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Wireless communication transmits information over a radio channel rather than a cable. The subject concerns managing attenuation with distance, multipath propagation, the effects of movement, and interference from other users, through link design, modulation, coding and multiple access schemes. Almost every technique in the field addresses one of those four problems.

Path loss is the reduction in signal power between transmitter and receiver. In free space it increases with the square of distance, but in real environments obstruction and reflection make it grow faster, which is modelled using a path loss exponent typically between two and five.

A link budget sums all gains and losses along a radio path in decibels to find received power, then compares it with receiver sensitivity to give a link margin. The margin is what allows the link to survive fading, weather and hardware ageing.

A decibel expresses a ratio between two powers, so it describes gain or loss. dBm is an absolute power level referenced to one milliwatt. Gains in dB can be added to a dBm level, but adding two dBm values together has no physical meaning.

Multipath occurs when a transmitted signal reaches the receiver by several routes after reflection, diffraction and scattering. The copies arrive at different times and phases and add constructively or destructively, producing rapid variation in received power as anything in the environment moves.

Rayleigh fading models a channel where many scattered paths arrive and none dominates, typical of dense urban areas without line of sight. Rician fading applies where a strong direct path exists alongside the scattered components, which gives noticeably less severe amplitude variation and a better average link quality.

Slow fading, or shadowing, is the gradual variation caused by obstruction from buildings and terrain, usually modelled as log-normal. Fast fading is the rapid variation caused by multipath components adding and cancelling as the receiver or the scatterers around it move, and it can change over fractions of a wavelength.

Delay spread is the difference in arrival time between the earliest and latest significant multipath components. When it becomes comparable to the symbol period it causes intersymbol interference, which effectively limits the symbol rate a single-carrier system can use without equalisation or a multicarrier scheme.

Relative motion between transmitter and receiver shifts the received frequency, and with multipath this produces a spread of shifts rather than one. Doppler spread determines how quickly the channel changes, and therefore how often channel estimation must be repeated for the receiver to stay accurate.

Frequency reuse assigns the same frequencies to cells far enough apart that mutual interference is acceptable, allowing a limited spectrum to serve a large area. Cells are grouped into clusters, and the cluster size sets both capacity per cell and interference margin.

Because a smaller cluster gives each cell more channels but places cells reusing the same frequencies closer together, raising co-channel interference. The reuse ratio falls as cluster size falls, so capacity is gained at the cost of signal quality and margin.

All CDMA users share the same band, so each one raises the noise floor for the others. A handset close to the base station transmitting at high power can swamp a distant one, so accurate and fast power control is essential rather than optional.

OFDM splits a wide channel into many narrow, orthogonal subcarriers, each narrow enough that its fading is effectively flat. A cyclic prefix longer than the channel delay spread removes intersymbol interference, replacing one difficult equalisation problem with many simple ones.

It is sensitive to carrier frequency offset and timing error, because these break subcarrier orthogonality, and it has a high peak-to-average power ratio, which forces power amplifiers to operate inefficiently or risk distortion. The cyclic prefix also consumes throughput that carries no data.

Diversity sends the same data over multiple antenna paths to improve reliability against fading. Spatial multiplexing sends different data streams simultaneously to raise throughput, but it requires sufficiently uncorrelated paths, so its gain collapses in a strong line-of-sight channel with little scattering.