The greatest achievement of LTE is the high data rate it can provide over a wireless channel.
In our last few videos we have seen various techniques such as MIMO, CoMP, Hetnet which
are used in LTE to increase the throughput.
So have you ever wondered what factors contribute to the data throughput that we get on LTE?
Hello Everyone welcome back to the fundamentals of LTE.
Today we will discuss how Throughput is calculated in LTE.
If we go by the Shannon-hartley formulae for channel capacity, we see that that the throughput
in bits per second depends on the channel bandwidth and the signal quality (SNR).
This formula indicates the highest possible throughput of a noisy channel and it applies
to every communication channel be it GSM/2G or LTE/4G.
We can see here that although Bandwidth is an indispensable factor in the overall throughput,
it all depends on how we use the SNR conditions for increasing throughput.
For example, Higher SNR means, we can use a higher order modulation scheme, thus increasing
the number of bits that can be transmitted in the same bandwidth.
Thus, a channel using 64 QAM can have One and half times higher throughput than 16QAM
and 3 times higher throughput than a channel using QPSK.
Now, LTE uses adaptive modulation which ensures dynamic selection of Modulation scheme based
on the SNR conditions.
So the first factor which affects the data throughput is the Modulation coding Scheme.
Now, based on the Modulation Scheme the bits are mapped on the Resource Element.
These Resource elements in turn aggregates into the Resource Block.
We know that in LTE we can have variable bandwidth.
And higher the bandwidth means more number of Resource Blocks.
So, The Second Factor is the number of Resource Blocks used.
The Third factor that comes into play here is MIMO.
We have seen in an earlier video how MIMO increases the throughput multiple folds.
Now Nyquist Theory suggests that a Channel of Bandwidth B can provide a maximum symbol
rate of 2B symbols/second.
However for practical consideration the symbol rate is usually taken as B Symbols/second.
So Let's assume LTE bandwidth of 20MHz is using 64 QAM over its whole spectrum along
with 2×2 MIMO.
Then considering the Nyquist Theory the throughput will be 20 mega symbols/second multiplied
by 6 bits for each symbol as we have assumed 64QAM, multiplied by 2 for 2×2 MIMO giving
a value of 240 Megabits/second.
This is ONLY a rough estimation of the total throughput an ENB can provide, over one of
its sector with given scenarios.
Now let us try to calculate it from LTE perspective using a more precise approach-
In order to calculate the throughput we first need to calculate the number of resource elements
in a given Bandwidth.
Each Resource block in LTE consist of 12 Subcarriers and 6 or 7 OFDM Symbols.
So each Resource Block has 84 Resource Elements.
Where 1 Resource element equal 1 subcarrier and 1 symbol.
- In an LTE Bandwidth of 20Mhz we can have a maximum of 100 Resource Blocks thus a total
of 8400 Resource element.
These 8400 Resource Elements are available for 0.5 milliseconds.
So in 1 second a total of 8400*2000 Resource elements are available.
Now, in 64 QAM modulation scheme 6 bits can be transmitted on each Resource Element and
assuming that 2X2 MIMO is used, we get a total throughput of 6*2*8400*2000 i.e 201.6Mbps.
This ideal throughput is based on physical layer operation and not on upper layers such
as IP layer.
Also we have assumed that, there is no physical layer overhead i.e every resource element
is being used for User data transmission.
Now if we want to stream something on NetFlix, then can we expect this much of data throughput ?
Simple answer would be yes, but in reality
we have to consider several other factors which affects the throughput significantly.
When a stream of data comes from IP layer to the physical layer, several overheads are
being added e.g, PDCP header, RLC header, MAC header etc.
So, the IP layer throughput gets reduced from the physical layer.
For the scenarios that we have considered, approximately 6% of total resource elements
is occupied by Cyclic Prefix, 10% by Guard Bands, 10% by signalling overhead and another
14% for Pilot Overhead.
If we add them all, we can see that 40% of the 201.6 Mbps is utilized for overheads.
Thus only 120 Mbps is available for user data.
Now we have to keep in mind that this is the cumulative throughput available for all the
users attached to one sector of the eNodeB.
That means if 10 users are connected to the same sector of the same eNodeB then the throughput
gets divided.
So even with good RF coverage your NEtFlix stream can suffer buffering if the number
of users connected is more.
Same way we can calculate throughput for uplink transmission as well.
For even more accurate calculations of throughput you can refer to 3GPP document 36.213 the
link to which has been provided in the description section.
So friends today we have seen how throughput is calculated in LTE and how various factors
affect the actual throughput that an end user experiences.
From our next video we will finally dive into the ocean of LTE starting with the architecture.
Happy Learning.