Data Throughput Overview

I have been being asked to troubleshot the throughput issue so many times.
Unfortunately my experience says "There is no clear/logical/deterministic
way to troubleshoot for throughput test".

Then what are we supposed to do ? Are we supposed to rely on "Hit and

Miss" strategy everytime we do the throughput test ? Is this process totally
random ?

No at least we are not in such a worst case, fortunately. I think we can set
some guidelines at least.

 What to expect ?

o Typical condition for LTE max throughput

o Typical Condition for 5G/NR max throughput

 First thing to remember for throughput troubleshooting

 What number you want to get ?

 Milestones in the history of throughput evolution

o LTE

 LTE Category 3 : 100 Mbps

 LTE Category 4 : 150 Mbps

 LTE Category 9 : 450 Mbps

 LTE Category 12 : 1 Gbps

 LTE Category 18,19,20 : 1.2.1.6,2.0 Gbps

o 5G/NR

What to expect ?
Almost every time when a new technology comes, what we we hear most
frequently would be 'you can download a movie in less than a minuite (in
early LTE case)' or 'you can download a movie in a few seconds (in early 5G
case)'.

Usually the most common reaction to this kind of (marketing ?) statement

would be

 Do we really need such an unbelievable throughput ?

 Why I don't see the same throughput on my phone ?

I don't think I need to answer to the first question. Everybody says 'who
would need such a high speed ?', but nobody complains when it really comes
and start complaining about the low throughput a few years later. However
high speed you get, a lot of new applications or use cases comes that
let such a big pipe look limiting. In my personal experience, I think I've heard
the same question (i.e, 'who would need such a high speed) since early
HSDAP which is just a few Mbps) and I am still hearing complaints about the
low throughput at the period of tens of Mbps. So.. my answer to this question
is 'don't worry, you will soon want to get the big pipe'.

What I am trying to do in this section is more about answering the second

question : Why I don't see the same throughput on my phone ?

In short, it is very unlikely that you would see the same throughput on your
phone if you are trying it in live network as a regular subscriber. Why not ?
There are two common reason as below

 The speed that they are talking is from the test with very special
condition that is very unlikely available to you

 Often people (especially media) talks about the speed that will be
available when the technology gets fully evolved, but not now.

For example, if you are talking about around 6 Gbps speed for 5G(NR), it
would belong to the first case. If you are talking about 20 Gbps, it would
belong to the second case at the time of writing this (Nov 2020).

I am not going to talk more about the second case. I will be focusing more on
the first case.... i.e, what would be the special condition that let such a
tremendous speed come true ?
There are several factors that is critical to determine the throughput can be
summarized in an illustration as shown below. This logic applies at least to
4G and 5G. 3G throughput (i.e, HSPA) is determined by a little bit different
set of factors(parameters) but I would focus more on 4G/5G throughput. If
you are interested in 3G throughput, refer to this note.

Let me comment a little bit about this illustration this illustration. The
comments written below is just written description of the illustration..
meaning the description and the illustration represents a same thing.

 The first main factor you can think of is 'how many carriers are used for
the data traffic'. Assuming that the bandwidth of each carrier is same,
the more carriers are used the higher throughput you can expect
(NOTE : In reality, there are cases where the bandwidth of each carrier
would not be same. In this case, total bandwidth (called 'aggregated
bandwidth) can be the determining factor.

 Next factor you should consider is that how many resource blocks
within each of the carrier are used for the traffic. In case of estimating
the max throughput (theoretical throughput), we assume that every
resource blocks of each of the carriers are used for the traffic.

 Then you should think of what kind of modulation scheme is used for
the traffic. Simply put, the data rate goes higher as the modulation
schedule progress in QPSK -> 16QAM -> 64QAM -> 256QAM (NOTE : in
reality, not only modulation scheme but also coderate affects the data
rate, a specific parameter defined in 3GPP spec called MCS
(modulation coding scheme) determines both modulation scheme and
coderate).

 Next major factor is how many layers are used for the traffic. You can
think of this factor as 'what type of MIMO is used'. Roughly put, you
can think of SISO as single Layer, 2x2 MIMO as 2 layers and 4x4 as 4
layer etc. Multiple layer imply multiple data streams are transmitted
simultaneously meaning higher data throughput.

 One last think you should consider especially in real life traffic is 'how
many UEs are connected to the same network at the same time'. The
more UEs are connected to the network, the less throughput you would
get on your device since the physical resources for the traffic should be
split among multiple devices.
Typical condition for LTE max throughput

At the point of writing this part (Nov 2020), I think the typical condition for
achieving the max throughput in LTE can be illustrated as follows.
 Number of Carriers : 5

 Bandwidth of each carrier : 20 Mhz (this is max BW for one LTE channel
defined in 3GPP)

 Number of RB for each carrier : 100

 Modulation Scheme : 256 QAM, corresponding to MCS 27

 Number of layers : 4 (4x4 MIMO)