PE1-5.1 Introduction to Software Defined Radio

What this is and why it exists

A traditional radio is built for one standard. Its filters, its mixers and its demodulator all assume that standard, and none of them can be changed after manufacture.

Software defined radio moves as much of that as possible into code. The case for doing so is economic before it is technical.

The vocabulary

• Software defined radio — a radio whose signal processing is done in software that can be changed.
• Multi-standard terminal — one device able to work with several different radio standards.
• Reconfiguration — changing what a deployed radio does, without changing its hardware.
• Adaptive modulation and coding — changing the scheme in use as conditions change.
• Handset model — the design case where cost and battery life dominate.
• Base station — the fixed infrastructure side, on mains power and with room to spare.
• Separation of digital and RF — putting the radio parts and the processing parts in different places.
• Tower-top mounting — placing the radio electronics at the antenna rather than at the base.
• Smart antenna — an antenna array whose pattern can be steered.
• Calibration — correcting for drift in the analog parts so the digital parts see what they expect.

The mental model

Start with the commercial problem, because that is what drives the field. A handset that speaks one standard sells in one market. An operator with several standards needs several sets of equipment in the cabinet, each bought, powered and maintained separately. Both are expensive in the same way.

One platform that software retargets to many standards solves both. It sells everywhere, which is the economy of scale argument. It lets a traveller's phone work in another country, which is the roaming argument. And it lets a deployed network gain a new capability by loading new software, rather than by sending someone up a tower.

Notice that none of those is a performance argument. A software radio is not more sensitive than a dedicated one. It is more flexible, and flexibility is what is being bought. Getting that straight early prevents a lot of confusion later.

Adaptive modulation and coding is the capability that most distinguishes the approach. A radio that can change its scheme uses a fast one when conditions are good and a robust one when they are not. A fixed radio must settle for whatever works in the worst case.

Reconfiguring a live radio is harder than it sounds. The radio must not transmit rubbish during the change, and must not interfere with anything else. It must also return to a working state if the change fails. Those requirements are what make reconfiguration a designed mechanism rather than a matter of loading a file.

The architectural consequence is the separation of digital from radio. Once the signal is digital it can be moved over a cable or a fibre. So the parts that must sit near the antenna and the parts that need not can be placed separately.

Two practical arrangements follow. Tower-top mounting puts the radio electronics at the antenna, which removes the long cable run and the loss that goes with it. Base station hotelling puts many processing units in one building, connected to distant antennas. The equipment is then powered, cooled and maintained in one place.

Smart antennas steer rather than radiating uniformly. An array with controlled phases can point energy where a user is and away from where interference is. That raises capacity and it also raises the processing load, which is a cost the later topics have to account for.

The syllabus includes an honest counterweight, and it is worth respecting. Flexibility is paid for in watts. A general-purpose processing chain doing a job that fixed hardware could do will consume more power, and on a handset that is the binding constraint. Calibration is the second cost. A flexible radio has more analog parts that can drift, and correcting for that drift is continuous work.

What you should now be able to explain or do

State what software defined radio is and give the economic case for it. Say why flexibility rather than performance is the benefit. Explain adaptive modulation and coding and what it changes. Say why reconfiguring a live radio needs a designed mechanism. Describe the separation of digital and radio, tower-top mounting and hotelling, and state the two costs of flexibility.

Check yourself