PE1-5.2 RF System Design & Link Budgets

What this is and why it exists

Before any of the software matters, one calculation decides whether a radio system can exist. Add up every gain and loss between the transmitter and the receiver, and see whether enough signal is left.

That is the link budget. It is the same shape of accounting as the optical link budget, and it is what turns a proposal into a design.

The vocabulary

• Band plan — the national allocation saying who may transmit in which frequencies.
• Noise floor — the noise power a receiver sees with no signal present.
• Free space loss — the loss of a wave spreading out, with nothing in the way.
• Practical loss model — an empirical model adding terrain, buildings and weather.
• Fade margin — spare signal kept for conditions worse than the model predicts.
• Link budget — the full accounting from transmitter power to received signal.
• Multicarrier amplifier — one amplifier carrying several signals at different frequencies at once.
• Linearity — how faithfully an amplifier reproduces its input.
• Intermodulation — unwanted products created when a nonlinear amplifier carries several signals.
• Back-off — running an amplifier below its maximum output to keep it linear.

The mental model

Band plans come first because they are the regulatory reality behind the whole flexibility argument. Countries do not allocate spectrum identically, so a device intended for several markets must cover several sets of frequencies. That is a hardware requirement no amount of software solves.

The noise floor sets what you are competing against. It follows from temperature and bandwidth, and the receiver adds some of its own. Together with the required signal-to-noise ratio, it sets the least received power the link can work with. Everything else in the budget works backwards from that number.

Free space loss is the optimistic bound. A wave spreading in every direction has its power distributed over a growing sphere, so the received power falls with the square of the distance. It also falls with the square of the frequency for a fixed antenna size. That is why higher frequency links are harder to sustain over distance.

Practical loss models exist because the real world adds terrain, buildings, foliage and rain, and none of that is in the free space calculation. These models are empirical. They come from measurement campaigns in particular kinds of environment, and knowing they are empirical is important. A model fitted in one city does not automatically describe another.

A full link budget then sums everything. Transmitter power, transmitting antenna gain, all the losses, receiving antenna gain, and the receiver's own noise contribution. What is left must exceed the required received power, and the difference is the fade margin. Working one through end to end is the exercise this topic exists for.

A real standard turns that into a specification. It states sensitivity, selectivity, spurious emission limits and error rate requirements in numbers. Those numbers are what a design must meet, not a target it aims at.

The hardest analog problem in the field closes the topic. A base station carrying several carriers can amplify them separately or together, and amplifying them together is cheaper and smaller. It is also difficult. Any nonlinearity in the amplifier creates products at frequencies that were not there, and some of them land in other people's bands.

So the amplifier must be linear. The trouble is that amplifiers are most efficient near their maximum output, and least linear there. Keeping them linear means running them well below maximum, which is back-off, and back-off wastes power as heat. That trade between linearity and efficiency drives the whole radio-frequency design, and it is why techniques for improving amplifier linearity are worth so much.

Finally, the processing itself has a capacity cost. More carriers, more antennas and more adaptivity all mean more computation, and computation is power. That cost sets up the device comparison two topics later.

What you should now be able to explain or do

Say why band plans constrain a multi-market design. Compute a noise floor and the least usable received power. Distinguish free space loss from empirical models and say why the distinction matters. Build a full link budget and identify the fade margin. Explain the linearity against efficiency trade in a multicarrier amplifier, and what back-off costs.

Check yourself