With over 30 wins, Small Cells are strategic for Radisys.

We invested early (and heavily) in Trillium software and ATCA platforms to deliver deployment-ready Small Cell Access Point reference platforms and ATCA solutions that support Small Cell Gateway applications.

Why Small Cells?

An average consumer spends 50 to 60% of his or her time in an indoor environment, and 70% of wireless calls originate and terminate indoors. But, many subscribers are frustrated by bad coverage and poor data rates within their homes and/or offices — and operators are frustrated by rising data consumption rates that do not yield additional revenue.

What to do?

Small cells, of course. As opposed to a traditional macrocell on a hilltop or a tall tower, small cell is a wireless base station that resides in the consumer’s home or office and transmits at very low power. Small cells typically use an IP broadband connection (e.g., cable, DSL, fiber) for backhaul and eliminate the need for dual-mode handsets as virtually any existing wireless handset should seamlessly work with a small cell offered by the carrier.

Small cells not only provide excellent indoor coverage, but also free up capacity in the macro-cellular network. In other words, indoor subscribers’ cell phone traffic is parked on a femto (small) cell as opposed to on a macrocell.

Solution Story

In large cities like Beijing, many people live in high-rise apartment buildings made of concrete, which creates both coverage and quality challenges for mobile operators transitioning to high frequency technologies such as 4G/LTE. To improve service, operators are circumventing the local macrocell base station and using residential femtocells to transmit calls over the household’s broadband connection.

A Tier 1 telecom equipment manufacturer (TEM) with highly reputable network core and macro base station solutions decided to design and manufacture femtocells. The company’s strategy was to reuse its base station software by downsizing it, going from supporting thousands of users to just a few. Another requirement was the femtocell had to use an internally-developed physical layer (PHY) and its application programming interface (API). The system also had to be FDD (frequency division duplexing) and TTD (time division duplexing) compliant.

Unfortunately, scaling down the software was more difficult and resource-intensive for the customer than anticipated. In addition, the prototype femtocell performance was unacceptable, due to unidentified PHY and MAC (Media Access Control) integration issues. The TEM changed course and adopted the Trillium LTE Femtocell Software from Radisys and contracted Radisys Professional Services to assist in integrating the software with their proprietary PHY. A key factor was the Trillium LTE software supported both FDD and TDD, a capability that was not available from other vendors.

Difficulties Scaling Down Macro Base Station Code

Although conceptually appealing, reusing macro base station code on a relatively low performance femtocell, after rightsizing it, can be a real challenge. This is what a Tier 1 TEM discovered, contrary to early expectations the project would be straightforward. At issue is that base station development teams typically focus on minimizing latency and getting packets out as quickly as possible for a large number of users. They use a hardware platform with multiple high-performance processors, each with large caches and memory subsystems, capable of providing the software with a lot of CPU cycles.

Trillium software was designed to support scaling from small to large deployments, and over the years it has been optimized to run on multiple architectures. In particular, Trillium LTE Femtocell Software was designed from the ground up to run on a single processor with limited resources. The software stacks were reviewed in detail and code cut out when possible. Radisys engineers carefully examined the interlayer communications and implemented strategies that greatly reduced overhead. Essentially, the protocol stacks and applications must be frugal in their use of computing resources — otherwise the hardware platform will be overburdened and slow.

Tuning Communications between the MAC and PHY

Another difficulty the Tier 1 TEM faced was minimizing the latency between their PHY and the Trillium MAC software. The company sought the assistance of Radisys Professional Services, who had experienced similar integration efforts with over a dozen silicon providers. Radisys sent four engineers on-site to assist with the integration, which was completed in a few months. The Radisys team knew how the interaction between PHY and MAC could be logically perfect, while timing issues caused by misaligned parameters and APIs remain, resulting in subpar performance.

Based on Radisys’ experience, just writing the code isn’t sufficient because there needs to be time alignment in the sequence that exchanges information between Layer 1 and Layer 2. For instance, a latency issue could be caused when the user entity (UE) and the network get out of sync and both attempt to independently resolve the problem. To optimize time alignment, Radisys has an IP-based simulator to help finalize synchronization and the flow of messages. The Tier 1 TEM had access to the tool and could replicate issues, model the message flow and correlate the bench setup with actual lab setup.

Comparing FDD and TDD

Demand for equipment supporting TDD started primarily from China and Japan due to the greater flexibility in how frames are transmitted over an open air interface. TDD makes it relatively easy to dynamically change the capacity ratio between uplink and downlink to reallocating time slots. In most instances, network operators will desire more downlink capacity than uplink, since users more frequently download content, like video and web pages, than upload content they created.

TDD wasn’t deployed in 3G networks, but it has great potential in LTE. The industry may actually gravitate to mixed LTE deployments, where femtocells and other small cells employ TDD, and macro base stations use FDD, a heterogeneous network topology that avoids interference issues. Quite simply, the laws of physics dictate that whenever two transmitters send signals over the same spectrum, they are bound to interfere. Today, FDD for base stations is dominant, as all major operators around the world are already acquiring wide bands of FDD spectrum for their 4G LTE networks. In the future, implementing TDD on femtocells can lead to improved quality signal and optimized bandwidth allocation, thus delivering a high quality of experience to end users. For TEMs seeking to serve all markets worldwide, Trillium LTE software is a safe choice because it supports both FDD and TDD.

Solution Example: LTE TDD Femtocell

Customer Profile & Business Environment

• Wireless equipment manufacturer
• One of the largest telecommunications solution suppliers in the world
• Far eastern counties are requesting TDD-based femtocells to manage capacity

Customer Challenge

• Development of complete LTE TDD small cell infrastructure with an aggressive deployment schedule
• Needed proven LTE solutions and subject matter expertise
• Needed faster time to market

Radisys Solution

• Trillium LTE Femtocell Software and PHY/MAC integration
• Professional Services for development and system integration

Why Radisys Won

• Offered scalable, redundant, and flexible software solution for 4G/LTE networks
• Trillium LTE software supports both FDD and TDD, a capability not available from other vendors
• Only company to offer a full complement of embedded software, hardware, and subject matter experts for LTE

Customer Benefits

• Company saved considerable time and development cost compared to doing everything in-house.