Wireless foundations

Overview

A radio link carries information between two points by changing some property of an electromagnetic wave. Every system covered on Wireless Field Guide, from a Bluetooth Low Energy sensor to a 5G NR base station, uses the same chain. Bits are coded, mapped to symbols, shaped into a band-limited waveform, moved up to a carrier frequency and radiated. The receiver reverses each step, working against noise, a distorting channel and a clock that never quite matches the transmitter’s.

This hub is the starting point for engineers who are new to radio, or who come from software, networking or digital design. It names the stages of the chain, the few quantities that limit every link, and the main families of systems. The hubs that follow each take one stage and go deeper.

Key ideas

The signal chain

The transmitter has a digital half and an analogue half. In the digital half a channel encoder adds structured redundancy, a mapper turns groups of bits into complex symbols, and a pulse-shaping filter limits the bandwidth the symbols occupy. A digital-to-analogue converter (DAC) then produces the in-phase (I) and quadrature (Q) baseband waveforms. In the analogue half a mixer driven by a local oscillator (LO) moves them to the carrier, a power amplifier raises the level, and the antenna launches the wave.

The receiver captures a signal that is often below −90 dBm, a billionth of a milliwatt. A low-noise amplifier sets the noise floor, a mixer brings the signal back to baseband and an ADC samples it. Digital processing then synchronises in time and frequency, estimates and equalises the channel, demaps symbols to soft bits and decodes them. Most of the hard engineering sits in that last block.

Frequency, wavelength and bands

Carrier frequency f and wavelength λ are linked by λ = c/f. At 2.4 GHz the wavelength is about 12.5 cm, at 3.5 GHz about 8.6 cm and at 28 GHz about 10.7 mm. Wavelength sets the size of a resonant antenna, how readily a wave bends around obstacles, and how many antenna elements fit in a given panel. Higher bands offer wider channels but lose more to walls, foliage and rain, so they serve shorter links.

Spectrum is assigned by national regulators within the international framework kept by ITU-R. Licensed bands are given to one operator for one area. Licence-exempt bands, used by Wi-Fi and Bluetooth, are shared under power and access rules that differ by country. For 5G NR, 3GPP defines frequency range 1 (FR1) as 410 MHz to 7125 MHz and FR2 as 24250 MHz to 71000 MHz, split into two sub-ranges at 52600 MHz, in TS 38.104 (Release 19 version). The spectrum and regulation hub covers allocations country by country.

Bandwidth, SNR and capacity

Two numbers bound what any link can carry: the bandwidth B it occupies and the signal-to-noise ratio at the receiver. Shannon’s 1948 result gives the capacity of a channel with additive white Gaussian noise as C = B log2(1 + SNR). For a 20 MHz channel at 20 dB SNR that is about 133 Mbit/s. At 10 dB it falls to about 69 Mbit/s.

Bandwidth and SNR are not independent. Thermal noise power is about −174 dBm/Hz at 290 K, so the noise in a 20 MHz channel is about −101 dBm before the receiver adds its own noise figure. Doubling the bandwidth at fixed transmit power doubles the noise and costs 3 dB of SNR. Real systems also run below the Shannon bound because of coding overhead, reference signals, guard bands and implementation loss. The link budgets and noise hub works through these numbers.

System families

Each family picks a different point in the trade between range, data rate, power and cost.

• Cellular (LTE and 5G NR) is specified by 3GPP, runs mostly in licensed spectrum and is planned and operated by network operators. ITU-R Recommendation M.2083 grouped IMT-2020 use into enhanced mobile broadband, ultra-reliable low-latency communication and massive machine-type communication. M.2160 extends these scenarios for IMT-2030.
• Wireless LANs follow the IEEE 802.11 standards and Wi-Fi Alliance certification, and run in licence-exempt bands.
• Short-range links such as Bluetooth Low Energy and UWB trade range for low power and, for UWB, precise ranging.
• Low-power wide-area networks such as LoRaWAN and NB-IoT send small amounts of data over kilometres on battery-powered devices.
• Satellite and non-terrestrial networks cover areas that terrestrial networks do not reach.

Where to start

Read /learn/beginner/01-what-is-a-radio-link/ first. Then work through /learn/beginner/03-decibels-and-link-budgets/ and /learn/beginner/04-noise-snr-and-capacity/, which put numbers on every arrow in the diagram above. /learn/beginner/02-complex-baseband-and-iq/ explains the I and Q signals that every later lesson uses. Keep /reference/decibels-and-link-budgets/ and the glossary open. Intermediate lessons that build on this track are in preparation.

Tools and platforms

Radio specifications are collected on /reference/hardware/, and the wider tool list is on /reference/software/.

Common mistakes

• Writing dB when you mean dBm. A ratio and an absolute power are different quantities.
• Quoting a peak physical-layer rate as user throughput. Overheads, scheduling and retransmissions take a large share.
• Comparing two receivers’ sensitivity figures without checking that they used the same bandwidth and error-rate target.
• Assuming licence-exempt band rules are the same everywhere. They differ by country and by band.
• Assuming a network implements every feature of the 3GPP release it claims to support.

Going deeper

• C. E. Shannon, A mathematical theory of communication, Bell System Technical Journal 27(3), 1948.
• D. Tse and P. Viswanath, Fundamentals of Wireless Communication, Cambridge University Press, 2005. Chapters are free to download from the authors’ page.
• J. G. Andrews et al., What will 5G be?, IEEE Journal on Selected Areas in Communications 32(6), 2014.
• ITU-R Recommendation M.2083 (the IMT-2020 vision) and Recommendation M.2160 (the framework for IMT-2030).
• 3GPP TS 38.300, the overall description of NR and NG-RAN, and TR 21.905, the vocabulary used across 3GPP specifications.