Interference management is a key issue for heterogeneous network deployments of macro- cells and small cells in LTE. The problem is most acute for femtocells operating in closed access mode on the same carrier frequency as the macrocells. Interference is typically more manageable in systems with picocells or other open- or hybrid-access small cells because UEs which are causing or suffering from interference can be handed over freely between the macro and small cells. For example, if a Macro UE (MUE) (i.e. a UE connected to a macrocell) is near the edge of the macrocell and is also close to a CSG femtocell, then it will be transmitting at high power and may interfere with the uplink of the small cell.
If the MUE can be handed over to the small cell, the interference to the small cell would be eliminated. Similarly in the downlink, a MUE near the macrocell edge might suffer interference from a nearby small cell, which would be eliminated if the MUE could be handed over to the femtocell. The most severe interference occurs when the small cells and macrocells are deployed on the same carrier frequency; otherwise, the interference depends upon the Spectrum Emission Mask (SEM), and, in the case of adjacent channel operation, on the Adjacent Channel Leakage Ratio (ACLR) and Adjacent Channel Selectivity (ACS).
Therefore in this section we focus on the case of co-channel deployments of femtocells and macrocells. Section 31.2 addresses co-channel deployments of picocells and macrocells.
Femtocells are usually installed by the consumer in an ad-hoc fashion, rather than being part of a planned deployment. They are therefore designed to be self-configuring, in that they are required to sense their environment (e.g. to detect and measure neighbouring macrocells and femtocells) and adapt their operation accordingly (see, for example, Section 24.4.1 regarding adjacent channel protection. If the femtocell can choose its carrier frequency, then interference may be controlled by the appropriate carrier selection (for example based on measured Reference Signal Received Power (RSRP) and cell reselection priority information).
Note that co-channel interference between femtocells situated close to each other may also occur.
24.3.1 Interference Scenarios
Here we outline the possible interference scenarios of relevance./footnoteNote that these scenarios apply for both FDD and TDD systems; in the case of TDD it is assumed that synchronization of uplink/downlink switching points is achieved between the macrocell and small cells.)
Downlink transmissions from the femtocell suffer interference from transmissions from a macrocell as shown by interference path A in Figure 24.4.
Macro to Femto, Downlink. UEs connected to femtocells (known as Femto UEs or FUEs) are more susceptible to this interference when they (and their associated HeNB) are closer to the macrocell, since the transmit power of the macrocell is much higher than that of the femtocell and thus the received interference power will be higher. FUEs are also more susceptible when located far from the serving HeNB, especially if they are outside the house or apartment the femtocell is designed to cover.
FUE
Figure 24.4: Macro/femto downlink interference scenarios.
Potential interference mitigation approaches for this scenario may include:
• Control channel protection (Physical Downlink Control CHannel (PDCCH), syn- chronization signals (PSS/SSS), Physical Broadcast CHannel (PBCH)) by arranging control channels to be orthogonal in time and/or frequency, e.g. by applying a subframe boundary offset in the femtocells relative to the macrocells.
• Data channel protection (PDSCH). If partial frequency re-use (see Section 12.5) is employed in the macrocell then a HeNB can schedule data on the RBs with low transmission power from the macrocell (e.g. RBs being used for cell centre UEs by the macrocell). The HeNB could ascertain the frequency partition information of the MeNB by various means, e.g. by configuration or by monitoring the macrocell transmissions.
Femto to Macro, Downlink. Downlink transmissions from a macrocell suffer interference from transmissions from a HeNB as shown by interference path B in Figure 24.4. This can cause a ‘deadzone’ around a HeNB within which a MUE is unable to receive transmissions from the macrocell. Such deadzones are larger for HeNBs near the edge of the macrocell, where the signal received from the macrocell is weakest, or for MUEs located indoors in the coverage of a CSG HeNB. Potential interference mitigation approaches for this scenario may include:
• Enabling hybrid or open access if possible. In the case of hybrid access, the power settings of the HeNB could be adapted differently to the closed access case, taking into account the total system performance (macro + hybrid cell) and the resources consumed by ‘visiting’ non-CSG UEs.
• Downlink power setting. The HeNB can limit the maximum downlink power (or power per RB) according to its environment. In the case of co-channel deployment of macrocells and femtocells with closed access, there are a number of possible ways this could be achieved [3], such as setting the power to achieve a trade-off between coverage and interference, based on the estimated path-loss between the HeNB and the victim MUEs, and the coverage requirements of the femtocell. Such a solution could also involve detecting the presence of nearby victim MUEs and correspondingly reducing the downlink transmission power; this could be done by detecting uplink transmissions at the HeNB, or by means of measurement reports from victim MUEs to the serving MeNB if it is possible to then signal this information to the HeNB.
• Time-domain coordination making use of Almost Blank Subframes (ABS), a concept introduced in Release 10 (see Section 31.2.2). ABSs contain only certain essential transmissions, leading to a reduction in interference to victim UEs. Typically, an aggressor HeNB would set up a pattern of ABSs resulting in reduced interference to victim MUEs. In Release 10 there is no X2 interface between HeNBs and macro eNodeBs, and therefore ABS patterns at a HeNB would need to be configured either by Operation and Maintenance (O&M) or autonomously by the HeNB; for example, an ABS pattern could be set up at an HeNB to protect subframes containing PSS/SSS/PBCH and paging occasions at a macro eNodeB, assuming the HeNB has obtained time synchronization with the macro eNodeB. Additional mitigation of the residual interference due to the essential transmissions is possible e.g. by arranging the frequency positions of CRS to be different in victim and aggressor cells.
System simulation results [3] are given in Table 24.1 showing the effect of introducing HeNBs, setting the HeNB power appropriately and enabling hybrid access. The system bandwidth is 5 MHz with a co-channel deployment of macrocells (with 1 km inter-site distance) and HeNBs, with 22 UEs per macrocell, 12 of which are FUEs.
Table 24.1: Effect of various small cell configurations on system throughput.
Outage Probability Worst 20% UE Median (SNR<−6 dB) throughput (kbps) throughput (kbps)
No HeNB 12.7% 35 150
CSG HeNB, 18.9% 100 5600
Fixed Power 8 dBm
CSG HeNB, 9.8% 250 3300
Power Control (−10 ,+10 dBm)
Hybrid HeNB, 2% 900 5100
Fixed Power 8 dBm
Hybrid HeNB, 3% 400 3400
Power Control (−10 ,+10 dBm)
Macro to Femto, Uplink. Uplink transmissions from an FUE suffer interference from transmissions from a MUE as shown by interference path C in Figure 24.5.
FUE
Figure 24.5: Macro/femto uplink interference scenarios.
Femtocells are more susceptible to this interference when they are located close to the edge of the macrocell edge since MUEs in the vicinity of the HeNB will be transmitting at higher power in this case. System simulations [3] have suggested that this scenario does not create a significant interference impact because in most cases MUEs cannot operate very
close to the HeNB, and therefore the FUEs will typically be closer to the HeNB than the MUEs are. Nevertheless, the interference may be severe if the MUE and HeNB are both situated indoors; in this case the following potential interference mitigation approaches may be relevant:
• Uplink power control: the HeNB can control the power of its FUEs to overcome the interference from neighbouring MUEs;
• Control channel protection (PUCCH): if the PUCCH resources are over-provisioned at the HeNB then the RBs used for PUCCH at the HeNB can be moved away from the band edges (see Section 16.3.1) such that they do not overlap with the RBs used for PUCCH by the macrocell.
Femto to Macro, Uplink. Uplink transmissions from an MUE suffer interference from transmissions from an FUE as shown by interference path D in Figure 24.5. The impact of this interference increases as the density of femtocells within the macrocell coverage area increases. The severity of the impact also depends on whether the FUEs are operating outdoors (in which case the FUEs will tend to be transmitting at higher power and the path- loss to the macrocell will be lower). Potential interference mitigation approaches for this scenario include:
• Uplink power control: the HeNB can control the power of its FUEs to limit the interference to neighbouring macrocells. This can, for example, be based on the estimated path-loss between the FUEs and the macrocells or the estimated path-loss between the FUEs and the HeNB [3].
• Control channel protection (PUCCH): as described above for mitigating macro-to- femto uplink interference.
Femto to Femto, Downlink Downlink transmissions from one femtocell suffer interfer- ence from transmissions from another femtocell as shown by interference paths E and F in Figure 24.6. This figure assumes an apartment scenario, with just two apartments for simplicity; however, it should be borne in mind that in practice interference can occur between multiple apartments on the same and different floors.
Potential interference mitigation approaches for this scenario include setting up ABS patterns and/or frequency re-use schemes whereby each HeNB determines its neighbours (from measurements made at the HeNB or FUE) and the associated path-losses. This neighbour information is then used to construct orthogonal patterns of RBs or ABSs to be used by neighboring HeNBs. Both distributed approaches (with or without information exchange between HeNBs [4,5]) and centralized approaches (e.g. at the HeNB GW [3]) have been proposed for constructing the orthogonal sets. Direct X2 connectivity between HeNBs was added inRelease 10 for the support of mobility, and this also allows coordination of ABS patterns between HeNBs.
Femto to Femto, Uplink. Uplink transmissions from one FUE suffer interference from transmissions from another FUE connected to another HeNB as shown by interference paths G and H in Figure 24.6. Uplink power control is one potential interference mitigation technique for this scenario.
FUE2
FUE1
Figure 24.6: Femto/femto interference scenarios.
24.3.2 Network Listen Mode
Many of the interference mitigation techniques discussed above require the HeNB to make measurements of surrounding macrocells and femtocells. HeNBs are therefore commonly designed to have a Network Listen Mode (NLM) of operation which involves making measurements and decoding system information from neighbouring eNodeBs. This may be done at initial system setup and periodically thereafter. Examples of measurements made in NLM include:
• Uplink interference power, to infer the presence of nearby MUEs;
• Determination of cell IDs and CSG status/ID, by decoding the SI of the neighbour cells;
• Co-channel RSRP and RS transmission power to estimate the path-loss to neighbour cells, which in turn may be used for uplink and downlink power control at the HeNB;
• Reference Signal Received Quality (RSRQ) which can be used together with RSRP to determine the reliability of coverage of a macrocell, to help determine a suitable power setting for a femtocell operating in hybrid access mode.
In addition, TDD femtocells may obtain time synchronization from neighbouring macrocells.