RF Signals in Wireless Communication

• Communication in wireless networks depends on radio frequency (RF) signals, electromagnetic waves sent through the air.
• Transmitters send RF signals, and stations receive them.
• Wireless devices usually have both a radio transmitter and receiver.
• Wireless clients' transmitters and antennas are smaller than those in wireless access points (WAPs).

RF Signal Characteristics

• Frequency and amplitude are the main characteristics of RF signals.
• Frequency is the number of electromagnetic wave cycles in a second, measured in Hertz (Hz).
• Wireless networks operate in the Gigahertz (GHz) range, meaning millions of cycles per second.
• Amplitude measures the strength of an RF signal, either in absolute power (watts) or relative power (decibels).
• Wireless devices use milliwatts (mW).
• Decibel (dB) is a relative unit of signal strength, using a logarithmic ratio compared to a reference source.
• Phase measures timing shift between signal cycles.
• In-phase RF signals add to signal strength.
• Out-of-phase RF signals have a negative effect.

Frequency Bands and Channels

• RF signal use is regulated, requiring compliant devices to operate within allocated frequencies (bands).
• Devices operate on channels within a band.
• Channels are distinct frequencies that keep RF signals separated.
• Bandwidth is the frequency range needed to support an RF signal.
• Bandwidth overlap between channels can cause issues due to limited allocated channels.

Amplitude and Signal Strength

• Amplitude represents the signal's strength at a point, like a transmitter.
• Logarithmic dB scale simplifies power discussion, with these rules:
  • Signals of equal strength have 0 dB difference.
  • Doubling signal strength adds 3 dB.
  • Tenfold increase adds 10 dB.
• Signal strength halving results in 3 dB decrease.
• Tenfold reduction gives 10 dB decrease.
• Relative power is measured against 1mW, creating the dB-milliwatt (dBm) unit.
• RF signals can be weakened or strengthened between source and destination.
• Antennas strengthen signals.
• Free space and cable resistance weaken signals
• Increase in amplitude = gain
• Reduction in amplitude = loss.
• Signal loss is affected by distance and frequency.
• Frequency affects a signal's wavelength and its ability to pass through media.

Signal vs. Noise

• An RF receiver has a minimum signal strength, or sensitivity, it can discern.

• Signals below the receiver's sensitivity are indistinguishable from noise.

• Received Signal Strength Indicator (RSSI) measures signal strength at the receiver.

• RSSI varies as conditions change.

• Comparing RSSI to average noise is more helpful.

• Noise consists of any electromagnetic radiation other than the signal, often caused by interference.

• Interference may come from devices on the same frequency/channel.

• More devices on the same channel increase the chance of interference.

• Microwave ovens, cordless phones, and power lines can also cause interference.

• Metal objects can block signals

• The noise floor is the average noise amount.

• If the difference between noise floor and RSSI is too small, the signal is hard to distinguish.

• Signal-to-noise ratio (SNR) is the RSSI and noise floor difference, expressed in dBm.

Wireless Bands and Channels

• WLAN communication uses the 2.4-GHz and 5-GHz bands.

• The 2.4-GHz band is divided into 14 channels, each 24 MHz wide.

• Channel availability can be limited by regulators; for example, in the U.S., only channels 1-11 are allowed.

• 2.4-GHz channels overlap and can interfere.

• Non-overlapping channels are recommended and in the U.S., these are only channels 1, 6, and 11.

• Channels 12 and 13 are usable in most of the world, and channel 14 is exclusive to Japan.

• The 5-GHz spectrum is in four bands each band has non-overlapping 20 MHz wide channels.

• U-NII-1: 5.150 – 5.250 GHz (Channels 36, 40, 44, 48)

• U-NII-2: 5.250 – 5.350 GHz (Channels 52, 56, 60, 64)

• U-NII-2 Extended: 5.470 – 5.725 GHz (Channels 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140)

• U-NII-3: 5.725 – 5.825 GHz (Channels 149, 153, 157, 161)

• Gaps between ranges are intended for other purposes, but some efforts are being made to repurpose them for wireless use.

• 5-GHz channels don't interfere and have less interference than 2.4-GHz band because the 2.4-GHz band have more devices operating on it than the 5-GHz band.

Modulation Techniques

• WLANs use RF signals as carriers for network data.
• Frequency Hopping Spread Spectrum (FHSS) was originally used, which changes radio channels in a synchronized pattern.
• FHSS prevents eavesdropping but doesn't scale well with high data rates.
• Direct Sequence Spread Spectrum (DSSS) and Orthogonal Frequency Division Multiplexing (OFDM) superseded FHSS.
• DSSS uses chipping codes to spread data transmissions across the spectrum.
• Chipping ensures data is received even if the entire chip stream is not, increasing resistance to signal loss.
• OFDM divides a frequency range into 20-MHz channels and sub-divides these into a series of subcarriers.
• OFDM channels are divided in to 52 subcarriers per 20-MHz channel, with each subcarrier 300 kHz wide.
• By dividing channels into subcarriers, OFDM can provide higher transmission rates.
• OFDM gives protection against attenuation, fading and interference.
• Quadrature amplitude modulation (QAM) is used for amplitude and phase modulation.

Wireless Standards

• IEEE released the first WLAN standard, 802.11-1997, in 1997.
  • Operates at 2.4 GHz with a maximum throughput of 2 Mbps.
• Amendments to the original 802.11-1997 standard:
  • 802.11b, 802.11a, 802.11g, 802.11n, 802.11ac
• The 802.11ax specification release date expected in late 2020.
• 802.11a and 802.11b were both introduced in 1999.
  • 802.11a operates at 5.0 GHz with 54 Mbps max data throughput.
  • 802.11b operates at 2.4 GHz, uses DSSS, and has 11 Mbps max data throughput.
• 802.11b gained more acceptance than 802.11a.
• 802.11g was introduced in 2003.
  • Operates at 2.4 GHz and coexists with 802.11b using DSSS.
  • Uses OFDM for faster data rates, similar to 802.11a.
• 802.11n was introduced in 2009.
  • Can operate in both 2.4 GHz and 5 GHz bands and is compatible with 802.11a/b/g.
  • It Uses OFDM
  • Supports Multiple Input, Multiple Output (MIMO) and channel bonding.
  • MIMO and channel bonding increased the maximum data rate to 600 Mbps.
• 802.11ac was introduced in 2013.
  • Operates at 5 GHz.
  • MU-MIMO and extended channel bonding increased transmission speeds to 3.5 Gbps.
• 802.11ax was introduced in 2019.
  • Operates primarily in 2.4-GHz and 5-GHz bands at 9.6 Gbps.
  • Improved power-control methods reduce interference and consumption.
• The Wi-Fi Alliance labelled 802.11ax as Wi-Fi 6, 802.11n as Wi-Fi 4, and 802.11ac as Wi-Fi 5.
• Wi-Fi 6E enhances Wi-Fi 6 by extending 802.11ax into the 6-GHz band.
• Wi-Fi 6 and 6E require Wi-Fi Protected Access 3 (WPA3) for security.

Antenna Characteristics

• An antenna radiates an RF signal as an electromagnetic wave when alternating current passes through it.
• Antennas focus RF signals instead of amplifying them.
• Gain measures how well an antenna focuses a signal.
  • Gain is used to offset signal loss between transmitter and antenna.
• Gain is measured in dBi (dB isotropic) relative to a theoretical isotropic antenna.
• Higher gain antennas reach longer distances than lower gain antennas.
• Effective Isotropic Radiated Power (EIRP) calculates energy radiated from antenna.
  • EIRP = Transmission power + Gain - Cable loss
• Transmission power measures total energy emitted from an antenna.
• Polarization describes a wave's directional orientation through open air:
  • Vertical, horizontal, and circular
• Vertically polarized antennas emit waves moving up and down; omnidirectional antennas are vertically polarized.
• Horizontally polarized antennas emit waves moving left and right; dipole antennas are horizontally polarized.
• Circularly polarized antennas emit waves in a continuous circle.
• Antenna polarization determines the electrical field's orientation and should be correctly oriented.
• Beamwidth is the width of an emitted signal, representing the radiation pattern's spread.
• Beamwidth measures the distance between half-power points of a radiation pattern.
• Common antenna designs: dipole, integrated omnidirectional, patch, Yagi, and parabolic dish.
• Azimuth plot is the horizontal plot showing the radiation pattern
• Elevation plot is the vertical plot showing the radiation pattern

Dipole Antennas

• Dipole antennas are omnidirectional with low gain, typically 2-5 dBi.
• Provide comprehensive coverage over small areas.
• Wide beamwidth suits maximizing RF coverage in homes or offices.
• Commonly found on consumer and SOHO wireless routers/APs.

Integrated Omnidirectional Antennas

• Low gain antennas providing between 2-5 dBi.gain
• Integrated omnidirectional antennas area of coverage is roughly spherical.
• Unlike dipole antennas, integrated omnidirectional antennas are internal and cannot be adjusted without moving the entire device.
• Commonly found in enterprise wireless routers/APs.

Patch Antennas

• Directional antennas with 6 to 10 dBi gain.
• Designed to focus RF energy in a semi-spherical area and provide coverage to restricted areas.
• Patch antennas are an excellent solution for providing coverage within a warehouse without inadvertently extending coverage to external areas such as parking lots
• Commonly found in industrial and enterprise deployments.

Yagi Antennas

• Yagi antennas, also known as Yagi-Uda, are high gain directional antennas
• They typically provide 10 to 14 dBi of gain within their narrowly focused beamwidth
• Common use is site-to-site wireless deployments than client connectivity.
• Yagi antennas may have sufficient beamwidth for general client connectivity under certain conditions.

Parabolic Dish Antennas

• Provide the highest gain among common directional antennas, typically 20-30 dBi.
• Feature an extremely narrow beamwidth.
• Almost exclusively used for long-range wireless solutions, such as connecting a branch office to a headquarters location