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SDR: Today's myth, tomorrow's reality?

Posted: 01 Feb 2006 ?? ?Print Version ?Bookmark and Share

Keywords:cedric paillard? semiconductor insights? sdr? sdr technology? transceiver?

The potential impact of SDR on the wireless industry is significant, and while it was just a buzz word five years ago amongst a small group of RF designers, it is now a key technology.

SDR impacts not only consumer wireless devices and their support networks, but also governmental, military, public safety and transportation wireless operators. The SDR forum (www.sdrforum.org) now has more than 100 members. The business case for SDR has already been accepted and technology gurus are working to deploy SDR across wireless platforms, probably starting with cellphones.

According to the FCC, SDR is alarmingly simple. It states, "In an SDR, functions that were formerly carried out solely in hardwaresuch as the generation of the transmitted signal and tuning and detection of received radio signalsare performed by software that controls high-speed signal processors."

Similarly, the SDR Forum defines an SDR device as one that functions independently of carrier frequencies and can operate within a range of transmission-protocol environments.

Architecturally, these definitions suggest transceivers that perform up/down conversion between baseband and RF exclusively in the digital domain, reducing the RF interface to a transmit-channel power amplifier, LNA for the receive path and minimal analog filtering.

The forum defines different classes of SDRs based on the level (or tier) of capability and flexibility. Tier-0 devices, known as hardware radios, implement all the radio functionality in hardware and any changes in functionality require physical intervention to implement. All radios can be tuned to pick up a specific, limited frequency range and it takes some amount of time to change the tuning. The goal is to make these characteristics programmable.

Tier-1 devices are known as software-controlled radios (scrs). At this level of the SDR hierarchy, only the control functions are implemented in software. For a given modulation standard, the baseband processing and radio front-end are fixed. Using multiple transceivers in the same device provides multistandard support. The software controls which transceiver to activate.

Reconfigurable SDRs are the natural progression from SCRs and are now common in base-station applications. Known as Tier-2 SDRs, these devices use software to control various modulation techniques, wideband and narrowband operation, security, and the waveform requirements of current and evolving standards over a broad frequency range. When people talk about SDR, they're generally referring to Tier-2 devices.

The ideal software radio, known as Tier-3, includes all the features of Tier-2 reconfigurable SDRs, but eliminates the analog amplification or heterodyne mixing prior to D/A conversion. Programmability extends over the entire system, with all analog conversions taking place at the antenna, speaker and microphone in the case of a cellphone.

Looking at how far the industry is from delivering a Tier-3 radio, we must quantify the gap between today's SDRs and the Tier-3 radio. This requires the definition of some measurements metrics. One criterion is the RF circuitry's complexity. A larger quantity of RF/analog functionality provides less SDR flexibility, while reduced RF/analog circuitry implies more SDR flexibility. A potentially more effective criterion is to characterize the radio's reconfigurability, which comes from digitizing the signal-processing functions, typically, in the RF/analog domain.

SDR technology can be considered as the driving force behind next-generation RF systems. For today's SDR applications, much of the computational power comes from improvements in baseband-processing designs, faster interface buses and process-node advances. Software radios are still limited by specialized RF chips that target particular frequency bands and waveforms. To realize low-cost solutions, digital functionality must use the latest process node in deep-submicron CMOS technology, such as 130-, 90- and 65nm in the future.

To provide the required dynamic range and power, RF and analog circuits require higher supply voltages that deep-submicron CMOS technologies can support. With supply voltages in digital processes falling to 1.5V and below and threshold voltages around 600mV, there's little headroom available to conduct significant signal processing in the analog domain. As digital processing moves to the latest process node to reduce cost and increase functional density, analog and RF functions lag behind at earlier process nodes, resulting in lower system-integration levels, increased system cost and reduced SDR capabilities.

To quote Bogdan Staszewski from TI's Wireless Analog Technology Center at RF IC 2005, "When designing highly-integrated RF circuits in deep-submicron CMOS processes, we are faced with the paradigm shift that the time domain resolution of a digital-signal edge transition is superior to the voltage resolution of analog signals."

TI has exploited this paradigm shift extensively in their new digital radio processor architecture. The company has created a design that compensates for the weaknesses of modern digital process nodes with respect to analog and RF functions by exploiting transition speed and logic density, the strengths of the process. Traditional analog and RF functions are being migrated into the digital domain to increase integration levels, reduce system cos, and ultimately push toward the goal of Tier-2 and beyond SDR capabilities. These and other advances in software-tunable/configurable RF technology will lead to cost-effective front-end designs. At that point, the commercialization of SDRs will really begin.

Most commercial SDR technologies are implemented in base-station systemsit's only recently that SDR technology is being developed for the cellphone market. The performance and cost challenges related to RF chip design for cellphones and the requirement to pack more functionality on a die may be addressed with SDR by using digital logic to implement traditional analog functions. For such designs, the key trade-offs shift from performance and design cost to flexibility, small form factor and product variances. While work is progressing, practical and commercially viable Tier-2 and 3 SDR systems are still a few years off.

- Cedric Paillard
Semiconductor Insights




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