The Key to a Femtocell in every Home and Office

Erik L. Org, Marketing Manager - BitWave Semiconductor

Introduction

Wireless devices have become omnipresent in both business and the home. Unfortunately, pervasive is not synonymous with simple. Multiple protocols, multiple frequency bands and multiple devices for multiple applications invariably results in a briefcase full of complex devices. What users would like is for device vendors and network providers to flatten the hierarchy of networks and devices which confront them in today's world. Users want to take advantage of the broad set of applications available to them but want a simple and transparent platform to connect to those applications.  The OEMs challenge is therefore to deploy low cost, easy-to-provision infrastructure with technology which satisfies consumer's ever-growing need for quality anywhere anytime connectivity. One solution is the femtocell, a flexible wireless access point with a small wireless footprint which is suitable for the home and office. The requirement for anytime anywhere connectivity implicitly demands that femtocells operate over the full frequency range and protocols used in today's wireless applications. Although the challenge is daunting, it's achievable. BitWave's Softransceiver™ RFIC offers one possible path to that goal.

Pervasive Wireless

Pervasive wireless connectivity has changed the way people communicate, affecting the way that they interact socially and in business. The growth in all types of mobile connectivity has exceeded expectations. Almost twenty five years have passed since Motorola introduced the first commercial portable cellular phone. Today it's clear that mobile communications are ubiquitously present in our society:

·        Informa's WCIS reported over 2 billion global subscribers in 2005

·        GSM World reports over 1+ trillion SMS were sent in 2005[ii]

·        > 53% of computers sold in May 2005 were laptops; Over 95% shipped with a WiFi adapter![iii]

Multiple Applications require Multiple Protocols deployed over Multiple Frequency Bands

Today's global wireless market is complex; there are over 2 billion handsets deployed on multiple bands and protocols.  Wireless carriers continue to roll out new service offerings based upon new air interface protocols and frequencies.  Mergers and acquisitions in the industry mean that carrier may now have multiple licensed bands and protocols that they need to integrate in their service offerings. The Sprint/Nextel merger is a prime example.

As consumers of wireless services, most individuals expect seamless wireless connectivity for the applications they are using and have little patience for dropped calls. Consumers demand the network to be as transparent as possible. Users don't want to understand the standard deployed in their service provider's network; they only want it to work and to be offered at a reasonable price. Carriers must therefore balance their customer's desires against the economics of deploying a network. Users require voice, data and multi-media connectivity in wide area networks (WANs) as well as in metropolitan area networks (MANs), local area networks (LANs) and personal area networks (PANs). Carriers respond by deploying multiple sets of infrastructure to support all the bands and protocols or by partnering with other carriers to offer the pervasive connectivity that users are looking for.

Figure 2 - Segmenting the Wireless World

 

However, the base station is a complex high performance electronic system.  A base station must be designed to support the ever growing protocols and frequency bands deployed across a carriers network and deliver the performance necessary to support ubiquitous connectivity and superior link quality. Luckily, until now, the economics of a base station allowed the cost to be amortized across a fairly wide user base.  But as users demand steady improvement in network coverage, carriers are forced to deploy more base stations within smaller coverage zones.

With the advent of residential broadband connectivity, it's now possible to envision small base stations deployed in each house and backhauled through the residential broadband connection. These in-home base stations, or femtocells, are lower power, lower performance and lower cost versions of their macro and microcell cousins. They are similar in nature to WiFi access points. Consumers often have difficulty with cell phone coverage in the home.[iv] The extra attenuation provided by walls and property inside the home is often enough to impact performance. Of course, inside the home is where many consumers use their device the most. Consumers would clearly benefit from more complete in-home coverage for all their wireless devices. While there isn't a femtocell in every living room yet, carriers appear to be interested in the idea.

However residential equipment must be marketed with a consumer-friendly price tag. The typical consumer electronics price point is still below what can be achieved with today's macro and microcell technology repackaged into a femtocell. Radios for infrastructure are more expensive because of the performance requirements and because the sales and production volume is much lower than handset volumes.  Until now, handset and infrastructure RF components have not been interchangeable (infrastructure frequency bands are reversed as compared to a mobile device). A single, programmable radio platform would significantly simplify this issue.

 

Radio Technology Today

At the heart of every wireless device, including a Femtocell is a radio (Figure 3).  A Femtocell radio consists of three main components or modules; the baseband modem, the transceiver and the front-end module (FEM).  The baseband modem is a digital bulk CMOS semiconductor which is normally a combination of a digital signal processor and a Reduced Instruction Set Computer (RISC) processing core.  The baseband (BB) chip converts data from the incoming application data stream to modulated I/Q signals.  The transceiver is responsible for converting between the appropriate RF frequency, bandwidth, protocol and performance to the modulated I/Q signals the baseband requires.  In general, infrastructure components require multiple transceivers for multiple frequency, protocol and performance requirements.  Finally the front-end module (FEM) usually consists of duplexers, SAW band filters and power amplifiers.  The FEM connects the transceiver to the antenna and provides the band filtering and signal amplification required to meet modern day system and regulatory requirements.

Figure 3 - Typical Components of a Femtocell      

Initially, cellular communications operated over limited frequencies and protocols.  Radios needed only to work at one (1) frequency and one (1) protocol to be useful.  Times have changed.  Today, there are a multitude of frequency bands around the world for communications and a plethora of communication standards.  Companies such as Vanu (Cambridge, MA) have already proposed notional network-in-a-box architectures which support remote deployment (whether at home or in a remote work site) [v] and which would clearly benefit from reconfigurable transceiver technology in both cost and performance.

Of course this growth in wireless protocols has been led by and enabled by growth in semiconductor technology.  Through the last 20 years, the ongoing PC (r)evolution has been the engine of semiconductor demand.  Those advances have also driven a tremendous increase in the use of embedded digital technology in a wide range of electrical devices.  Moore's Law[vi] has enabled performance and capability increases at an ever lower cost.

To date, the benefits of Moore's Law have been most apparent in the baseband modem, which is the digital portion of the radio.  Baseband modems have leveraged the increases in gate density which has resulted from ever-smaller process geometries to now include both RISC processor and DSP processor functionality on the same die. On the base station front, companies such as ST Microelectronics have developed multi-standard baseband modem solutions for base stations. Solutions such as the STW51000 cover all major wireless standards and offer high voice and data channel density in a single System-on-Chip (SoC).[vii]

RF functionality has remained the province of the RF, analog and mixed signal engineers.  The RF portion of the radio; antenna, power amplifier and transceiver have remained a challenge.  Engineers have continued to wrestle with the difficult problem of achieving infrastructure performance targets while delivering semiconductor products at handset costs.  Achieving the power efficiency and performance goals for radios has continually driven base station OEMs towards custom designs.

The high performance multi-channel architectures deployed in today's macro and microcells have led base station transceiver vendors to provide multiple analog transceivers. They build multi-chip modules, multi-die packages, as well as multiple transceivers on a line card. Unfortunately for them, these approaches all come with limitations. They all require more die area and more line cards for each additional band. Each additional transceiver draws additional power. Also, each additional transceiver may require an additional antenna and matching network. At the end of all this, they have a high performance, power hungry and EXPENSIVE chip. This doesn't complement a femtocells low power and low cost goals at all.

On the other extreme, a traditional software-defined radio (SDR), takes a completely different approach to multi-band yet still runs into the same problems that the multi-transceiver/multi-die/multi-chip module approaches all do. The classical SDR architecture requires a high sampling rate, wide bandwidth and power hungry analog to digital converter (ADC) as well as a high performance, high power digital drop receiver (DDR).  For consumer applications, the high cost associated with these low volume components becomes prohibitive as well.

 

Designing multi-band femtocells for the home and office

The trick is to come up with a low cost low power transceiver architecture that is flexible enough to support a wide variety of signal bandwidths, modulation formats, signal levels and blocking specifications. As an example, the cellular standards have low to medium bandwidths, but have very high dynamic range requirements due to challenging blocker environments.   Data standards such as WiMax have high signal bandwidths and high order modulation (requiring higher SNR).

When femtocells appear in the market, the key metric will be cost. Each femtocell deployed in a home or office location will have minimal users and (relatively) low utilization. There is not a big opportunity for a carrier to amortize the cost across many customers. To be an economically viable solution, the retail price must be low enough to be attractive to retail customers who would be the primary beneficiary of the extended wireless coverage.

 

A new approach

BitWave's long experience with SDR has enabled it to develop a new solution to this complex problem.

A Softransceiver must be low cost. To do that, it must be able to leverage the economies of scale that result from aggregating the volume required by multiple markets (i.e. GSM/CDMA/WiFi etc) and optimize itself for each unique protocol; i.e. be easily controlled with digital technology by the baseband through a simplified interface.

BitWave's Softransceiver technology was created with the preceding ideas in mind: it provides a way to increase flexibility while at the same time reducing cost.

Figure 4 - BitWave's Softransceiver Functional Architecture

The Softransceiver can quickly reconfigure its operating characteristics in real time under software control.  It can shift the center frequency, modify the bandwidth and sampling rate, and change the linearity and noise figure of a transceiver channel in real time.  One programmable Softransceiver™ RFIC could replace the many fixed transceivers required by a multi-mode multi-band Femtocell. The Softransceiver stands apart from other multi-band solutions in that it is competitive in performance even against single standard transceivers.

BitWave's unique approach to reconfiguring the individual components is what enables BitWave to meet industry performance benchmarks for multiple standards in the same small device.  The patent pending techniques allow optimization of system level performance through dynamic control of individual stages in the transceiver chain.

Optimization and reconfiguration can be done in real time (in-call) as well as between mode switches. The circuit level overhead associated with implementing flexibility using the BitWave IP is minimal and scales with process technology node. These factors combine to allow the resolution of system level performance issues to occur in the software abstraction layer rather than in silicon.

Figure 5 – Reconfigurable Receiver

Since each standard that the Softransceiver supports has an optimal architecture for implementation, a flexible transceiver provides the designer the option of selecting between multiple architectures. For example, most GSM standards can be more easily implemented in a Low IF (LIF) architecture while other standards such as WiFi or WiMax may be more easily implemented with a Zero IF (ZIF) architecture.  The Softransceiver allows the selected standard to be demodulated with whichever works best.

 

Additional Benefits

BitWave recognizes that infrastructure vendors will require low power portable solutions for multiple protocols and frequency bands. SDR solutions can have more impact than simply flattening wireless boundaries. SDR also offers performance benefits. Radios which can readjust their performance (for example in the fringe at the edge of a cell) and can develop improved SNR while burning more power offer new paths for carriers to satisfy their customers.

 

Summary

While many different approaches have been taken over the years, neither of the two primary approaches … parallel hardware architecture and traditional SDR (supported by high bandwidth ADCs) … has been successful in enabling a low power reconfigurable radio. Both parallel hardware architectures as well as SDR have driven designers to make undesired tradeoffs between flexibility, power, performance and cost.

The Softransceiver ™ RFIC offers a new solution for low cost flexible radios. Reconfigurable analog components and programmable digital processing elements have been integrated into a complete transceiver solution which can benefit the entire wireless value chain.

By using the Softransceiver™ RFIC along with a programmable baseband modem, wireless carriers will be able to deploy flexible femtocells on premise at the right consumer electronics price point.

Erik Org is a marketing manager at BitWave Semiconductor, Inc. He has more than 10 years of experience with wireless markets and technology and has worked at companies such as Motorola, Qualcomm and Kyocera Wireless. He earned an MBA from Columbia Business School, as well as a BS in Electrical Power Engineering from Rensselaer Polytechnic Institute.

Email: org@bitwavesemiconductor.com

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www.bitwavesemiconductor.com