When mobile operators deploy small cells, they often find that they do not deliver the expected user experience. A primary reason for degraded quality of service with HetNets is poor cell-edge performance due to the lack of traffic coordination and interference management between small cells and macrocells. The 3GPP standards for LTE-Advanced incorporate a range of techniques for mitigating cell-edge interference issues, but these present many challenges both to during implementation and then in the ability to validate the user experience improvement once they are deployed in the network.
How can eICIC, feICIC and CoMP techniques help reduce cell-edge issues, and how can the operator test these features in the network to ensure that they are delivering the required improvement in user experience under real traffic conditions?
3GPP Releases 8 and 9 introduced ICIC (Inter-Cell Interference Coordination) to reduce interference at the cell edges by using radio resource management (RRM) techniques in the frequency domain – using an autonomous scheduler to dynamically distribute bandwidth and power resources between cell users. Users are categorised according to their Signal-to-Noise-plus-Interference Ratio (SNIR), and different reuse factors are applied according to this basic indicator of interference level.
In Release 10 (Rel-10) this scheme was extended to include time domain management of the interference. This is called eICIC (enhanced ICIC), which is much more effective for users in close proximity to a small cell. eICIC is designed to improve cell edge performance and coverage in HetNet deployment scenarios where nodes of different types – macrocells, small cells and RRH - have coverage areas that partially overlap, and differs from ICIC in that it is not transparent to the mobile handset or UE (User Equipment).
eICIC requires coordination between each of the network nodes that communicate with each other through the X2 interface. In a typical application, a macrocell whose coverage area overlaps with that of one or more small cells can coordinate its transmissions with these nodes. The coverage of the small cell can be extended by applying cell selection bias offset values, commonly known as cell range expansion (CRE) bias, which offloads traffic (from UEs that would not otherwise be considered to be in the small cell coverage area) from the heavily-loaded macrocells to the more lightly-loaded small cells in order to achieve better system performance in the HetNet. In Rel-10 eICIC, the maximum CRE bias value is 6 dB, since a cell may not be detectable under -6 dB signal-to-noise ratio (SNR).
With CRE bias, a UE operating at the cell edge of a picocell will experience significant interference from the macrocell. This interference can be mitigated by limiting the macrocell transmissions to DL Common Reference Signal (CRS) alone, without scheduling any data transmission, during certain subframes – these are called Almost Blank Subframes (ABS). This technique results in lower interference being apparent in the UEs at the cell edge of the microcell or picocells, and gives the microcell or picocells the opportunity to perform CRE to increase the coverage area during these protected subframes. Cell range expansion techniques are used for offloading some of the UEs from the macrocell to the smaller cell when the macrocell is loaded too heavily, and are used to achieve better load balancing. The UE that has been offloaded needs to be scheduled from the smaller cell during the low interference ABS. Load balancing is an important constituent of SON, the collective term for a range of techniques that promote the overall improvement of network performance and energy saving.
Testing a network employing eICIC means that the network tester must be able to apply the relevant UE measurement procedures in order to feed back correct and reliable information to the network. The TM500 network tester family from Cobham Wireless can be used to verify the correlation between the measurements reported by the UE and eNodeB signalling (ABS patterns), verifying the benefits of interference management. This includes logging the eICIC specific protocol messages and physical layer measurements.
In LTE Rel-11 eICIC is evolved to further enhanced ICIC (feICIC). This enables even further cell range expansion of the small cell – increasing the CRE from 6 dB to 9 dB - by focusing on interference handling by the UE through inter-cell interference cancellation.eICIC and feICIC are especially important when Carrier Aggregation (CA) is not used.
Coordinated Multipoint transmission/reception (CoMP) is another major feature of 3GPP LTE-A Release 11. Although applying eICIC empowers mobile network operators to achieve better overall network capacity, the addition of CoMP goes one step further - by coordinating transmission and reception between different transmitting and receiving cells. It achieves this through the use of load balancing, coordinated scheduling, and the management of signal power and interference. In the downlink, each mobile terminal sees improved data throughput, especially near the cell edges, due to reduced interference and an increase in received power. Similarly, for the uplink, received signal quality and cell edge coverage is improved by simultaneous coordinated reception from different receiving points on the network side.
CoMP requires rapid information exchange and the coordination of shared and centralized processing between multiple transmitting points - eNodeBs, remote radio heads (RRH) and small cells. It is both time-critical and computationally intensive, requiring reports from each UE to be processed for different points in order to make centralized decisions on scheduling and load balancing. These decisions are then rapidly executed to adjust the configuration and the number of points that are active at any time, based on instantaneous channel and interference conditions.
The TM500 provides decoding of received downlink signals from multiple transmission points to test the CoMP algorithms under inter-cell interference conditions. For the uplink, it provides multi-point reception testing from multiple mobile terminals in parallel. Each link simulates independent channel conditions, allowing the uplink CoMP algorithms to be validated for distributed reception, with different channel quality at each terminal. A full protocol log provides debugging of Radio Resource Control (RRC) messages and configuration. The test feature suite offers functional testing, including over-the-air (OTA), different modes of operation, manipulation of messages, overriding network parameters, logging, and scripting.
The TM500 has been designed to help mobile operators and vendors deploy HetNets by emulating and validating realistic usage scenarios with support for eICIC and CoMP, and feICIC. It allows them to perform lab and field trials incorporating realistic performance tests in order to improve the user experience under challenging cell edge and interference conditions.
