Why do we need DC?
In Het Nets, when Macro and small cells are deployed on the same frequency (Scenario 1) /different frequency (Scenario 2) and connected via non-ideal backhaul, some of the below challenges are seen
1. Increased Handover Failures (HOF) or Radio Link Failures (RLF) in mobility between macro and small cells
2. UL/DL imbalance between macro and small cells
3. Increased CN signaling load due to frequent handovers
4. Difficulty in improving per-user throughput even after utilizing resources from more than one eNB
5. Network planning and configuration effort
When only small cells on one of more carrier frequencies are connected via non-ideal backhaul (Scenario 3), above 1, 3 and 5 challenges are observed.
To address some of the above challenges, one of the solutions proposed is Dual Connectivity (DC).
Below are the various terms used in the context of DC (based on 3GPP TR 36842).
Bearer Split: in
dual connectivity, refers to the ability to split a bearer over multiple eNBs.
Dual Connectivity:
Operation where a given UE consumes radio
resources provided by at least two different network points (Master and Secondary
eNBs) connected with non-ideal backhaul while in RRC_CONNECTED.
Master Cell Group: the group of the serving cells associated
with the MeNB.
Master eNB: in
dual connectivity, the eNB which terminates at least S1-MME and therefore act as mobility anchor
towards the CN.
Secondary Cell Group: the group of the serving cells associated
with the SeNB.
Secondary eNB: in
dual connectivity, an eNB providing
additional radio resources for the UE, which is not the Master eNB.
Xn: interface
between MeNB and SeNB. Since the current
E-UTRAN architecture was selected as baseline in this study, Xn in this TR
means X2.
To address the above 4), improve per-user throughput for the scenario 2, Inter-node radio resource aggregation is considered as potential solution. Here, higher user throughput can be achieved by aggregating radio resources in more than one eNB.
Based on the experiments mentioned in 3GPP TR 36842, to achieve higher throughput for UEs in Pico cells, Network should deploy both Pico and macro on different frequencies, as in Scenario B. The gain is achieved due to lack of strong interference in Pico cells from Macro cells.
To achieve higher throughput in macro UEs, Network should deploy both eNBs on the same frequency as in Scenario A.
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| Simulation scenarios for inter-node radio resource aggregation |
RLF challenge or mobility robustness, above 1), could be addressed with RRC diversity. In this case, RRC signalling related to HO could additionally be sent from the potential target cell when the UE is in "handover region or RRC diversity region", this way UE maintains connection with at least one cell and RLF could be prevented as shown below. From the simulation results, it can be seen that this proposal gives significant gains for both Scenario 1 and 2.
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| RRC Diversity |
Another challenge UL/DL imbalance, due to large difference in the transmit power of the macro and pico, is address via load balancing from Macro and Pico cells. Using DC this could be achieved by allowing the UE to connected in DL to the cell which offers highest DL throughput while being connected in UL to the cell which offers highest UL throughput. In this way, network has the flexibility to shift more DL traffic to macro eNB and UL traffic to Pico eNB (when macro eNB is loaded in UL) for both intra and inter frequency cases.
There are other solutions proposed to improve user throughput using CA+eICIC for scenario 2, but down prioritized due to low cost Pico cell deployment.
Another solution being the mobility anchor with the intention to reduce/hide the signalling load towards the CN by hiding the subsequent mobility involving SeNBs which seems independent of DC solution.
Part 2 will explain the architecture and protocol enhancements to realize the solutions described above.
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