Software Defined Radio, SDR, Tutorial

- tutorial and information about the basics of the software defined radio, SDR, and links of software defined radios to JTRS, and general SDR receiver technology.

The software defined radio, SDR, sometimes called a software radio has been the aim of many radio developments for a number of years. The roots of software defined radios can be traced back to the days when software was first used within radios and radio technology.

The basic concept of the SDR software radio is that the radio can be totally configured or defined by the software so that a common platform can be used across a number of areas and the software used to change the configuration of the radio for the function required at a given time. There is also the possibility that it can then be re-configured as upgrades to standards arrive, or if it is required to meet another role, or if the scope of its operation is changed.

One major initiative that uses the SDR, software defined radio, is a military venture known as the Joint Tactical Radio System, JTRS. Using this a single hardware platform could be used and it could communicate using one of a variety of waveforms simply by reloading or reconfiguring the software for the particular application required. This is a particularly attractive proposition, especially for coalition style operations where forces from different countries may operate together. Radios could be re-configured to enable communications to occur between troops from different countries, etc.

The SDR software radio concept is equally applicable for the commercial world as well. One application may be for cellular base stations where standard upgrades frequently occur. By having a generic hardware platform, upgrades of standards can easily be incorporated. Migrations for example from UMTS to HSPA and on to LTE could be accommodated simply by uploading new software and reconfiguring it without any hardware changes, despite the fact that different modulation schemes and frequencies may be used.

There are many opportunities for considering the use of the software defined radio, SDR, concept. As time progresses and the technology moves forward, it will be possible to use the concept in new areas.

Software defined radio definition

Although it may sound a trivial exercise, creating a definition for the software defined radio is not as simple as it seems. It is also necessary to produce a robust definition for many reasons including regulatory applications, standards issues, and for enabling the SDR technology to move forwards more quickly.

Many definitions have appeared that might cover a definition for a software defined radio, SDR. The SDR Forum themselves have defined the two main types of radio containing software in the following fashion:

  • Software Controlled Radio:   Radio in which some or all of the physical layer functions are Software Controlled. In other words this type of radio only uses software to provide control of the various functions that are fixed within the radio.
  • Software Defined Radio:   Radio in which some or all of the physical layer functions are Software Defined. In other words, the software is used to determine the specification of the radio and what it does. If the software within the radio is changed, its performance and function may change.

Another definition that seems to encompass the essence of the Software Defined radio, SDR is that it has a generic hardware platform on which software runs to provide functions including modulation and demodulation, filtering (including bandwidth changes), and other functions such as frequency selection and if required frequency hopping. By reconfiguring of changing the software, then the performance of the radio is changed.

To achieve this the software defined radio technology uses software modules that run on a generic hardware platform consisting of digital signal processing (DSP) processors as well as general purpose processors to implement the radio functions to transmit and receive signals.

In an ideal world the signal at the final frequency and at the correct level would emanate, and similarly for reception, the signal from the antenna would be directly converted to digits and all the processing be undertaken under software control. In this way there are no limitations introduced by the hardware. To achieve this, the Digital to Analogue conversion for transmission would need to have a relatively high power, dependent upon the application and it would also need to have very low noise for receive. As a result full software definition is not normally possible.

Levels of SDR

It is not always feasible or practicable to develop a radio that incorporates all the features of a fully software defined radio. Some radios may only support a number of features associated with SDRs, whereas others may be fully software defined. In order to give a broad appreciation of the level at which a radio may sit, the SDR Forum (now called the Wireless Innovation Forum, WINNF) has defined a number of tiers. These tiers can be explained in terms of what is configurable.

  • Tier 0:   A non-configurable hardware radio, i.e. one that cannot be changed by software.
  • Tier 1:   A software controlled radio where limited functions are controllable. These may be power levels, interconnections, etc. but not mode or frequency.
  • Tier 2:   In this tier of software defined radio there is significant proportion of the radio is software configurable. Often the term software controlled radio, SCR may be used. There is software control of parameters including frequency, modulation and waveform generation / detection, wide/narrow band operation, security, etc. The RF front end still remains hardware based and non-reconfigurable.
  • Tier 3:   The ideal software radio or ISR where the boundary between configurable and non-configurable elements exists very close to the antenna, and the "front end" is configurable. It could be said to have full programmability.

    Block digram of an ideal Software Defined Radio

    Block diagram of an 'Ideal' Software Defined Radio
  • Tier 4:   The ultimate software radio or USR is a stage further on from the Ideal Software Radio, ISR. Not only does this form of software defined radio have full programmability, but it is also able to support a broad range of functions and frequencies at the same time. With many electronic items such as cellphones having many different radios and standards a software definable multifunction phone would fall into this category.

Although these SDR tiers are not binding in any way, they give a way of broadly summarising the different levels of software defined radios that may exist.

SDR waveform portability

Apart from the fact that the software defined radio can reconfigure itself, another major advantage is that of waveform portability. There are several reasons for the need for SDR waveform portability:

  • Cost savings:   With the waveforms for various transmissions, military and commercial, costing huge sums to develop, there is a real need to be able to re-use waveforms on different projects and this is likely to involve very different platforms
  • Obsolescence mitigation:   A similar requirement comes as hardware technology develops and it is necessary to transfer existing waveforms onto newer platforms
  • Interoperability   To provide complete interoperability a customer may request the use of a particular waveform being used across the equipment from several manufacturers.

Complete SDR waveform portability is not always easy to achieve. However it is necessary to incorporate measures at the earliest stages of the design to ensure the optimum level portability. Elements such as the use of SCA - Software Communications Architecture, and CORBA, a form of middleware associated with SCA..In addition to the use of SCA and CORBA general good structured programming techniques are needed - short-cuts that may work on one platform are certainly not likely to work on another. It is often necessary to be able to re-compile the code for use the different platforms, so all code should be in a format that can be compiled on the foreseeable platforms.

SDR security

Another area of growing importance is that of SDR security. Many military radios, and often many commercial radio systems will need to ensure the transmissions remain secure, and this is an issue that is important for all types of radio. However when using a software defined radio, SDR, there is another element of security, namely that of ensuring that the software within the radio is securely upgraded. With the growing use of the Internet, many SDRs will use this to medium to deliver their updates. This presents an opportunity for malicious software to be delivered that could modify the operation of the radio or prevent its operation altogether. Accordingly SDR software security needs to be considered, if the Internet is used for software delivery or where there could be security weaknesses that could be employed maliciously.

SDR interoperability testing

With the need to transfer waveforms from one radio or platform to another it is necessary to undertake full interoperability testing. This needs to assure that the code can be transported from one platform to another and provides the correct functionality for the particular waveform in case. To achieve this the waveforms generally need to be certified and accredited.

The SDR, software defined radio is a reality today, and it is being used in many areas. However there are a number of limitations that prevent them being used in as many applications as some would like. One is the sheer processing power that is required, and the resulting power consumption. It is necessary to undertake a power consumption / processing power trade-off, and this is one of the core decisions that needs to be made at the outset. As a result of this it is not feasible to use SDR for cellphone designs, but cellphone base-stations are using them as power consumption and space are normally not issues and the software can be upgraded to enable the moving standards to be tracked. Also software defined radios are being used by the military, and already some handheld designs are appearing. As technology progresses software defined radios will be used in more applications, yet there will always be a decision to be made as the SDR is not the right decision for all radios. For small cheap radios where changes will be few, the SDR is definitely not right. But for more complicated systems where length of service is an issue and where change is likely, then the SDR is definitely a good option to be considered.

By Ian Poole

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