Unit
- Definition and scope: Wireless communication transfers information between two or more points without a physical connection. It spans radio, microwave, infrared, and visible light bands. Applications include telephony, data networking, broadcasting, and remote sensing.
- Historical milestones: Guglielmo Marconi's 1901 transatlantic transmission proved radio waves could cross oceans. The 1940s brought mobile telephony; the 1970s introduced cellular concepts by Bell Labs. The IEEE 802.11 standard (1997) launched WLANs, and 4G LTE (2009) enabled mobile broadband.
- Key advantages: Mobility allows users to communicate while moving. Installation speed exceeds wired networks, especially in difficult terrain. Scalability is simpler: adding a new device often requires just power and registration.
- Inherent limitations: Spectrum is a finite, regulated resource. Interference from other transmitters degrades link quality. Signal attenuation and fading reduce range and throughput. Security is harder to enforce over broadcast media.
- OSI model alignment: Wireless communication operates primarily at the physical and data link layers. The physical layer handles modulation, frequency, and power. The MAC sublayer governs access to the shared medium, collision avoidance, and addressing.
- Propagation environment: The path between transmitter and receiver includes free space, obstacles, and reflecting surfaces. Signals can be reflected, diffracted, or scattered. These effects cause time-varying channel conditions that the system must handle.
- Systems engineering perspective: A wireless link involves a transmitter, a receiver, and a channel. The transmitter encodes and modulates data onto a carrier wave. The receiver demodulates and decodes. The channel introduces noise, interference, and distortion.
- Frequency reuse: Cellular systems divide service areas into cells, each using a subset of frequencies. Adjacent cells use different frequencies to avoid co-channel interference. The same frequencies can be reused in non-adjacent cells, increasing capacity.
- Modulation and coding: Digital modulation schemes like QPSK, 16-QAM, and OFDM map data to carrier waveforms. Higher order modulation increases bit rate but requires better signal-to-noise ratio. Forward error correction codes (e.g., convolutional, LDPC) add redundancy to correct bit errors caused by noise and fading.
- Multiple access techniques: Wireless systems share the medium among users. FDMA assigns distinct frequency bands, TDMA uses time slots, and CDMA spreads signals with orthogonal codes. Modern systems like 4G and 5G use OFDMA, dividing spectrum into subcarriers allocated dynamically to users.
Code Example
Python
def free_space_path_loss(distance_m, frequency_hz):
# Friis transmission equation for isotropic antennas
wavelength = 3e8 / frequency_hz
loss_db = 20 * log10(4 * pi * distance_m / wavelength)
return loss_db
# Example: 2.4 GHz at 100 meters
loss = free_space_path_loss(100, 2.4e9)
print('Path loss: {:.1f} dB'.format(loss))This Python function computes free-space path loss using the Friis equation. At 100 m and 2.4 GHz, loss is about 80.0 dB. Real environments add extra loss from walls and multipath.
System Architecture
Pattern: Point-to-Point Wireless Link
Components:
- Transmitter: converts data to a modulated RF signal at a specific frequency and power level.
- Channel: the physical medium (air) that carries the wave; includes noise sources and obstacles.
- Receiver: captures the RF signal, filters out interference, and demodulates to recover data.
- Antennas: couple energy between the circuit and free space; gain and directivity shape coverage.
Quick Revision
- Wireless communication enables mobility and rapid deployment at the cost of spectrum limitations and interference.
- Historical evolution from Marconi to 4G/5G shows increasing data rates and spectral efficiency.
- The OSI physical and MAC layers are most affected by wireless propagation characteristics.
- Frequency reuse underpins cellular capacity while avoiding co-channel interference.