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Tuesday, March 29, 2011

Femtocells: Sensing the Radio Channel


In order to cope with potential interference issues, it’s assumed that femtocells will use some sort of self-organizing approach to cope with potential interference issues. The idea is that femtocells will scan the radio environment (either scanning it themselves or throughout measurement reports from the UEs) and use this information in a distributive manner (cooperatively or not).

Femtocells must be aware of the presence of neighbouring cells and their power and spectrum allocation in order to maintain the femtocell coverage and avoid interference. Different strategies can be used to achieve this cognitive radio stage, in which the femtocell is able to learn about the structure and behaviour of the network and the channel conditions. Some of the approaches used to gather information needed for self-organizing functions related to avoiding interference and ensuring coverage include;





Network Listening Mode:
In this approach, the sensing capability is implemented in the femtocell device itself. This way, the femtocell will be able to scan the air interface, detect neighbouring cells and tune its network and RF parameters accordingly.
With this network listening capability, Monitoring the channel, both femtocells will be aware of the presence of the other, and they can coordinate their resource allocation (for example, subchannel assignment in case of OFDMA femtocells) in order to minimize interference. The implementation of a sniffer or network listening capability is essential in order to automate the tasks of cell planning and optimization within a femtocell network. Using this functionality, the femtocell will periodically switch on the sniffer to check network settings, synchronization and interference conditions.

Message Exchange:
Femtocells might be able to broadcast information messages that will be received by their neighbouring femtocells, containing interference measurements (receive signal strength) or information about the power and/or scrambling/sub-channel allocation of the broadcaster, e.g. the priority and probability of usage of different RF resources could be broadcast. This way, femtocells will be aware of the present actions and future intentions of neighbouring femtocells, and they can act cooperatively.

By exchanging messages, both femtocells will be aware ofthe actions of the other, and they can coordinate their resource allocation (for example, subchannelassignment in case of OFDMA femtocells) in order to minimize interference.

It should be noted however that these two techniques, network listening mode and message exchange, are limited by the coverage area of the femtocells. For example, if two femtocells are not within range of each other, they will not be able to notice the presence of the other or exchange information (over direct interface) with them. As a result, these femtocells will not be able to coordinate their resource allocation, and users located in the cell edge of these two overlapping femtocells will suffer from physical cell ID collision, inter-cell interference, etc. This scenario is shown below.
As a result, the user situated in the overlapping area suffers from interference due to uncoordinated resource allocation. For example, in case of OFDMA femtocells, users 1 and 2 may be assigned to the same subchannels, resulting in an unsatisfactory performance for user 1 located in the overlapping coverage area of both femtocells

Measurement Reports:
To solve the hidden femtocell problem, Measurement Reports (MRs) created by the terminals and reported to the femtocells can be used. In this way, a user situated in the cell edge of two overlapping cells can indicate to its serving femtocell the presence and the actions (power and frequency) of other overlapping macrocells and/or femtocells, as illustrated below;
As seen from this illustration, when using measurement reports, the user situated in the overlapping
area can alert its serving femtocell of the presence and actions of other hidden femtocells.
As a result, the femtocells can coordinate their resource allocation. For example, in case of OFDMA
femtocells, knowing that femtocell 1 is using a subset of subchannels for user 1, femtocell 2 will
allocate user 2 in other ones. This way, interference is mitigated.

When a connection is active, the UEs periodically report its signal quality to the femtocell via an MR, which also includes signal measurements from neighbouring cells. If the UE is reporting good signal quality, the femtocell will not take further actions. However, if the signal quality is weak, the femtocell might hand-off the connection to another scrambling/sub-channel/time slot, or initiate a handover to another macrocell or femtocell. Measurements collected through the users attached to the femtocell can indicate, for example, the DL received signal strength of the co-channel and adjacent carrier towards the user terminals.

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