The femtocell is a wireless network which shares the licensed wireless spectrum with the macrocell. Both networks are connected through an IP-based backhaul link. In contrast to the optimized deployment of base stations in the macrocell, the femto-access point (FAP) is installed in the households by the end-user without the supervision of the macrocell. Under those circumstances, the deployment of a large number of femtocells imposes an efficient administration of the interactions between both types of networks. FREEDOM will investigate advanced interference-aware PHY techniques (scaling as the quality of the backhaul link) and the enhancement of the control plane procedures. The devised algorithms will be evaluated at system level, outlining the benefits of the femto-based networks and giving some network planning recommendations. Additionally, the candidate algorithms/protocols will be assessed in terms of hardware feasibility and on-field demonstration.
FREEDOM deals with several challenges related with the introduction of the femtocells, addressing them at different levels, in order to clarify a global view of the benefits of a femto-based network in the next generation wireless system.
Advantages of advanced interference-aware PHY techniques
The challenges faced at the PHY layer in FREEDOM are:
- human activity impact on the channel models in a femto-cell context;
- synchronization (time, carrier frequency and carrier phase);
- interference power modelling;
- transmission/reception schemes based on the quality of the backhaul link.
Since the femtocells are installed inside of the buildings, the moving people will block intermittently the signal transmitted from the femtocell or the interfering signals coming from the macrocell. Those issues are currently investigated in the IEEE 802.11n and COST2100. One of the objectives pursued by FREEDOM is to get a realistic channel model for indoor and outdoor-to-indoor transmissions.
The synchronization is a relevant topic addressed in this project and it will be tackled from two different points of views: time synchronization and carrier frequency/phase synchronization. The femtocells are autonomous (cheap) entities connected to the macrocell through an IP-based backhaul. At the macrocell, the base stations have high-accuracy oscillators calibrated periodically by timing signals sent from a central controller over T1 lines (more reliable than IP lines). The accuracy of the oscillators installed at the FAPs is significantly lower than the BSs in the macrocell. The lack of time synchronization among femtocells (GPS-assisted synchronization is very seldom viable in indoor scenarios) motivates the generation of the interference due to the uplink/downlink transmissions. FREEDOM will consider the adoption of distributed techniques for time synchronization at frame level and will evaluate how the synchronization is related with the generated interfering power. The second type of synchronization (carrier frequency/phase) is required for implementing distributed precoding solutions when two or more network elements (nodes) decide to cooperate in transmission. The promising gains of the distributed precoding techniques can be dramatically reduced when the nodes are not synchronized.
The derivation of a statistical model for the interfering power received at the femtocell and macro-cell is necessary to design robust transmission schemes for minimizing the impact of the outage events associated to the current (unknown) values of the interference. Moreover, this model will be useful for the system level evaluations of the femto-based network.
The transmission/reception techniques are grouped as a function of the quality of the IP-based backhaul link: minimal, medium and high quality. When the quality of the backhaul link is minimal it is not possible to exchange much information among nodes (femto-femto, femto-macrocell), so the signals received from neighbouring femtocells and/or macrocells are tackled as interference. To this end, intelligent sensing algorithms will be developed, exploiting the compressive sensing. Using that knowledge, the resource allocation can be designed under a game-theory approach as a competitive game. The interfering signals also can be combated by means of hybrid access schemes, defining regions where several femtocells can reuse the resources. Under the assumption of a medium-quality level of the backhaul link, the nodes are capable of exchanging messages at control-plane level, deriving coordinated strategies, where several nodes can collaborate in order to identify the interference; on the other hand, a dynamic resource management will reveal which nodes are able to reuse the same resources. Finally, a high-quality backhaul link allows exchanging messages at data-plane level. Hence transmitter cooperation strategies can be devised such as distributed beamforming (under carrier frequency/phase synchronization) or space-time coding, where all the cooperative nodes must know all the messages. If one FAP is assisting another one, then decode-and-forward relay-based protocols can be explored. Likewise, several FAPs can cooperate at the received side by transmitting their received signals to a central processor (compress-and-forward). In such a case, there are as many virtual receiving antennas as the number of FAPs. Notice that in the transmitter/receiver cooperation one of the links employed in the communication is the IP-backhaul.
Enhancements in the control plane procedures
Strategies for seamless handover: the fast seamless handover generating minimum signalling overhead for femto-based networks with coordinated femtocells will be designed. This procedure should allow handover among macro and femtocells as well as among femtocells. The femtocells have some specifics (such as fast decrease of signal strength) that will be reflected in the proposed handover management procedure. Therefore, advanced methods for handover decision and initialization will be designed considering either parameters from different layers or passive scanning. Also in this case the exploitation of the coordination paradigm enables the design of significantly more promising femto-specific coordinated handover mechanisms. However, as in the case of the interference management, also not-coordinated solutions will be proposed, as the lack of sufficient quality of the backhaul is a worst-case, but still valid, working hypothesis. Finally, since the femtocells can support both 4G candidates, LTE-A and WiMAX, procedures for movement of users among networks with different radio access technologies will be also supported.
MAC control procedures: the control procedures for radio resource management in networks with femtocells should manage radio resources despite the limited backbone capacity. To achieve an effective utilization of backbone capacity new scenarios will be considered for routing of data among users served by same FAP. Besides routing, spectrum-efficient techniques for power control, scheduling and broadcast services transmission will be designed. Additionally, novel user admission policies and FAPs identification techniques will be defined keeping in consideration the scalability of the system. The designed control procedures will respect FREEDOM environment and will get the benefit from cooperation and coordination among femtocells and macrocell designed in WP3.
Indications from the system level evaluation
The impact and the benefits of the advanced PHY techniques for the interference avoidance and the control plane procedures must be tested at system level and this poses new challenges such as the adoption of realistic interference system-level models, the system scalability in case of sense deployments and the optimisation of the routing and security mechanisms to be implemented on the ISP backhaul. These aspects will be faced in WP5 by adopting a systematic approach that will consider the dynamic femtocells clustering as a viable route to cope with the scalability issues by introducing cluster-aggregated metrics, in order to minimise the impact on the ISP backhaul bandwidth requirements. However, beside the management of the additional overhead on the ISP bandwidth due to the implemented cooperation/coordination, the scalability also addresses the adoption of strategies for the optimisation of the routed data flow and of the secure transport protocols. These will be faced by access control systems and QoS enforcement policies and by the smart aggregation of multiple SRTP streams to significantly increase the bandwidth efficiency.
Beside the objectives directly related with the system level evaluations of an optimised femto-based network, a problem (to our knowledge) not yet faced in the SoA is the comparison of a femto-based network against potentially competing technologies. Many studies and technologic developments (e.g. [WNe08], [AlS08], [Dop08], [Sch08]]) demonstrate that a relay-based network improves the indoor coverage: this could make the FAPs obsolete. A specific activity targets at giving a tradeoff evaluation between the two competing technologies ad the results will drive the business model to be developed in 2A3.
The devised algorithms will be implemented and verified through a hardware prototyping. The hardware feasibility study will suggest which techniques are chosen as a basis of the first femto access point (AP) prototype realization. The latter will address the selected interference mitigation techniques and routing mechanisms to be proven in order to refine the engineering rules of the femtocell deployment. The manufacturers will test and verify the implemented techniques in standalone manner so that there is no requirement to integrate the FAPs with the LTE/WiMAX core networks. These will prove the chosen techniques individually without considering the integration issue.
Another approach will further enrich the proof of concept activities. The industrial partner will invite several interested infrastructure vendors to give their comment on the feasibility study outcomes, prior to a laboratory trial. A laboratory trial then will be performed to study the interference characteristics, power control mechanism and handover performance (if applicable) between femto-to-femto and/or femto-to-macro cell and their impact to the system coverage and capacity in a reduced-scale environment. The integration between femtocell and xDSL/Metro-Ethernet network system will be provided within the TELKOM R&D Centre testbed facility in Indonesia. The trial activities goal may not be limited to prove the selected new techniques implemented in the prototypes (due to the interoperability issue), but extended to refine the engineering rules of the femtocell deployment.