Chapter 1

INTRODUCTION TOSOFTWARE RADIO

CONCEPTS

1.1 The Need for Software Radios

With the emergence of new standards and protocols, wireless communications is develop-ing at a furious pace. Rapid adoption of the wireline-base Internet has led to demand forwireless Internet connectivity but with added capabilities, such as integrated services thatoffer seamless global coverage and user-controlled quality of service (QoS). The challengein creating sophisticated wireless Internet connectivity is compounded by the desire forfuture-proof radios, which keep radio hardware and software from becoming obsolete asnew standards, techniques, and technology become available. The concept of integratedseamless global coverage requires that the radio support two distinct features: first, globalroaming or seamless coverage across geographical regions; second, interfacing with dif-ferent systems and standards to provide seamless services at a fixed location. Multimodephones that can switch between different cellular standards like IS-95 and Global SystemMobile (GSM) fall in the first category, while the ability to interface with other serviceslike Bluetooth or IEEE 802.11 networks falls in the second category. Further, the rate oftechnology innovation is accelerating, and predicting technological change and its ramifi-cations to business is especially problematic. As a result, to keep their systems up to date,wireless systems manufacturers and service providers must respond to changes as they oc-cur by upgrading systems to incorporate the latest innovations or to fix bugs as they arediscovered. Many manufacturers tell horror stories of releasing hundreds of thousands ofdefective phones that had to be recalled and discarded. Since frequent redesign is expen-sive, time-consuming, and inconvenient to end users, interest is increasing in future-proofradios.

Existing technologies for voice, video, and data use different packet structures, datatypes, and signal processing techniques. Integrated services can be obtained with eithera single device capable of delivering various services or with a radio that can commu-nicate with devices providing complementary services. The supporting technologies and

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2 Introduction to Software Radio Concepts Chapter 1

networks that the radio might have to use can vary with the physical location of the user.To successfully communicate with different systems, the radio has to communicate and de-code the signals of devices using different air-interfaces. Furthermore, to manage changesin networking protocols, services, and environments, mobile devices supporting reconfig-urable hardware also need to seamlessly support multiple protocols, such as IP (InternetProtocol) and MExE (Mobile Execution Environment). Such radios can be implementedefficiently using software radio architectures in which the radio reconfigures itself based onthe system it will be interfacing with and the functionalities it will be supporting.

Second-generation (2G) wireless technology consists of a handful of incompatible stan-dards, and the goal behind the development of third-generation (3G) standards is compat-ibility among these standards within and between different generations’ standards. Evenif cellular standards globally converge, 3G systems require multimode operation and au-tomatic mode selection. With fourth-generation (4G) and possibly 3G systems, the user’sapplication will likely have the ability to control the quality of service and obtain a higherQoS for a higher cost. Higher QoS can be achieved through priority scheduling of packets,changes in data packaging, improved protection coding, better channel equalization tech-niques, implementation of smart antennas, and so on. The mobile subscriber must have theability to select the network provider as well as the services needed.

1.2 What Is a Software Radio?

The term software radio was coined by Joe Mitola in 1991 to refer to the class of repro-grammable or reconfigurable radios [1]. In other words, the same piece of hardware canperform different functions at different times. The SDR Forum defines the ultimate soft-ware radio (USR) as a radio that accepts fully programmable traffic and control informationand supports a broad range of frequencies, air-interfaces, and applications software. Theuser can switch from one air-interface format to another in milliseconds, use the GlobalPositioning System (GPS) for location, store money using smartcard technology, or watcha local broadcast station or receive a satellite transmission.

The exact definition of a software radio is controversial, and no consensus exists aboutthe level of reconfigurability needed to qualify a radio as a software radio. A radio thatincludes a microprocessor or digital signal processor (DSP) does not necessarily qualifyas a software radio. However, a radio that defines in software its modulation, error cor-rection, and encryption processes, exhibits some control over the RF hardware, and can bereprogrammed is clearly a software radio. A good working definition of a software radiois a radio that is substantially defined in software and whose physical layer behavior canbe significantly altered through changes to its software. The degree of reconfigurability islargely determined by a complex interaction between a number of common issues in ra-dio design, including systems engineering, antenna form factors, RF electronics, basebandprocessing, speed and reconfigurability of the hardware, and power supply management.

The term software radio generally refers to a radio that derives its flexibility throughsoftware while using a static hardware platform. On the other hand, a soft radio denotesa completely configurable radio that can be programmed in software to reconfigure thephysical hardware. In other words, the same piece of hardware can be modified to perform

Section 1.3 Characteristics and Benefits of a Software Radio 3

different functions at different times, allowing the hardware to be specifically tailored to theapplication at hand. Nonetheless, the term software radio is sometimes used to encompasssoft radios as well.

The functionality of conventional radio architectures is usually determined primarily byhardware with minimal configurability through software. The hardware consists of the am-plifiers, filters, mixers (probably several stages), and oscillators. The software is confinedto controlling the interface with the network, stripping the headers and error correctioncodes from the data packets, and determining where the data packets need to be routedbased on the header information. Because the hardware dominates the design, upgradinga conventional radio design essentially means completely abandoning the old design andstarting over again. In upgrading a software radio design, the vast majority of the newcontent is software and the rest is improvements in hardware component design. In short,software radios represent a paradigm shift from fixed, hardware-intensive radios to multi-band, multimode, software-intensive radios.

1.3 Characteristics and Benefits of a Software Radio

Implementation of the ideal software radio would require either the digitization at the an-tenna, allowing complete flexibility in the digital domain, or the design of a completelyflexible radio frequency (RF) front-end for handling a wide range of carrier frequenciesand modulation formats. The ideal software radio, however, is not yet fully exploited incommercial systems due to technology limitations and cost considerations.

A model of a practical software radio is shown in Figure 1.1. The receiver begins with asmart antenna that provides a gain versus direction characteristic to minimize interference,multipath, and noise. The smart antenna provides similar benefits for the transmitter. Mostpractical software radios digitize the signal as early as possible in the receiver chain whilekeeping the signal in the digital domain and converting to the analog domain as late as pos-sible for the transmitter using a digital to analog converter (DAC). Often the received signalis digitized in the intermediate frequency (IF) band. Conventional radio architectures em-ploy a super heterodyne receiver, in which the RF signal is picked up by the antenna alongwith other spurious/unwanted signals, filtered, amplified with a low noise amplifier (LNA),and mixed with a local oscillator (LO) to an IF. Depending on the application, the numberof stages of this operation may vary. Finally, the IF is then mixed exactly to baseband.Digitizing the signal with an analog to digital converter (ADC) in the IF range eliminatesthe last stage in the conventional model in which problems like carrier offset and imagingare encountered. When sampled, digital IF signals give spectral replicas that can be placedaccurately near the baseband frequency, allowing frequency translation and digitization tobe carried out simultaneously. Digital filtering (channelization) and sample rate conversionare often needed to interface the output of the ADC to the processing hardware to imple-ment the receiver. Likewise, digital filtering and sample rate conversion are often necessaryto interface the digital hardware that creates the modulated waveforms to the digital to ana-log converter. Processing is performed in software using DSPs, field programmable gatearrays (FPGAs), or application specific integrated circuits (ASICs). The algorithm usedto modulate and demodulate the signal may use a variety of software methodologies, such

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Section 1.3 Characteristics and Benefits of a Software Radio 5

as middleware, e.g., common object request broker architecture (CORBA), or virtual radiomachines, which are similar in function to JAVA virtual machines. This forms a typicalmodel of a software radio.

The software radio provides a flexible radio architecture that allows changing the radiopersonality, possibly in real-time, and in the process somewhat guarantees a desired QoS.The flexibility in the architecture allows service providers to upgrade the infrastructure andmarket new services quickly. This flexibility in hardware architecture combined with flex-ibility in software architecture, through the implementation of techniques such as object-oriented programming and object brokers, provides software radio with the ability to seam-lessly integrate itself into multiple networks with wildly different air and data interfaces. Inaddition, software radio architecture gives the system new capabilities that are easily im-plemented with software. For example, typical upgrades may include interference rejectiontechniques, encryption, voice recognition and compression, software-enabled power mini-mization and control, different addressing protocols, and advanced error recovery schemes.Such capabilities are well-suited for 3G and 4G wireless requirements and advanced wire-less networking approaches. In summary, five factors are expected to push wider accep-tance of software radio.

1. Multifunctionality—With the development of short-range networks like Bluetoothand IEEE 802.11, it is now possible to enhance the services of a radio by leverag-ing other devices that provide complementary services. For instance, a Bluetooth-enabled fax machine may be able to send a fax to a nearby laptop computer equippedwith a software radio that supports the Bluetooth interface. Software radio’s recon-figuration capability can support an almost infinite variety of service capabilities ina system.

2. Global mobility—A number of communication standards exist today. In the 2Galone, there are IS-136, GSM, IS-95/CDMA1, and many other, less well known stan-dards. The 3G technology tried to harmonize all the standards. However, there aremany standards under the 3G umbrella. The need for transparency, i.e., the abilityof radios to operate with some, preferably all, of these standards in different geo-graphical regions of the world has fostered the growth of the software radio concept.Military services also face a similar issue with incompatible radio standards existingbetween as well as within branches of the military.

3. Compactness and power efficiency—Multifunction, multimode radios designedusing the “Velcro” approach of including separate silicon for each system can be-come bulky and inefficient as the number of systems increases. The software radioapproach, however, results in a compact and, in some cases, a power-efficient design,especially as the number of systems increases, since the same piece of hardware isreused to implement multiple systems and interfaces.

4. Ease of manufacture—RF components are notoriously hard to standardize and mayhave varying performance characteristics. Optimization of the components in terms

6 Introduction to Software Radio Concepts Chapter 1

of performance may take a few years and thereby delay product introduction. Ingeneral, digitization of the signal early in the receiver chain can result in a de-sign that incorporates significantly fewer parts, meaning a reduced inventory for themanufacturer.

5. Ease of upgrades—In the course of deployment, current services may need to beupdated or new services may have to be introduced. Such enhancements have to bemade without disrupting the operation of the current infrastructure. A flexible archi-tecture allows for improvements and additional functionality without the expense ofrecalling all the units or replacing the user terminals. Vocoder technology, for exam-ple, is constantly improving to offer higher quality voice at lower bit rates. As newvocoders are developed, they can be quickly fielded in software radio systems. Fur-thermore, as new devices are integrated into existing infrastructures, software radioallows the new devices to interface seamlessly, from the air-interface all the way tothe application, with the legacy network.

Users/customers expect service regardless of the geographical areas in which they traveland the wireless technologies that are in use in different regions in the world, but carryingseveral devices that cover the broad range of technology alternatives is impractical. Usersexpect one device to utilize services in all regions, which is possible only by reconfiguringthe receiver to the air-interface standards used in the respective regions. By dynamicallydownloading the software to cover the needed air-interface standard, perhaps through trans-mission of the software configuration to the remote terminal, such over-the-air updates willallow for speedy implementation of software upgrades and new features.

1.4 Design Principles of a Software Radio

Radio design has always required a broad set of design skills. Although one might initiallyassume that software radios would require simply a higher level of digital signal processingprogramming skill than conventional radio design, this is not the case; a higher skill levelis needed for almost all aspects of the radio design because of the dependency of the radiosubsystems.

Software radios derive their benefits from their flexibility, complete and easy reconfig-urability, and scalability. It is important to ensure that these characteristics are present inthe final product. A generic design procedure for software radios follows and demonstratesthe interaction between the various subsystems of the radio design. Subsequent chapters inthis book focus on the details of these design procedures.

• Step 1: Systems engineering—Understanding the constraints and requirements ofthe communication link and the network protocol allows the allocation of sufficientresources to establish the service given the system’s constraints and requirements.For instance, constraints on the range and transmit power constrain the modulationtypes and data rate that can be supported. For a well-defined standard, the systemsengineering aspects, such as the routing protocol, are to a great extent predetermined.However, as additional flexibility is allowed in defining the network, systems engi-neering and optimization becomes a more complex task. In an ideal software radio

Section 1.4 Design Principles of a Software Radio 7

with the ability to change a number of system parameters in real-time, optimizing anactive communications session is a major challenge.

• Step 2: RF chain planning—The ideal RF chain for the software radio should in-corporate simultaneous flexibility in selection of power gain, bandwidth, center fre-quency, sensitivity, and dynamic range. Achieving strict flexibility is impractical andtrade-offs must be made. If the communication system is constrained to selectedcommercial or military bands, this optimization problem is simplified. Nevertheless,with a software radio design, it is possible to compensate for some of the inadequa-cies of the RF components in the digital domain. Compensations for power amplifierdistortion or power management of the RF circuitry, for example, can be accom-plished in the digital domain.

• Step 3: Analog to digital conversion and digital to analog conversion selection—Analog to digital conversion and digital to analog conversion for the ideal softwareradio is difficult to achieve, and in practice, the selection requires trading power con-sumption, dynamic range, and bandwidth (sample rate). Analog to digital conversionand digital to analog conversion selection is closely tied to the RF requirements fordynamic range and frequency translation. Channelization requirements also impactthe selection of the analog to digital conversion and digital to analog conversion.Current conversion technology is very limited and is often the weak link in the over-all system design. There are post-digitization techniques based on multirate digitalsignal processing that can be used to improve the flexibility of the digitization stage.

• Step 4: Software architecture selection—The software architecture is an importantconsideration to ensure maintainability, expandability, compatibility, and scalabilityfor the software radio. Ideally, the architecture should allow for hardware indepen-dence through the appropriate use of middleware, which serves as an interface be-tween applications-oriented software and the hardware layer. The software needs tobe aware of the capabilities of the hardware (both DSP and RF hardware) at bothends of the communications link to ensure compatibility and to make maximum useof the hardware resources. Furthermore, given that the software radio will operatein an existing data infrastructure, it must interface quickly and efficiently with thisinfrastructure. This means that the software radio needs to control issues such asattribute naming, error management, and addressing, regardless of the protocol usedin the infrastructure. Partitioning the radio functions into objects can help with theseissues as well as aid in portability and maintenance of the software. Example objectsmight include the blocks of the model software radio shown in Figure 1.1. Securityis an important issue to ensure that software downloads are legitimate. Finally, giventhat higher-layer protocols such as TCP have constraints inherent to the way in whichthey manage a session, the software architecture should consider latency and timingfor the whole protocol stack.

• Step 5: Digital signal processing hardware architecture selection—The core dig-ital signal processing hardware can be implemented through microprocessors, FP-GAs, and/or ASICs. Typically microprocessors offer maximum flexibility, highest

8 Introduction to Software Radio Concepts Chapter 1

power consumption, and lowest computational rate, while ASICs provide minimalflexibility, lowest power consumption, and highest computational rate. FPGAs, onthe other hand, lie somewhere between an ASIC and a DSP in these characteristics.The selection of the core computing elements depends on the algorithms and theircomputational and throughput requirements. In practice, a software radio will useall three core computing elements, yet the dividing line between the implementationchoices for a specific function depends on the particular application being supported.

• Step 6: Radio validation—This step is perhaps the most difficult. It is essentialto ensure not only that the communicating units operate correctly, but also that aglitch does not cause system-level failures. Interference caused by a software radiomobile unit to adjacent bands is an example of how a software radio could cause asystem-level failure, and this is of great concern to government regulators [2]. Giventhe many variable parameters for the software radio and the desire for an open andvaried source of software modules, it is very difficult to ensure a fail-proof system.Testing and validation steps can be taken to help minimize risk. Structuring thesoftware to link various modules with their limitations and deficiencies can help intesting compatibility of software modules.

As you can see from the cartoon in Figure 1.2, Dilbert is skeptical of the ideal softwareradio. This skepticism is understandable; software radios require a much higher level ofsystems-level engineering than today’s products. To carry out this cooperative interdisci-plinary design, engineers must understand the ramifications of their design on the overallsystem and be willing to have their subsystem control and be controlled by other subsys-tems, and they must be knowledgeable in a variety of technical disciplines.

Figure 1.2: Dilbert’s View of Software Radios.

SOURCE: S. Adams, “Dilbert,” 4/11/1994. c© United Feature Syndicate, 1994. Used byPermission.

1.5 Questions

Fill in the design matrix in the following table to show how one design step may be relatedto another design step. For the sake of illustration, some examples are given.

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