Research On The Techniques Of Interference Management In Femtocell System

Femtocell splits the macro-cell by inserting multiple indoor stations. Since thedistance between the transmitting antennas and the receiving antennas is shortened，thebattery endurance of the user equipment is prolonged in view of the decreasedtransmission power. It is difficult for the operator to realize network planning andoptimization owing to the random employment of Femtocell by the consumers. In orderto ensure that the macro base station is free from the interference introduced byFemtocells, the efficient interference management techniques become the primaryconcern.Motivated by the previous studies on the interference management technologiesabout the Femtocell, we focus on the following interference management schemes.1. Phase rotation-based differential interference detection algorithmFor the dynamic frequency selection (DFS) technique, the sniffers have to detect theradio frequency condition, which result in high complexity, large delay time, and lowaccuracy. To overcome these problems, phase rotation-based differential interferencedetection algorithm, which utilizes the equal2-normal of pilot symbols in LTE system,is proposed. The received pilots in the consecutive OFDM symbols are rotated to makethe expected signal approximate identical, whereas the power of interference and noisekeeps unchanged. The interference and noise can be directly detected after the expectedsignal has been cancelled by differential operation. The differential operation canimprove the number of the samples and accumulate the interference and noise in thesame direction to enhance the accuracy. By extending phase rotation technique tomultiple antennas, we propose a total interference detection algorithm according tomaximal ratio combining detection algorithm. Simulation results demonstrate phaserotation-based differential interference detection algorithm can improve the accuracy ofthe power estimation of the interference plus noise. As a result, false-alarm probabilitycan be greately decreased with relatively small interference power.2. Orthogonal complement space-based interference management tecniqueA schedule algorithm based on orthogonal complement space is proposed tomitigate the interference received by Femtocell users. The algorithm estimates theinterference channels and performs singular value decomposition. The maximal leftsingular vector is selected as the signature. An interference space is spanned by thesignatures of interference channels. For mitigating the interference, the algorithmschedules the users whose signatures lie in the orthogonal complement space of the interference space. Simulation results indicate the orthogonal complement space-basedschedule can expand the useable frequency and enhance the capacity of Femtocellsystem.3. Beamforming schemes in uplink and downlink based on multiple Femtocellscooperationa) A beamforming cooperation scheme with transmitter-receiver processing isproposed to mitigate the downlink interference among cooperation Femtocells. Ourscheme adopts the maximal left singular vector of the channel as the receiver vector toreduce the channel state information (CSI) feedback, and the interference between thecooperation Femtocells can be mitigated by beamforming vectors which are constructedorthogonal to each other based on the feedback CSI. Simulation results show that thescheme can increase the useable frequency and significantly enhance the capacity of theFemtocell system.b) A beamforming cooperation scheme based on null space is also proposed tomitigate the uplink interference to Femtocells. Our scheme can expand the dimensionsof receiver antennas to make sure that the receiver vectors in Femtocells lie in the nullspace of the macro-user interference space to mitigate the uplink interference from themacro-users. The interference among the Femtocells is cancelled by multi-userdetection schemes. Simulation results verify that the scheme can mitigate the uplinkinterference from the macro-cell and another Femtocell simultaneously. Both thefrequency reuse factor and the uplink capacity of Femtocell system are improved.4. Power control in the downlinka) Power control in the downlink of single Femtocell is proposed based on thedistribution of macro-users. Femtocell increases its power if macro-users are far from it,on the contrary, Femtocell degrades its power. Simulation results prove that the schemeensures the loss of the capacity of macro-users under the threshold and can start theFemtocell with high probability.b) Power control scheme in downlink in cooperation Femtocells is proposed basedon the distribution of macro-users. The scheme firstly determines the maximuminterference threshold of macro-users introduced by multiple Femtocells. Femtocellsfurther set their power to maximize the capacity of Femtocells by game theory.Simulation results confirm that the scheme can limit the interference to macro-users.