Femtocell Radio Technology

In this section:
Interference Management in Femtocells
- Interference Avoidance Through Frequency Planning
- Interference Mitigation in Co-Channel Femtocell Deployments
Seamless Mobility Across Femtocell-Macrocell Boundaries
Mobility in Standby Mode & Access Control
- Handoffs at Femtocell Coverage Boundaries

Interference Mitigation in Co-Channel Femtocell Deployments
Sharing all available spectrum between femtocell and macrocell networks ultimately leads to the most efficient utilization of available spectrum, provided proper interference mitigation techniques are employed. In the discussion of interference mitigation techniques below, we consider separately interference on the Downlink (DL) — or equivalently Forward Link (FL) — from the base station to the mobile device and interference on the Uplink (UL) — or equivalently Reverse Link (RL) — from the mobile device towards the base station. Also, we consider separately interference affecting users of the femtocell and users of macrocell.

Forward Link/Downlink Interference Mitigation

A femtocell must set its FL/DL transmission power high enough to overcome the interfering macrocell signal within the femtocell’s target coverage area. But the femtocell cannot arbitrarily increase its transmission power, as this would generate interference to mobile devices nearby that are operating on the same radio channel but are being served by a macrocell base station or another femtocell. To deal with this conundrum femtocells set their transmission power adaptively. They measure the strength of the signals received from nearby macrocells and other femtocells and set the FL/DL transmission power level just high enough to achieve acceptable SNR inside the target coverage area. Airvana femtocells also obtain measurement reports from nearby mobile devices, and track the presence of mobile devices being served by macrocell base stations to fine tune the FL/DL transmission power level and deliver the best possible grade of service to all femtocell and macrocell users nearby.

A femtocell transmitting at too high a power level creates interference to a nearby mobile device that is being served on the same radio channel by a far away macrocell. This can create a “dead zone” where even basic voice communication with the macrocell base station may become impossible. Airvana’s femtocells avoid this scenario by either pushing these mobile devices away to another radio channel on the macrocell network while they are still in standby mode or by allowing them to park on the femtocell in standby mode, and handing them out to a different radio channel on the macrocell network whenever they turn active for a voice or data call. This way all macrocell calls in the close vicinity of the femtocell always take place on a different radio channel and thus avoid any interference from the femtocell.

In the scenario where a mobile device starts a call with the macrocell outside the femtocell coverage area and moves close to the femtocell while still active on the call, the call quality can degrade as it approaches the femtocell. Upon detecting the strong pilot signal of the femtocell, the mobile device will report to its macrocell base station that the SNR it is experiencing has decreased and that another cell with a stronger signal is nearby. A well-designed macrocell radio controller can recognize (based on the “CDMA2000 PN Offset” or “UMTS Scrambling Code” identifier provided by the mobile device) that the interfering cell is a femtocell, and redirect the mobile device to another radio channel, i.e., perform inter-frequency handoff, thus avoiding any interference that may be caused by the femtocell.

Reverse Link/Uplink Interference Mitigation

Macrocell base stations maintain system stability on the RL/UL by controlling the total received RL/UL power. The transmission power of mobile devices that are being served by the macrocell base station are controlled in such a way that the rise in total received power over the equivalent thermal (ambient) noise level is maintained at or below a pre-determined threshold. This threshold, also known as the Rise-Over-Thermal (RoT), is typically set at between 5 and10 dB. Power control equalizes the strength of signals being received from mobile devices that are at different distances from the base station, and thereby maintains system stability. Soft handoff procedures also allow multiple base stations to control the transmission power of the mobile device located at a cell boundary.

Mobile devices that are being served by macrocells will set their transmission power in a way that is oblivious to the presence of femtocells. Because the distance between a mobile device and a macrocell is typically much larger than that between the device and a nearby femtocell, the RL/UL signal received by the femtocell from such a device can be very high, raising the interference level up to 30 or 40 dB above levels typically seen in macrocell base station receivers.

The femtocell receiver hardware is designed to handle such high levels of interference from nearby mobile devices, without suffering from any saturation effects. The femtocell will instruct the mobile devices it is serving to raise their transmission power to overcome the interference from nearby mobile devices being served by a macrocell base station, using a variation of the power control algorithm used in macrocell base stations. Having to raise its transmission power poses no problem for the mobile device being served by the femtocell because its transmitter is designed for operation with distant macrocell base stations. It generally has plenty of power available to reliably communicate with a nearby femtocell, even in presence of interference from other nearby mobile devices transmitting to a macrocell at a high power. In fact, in the absence of any interference from other mobile devices, a mobile device being served by a femtocell uses very little transmission power relative to what it uses on a macrocell network, which leads to longer battery life (or more specifically, talk time) and avoids RL/UL interference to nearby macrocell base stations.

However in the scenario where a mobile device being served by femtocell has to raise its transmission power in response to interference being caused by a mobile device being served by a far away macrocell base station, elevated interference levels can occur in the macrocell base station. As macrocell base stations are designed to operate in a power-controlled environment, unplanned interference from such a mobile device can cause mobile users being served by the macrocell near the cell edge to experience lower data throughput and call drops. Femtocells avoid this phenomenon by constantly evaluating the interference its mobile devices are causing to nearby macrocell base stations, and ensure that such interference does not reach levels where they affect macrocell user experience. This is done by using measurement reports from mobile devices to evaluate their path loss to the nearest macrocell base station, and by limiting their transmission power using power control algorithms.[source]


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