing in-building coverage from an outdoor cell is very costly in terms of system capacity. On the other hand the indoor environment is precisely the most likely place where a user will want to take advantage of the type of advanced data services potentially available through UMTS. Indoor users demand the largest proportion of available transmit power from a Node B and at the same time need to transmit much higher powers. Additionally, the cost for the subscriber of using a mobile network for data services can be very high compared to other access network options.
Indoor voice traffic loads network disproportionately
Given that most subscribers are inside most of the time, the overall result is that traffic (still largely voice) generated by indoor subscribers tends to load the network disproportionately. Further, limited spare network capacity for data and high access costs mean that a lot of potentially revenue-generating data traffic is lost to alternative access mechanisms. Chiefly, data traffic will be lost to fixed-line and Wi-Fi-based options, both of which are often readily accessible in the indoor environment.
Internet and content
Data revenue lost to fixed DSL/Wi-Fi alternatives
The Femtocell A possible solution for this is the femtocell concept. This is a small device purchased by a subscriber, or provided as part of a contract bundle, and installed in their home or small office. The device looks and behaves rather like a wireless router. The femtocell is a compact low-power base station, but it also contains other RAN functionality. It is plugged into a broadband fixed-line connection over which it establishes a connection to the core network belonging to the subscriber’s mobile operator. Control functions in the core network configure and control the femtocell as if it were part of the operator’s network; which in effect it is.
August 2008
Home or small office indoor environment
ISP
Generic Access Network Controller (GANC)
ADSL 3GPP Core Network
Femtocell
UTRAN Outdoor environment
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Briefing
Technical Briefing
Technical Briefing
Technical
The subscriber has control over which phones can access the femtocell and under what circumstances. When the subscriber is away from home their phone or data card works in the normal way via the UTRAN. When they are at home their equipment will communicate via their own femtocell. The idea is that the mobile operator will offer reduced rates for communication via the femtocell; probably in line with fixed communication charges. This raises the possibility of the mobile operator becoming the subscriber’s sole communication provider. For the subscriber it means a seamless transition between fixed and mobile communication options. For the operator it means a much larger coverage footprint with low infrastructure costs, greatly increased capacity with low backhaul cost and, importantly, entry into the fixed telecommunications market. Femtocell Implementation In theory the femtocell concept is applicable to any cellular technology, but it makes most sense in the context of 3G, thus most current femtocell products are based on UMTS. The operator should think of the femtocells as an additional hierarchical layer within the established cellular network hierarchy. An operator needs consider how to utilise their available spectrum within all these hierarchical layers. However, the femtocell layer differs from other layers in that the operator has limited control over the location and coverage of any given femtocell. Therefore some consideration is needed regarding the possible interference effects resulting from the use of femtocells. There are essentially two solutions: self channel selecting devices and fixed spectrum partitioning.
Macrocell
Picocell
Femtocell
Microcell
Self channel selecting femtocells operate rather like Wi-Fi routers or Bluetooth devices. They scan the operator’s spectrum allocation to find the least interfered channel. They may also set their own transmit power level in sympathy with measurements of the outdoor cells. Fixed spectrum partitioning involves the operator making a specific allocation of spectrum for use on femtocells. Given the limited amount of spectrum available to most operators this may be difficult. However, there are options for some operators. For example, the femtocell could be based on the relatively unused spectrum for the TDD mode of UMTS, or spectrum could be made available through the refarming of current 2G spectrum.
Need more? Delivered on your site or as a regularly scheduled public course, Wray Castle offers a wide range of radio planning and optimization training
Cell Planning for GSM Networks Introduction to GSM Optimization Cell Planning for UMTS Networks
Introduction to UMTS Optimization 2G/3G Indoor Coverage Planning Drive-Test Data Capture and Analysis
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August 2008
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