Stephen Hire, APAC Sales Director at Cobham Wireless, says a significant proportion of the network problems that are experienced by 4G operators can be avoided by rigorous validation during the development and planning phase, and ideally this should include real-life data scenarios to emulate the applications traffic the network will be expected to carry.

Many of the features of LTE-Advanced, including deploying small cells in heterogeneous networks (HetNets) and Self-Organizing Networks (SON) are effectively addressing the need for increased network capacity, but bring additional challenges in ensuring that they deliver the expected user experience. Other LTE-A features that increase data rates and capacity include Carrier Aggregation (CA), Higher Order MIMO, and 256 QAM, all of which increase the complexity of testing as the validation system needs to be able to emulate all these features.

Carrier aggregation and higher-order MIMO

Carrier aggregation combines blocks of spectrum known as component carriers (CC), with bandwidths up to 20 MHz, enabling the use of fragmented spectrum to increase data rates. Carrier aggregation has so far been implemented using both two carriers (2CC) and three carriers (3CC), and also by combining time-division duplexing (TDD) and frequency-division duplexing (FDD) spectrum, and demonstrations have shown aggregation of up to 5CC achieving data rates of up to 1 Gbps. Several carrier aggregation application scenarios are possible – using one frequency to increase coverage and another to boost the data rate; using both frequencies to increase cell throughput; or using one frequency to provide macro coverage and a higher frequency to boost throughput in hotspots.

Higher Order MIMO (HOM) allows increased spectral efficiency, in terms of bps per Hz, to be achieved, and is again a hardware upgrade. Several clever schemes in uplink and downlink are possible, for both single- and multi-user. MIMO requires multiple antennas to be used on both base stations and user devices ‑ eight streams require eight separate antennas on the device.

Interference coordination

One of the main reasons that quality of service can be degraded with HetNets is poor cell-edge performance due to the lack of traffic coordination and interference management between small cells and macrocells. A range of techniques – including eICIC, feICIC and CoMP – have been included in the LTE-A standard to help mitigate cell-edge interference issues, but these also present many challenges both during implementation and in the ability to validate the user experience improvement once they are deployed in the network.

3GPP Releases 8 and 9 introduced ICIC (Inter-Cell Interference Coordination) to reduce interference at the cell, which was extended in Release 10 to 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.

eICIC requires coordination between each of the network nodes that communicate with each other through the X2 interface. The coverage of the small cell can be extended by a technique known as cell range expansion (CRE) bias, which offloads traffic from the heavily-loaded macrocells to the more lightly-loaded small cells in order to achieve better system performance in the HetNet. In LTE Release 11 eICIC is evolved to further enhanced ICIC (feICIC), which extends the range of cell range expansion of the small cell eICIC and feICIC are especially important when CA is not being used.

Cell range expansion techniques are used for offloading some of the mobile devices, or UEs, from the macrocell to the smaller cell when the macrocell is loaded too heavily, and are used to better balance the load. 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.

CoMP

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.

Test considerations

One of the major challenges for wireless network testing is in keeping up with the increasingly rapid introduction of new features in the roadmap. From a network test point of view, it is crucial that support for new features is made available for R&D as soon as possible, so that network performance can be validated under realistic user scenarios before the features are introduced on real mobile terminals.

HetNets require the flexibility to test heterogeneous deployments to assess the performance of the network. Performance-related features such as FeICIC and CoMP have mobility KPIs to indicate a percentage of early and late handovers, with thresholding to indicate outliers. This is validated by using tools that predict the UE feedback depending on the UE locations, to set target UE handover rates even under different fading profiles

A key requirement is the ability to test mixed features to evaluate the network performance and Quality of Service (QoS) for groups of UEs at the application layers, and being able to flag up where the unified throughput does not meet the expected performance. This means that a tool must be able to indicate which UE in a cell or aggregated cell does not meet a performance requirement.

It is necessary not only to test features in isolation, but crucially also the interaction between the features, for example testing downlink CoMP in combination with carrier aggregation. As the demand on the networks increases, testing needs to take place for significantly larger numbers of UEs, in order to ensure that the system and user KPIs are met when the network is loaded. This is particularly challenging at the cell edge where interference is prominent.

As capacity becomes an even greater challenge, intelligent debugging capabilities are being developed to validate the system performance and specific of the stack under high load conditions. However as feature interaction and capacity testing place ever increasing demands on validation, it is important to provide KPI metrics that do not create ‘information overload’, but instead genuinely benefit testers and help them to identify where there is a performance bottleneck.

The TM500 is able to measure quality of experience for each individual UE on each type of emulated application traffic. Providing support for all of the key features ahead of the market need is helping operators to continue to provide the quality of experience their users demand, and to cope with increasing demand for data rates and capacity right up to, and beyond, the launch of 5G.

By Stephen Hire, APAC Sales Director at Cobham Wireless