Microwave Engineering Europe - November 2007 - (Page 20)

20 SOFTWARE DEFINED RADIO High-speed ADC technology paves the way for software deﬁned radios By Yiannis N. Papantonopoulos, Systems and Applications Manager, Texas Instruments oftware Deﬁned Radio (SDR) addresses the tremendous capital expenditure demands placed on operators as wireless standards continue to evolve and change. The cost to install infrastructure is considerable, and it’s this cost that inhibits rapid adoption and deployment of new wireless technologies. This poses a signiﬁcant hindrance to the agility of operators in offering new and improved services to their subscribers. The ultimate goal of a fully re-conﬁgurable radio that can adapt to a new standard or accommodate multiple standards simply through software upgrades is, paradoxically, not limited by software. Indeed, it is the analog domain and its bridge to the digital world that presents system designers with their biggest challenge. The focus of this article is on the challenges of analog-to-digital (A/D) conversion as they pertain to SDR implementations, and how breakthroughs in analog-to-digital converters (ADCs) can bring true SDR closer to reality. The problem The big promise of SDR for operators is that, eventually, it will allow them to deploy one network, and one set of infrastructure capable of handling a broad range of radio frequencies, standards and their future evolutions. This requires the radio design to be ﬂexible enough to allow for wider frequency coverage than usual. Additionally, it has to offer dynamic range beyond the one necessary for narrow band applications. So, ultimately, we could deal with a multi-carrier environment with carriers of different modulation types and bandwidths, blocking requirements, etc. The advances in digital signal processing (DSP) technology have elevated the digital backend capabilities of radios to levels that can be amenable to SDR implementations. Hence, the missing piece of the puzzle is getting the extremely sensitive analog signals converted to the comfort of the digital domain. A/D conversion in these radios is pivotally important in trying to S Figure 1: Processing gain versus wanted signal bandwidth for an ADC sampling at Fs = 500 MSPS. realize the end-goal. ADCs are utilized in both the receiver (Rx) and the transmitter (Tx) sections of the radio, and are the enabling components for SDR development Key ADC speciﬁcations Among the primary speciﬁcations driving the design of the Rx section of the radio are sensitivity and usable bandwidth. Simply stated, sensitivity refers to the radio’s ability to effectively process very low-level signals at the antenna input, expressed in dBm. For the ADC, this most commonly translates into signal-to-noise ratio (SNR) speciﬁcations expressed in dBc or dBFS (dBc is the ratio of signal to noise expressed in reference to the carrier, whereas dBFS refers back to the full scale input of the ADC). Closely related to the radio’s capability to receive small signals and reject larger interferers is the spurious-free dynamic range (SFDR) of the ADC. This is the ratio of the wanted signal (carrier) to the next highest spurious component in the ADC’s output, be it harmonic or not, expressed in dBc. Finally, the usable bandwidth of the converter, a term really not speciﬁed effectively, deals with the actual signal bandwidth that the ADC can digitize with adequate SNR and SFDR performance. In standard industry practice, ADCs are speciﬁed to their –3 dB point of their analog input ‘frequency response.’ However, a lot of modern day converters show dramatically decreased performance as the analog input frequency increases past 200 to 300 MHz, even though their bandwidth is rated to several hundreds of MHz. It’s all about bandwidth One of the key advantages of SDR is its ability to handle a larger than usual frequency range without the need for new hardware. This is particularly appealing given the nature of today’s frequency map across the world. Each wireless standard has multiple frequencies deﬁned for operation. For example, GSM alone can operate at frequencies around 400 MHz, 850 MHz, 900 MHz, 1800 MHz, 1900 MHz, and even 2500-2690 MHz for the GSM extension band. 3GPP frequencies include 1800 MHz, 1900 MHz and 2100 MHz, while WiMAX frequencies exist in the 2500 MHz, 3500 MHz, and all the way to 5 GHz, with more coming. With such a plethora of frequencies, digitizing as large a signal bandwidth as possible through the ADC becomes a huge advantage. Therefore, it is the ADC sample rate that becomes critical in such implementations. The Nyquist criterion limits the bandwidth an ADC can effectively digitize without aliasing (a process whereby the wanted signal after digitization folds over on itself thus producing distortion) to half its sample rate (Fs/2). Thus, for an Microwave Engineering Europe ● November 2007 ● www.mwee.com 020_021_022_MWEE.indd 20 24/10/07 14:00:12 http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - November 2007

Microwave Engineering Europe - November 2007 Contents News Comment Metamaterials: Metamaterials Tackle Communications Wavelengths Microwave Components — EM tools: Microwave Component Design Easier With New EM and EDA Tools Cover Feature: RF Testing for OFDMA in LTE Base-Stations Startup Eyes Battery-Free Wireless Sensor Nets High-speed ADC Technology Paves the Way for Software Defined Radios Planning a WiMAX network: Maximising the ROI by Using Advanced Optimisation Tools Transporting Video Over Wireless Networks Ultrawideband Under the Gun Specifying the Proper SAW Filter Products Product Feature: RF Test Solution Supports Emerging 4x4 MIMO as Well as Multiple Commercial Standards Calendar

Microwave Engineering Europe - November 2007

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