mmWave Free Space Link Budget

Radio frequency signals in the band 30 – 300 gigahertz are often called millimeter Wave (mmWave) frequencies. Traditional dual-band WLAN systems operate in 2 Ghz and 5 Ghz frequency bands.Tri-band WLAN systems will operate in one more additional band located at 60 Ghz frequency Operating Wireless communication link distance between transmitter and receiver is regulated by the transmitter power, antenna gains and path loss fading. Wireless communication Link budget describes the received power (Pr dBm) as a function of transmitter power (Pt dBm), antenna gain (Ga dB) and path loss (Lp dB). Simple link budget equation looks like
Pr = Pt + Ga – Lp

For the sake of discussion, consider an isotropic antenna at both ends of wireless link, which will reduce the link budget equation to Pr = Pt – Lp
For a fixed transmitter power, link budget can be computed by knowing the path loss Lp.
Free-space path loss is proportional to the square of the distance (d) between the transmitter and receiver, and also proportional to the square of the frequency (f) of the radio signal.
The equation for Free space path loss is described as below, where c is speed of light
Using the free space path loss expression, Free space AWGN channel Capacity (b/s) for any wireless communication link can be computed as,

where B is channel bandwidth and No is noise power spectral density. Using the above expressions, channel capacity densities for different radio frequencies are computed for 30 dBm transmit power and 20 Mhz channel bandwidth.

Inferences from above plot
  • 10.8 b/s/hz Capacity of 2 Ghz radio signal at 1000 meter distance is considered as reference
  • 5 Ghz radio signal can achieve the reference capacity only below 400 meters range
  • 10 Ghz radio signal can achieve the reference capacity only below 200 meters range
  • 20 Ghz radio signal can achieve the reference capacity only below 100 meters range
  • 60 Ghz radio signal can achieve the reference capacity only below 33 meters range
These capacities are computed for only free space line-of-sight (LOS) wireless links. mmWave communication links are severely degraded by atmospheric absorption.Transmit power and Channel bandwidth are the resources, that can help improving the capacity of a wireless link. Capacity is limited by the noise in AWGN channel. It can be observed from above figure, that capacity density does not grow with increased channel bandwidth, indeed capacity density becomes independent of bandwidth and solely guided by path attenuation at mmWave frequencies IEEE 802.11ad protocol PHY layer is defined as mmWave radio signal operating in 60 Ghz frequency band. IEEE 802.11ad protocol defines 3 different Physical layer (PHY) modulation methods CS, SC and OFDM. I will conclude this article with the link range description for different modulation methods implemented in IEEE 802.11ad PHY.

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