By Cheryl Ajluni

Nothing has been left unscathed in the current global economic downturn, and that includes femtocell deployments.

It was just last year that femtocells were being proclaimed a 2009 “killer app,” along with LTE and WiMAX. But what was once viewed as the next great thing has instead faced a tough road with more than a few large-scale deployments by major mobile operators being put on hold. Despite the slowdown, femtocells remain a viable part of the road to 4G. According to Aditya Kaul, senior analyst at ABI Research, while the pace of adoption has been slow, “deployments in 2010 will pick up.” In fact, ABI Research forecasts that the total available femtocell market in 2010 will reach 2.3 million units and rise to 40 million units by 2014.

Adding fuel to the slow-burning fire of the femtocell market is movement on the part of carriers, which may signal that things are heating up once more. Comcast, for example, announced that it is testing WiMAX femtocells. And across the globe, China Mobile has partnered with Nokia Siemens Networks to test a TD-LTE femtocell. This year, six major network operators have launched femtocell services that cover the USA, Europe and Asia.

Such activities bode well for the emerging femtocell market, but do little to address designer’s challenges at a system level. From a technical perspective, femtocells are no more complex than a full macro base station, yet they demand the integration level of a WLAN in order to meet various cost, power and footprint requirements. And, since femtocell products are likely to appeal to many different users around the world, each with potentially different needs and requirements, different models will need to be developed. Each model (e.g., a W-CDMA, WiMAX or LTE femtocell) will have its own unique design requirements and challenges determined by the standard it supports. Such challenges make designing femtocell products a difficult and risky proposition, and demand solutions that can ease the burden on the system designer.

As Caroline Gabriel of Rethink Research, explains “The femtocell market is starting to mature. While the sophisticated early entrants wrote their own solutions, this is bringing an inevitable demand from developers for more complete system solutions that reduce risk, enable differentiation and speed time-to-market. This is a value chain dynamic that other semiconductor markets such as Wi-Fi and DSL also experienced as they hit the mass-market.”

picoChip is working on just such a solution—an end-to-end femtocell reference solution for both HSPA and LTE that’s aimed at tackling the designers’ system-level challenges, including interference management, security, timing, and provisioning. The solution integrates optimized versions of Continuous Computing’s protocol stacks with picoChip’s SoC products (Figure 1). It also includes sophisticated femtocell management software for the complex control functionality required by femtocells.

Figure 1. The four-user PC302 residential femtocell is the first in picoChip’s PC3xx family of highly integrated baseband SoCs. It implements a complete 3GPP Release 7 access point, is compliant to TR25.820 and the newly standardized Iuh interface, and is built using an advanced 65-nm manufacturing process.

Of course, even the availability of a complete femtocell solution with things like automated interference management and network self organization, does not exempt the designer from having to consider some fairly difficult issues during femtocell design. Some of these issues include:

Power Consumption: By definition, femtocells are low-cost, low-power wireless access points that operate in licensed spectrum to connect standard mobile devices to a mobile operator’s network using residential DSL or cable broadband connections. Power is an especially critical concern since in a residential situation the end user has to pay the bill for the electrical energy consumed by the femtocell base station. Its power consumption therefore, has to be low enough to not significantly impact the user’s bill. Various low-power design techniques such as comprehensive power-down modes and clock gating, as well as effective power management can be employed to achieve this goal.

Interference: Because femtocells utilize spectrum currently employed by macro networks, interference can result between cells (e.g., a macrocell with a femtocell or a femtocell with another femtocell, when two units are in close proximity). Interference affects operators as well as consumers who will likely move their femtocells around or place them next to other devices.

There are a couple of ways to mitigate the risk of interference. The first, cognitive radio technology, essentially allows the femtocell’s radio to constantly monitor its RF environment and set itself up accordingly. Another option calls on the femtocell to intelligently set its output power when it’s in the presence of nearby femtocells. In this case, each femtocell would need to transmit at lower power to avoid same-frequency interference. To avoid interference with signals from neighboring macrocell base stations operating on an adjacent channel, the femtocell can be designed to measure the power in the adjacent channel downlink and set its own power accordingly.

Standards: These are key to taking a technology from a niche application to wide-scale adoption. The question with regard to femtocell technology is which standard—a proprietary approach, an existing standard from the telecom industry (e.g., the session initiation protocol (SIP)/IP multimedia subsystem standard for integration used in LTE networks), or some other standard altogether?

The Femto Forum, 3GPP and the Broadband Forum think they have the answer. It’s the world’s first femtocell standard and they created it together. The standard, part of 3GPP’s Release 8 and interdependent with Broadband Forum extensions to its Technical Report-069 (TR-069), was officially released in April and paves the way for not only the development and production of large volumes of standardized femtocells, but also for ensuring interoperability between different vendors’ access points and femto gateways. It covers four main areas: network architecture (the Iuh), radio & interference aspects, femtocell management/provisioning via the popular TR-069 protocol, and security (e.g., IKEv2 and IPsec).

Handovers: These can be tricky, especially in the case of femtocells where precise timing and synchronization is need to properly manage handovers with the macro network. The process is complicated by the fact that there may be millions of femtocells deployed and end-users may move them around their homes, entering into areas where the signal strength from the macrocell is greater than that of the femtocell. Unfortunately, existing macrocell RF planning techniques offer no real solution. Instead, femtocell handovers require sophisticated algorithms capable of ensuring that the network quality is not impacted by inefficient handovers and wasted capacity.

These issues are just some of the challenges system designers face when developing femtocell products today. Addressing them, whether by using the techniques suggested or through a complete femtocell solution like the one picoCell is working on, will be critical to ensuring not only successful femtocell products, but a successful femtocell industry that can take its rightful place on the road to 4G.

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.