channel in wireless communication

In wireless communication, a channel refers to the medium through which signals are transmitted from a sender to a receiver. The concept of a channel is pivotal in understanding how wireless communication systems work because the channel introduces various impairments and characteristics that affect the quality and reliability of transmitted signals.

Let's delve into the technical details.

Types of Wireless Channels

1. Wireless Propagation Channels: These channels describe how electromagnetic waves propagate from the transmitter to the receiver. Depending on the environment and the frequency of operation, these channels can be:
   • Line-of-Sight (LOS): Direct propagation without significant obstacles.
   • Non-Line-of-Sight (NLOS): Propagation with reflections, diffractions, and scattering due to obstacles.
2. Multipath Channels: In most environments, signals don't follow a single path; instead, they bounce off objects, leading to multiple versions (or paths) of the transmitted signal reaching the receiver. This phenomenon is called multipath propagation.

Characteristics and Impairments

1. Path Loss: As a signal travels through a medium, it loses its power due to factors like distance, free-space loss, and obstacles. The path loss can be modeled using various path loss models such as the Free-Space Path Loss (FSPL) model or the Log-distance path loss model.
2. Shadowing: Due to obstacles like buildings, trees, or terrain, there can be localized variations in signal strength. This phenomenon, where the received signal strength varies because of obstacles, is known as shadowing.
3. Multipath Fading: When multiple versions of a signal arrive at the receiver with different delays and phases, they can interfere constructively or destructively. This interference is known as multipath fading. It can be divided into:
   • Flat Fading: Occurs when the channel's bandwidth is larger than the coherence bandwidth, making all frequencies experience the same fading.
   • Frequency-Selective Fading: Occurs when the channel's bandwidth is smaller than the coherence bandwidth, causing different frequencies to fade differently.
4. Doppler Spread: When there's relative motion between the transmitter and receiver, the frequency of the received signal can shift due to the Doppler effect. This phenomenon can lead to frequency-selective fading, especially in mobile communication scenarios.

Channel Models

To design and analyze wireless communication systems, engineers use mathematical models to represent the wireless channel. Some popular models include:

1. Deterministic Models: These models attempt to capture the exact behavior of the channel based on known environmental factors and geometries.
2. Statistical Models: Given the unpredictable nature of wireless channels, statistical models describe the channel using probabilistic distributions, often based on empirical measurements.

Mitigation Techniques

Given the challenges posed by wireless channels, various techniques are employed to mitigate their effects:

1. Diversity Techniques: Using multiple antennas or paths to improve signal reliability by exploiting the fact that fading affects different paths differently.
2. Equalization: Algorithms and techniques like Maximum Likelihood Sequence Estimation (MLSE) are used to reverse the effects of channel distortion.
3. Adaptive Modulation and Coding: Based on the channel conditions, the system dynamically adjusts the modulation scheme and coding rate to ensure optimal performance.