An overview of Software Defined Radio, SDR

Software Defined Radio (SDR) is a radio system where functionality is defined by software, allowing a common hardware platform to support multiple standards and roles through reconfiguration. It offers flexibility for upgrades, standard changes, and role adaptation. Applications of SDR include:

Software Defined Radio definition

Creating a precise definition for SDR is complex but essential for regulatory, standardisation, and technological progress.

There are two main types of radio containing software (as defined by the SDR Forum):

• Software Controlled Radio: Software manages fixed physical layer functions.
• Software Defined Radio: Software defines the actual specification and behaviour of the radio; changing the software alters its performance and capabilities.

SDR uses a generic hardware platform, e.g. Digital Signal Processing (DSP) and general purpose processors, where software performs key radio functions like modulation, demodulation, filtering (including bandwidth changes), and frequency selection and hopping.

Ideally, all signal processing- transmit and receive- would be handled in software, eliminating hardware limitations. However, practical constraints like high-power Digital to Analogue conversion for transmission and low-noise requirements for reception make full software implementation difficult in reality.

Levels of SDR

Not all radios fully implement SDR capabilities. To categorise the range, the Wireless Innovation Forum (WINNF) defines tiers based on how configurable a radio is:

• Tier 0: Fixed hardware radio – No software control or configurability.

• Tier 1: Software controlled radio – Limited control via software (e.g. power levels, interconnections), but no control over mode or frequency.

• Tier 2: Partially software defined – Software can control key parameters (e.g. frequency, modulation and waveform generation / detection, wide/narrow band operation, security), but the RF front end remains fixed hardware.

• Tier 3: Ideal Software Radio (ISR) – Near full programmability; only minimal hardware remains fixed, close to the antenna. The front end is configurable.

Block diagram of an ‘Ideal’ Software Defined Radio

• Tier 4: Ultimate Software Radio (USR) – Fully programmable and capable of supporting multiple functions and frequencies simultaneously, e.g. a multifunction phone replacing several radios.

These tiers aren’t mandatory standards but offer a useful way to classify SDR capabilities.

SDR waveform portability

Beyond reconfigurability, a key advantage of Software Defined Radio (SDR) is waveform portability- the ability to reuse waveforms across different platforms and projects. This offers several benefits:

• Cost savings: Reusing expensive waveforms across multiple military or commercial systems reduces development costs.

• Obsolescence mitigation: Enables migration of waveforms to new hardware as technology evolves.

• Interoperability: Ensures consistent communication across equipment from different manufacturers.

However, full portability is challenging and requires early design planning using tools like SCA (Software Communications Architecture) and CORBA middleware, along with structured, portable code that can be recompiled across platforms.

SDR security

Security is critical in both military and commercial radio systems, and in SDRs, it extends beyond transmission protection to include software integrity. The software within the radio must be securely updated, especially when updates are delivered via the Internet, as this presents an opportunity for malicious software which could compromise or disable radio operation. Robust software security measures must be put in place to prevent unauthorised access or tampering during updates or through potential system vulnerabilities.

SDR interoperability testing

To ensure waveform portability across different SDR platforms, full interoperability testing is essential, including certification and accreditation of the waveforms.

Software Defined Radios (SDRs) are widely used today, especially in military and cellular base stations, but their broader adoption is limited by:

• High processing and power requirements, making them unsuitable for mobile phones.

• A necessary trade-off between power consumption and processing capability.

SDRs are ideal for complex, long-lifespan systems where upgradability and flexibility are important.
However, for simple, low-cost, static applications, traditional fixed-function radios remain more practical.

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