UMTS: Alive and Well - 4G Americas

C.2.2.5INTERFERENCE MITIGATION TECHNIQUESThis section discusses interference mitigation techniques for improving spectral efficiency and/or celledge user experience. It should be noted that the techniques discussed in this section are not mandatoryfor LTE, but randomization and coordination/avoidance are supported by specification in the standard.The techniques can be viewed as enhancements or optimizations that can be used for LTE to improveperformance. However, the interference mitigation techniques discussed in this section are particularlybeneficial for managing interference in LTE deployments using frequency reuse 1 (i.e. deployments thatare typically interference limited).As identified in the LTE work there are basically three approaches to inter-cell interference mitigation:1. Inter-cell-interference randomization2. Inter-cell-interference cancellation3. Inter-cell-interference coordination/avoidanceIn addition, the use of Beamforming antenna solutions is a general method that can also be seen as ameans for downlink inter-cell-interference mitigation. These approaches can be combined and they arenot necessarily mutually exclusive.INTERFERENCE RANDOMIZATIONInter-cell-interference randomization aims at randomizing the interfering signal(s), which can be done byscrambling, applying (pseudo) random scrambling after channel coding/interleaving or by frequencyhopping on either the slot boundary (every 0.5 ms), subframe boundary (every 1 ms) and across HARQretransmissions.The randomization in general makes the interference more uniform so that a single strong interferingsignal (e.g. generated from a cell edge user) will tend to have a small/tolerable impact on a large numberof users in adjacent cells, rather than a large/destructive impact on a few users in adjacent cells (thusincreasing outage).INTERFERENCE CANCELLATIONInterference at the receiver can be considered irrespective of the interference mitigation scheme adoptedat the transmitter.Two methods can be considered:1. Interference cancellation based on detection/subtraction of the inter-cell interference by explicitlymodeling the interfering symbols. In order to make inter-cell-interference cancellation complexityfeasible at the receiver, it is helpful that the cells are time-synchronized.2. Spatial suppression by means of multiple antennas at the UE: It should be noted that theavailability of multiple UE antennas is an assumption for E-UTRA. This can be done without asynchronization of the cells and the corresponding receiver is usually called InterferenceRejection Combining (IRC)-Receiver.www.3GAmericas.org February 2010 Page 192