Table of Contents
Overview
Wireless transmission is the sending of data through the air without a physical cable. At the physical layer this means converting bits into electromagnetic waves and sending them through space where a receiver captures and converts them back into bits. This chapter focuses on how that process works in general, not on Wi‑Fi standards or security, which have their own chapters.
Wireless transmission is used in Wi‑Fi, mobile networks, Bluetooth, satellite links, and many other systems. The same basic ideas appear in all of them: frequencies, waves, modulation, antennas, and the way signals weaken and interfere.
Electromagnetic Waves and Frequency
Wireless networks use electromagnetic (EM) waves. These waves do not need a medium like a wire. They travel through air and even through vacuum.
Each wave repeats a pattern. The number of repetitions per second is the frequency. Frequency is measured in hertz (Hz). One hertz is one cycle per second. In networks you will see kilohertz (kHz), megahertz (MHz), and gigahertz (GHz).
| Unit | Meaning | Example use in networking |
|---|---|---|
| Hz | 1 cycle per second | Very low frequency control systems |
| kHz | $10^3$ Hz | Some legacy radio links |
| MHz | $10^6$ Hz | Some wireless control, older Wi‑Fi channels |
| GHz | $10^9$ Hz | Modern Wi‑Fi, cellular, microwave links |
A related property is the wavelength, which is the distance between two peaks of the wave. Wavelength $\lambda$ and frequency $f$ are linked by the speed of light $c$:
$$
\lambda = \frac{c}{f}
$$
with $c \approx 3 \times 10^8 \text{ m/s}$.
Important rule: Higher frequency means shorter wavelength, and often supports higher data rates, but usually has shorter range and is blocked more easily by obstacles.
Modulation of Wireless Signals
Bits cannot be sent directly. Instead, a radio creates a carrier wave at a chosen frequency and then modulates it. Modulation means changing one or more properties of the carrier to represent data.
Common properties used in wireless modulation are:
- Amplitude, the strength of the wave.
- Frequency, how fast the wave oscillates.
- Phase, the relative position of the wave in its cycle.
In simple systems one bit might be represented by one change, for example a higher amplitude for 1 and a lower amplitude for 0. In modern systems, multiple bits are encoded in each symbol by combining phase and amplitude changes. This is called higher order modulation.
A simple example is binary phase shift keying (BPSK). The transmitter sends the same frequency but uses two possible phases. One phase means bit 0 and the opposite phase means bit 1. More advanced techniques such as quadrature amplitude modulation (QAM) use many different phase and amplitude combinations so that each symbol can carry several bits at once.
Key idea: Modulation converts digital bits into changes of a radio wave. More complex modulation can send more bits per symbol but usually needs a cleaner signal and higher signal to noise ratio.
Antennas and Radiation Patterns
Antennas are the physical devices that send and receive radio waves. In a wireless link the transmitter drives an antenna with an electrical signal. The antenna converts it into an electromagnetic wave that radiates into space. The receiving antenna does the reverse process.
Antennas have radiation patterns, which describe how they send energy in different directions. Two simple categories are:
- Omnidirectional antennas, which radiate roughly equally in all horizontal directions, like a doughnut shape. These are common in home Wi‑Fi routers because they serve users in many directions.
- Directional antennas, which focus energy into a narrower beam, like a flashlight. These are used for point to point links where you want longer range and less interference outside the beam.
The gain of an antenna is a measure of how much it concentrates energy in certain directions compared to a theoretical reference antenna. Higher gain in one direction usually means less radiation in other directions.
Path Loss and Range
As a wireless signal travels away from the antenna it spreads out. The power density at a given point decreases as distance increases. This is called path loss. Even in perfect free space, without buildings or trees, path loss grows quickly with distance.
In simple free space conditions, received power $P_r$ can be related to transmitted power $P_t$ by the free space path loss formula, which depends on distance $d$ and wavelength $\lambda$. You do not need to know the full formula here, but the main result is that doubling the distance reduces the received power sharply.
In the real world, signals do not travel in perfect free space. They pass through or around walls, furniture, and other objects. Each object absorbs or reflects part of the signal and increases path loss. That is why a Wi‑Fi signal can be strong in one room but weak or unusable two rooms away.
Devices have receiver sensitivity, which is the lowest signal level at which they can still decode data reliably. If the path loss plus any other losses reduce the signal below this sensitivity level, the link fails or falls back to a lower data rate.
Rule: Greater distance, more obstacles, and higher frequency all tend to increase path loss and reduce useful range.
Multipath, Reflection, and Fading
Because wireless signals can reflect off surfaces, there is often more than one path from transmitter to receiver. Some copies of the signal may arrive slightly later after bouncing off walls or metal surfaces. This is multipath.
Multipath can cause several effects:
- Constructive interference, where waves line up and boost the received signal.
- Destructive interference, where waves cancel each other and reduce the signal.
- Time spreading, where different copies arrive at slightly different times.
The result is fading, where the received signal strength changes over time or over small movements of the device. You might see this when moving slowly and the signal suddenly becomes worse or better by a lot even when you have not moved far.
Modern wireless systems use techniques like equalization, guard intervals, and multiple antennas to handle multipath and reduce its negative effects. The details are covered elsewhere, but it is important to know that multipath is a core challenge in wireless transmission.
Noise, Interference, and SNR
Wireless signals always share the medium with other electromagnetic energy. Some of that energy is noise from natural sources such as the sun or electrical devices. Some is interference from other transmitters that are using the same or nearby frequencies.
Noise and interference add unwanted variations to the signal. The receiver needs to distinguish the intended signal from this background. The key measure is the signal to noise ratio, or SNR.
SNR is often expressed in decibels, written as dB. A higher SNR means the signal is much stronger than the noise, which allows more complex modulation and higher data rates. A low SNR forces the system to use simpler modulation and lower speeds, or it may fail entirely.
Important concept: For reliable wireless transmission you need sufficient SNR. If noise or interference increase, or if the signal becomes weaker, the available data rate will drop and errors will rise.
Interference can come from:
- Other Wi‑Fi networks using the same channel.
- Non Wi‑Fi devices like microwave ovens or cordless phones in similar frequency bands.
- Industrial machines, poorly shielded electronics, or power equipment.
Careful channel selection and proper placement of access points help to reduce interference.
Channelization and Shared Medium
Wireless systems divide the available frequency band into channels. A channel is a specific range of frequencies used by one or more transmitters. Devices tune to a channel in order to send and receive.
The exact channel structure depends on the technology and region. For example, there are defined channel grids in the 2.4 GHz and 5 GHz bands for Wi‑Fi, but those details are discussed in later chapters.
Unlike a dedicated cable, a wireless channel is usually a shared medium. Many devices in range of each other must share the same channel. If two transmit at the same time on the same channel and in the same place, their signals can collide and corrupt each other. Wireless protocols use methods to coordinate access to the channel and reduce collisions.
Line of Sight and Obstacles
Some wireless links require a clear line of sight between antennas to work well, especially at very high frequencies. Others can tolerate obstacles and use reflections or diffraction to reach the receiver.
Materials affect wireless signals differently. For example:
| Material | Typical effect on RF signals |
|---|---|
| Open air | Lowest attenuation, best propagation |
| Wood | Moderate attenuation, some reflection |
| Brick | Higher attenuation, significant weakening |
| Concrete | Very high attenuation, often blocks many signals |
| Metal | Strong reflection, can block or redirect signals |
| Glass | Usually moderate, can be more severe for some coatings |
The thickness of the material and the frequency used are both important. Higher frequencies are more easily blocked by walls and objects.
For outdoor point to point links, network designers consider the Fresnel zone. This is an area around the direct line of sight path. Obstacles in this zone can still disturb the link even if the visual line of sight seems clear. The general goal is to keep most of the Fresnel zone free of obstacles to reduce additional path loss.
Power Limits and Regulations
Wireless transmission uses a shared public medium, so governments regulate how much power devices can transmit and which frequencies they can use. These regulations help avoid harmful interference between different services such as Wi‑Fi, mobile networks, radar, and satellite communications.
Each country or region has an authority that defines rules for:
- Allowed frequency bands for unlicensed use, for example common Wi‑Fi bands.
- Maximum transmit power and antenna gain for devices in each band.
- Special restrictions around airports, military zones, and other sensitive areas.
From a network perspective, this limits how far a standard device can reach and how dense deployments can be. Designers often must work within those limits rather than simply increasing power.
Practical Behavior of Wireless Links
In real networks wireless transmission has some common characteristics that differ from wired links:
- Variable data rates. Devices often adapt their modulation and coding based on SNR. As conditions change, the link speed changes.
- Higher error rates. Even with error correction, wireless frames are more likely to be lost or corrupted than frames on a good cable.
- Asymmetry. The link quality in one direction can differ from the other direction because of different transmit powers or antenna placements.
- Sensitivity to movement. Users walking, doors opening, or people standing in different places can affect multipath and attenuation.
Protocols above the physical layer are designed with these behaviors in mind. For example, retransmissions and rate adaptation are used to keep communication reliable despite changing radio conditions.
Summary
Wireless transmission at the physical layer is the process of turning bits into electromagnetic waves, sending them through space, and turning them back into bits at the receiver. Frequency, wavelength, modulation, antennas, path loss, multipath, SNR, and obstacles all shape how well a wireless link performs.
Later chapters on radio fundamentals and Wi‑Fi standards will build on these ideas and describe how specific wireless networking technologies implement them.