RF, Density & Tuning

A transmitted signal spreads out as it travels, so the power reaching a receiver falls off with distance even in a vacuum. The theoretical minimum is free-space path loss (FSPL), the loss from spreading alone with no obstacles. In decibels, with distance in kilometers and frequency in MHz:

FSPL(dB) = 20*log10(d_km) + 20*log10(f_MHz) + 32.45

The square terms mean two things matter equally. Loss rises with the square of distance: doubling the range adds about 6 dB. Loss rises with the square of frequency: doubling the frequency also adds about 6 dB. This is why, all else equal, a 5 GHz signal does not reach as far as a 2.4 GHz signal, and a 6 GHz signal reaches even less. The higher band is not weaker at the transmitter; it simply loses more to spreading and, because its wavelength is shorter, is absorbed more readily by walls and furniture.

Real indoor links lose far more than FSPL because the signal passes through materials. Published measurements vary with thickness, density, and moisture, but representative figures are roughly 3 dB for plasterboard or drywall, around 4 dB for a cinder block wall, around 6 dB for a metal-framed glass wall or a metal door, and 12 dB or more for a metal door set in brick. Reinforced concrete and dense masonry are far worse. Attenuation generally increases with frequency, so a wall that costs a 2.4 GHz signal a few dB can cost a 6 GHz signal noticeably more. Every 3 dB of loss halves the received power; every 6 dB halves the usable range. This is the budget you are spending when you place an AP on the wrong side of a wall.

Wi-Fi operates in three unlicensed band groups. Each is divided into channels of a base 20 MHz width that can be bonded into wider channels for more throughput.

2.4 GHz sits in the ISM band from 2400 to 2483.5 MHz. It is divided into channels spaced 5 MHz apart: channel 1 is centered at 2412 MHz, channel 6 at 2437 MHz, channel 11 at 2462 MHz. The US allows channels 1 through 11; most other countries allow 1 through 13, and channel 14 exists only in Japan for legacy 802.11b. The band penetrates walls best and reaches farthest, which is exactly why it is the most crowded and least useful for capacity.

5 GHz spans the U-NII bands: U-NII-1 (5150-5250 MHz, channels 36-48), U-NII-2A (5250-5350 MHz, channels 52-64), U-NII-2C (5470-5725 MHz, channels 100-144), and U-NII-3 (5725-5850 MHz, channels 149-165). It offers roughly 25 non-overlapping 20 MHz channels, far more room than 2.4 GHz. The cost is reach and the DFS obligation: channels in U-NII-2A and U-NII-2C are shared with radar, so an AP must run Dynamic Frequency Selection, listening for radar before and during use and vacating a channel within seconds if it detects a pulse.

6 GHz (Wi-Fi 6E and Wi-Fi 7) adds 1200 MHz from 5925 to 7125 MHz, split into U-NII-5, U-NII-6, U-NII-7, and U-NII-8. It carries 59 contiguous 20 MHz channels with no legacy clutter and no DFS, the cleanest spectrum Wi-Fi has. It is also the shortest-reaching band, and access is gated by power class (below).

The base channel is 20 MHz. Channels can be bonded into 40, 80, 160, and, with Wi-Fi 7, 320 MHz blocks. Each doubling of width roughly doubles peak throughput because it carries roughly twice the OFDM subcarriers: a 20 MHz channel uses 242 subcarriers, an 80 MHz channel uses 996. The trade-off is twofold. First, a wider channel consumes more of the band, so fewer non-overlapping channels remain for reuse: 2.4 GHz cannot fit even one clean 40 MHz channel alongside others, and a single 160 MHz channel eats most of the usable 5 GHz spectrum. Second, doubling the width raises the noise floor by about 3 dB, which lowers the signal-to-noise ratio at the receiver and shrinks the range at which the highest data rates hold. The 6 GHz band is the only place wide channels are practical at scale: it fits 14 channels at 80 MHz, 7 at 160 MHz, or a small number at 320 MHz.

Two APs share air in two very different ways, and the distinction governs every channel-planning decision.

Co-channel interference (CCI) happens when two APs (and their clients) use the same channel and can hear each other. Because 802.11 is half-duplex and uses CSMA/CA (carrier sense multiple access with collision avoidance), radios that hear one another take turns: each listens, defers while the channel is busy, and transmits when it is clear. CCI is not corruption. It is contention. The network keeps working, but the airtime is divided among everyone on the channel, so throughput per device falls as the population grows. CCI is the manageable, designed-for failure mode.

Adjacent-channel interference (ACI) happens when two APs use overlapping channels, for example channels 1 and 3 in 2.4 GHz. Their transmissions partially land on top of each other but are not cleanly decodable as the same channel, so CSMA/CA cannot arbitrate them. The result is raised noise, corrupted frames, and Layer 2 retransmissions. ACI is the worse failure mode because it degrades the link itself rather than merely sharing time on it. Sound channel planning trades ACI for CCI on purpose: it is always better to reuse a clean channel than to squeeze into an overlapping one.

The 2.4 GHz channels are numbered every 5 MHz, but each 20 MHz channel occupies four channel slots of spectrum on either side of its center. Channel 1 (2412 MHz) therefore spreads across roughly 2401-2423 MHz, channel 6 (2437 MHz) across roughly 2426-2448 MHz, and channel 11 (2462 MHz) across roughly 2451-2473 MHz. Those three are the only set spaced 25 MHz apart that fits inside the ~83.5 MHz of usable band without any pair overlapping. Any other choice, such as 1/5/9 or 1/4/8/11, forces overlap and turns CCI into ACI. This is the single most important rule for 2.4 GHz: design on 1, 6, 11 only, everywhere, every time.

It is tempting to fix poor coverage by adding access points. Past a point this makes things worse, because every added AP on a reused channel adds another contender for the same airtime. More APs raise coverage (signal is present in more places) but not necessarily capacity (useful throughput delivered), and in 2.4 GHz, where only three channels exist, a fourth nearby AP must reuse 1, 6, or 11 and so competes directly with an existing one.

The lever that converts density into capacity is channel reuse: lay out cells so that any two APs sharing a channel are far enough apart, or separated by enough walls, that they barely hear each other, while neighboring cells use different channels. A clean 5 or 6 GHz plan supports this because it has many channels; 2.4 GHz fundamentally cannot, which is why high-density designs lean on 5 and 6 GHz for capacity and treat 2.4 GHz as a thin coverage layer or disable some of its radios entirely. The goal is not maximum APs or maximum range; it is the right number of right-sized cells.

Cell sizing. A cell is the area one AP serves. Smaller cells mean each AP covers fewer clients and reuses channels sooner, which raises aggregate capacity but needs more APs and tighter channel planning. The cell is sized mainly by transmit power and AP spacing, not by adding range.

Transmit power. Turning an AP to maximum power is a common mistake. It enlarges the cell and so increases CCI overlap with neighbors, and it creates asymmetry: the AP shouts farther than a battery-powered client can answer, so a device may show full bars yet fail to transmit reliably. Better practice is to lower AP power toward the client's realistic power and keep neighboring cells comparable, so the downlink and uplink ranges match and cells stay contained. Regulatory ceilings cap this regardless: in 6 GHz, FCC power classes set the limits. Standard power (up to 36 dBm EIRP, 23 dBm/MHz PSD) requires an AFC (Automated Frequency Coordination) system and is allowed only in U-NII-5 and U-NII-7. Low-power indoor (LPI) is up to 30 dBm EIRP (5 dBm/MHz PSD), indoor only, no AFC. Very low power (VLP) is up to 14 dBm EIRP, indoor or outdoor, no AFC.

Data-rate tuning. A client far from its AP falls back to a lower MCS (modulation and coding scheme); higher MCS rates carry more bits per symbol but require a higher SNR to decode. A slow client is costly because of airtime fairness: a station forced down to 6 Mbps occupies the channel about nine times as long to move the same data as a station at 54 Mbps, and that airtime is taken from everyone on the channel. Disabling the lowest legacy rates raises the minimum data rate, which both trims management overhead and prunes distant, slow clients toward a nearer AP, shrinking effective cells without touching power.

Throughput in a busy network is governed less by any one link's speed than by how the shared channel is divided. Under CSMA/CA every transmitter that can hear the channel must wait its turn, so the channel's airtime is the real budget. As clients and neighboring APs multiply, contention rises and per-device throughput falls even when each individual link is fast. Three habits keep dense air healthy: plan channels so co-channel APs are spaced for reuse rather than overlapping (CCI, not ACI); raise the minimum data rate so slow clients do not monopolize airtime; and prefer the band with the most channels (6 GHz, then 5 GHz) for capacity, leaving 2.4 GHz as a fallback. The objective is to maximize delivered capacity per unit of airtime, not to maximize the speed shown on any single device.

• Open networks and the bands they ride on
• WPA2-Personal, WPA3-Personal and the security layered over RF
• Glossary for terms such as DFS, EIRP, MCS, and SNR

• IEEE 802.11 Working Group (Wireless LAN MAC and PHY standards): https://www.ieee802.org/11/
• Wi-Fi Alliance, Wi-Fi 6 in the 6 GHz band: https://www.wi-fi.org/news-events/newsroom/wi-fi-alliance-brings-wi-fi-6-into-6-ghz
• FCC, Unlicensed Use of the 6 GHz Band (power classes and AFC): https://docs.fcc.gov/public/attachments/DOC-397315A1.pdf
• FCC, Report and Order, Unlicensed Use of the 6 GHz Band: https://docs.fcc.gov/public/attachments/DOC-363490A1.pdf
• NTIA, evaluation of the 5 GHz bands (U-NII / DFS): https://www.ntia.gov/files/ntia/publications/ntia_5_ghz_report_01-25-2013.pdf
• FCC 13-22, unlicensed devices in the 5 GHz band: https://docs.fcc.gov/public/attachments/FCC-13-22A1.pdf
• Free-space path loss (FSPL formula and scaling): https://en.wikipedia.org/wiki/Free-space_path_loss
• List of WLAN channels (band/channel center frequencies): https://en.wikipedia.org/wiki/List_of_WLAN_channels
• NIST, Electromagnetic Signal Attenuation in Construction Materials: https://www.nist.gov/publications/electromagnetic-signal-attenuation-construction-materials