Achieving Successful LTE Design and Test

How do you create a successful system-level design when you don’t know what the new interfaces are going to look like?

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By Cheryl Ajluni

In spite of all of its hype, WiMAX is not the only standard causing a stir these days or being called a “killer app.” Another technology that has achieved this illustrious title is Long Term Evolution (LTE), the Third Generation Partnership Project’s (3GPP’s) air interface for wireless access.

Granted, WiMAX does have the advantage of a head start in development, testing and deployment, but LTE is gaining momentum. According to a new ABI Research report, more than 18 operators globally have announced LTE deployment plans, and the tough economy seems to have done little to dampen their enthusiasm. Verizon accelerated its LTE deployment timetable, moving its launch forward from 2010 to 2009. NTT also is likely to deploy LTE in Japan in 2009. By 2013, operators are expected to spend over $8.6 billion on LTE base station infrastructure alone.

The difficulty with these projections is that LTE is an evolving technology (e.g., its MAC and upper layers are still be finalized) and therefore subject to change and interpretation. Specifications for the LTE radio interface are stabilizing, but this uncertainty leaves room for error and further complicates an already challenging design and test process. Nevertheless, chipsets, infrastructure and devices currently are being developed for commercial launch. Much of the pressure for successful development falls to the system-level engineer, who must accurately and cost-effectively design and test for the moving target that is LTE. How can this goal be achieved? Let’s take a closer look.

Understanding the Options

While LTE is expected to offer both consumers and operators a number of key benefits (e.g., lower costs, better services and an increase in data rate with lower latency), the complexity resulting from its use of technologies like SC-FDMA in the uplink, multiple antenna configurations and OFDMA, presents a host of engineering challenges to the engineer. LTE’s variable channel bandwidths further add to this complexity. Challenges also stem from the dependence of LTE system performance on its baseband and RF subsystems, both of which are subject to impairments like nonlinearities, multi-path and fading.

Dealing with this complexity and the resulting challenges is no easy task. As Frank Ditore, product marketing manager at Agilent Technologies points out, “For the system-level engineer working with LTE, or any emerging technology for that matter, there is simply nothing to validate their designs against. There is no LTE base station against which a designer can test their handset design. So, right from the very beginning the engineer faces uncertainty.”

Anritsu offers a solution to this dilemma. Its new MD8430A Signalling Tester is intended for developers who want to verify the operation of a new LTE terminal, but are unable to connect to an actual base station. As a base station simulator, this solution offers the functions needed to test the performance of 3.9G mobile terminals supporting the LTE standard.

What are some of the designer’s other options? The first alternative is to guess. In this case, the engineer builds a device with LTE functionality and hopes the design is correct. If the device was not designed properly, the engineer would unfortunately not realize this until after the design was fabricated. The design would then need to be fixed and fabricated again—a costly and time consuming process and one that’s not likely to receive much support given the current economic situation.

The other alternative is to use early design solutions with algorithms created by a company that’s closely involved with the LTE specification. Granted these solutions and the algorithms on which they are based will not be perfect as LTE is not yet finalized, but they do increase the engineer’s confidence that his/her design is correct. Over time these algorithms will become more mature and the design solutions that employ them will likewise mature, further raising the engineer’s confidence. And, since algorithms used in early design solutions ultimately find their way into measurement solutions, test equipment like signal analyzers, signal generators and network emulators that employ these algorithms also will be mature. Using design tools and measurement solutions from the same company is one way to ensure access to the most mature algorithms.

Agilent Technologies is one company offering solutions that span the entire LTE development lifecycle. In addition to its Advanced Design System (ADS) and the ADS Wireless LTE Library for design simulation and verification, the company also offers a range of pattern generators, logic analyzers, signal generators, signal analyzers, and network emulation and protocol development tools—all of which support early R&D in components, base station equipment and user equipment.

Successful Design And Test

Regardless of which company’s design and test solutions that are used, there are a few key tips for the engineer to keep in mind:

  1. Design simulation can be a valuable ally in addressing LTE development challenges and in verifying the engineer’s interpretation of the LTE standard. Its uses are multi-purpose: enabling the engineer to perform system-level trade-offs early in the design cycle to determine design requirements and specifications, and enabling evaluation of the system’s RF/mixed-signal performance by simulating RF and baseband designs together in one simulation environment. Additionally, combining design simulation with test equipment provides added flexibility in addressing testing needs for LTE.

    One solution capable of enabling such functionality is Agilent’s SystemVue 2008 (see Figure 2). This new electronic design automation platform provides an easy-to-use environment with simulator and modeling technologies, along with links to hardware implementation and test. It allows algorithm creation and prototyping for challenging communications system architectures at the physical layer. It also bridges the design flow gap between algorithm developers and the mainstream design community and lowers the cost of ownership by unifying a disjointed flow at an affordable price.

  2. For design and test accuracy, select tools from a company with known good algorithms and models.

  3. Consider purchasing design automation tools and measurement solutions from the same company, as its algorithms will become much more mature as they trickle down from design automation tool to measurement solution.

  4. Foster a close working relationship with the company from whom you purchase design tools and/or measurement solutions. You want to know what your vendor is doing to address changes in the LTE specification and that they are fully committed to making updates to their solutions, as necessary, in a quick and efficient manner.

  5. According to Andrew Kodarin, business development manager, Anritsu, another key tip is to “verify that the solutions you purchase are future proof and will preserve your investment.” In other words, ensure that the tools can be expanded to support future developments in the standard and that you won’t have to buy a new solution every time the specification changes.

Summary

There is no denying the current buzz surrounding LTE. Despite this, its true test will come on the first day of its commercial launch, when user’s expectations will be at the highest. How well LTE can meet those expectations will ultimately determine its long-term success. Much of this burden will fall to the system-level engineer tasked with designing and testing LTE devices. While some uncertainty in this process is inevitable given the changing nature of the standard, some tips (e.g., using design simulation with known, good algorithms and models) can prove especially useful in helping the engineer achieve a successful design.