Archive for SDR
RF Receiver Front-End Topologies for Software Radios
Posted by: | CommentsA number of different RF front-end topologies are appropriate for software radios, each with its own advantages and disadvantages. This article explores the tradeoffs involved with each approach.
By Jeffrey H. Reed, Virginia Tech
The most common types of RF front-ends for software radios are dual conversion, single conversion, and tuned radio frequency receivers. The suitability of a particular receiver topology depends on a number of parameters that may include the following.
- Sensitivity defines the weakest signal level that a receiver can detect and is usually determined by the various noise sources in the receiver.
- Selectivity- represents the ability of the receiver to detect the desired signal and reject all others.
- Stability indicates the lack of change in the receiver gain and operating frequency with temperature, time, voltage, etc.
- Dynamic range is the difference in power between the weakest signal that the receiver can detect and the strongest signal that can be supported (either in-band or out-of- band) on the receiver without detrimental effects.
- Spurious response is a receiver’s freedom from interference due to internally generated spurious signals or to their interaction with external signals.
Topologies
Tuned RF
The tuned radio frequency (TRF) receiver, shown in Figure 1, consists of an antenna connected to an RF bandpass filter (BPF). The BPF selects the signal and the LNA with the automatic gain control (AGC) raises the signal level for compatibility with the ADC. This BPF bandwidth relative to the carrier frequency can be quite narrow, while in absolute bandwidth, it may be quite broad. For example, a second-order inductor and capacitor filter would require a filter quality factor of 107 to extract a 30 kHz signal at 900 MHz with 60 dB of attenuation for a channel 60 kHz away, which is highly impractical.
IMEC’s Cognitive Radio Platform Brings SDR Closer to Handsets
Posted by: | CommentsThis week (June 8, 2010) as part of its Green Radio program IMEC introduced a reconfigurable cognitive radio baseband architecture (COBRA) that promises to meet 4G requirements at up to 1 Gbit/s throughput with multiple asynchronous concurrent data streams on mobile handsets, basestations and small cells.
The COBRA architecture can be customized to meet the requirements for Wi-Fi (WLAN (IEEE802.11n to .11ac), cellular (LTE to LTE-advanced), and broadcasting (DVB-T/H to DVB-T2) and other air interfaces. With idle power in the range of 2mW in 65nm low-power CMOS technology for the baseband platform, COBRA could prove to be a boon for both handset makers and carriers alike.
It’s hard to see how multi-band, multi-protocol handsets can be implemented without software-defined radio (SDR) techniques. To date the major problem has been that SDR radios have been power hungry, since realizing a bullet-proof front end with sufficient bandwidth means cranking up the power to the ADC. In that regard alone IMEC’s SDR implementation is a big step forward.
SDR also offers carriers the possibility of doing a simple firmware update over phone lines to thousands of cellular basestations when they want to introduce a new air interface instead of spending millions of dollars doing truck rolls to upgrade the hardware.
The RF Front End
Scaldio-2B consists of a reconfigurable frequency synthesizer and receiver in 40nm digital TSMC CMOS technology. The single-chip flexible receiver is fully software configurable across all channels in the frequency bands between 100MHz and 5GHz. Its properties (such as the RF carrier frequency, channel bandwidth, noise figure, linearity, filter characteristics) can be adapted to the requirements of the standards that are used.
To meet the additional demands implementing a cognitive radio in 4G, COBRA has added a novel ASIP (application-specific integrated processor)-based digital front end enabling hierarchical platform activation—presumably to swap in different waveform images from software as the need arises—in addition to flexible filtering, synchronization and spectrum sensing. According to Liesbet Van Der Perre, IMEC’s Program Director for Wireless Communication, the goal of COBRA is to enable “Devices that can negotiate and switch between frequencies to optimally use the available spectrum. Devices that switch between standards, choosing the best option depending on location, user environment and user application.”
Cognitive SDR techniques will enable both multi-band, multi-mode handsets as well as far more efficient use of available spectrum—a major issue as the number of cell phone users continues to increase geometrically.
Baseband Processing
COBRA utilizes IMEC’s 3rd generation reconfigurable ADRES processor (Architecture for Dynamically Reconfigurable Embedded Systems). ADRES is a processor architecture designed for wireless and multimedia processing in single- and multiprocessor systems. Through an XML template, designers can create the ADRES processor instance that is best suited for their applications. Applications for an ADRES processor can be completely programmed in a high-level programming language (C) and compiled with IMEC’s DRESC C compiler.
The 2nd generation ADRES processor was designed to support 600Mbps 802.11.n on two cores with a total power use of 220mW using 40nm technology. For this application IMEC got rid of the central bus and went to a crossbar architecture and added multi-threading and wide SIMD (single instruction, multiple data) capabilities. They also added support in their DRESC compiler for instruction-, data- and task-level parallelism, which in itself is a major step forward. According to Van Der Perre, “We expect to have a first instantiation of this processor ready in the course of 2010”.
To cope with a diversity of high-speed protocols, IMEC has added a flexible forward-error-correction (FlexFEC) processor template to achieve both high-speed turbo and low-density parity check (LDPC). An LDPC-specific instance for multi-standard broadcasting has also been derived to further optimize power and area.
Looking Ahead
IMEC’s COBRA architecture is far from the only SDR architecture out there, but to my knowledge it’s the first one to bring cognitive techniques close to the silicon. It’s also a big step forward in terms of energy efficiency. With IMEC partners Samsung, Renasas, Panasonic and TSMC working on implementing CORBA in both software and silicon, commercial products shouldn’t be far off.
Build Your Own Cellular Network
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The June issue of the MIT Technology Review has a short article on how to build your own cellular network (registration required). The idea of going into competition with Verizon for a few hundred dollars was just too much fun to overlook.
Far from being just a fun hack, OpenBTS’s founders Harvind Samra and David Burgess set out to design a cellular basestation that would reduce the cost of GSM service provision in rural areas and the developing world to below $1 per month per subscriber. They’ve already successfully field tested their DIY basestation at the Burning Man festival in Nevada and on the tiny island nation of Niue in the South Pacific. Now you can put your own island on the digital map for around $1,000 or so, depending on your patience and your junkbox.
OpenBTS is an open-source Unix application that uses the Universal Software Radio Peripheral (USRP) to present a GSM air interface to standard GSM handset and uses the Asterisk software PBX to connect calls. The combination of the ubiquitous GSM air interface with VoIP backhaul could form the basis of a new type of cellular network that could be deployed and operated at substantially lower cost than existing technologies in greenfields in the developing world.
OpenBTS systems draw only about 60W, which can easily be supplied by a few marine batteries in remote locations, topped up by solar panels or a small wind generator. And while the USRP hardware—which is the basis for a large number of SDR deployments—can be programmed for many waveforms other than GPS, GPS is the dominant standard worldwide—particularly in developing countries—and besides Asterisk is based on it. And Asterisk, which can run on a notebook computer, is free vs. $250,000 or so for comparable commercial infrastructure.
Your output power and antenna will clearly determine—or be determined by—the range you wish to cover. The USRP’s WBX daughterboard puts out a modest 50-100mW (17-20dBm) from 50 MHz to 1.2 GHz, which covers most international cellular bands, though 100mW won’t get you very far. Add a PA and a small array of yagis and the sky’s the limit.
There is, however, the little matter of FCC approval. For the Burning Man field test in 2008 the FCC issued Experimental Special Temporary Authorization license WD9XKN to Kestrel Signal Processing Inc., Samra and Burgess’ consulting firm. The FCC issued a second temporary license for a larger test at the 2009 Burning Man festival. Both tests proved quite successful. OpenBTS is now providing cell phone coverage to Niue’s 1,700 residents and an unknown number of tourists, using Telecom Niue for Internet backhaul. Tests are currently under way in South America and Asia.
Ready to roll your own basestation? Samra and Burgess have the Burning Man covered, but maybe you could talk the FCC into letting you cover Coachella or Austin’s South by Southwest (SXSW) music festival. You can download all the necessary software here and browse for hardware at the Kestrel OpenBTS store. You’re on your own for the batteries.
Good luck, have fun and don’t forget to call!
–John Donovan
Welcome
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Welcome to Low-Power Wireless, your resource for RF design. In these pages we’ll bring the latest news, views, products and prognostications for engineers involved in designing state of the art low-power RF devices. We’ll also be doing hands-on evaluations of RF development kits from leading vendors.
At the heart of our value proposition are detailed design articles addressing some of the knottier issues facing RF designers. We’ll attempt to cover every protocol known to man and then some, showing how to utilize them in practical designs. We’ll admit going in to an enduring interest in software-defined radio (SDR), which we’ll attempt to cover extensively. SDR is where hardware and software designers are working together to resolve the most challenging and interesting RF problems. SDR is the future of the wireless world, and it’s here now.
We’re just getting started, so please don’t be put off if this site is still pretty sparse. We’ll be adding more news, products and articles daily, so check back regularly. And let us know how to make this site more useful to you. Meanwhile check out the outstanding articles highlighted below that we’re proud to feature in Low-Power Wireless.
“Live long and prosper.”
Best regards,
John Donovan, Editor/Publisher
Challenges with Measuring Current when Developing of Power Management Schemes for Battery-Powered Devices (Part 1)
In order to understand if a design has achieved an improvement in runtime, an engineer needs to be able to measure current being consumed by the device and determine if his optimized design has indeed lowered overall current consumption. In this article, we will cover methods used to measure current flowing from the battery into the electronic device (or within sub-circuits of the device) and how modern power management schemes create challenges for measuring these currents. Read
Designing RF Mixed Technology Boards
For most of the century-plus that RF circuits have been designed and built, RF has been treated as a highly specialized task, and as such has been more-or-less been isolated, both in design and on the printed circuit board (PCB). With the advent of the handheld cell phone, there was no longer the luxury of having an isolated PCB just for the radio components. Read
Frequency Hopping Diversity Improves Low-Power Wireless System Performance
The motivation for the article is to present methods of enhancing a wireless link in a typical environment of simple low-power wireless transceivers. This article focuses on maximizing the reliability of a wireless link, while keeping the overall hardware costs down. Read
Joint Tactical Radio System: AMF, GMR, HMS
The JPEO JTRS team at SPAWAR San Diego and PEOC3T Fort Monmouth, NJ is leading the development and testing of the Airborne Maritime Fixed (AMF), Ground Mobile Radio (GMR) and Handheld, Manpack and Small Form Factor (HMS) radios. These radios will be capable of communicating in the Global Information Grid using 9 waveforms and 13 radio form factors, including 7 small form factors (SFF). Read
Key Priorities for Sub-GHz Wireless Deployment
Wi-Fi, Bluetooth and ZigBee technologies are heavily marketed 2.4 GHz protocols used extensively in today’s markets. However, for low-data-rate applications, such as home security/automation and smart metering, sub-GHz wireless systems offer several advantages, including longer range, reduced power consumption and lower deployment and operating costs. Read
Simplifying Android Migration
Android is not just an operating system (OS), but a complete handset platform, combining a mobile OS kernel, a Java run-time (Dalvik), a telephony interface, and other middleware, plus browser and application environment. Integration of such a comprehensive package in a handset design would appear to demand an all-or-nothing approach. As tempting as developers might find a blank slate, existing investments, innovations, and expertise built on current platforms cannot realistically be abandoned in favor of an all-encompassing new technology. Read
CMOS Power Amplifier Technology
Traditionally, the power amplifier (PA) has been the last bastion of non-CMOS technology. Typically, this block is manufactured using a specialty GaAs or LDMOS process coupled with a hybrid module packaging technology, in total an expensive manufacturing flow, which has made it a substantial part of the cell phone bill of materials. The specialty semiconductor process is required to provide a high gain, high frequency transistor element with a high breakdown voltage. The hybrid packaging technology provides high Q passive components to generate the 50 Ohm matching circuit. Read
Putting Intelligence in ‘Bricks’The size and weight of early public safety portable radios led them to often be referred to as “bricks”; while that reference had more to do with the physical characteristics of a radio, it also implied that the radios were not all that sophisticated. Today’s public safety radios, however, incorporate a significant amount of software that processes RF signals. In fact, most high-end portables fit typical engineering definitions of software defined radios. Read
