Microwave Engineering Europe - April 2008 - (Page 18)

18 MILITARY/AEROSPACE FOCUS Hardware needs limit software radio By David Lipets, Tadiran Communications Ltd., www.tadconn he ultimate goal of software radio design is to build a device ﬂexible enough to run every possible waveform, without restrictions on operating frequency range, signal bandwidth, modulation scheme, data rate, networking requirements, etc., while adequately addressing collocation issues and radio channel impairments, and all of these must be achieved simply by changing its software. Since 1995, when Joseph Mitola III ﬁrst coined this term, extensive research and development efforts have been invested toward this goal, leaded by the Joint Tactical Radio System Joint Program Ofﬁce (JTRS JPO) and the SDR Forum. As of today, these efforts led to signiﬁcant progress in software communication architecture (SCA), but yielded little in other areas. As a result, many radio functions that are critical in implementing a successful, high performance radio design, cannot be considered as software implementable. Antennas, power ampliﬁers, oscillators, passive ﬁlters, power supplies, to name just a few of these functions, must still be implemented in hardware, yet these functions add up to a signiﬁcant fraction of the radio system development and production costs, as well as to radio size and weight. This paper presents major design and implementation issues and reaches the conclusion that even using intensive software processing, the attempt to design a real world software radio will result, contrary to common expectations, in a big, heavy, power hungry and very expensive device. Actually, it seems that radio hardware design engineering expertise is ever more relevant and challenging in the software radio world. Physics or mathematics? First, let’s deﬁne what is to be considered a physical operation and what a mathematical one. Energy transformation (occurring in any microphone, loudspeaker, etc.) is obviously a physical operation. Clock signal generation — creating periodic oscillations of an electrical signal — is also a physical operation, because it exploits electrical or electromechanical resonance phenomena. Operations on data represented as input and output ﬁles (for example, numerical arrays), in which the output data is determined T Figure 1: ISWR transmitter. Figure 2: Practical implementation of ideal DAC functionality. by an arbitrary user deﬁned function of the input data, are obviously mathematical operations. Such operations are routinely carried out in today’s electronic equipment by software using an embedded digital computing platform. Input ﬁles are created in the normal course of equipment operation or received from other devices. For example, most of the baseband signal processing in a contemporary radio system is mathematically performed by software. Complex networking algorithms are also routinely implemented in software. Hardware challenges of ideal software radio • ADC and DAC The common perception of the ideal software radio (ISWR) is that its receive path samples the analog input signal at the antenna connector, and converts it to a mathematically processable format by means of an analog to digital converter (ADC). The transmit path is expected to use a DAC to convert digital data to an analog transmit signal that is then applied to the antenna connector. It is often suggested that only the technological limitations of the ADC and DAC components are barriers on the way to a successful ISWR implementation. Thus, sometime in the future, it is expected that the ongoing process of technologically improving ADC and DAC components will remove these limitations and a true software radio will be within reach. But, to anyone familiar with ADC and DAC functions, this is a clear misconception. Actually, ADC and DAC components are complex analog devices performing linear conversion of input signals. Even today, suitable devices can be built using discrete parts. However, such implementations will be very costly in terms of materials, space, and power consumption. On the other hand, despite the rapid pace of progress in semiconductor technology (whether based on silicon or other materials), the integration of truly high power signal handling circuits on the same chip with low level processing devices does not seem to become practical in the foreseeable future. Anyway, we will show below that an ISWR implementation based on A/D and D/A conversion suffers from inherent drawbacks. • The transmit path Let’s start with the transmission path, using the generic block diagram of an ISWR transmitter shown in Figure 1 for reference. In the architecture of Figure 1, the output DAC performs pure physical, analog operations: it accepts a digital data stream, and Microwave Engineering Europe ● April 2008 ● www.mwee.com 018-019-020-021-022_MWEE.indd 18 26/03/08 18:06:53 http://www.tadconn.com http://www.mwee.com

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Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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