Inside a cellular femtocell

-December 15, 2009

Cellular-service providers are trying to shore up their service by selling femtocells to customers. These little boxes have to sell for at most a few hundred dollars and be both user-installed and self-configuring. And they must provide most of the functions of a full cellular base station. On power-up, the femtocell configures itself to take advantage of the most available radio frequencies at its current location, establishes an IPsec (Internet Protocol-security) connection through the user’s Wi-Fi or wired Ethernet back to the carrier’s femtocell gateway, and checks itself into the carrier’s wireless network.

To get all of these functions into this price range and into a form factor similar to that of a Wi-Fi hub, designers lean heavily on both a highly integrated, application-specific system chip and carefully optimized radio and power circuits. This evaluation-level reference design of an HSPA (high-speed-packet-access) femtocell from silicon vendor PicoChip illustrates the implementation and the care that will go into femtocell designs. Note that this unit is a lab development board, not an optimized product. As such, it has a lot of test points, debugging connectors, and so on. It also has some duplication—for example, several memory types.

1. Some carriers use the optional SIM (subscriber-identity-module) card for authentication and security.

2. The femtocell requires a variety of radios: an HSPA Node B transmitter and receiver for femtocell operation, an HSPA UE (user-equipment) receiver for hand-over and network-monitor functions, and a GSM (global-system-for-mobile) communications UE receiver for hand-over.

3. An Ethernet broadband connection routes traffic back onto the wired Internet.

4. The board requires a variety of power supplies at the lowest possible cost.

5. The low-cost, low-power, fully integrated PicoChip PC302 baseband SOC (system on chip) integrates a Node B modem; a controller, including an RNC (radio-network controller) and all associated functions; interference management; and peripherals.

6. Either an NTP (network-time protocol) from the left-hand module or an A-GPS (assisted-global-positioning-system) circuit from the right-hand module provides timing and synchronization for stationary indoor applications.

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