There have been several system architectures proposed and developed by different small cell vendors, but the industry has standardised on common solutions published by the 3GPP standards committees. There are three architectures, one for each of the three main different radio technologies.
- UMTS - the most common and used for today's 3G UMTS phones
- CDMA - applies to 3G CDMA femtocells compatible with 3G CMDA phones
- LTE - the 4G standard uses a different approach, having incorporated small cells into the system from the outset
Maintaining compatibility with mobile devices and services
In order to ensure compatibility with existing mobile phone networks, phones and services, the radio interface to the mobile phone and to the core network are the same. For 3G UMTS, the small cell subsystem connects into the operator's core network using the same Iu interface that traditional outdoor cellsites use. For LTE, the standard S1 interface to the core is used. Small cells also conform to the standard radio transmission frequencies and protocols used today.
The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate with the small cell controller in the same way as other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user.
The small cell appears to the standard 3G or LTE phone as just another cellsite from the host mobile operator, and can be used by almost any phone including roamers visiting from other countries.
Clear interface between small cell gateways and small cells themselves
The connection between the small cell and the small cell controller/gateway is termed the Iu-h interface and it uses a secure IP encryption (IPsec) to avoid interception. LTE uses the standard S1 interface protected by IPsec. There is also authentication of the small cell itself to ensure it is a valid access point.
Each small cell connects over broadband IP with a small cell gateway which may handle hundreds of thousands of small cells. These are consolidated into a single Iu or S1 interface which can carry thousands of concurrent calls and data sessions.
The standardisation of the Iu-h and S1 interfaces was included in the 3GPP Release 8 standard approved in 2009 and refined in subsequent releases. These interfaces allow an operator to procure small cells from several different vendors and connect them to a single gateway. The ensuing competition drives cost down and encourages innovation.
Next, how small cells work
