Wireless communication impairments

Wireless communication is affected by several factors that can reduce signal quality:

Impairment	Description
Signal attenuation	Signal attenuation is the reduction in signal strength as the signal travels. It occurs because: electromagnetic radiation loses strength due to absorption, reflection, and scattering when passing through matter even in free space, the signal spreads out as distance increases the reduction in received signal power due to propagation over distance is called path loss.
Interference	Interference occurs when unwanted signals overlap with the desired signal in the same frequency band. Examples include: other wireless devices using the same frequency, e.g., 2.4 GHz wireless phones and 802.11b Wi-Fi LANs electrical devices that generate electromagnetic noise, e.g., microwaves or electric motors Many newer 802.11 standards operate in the 5 GHz band because it provides more channels and typically experiences less interference than the crowded 2.4 GHz band.
Multipath propagation	The electromagnetic wave can bounce off objects or the ground and reach the receiver through different paths. As a result, the received signal becomes distorted due to delayed copies of the transmitted signal arriving at the receiver at different times.

The receiver obtains an electromagnetic signal that consists of:

1. a degraded version of the original signal, affected by attenuation and multipath propagation
2. background noise from the environment

Error handling in wireless links

Because wireless links experience more bit errors than wired links, wireless protocols use:

• CRC error detection codes to detect corrupted frames
• link-level reliable-data-transfer protocols to retransmit damaged frames

SNR

The Signal-to-Noise Ratio (SNR) is the ratio of received signal power to noise power:

• expressed in decibels (dB)
• higher SNR means the signal is easier to distinguish from the noise
• therefore, a higher SNR generally leads to more reliable communication

BER

The Bit Error Rate (BER) is the probability that a transmitted bit is received incorrectly:

• lower BER = fewer transmission errors
• higher BER = less reliable communication

Relationship between SNR, BER, and modulation

Different modulation techniques (such as BPSK and QAM16) behave differently depending on the SNR.

BER versus SNR for 3 different physical-layer modulation techniques:

For a given modulation technique (e.g. BPSK):

• as SNR increases, BER decreases
• communication becomes more reliable
• one way to increase SNR is to transmit with more power, but this has disadvantages:
  • higher energy consumption
  • higher transmit power increases the signal range, making it more likely to interfere with nearby transmissions on the same frequency

For a given SNR:

• higher-speed modulation techniques produce a higher BER
• when SNR = 10 dB
  • BPSK (1 Mbps): BER < 10^-7 (very reliable)
  • QAM16 (4 Mbps): BER ≈ 10^-1 (far too many errors for practical use)
• when SNR = 20 dB
  • QAM16 (4 Mbps): BER < 10^-7 (now reliable enough to use)
  • BPSK (1 Mbps): BER is even lower (below the plotted range)
• key idea:
  • at low SNR, slower modulation methods are more reliable
  • at high SNR, faster modulation methods become practical and provide higher data rates

Adaptive modulation:

• wireless channel conditions change over time (causing the available SNR and achievable BER to vary) due to:
  • mobility
  • obstacles
  • changes in the environment
• wireless systems (including 802.11 Wi-Fi and 4G/5G networks) automatically switch between modulation techniques
• its goal is to maximize the transmission rate while keeping the BER below an acceptable level based on the current channel conditions