Wireless Basics - Max devices?

[jasons520]

Hi Everyone,

I was wondering if anyone could give me some advice on what a theoretical maximum receiving devices would be?
Hypothetically, if i were to transmit a 2.4ghz signal, at 5w, how many devices could i expect to be able to receive that signal?
Obviously it will be effected by numerous things like humidity and objects in the way of the signal etc.

But how does an RF engineer determine how many devices will be able to receive a signal? Even like FM radio.

I am really stuck on trying to understand this concept.

Thanks for anyone's time if they reply

[helius]

How many receivers can you fit into a solid sphere around the transmitter?

[jasons520]

Thanks for the reply,

but, how would you calculate the size of that solid sphere around the transmitter? Would free space loss etc be used?
Or is there another method i don't know off?

[helius]

In the far field, there is effectively no coupling between transmitter and receiver, and you would indeed calculate the loss budget based on free space attenuation, fading, polarization, etc. Then construct a sphere at that radius. Of course, the transmitter gain will not be perfectly isotropic, so you may integrate over a parametric surface based on its polar pattern.
In the real world, receiving antennas will re-emit some energy as interference so there may be a minimum spacing. I don't really know how you would calculate it.

[ataradov]

Theoretical formula to calculate all of  this is free-space path loss ( https://en.wikipedia.org/wiki/Free-space_path_loss ). Practice will be influenced by a lot of other things, of course.

[ejeffrey]

Theoretical maximum?  Like how theoretical?

According to some random chart on the internet I found, -60 dBm is a "good, reliable signal".  1 watts of transmitted power is 30 dBm.   By that logic if you surround the transmitter with receivers so that 100% of the signal is received, you can have approximately 1 billion receivers.  Arranging them properly is left as an exercise for the reader

Number of receivers is not a useful metric.  For the most part, individual receivers have negligible interaction--it isn't like putting an extra receiver is going to use up the signal.  What designers normally care about is the communication range.  If you have your FM transmitter or wifi base station, you try to determine how the signal falls off with distance.  This can be just inverse-square fall off, obstructions, the curvature of the earth, or diffraction.  So of this you calculate, some you determine by a site survey, and you add some design margin for variable effects.  Then you determine how small of a signal a reasonable receiver can operate with.  This depends on the size and design of the antenna, the noise floor of the receiver, and possible interference from other sources.  You put all that together and you get a service radius.  People within that radius can expect to receive your signal, those outside may not.

[hagster]

The theoretical maximum sensitivity for a receiver is set by the Shannon Limit and the tollerable bit error rate. The aim of sucessful radio design is to build something that can approach this theoretical limit. Today many comercial systems already operate very close to this performance limit.

Fundementally it takes a certain amount of ENERGY to decode(at a given error rate) a single bit of energy.

Note that I use the term ENERGY not power. Energy = Power x Time. Hence for a given power transmitter you can improve your potential rx sensitivity by increasing the transmit time of each bit. Or slow data rate ~= better sensitivity.

To answer the original question of how many receivers could theoretically could receive a signal we can make a few theoretical(ie totally unrealistic assumptions).

Say we have a 0dBm(1mW) transmitter and receivers with a sensitivity of -100dBm.

Assuming no loss in the tx medium (a good assumption for dry air or outer space) and we can arrange our receivers such that no energy escapes our system(perhaps we create a metal sphere of conjoined horn antenna).

In this perfect scenario we can divide the Tx power by the Rx sensitivity and get our figure. I make this 10,000,000,000 recevers.