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Boost SDR performance

Posted: 04 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:software defined radio? signal processing? bit error rate?

The concept of software defined radio (SDR) has been around for many years. Consequently, there are many definitions for SDR. A concise one is that SDR is a radio in which some or all of the physical layer functions are software-defined. The physical layer function is the layer within the wireless protocol in which processing of RF, IF, or base band signals (including channel coding) occurs. Many of today's SDRs have part of the signal processing implemented in software.

The ability to implement in software numerous waveforms and receiver functions is appealing from the perspective of cost, complexity, programmability, and more. A goal of SDR is to produce an entire radio in software, down-converting the received analogue signal to base band at the antenna port and eliminating much of the hardware in between. Today, SDR is increasingly implemented in both military and commercial networks in more modest ways. The challenge with any SDR design is that it is a complex system that requires analysis from the component through system levels in order to achieve the desired performance. This article explains the attributes of SDR, the design challenges, and it includes hands-on examples of how AWR's Visual System Simulator (VSS) software for system simulation of RF end-to-end architecture design can be used to address these challenges.

SDR in brief
There are many specific processes that define a radio as an SDR. A modern day cell phone, for example, can be considered an SDR because most (if not all) of the base band processing is performed by a digital signal processing (DSP) chip. SDRs enable a single transceiver to accommodate many different waveforms, which makes it an essential ingredient in creating, for instance, a universally-compatible nationwide public safety network. SDR has long been attractive to manufacturers, commercial wireless carriers, and the military because it enables a single hardware platform to accommodate a wide array of modulated waveforms by implementing them in software. As a result, it eliminates physical components such as mixers, filters, amplifiers, modulators and demodulators, and detectors. In addition, it creates a multi-mode, multi-band, multi-functional communications platform that can be dynamically programmed locally or remotely to accommodate new waveforms and enhancements to existing ones.

Simulating SDRs
In order to understand and predict the behaviour of a particular SDR design, it is necessary to construct the signals per the base band physical layer specifications, such as WCDMA, WiMAX, or LTE. This requires designers to implement channel coding, interleaving, and pulse shaping. Once these signals are generated, the simulation can be used to determine the impact of impairments on the RF/IF portion of the SDR, making measurements such as error vector magnitude (EVM) or bit error rate (BER) for example. With this information, designers can make trade offs between different components or circuit implementations while injecting interfering signals and adjacent channels into the RF path to simulate a real-world environment.

SDR systems must inherently accommodate a variety of modulation schemes that vary in their complexity within the same system architecture. For instance, higher-order schemes such as 64QAM are used when signal conditions permit in order to deliver higher data rates. So, the impact of an RF link on the BER performance of a 16QAM versus that of a 64QAM signal is important to understand. Unfortunately, understanding the impact that RF impairments have on today's complex modulated signals is not a straightforward procedure, nor can it be accurately predicted with 'gut calls' or back-of-the-envelope calculations.

Critical SDR parameters required in a simulation tool include:

???Modulation accuracy or EVM
???Fixed-point implementation
???ADC/DAC quantisation level
???BER performance
???Carrier to noise ratio (CNR)
???Spurious-free dynamic range (SFDR)
???Adjacent channel-power ratio (ACPR)
???Carrier to third-order-intermodulation ratio (C/IM3dBc)
???Spectral mask requirement

Simulation example
In the following example, AWR's VSS software is used to virtually modify the values of components to achieve results for an SDR receiver designed to capture 16QAM and 64QAM signals. The goal is to use VSS to determine a suitable P1dB for the front-end amplifier as well as a phase noise mask for the local oscillator (LO). Determining a suitable P1dB prevents a device from being driven into compression as well as getting reasonable power added efficiency (PAE). Achieving a tolerable phase-noise level will enable the device to meet BER requirements.

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