Femtocell Radio Technology
In this section:
Introduction
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
The tiny cells created by femtocells typically lie inside larger cells served by nearby macrocell base stations. To operate such an underlay network reliably, femtocells need to avoid or strongly mitigate any interference with macrocells and provide a seamless experience to users as they roam in and out of femtocell coverage. Since existing macrocell networks and mobile devices have been designed without awareness of femtocells, these requirements must be met without requiring any changes to mobile devices.
The underlying femtocell radio technologies used in CDMA and UMTS femtocells are quite similar. Therefore, we will cover them together in this section noting differences where relevant. An important difference between these two systems to note at the outset is that in UMTS, voice and high-speed data (HSxPA) services can be delivered on the same 5 MHz wide radio channel, whereas in CDMA these services are delivered on different 1.25 MHz wide radio channels using two different air interfaces: CDMA2000 1x is used for voice services and CDMA2000 EV-DO is used for broadband data.
Interference Management in Femtocells
Femtocells implement several layers of interference management to optimize user experience across both femtocell and macrocell networks.
Interference Avoidance through Frequency Planning
A mobile operator has three basic options for allocating available frequencies in femtocell deployments. These are represented by the scenarios A, B and C in Figure 1., below.
Scenario A represents a dedicated radio channel femtocell deployment that provides separate macrocell and femtocell radio channels. This has the advantage of minimizing interference between the two networks and simplifies initial deployment of femtocells. Scenario A is typically more suitable in rural areas where the mobile operator may have unused radio channels. Scenario A is also more feasible in CDMA2000 systems where radio channels are more numerous because they have a narrower bandwidth.
Scenario C shares all available radio channels between the macrocell and femtocell networks. This has the advantage of providing more degrees of freedom to manage interference between femtocells, especially in dense urban deployments, but also requires the greatest degree of interference management to ensure minimal impact on the macrocell network from the co-channel femtocells.
Scenario B represents a compromise between scenarios A and C in which some radio channels are shared between the macrocell and femtocell networks and other radio channels are reserved for the macrocell network only. In Scenario B the macrocell can redirect the mobile devices it is serving on the shared radio channel to a dedicated macrocell radio channel when they approach a femtocell. One way of deploying scenario B is to use the shared radio channel primarily for macrocell data services (HSxPA), and for femtocell voice and data services, and leave the dedicated radio channels for macrocell voice services.
Since mobile operators may not be able to dedicate radio channels to femtocells in many of their markets, femtocells need to be designed with advanced interference mitigation techniques that allow reliable operation when femtocell and macrocell networks share the same radio channels—Scenario C. This is the scenario that we discuss next.
Figure 1. Deployment Scenarios for Sharing Radio Channels Between Macrocell and Femtocell Networks
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.
Seamless Mobility Across Femtocell-Macrocell Boundaries
Mobility in Standby Mode
A mobile device in standby mode changes the cell that it is camped on as it moves across cell boundaries. This process is often helped by parameters broadcast by the cell sites.
It is desirable for the mobile device to switch to its femtocell when the signal received from the femtocell is strong enough to support reliable service. This needs to occur even when the macrocell network can still provide reliable service, because switching to the femtocell will improve the mobile user experience and ensure that he/she can instantly take advantage of any subscribed flat-rate femtozone calling plan.
In either CDMA or UMTS, the mobile device will switch to a new cell on the same radio channel based on continuous measurement of pilot signals from neighboring cells. But mobile operators in most markets use multiple radio channels and switching to a cell on a different radio channel has more stringent requirements. To increase battery standby time the mobile device scans other radio channels only when the signal-to-noise ratio of the current cell is lower than a certain threshold. In UMTS systems, this threshold is determined by a system parameter called SIntersearch. Setting SIntersearch to a higher value can force all mobile devices to perform inter-frequency searches under more circumstances, thus increasing their battery drainage. On the other hand, if SIntersearch is set to a lower value, the inter-frequency scans required to detect the femtocell may not be triggered if the signal received from the macrocell base station is strong and a mobile device may never switch to the femtocell. Thus operator must optimize how they set SIntersearch. UMTS also includes a feature known as Hierarchical Cell Structures (HCS) in which femtocells can be given a higher priority than macrocells. HCS can be used to accelerate the selection of the femtocell, but it does not increase the frequency of inter-frequency searches.
CDMA2000 femtocells solve this problem by including a special transmitter, called a beacon, which makes the femtocell’s presence known on all macrocell radio channels except the one used by the femtocell. Beacon approaches are also being considered for UMTS femtocells. The beacon is a special signal that consists essentially of a low-power pilot signal along with a broadcast signal that forces mobile devices to camp on the femtocell radio channel. In UMTS this is done by broadcasting appropriate cell reselection parameters while in CDMA a command is sent to all mobile devices or only to those devices authorized to use the femtocell.
Since most CDMA2000 devices support both CDMA2000 1x and EV-DO, a mobile device needs to camp on the femtocell in both 1x and EV-DO systems simultaneously. Airvana’s CDMA femtocells implement a unique beacon solution to allow a device entering femtocell coverage area to attach and remain attached to both systems.
The overall femtocell user experience will be best when the beacon range is somewhat smaller than the service range most of the time and occasionally equal to the service range, as illustrated in Figure 1. The operation of the beacon is optimized through detailed simulations and lab/field testing to ensure that target mobile devices attach to the femtocell as quickly as possible and avoid unnecessary interference to other mobile devices that are being served by the macrocell base station.
Figure 2 Illustration of beacon and service coverage areas
Femtocell Access Control
Femtocells support flexible access control mechanisms.
In restricted access, the femtocell owner can restrict femtocell usage to members of the household and frequent visitors and avoid sharing Internet backhaul with others. CDMA femtocells implement restricted access by having its beacon redirect only authorized mobile devices, thus leaving unauthorized mobile devices continue to operate normally on other radio channels and attached to the macrocell. Unauthorized mobile devices on the same radio channel as the femtocell who detect the femtocell are allowed to camp on the femtocell and when they turn active they are handed out to the macrocell on a different radio channel. This approach ensures that no unauthorized active user is allowed to use the femtocell and they are moved away from the femtocell radio channel to avoid interference.
UMTS femtocells implement access control by sending an appropriate “rejection” message, which causes the mobile device to switch to another radio channel on the macrocell network.
Mobile operators may choose to provide commercial incentives to femtocell owners to have them configure their femtocells in open access mode, where any mobile device can receive service from the femtocell. Open access not only makes the femtocell experience available to more users, it also avoids many of the interference scenarios discussed in the previous section. In open access, the femtocell beacon will redirect all mobile devices within its coverage area to the femtocell radio channel.
Hybrid access is similar to open access except here certain mobile devices selected by the femtocell owner are given preferential treatment over other mobile devices that can use the femtocell on a best-effort basis.
Figure 3 Simplified CDMA2000 1x Circuit-switched Services Femtocell Architecture
Handoffs at Femtocell Coverage Boundaries
When a user on a call moves in or out of femtocell coverage, a handoff procedure is executed to maintain call continuity. Such hand-in (from macrocell to femtocell) or hand-out (from femtocell to macrocell) procedures greatly enhance femtocell experience as users enjoy seamless call continuity when they cross network boundaries.
Hand-out can be supported with relative ease without any changes to the existing macrocell network or to mobile devices. This is achieved by making the femtocell network behave just like a macrocell network towards the macrocell Mobile Switching Center (MSC), essentially making the legacy equipment think that it is handling a handoff between two macrocell networks. Except when the femtocell and macrocell networks are sharing a single available radio channel, hand-out is generally performed to a macrocell radio channel that is different from the femtocell radio channel. In UMTS systems, when UMTS macrocell coverage is not available, a voice call can also be handed out to the ubiquitous GSM system.
Hand-in on the other hand is more difficult because there is no simple mechanism for the macrocell network to determine the identity of the target femtocell from the measurement reports sent by the mobile device approaching the femtocell.
In CDMA2000 systems, the measurement report contains the strength of pilot signals seen by the mobile device and uses the so-called PN offset to identify the target cell. However, target femtocells cannot be identified without ambiguity based solely on PN offset report because out of the available 512 distinct PN offsets only a small number will be allocated to femtocells and these are re-used amongst them. Thus with possibly hundreds of femtocells per macrocell it is not possible to uniquely identify the handoff target.
CDMA femtocells can solve this problem by measuring the RL signal of the approaching mobile device. First, using cdma2000 1x signaling protocols the Base Station Controller (BSC) in the macrocell network triggers the handoff and forwards the mobile device’s RL scrambling code information and the target PN offset to the Femtocell Convergence Server (FCS) via the MSC. Based on this and other available information, the FCS then requests a subset of the femtocells it is serving to listen for the mobile device based on the RL scrambling code. All femtocells with the same target PN offset then report back the mobile device’s RL signal quality together with the femtocell’s FL pilot transmission power level. Based on the reports from the various femtocells, the FCS determines the correct target femtocell and signals the macrocell BSC to order the mobile device to handoff to the femtocell.
The same femtocell identification issue occurs in UMTS femtocells. A scalable hand-in solution requires changes to existing standards, thus can work with only future devices that will be compliant with these new standards. A proposed 3GPPstandard solution is based on “autonomous gaps”, which allows the mobile device in a call to break from the call for a sufficiently long period of time to allow it to search for a femtocell on a different radio channel and decode its broadcast channel. Based on the cell identity and the measurements, the macrocell network can trigger a hand-in to the femtocell.