The RF challenges of LTE-Advanced

LTE’s more advanced features will bring with them a number of headaches in the form of challenging RF environments.

Most people in the critical communications industry see LTE as the logical next step to provide enhanced capabilities to a host of operators, such as public safety, mining and utilities. But LTE’s more advanced features bring with them a number of challenges, which this article will explore.

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With nearly 1.5 billion LTE subscriptions worldwide expected by 2018 according to ABI Research, operators are scrambling to add the speed- and capacity-increasing features of LTE-Advanced to their almost-new LTE networks. Millions of smartphones, tablets and other devices are already devouring bandwidth. Without upgrades, operators may not be able to deliver a reliable, consistent end-user experience if traffic loads continue to grow exponentially as predicted.

First on every operator’s list of what to implement is carrier aggregation (CA), a feature that enables bundling of diverse frequencies into a larger, single-channel bandwidth to achieve significantly higher data rates. This feature could be a game-changer for operators with limited spectrum and no new allotments on the horizon.

Other anticipated LTE-Advanced features include techniques for managing interference among large and small cells in heterogeneous networks (HET-NETs) and incorporation of higher order MIMO (multiple input, multiple output) antenna systems for higher data rates and better connections.

The benefits of LTE-Advanced come at the cost of adding more complexity to an already complex RF environment. Base station (BS) and user equipment (UE) developers are tackling new architectures for carrier aggregation, 8x8 MIMO and other LTE-Advanced options, making the new technology work on multiple frequency bands and alongside other communication formats … all while maintaining or improving the power efficiency of the equipment.

Figure 1. LTE-Advanced adds even more complexity to an already-challenging cellular environment.

Not all of these challenges are unique to LTE-Advanced, but each intensifies the development effort, especially when added to the operators’ higher data throughput, system capacity and time-to-market needs.

UEs, BSs and HET-NETs

Advanced radio access techniques such as MIMO require near-ideal signal environments with high signal-to-noise ratio and power. These conditions are usually found close to the base station; UE performance goes down at the cell edge. Adding coverage with traditional base stations (macrocells) is expensive; finding locations for macrocells is often difficult. Therefore, LTE-Advanced supports the use of relay nodes and small cells (microcells, picocells and femtocells), which are much less expensive to acquire and operate, and relatively easy to deploy. The resulting HET-NET may encompass many radio access technologies, from cellular (LTE/LTE-Advanced, UMTS, GSM) to Wi-Fi, as well as remote radio heads and distributed antenna systems.

Operators face a huge network management challenge - not least of which is handling the interference that will be generated by the interactions of multiple layers of cells and other RF-emitting devices sharing the same frequency. New MIMO transceivers in the UE will contribute to the interference as will new co-channel deployments defined in LTE-Advanced - open subscriber group (OSG) and closed subscriber group (CSG).

Figure 2. A heterogeneous network (HET-NET) supports the deployment of small cells and relay nodes, each optimised for different communication requirements.

Each interference scenario requires a different solution, and enhanced interference mitigation in HET-NETs is the focus of LTE-Advanced standards work. Meticulous design of network devices and rigorous interference testing from design through deployment will be key to controlling this problem.

Maximising power efficiency

Battery life in a device is critical, but making the battery larger - for example, to accommodate the extra transceivers required by MIMO - is not an option. Furthermore, base stations and small cells need to operate as efficiently as possible for economic and ecological reasons. So developers optimise power in their products using RF, baseband, and system-level design techniques.

Power amplifiers (PAs) account for much of the energy consumed and heat generated by the RF front end. These essential components affect overall performance and throughput of wireless systems and are inherently nonlinear. Several techniques enable PAs to operate near saturation, where they are most efficient but also more nonlinear. Crest factor reduction (CFR) and digital pre-distortion (DPD), for example, improve PA linearity allowing the PA to be operated at its high power added efficiency (PAE) region, near saturation, without significant signal distortion.

Average power tracking (APT) and envelope tracking (ET) are newer techniques for improving performance and efficiency by adjusting voltage in modern PAs with switched high- and low-power operation. APT and ET will be adopted widely over the next 12-18 months.

Figure 3. Envelope tracking is a technique that improves power amplifier performance by dynamically adjusting the supply voltage to track the magnitude of the RF input signal envelope.

Power is required not just for the primary radio but also for multiband and multiRAT support, receive diversity, MIMO, interference cancellation, high data rates, Wi-Fi, Bluetooth, GPS, HD displays and so forth. Fortunately, advanced solutions are available to analyse battery current drain and optimise UE run times.

The dramatic increase in design and verification

The evolving standards for LTE and LTE-Advanced are subject to change and interpretation. Technically speaking, the use of multiple antenna configurations makes the design of UE more complicated, as do the new uplink modulation scheme introduced in LTE and the addition of carrier aggregation.

Along with development challenges specific to LTE and LTE-Advanced are those generally associated with wireless design. Products are often mixed-signal, containing baseband and RF sections. Each component type is vulnerable to particular impairments - nonlinearity and effective noise figure in RF upconverters and downconverters; phase and amplitude distortion from the PA; multipath and fading in the channel; and impairments associated with the fixed bit-width of baseband hardware.

With exceptionally high performance targets for LTE-Advanced, developers have to allocate resources to cover each critical part of the transmit and receive chain. Astute decisions regarding system performance budgets will be key in meeting system-level specifications and time-to-market goals. Managing the effort required in the design and verification process will be a major challenge at every step of the product development life cycle.

Simulation tools can address the magnitude of the design effort and verify interpretations of the standard. Baseband and RF sections can be evaluated individually and together to minimise problems during system integration.

Design and test integration provides power and flexibility for hardware testing. Signal creation and analysis software in simulation along with test instruments (logic analysers, digital oscilloscopes, RF signal analysers) can provide a common test methodology for diagnosing issues along the transmitter and receiver chain. Potential issues will be identified earlier in the cycle, when they are easiest and least costly to fix.

Figure 4. Combining simulation and test facilitates measurement and troubleshooting at various stages along the RF and mixed signal transmitter and receiver chain of a product design.

Cart before the horse

LTE and LTE-Advanced conformance test definitions have lagged behind publication of the core specs. That means manufacturers are often trying to bring products to market ahead of official validation testing - a risky proposition since finding compliance problems late in the design or production phase is very costly.

The number of frequency bands specified for LTE and LTE-Advanced, the option for FDD- or TDD-based systems, and the use of multiple subcarriers and multiple bandwidths create a seemingly endless number of possible test configurations. The specifications define only a limited number of test scenarios and of those, the organisations that certify devices choose only a subset.

Work continues on the test definitions, however, so manufacturers may find that the tests for their particular configuration do not yet exist or the tests change during the course of product development. Test equipment vendors who provide standards-compliant test platforms can be of help at this time by providing knowledge of the most important types of test and acceptable test procedures.

Figure 5. Example of required RF tests for LTE UE transmitters and receivers - a subset of the tests required for industry certification. This list continues to evolve with each new release of the 3GPP specifications.

Passing acceptance tests

Operators have become network gatekeepers who put demanding performance, quality and security metrics as entry qualifications. Acceptance tests are built around specific characteristics and conditions of a network. These tests go beyond the conformance-test requirements to cover a multitude of scenarios that stress both the network and the UEs, aiming to detect and resolve any problems before end users are affected.

As more devices are submitted to operators for testing, the strain on operator test facilities is increasing. Therefore, test methods must be highly effective, efficient, reliable and repeatable. While complex stress tests provide the best way to understand UE behaviour in the network, operators may wish to automate as much of their test plan as possible.

It's expected that UE vendors or operator-approved third-party labs will eventually take over the acceptance test process. Due to the complexity and proprietary nature of acceptance tests, operators will want to approve the exact implementation of the test systems used. An approved list of test systems ensures that all tests carried out by other parties will be done to the same standards as used by the operator.

Formula for the right solution

Developing RF components, devices and systems that balance all of the LTE-Advanced requirements calls for sophisticated design and test capability. Partnering with the right design and test equipment vendor can increase the likelihood of success. The best vendors will offer more than a generic set of hardware and software tools; they will provide application-focused solutions. Here’s what to look for:

• The vendor sells both design software and test solutions optimised for LTE-Advanced. Such vendors have to stay on the leading edge of standards requirements. Check out their level of participation in the key standards bodies.
• The vendor’s LTE-Advanced solutions provide best-in-class features and functions optimised for specific applications (for example, device design or network design). Check their track record for evolving over time to meet your application’s changing needs.
• Every solution involves trade-offs, so look for a vendor who can offer multiple solution options for both hardware and software. You’re much more likely to find one that is right for your unique situation.
• Does the vendor provide easy access to measurement expertise related to your LTE-Advanced application? Check the depth of their application notes, the frequency and quality of their webcasts, and the value of their technical articles to you.
• Since the design of wireless devices and networks increasingly requires collaboration between global teams, make sure your vendor has the resources to support your LTE-Advanced application wherever and whenever needed.