At the heart of any wireless communication is the idea of transmission. When we send a wireless signal from an access point down to a user device, we understand that we're transmitting the signal. We often refer to this as 'Tx.' So, there's a transmission, and then there's the reception of that signal, which we abbreviate as 'Rx.' Now, something many of us intuitively understand, due to the prevalence of wireless devices these days, is that the further we move away from this access point, the lower the signal strength will be.
This phenomenon occurs because of
the concept of free space path loss and possibly attenuation as the signal
passes through objects. Ultimately, the power level decreases over distance and
time. Consequently, we might need to increase the power of this access point,
ensuring it's powerful enough to create what we call a 'cell.'
When we install an access point,
whether in an office hallway or in a house, we typically draw a circle around
it and designate it as the cell for that wireless access point. What we mean by
this is that the power level starts off very strongly near the access point,
but we understand that it will weaken as we move away (as shown on the RF
Heatmap). When we draw the line at this point, it represents the threshold at
which we transition from a good-quality signal to a signal that is usable but
not ideal. So, if I want to ensure that devices located farther away receive a
wireless signal, I may need to go to the access point and increase its power.
So, if the wireless receiver in
this case turns around and transmits back, but its power level is lower than
the access point's, it might not actually be able to reach the AP. We need to
make sure that our transmit power between the two devices—note that they don't
have to be equal in many cases, but they need to be strong enough to reach each
other. This is the type of thing that can disrupt our wireless networks: if our
access points are broadcasting so powerfully that the signal goes everywhere,
our wireless devices can't talk back.
Interestingly, as we try to
minimize our footprint, a common feature in wireless networks these days, we
might find that the access point's cell is smaller than the device's cell. This
device might have enough power to communicate with the access point, but the
access point can't communicate back.
Now, one thing to understand about power levels is that we measure power in watts (W). When we express it mathematically, power is equal to the rate of energy expenditure over time. In simpler terms, if we input the same amount of energy into something but do it in a significantly shorter time, the power increases. Think of it as the 'oomph' factor, representing the effort we put into something.
For example, consider my front door, which sometimes gets stuck. Instead of just pushing it with a sustained effort, I could exert a burst of energy in a very short time by lowering my shoulder and giving it a strong shove. It illustrates the concept of power. This concept is quantified in watts (W).
The unit 'Watt' is named after a
Scottish inventor, James Watt. Since it's named after a person, the 'W' in
'Watt' is always capitalized. So, when expressing power, if I'm transmitting,
for example, at one watt, I don't write it as '1 watt' with a lowercase 'w'; I
must write it as '1 Watt' with a capital 'W.'
Now, the concept of Watt applies
to all types of wireless radiation. For instance, when I think about a
microwave in my kitchen that heats up my food, it requires a substantial amount
of power. Typically, the microwaves used for heating food operate in a range of
around 700 to 1,200 Watts.
Of course, there are appliances
both smaller and larger than this range, but the point is that we are dealing
with power levels in the hundreds and even exceeding 1,000 Watts. On the other
hand, in the realm of Wi-Fi, our transmissions are typically in the order of
0.1 Watts, or possibly even smaller.
We might transmit at 0.05 Watts
or 0.025 Watts. At this point, you might wonder why we still call them 'Watts.'
It might be more appropriate to use 'milliwatts.' That's exactly what we're
going to do when discussing wireless transmissions.
So, when I say I'm transmitting at 0.1 Watts, it's equivalent to 100 milliwatts (mW). It's important to note that 'm' in 'milli' can be lowercase, but the 'W' in 'Watts' must be uppercase. Therefore, it's 100 milliwatts (mW). Similarly, this is 50 milliwatts (mW), and finally, we have 25 milliwatts (mW).
Milli' here signifies one
one-thousandth (1/1000) of a Watt. Therefore, 100 milliwatts (mW) is equivalent to 0.1 Watt.
We still face a problem when it
comes to how power radiates outward into free space. Think of it this way: a
wireless transmitter, often depicted as a two-dimensional concept, operates
within a three-dimensional space. Without any antennas altering the signal's
propagation, it's like the concept of a balloon inflating as the signal
propagates outward.
I like using the balloon analogy
because, when I hold a deflated balloon, it contains a significant amount of
latex in a small space. So, if I examine a square centimeter of the balloon's
surface, I see a dense concentration of latex. However, as I inflate the
balloon, it grows larger, and that same square centimeter contains much less
latex. Eventually, it might even become so thin that it pops, unable to
maintain its structural integrity. This is like blowing bubbles with bubblegum.
The concept here is that the
power transmitted by a radio is akin to the latex in the balloon. During the
initial transmission, let's say I have 100 milliwatts of power in a confined
space. But as the signal propagates, that 100 milliwatts spread out, covering
the size of a room or even a house. Wireless signals have impressive reach.
When I'm right next to the wireless device, I should receive a strong signal. However,
as I move farther away, just like the square centimeter on the balloon, the
power diminishes significantly.
What might surprise us is the
extent of power loss. For example, right next to this access point, we might
have 100 milliwatts of power. However, if I move just a few feet away and sit
at my desk with my laptop, I could find that the signal strength drops
significantly, perhaps to as low as 0.00001 milliwatts. This is much lower than
what I might have expected, perhaps around 50 or 30 milliwatts. When I first
started studying wireless, I assumed that the signal would degrade in a linear
fashion, but it's exponential.
So, as I move farther away from the access point, the signal strength decreases rapidly, not linearly. We're talking about exponential drop-offs. This exponential drop-off emphasizes the need for a better way to measure wireless signals. When I see numbers like 0.00001 milliwatts compared to six zeros followed by one milliwatts, it's challenging to determine which one represents a good signal, and which one is poor. While I can identify which one is worse, it's still difficult to judge their quality. Looking at a series of zeros doesn't provide a clear indication of signal strength, and it doesn't help me understand the gradual decline in signal strength either.
So, we need a way to convert this over to a linear measurement system, and that would be the decibel ‘dB.’
Happy Learning!!
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